nanoparticle synthesis methods review

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Nanoparticle synthesis articles from across Nature Portfolio

Nanoparticle synthesis refers to methods for creating nanoparticles. Nanoparticles can be derived from larger molecules, or synthesized by ‘bottom-up’ methods that, for example, nucleate and grow particles from fine molecular distributions in liquid or vapour phase. Synthesis can also include functionalization by conjugation to bioactive molecules.

Latest Research and Reviews

Simplified synthesis and identification of novel nanostructures consisting of cobalt borate and cobalt oxide for crystal violet dye removal from aquatic environments

• Asma S. Al-Wasidi
• Hesham H. El-Feky
• Ehab A. Abdelrahman

Efficient removal of Zn(II) ions from aqueous media using a facilely synthesized nanocomposite based on chitosan Schiff base

• Zahrah Alhalili

Ultrasound-assisted Cu(II) Strecker-functionalized organocatalyst for green azide–alkyne cycloaddition and Ullmann reactions

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Water-dispersable photoreactors based on core–shell mesoporous silica particles

• Andrzej Baliś
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Emerging trends in chiral inorganic nanomaterials for enantioselective catalysis

Considerable attention has been directed towards chiral nanocatalysts due to their significant role in facilitating asymmetric organic transformations. Here the authors highlight the recent advancements and notable examples in the field of chiral inorganic nanocatalysts.

• Chuanlai Xu

Narrow bandgap silver mercury telluride alloy semiconductor nanocrystal for self-powered midwavelength-infrared photodiode

Infrared colloidal quantum dots are interesting due to their low-cost fabrication and wavelength tunability for optoelectronic applications. Here, air-stable low-noise mid-infrared photodiode devices are fabricated using hole-doped Ag-HgTe nanocrystals.

• Haemin Song
• So Young Eom
• Kwang Seob Jeong

News and Comment

Let’s twist again

Drawing inspiration from helical structures in nature, researchers have developed a cobalt-based complex able to twist and untwist, converting between nanohelix and nanowire structures.

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Paving the path to the future of carbogenic nanodots

Insufficient purification and incomplete characterization pose a serious problem for attributing photoluminescence properties to carbogenic nanodots, especially those synthesized by bottom-up approaches. Here, we provide a roadmap for the successful future of these nanodots.

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Self-assembled ‘silicages’

Sustainable magnetic nanoparticle synthesis.

DNA nanomaterials: Absolute control

The controlled growth of well-defined DNA crystals on silica substrates could open new applications in nanoelectronics.

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A review on nanoparticles: characteristics, synthesis, applications, and challenges

The significance of nanoparticles (NPs) in technological advancements is due to their adaptable characteristics and enhanced performance over their parent material. They are frequently synthesized by reducing metal ions into uncharged nanoparticles using hazardous reducing agents. However, there have been several initiatives in recent years to create green technology that uses natural resources instead of dangerous chemicals to produce nanoparticles. In green synthesis, biological methods are used for the synthesis of NPs because biological methods are eco-friendly, clean, safe, cost-effective, uncomplicated, and highly productive. Numerous biological organisms, such as bacteria, actinomycetes, fungi, algae, yeast, and plants, are used for the green synthesis of NPs. Additionally, this paper will discuss nanoparticles, including their types, traits, synthesis methods, applications, and prospects.

1. Introduction

Nanotechnology evolved as the achievement of science in the 21st century. The synthesis, management, and application of those materials with a size smaller than 100 nm fall under the interdisciplinary umbrella of this field. Nanoparticles have significant applications in different sectors such as the environment, agriculture, food, biotechnology, biomedical, medicines, etc. like; for treatment of waste water ( Zahra et al., 2020 ), environment monitoring ( Rassaei et al., 2011 ), as a functional food additives ( Chen et al., 2023 ), and as a antimicrobial agents ( Islam et al., 2022 ). Cutting-edge properties of NPs such as; nature, biocompatibility, anti-inflammatory and antibacterial activity, effective drug delivery, bioactivity, bioavailability, tumor targeting, and bio-absorption have led to a growth in the biotechnological, and applied microbiological applications of NPs.

A particle of matter with a diameter of one to one hundred nanometers (nm) is commonly referred to as a nanoparticle or ultrafine particle. Nanoparticles frequently exhibit distinctive size-dependent features, mostly due to their tiny size and colossal surface area. The periodic boundary conditions of the crystalline particle are destroyed when the size of a particle approaches the nano-scale with the characteristic length scale close to or smaller than the de Broglie wavelength or the wavelength of light ( Guo et al., 2013 ). Because of this, many of the physical characteristics of nanoparticles differ significantly from those of bulk materials, leading to a wide range of their novel uses ( Hasan, 2015 ).

2. Emergence of nanotechnology

Nanotechnology emerged in the 1980s due to the convergence of experimental advances such as the invention of the scanning tunneling microscope in 1981 and the discovery of fullerenes in 1985 ( Bayda et al., 2019 ), with the elucidation. The popularization of a conceptual framework for nanotechnology goals began with the publication of the book Engines of Creation in 1986 ( Bayda et al., 2019 ).

2.1. Early stage of NPs

Carbon nanotubes have been discovered in pottery from Keeladi, India, dating from around 600–300 BC ( Bayda et al., 2019 ; Kokarneswaran et al., 2020 ). Cementite nanowires have been discovered in Damascus steel, a material that dates back to around 900 AD; nevertheless, its origin and creation method are unclear ( Kokarneswaran et al., 2020 ). However, it is unknown how they developed or whether the material containing them was used on purpose.

2.2. Discovery of C, Ag, Zn, Cu, and Au nanoparticles

Carbon NPs were found in 1991, and Iijima and Ichihashi announced the single-wall carbon nanotube synthesis with a diameter of 1 nanometer in 1993 ( Chen et al., 2021 ). Carbon nanotubes (CNTs), also known as Bucky tubes, are a kind of nanomaterial made up of a two-dimensional hexagonal lattice of carbon atoms. They are bent one way and joined to produce a hollow cylindrical cylinder. Carbon nanotubes are carbon allotropes that fall between Fullerene (0 dimensional) and Grapheme (2 dimensional) ( Chen et al., 2021 ).

In addition, M. C. Lea reported that the synthesis of citrate-stabilized silver colloid almost 120 years ago ( Nowack et al., 2011 ). This process produces particles with an average diameter of 7 to 9 nm. Nanoscale size and citrate stabilization are analogous to recent findings on nanosilver production employing silver nitrate and citrate ( Majeed Khan et al., 2011 ). The use of proteins to stabilize nanosilver has also been documented as early as 1902 ( Nowack et al., 2011 ; Beyene et al., 2017 ). Since 1897, a nanosilver known as “Collargol” has been made commercially and used for medicinal purposes ( Nowack et al., 2011 ). Collargol, a type of silver nanoparticle, has a particle size of about 10 nanometers (nm). This was determined as early as 1907, and it was found that the diameter of Collargol falls within the nanoscale range. In 1953, Moudry developed a different type of silver nanoparticle called gelatin-stabilized silver nanoparticles, with a diameter ranging from 2–20 nm. These nanoparticles were produced using another method than Collargol. The necessity of nanoscale silver was recognized by the creators of nanosilver formulations decades ago, as seen by the following remark from a patent: “for optimal efficiency, the silver must be disseminated as particles of colloidal size less than 25 nm in crystallite size”( Nowack et al., 2011 ).

Gold NPs (AuNPs) have a long history in chemistry, going back to the Roman era when they were used to decorate glassware by staining them. With the work of Michael Faraday, who may have been the first to notice that colloidal gold solutions have characteristics different from bulk gold, the contemporary age of AuNP synthesis began more than 170 years ago. Michael Faraday investigated the making and factors of colloidal suspensions of “Ruby” gold in 1857. They are among the magnetic nanoparticles due to their distinctive optical and electrical characteristics. Under specific illumination circumstances, Faraday showed how gold nanoparticles might create solutions of various colors ( Bayda et al., 2019 ; Giljohann et al., 2020 ).

3. Classification of NPs

Nanoparticles (NPs) are categorized into the following classes based on their shape, size, and chemical characteristics;

3.1. Carbon-based NPs

Fullerenes and carbon nanotubes (CNTs) are the two essential sub-categories of carbon-based NPs. NPs of globular hollow cages, like allotropic forms of carbon, are found in fullerenes. Due to their electrical conductivity, high strength, structure, electron affinity, and adaptability, they have sparked significant economic interest. These materials have organized pentagonal and hexagonal carbon units, each of which is sp2 hybridized. While CNTs are elongated and form 1–2 nm diameter tubular structures. These fundamentally resemble graphite sheets rolling on top of one another. Accordingly, they are referred to as single-walled (SWNTs), double-walled (DWNTs), or multi-walled carbon nanotubes (MWNTs) depending on how many walls are present in the rolled sheets ( Elliott et al., 2013 ; Astefanei et al., 2015 ).

3.2. Metal NPs

Metal NPs are purely made of metals. These NPs have distinctive electrical properties due to well-known localized surface Plasmon resonance (LSPR) features. Cu, Ag, and Au nanoparticles exhibit a broad absorption band in the visible region of the solar electromagnetic spectrum. Metal NPs are used in several scientific fields because of their enhanced features like facet, size, and shape-controlled synthesis of metal NPs ( Khan et al., 2019 ).

3.3. Ceramics NPs

Ceramic NPs are tiny particles made up of inorganic, non-metallic materials that are heat-treated and cooled in a specific way to give particular properties. They can come in various shapes, including amorphous, polycrystalline, dense, porous, and hollow, and they are known for heat resistance and durable properties. Ceramic NPs are used in various applications, including coating, catalysts, and batteries ( Sigmund et al., 2006 ).

3.4. Lipid-based NPs

These NPs are helpful in several biological applications because they include lipid moieties. Lipid NPs typically have a diameter of 10–1,000 nm and are spherical. Lipid NPs, i.e., polymeric NPs, have a solid lipid core and a matrix consisting of soluble lipophilic molecules ( Khan et al., 2019 ).

3.5. Semiconductor NPs

Semiconductor NPs have qualities similar to metals and non-metals. That is why Semiconductor NPs have unique physical and chemical properties that make them useful for various applications. For example, semiconductor NPs can absorb and emit light and can be used to make more efficient solar cells or brighter light-emitting diodes (LEDs). They can make smaller and faster electronic devices, such as transistors, and can be used in bio imaging and cancer therapy ( Biju et al., 2008 ).

3.6. Polymeric NPs

Polymeric NPs with a size between 1 and 1,000 nm can have active substances surface-adsorbed onto the polymeric core or entrapped inside the polymeric body. These NPs are often organic, and the term polymer nanoparticle (PNP) is commonly used in the literature to refer to them. They resemble Nano spheres or Nano capsules for the most part ( Khan et al., 2019 ; Zielińska et al., 2020 ).

4. Types of different metal-based NPs

Metal NPs are purely made of metal precursors. Due to well-known localized surface plasmon resonance (LSPR) characteristics, these NPs possess unique optoelectrical properties. NPs of the alkali and noble metals, i.e., Cu, Ag, and Au, have a broad absorption band in the visible zone of the solar electromagnetic spectrum. The facet, size, and shape-controlled synthesis of metal NPs are essential in present-day cutting-edge materials ( Dreaden et al., 2012 ; Khan et al., 2019 ).

4.1. Silver nanoparticles (AgNPs)

AgNPs are particles with a size range of 1–100 nanometers made of silver. They have unique physical and chemical properties due to their small size, high surface area-to-volume ratio, and ability to absorb and scatter light in the visible and near-infrared range. Because of their relatively small size and high surface-to-volume ratios, which cause chemical and physical differences in their properties compared to their bulk counterparts, silver nanoparticles may exhibit additional antimicrobial capabilities not exerted by ionic silver ( Shenashen et al., 2014 ).

Besides, AgNPs can be created in various sizes and forms depending on the manufacturing process, the most common of which is chemical reduction. The AgNPs were created by chemically reducing a 12 mM AgNO3 aqueous solution. The reaction was carried out in an argon environment using 70 mL of this solution containing PVP (keeping the molar ratio of the repeating unit of PVP and Ag equal to 34) and 21 mL of Aloe Vera. The mixture was agitated in ultrasonic for 45 min at ambient temperature, then heated 2°C/min to 80°C and left for 2 h to generate a transparent solution with tiny suspended particles that must be removed by simple filtering ( Shenashen et al., 2014 ; Gloria et al., 2017 ).

4.2. Zinc nanoparticles (ZnONPs)

Zinc nanoparticles (ZnONPs) are particles with a size range of 1–100 nm made of zinc. Zinc oxide (ZnO) NPs are a wide band gap semiconductor with a room temperature energy gap of 3.37 eV. Its catalytic, electrical, optoelectronic, and photochemical capabilities have made it widely worthwhile ( Kumar S.S. et al., 2013 ). ZnO nanostructures are ideal for catalytic reaction processes ( Chen and Tang, 2007 ). Laser ablation, hydrothermal methods, electrochemical depositions, sol-gel method, chemical vapor deposition, thermal decomposition, combustion methods, ultrasound, microwave-assisted combustion method, two-step mechanochemical-thermal synthesis, anodization, co-precipitation, electrophoretic deposition, and precipitation processes are some methods for producing ZnO nanoparticles ( Madathil et al., 2007 ; Moghaddam et al., 2009 ; Ghorbani et al., 2015 ).

4.3. Copper nanoparticles (CuNPs)

Copper nanoparticles (CuNPs) comprise a size range of 1–100 nm of copper-based particles ( Khan et al., 2019 ). Cu and Au metal fluorescence have long been known to exist. For excitation at 488 nm, a fluorescence peak centering on the metals’ interband absorption edge has been noted. Additionally, it was noted that the fluorescence peaked at the same energy at two distinct excitation wavelengths (457.9–514.5 and 300–400 nm), and the high-energy tail somewhat grows with increased photon energy pumping. A unique, physical, top-down EEW approach has been used to create Cu nanoparticles. The EEW method involves sending a current of *1,010 A/m2 (1,010 A/m2) across a thin Cu wire, which explodes on a Cu plate for a duration of 10–6 s ( Siwach and Sen, 2008 ).

4.4. Gold nanoparticles (AuNPs)

Gold nanoparticles(AuNPs) are nanometers made of gold. They have unique physical and chemical properties and can absorb and scatter light in the visible and near-infrared range ( Rad et al., 2011 ; Compostella et al., 2017 ).

Scientists around the turn of the 20th century discovered anisotropic AuNPs. Zsigmond ( Li et al., 2014 ) said that gold particles “are not always spherical when their size is 40 nm or lower” in his book, released in 1909. Additionally, he found anisotropic gold particles of various colors. Zsigmondy won the Nobel Prize in 1925 for “his demonstration of the heterogeneous character of colloidal solutions and the methods he utilized” and for developing the ultramicroscope, which allowed him to see the forms of Au particles. He noticed that gold frequently crystallized into a six-sided leaf shape ( Li et al., 2014 ).

AuNPs are the topic of extensive investigation due to their optical, electrical, and molecular-recognition capabilities, with numerous prospective or promised uses in a wide range of fields, including electron microscopy, electronics, nanotechnology, materials science, and biomedicine ( Rad et al., 2011 ).

4.5. Aluminum nanoparticles (AlNPs)

Aluminum nanoparticles (AlNPs) are nanoparticles made of aluminum. Aluminum nanoparticles’ strong reactivity makes them promising for application in high-energy compositions, hydrogen generation in water processes, and the synthesis of alumina 2D and 3D structures ( Lerner et al., 2016 ).

4.6. Iron nanoparticles (FeNPs)

Iron nanoparticles(FeNPs) are particles with a size range of 1−100 nanometers ( Khan et al., 2019 ) made of iron. FeNPs have several potential applications, including their use as catalysts, drug delivery systems, sensors, and energy storage and conversion. They have also been investigated for use in photovoltaic and solar cells and water purification and environmental remediation. FeNPs can also be used in magnetic resonance imaging (MRI) as contrast agents to improve the visibility of tissues and organs. They can also be used in magnetic recording media, such as hard disk drives ( Zhuang and Gentry, 2011 ; Jamkhande et al., 2019 ).

As with any NPs, there are potential health and safety concerns associated with using FeNPs, e.g., FeNPs are used to deliver drugs to specific locations within the body, such as cancer cells and used in MRI, and used to remove contaminants from water ( Farrell et al., 2003 ; Zhuang and Gentry, 2011 ). Tables 1 , ​ ,2 2 show the characteristics of metal-based nanoparticles and the techniques to study their characteristics, respectively.

Characteristics of metal based nanoparticles.

Different analytical techniques and their purposes in studying nanoparticles.

5. Approaches for the synthesis of metal NPs

There are mainly three types of approaches for the synthesis of NPs: the physical, chemical, and biological approaches. The physical approach is also called the top-down approach, while chemical and biological approaches are collectively called the bottom-up approach. The biological approach is also named green systems of NPs. All these approaches are further sub-categorized into various types based upon their method adopted. Figure 1 illustrates each approach’s reported methods for synthesizing NPs.

Approaches of NPs synthesis.

5.1. Top down/physical approach

Bulk materials are fragmented in top-down methods to create nano-structured materials ( Figure 2 ). They are additionally known as physical approaches ( Baig et al., 2021 ). The following techniques can achieve a top-down approach;

Difference between top-down and bottom-up approaches.

5.1.1. Mechanical milling

The mechanical milling process uses balls inside containers and may be carried out in various mills, typically planetary and shaker mills, which is an impact process with high energy ( Gorrasi and Sorrentino, 2015 ). Mechanical milling is a practical approach for creating materials at the nanoscale from bulk materials. Aluminum alloys that have been strengthened by oxide and carbide, spray coatings that are resistant to wear, nanoalloys based on aluminum, nickel, magnesium, and copper, and a variety of other nanocomposite materials may all be created mechanically. A unique class of nanoparticles known as ball-milled carbon nanomaterials has the potential to meet the needs for energy storage, energy conversion, and environmental remediation ( Yadav et al., 2012 ; Lyu et al., 2017 ).

5.1.2. Electrospinning

Typically, it is used to create nanofibers from various materials, most often polymers ( Ostermann et al., 2011 ). A technique for creating fibers called electrospinning draws charged threads from polymer melts or solutions up to fiber sizes of a few hundred nanometers ( Chronakis, 2010 ). Coaxial electrospinning was a significant advancement in the field of electrospinning. The spinneret in coaxial electrospinning is made up of two coaxial capillaries. Core-shell nanoarchitectures may be created in these capillaries using two viscous liquids, a viscous liquid as the shell and a non-viscous liquid as the core ( Du et al., 2012 ). Core-shell and hollow polymer, inorganic, organic, and hybrid materials have all been developed using this technique ( Kumar R. et al., 2013 ).

5.1.3. Laser ablation

A microfeature can be made by employing a laser beam to vaporize a single material ( Tran and Wen, 2014 ). Laser ablation synthesis produces nanoparticles by striking the target material with an intense laser beam. Due to the high intensity of the laser irradiation used in the laser ablation process, the source material or precursor vaporizes, causing the production of nanoparticles ( Amendola and Meneghetti, 2009 ). Laser ablation is an environmentally friendly for producing noble metal nanoparticles ( Baig et al., 2021 ). This method may be used to create a wide variety of nanomaterials, including metal nanoparticles, carbon nanomaterials, oxide composites, and ceramics ( Su and Chang, 2018 ; Baig et al., 2021 ).

5.1.4. Sputtering

Microparticles of a solid material are expelled off its surface during the phenomenon known as sputtering, which occurs when the solid substance is assaulted by intense plasma or gas particles ( Behrisch, 1981 ). According to the incident gaseous ion energy, energetic gaseous ions used in the sputtering deposition process physically expel tiny atom clusters off the target surface ( Muñoz-García et al., 2009 ). The sputtering method is intriguing because it is more affordable than electron-beam lithography, and the composition of the sputtered nanomaterials is similar to the target material with fewer contaminants ( Baig et al., 2021 ).

5.1.5. Electron explosion

In this technique, a thin metal wire is subjected to a high current pulse that causes an explosion, evaporation, and ionization. The metal becomes vaporized and ionized, expands, and cools by reacting with the nearby gas or liquid medium. The condensed vapor finally forms the nanoparticles ( Joh et al., 2013 ). Electron explosion method because it produces plasma from the electrical explosion of a metallic wire, which may produce nanoparticles from a Pt solution without using a reducing agent ( Joh et al., 2013 ).

5.1.6. Sonication

The most crucial step in the creation of nanofluids is sonication. After the mixture has been magnetically stirred in a magnetic stirrer, sonication is performed in an ultrasonication path, ultrasonic vibrator, and mechanical homogenizer. Sonicators have become the industry standard for Probe sonication and are noticeably more powerful and effective when compared to ultrasonic cleaner baths for nanoparticle applications. Probe sonication is highly effective for processing nanomaterials (carbon nanotubes, graphene, inks, metal oxides, etc.) ( Zheng et al., 2010 ).

5.1.7. Pulsed wire discharge method

This is the most used method for creating metal nanoparticles. A pulsating current causes a metal wire to evaporate, producing a vapor that is subsequently cooled by an ambient gas to form nanoparticles. This plan may quickly produce large amounts of energy ( Patil et al., 2021 ).

5.1.8. Arc discharge method

Two graphite rods are adjusted in a chamber with a constant helium pressure during the Arc Discharge procedure. It is crucial to fill the chamber with helium because oxygen or moisture prevents the synthesis of fullerenes. Arc discharge between the ends of the graphite rods drives the vaporization of carbon rods. Achieving new types of nanoparticles depends significantly on the circumstances in which arc discharge occurs. The creation of several nanostructured materials may be accomplished with this technique ( Berkmans et al., 2014 ). It is well-recognized for creating carbon-based materials such as fullerenes, carbon nanohorns (CNHs), carbon nanotubes ( Shi et al., 2000 ), few-layer graphene, and amorphous spherical carbon nanoparticles ( Kumar R. et al., 2013 ).

5.1.9. Lithography

Lithography typically uses a concentrated beam of light or electrons to create nanoparticles, a helpful technique ( Pimpin and Srituravanich, 2012 ). Masked and maskless lithography are the two primary categories of lithography. Without a mask, arbitrary nano-pattern printing is accomplished in maskless lithography. Additionally, it is affordable and easy to apply ( Brady et al., 2019 ).

5.2. Bottom-up approach

Tiny atoms and molecules are combined in bottom-up methods to create nano-structured particles ( Figure 2 ; Baig et al., 2021 ). These include chemical and biological approaches:

5.2.1. Chemical vapor deposition (CVD)

Through a chemical process involving vapor-phase precursors, a thin coating is created on the substrate surface during CVD ( Dikusar et al., 2009 ). Precursors are deemed appropriate for CVD if they exhibit sufficient volatility, high chemical purity, strong evaporation stability, cheap cost, a non-hazardous nature, and long shelf life. Additionally, its breakdown should not leave behind any contaminants. Vapor phase epitaxy, metal-organic CVD, atomic layer epitaxy, and plasma-enhanced CVD are only a few CVD variations. This method’s benefits include producing very pure nanoparticles that are stiff, homogeneous, and strong ( Ago, 2015 ). CVD is an excellent approach to creating high-quality nanomaterials ( Machac et al., 2020 ). It is also well-known for creating two-dimensional nanoparticles ( Baig et al., 2021 ).

5.2.2. Sol-gel process

A wet-chemical approach, called the sol-gel method, is widely utilized to create nanomaterials ( Das and Srivasatava, 2016 ; Baig et al., 2021 ). Metal alkoxides or metal precursors in solution are condensed, hydrolyzed, and thermally decomposed. The result is a stable solution or sol. The gel gains greater viscosity as a result of hydrolysis or condensation. The particle size may be seen by adjusting the precursor concentration, temperature, and pH levels. It may take a few days for the solvent to be removed, for Ostwald ripening to occur, and for the phase to change during the mature stage, which is necessary to enable the growth of solid mass. To create nanoparticles, the unstable chemical ingredients are separated. The generated material is environmentally friendly and has many additional benefits thanks to the sol-gel technique ( Patil et al., 2021 ). The uniform quality of the material generated, the low processing temperature, and the method’s ease in producing composites and complicated nanostructures are just a few of the sol-gel technique’s many advantages ( Parashar et al., 2020 ).

5.2.3. Co-precipitation

It is a solvent displacement technique and is a wet chemical procedure. Ethanol, acetone, hexane, and non-solvent polymers are examples of solvents. Polymer phases can be either synthetic or natural. By mixing the polymer solution, fast diffusion of the polymer-solvent into the non-solvent phase of the polymer results. Interfacial stress at two phases results in the formation of nanoparticles ( Das and Srivasatava, 2016 ). This method’s natural ability to produce high quantities of water-soluble nanoparticles through a straightforward process is one of its key benefits. This process is used to create many commercial iron oxide NP-based MRI contrast agents, including Feridex, Reservist, and Combidex ( Baig et al., 2021 ; Patil et al., 2021 ).

5.2.4. Inert gas condensation/molecular condensation

Metal NPs are produced using this method in large quantities. Making fine NPs using the inactive gas compression approach has been widespread, which creates NPs by causing a metallic source to disappear in an inert gas. At an attainable temperature, metals evaporate at a tolerable pace. Copper metal nanoparticles are created by vaporizing copper metal inside a container containing argon, helium, or neon. The atom quickly loses its energy by cooling the vaporized atom with an inert gas after it boils out. Liquid nitrogen is used to cool the gases, forming nanoparticles in the range of 2–100 nm ( Pérez-Tijerina et al., 2008 ; Patil et al., 2021 ).

5.2.5. Hydrothermal

In this method, for the production of nanoparticles, hydrothermal synthesis uses a wide temperature range from ambient temperature to extremely high temperatures. Comparing this strategy to physical and biological ones offers several benefits. At higher temperature ranges, the nanomaterials produced by hydrothermal synthesis could become unstable ( Banerjee et al., 2008 ; Patil et al., 2021 ).

5.2.6. Green/biological synthesis

The synthesis of diverse metal nanoparticles utilizing bioactive agents, including plant materials, microbes, and various biowastes like vegetable waste, fruit peel waste, eggshell, agricultural waste, algae, and so on, is known as “green” or “biological” nanoparticle synthesis ( Kumari et al., 2021 ). Developing dependable, sustainable green synthesis technologies is necessary to prevent the formation of undesirable or dangerous byproducts ( Figure 3 ). The green synthesis of nanoparticles also has several advantages, including being straightforward, affordable, producing NPs with high stability, requiring little time, producing non-toxic byproducts, and being readily scaled up for large-scale synthesis ( Malhotra and Alghuthaymi, 2022 ).

Schematic diagram for biosynthesis of NPs.

5.2.6.1. Biological synthesis using microorganisms

Microbes use metal capture, enzymatic reduction, and capping to create nanoparticles. Before being converted to nanoparticles by enzymes, metal ions are initially trapped on the surface or interior of microbial cells ( Ghosh et al., 2021 ). Use of microorganisms (especially marine microbes) for synthesis of metalic NPs is environmental friendly, fast and economical ( Patil and Kim, 2018 ). Several microorganisms are used in the synthesis of metal NPs, including:

Biosynthesis of NPs by bacteria: A possible biofactory for producing gold, silver, and cadmium sulfide nanoparticles is thought to be bacterial cells. It is known that bacteria may produce inorganic compounds either inside or outside of their cells ( Hulkoti and Taranath, 2014 ). Desulforibrio caledoiensis ( Qi et al., 2013 ), Enterococcu s sp. ( Rajeshkumar et al., 2014 ), Escherichia coli VM1 ( Maharani et al., 2016 ), and Ochrobactrum anhtropi ( Thomas et al., 2014 ) based metal NPs are reported previously for their potential photocatalytic properties ( Qi et al., 2013 ), antimicrobial activity ( Rajeshkumar et al., 2014 ), and anticancer activity ( Maharani et al., 2016 ).

Extracellular synthesis of NPs by bacteria: The microorganisms’ extracellular reductase enzymes shrink the silver ions to the nanoscale range. According to protein analysis of microorganisms, the NADH-dependent reductase enzyme carries out the bio-reduction of silver ions to AgNPs. The electrons for the reductase enzyme come from NADH, which is subsequently converted to NAD+. The enzyme is also oxidized simultaneously when silver ions are reduced to nanosilver. It has been noted that bio-reduction can occasionally be caused by nitrate-dependent reductase. The decline occurs within a few minutes in the quick extracellular creation of nanoparticles ( Mathew et al., 2010 ). At pH 7, the bacterium R. capsulata produced gold nanoparticles with sizes ranging from 10−20 nm. Numerous nanoplates and spherical gold nanoparticles were produced when the pH was changed to four ( Sriram et al., 2012 ). By adjusting the pH, the gold nanoparticles’ form may be changed. Gold nanoparticle shape was controlled by regulating the proton content at various pH levels. The bacteria R. capsulata ’s release cofactor NADH and NADH-dependent enzymes may cause the bioreduction of Au (3+) to Au (0) and the generation of gold nanoparticles. By using NADH-dependent reductase as an electron carrier, it is possible to start the reduction of gold ions ( Sriram et al., 2012 ).

Intracellular synthesis of NPs by bacteria: Three processes are involved in the intracellular creation of NPs: trapping, bioreduction, and capping. The cell walls of microorganisms and ions charge contribute significantly to creating NPs in the intracellular route. This entails specific ion transit in the presence of enzymes, coenzymes, and other molecules in the microbial cell. Microbes have a range of polysaccharides and proteins in their cell walls, which function as active sites for the binding of metal ions ( Slavin et al., 2017 ). Not all bacteria can produce metal and metal oxide nanoparticles. The only ions that pose a significant hazard to microorganisms are heavy metal ions, which, in response to a threat, cause the germs to react by grabbing or trapping the ions on the cell wall via electrostatic interactions. This occurs because a metal ion is drawn to the cell wall’s carboxylate groups, including cysteine and polypeptides, and certain enzymes with a negative charge ( Zhang et al., 2011 ).

Additionally, the electron transfers from NADH via NADH-dependent educates, which serves as an electron carrier and is located inside the plasma membrane, causing the trapped ions to be reduced into the elemental atom. The nuclei eventually develop into NPs and build up in the cytoplasm or the pre-plasmic space. On the other hand, the stability of NPs is provided by proteins, peptides, and amino acids found inside cells, including cysteine, tyrosine, and tryptophan ( Mohd Yusof et al., 2019 ).

Biosynthesis of NPs by fungi: Because monodisperse nanoparticles with distinct dimensions, various chemical compositions, and sizes may be produced, the biosynthesis of nanoparticles utilizing fungus is frequently employed. Due to the existence of several enzymes in their cells and the ease of handling, fungi are thought to be great candidates for producing metal and metal sulfide nanoparticles ( Mohanpuria et al., 2008 ).

The nanoparticles were created on the surface of the mycelia. After analyzing the results and noting the solution, it was determined that the Ag + ions are initially trapped on the surface of the fungal cells by an electrostatic interaction between gold ions and negatively charged carboxylate groups, which is facilitated by enzymes that are present in the mycelia’s cell wall. Later, the enzymes in the cell wall reduce the silver ions, causing the development of silver nuclei. These nuclei then increase as more Ag ions are reduced and accumulate on them.

The TEM data demonstrate the presence of some silver nanoparticles both on and inside the cytoplasmic membrane. The findings concluded that the Ag ions that permeate through the cell wall were decreased by enzymes found inside the cytoplasm and on the cytoplasmic membrane. Also possible is the diffusion of some silver nanoparticles over the cell wall and eventual cytoplasmic entrapment ( Mukherjee et al., 2001 ; Hulkoti and Taranath, 2014 ).

It was observed that the culture’s age does not affect the shape of the synthesized gold nanoparticles. However, the number of particles decreased when older cells were used. The different pH levels produce a variety of shapes of gold nanoparticles, indicating that pH plays a vital role in determining the shape. The incubation temperature also played an essential role in the accumulation of the gold nanoparticles. It was observed that the particle growth rate was faster at increased temperature levels ( Mukherjee et al., 2001 ; Ahmad et al., 2003 ). The form of the produced gold nanoparticles was shown to be unaffected by the age of the culture. However, when older cells were utilized, the particle count fell. The fact that gold nanoparticles take on various forms at different pH levels suggests that the pH is crucial in determining the shape. The incubation temperature significantly influenced the accumulation of the gold nanoparticles. It was found that higher temperatures caused the particle development rate to accelerate ( Mukherjee et al., 2001 ; Ahmad et al., 2003 ). Verticillium luteoalbum is reported to synthesize gold nanoparticles of 20–40 nm in size ( Erasmus et al., 2014 ). Aspergillus terreus and Penicillium brevicompactum KCCM 60390 based metal NPs are reported for their antimicrobial ( Li G. et al., 2011 ) and cytotoxic activities ( Mishra et al., 2011 ), respectively.

Biosynthesis of NPs using actinomycetes: Actinomycetes have been categorized as prokaryotes since they share significant traits with fungi. They are sometimes referred to as ray fungi ( Mathew et al., 2010 ). Making NPs from actinomycetes is the same as that of fungi ( Sowani et al., 2016 ). Thermomonospora sp., a new species of extremophilic actinomycete, was discovered to produce extracellular, monodispersed, spherical gold nanoparticles with an average size of 8 nm ( Narayanan and Sakthivel, 2010 ). Metal NPs synthesized by Rhodococcus sp. ( Ahmad et al., 2003 ) and Streptomyces sp. Al-Dhabi-87 ( Al-Dhabi et al., 2018 ) are reported for their antimicrobial activities.

Biosynthesis of NPs using algae: Algae have a high concentration of polymeric molecules, and by reducing them, they may hyper-accumulate heavy metal ions and transform them into malleable forms. Algal extracts typically contain pigments, carbohydrates, proteins, minerals, polyunsaturated fatty acids, and other bioactive compounds like antioxidants that are used as stabilizing/capping and reducing agents ( Khanna et al., 2019 ). NPs also have a faster rate of photosynthesis than their biosynthetic counterparts. Live or dead algae are used as model organisms for the environmentally friendly manufacturing process of bio-nanomaterials, such as metallic NPs ( Hasan, 2015 ). Ag and Au are the most extensively researched noble metals to synthesized NPs by algae either intracellularly or extracellularly ( Dahoumane et al., 2017 ). Chlorella vulgaris ( Luangpipat et al., 2011 ), Chlorella pyrenoidosa ( Eroglu et al., 2013 ), Nanochloropsis oculata ( Xia et al., 2013 ), Scenedesmus sp. IMMTCC-25 ( Jena et al., 2014 ) based metal NPs are reported for their potential catalytic ( Luangpipat et al., 2011 ; Eroglu et al., 2013 ) and, antimicrobial ( Eroglu et al., 2013 ; Jena et al., 2014 ) activities along with their use in Li-Ion batteries ( Xia et al., 2013 ).

Intracellular synthesis of NPs using algae: In order to create intracellular NPs, algal biomass must first be gathered and thoroughly cleaned with distilled water. After that, the biomass (living algae) is treated with metallic solutions like AgNO3. The combination is then incubated at a specified pH and a specific temperature for a predetermined time. Finally, it is centrifuged and sonicated to produce the extracted stable NPs ( Uzair et al., 2020 ).

Extracellular synthesis of NPs using algae: Algal biomass is first collected and cleaned with distilled water before being used to synthesize NPs extracellularly ( Uzair et al., 2020 ). The following three techniques are frequently utilized for the subsequent procedure:

(i) A particular amount of time is spent drying the algal biomass (dead algae), after which the dried powder is treated with distilled water and filtered.

(ii) The algal biomass is sonicated with distilled water to get a cell-free extract.

(iii) The resultant product is filtered after the algal biomass has been rinsed with distilled water and incubated for a few hours (8–16 h).

5.2.6.2. Biological synthesis using plant extracts

The substance or active ingredient of the desired quality extracted from plant tissue by treatment for a particular purpose is a plant extract ( Jadoun et al., 2021 ). Plant extracts are combined with a metal salt solution at room temperature to create nanoparticles. Within minutes, the response is finished. This method has been used to create nanoparticles of silver, gold, and many other metals ( Li X. et al., 2011 ). Nanoparticles are biosynthesized using a variety of plants. It is known that the kind of plant extract, its concentration, the concentration of the metal salt, the pH, temperature, and the length of contact time all have an impact on how quickly nanoparticles are produced as well as their number and other properties ( Mittal and Chisti, 2013 ). A leaf extract from Polyalthia longifolia was used to create silver nanoparticles, the average particle size was around 58 nm ( Kumar and Yadav, 2009 ; Kumar et al., 2016 ).

Acacia auriculiformis ( Saini et al., 2016 ), Anisomeles indica ( Govindarajan et al., 2016 ), Azadirachta indica ( Velusamy et al., 2015 ), Bergenia ciliate ( Phull et al., 2016 ), Clitoria ternatea , Solanum nigrum ( Krithiga et al., 2013 ), Coffea arabica ( Dhand et al., 2016 ), Coleus forskohlii ( Naraginti et al., 2016 ), Curculigo orchioides ( Kayalvizhi et al., 2016 ), Digitaria radicosa ( Kalaiyarasu et al., 2016 ), Dioscorea alata ( Pugazhendhi et al., 2016 ), Diospyros paniculata ( Rao et al., 2016 ), Elephantopus scaber ( Kharat and Mendhulkar, 2016 ), Emblica officinalis ( Ramesh et al., 2015 ), Euphorbia antiquorum L. ( Rajkuberan et al., 2017 ), Ficus benghalensis ( Nayak et al., 2016 ), Lantana camara ( Ajitha et al., 2015 ), Cinnamomum zeylanicum ( Soni and Sonam, 2014 ), and Parkia roxburghii ( Paul et al., 2016 ) are the few examples of plants which are reported for the green synthesis of metal NPs (i.e., AgNPs). These were evaluated for their antifilaria activity ( Saini et al., 2016 ), mosquitocidal activity ( Govindarajan et al., 2016 ), antibacterial activity ( Velusamy et al., 2015 ), catalytic activity ( Edison et al., 2016 ), antioxidant activity ( Phull et al., 2016 ), and Cytotoxicity ( Patil et al., 2017 ).

5.2.6.3. Biological synthesis using biomimetic

“Biomimetic synthesis” typically refers to chemical processes that resemble biological synthesis carried out by living things ( Dahoumane et al., 2017 ). In the biomimetic approach, proteins, enzymes, cells, viruses, pollen, and waste biomass are used to synthesize NPs. Two categories are used to classify biomimetic synthesis:

Functional biomimetic synthesis uses various materials and approaches to emulate particular characteristics of natural materials, structures, and systems ( Zan and Wu, 2016 ).

Process biomimetic synthesis is a technique that aims to create different desirable nanomaterials/structures by imitating the synthesis pathways, processes, or procedures of natural chemicals and materials/structures. For instance, several distinctive nano-superstructures (such as satellite structures, dendrimer-like structures, pyramids, cubes, 2D nanoparticle arrays, 3D AuNP tubes, etc.) have been put together in vitro by simulating the protein manufacturing process ( Zan and Wu, 2016 ).

6. Applications of NPs

6.1. applications of nps in environment industry.

Due to their tiny size and distinctive physical and chemical characteristics, NPs appeal to various environmental applications. The properties of nanoparticals and their advantages are illustrated in Figure 4 . The following are some possible NP uses in the environment.

Properties of nanoparticals and their advantages.

6.1.1. Bioremediation

Nanoparticles (NPs) can remove environmental pollutants, such as heavy metals from water or organic contaminants from soil ( Zhuang and Gentry, 2011 ). For example, silver nanoparticles (AgNPs) effectively degrade certain pollutants, such as organic dyes and compounds found in wastewater. Several nanomaterials have been considered for remediation purposes, such as nanoscale zeolites, metal oxides, and carbon nanotubes and fibers ( Zhuang and Gentry, 2011 ). Nanoscale particles used in remediation can access areas that larger particles cannot. They can be coated to facilitate transport and prevent reaction with surrounding soil matrices before reacting with contaminants. One widely used nanomaterial for remediation is Nanoscale zerovalent iron (nZVI). It has been used at several hazardous waste sites to clean up chlorinated solvents that have contaminated groundwater ( Elliott et al., 2013 ). Removing heavy metals such as mercury, lead, thallium, cadmium, and arsenic from natural water has attracted considerable attention because of their adverse effects on environmental and human health. Superparamagnetic iron oxide NPs are an effective sorbent material for this toxic soft material. So, no measurements of engineered NPs in the environment have been available due to the absence of analytical methods able to quantify the trace concentration of NPs ( Elliott et al., 2013 ).

6.1.2. Sensors in environment

Nanotechnology/NPs are already being used to improve water quality and assist in environmental clean-up activities ( Pradeep, 2009 ). Their potential use as environmental sensors to monitor pollutants is also becoming viable NPs can be used as sensors to detect the presence of certain compounds in the environment, such as heavy metals or pollutants. The nano-sensors small size and wide detection range provide great flexibility in practical applications. It has been reported that nanoscale sensors can be used to detect microbial pathogens and biological compounds, such as toxins, in aqueous environments ( Yadav et al., 2010 ). NPS can be designed to selectively bind to specific types of pollutants, allowing them to be detected at low concentrations. For example, gold nanoparticles (AuNPs) have been used as sensors for the detection of mercury in water ( Theron et al., 2010 ).

6.1.3. Catalysts in environment

Nanoparticles (NPs) are used as catalysts in chemical reactions, such as in the production of biofuels or environmental remediation processes, and to catalyze biomass conversion into fuels, such as ethanol or biodiesel. For example, platinum nanoparticles (PtNPs) have been explored for use in the production of biofuels due to their ability to catalyze the conversion of biomass into fuels ( Lam and Luong, 2014 ). PtNPs also showed promising sensing properties; for example, Using Pt NPs, the Hg ions were quantified in the range of 50–500 nM in MilliQ, tap, and groundwater samples, and the limit of quantifications for Hg ions were 16.9, 26, and 47.3 nM. The biogenic PtNPs-based probe proved to be applicable for detecting and quantifying Hg ions ( Kora and Rastogi, 2018 ).

Overall, NPs have significant potential for use in the environment and are being actively researched for a variety of applications.

6.2. Applications of NPs in medicine industry

Nanoparticles (NPs) have unique physical and chemical properties due to their small size, making them attractive for use in various applications, including the medicine industry. Some potential applications of NPs in medicine include:

6.2.1. Drug delivery

Technological interest has been given to AuNPs due to their unique optical properties, ease of synthesis, and chemical stability. The particles can be used in biomedical applications such as cancer treatment ( Sun et al., 2014 ), biological imaging ( Abdulle and Chow, 2019 ), chemical sensing, and drug delivery. Sun et al. (2014) mentioned in detail about two different methods of controlled release of drugs associated with NPs, which were (1) sustained (i.e., diffusion-controlled and erosion-controlled) and (2) stimuli-responsive (i.e., pH-sensitive, enzyme-sensitive, thermoresponsive, and photosensitive). Figure 5 illustrates that how NPs acts as targeted delivery of medicines to treat cancer cells ( Figure 5A ) and therapeutic gene delivery to synthesis proteins of interests in targeted cells ( Figure 5B ). NPs can deliver drugs to specific body areas, allowing for more targeted and effective treatment ( Siddique and Chow, 2020 ). For example AgNPs have been explored for use in drug delivery due to their stability and ability to accumulate in certain types of cancerous tumors ( Siddique and Chow, 2020 ). ZnONPs have also been explored for drug delivery due to their ability to selectively target cancer cells ( Anjum et al., 2021 ). CuNPs have been shown to have antimicrobial properties and are being explored for drug delivery to treat bacterial infections ( Yuan et al., 2018 ). AuNPs have unique optical, electrical, and catalytic properties and are being explored for drug delivery due to their ability to accumulate in certain cancerous tumors. Silver NPs (AgNPs) have been incorporated into wound dressings, bone cement, and implants ( Schröfel et al., 2014 ).

Application of nanoparticles as; targated drug delivery (A) , and therapeutic protein generation in targated cells (B) .

6.2.2. Diagnostics

Nanoparticles (NPs) can be used as imaging agents to help visualize specific body areas. For example, iron oxide nanoparticles (Fe 3 O 4 NPs) have been used as magnetic resonance imaging (MRI) contrast agents to help visualize tissues and organs ( Nguyen et al., 2013 ). AuNPs have unique optical, electrical, and catalytic properties and are being explored for diagnostics due to their ability to accumulate in certain cancerous tumors ( Siddique and Chow, 2020 ).

6.2.3. Tissue engineering

Nanoparticles (NPs) can help stimulate the growth and repair of tissues and organs. For example, titanium dioxide nanoparticles (TiO2 NPs) have been explored for tissue engineering due to their ability to stimulate the growth of bone cells ( Kim et al., 2014 ).

6.2.4. Antimicrobials

Some NPs, such as silver nanoparticles (AgNPs) and copper nanoparticles (CuNPs), have strong antimicrobial properties and are being explored for use in a variety of medical products, such as wound dressings and medical devices ( Hoseinzadeh et al., 2017 ).

Overall, NPs have significant potential for use in the medical industry and are being actively researched for various applications. However, it is essential to carefully consider the potential risks and benefits of using NPs in medicine and ensure their safe and responsible use.

6.3. Applications of NPs in agriculture industry

There are several ways in which nanoparticles (NPs) have the potential to alter the agricultural sector. NPs may be used in agriculture for a variety of reasons, including:

6.3.1. Pesticides and herbicides

Nanoparticles (NPs) can be used to deliver pesticides and herbicides in a targeted manner, reducing the number of chemicals needed and minimizing the potential for environmental contamination ( Khan et al., 2019 ). AgNPs and CuNPs have antimicrobial properties, making them potentially useful for controlling pests and diseases in crops. They can also be used as delivery systems for active ingredients, allowing for more targeted application and reducing the potential for environmental contamination ( Hoseinzadeh et al., 2017 ; Dangi and Verma, 2021 ).

It is important to note that using metal NPs in pesticides and herbicides is still in the early stages of development. More research is needed to understand their potential impacts on human health and the environment ( Dangi and Verma, 2021 ).

6.3.2. Fertilizers and plant growth

Nano fertilizers offer an opportunity for efficiently improving plant mineral nutrition. Some studies have shown that nanomaterials can be more effective than conventional fertilizers, with a controlled release of nutrients increasing the efficiency of plant uptake and potentially reducing adverse environmental outcomes associated with the loss of nutrients in the broader environment. However, other studies have found that nanomaterial has the same or even less effective effectiveness than conventional fertilizers. NPs used to deliver fertilizers to plants more efficiently, reducing the amount of fertilizer needed, and reducing the risk of nutrient runoff ( Kopittke et al., 2019 ).

Ag ( Jaskulski et al., 2022 ), Zn ( Song and Kim, 2020 ), Cu, Au, Al, and Fe ( Kopittke et al., 2019 ) based NPs have been shown to have fertilizing properties and plant growth-promoting properties, and may help provide essential nutrients to plants and improve plant growth and yield. It is important to note that the use of NPs in fertilizers is still in the early stages of development. More research is needed to understand their potential impacts on human health and the environment.

6.3.3. Food safety

Nanoparticles (NPs) can detect and eliminate pathogens in food products, improving food safety, and reducing the risk of foodborne illness ( Zhuang and Gentry, 2011 ).

6.3.4. Water purification

Nanoparticles (NPs) can purify irrigation water, reducing the risk of crop contamination and improving crop yield ( Zhuang and Gentry, 2011 ). Using NPs in agriculture can improve crop yields, reduce agriculture’s environmental impact, and improve food products’ safety and quality.

6.4. Applications of NPs in food industry

Numerous applications for nanoparticles (NPs) in the food sector are possible, including:

6.4.1. Food processing and food preservation/food packaging

Nanoparticles (NPs) can be used to improve the efficiency and performance of food processing operations, such as grinding, mixing, and drying, e.g., AgNPs have been used as a natural antimicrobial agent in food processing operations, helping to prevent the growth of bacteria and other microorganisms ( Dangi and Verma, 2021 ) and also NPs are used to enhance the performance of materials used in food packaging, making them more resistant to pollutants like moisture and gases.

6.4.2. Food fortification

Nanoparticles (NPs) can deliver essential nutrients to food products, such as vitamins and minerals, more efficiently and effectively. e.g., Fe 2 O 3 , and CuNPs have been used to fortify food products with iron, and Cu is an essential nutrient necessary for the metabolism of iron and other nutrients. Iron is an essential nutrient often lacking in many people’s diets, particularly in developing countries ( Kopittke et al., 2019 ).

6.4.3. Sensors

Nanoparticles (NPs) used to improve the sensitivity and specificity of food sensors, allowing them to detect a broader range of substances or signals ( Yadav et al., 2010 ).

Overall, using NPs in the food industry can improve the performance, safety, and nutritional value of a wide range of food products and processes.

6.5. Applications of NPs in electronics industry and automotive industry

In many aspects, nanoparticles (NPs) can transform the electronics sector. NPs may be used in a variety of electrical applications, such as:

6.5.1. Display technologies/storage devices

Nanoparticles (NPs) can be used to improve the performance of displays ( Park and Choi, 2019 ; Bahadur et al., 2021 ; Triana et al., 2022 ), such as LCD and OLED displays, by enhancing the brightness, color, and contrast of the image, such as silver NPs and gold NPs, have been explored for use in LCD and OLED displays as a means of improving the conductivity of the display ( Gwynne, 2020 ). NPs improve the performance and durability of energy storage devices, such as batteries and supercapacitors, by increasing energy density and charging speed. Zinc oxide nanoparticles (ZnO NPs) have the potential to be used in energy storage devices, such as batteries and supercapacitors, due to their ability to store and release energy ( Singh et al., 2011 ).

6.5.2. Data storage

Nanoparticles (NPs) can improve the capacity and speed of data storage devices, such as hard drives and flash drives. Magnetic NPs, such as iron oxide NPs, have been explored for use in data storage devices, such as hard drives, due to their ability to store, and retrieve data using magnetism. These NPs are often composed of a magnetic metal, such as iron, cobalt, or nickel. They can be magnetized and demagnetized, allowing them to store and retrieve data ( Ahmad et al., 2021 ).

Overall, the use of NPs in electronics has the potential to improve the performance and efficiency of a wide range of electronic devices and systems.

Applications of NPs in chemical industry: The chemical industry might be entirely transformed by nanoparticles (NPs) in various ways. The following are potential uses for NPs in the chemical industry ( Salem and Fouda, 2021 ).

6.5.3. Chemical processing/catalysis

Nanoparticles (NPs) can be used as catalysts in chemical reactions, allowing them to be carried out more efficiently and at lower temperatures. Some examples of metal NPs that have been used as catalysts in the chemical industry include: PtNPs have been used as catalysts in a variety of chemical reactions, including fuel cell reactions ( Bhavani et al., 2021 ), hydrogenation reactions, and oxidation reactions ( Lara and Philippot, 2014 ), PdNPs have been used as catalysts in a variety of chemical reactions, including hydrogenation reactions and cross-coupling reactions ( Pérez-Lorenzo, 2012 ), FeNPs have been used as catalysts in a variety of chemical reactions, including hydrolysis reactions ( Jiang and Xu, 2011 ), and oxygen reduction reactions, NiNPs have been used as catalysts in a variety of chemical reactions, including hydrogenation reactions, and hydrolysis reactions ( Salem and Fouda, 2021 ).

6.5.4. Separation and purification

NPs are used to separate and purify chemicals and other substances, such as gases and liquids, by exploiting their size-based properties ( Hollamby et al., 2010 ). Several types of metal nanoparticles (NPs) have been explored for use in separation and purification processes in the chemical industry, including Fe 2 O 3 NPs have been used to separate and purify gases, liquids, and chemicals. They have also been used to remove contaminants from water ( Pradeep, 2009 ; Siddique and Chow, 2020 ). AgNPs have been used to purify water and remove contaminants ( Pradeep, 2009 ), such as bacteria and viruses. They have also been used to remove heavy metals from water and other substances ( Zhuang and Gentry, 2011 ). AuNPs have been used to purify water and remove contaminants, such as bacteria and viruses ( Siddique and Chow, 2020 ). They have also been used to separate and purify gases and liquids ( Zhuang and Gentry, 2011 ). AlNPs have been used to remove contaminants from water and other substances, such as oils and fuels. They have also been used to purify gases ( Zhuang and Gentry, 2011 ).

6.6. Applications of NPs in defense industry

Nanoparticles (NPs) can be used to improve the efficiency and performance of chemical processing operations, such as refining and synthesizing chemicals ( Schröfel et al., 2014 ). Nanoparticles (NPs) have the potential to be used in the defense industry in several ways, including:

6.6.1. Sensors

Nanoparticles (NPs) can improve the sensitivity and specificity of sensors used in defense systems, such as sensors for detecting chemical, biological, or radiological threats ( Zheng et al., 2010 ).

6.6.2. Protective coatings

Nanoparticles (NPs) can improve the performance and durability of protective coatings applied to defense equipment, such as coatings resistant to chemical or biological agents. For example, metal NPs can improve the mechanical properties and durability of the coating, making it more resistant to wear and corrosion. For example, adding Al or Zn based NPs to a polymer coating can improve its corrosion resistance. In contrast, adding Ni or Cr-based NPs can improve their wear resistance ( Rangel-Olivares et al., 2021 ).

6.6.3. Weapons

Nanoparticles (NPs) are used as weapons against viruses, bacteria, etc, ( Ye et al., 2020 ) and as well as in the development of armor and protective materials. There have been some reports of the potential use of NPs in military and defense applications, such as in the development of armor and protective materials. For example, adding nanoparticles, such as ceramic or metal NPs, to polymers or other materials can improve their mechanical properties and make them more resistant to damage. In addition, there have been reports of the use of NPs in developing sensors and detection systems for defense purposes.

6.6.4. Manufacturing

Nanoparticles (NPs) can improve the performance and durability of materials used in defense equipment, such as armor or structural materials. Metal NPs can be used in materials by adding them as a filler or reinforcement in polymers. For example, the addition of metal NPs such as aluminum (Al), copper (Cu), or nickel (Ni) to polymers can improve the mechanical properties, thermal stability, and electrical conductivity of the resulting composite material ( Khan et al., 2019 ).

Metal NPs can also make functional materials, such as catalysts and sensors. For example, metal NPs, such as gold (Au), and platinum (Pt), can be used as catalysts in various chemical reactions due to their high surface area and ability to adsorb reactants ( Zheng et al., 2010 ).

6.6.5. Energy storage

Nanoparticles (NPs) can improve the performance and efficiency of energy storage systems used in defense systems, such as batteries or fuel cells ( Morsi et al., 2022 ). In batteries, nanoparticles can be used as a cathode material to increase the battery’s energy density, rate capability, and cycling stability. For example, lithium cobalt oxide (LiCoO 2 ) nanoparticles have been used as cathode materials in lithium-ion batteries due to their high capacity and good rate performance. In addition, nanoparticles of transition metal oxides, such as iron oxide (Fe 2 O 3 ), and manganese oxide (MnO 2 ), have been used as cathode materials in rechargeable lithium batteries due to their high capacity and good rate performance. In supercapacitors, nanoparticles can be used as the active material in the electrodes to increase the specific surface area, leading to an increase in the device’s capacitance ( Morsi et al., 2022 ). Using NPs in the defense industry can improve defense systems’ performance, efficiency, and safety.

7. Future perspectives

Metal nanoparticles (NPs) have many potential applications in various fields, including electronics, energy storage, catalysis, and medicine. However, there are also several challenges and potential future directions for developing and using metal NPs.

One major challenge is synthesizing and processing metal NPs with precise size and shape control. Many methods for synthesizing metal NPs involve high temperatures and harsh chemical conditions, which can be challenging to scale up for large-scale production. In addition, the size and shape of metal NPs can significantly impact their properties and potential applications, so it is essential to synthesize NPs with precise size and shape control.

Another challenge is the environmental impact of metal NPs. Some metal NPs, such as silver NPs, can be toxic to aquatic life and may have other environmental impacts. There is a need for more research on the environmental effects of metal NPs and the development of more environmentally friendly (Green) synthesis and processing methods.

In terms of future directions, one promising area is the use of metal NPs for energy storage, conversion, and protection of the environment. For example, metal NPs could be used to improve batteries’ performance or develop more efficient solar cells. In addition, metal NPs could be used in catalysis to improve the efficiency of chemical reactions. There is also ongoing research on metal NPs in medicine, including drug delivery and cancer therapy.

Author contributions

KAA: conceptualization, methodology, validation, formal analysis, investigation, writing – original draft, writing – review and editing, and visualization.

Acknowledgments

The author thanks Prof. Dr. Mona M. Sobhy, Department of Reproductive Diseases, Animal Reproduction Research Institute, ARC, Giza, Egypt, and Dr. Omar Hewedy, University of Guelph, Canada, for the critical reading of the manuscript.

Conflict of interest

The author declares that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

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• Published: 07 June 2022

Nanoparticle classification, physicochemical properties, characterization, and applications: a comprehensive review for biologists

• Nadeem Joudeh 1 &
• Dirk Linke 1  

Journal of Nanobiotechnology volume  20 , Article number:  262 ( 2022 ) Cite this article

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Interest in nanomaterials and especially nanoparticles has exploded in the past decades primarily due to their novel or enhanced physical and chemical properties compared to bulk material. These extraordinary properties have created a multitude of innovative applications in the fields of medicine and pharma, electronics, agriculture, chemical catalysis, food industry, and many others. More recently, nanoparticles are also being synthesized ‘biologically’ through the use of plant- or microorganism-mediated processes, as an environmentally friendly alternative to the expensive, energy-intensive, and potentially toxic physical and chemical synthesis methods. This transdisciplinary approach to nanoparticle synthesis requires that biologists and biotechnologists understand and learn to use the complex methodology needed to properly characterize these processes. This review targets a bio-oriented audience and summarizes the physico–chemical properties of nanoparticles, and methods used for their characterization. It highlights why nanomaterials are different compared to micro- or bulk materials. We try to provide a comprehensive overview of the different classes of nanoparticles and their novel or enhanced physicochemical properties including mechanical, thermal, magnetic, electronic, optical, and catalytic properties. A comprehensive list of the common methods and techniques used for the characterization and analysis of these properties is presented together with a large list of examples for biogenic nanoparticles that have been previously synthesized and characterized, including their application in the fields of medicine, electronics, agriculture, and food production. We hope that this makes the many different methods more accessible to the readers, and to help with identifying the proper methodology for any given nanoscience problem.

Nano etymology

The prefix nano is derived from the Greek word nanos, “a dwarf”. In 1947, at the 14th conference of the International Union of Pure and Applied Chemistry (IUPAC), the prefix nano was officially adopted to describe the one-billionth part (10 –9 ) of a unit Footnote 1 . In scientific literature, the prefix nano has been adopted as a popular label in many fields of modern science to describe small entities and processes. These terms include, but are not limited to nanoscience, nanotechnology, nanorobots, nanomagnets, nanoelectronics, nanoencapsulation, etc. [ 1 ]. In all of these cases, the prefix nano is used to describe “very small” entities or processes, most often at actual nanometer scale.

Definitions

Nanoscience is a branch of science that comprises the study of properties of matter at the nanoscale, and particularly focuses on the unique, size-dependent properties of solid-state materials [ 2 ]. Nanotechnology is the branch that comprises the synthesis, engineering, and utilization of materials whose size ranges from 1 to 100 nm, known as nanomaterials [ 3 ]. The birth of nanoscience and nanotechnology concepts is usually linked to the famous lecture of Nobel laureate Richard Feynman at the 1959 meeting of the American Physical Society, ‘‘There’s Plenty of Room at the Bottom’’ [ 4 ]. However, the use of nanotechnology and nanomaterials goes back in history long before that.

History of nanotechnology

Long before the era of nanotechnology, people were unknowingly coming across various nanosized objects and using nano-level processes. In ancient Egypt, dyeing hair in black was common and was for a long time believed to be based on plant products such as henna [ 5 ]. However, recent research on hair samples from ancient Egyptian burial sites showed that hair was dyed with paste from lime, lead oxide, and water [ 6 ]. In this dyeing process, galenite (lead sulfide, PbS) nanoparticles are formed. The ancient Egyptians were able to make the dyeing paste react with sulfur (part of hair keratin) and produce small PbS nanoparticles which provided even and steady dyeing.

Probably the most famous example for the ancient use of nanotechnology is the Lycurgus Cup (fourth century CE). This ancient roman cup possesses unusual optical properties; it changes its color based on the location of the light source. In natural light, the cup is green, but when it is illuminated from within (with a candle), it becomes red. The recent analysis of this cup showed that it contains 50–100 nm Au and Ag nanoparticles [ 7 ], which are responsible for the unusual coloring of the cup through the effects of plasmon excitation of electrons [ 8 ]. The ancient use of nanotechnology does not stop here, in fact, there is evidence for the early use of nanotechnology processes in Mesopotamia, Ancient India, and the Maya [ 9 , 10 ].

Why nanomaterials are different

Today, due to their unique properties, nanomaterials are used in a wide range of applications, such as catalysis, water treatment, energy storage, medicine, agriculture, etc . [ 11 , 12 , 13 ]. Two main factors cause nanomaterials to behave significantly differently than the same materials at larger dimensions: surface effects and quantum effects [ 14 ]. These factors make nanomaterials exhibit enhanced or novel mechanical, thermal, magnetic, electronic, optical, and catalytic properties [ 1 , 15 , 16 ].

Nanomaterials have different surface effects compared to micromaterials or bulk materials, mainly due to three reasons; (a) dispersed nanomaterials have a very large surface area and high particle number per mass unit, (b) the fraction of atoms at the surface in nanomaterials is increased, and (c) the atoms situated at the surface in nanomaterials have fewer direct neighbors [ 1 , 14 ]. As a consequence of each of these differences, the chemical and physical properties of nanomaterials change compared to their larger-dimension counterparts. For instance, having fewer direct neighbor atoms for the atoms situated at the surface results in lowering the binding energy per atom for nanomaterials. This change directly affects the melting temperature of nanomaterials following the Gibbs–Thomson equation, e.g., the melting point of 2.5 nm gold nanoparticles is 407 degrees lower than the melting point of bulk gold [ 14 ]. Larger surface areas and larger surface-to-volume ratios generally increases the reactivity of nanomaterials due to the larger reaction surface [ 1 ], as well as resulting in significant effects of surface properties on their structure [ 17 ]. The dispersity of nanomaterials is a key factor for the surface effects. The strong attractive interactions between particles can result in the agglomeration and aggregation of nanomaterials, which negatively affects their surface area and their nanoscale properties [ 18 ]. Agglomeration can be prevented by increasing the zeta potential of nanomaterials (increasing the repulsive force) [ 19 ], optimizing the degree of hydrophilicity/hydrophobicity of the nanomaterial, or by optimizing the pH and the ionic strength of the suspension medium [ 20 ].

Nanomaterials display distinct size-dependent properties in the 1–100 nm range where quantum phenomena are involved. When the material radius approaches the asymptotic exciton Bohr radius (the separation distance between the electron and hole), the influence of quantum confinement becomes apparent [ 17 ]. In other words, by shrinking the size of the material, quantum effects become more pronounced, and nanomaterials become quantal. Those quantum structures are physical structures where all the charge carriers (electrons and holes) are confined within the physical dimensions [ 21 ]. As a result of quantum confinement effects, for instance, some non-magnetic materials in bulk such as palladium, platinum, and gold become magnetic in the nanoscale [ 14 ]. Quantum confinement can also result in significant changes in electron affinity or the ability to accept or donate electrical charges, which is directly reflected on the catalytic properties of the material. For example, the catalytic activity of cationic platinum clusters in N 2 O decomposition is dictated by the number of atoms in the cluster. 6–9, 11, 12, 15, and 20 atom-containing clusters are very reactive, while clusters with 10, 13, 14, and 19 atoms have low reactivity [ 14 ].

Classification of nanomaterials

The key elements of nanotechnology are the nanomaterials. Nanomaterials are defined as materials where at least one of their dimensions is in the nanoscale, i.e. smaller than 100 nm [ 22 ]. Based on their dimensionalities, nanomaterials are placed into four different classes, summarized in Fig.  1 .

Zero-dimensional nanomaterials (0-D): the nanomaterials in this class have all their three dimensions in the nanoscale range. Examples are quantum dots, fullerenes, and nanoparticles.

One-dimensional nanomaterials (1-D): the nanomaterials in this class have one dimension outside the nanoscale. Examples are nanotubes, nanofibers, nanorods, nanowires, and nanohorns.

Two-dimensional nanomaterials (2-D): the nanomaterials in this class have two dimensions outside the nanoscale. Examples are nanosheets, nanofilms, and nanolayers.

Three-dimensional nanomaterials (3-D) or bulk nanomaterials: in this class the materials are not confined to the nanoscale in any dimension. This class contains bulk powders, dispersions of nanoparticles, arrays of nanowires and nanotubes, etc .

Nanomaterials classification based on dimensionality

Nanoparticles (NPs)

The International Organization for Standardization (ISO) defines nanoparticles as nano-objects with all external dimensions in the nanoscale, where the lengths of the longest and the shortest axes of the nano-object do not differ significantly. If the dimensions differ significantly (typically by more than three times), terms such as nanofibers or nanoplates maybe preferred to the term NPs Footnote 2 .

NPs can be of different shapes, sizes, and structures. They can be spherical, cylindrical, conical, tubular, hollow core, spiral, etc., or irregular [ 23 ]. The size of NPs can be anywhere from 1 to 100 nm. If the size of NPs gets lower than 1 nm, the term atom clusters is usually preferred. NPs can be crystalline with single or multi-crystal solids, or amorphous. NPs can be either loose or agglomerated [ 24 ].

NPs can be uniform, or can be composed of several layers. In the latter case, the layers often are: (a) The surface layer, which usually consists of a variety of small molecules, metal ions, surfactants, or polymers. (b) The shell layer, which is made of a chemically different material from the core layer. (c) The core layer, which is the central portion of the NP [ 25 ].

Classification of NPs

Based on their composition, NPs are generally placed into three classes: organic, carbon-based, and inorganic [ 23 ].

Organic NPs

This class comprises NPs that are made of proteins, carbohydrates, lipids, polymers, or any other organic compounds [ 26 ]. The most prominent examples of this class are dendrimers, liposomes, micelles, and protein complexes such as ferritin (shown in Fig.  2 ). These NPs are typically non-toxic, bio-degradable, and can in some cases, e.g., for liposomes, have a hollow core. Organic NPs are sensitive to thermal and electromagnetic radiation such as heat and light [ 23 ]. In addition, they are often formed by non-covalent intermolecular interactions, which makes them more labile in nature and offers a route for clearance from the body [ 27 ]. There are different parameters that determine the potential field of application of organic NPs, e.g., composition, surface morphology, stability, carrying capacity, etc . Today, organic NPs are mostly used in the biomedical field in targeted drug delivery [ 23 ] and cancer therapy [ 28 ].

Types of organic NPs. A Dendrimers; B liposomes; C micelles; and D ferritin

Carbon-based NPs

This class comprises NPs that are made solely from carbon atoms [ 23 ]. Famous examples of this class are fullerenes, carbon black NPs, and carbon quantum dots (shown in Fig.  3 ). Fullerenes are carbon molecules that are characterized by a symmetrical closed-cage structure. C 60 fullerenes consist of 60 carbon atoms arranged in the shape of a soccer ball [ 29 ], but also other types of fullerenes such as C 70 and C 540 fullerenes have been described [ 30 ]. Carbon black NPs are grape-like aggregates of highly fused spherical particles [ 31 ]. Carbon quantum dots consist of discrete, quasi-spherical carbon NPs with sizes below 10 nm [ 32 ]. Carbon-based NPs unite the distinctive properties of sp 2 -hybridized carbon bonds with the unusual physicochemical properties at the nanoscale. Due to their unique electrical conductivity, high strength, electron affinity, optical, thermal, and sorption properties [ 25 , 33 ], carbon-based NPs are used in a wide range of application such as drug delivery [ 34 ], energy storage [ 35 ], bioimaging [ 36 ], photovoltaic devices, and environmental sensing applications to monitor microbial ecology or to detect microbial pathogens [ 33 ]. Nanodiamonds and carbon nano onions are more complex, carbon-based NPs. Due to their characteristic low toxicity and biocompatibility, they are used in drug delivery and tissue engineering applications [ 37 , 38 ].

Different types of carbon-based NPs. A C 60 fullerene; B carbon black NPs; and C carbon quantum dots

Inorganic NPs

This class comprises NPs that not made of carbon or organic materials. The typical examples of this class are metal, ceramic, and semiconductor NPs. Metal NPs are purely made of metal precursors, they can be monometallic, bimetallic [ 39 ], or polymetallic [ 40 ]. Bimetallic NPs can be made from alloys or formed in different layers (core–shell) [ 39 ]. Due to the localized surface plasmon resonance characteristics, these NPs possess unique optical and electricals properties [ 25 ]. In addition, some metal NPs also possess unique thermal, magnetic, and biological properties [ 23 ]. This makes them increasingly important materials for the development of nanodevices that can be used in numerous physical, chemical, biological, biomedical, and pharmaceutical applications [ 41 , 42 ] (these applications are discussed in detail later in the applications section of the review). In present days, the size-, shape-, and facet-controlled synthesis of metal NPs is important for creating cutting-edge materials [ 43 ].

Semiconductor NPs are made of semiconductor materials, which possess properties between metals and non-metals. These NPs possess unique wide bandgaps and show significant alteration in their properties with bandgap tuning compared to bulk semiconductor materials [ 25 ]. As a result, these NPs are important materials in photocatalysis, optic, and electronic devices [ 44 , 45 ]. Ceramic NPs are inorganic solids made of carbonates, carbides, phosphates, and oxides of metals and metalloids, such as titanium and calcium [ 46 ]. They are usually synthesized via heat and successive cooling and they can be found in amorphous, polycrystalline, dense, porous or hollow forms [ 25 ]. They are mainly used in biomedical applications due to their high stability and high load capacity [ 47 ]. Nevertheless, they are also used in other applications such as catalysis, degradation of dyes, photonics and optoelectronics [ 46 , 48 ].

Physicochemical properties of NPs

As mentioned earlier, NPs can be used in a long list of applications due to their unique physical and chemical properties that do not exist in their larger-dimension counterparts of the same materials. The following sections summarize the most import physicochemical properties that are changing on the nanoscale.

Mechanical properties

Mechanical properties refer to the mechanical characteristics of a material under different conditions, environments, and various external forces. As for traditional materials, the mechanical properties of nanomaterials generally consist of ten parts: strength, brittleness, hardness, toughness, fatigue strength, plasticity, elasticity, ductility, rigidity, and yield stress [ 49 ]. Most inorganic, non-metallic materials are brittle materials and do not have significant toughness, plasticity, elasticity, or ductility properties. Organic materials on the other hand, are flexible materials and do not necessarily have brittleness and rigidity properties.

Due to surface and quantum effects, NPs display different mechanical properties compared to bulk materials [ 49 ]. For example, conventional FeAl powder which is composed of microparticles (larger than 4 µm), is brittle, while ultrafine FeAl alloy powder displays a good combination of strength and ductility as well as enhanced plasticity [ 50 ]. These new properties are believed to arise due to the diverse interaction forces between NPs or between them and a surface. The most important interaction forces involved are van der Waals forces, which consist of three parts, Keesom force, Debye force, and London force [ 51 , 52 , 53 ]. Other relevant interaction forces are electrostatic and electrical double layer forces, normal and lateral capillary forces, solvation, structural, and hydration forces [ 54 ].

There are different theories on how the interaction forces between NPs give them new mechanical properties, such as the DLVO (Derjaguin–Landau–Verwey–Overbeek) theory, JKR (Johnson–Kendall–Roberts) theory, and DMT (Derjaguin–Muller–Toporov) theory. The DLVO theory combines the effects of van der Waals attraction and electrostatic repulsion to describe the stability of colloidal dispersions [ 54 ]. This theory can explain many phenomena in colloidal science, such as the adsorption and the aggregation of NPs in aqueous solutions and the force between charged surfaces interacting through a liquid medium [ 55 , 56 ]. Nevertheless, the DLVO theory is inadequate for the colloidal properties in the aggregated state [ 54 ].

When the size of objects decreases to the nanoscale, the surface forces become a major player in their adhesion, contact, and deformation behaviors. The JRK theory is applicable to easily deformable, large bodies with high surface energies, where it describes the domination of surface interactions by strong, short-range adhesion forces. In contrast to this, the DMT theory is applicable to very small and hard bodies with low surface energies, where it describes the adhesion being caused by the presence of weak, long-range attractive forces. Although the DLVO, JKR and DMT theories have been widely used to describe and study the mechanical properties of NPs [ 57 , 58 ], it is still a matter of debate whether or not continuum mechanics can be used to describe a particle or collection of particles at the nanometer scale [ 54 ].

Thermal properties

Heat transfer in NPs primarily depends on energy conduction due to electrons as well as photons (lattice vibration) and the scattering effects that accompany both [ 59 ]. The major components of thermal properties of a material are thermal conductivity, thermoelectric power, heat capacity, and thermal stability [ 59 , 60 ].

NP size has a direct impact on electrical and thermal conductivity of NPs [ 60 ]. As the NP size decreases, the ratio of particle surface area respective to its volume increases hyperbolically [ 60 ]. Since the conduction of electrons is one of the two main ways in which heat is transferred, the higher surface-to-volume ratio in NPs provides higher number of electrons for heat transfer compared to bulk materials [ 61 ]. Moreover, thermal conductivity in NPs is also promoted by microconvection, which results from the Brownian motion of NPs [ 62 ]. Nevertheless, this phenomenon only happens when solid NPs are dispersed in a liquid (generating a Nanofluid) [ 63 ]. As an example, the addition of Cu NPs to ethylene glycol enhances the thermal conductivity of the fluid up to 40% [ 64 ].

The thermoelectric power of a material depends on its Seebeck coefficient and electrical conductivity ( \(P={S}^{2}\sigma \) , where P is thermoelectric power, S is the Seebeck coefficient, and \(\sigma \) is the electrical conductivity). The scattering of NPs in bulk materials (doping) is known to enhance the thermoelectric power factor [ 65 ]. This enhancement could come from the enhancement of the Seebeck coefficient or the enhancement of electrical conductivity. The embedding of size-controlled NPs in bulk thermoelectric materials helps to reduce the lattice thermal conductivity and enhances the Seebeck coefficient due to electron energy filtering [ 66 , 67 ]. Generally, the enhancement of electrical conductivity is accompanied by the reduction of the Seebeck coefficient and vice versa [ 65 ] However, the doping of InGaAlAs material with 2–3 nm Er NPs resulted in the significant increase of thermoelectric power of the material through the enhancement of the conductivity while keeping the Seebeck coefficient unchanged [ 65 ]. Depending on NP size, volume fraction, and band offset, a NP-doped sample can either enhance or suppress the electrical conductivity in comparison with undoped bulk sample.

Experimental studies have shown that the heat capacity of NPs exceeds the values of analogous bulk materials by up to 10% [ 68 ], e.g. in the case of Al 2 O 3 and SiO 2 NPs [ 69 , 70 ]. The major contribution to heat capacity at ambient temperatures is determined by the vibration degrees of freedom, i.e., the peculiarities of phonon spectra (vibrational energy that arises from oscillating atoms within a crystal) are responsible for the anomalous behavior of heat capacity of NPs [ 68 ]. NPs usually exhibit a significant decrease in melting temperature compared to their analogous bulk materials [ 71 ]. The main reason for this phenomenon is that the liquid/vapor interface energy is generally lower than the average solid/vapor interface energy [ 72 ]. When the particle size decreases, its surface-to-volume ratio increases, and the melting temperature decreases as a result of the improved free energy at the particle surface [ 73 ]. For instance, the melting temperature of 3 nm Au NPs is 300 degrees lower than the melting temperature of bulk gold [ 14 ]. In addition, NP composition plays an important role in thermal stability. For example, the thermal stability of Au in Au 0.8 Fe 0.2 is significantly higher than of pure Au or Au 0.2 Fe 0.8 [ 74 ]. Generally, bimetallic alloy NPs show higher thermal stabilities and melting temperatures than monometallic NPs due to the alloying effect [ 75 , 76 ].

Magnetic properties

All magnetic compounds include a ‘magnetic element’ in their formula, i.e., Fe, Co, or Ni (at ambient temperatures). There are only three known exceptions that are made from mixed diamagnetic elements, Sc 3 In, ZrZn 2 , and TiBe 2-x Cu x [ 77 , 78 , 79 , 80 ]. Otherwise, elements such as Pd, Au, or Ag are diamagnetic. This all changes in the nanoscale. Several materials become magnetic in the form of NPs as a result of uneven electronic distribution [ 25 ]. For instance, FeAl is not magnetic in bulk but in the form of NPs, it is becomes magnetic [ 50 ], other examples include Pd and Au [ 81 ]. In bulk materials, the key parameters for determining magnetic properties are composition, crystallographic structure, magnetic anisotropy, and vacancy defects [ 82 , 83 ]. However, on the nanoscale, two more important parameters are strongly involved, i.e., size and shape [ 84 ].

One of the interesting size-dependent phenomena of NPs is superparamagnetism [ 84 ]. As the size of the NPs decreases, the magnetic anisotropy energy per NP decreases. The magnetic anisotropy energy is the energy keeping the magnetic moment in a particular orientation. At a characteristic size for each type of NPs, the anisotropy energy becomes equal to the thermal energy, which allows the random flipping of the magnetic moment [ 85 ], in this case, the NP is defined as being superparamagnetic [ 86 ]. Superparamagnetic NPs display high magnetization only in the presence of a magnetic field, and once it is removed they do not retain any magnetization [ 87 ]. Superparamagnetism was long believed to form only in small ferromagnetic or ferrimagnetic NPs [ 88 ], but interestingly, other paramagnetic materials show magnetism in the nanoscale too [ 81 ].

NP size effects can also be observed in changes in magnetic coercivity, i.e., the resistance of a magnetic material to changes in magnetization (Fig.  4 ). In contrast to large particles or bulk materials, which possess multiple magnetic domain structures, small NPs possess single magnetic domain structures below a certain critical radius (r c ), where all magnetic spins in the NP align unidirectionally (blue arrows in Fig.  4 ). However, the NP radius has to be lower than the threshold radius for superparamagnetism (r sp ) in order to be superparamagnetic [ 89 ]. In the single-domain regime, between r sp and r c , the magnetic coercivity increases as the size of the NP increases until it reaches the maximum at r c [ 84 ]. In this size regime, due to the high magnetic coercivity, the NPs behave similarly as their larger dimension counterparts despite having a single domain structure, i.e., they become ferromagnetic for ferromagnetic materials or paramagnetic for paramagnetic materials etc . Above r c , the magnetic coercivity starts to decrease when multiple magnetic domains are formed in a single NP. The critical radius represents the size where it is energetically favored for the NP to exist without a domain wall [ 86 ]. The calculated critical radii for some common magnetic materials are 35 nm of Ni, 8 nm for Co, and 1 nm for Fe [ 90 ]. Above that point, multi-domain magnetism begins in which a smaller reversal magnetic field is required to make the net magnetization zero [ 84 ].

The change in magnetic coercivity of NPs as a function of particle radius. Figure adapted from Kalubowilage et al., 2019 [ 89 ]. rc critical radius, rsp threshold radius for superparamagnetism

The second key parameter for determining the magnetic properties of NPs is the shape of NPs. In comparison to the size parameter, there is significant less research on the effect of shape on the magnetic properties of NPs having the same volume [ 86 ]. However, large differences in coercivity were found between a set of cubic and spherical CoFe 2 O 4 NPs [ 91 ]. Unlike the curved topography in spherical CoFe 2 O 4 NPs, cubic CoFe 2 O 4 NPs have fewer missing oxygen atoms, and it was hypothesized that this led to less surface pinning and to lower coercivity for the cubic structures [ 86 ]. Other studies also found differences in magnetism between spherical and cubic Fe 3 O 4 NPs [ 92 , 93 ].

Similar to bulk materials, the composition also affects the magnetism of NPs. The magnetocrystalline phase of the NP is significant in determining its magnetic coercivity [ 94 ]. This effect can be observed in magnetic bimetallic core–shell or alloy NPs with anisotropic crystalline structures. For example, Co@Pt core–shell NPs composed of an isotropically structured face-centered cubic Co core and a non-magnetic Pt shell exhibit superparamagnetic behavior with zero coercivity at room temperature [ 95 ]. In general, the compositional modification of NPs by the adoption of magnetic dopants is known to significantly change the magnetism of NPs [ 96 ].

Electronic and optical properties

Metallic and semiconductor NPs possess interesting linear absorption, photoluminescence emission, and nonlinear optical properties due to the quantum confinement and localized surface plasmon resonance (LSPR) effect [ 97 , 98 ]. LSPR phenomena arise when the incident photon frequency is constant with the collective excitation of the conductive electrons [ 25 ].Due to this phenomenon, noble metal NPs exhibit a strong size-dependent UV–visible extinction band that is not present in the spectra of bulk metals. Generally, the optical properties of NPs depend on the size, shape, and the dielectric environment of the NPs [ 99 ].

The collective excitations of conductive electrons in metals are called plasmons [ 100 ]. Depending on the boundary conditions, bulk plasmons, surface-propagating plasmons, and surface-localized plasmons are distinguished (Fig.  5 A–C). Because of their longitudinal nature, the bulk plasmons cannot be excited by visible light. The surface-propagating plasmons propagate along metal surfaces in a waveguide-like fashion [ 98 ]. In the case of NPs, when they are irradiated by visible light, the oscillating electric field causes the conductive electrons to oscillate coherently. When the electron cloud is displaced relative to the nuclei, a restoring force rises from Coulomb attraction between electrons and nuclei that results in oscillation of the electron cloud relative to the nuclear framework [ 99 ]. This creates uncompensated charges at the NP surface (Fig.  5 D). As the main effect producing the restoring force is the polarization of the NP surface, these oscillations are called surface plasmons and have a well-defined resonance frequency [ 98 ].

Graphical illustration of the types of plasmons. A bulk; B surface propagating; and C surface localized plasmons (adapted from Khlebtsov et al., 2010 [ 98 ]). D graphical illustration of the localized surface plasmon resonance (LSPR) in NPs (adapted from Kelly et al., 2003 [ 99 ])

Experimental studies on Ag NPs showed significant differences in their optical properties based on the size of NPs. For Ag NPs with 30 nm radius, the main extinction peak was at 369 nm wavelength, while for Ag NPs with 60 nm radius, a totally different behavior was observed [ 99 ]. The same researchers found that the shape of the NPs also is critical for the optical properties, the plasmon resonance wavelength shifts to the red as the NPs become more oblate [ 99 ], demonstrating that plasmon resonance strongly depend on NPs shape. With respect to the dielectric environment of the NPs, both the surrounding solvent and the support (substrate) were found to be critical for the optical properties. For Ag NPs, both experimental and theorical studies on the effect of surrounding solvent show that plasmon wavelength linearly depends on the refractive index of the solvent [ 99 , 101 ]. At the same time, 10 nm Ag NPs supported on mica substrates displayed LSPR wavelength shifts to the red compared to unsupported NPs [ 102 ]. The biogenic synthesis of NPs can also improve the optical properties. Biologically produced CeO 2 NPs using Simarouba glauca leave extract were found to have different absorption bands and higher band gap energies compared to chemically produced CeO 2 NPs. These superior optical properties were attributed to the better crystallinity and small size of biogenic NPs compared to chemical NPs [ 103 ]. Biogenic NPs can also offer higher photocatalytic activities, e.g., ZnO NPs produced by Plectranthus amboinicus leaf extract had higher photocatalytic activity in the photodegradation of methyl red under UV illumination compared to chemical produced ZnO NPs [ 104 ].

Catalytic properties

Nano-catalysis, i.e., the use of NPs as catalysts, is a quickly evolving field within chemical catalysis. Significantly enhanced or novel catalytic properties such as reactivity and selectivity have been reported for NP catalysts compared to their bulk analogues. The catalytic properties of NPs depend on the size, shape, composition, interparticle spacing, the oxidation state, and the support of the NPs [ 76 ].

The dependency of catalytic activity on the size of NPs is well studied. The relation is an inverse one, i.e., the smaller the NPs the more catalytically active they are. This relationship was found e.g., in the electro-catalysis oxidation of CO by size-selected Au NPs (1.5, 4, and 6 nm) deposited on indium tin oxide. The researchers observed that the smallest NPs provided the highest normalized current densities [ 105 ]. The same relationship was also found in several other studies [ 106 , 107 , 108 , 109 , 110 ]. Goodman et al., 1998 [ 111 ] speculated originally that this behavior could be attributed to quantum-size effects generated by the confinement of electrons within a small volume. Later, size-dependent changes in the electronic structure of the clusters [ 112 ] and the resulting larger number of low-coordinated atoms available for interaction by the larger surface-to-volume ratios with smaller NPs were discussed [ 76 ].

The shape is also known to affect the reactivity and selectivity of the NPs. For the oxidation of CO by Au NPs, hemispherical NPs were found to be more active than spherical ones [ 113 ]. For the oxidation of styrene by Ag NPs, nanocubes were found to be fourteen times more efficient than nanoplates and four times more efficient than nanospheres [ 114 ]. The reason for these dramatical changes are attributed to the increase/decrease in the relative area of the catalytically active surface facets [ 76 ] or to the differences in stability for different NP shapes [ 115 ].

As for composition, several studies have shown that the use of alloys in NPs can enhance the catalytic activity as a result of the alloying effect causing changes in the electronic properties of the catalyst, decreasing poisoning effects, and providing distinct selectivities [ 76 ]. For example, the alloying of Pt with other metals such as Ru, Ni, and Co, was reported to enhance the hydrogenation and oxygen reduction activity of the NP catalyst material, as well as enhancing the resistance against CO poisoning [ 116 , 117 , 118 ]. However, the alloying of Pt with Fe, Ru, and Pd, resulted in reduced reactivity for methanol decomposition [ 119 ]. This reduction in reactivity was explained by the possible occupation of the surface with the addition metal atoms, since pure Fe, Ru, and Pd clusters are less reactive for methanol decomposition than similarly-sized pure Pt clusters. In general, the change in the composition of NPs changes the electronic structure of metal surfaces by the formation of bimetallic bonds as well as the modification of metal–metal bond lengths [ 76 ]. In addition, the charge-transfer phenomenon between different metals may favorably change the binding energy of adsorbents, lower the barriers for specific chemical reactions, and enhance resistance against poisoning [ 120 , 121 , 122 ].

The catalytic activity and stability of 2 nm Au NPs dispersed on polycrystalline TiC films displayed a strong dependence on interparticle spacing. In this study, Au NPs having two different interparticle spacing (30 and 80 nm) were analyzed by Thermal Desorption Spectroscopy. It was found that the sample with smaller interparticle spacing was poisoned and subsequently deactivated while the sample with longer interparticle spacing showed longer lifetime [ 123 ]. At the same time, the oxidation state of NPs was shown to affect the catalytic activities. Ru NPs under rich O 2 conditions and moderate temperatures oxidize and form RuO 2 , the reaction of CO oxidation was found to occur on the metal oxide surface not the metal surface [ 124 ]. A similar effect on CO oxidation was also observed with Pt NPs in which the reactivity of PtO 2 was found to be higher than Pt [ 125 ]. The reaction of CO oxidation was compared for several metal NPs (Ru, Pd, Ir, Os, and Pt) and their corresponding oxides, and the oxides were indeed more reactive than the metals [ 126 , 127 ]. The superior catalytic performance of RuO 2 over their metallic counterparts is generally agreed on, nevertheless, the same cannot be said for other catalytically active metals such as Pt [ 76 ]. In general, these differences in catalytic performance are attributed to the electron transfer processes at the metal/metal oxide interfaces. Consequently, the view that NP oxidation is an undesirable process that leads to the reduction of catalytic performance needs to be reconsidered [ 128 ].

An example for the effect of the support material is the role of the MgO support for Au NPs, where MgO was found to be important for CO oxidation and particularly, for controlling the rate of CO oxidation through oxygen vacancies [ 129 ]. Later, the process of electron charge transfer from oxygen vacancies at the metal-substrate interface of supported Au NPs was suggested to be an ideal environment for O 2 activation and oxidation reactions [ 130 ]. A similar behavior was also found in the decomposition of SO 2 and dissociation of water by Au NPs supported on CeO 2 , in which CeO 2 supports played a critical role [ 131 ]. The experiments showed that not only the chemical composition of the support affects the reactivity of the catalyst, but the crystal structure of the support, too [ 132 ]. Enhanced catalytic performance for CO oxidation and SO 2 dissociation have also been reported for Au NPs supported on metal carbides such as TiC [ 108 , 133 ]. In addition to enhanced catalytic reactivities, the support also plays an important role in NP stabilization [ 106 ], i.e., the stabilization of NPs against coarsening, the stabilization of metal oxides at the NP surface, and the stabilization of intermediate reactions species [ 76 ].

Characterization of NPs

The properties of NPs determine their potential applications. Hence, different methods and techniques are used for the analysis and characterization of the various physicochemical properties of NPs. Table 1 summarizes all characterization techniques mentioned in this review and shows what properties and features can be resolved by each technique.

Morphological and topographical characterization

The morphological and topographical features of NPs are of great interest since they influence most of the properties of NPs as described above. These features include the size, shape, dispersity, localization, agglomeration/aggregation, surface morphology, surface area, and porosity of the NPs. The following techniques are regularly used for the characterization of morphological and topographical features of NPs.

Electron microscopy (EM)

Scanning electron microscopy (SEM), scanning tunneling microscopy (STM), and transmission electron microscopy (TEM) are frequently employed for the analysis of NP size, shape, and surface. In SEM, an electron gun is used to produce a beam of electrons that is controlled by a set of lenses to follows a vertical path through the microscope until it hits the samples. Once the sample is hit by the beam, electrons and X-rays are ejected from the sample. Detectors are then used to collect the X-rays and scattered electrons in order to create a 3D image of the sample. SEM provides different information about the NPs such as size, shape, aggregation, and dispersion [ 134 ]. Similarly, TEM provides information about the size, shape, localization, dispersity, and aggregation of NPs in two-dimensional images [ 25 ]. TEM employs an electromagnetic lens that focuses a very fine beam of electrons into an ultrathin section of the sample. This beam passes through the specimen where the electrons either scatter or penetrate the sample and hit a fluorescent screen at the bottom of the microscope. The difference in electron densities is used for the contrast to create an image of the specimen. TEM can be also used for the characterization of NP crystal structure through the use of selected area electron diffraction (SAED), where the electron beam is focused on a selected area in the sample and the scattered electrons are used to obtain a diffraction pattern. STM is based on the phenomenon of quantum tunneling, where a metallic tip is brough very close to the sample surface and used to apply voltage. When voltage is applied, electrons from the sample surface are extracted creating an electrical current that is used to reconstruct an image of the surface with atomic resolution [ 135 ]. STM is mainly used to characterize the topography of NPs. For inorganic NPs, these techniques offer excellent approaches for the determination of morphological features of NPs. For organic NPs (or NPs coated with biological materials), these techniques require sophisticated sample preparations which constitute major restrictions to their use [ 136 ]. The sample preparation for these techniques might cause sample dehydration, which might lead e.g. to sample shrinking and aggregation [ 136 ].

Examples: TEM was used for the characterization of Ag NPs produced by Arbutus unedo leaf extract. In this example, the NPs have a spherical morphology with a uniform size of 30 nm. The NPs were found to agglomerate into small aggregates, each including 5–6 NPs. At the same time, the SAED approach was used to determine the crystal structure of the NPs. The majority of the NPs were found to be single crystalline cubic materials predominately oriented along their (111) direction [ 137 ]. For the characterization of Ag NPs produced by Diospyros kaki leaf extract, SEM helped to show that the NPs were also spherical and the size was 32 nm with some deviations [ 138 ]. STM is less frequently used for the characterization of biogenic NPs. The features of Ag NPs produced by lime, sweet-lime, and orange juices were compared using STM technique [ 139 ].

Dynamic light scattering (DLS)

This technique is a common approach for the analysis of NP size and size distribution. This approach involves the measurement of light interference based on the Brownian motion of NPs in suspension, and on the correlation of NP velocity (diffusion coefficient) with their size using Strokes-Einstein equation [ 140 ]. The size distribution range of NPs is shown as the polydispersity index, which is the output of an autocorrelation function [ 136 ]. The polydispersity index values lie between 0 and 1, where 0 represents a completely homogenous population and 1 represents a highly heterogeneous population. This technique also allows the analysis of non-spherical NPs through the use of multistage DLS [ 136 ]. This technique is also referred to as photon correlation spectroscopy (PCS) [ 141 ].

Examples: DLS was used to measure the size and the size distribution profile of a wide range of biogenic NPs. The average size of Ag NPs produced by Trichoderma koningii fungi was found to be around 25 nm and the size distribution profile was between 14 and 34 nm. The polydispersity index for those NPs was 0.681, which indicates that they are polydispersed [ 142 ]. While the average size of Ag NPs produced by potato ( Solanum tuberosum ) was found to be around 10–12 nm with a wider distribution profile between 3–65 nm [ 143 ]. In a different application, DLS was employed to study the size increase of biogenic MnO 2 NPs overtime, demonstrating that their size is 7.5 nm after 3 min of the initiation of the reaction, then their size grows overtime until it become 54 nm after 31 min [ 144 ].

Nanoparticle tracking analysis (NTA)

This method is used for the analysis of NP size in suspensions based on their Brownian motion. Like in DLS, the rate of NP movement is correlated with their size using Strokes-Einstein equation, allowing the measurement of size distribution profiles for NPs with 10–1000 nm diameter. Its advantage over DLS is that NP motion is analyzed by video. Individual positional changes of NPs are tracked in two dimensions, which are used to determine NP diffusion rates, and by knowing the diffusion coefficient, the hydrodynamic diameter of the particles can be calculated. In DLS, individual NPs are not visualized, but instead, the time-dependent intensity fluctuations caused by Brownian motion are used to calculate the polydispersity index [ 145 ]. NTA was found to be more precise for sizing monodisperse as well as polydisperse organic NPs compared to DLS [ 146 ].

Examples: NTA was used to measure the size and dispersity of Ag NPs produced by Camellia sinensis (green tea) powder, the NPs were found to be well dispersed in an aqueous medium with an average size of 45 ± 12 nm [ 147 ]. For Se NPs produced by lactic acid bacteria, NTA was employed to measure the size and the concentration of NPs. The average size was found to be 187 ± 56 nm with a concentration of (4.67 ± 0.30) × 10 9 Se NPs per ml [ 148 ].

Brunauer–Emmett–Teller (BET) method

This method is based on the adsorption and desorption principle developed by Stephen Brunauer, Paul Emmett, and Edward Teller, and it is considered one of the best methods for the analysis of NP surface area [ 25 ]. In BET analysis, a partial vacuum is created to produce adsorption between the sample and liquid N 2 (because the interaction between solid and gaseous phases is weak, the surface is cooled with liquid N 2 to obtain detectable amounts of adsorption). After the formation of adsorption monolayers, the sample is removed from the N 2 atmosphere and heated to cause the adsorbed N 2 to be released from the material (desorption) and quantified. The data collected is displayed in the form of isotherms (graphs representing the amount of N 2 adsorbed as a function of relative pressure at a constant temperature). The data is displayed in five isotherms where the information is used to determine the surface area of the sample [ 25 , 149 ]. Figure  6 graphically illustrates the principle of this method.

Principles of the BET and BJH methods. The BET method (steps 1–3) is based on the adsorption of nitrogen on the NP surface. After the formation of a monolayer, nitrogen is desorbed, and the surface area is calculated. The BJH method (steps 1, 2, 4, and 5) is based on the complete filling of NP pores with liquid nitrogen. When saturation is reached, nitrogen is desorbed, and pore size is calculated

Examples: The BET method was employed to measure the surface area of CeO 2 NPs produced by Eucalyptus globulus leaf extract. The surface area was found to be 40.96 m 2 /g of biogenic CeO 2 NPs, much higher than the commercial CeO 2 NPs (8.5 m 2 /g) [ 150 ]. BET was also used to measure the surface area of SiO 2 NPs produced by rice husk, CuO NPs produced by Leucaena leucocephala leaf extract, and Ag NPs produced by Acanthospermum hispidum leaf extract. In these examples, the surface area was 7.15 m 2 /g, 47.54 m 2 /g, and 9.91 m 2 /g, respectively [ 151 , 152 , 153 ].

Barrett–Joyner–Halenda (BJH) method

This method is based on the Barrett–Joyner–Halenda principle and is used for the determination of porosity (or pore size) of NPs. Similar to the BET method, this method also involves the use of N 2 gas to adsorb to the sample. In the BJH method, the process is extended so the gas condensates in the sample pores as pressure increases. The pressure is increased until a saturation point is achieved, at which all the pores of the sample are filled with liquid. Afterwards, the condensated gas is allowed to evaporate where the desorption data is calculated and correlated to the pore size using a modified Kelvin equation (Kelvin model of pore filling) [ 154 , 155 ]. Figure  6 graphically illustrates this method.

Examples: The BJH method was employed to study the pore size of a wide range of biogenic NPs, for instance, the pore size of CeO 2 NPs produced by Eucalyptus globulus leaf extract was found to be 7.8 nm [ 150 ], the pore size of CuO NPs produced by Leucaena leucocephala leaf extract was 2.13 nm [ 152 ], the pore size of SiO 2 NPs produced by rice husk and Ag NPs produced by Acanthospermum hispidum leaf extract were much larger, being 29.63 nm and 36.34 nm, respectively [ 151 , 153 ].

Structural and chemical characterization

The structural characterization of NPs and the study of their composition is of high interest due to the strong influence of these parameters on the physicochemical properties. The following techniques are commonly used for the analysis of NP composition, phase, crystallinity, functionalization, chemical state (oxidation), surface charge, polarity, bonding, and electrochemical properties.

X-ray diffraction analysis (XRD)

This technique is based on irradiating a material with incident X-rays and then measuring the intensities and scattering angles of the X-rays that leave the material [ 156 ]. This technique is widely used for the analysis of NP phase and crystallinity. However, the resolution and accuracy of XRD can be affected in cases where the samples have highly amorphous characteristics with varied interatomic distances or when the NPs are smaller than several hundreds of atoms [ 25 ].

Examples: For the characterization of biogenic Ag NPs, the XRD results of Ag NPs produced by Trichoderma koningii [ 142 ], Solanum tuberosum [ 143 ], and Acanthospermum hispidum leaf extract [ 153 ] displayed characteristic peaks occurring at roughly 2θ = 38 o , 44°, and 64 o corresponding to (111), (200), and (220) planes, respectively. These results are in good agreement with the reference to the face-centered cubic structure of crystalline silver. However, the XRD results of Ag NPs produced by Solanum tuberosum were not as clear as the other biogenic Ag NPs and had several impurities. The structural characterization of Pd NPs produced by Garcinia pedunculata Roxb leaf extract by XRD showed the distinct peaks of Pd, however, three other peaks were also observed at 2θ of 34.22˚, 55.72˚, and 86.38˚, indicating the presence of PdO phases along with Pd NPs [ 157 ].

Energy-dispersive X-ray spectroscopy (EDX)

This technique is based on the irradiation of the sample with an electron beam. Electrons of the electron beam when incident on the sample surface eject inner shell electrons, the transition of outer shell electrons to fill up the vacancy in the inner shell produces X-rays. Each element produces a characteristic X-ray emission pattern due to its unique atomic structure, and therefore can be used to perform compositional analysis [ 158 ]. The shortfall of EDX is that the resulting spectra give only qualitative compositional information (it shows the chemical elements present in the sample without quantification). However, the peak intensities to some extent give an estimate of the relative abundance of an element in a sample [ 159 ]. This technique does not require sophisticated additional infrastructures, usually it is a small device that is connected to an existing SEM or TEM. This allows the use of SEM or TEM for the morphological characterization and EDX is used simultaneously for the analysis of chemical composition [ 160 ].

Examples: The EDX technique is usually used for the confirmation of the presence of the element in question in biogenic NPs. For instance, EDX was used to confirm the presence of Au in Au NPs produced by Jasminum auriculatum leaf extract [ 161 ], the presence of Pd in Pd NPs produced by Pulicaria glutinosa extract [ 162 ], the presence of Te in Te NPs produced by Penicillium chrysogenum PTCC 5031 [ 163 ], and the presence of Ag in Ag NPs produced by Trichoderma viride [ 164 ].

High-angle annular dark-field imaging (HAADF)

This method is used for the elemental mapping of a sample using a scanning transmission electron microscope (STEM). The images are formed by the collection of incoherently scattering electrons with an annular dark-field detector [ 165 ]. This method offers high sensitivity to variations in the atomic number of elements of the sample, and it is used for elemental composition analysis usually when the NPs of interest consist of relatively heavy elements. The contrast of the images is strongly correlated with atomic number and specimen thickness [ 166 ].

Examples: The employment of HAADF-STEM in the characterization of biogenic Au–Ag–Cu alloy NPs confirmed the presence of the three elements in the same NP [ 167 ]. Similarly, this approach revealed that Ag NPs produced by Andrographis paniculata stem extract were coated with an organic polymer [ 168 ]. The employment of this approach in the characterization of Cu NPs produced by Shewanella oneidensis revealed that Cu NPs remained stable against oxidization under anaerobic conditions, but when they were exposed to air a thin shell of Cu 2 O develop around the NPs [ 169 ].

X-ray photoelectron spectroscopy (XPS)

This technique is considered the most sensitive approach for the determination of NP exact elemental ratios, chemical state, and exact bonding nature of NP materials [ 25 ]. XPS is based on the photoelectric effect that can identify the elements within a material, or covering a material, as well as their chemical state with high precision [ 170 ]. XPS can also be used to provide in-depth information on electron transfer, e.g., for Pt NPs supported on CeO 2 , it was found that per ten Pt atoms only one electron is transferred to the support [ 171 ].

Examples: The XPS technique can employed for different purposes. For instance, it was used for measuring the purity of Au NPs produced by cumin seed powder [ 172 ]. XPS was used for the determination of the oxidation states of Pt NPs produced by Nigella sativa seeds and Ag NPs produced by Rosa canina . XPS results of Pt NPs showed the presence of three oxidation states for Pt (Pt (0), Pt (II), and Pt (IV)) and two oxidation states for Ag NPs (Ag (0) and Ag (I)). In both cases, the zero-oxidation state was the abundant one, the presence of a small amount of the other oxidation states suggests that some of the NPs were oxidized or had unreduced species [ 173 , 174 ]. XPS was used for the determination of the exact elemental ratios and the bonding nature of FeS NPs produced by Shewanella putrefaciens CN32. For the exact elemental ratios, the researchers compared biogenic and abiotic FeS NPs and found that biogenic FeS NPs had a 2.3:1 Fe:S ratio while the abiotic NPs had a 1.3:1 Fe:S ratio. For the bonding nature, it was determined that the surface of NPs had Fe(II)-S, Fe(III)-S, Fe(II)-O, and Fe(III)-O bonds [ 175 ].

Fourier-transform infrared spectroscopy (FTIR)

This technique is based on irradiating a material with infrared light, where the absorbed or transmitted radiation is recorded. The resulting spectrum represents a unique fingerprint of samples, where information about the nature of the sample can be obtained such as the bonds involved, polarity, and oxidation state of the sample [ 176 , 177 ]. This technique is mainly used for the characterization of organic materials such as the surface chemical composition or functionalization of NPs. It is also used for the identification of contaminants when high purity is sought [ 178 ].

Examples: For biogenic NPs, FTIR is usually used for the identification of probable functional groups present on the surface of NPs that are responsible for the reduction and stabilization of the NPs. For plant-mediated NP synthesis, for instance for Ag NPs produced by Camellia sinensis , the FTIR results indicate the presence of Camellia sinensis phytocompounds, such as caffeine and catechin, on the surface of Ag NPs that could be responsible for the reduction of Ag or act as stabilizing agents [ 147 ]. For Ag NPs produced by Solanum tuberosum , the NPs were found to be capped by amide and amine groups [ 143 ]. For CeO 2 NPs produced by Eucalyptus globulus , the polyphenol groups present in Eucalyptus globulus extract were found on the surface of NPs suggesting their involvement in the reduction/stabilization process [ 150 ]. For microbe-mediated NP synthesis, FTIR results show the presence of protein residues on the surface of NPs confirming the involvement of different proteins in the reduction/stabilization process, such as in Ag NPs produced by Streptomyces sp. NH28 [ 179 ], in Te NPs produced by Penicillium chrysogenum PTCC 5031 [ 163 ], and in Se NPs produced by Azospirillum thiophilum [ 180 ].

Zeta potential analysis

Zeta potential measurements are used for the determination of NP surface charge in colloidal solutions. The surface charge of NPs attracts counter-ions that form a thin layer on the surface of the NPs (called Stern layer). This layer travels with the NPs as they diffuse thought the solution. The electric potential at the boundary of this layer is known as NP zeta potential [ 136 ]. The instruments used to measure this potential are called zeta potential analyzers [ 181 ]. Zeta potential values are indicative for NP stability, where higher absolute value of zeta potential indicate more stable NPs [ 136 ].

Examples: The zeta potential is a good indicator for the stability of NPs, where NPs with zeta potentials of more than + 30 mV or less than − 30 mV are considered stable. Zeta potentials have been measured for a wide range of biogenic NPs. The zeta potential for Ag NPs produced by Ziziphus jujuba leaf extract of − 26.4 mV [ 182 ]. Ag NPs produced by other organisms have different zeta potential values, for example, Ag NPs produced by Punica granatum peel extract have a zeta potential of − 40.6 mV indicating their higher stability [ 183 ], while Ag NPs produced by Aspergillus tubingensis have a zeta potential of + 8.48 indicating their relative instability [ 184 ]. The pH of the sample is another important parameter for zeta potential values, the higher pH the lower the zeta potential value [ 185 ]. Having different zeta potential values for the same type of NPs depending on the organism used for their synthesis is not unique to silver, Se NPs also show different potential values depending on the organism used for their synthesis [ 186 ].

Cyclic voltammetry (CV)

CV is an electrochemical technique for measuring the current response of redox-active solutions to a linearly cycled potential sweep between two or more set values. The CV technique involves the use of three electrodes: a working electrode, reference electrode, and counter electrode. These electrodes are introduced to an electrochemical cell filled with an electrolyte solution and where voltage is in excess, the potential of the working electrode is cycled and the resulting current is measured. This technique is used for determining information about the reduction potential of materials, the kinetics of electron transfer reactions, and the thermodynamics of redox processes [ 187 , 188 , 189 ].

Examples: The CV technique can be employed for two different purposes in the context of biogenic NP characterization. Firstly, it can be used for measuring the stability of NPs in electrocatalysis. For this purpose, the biogenic NPs are assembled on an electrode of the electrolysis cell and are tested for their electrocatalytic behavior against a redox reaction over different cycles. As an example, Ag NPs produced by Citrus sinensis were found to be stable in phenolic compounds redox reactions over multiple cycles [ 190 ]. Secondly, CV can be used for monitoring the progress of reduction of metallic NPs or for the determination of the reducing agent involved in the reduction. For example, for Ag NPs produced by Indian propolis, four cyclic voltammograms were recorded, one for a water extract of Indian propolis, another for an ethanol extract of Indian propolis, and two for the constituent flavonoids of Indian propolis (pinocembrin and galangin). The four cyclic voltammograms showed similar behaviors indicating the involvement of these flavonoids in the reduction of Ag and in forming Ag NPs [ 191 ].

Raman spectroscopy

This technique is based on irradiating a sample with monochromatic light emitted by a laser, in which the interactions between the laser light and molecular vibrations (photons and phonons) are recorded. The technique records the inelastically scattered photons, known as Raman scattering (named after the Indian physician C. V. Raman) [ 192 ]. The output of this technique is a unique fingerprint for each sample, which is used to characterize the chemical and intramolecular bonding of the sample. It can also be used to characterize the crystallographic orientation of the sample [ 193 ]. Surface-enhanced Raman spectroscopy (SERS) enhances Raman scattering of a sample and provides a more sensitive, specific, and selective technique for identifying molecular structures [ 194 ]. Both techniques are also used for the characterization of optical properties, where the recorded photons and phonons are used to understand the plasmonic resonance of NPs [ 25 ].

Examples: Raman spectroscopy was used to characterize Fe 3 O 4 NPs produced by Pisum sativum peel, the researchers found that the NPs were Fe 3 O 4 NPs with face centered cubic phase which was in agreement with their XRD measurements [ 195 ]. Other researchers used Raman spectroscopy for studying the trace deposits of carbohydrates on ferrihydrite NPs produced by Klebsiella oxytoca , the results showed that the pores of NPs had more deposits of carbohydrates that the surface of the NPs [ 196 ]. For Au NPs produced by Raphidocelis subcapitata (green algae), several biomolecules were suggested for their involvement in this process. SERS technique was used to study Au NPs surface-associated biomolecules in order to narrow down the list of biomolecules involved in the bioproduction process. The researchers found that several biomolecules such as, glutathione, β-carotene, chlorophyll a, hydroxyquinoline, and NAD were associated with Au NPs surface, thus, ruling out other molecules such as, glutaraldehyde fixing agent, saccharides, FAD, lipids, and DNA from the list [ 197 ].

Characterization of optical, electronic, and electrical properties

In addition to Raman spectroscopy and SERS, also other techniques can be employed to study and characterize the optical properties of NPs. These techniques give information about the absorption, reflectance, fluorescence, luminescence, electronic state, bandgap, photoactivity, and electrical conductance properties of NPs.

Ultraviolet–visible spectroscopy (UV–vis) and photoluminescence spectroscopy (PL)

In absorption spectroscopy such as UV–vis, the transition of electrons from the ground state to an excited state is measured, while in photoluminescence spectroscopy, the transition of electrons from the excited state to the ground state is measured [ 198 ]. UV–vis spectroscopy uses visible and UV light to measure the absorption or reflectance of a sample. In photoluminescence spectroscopy, usually UV light is used to excite the electron and then measure the luminescence or fluorescence properties of a sample [ 199 ].

Examples: UV–vis spectroscopy is a simple and common technique that is used for the characterization of the optical properties of NPs. For instance, for the characterization of the optical properties of Ag NPs produced by Trichoderma viride , the UV–vis spectrum showed that a Ag surface plasmon band occurs at 405 nm, which is a characteristic band for Ag NPs. The intensity of this band over the reaction time increased as a result of increasing Ag NP concentration in the solution. In the same study, the photoluminescence properties of these NPs were recorded, with an emission in the range between 320–520 nm, which falls in the blue-orange region [ 164 ]. For biogenic Cu NPs, the common absorption peaks are located between 530–590 nm. The difference in NP size and the bio-active molecules used for the reduction process are believed to be the reasons behind the differences in the absorption peaks [ 200 ]. For instance, 15 nm spherical Cu NPs produced by Calotropis procera have an absorption peak at 570 nm [ 201 ], while 76 nm spherical Cu NPs produced by Duranta erecta have an absorption peak at 588 nm [ 202 ]. The same applies to photoluminescence effects, where 27 nm spherical Cu NPs produced by Tilia extract emit light of 563 nm (dark brown) [ 203 ], while 19 nm spherical Cu NPs emit light of 430 nm (green) [ 204 ].

UV–vis diffuse reflectance spectroscopy (DRS)

This technique uses UV and visible light to measure the diffuse reflectance of a material (the reflection of light in many angles, as opposed to specular reflection). The resulting diffuse reflectance spectra are used to determine the electronic state of a sample, which is then used to calculate the bandgap [ 25 ]. Bandgap determination is crucial for determining conductance and photocatalytic properties especially for semiconductor NPs [ 205 ].

Examples: The DRS technique was used to calculate the bandgap for a wide range of biogenic NPs. For instance, TiO 2 NPs produced by Andrographis paniculata exhibit an optical energy bandgap of 3.27 eV [ 206 ]. Interestingly, biogenic ZnO NPs produced by different organism show different bandgaps, for example, ZnO NPs produced by Pseudomonas putida have a bandgap of 4 eV [ 207 ], while ZnO NPs produced by Calotropis procera leaf extract have a bandgap of 3.1 eV [ 208 ].

Spectroscopic ellipsometry

This technique is based on irradiating a sample with polarized light to measures changes in polarization. It is widely used to calculate the optical constants of a material (refractive index and extinction coefficient) [ 209 ]. This technique is also used to characterize the electrical conductivity and dielectric properties of materials [ 210 ].

Examples: Spectroscopic ellipsometry is not a common technique for the characterization of biogenic NPs. For chemically produced NPs, the optical properties for different-sized Au NPs partially embedded in glass substrate were measured by spectroscopic ellipsometry. In this example, a clear transition from LSPR to SPR mode was found as the thickness increases. Moreover, the partially-embedded Au NPs had much higher refractive index sensitivity compared to Au NPs fully immobilized in a glass substrate [ 211 ]. Spectroscopic ellipsometry was also used to measure the changes in the optical constants of a layer of 5 nm ZnO NPs induced by UV illumination. In this case, it was found that the UV illumination of ZnO NPs in inert atmospheres resulted in a clear blue shift in the absorption (Moss-Burstein shift). The UV illumination of ZnO NPs results in the desorption of O 2 from the NPs surface leading to the population of the lowest levels in conduction band with mobile electrons. This phenomenon is reversible, in which the exposure to O 2 from air results in the scavenging of these mobile electrons [ 212 ].

Characterization of magnetic properties

The magnetic properties of NPs are of high importance, as they potentially give NPs great advantages in catalysis, electronics, and medical applications. Several techniques were developed for the detection and quantification of small magnetic moments in NPs.

Magnetic force microscopy (MFM)

This technique is a variety of atomic force microscopy (AFM), in which a magnetic tip is used to scan the sample. The magnetic tip is approached very close to the sample, where the magnetic interactions between the tip and the sample are recorded [ 213 ]. At closer distances to the sample (0–20 nm), other forces such as van der Waals forces also interact with the tip. Therefore, MFM measurements are often operated with two-pass scanning method (also called lift height method) [ 214 ] (Fig.  7 ). In this method, the tip is firstly used to measure the topography of the sample including the molecular forces as van der Waals. Afterwards, the tip is lifted and a second scan is operated following the same topography outline. In the second scan, the short-ranged van der Waals forces disappear and the long-range magnetic forces are almost exclusively recorded. In an experimental study, researchers found that 22 nm was the optimal scanning height for the second scan, at which van der Waals forces are very weak while the distance is still small enough to measure the magnetic interactions for Pd-Fe bimetallic NPs [ 215 ].

Magnetic force microscopy lift height method. The first scan is done very close to the surface to obtain the topography of the sample. Then, the tip is lifted and a second scan is performed following the topography outline obtained in the first scan

Examples: MFM was heavily used for the characterization of magnetite NPs produced by magnetotactic bacteria. For instance, the size and orientation of the magnetic moment of magnetite NPs produced by Magnetospirillum gryphiswaldense strain MSR-1 were studied by MFM [ 216 ], in which the size of the magnetic moment was found to be 1.61 × 10 −17 Am 2 . In a different study, MFM was used to characterize the magnetic properties and to estimate the size of the magnetic kernel of the magnetosomes produced by the same strain, and it was determined that the NPs behaved like single mono-domain nanomagnets [ 217 ]. The magnetic properties of NPs made from materials such as Pd that only exhibit significant magnetism on the nanoscale can also be studied by MFM, however, the magnetic moment of these NPs is much lower than for ferromagnetic NPs. The magnetic decoration of Pd NP samples with Fe 2 O 3 NPs strongly enhances the weak magnetic signal of Pd NPs up to 15 times [ 218 ]. This approach could make the MFM technique useful for the characterization of weak magnetic NPs.

Vibrating-sample magnetometry (VSM)

This technique measures the magnetic properties of materials based on Faraday’s law of induction. In VSM, the sample is placed in a constant magnetic field in a special holder that vibrates vertically. As the holder starts vibrating, the magnetic moment of the sample creates a magnetic field that changes as function of time. The alternating magnetic field created in the sample induces an electric current that is recorded and used to calculate the magnetic properties of the sample [ 219 , 220 ].

Examples: For the characterization of Fe 2 O 3 NPs produced by Tridax leaf extract, VSM studies revealed that the NPs had a saturation magnetization of 7.78 emu/g, a remnant magnetization of 0.054 emu/g, and a coercivity of − 1.6 G [ 221 ]. In other studies, VSM was used to compare the magnetic properties of iron oxide NPs produced Moringa oleifera with the magnetic properties of the same NPs but coated with chitosan. The researchers found that saturation magnetisation, remnant magnetization, and coercivity have lower values when the NPs are coated with chitosan [ 222 ].

Superconducting quantum interference device (SQUID) magnetometry

This technique measures the magnetic properties of materials based on the Josephson effect. Niobium (Nb) or other metal alloys are used in the device which needs to be operated at temperatures very close to the absolute zero to main superconductivity, where liquid helium is used to maintain the cold environment [ 223 ]. However, other kinds of SQUID also exist where high-temperature superconductors are used [ 224 ]. After reaching superconducting environments, the Josephson junctions contained in the device help to create a supercurrent, which is recorded and used to calculate the magnetic properties of the sample [ 225 ].

Examples: For the characterization of iron oxide NPs produced by Cnidium monnieri seed extract, SQUID magnetometry revealed that the NPs had a saturation magnetization of 54.60 emu/g, a remnant magnetization of 1.15 emu/g, a coercivity of 11 Oe, and a magnetic susceptibility of + 1.69 × 10 –3 emu/ cm 3 ⋅ Oe at room temperatures, indicating the superparamagnetic behaviour of these NPs [ 226 ]. SQUID magnetometry was also used for the characterization of the magnetic properties of zinc incorporated magnetite NPs produced by Geobacter sulfurreducens , showing that the loading of only 5% zinc results in the enhancement of saturation magnetization of the NPs by more than 50% [ 227 ].

Electron spin resonance spectroscopy (ESR)

This technique measures the magnetic properties of materials by characterizing and quantifying the unpaired electrons in the sample. Electrons are charged particles that spin around their axis, which can align in two different orientations (+ ½ and − ½) when the sample is placed in strong magnetic field. These two alignments have different energies due to the Zeeman effect. Since unpaired electrons can change their spins by absorbing or emitting photons, in ESR the sample is irradiated with microwave pulses to excite electron spins until a resonance state is reached [ 228 ]. This technique is also referred to as electron paramagnetic resonance spectroscopy (EPR). It can be used to measure the ferromagnetic and antiferromagnetic properties of NPs [ 229 , 230 ].

Examples: ESR was used to characterize the magnetic properties of iron oxide NPs produced by Ficus carica . The trees naturally produce iron oxide NPs as a defence mechanism when are they are subjected to stress. The researchers found that the magnetic properties of iron oxide NPs produced by the same tree but grown in different environmental conditions have different magnetic properties. In addition, a magnetic anisotropy of the signal was visible as the magnetic properties of these NPs varied strongly at different temperatures [ 231 ]. ESR was also used to characterize the magnetic properties of Se nanomaterials produced by anaerobic granular sludge. The ESR results revealed the presence of Fe(III) atoms incorporated in the Se nanomaterial, which enhanced their overall magnetic properties, giving it ferromagnetic behaviour [ 232 ].

Characterization of thermal properties

Several techniques can be used for the characterization of the thermal properties of NPs, such as melting points, crystallization and structural-phase transition points, heat capacity, thermal conductivity, and thermal and oxidative stability.

Differential scanning calorimetry (DSC)

In this technique the analyte and a well-defined reference sample are put at the same temperature, then, the amount of heat required to increase the temperature of the sample and the reference in measured as a function of temperature. This technique is widely used to measure melting points [ 233 ], crystallization points, structural-phase transition points [ 234 ], latent heat capacity [ 235 ], heat of fusion [ 236 ], and oxidative stability [ 237 ].

Examples: For the characterization of Ag NPs produced by Rhodomyrtus tomentosa leaf extract, DSC showed three exothermic peaks at 44, 159, 243, and an endothermic peak at 441 °C. The first peak (at 44 °C) indicates that at this temperature the NPs face a gradual loss of water from their surface. The second peak (at 159 °C) shows that the thermal decomposition of the sample happens at this temperature. The last temperature (441 °C) indicates the melting temperature for those NPs [ 238 ]. For Ag NPs produced by Parthenium hysterophorus leaf extract, DSC showed that their melting temperature was at 750 °C. The researchers also found that these NPs had completely thermally decomposed and crystallized simultaneously [ 239 ].

Differential thermal analysis (DTA)

This technique is based on heating or cooling a sample and an inert reference under identical conditions, where any temperature difference between the sample and the reference is recorded. This technique is primarily used for the study of phase diagrams and transition temperatures [ 240 ]. However, it is also used to measure the melting points, thermal, and oxidative stability [ 241 , 242 ].

Thermogravimetric analysis (TGA)

This technique measures the change in the mass of a sample as a function of temperature and/or time in a controlled atmosphere [ 243 ]. This technique is mainly used to study the thermal stability of materials [ 244 ], in addition, it is also used to measure structural-phase transition points [ 245 ], thermal activation energies [ 246 ], and oxidative stability [ 247 ]. The resulting thermogram is unique for each compound and therefore can also be used for the determination of material composition [ 248 ]. TGA and DTA are usually combined in the same thermal analyzing instrument, called thermogravimetry/differential thermal analysis (TG/DTA) [ 244 ].

Examples: TG/DTA is a common technique for the characterization of thermal properties of biogenic NPs. For instance, the thermal properties of Ag NPs produced by Daphne mucronate leaf extract were studied in the range between 0–1000 °C where the sample was heated at a rate of 10 °C/min. The researchers found that between 400–500 °C the NPs faced a dominant weight loss, while the weight loss below 400 °C and above 500 °C was negligible. The DTA curve showed an intense exothermic peak in the range between 400–500 °C, this indicates that the crystallization of NPs happens in this temperature interval. Some minor weight loss events were seen below 400 °C, this may be caused by the evaporation of water or the degradation of the organic components [ 249 ]. In another study, the thermal properties of Ag NPs produced by two different plants ( Stereospermum binhchauensis and Jasminum subtriplinerve ) were compared. The researchers found that the major weight loss happens between 220–430 °C, which is attributed to the decomposition of biomolecules from the NP surface [ 250 ]. This shows that Ag NPs produced by these plants have much higher content of biomolecules on their surface than Ag NPs produced by Daphne mucronate. TG/DTA showed that Stereospermum binhchauensis Ag NPs crystallize at 315 °C and Jasminum subtriplinerve Ag NPs at 345 °C, around 100 °C less than Daphne mucronate Ag NPs [ 250 ].

Transient hot wire method (THW)

This method is used for the determination of thermal conductivity based on increasing the temperature of a material by a thin hot wire as a function of time, where the heating wire is located directly in the test sample. The advantage of this method over other thermal conductivity measurement methods is the very short measuring time, this gives high accuracy of thermal conductivity due to the negligible values of convection in such short times [ 251 ]. In this method, the NPs are added to a solution (usually water or ethylene glycol) forming a colloidal dispersion called a nanofluid. Then, the thermal conductivity of the nanofluid is measured and compared to the thermal conductivity of the base fluid, giving a thermal conductivity ratio which is used to evaluate the thermal conductivity of different NPs.

Examples: The thermal conductivity ratios of three different concentrations (0.12, 0.18, and 0.24%) of biogenic SnO 2 NPs produced by Punica granatum seed extract were measured in ethylene glycol at 303 K. The researchers found a linear relationship between NPs concentration and the thermal conductivity. The thermal conductivity enhancement of nanofluid to base fluid was between 6 and 24% [ 252 ]. In another study, the thermal conductivity of Fe 2 O 3 NPs produced by Psidium guajava leaf extract was measured in water and in ethylene glycol. The researchers found that the thermal conductivity enhancement in ethylene glycol was better than in water, the thermal conductivity enhancement for 0.025% Fe 2 O 3 NPs in water was 30% while in ethylene glycol was 34%. Moreover, the linear relationship between NPs concentration and thermal conductivity ratio was found for Fe 2 O 3 NPs in both water and ethylene glycol [ 253 ].

Characterization of mechanical properties

Several methods can be used for the characterization of mechanical properties of NPs, such as tensile and compressive strengths, elasticity, viscoelasticity, hardness, and stiffness.

Tensometery

The machine used for this method is called a universal testing machine (UTM) or a tensometer. It is used to measure the elasticity (elastic modulus), tensile and compressive strengths (Young’s modulus) of materials. In this machine, the sample is placed between grips and an extensometer, where changes in gauge length are recorded as a function of load [ 254 ]. However, other mechanical changes in addition to the change in gauge length are also recorded in this machine, such as the elasticity.

Examples: The mechanical properties of different biogenic NP-containing composites can be measured by this machine. For example, the mechanical properties of orthodontic elastic ligatures containing Ag NPs produced by Heterotheca inuloides were studied by comparing the maximum strength, tension, and displacement of the composite with and without the biogenic NPs. The researchers found that maximum strength, tension, and displacement have improved after the addition of Ag NPs [ 255 ]. Interestingly, the addition of biogenic Ag NPs produced by Diospyros lotus fruit extract to starch and polyvinyl alcohol hydrogel membranes resulted in an adverse effect. The tensile strength and modulus of the hydrogel membranes containing 50 and 100 ppm Ag NPs were much lower than of the neat hydrogel membrane. The researchers attributed this adverse effect to the possibility that the addition of Ag NPs could have resulted in blocking the crosslinking between starch and polyvinyl alcohol, or to the possibility of the formation of breakage points in the polymer matrix due to NPs agglomeration [ 256 ].

Instrumented indentation testing

This method is used to characterize the hardness features of materials by using a well-defined hard indenter tip typically made of diamond. The indenter tip is used to make an indentation in the sample by placing incremental loads on the tip, after which the area of indentation in the sample is measured and used to calculate the hardness features [ 257 ]. Light microscopy, SEM, or ATM technique are usually used to visualize the indentation in the sample. The method is also called micro- or nano-indentation testing.

Examples: This method was used to characterize the mechanical properties of calcite NPs produced by Ophiocoma wendtii brittlestar. The arm plates of this brittlestar are covered by hundreds of nanoscale calcite lenses that focus light onto photoreceptor nerve bundles positioned beneath the brittlestar. The researchers used the nanoindentation method to compare Young’s modulus, hardness and fracture toughness of biogenic calcite with geocalcite. The results showed that the biogenic calcite lenses have higher hardness and fracture toughness compared to geocalcite (more than twofold) [ 258 ]. Bamboo is well known for its high silica content in comparison to other wood species. It produces SiO 2 NPs and deposits it in its epidermis in the form of silica cells. The mechanical properties of silica cells compared to other types of cells of Moso bamboo ( Phyllostachys pubescens ) were studied by instrumented indentation testing. The researchers found that the cell wall of silica cells display higher hardness and elastic recovery compared to fibre and epidermal cells, which is attributed to the presence of biogenic SiO 2 NPs in the silica cells [ 259 ].

Dynamic mechanical analysis (DMA)

This method is used to study the mechanical properties of materials by measuring the strain of a material after applying a stress. This method helps to obtain three different values: storage modulus, loss modulus, and loss tangent. These values are important to give an overview about the stiffness and viscoelasticity behavior of materials [ 260 ].

Examples: The DMA method was used to characterize the mechanical properties of polymethyl methacrylate denture base polymer filled with Ag NPs produced by Boesenbergia rotunda . In this study frequency sweep test was used to determine the viscoelastic behavior of this nanocomposite where the temperature was constant at 37 °C and the frequency was increasing from 0.5 to 100 Hz in tension mode. The researchers found a frequency dependence for storage modulus, loss modulus, and loss tangent for the nanocomposite with various Ag NPs loading concentrations. The frequency dependence of storage modulus, loss modulus, and loss tangent indicates the viscoelastic response of this polymer. However, the results showed that the storage modulus for the nanocomposite is much higher than the loss modulus over the range of frequencies, indicating the elastic dominance of the nanocomposite. Moreover, the researchers found that storage and loss moduli increase with increasing Ag NPs loading concentrations, which is due to the interaction between polymethyl methacrylate and Ag NPs [ 261 ].

In a different study, DMA was used to determine the thermomechanical properties of pol(S-co-BuA) polymer filled with cellulose nanocrystals produced by Posidonia oceanica . In this case, the behaviour of storge modulus and loss tangent were studied as a function of temperature for different cellulose nanocrystals loading concentrations. The results showed that the unloaded polymer behaves like an amorphous polymer, the storage modulus remains constant until the temperature reaches 25 °C then it starts to sharply decrease due to glass–rubber transition. A relaxation process was also evident for the unloader polymer, where the loss tangent reaches its maximum at 35 °C then it starts to fall. The addition of cellulose nanocrystals to the polymer positively enhanced both effects. The dramatic drop of storage modulus at 25 °C was less for the nanocomposite, where the drop for the polymer loaded with 15% cellulose nanocrystals was almost cancelled. Similar positive enhancement was found for loss tangent. These enhancements could be attributed to the mechanical coupling effect, in which the NPs connect and form a stiff continuous network linked through hydrogen bonding [ 262 ].

Applications of NPs

NPs, due to their above-mentioned unique or enhanced physicochemical properties, are used in a wide range of applications in different fields. In addition, several potential applications are in research and development. Here we present some examples of these applications.

Applications in medicine and pharma

Metallic and semiconductor NPs have huge potential for cancer diagnosis and therapy based on their enhanced light scattering and absorption properties due to LSPR effect. For instance, Au NPs efficiently absorb light and convert it into localized heat, which can be exploited for selective photothermal therapy of cancer (cancer cell death by heat generated in tumor tissue) [ 263 , 264 ]. In addition, the unique optical properties of Au NPs make them a great candidate for the photodynamic therapy of cancer (the use of a drug that is activated by light to kill cancer cells) [ 265 ]. Gd based NPs have also shown great abilities in tumor growth inhibition [ 266 ], metastasis inhibition [ 267 ], and tumor-specific magnetic resonance contrast enhancement [ 268 ]. Targeted drug delivery is also an important potential application of NPs. ZnO and Fe 3 O 4 NPs were efficiently used for targeted drug delivery and selective destruction of tumor cells [ 269 , 270 , 271 ].

Moreover, NPs have been successfully used in different medical applications such as cellular imaging [ 272 ], or in biosensors for DNA, carbohydrates, proteins, and heavy metal ions [ 273 , 274 ], determination of blood glucose levels [ 275 ], and for medical diagnostics to detect bacteria [ 276 ] and viruses [ 277 ]. For instance, Au NPs were conjugated with SARS-CoV-2 antigens to rapidly detect the presence of SARS-CoV-2 IgM/IgA antibodies in blood samples within 10–15 min [ 278 ], At the same time, due to their antimicrobial and antibacterial activities, NPs such as TiO 2 , ZnO, CuO, and BiVO 4 are being increasing used in various medical products such as catheters [ 279 , 280 ].

Applications in electronics

NPs, due to their novel electronic and optical properties, have a wide range of potential applications in imaging techniques and electronics. For instance, Gd-based NPs can improve the imaging quality and the contrast agent administration dose of magnetic resonance imaging (MRI). The use of Gd 2 O 3 NPs as a contrasting agent was found to be more efficient than the commonly used agent (Gd-DOTA) at the same concentration [ 281 ]. At the same time, GdPO 4 NPs were successfully used for tumor detection using MRI in 1/10 of the dose typically used with Gd-DTPA agent [ 282 ]. Interestingly, NPs also offer the ability to image and track a single molecule, which can reveal important information about cellular processes such as membrane protein organization and interaction with other proteins. For example, Eu 3+ -doped oxide NPs were used to track a single toxin receptor with a localization precision of 30 nm [ 283 ].

Regarding applications in batteries, an important component in lithium-ion batteries is the separators. Their main function is to prevent the physical contact of anode and cathode, and to provide channels for the transport of ions. The commonly used commercial material in battery separators, a polyolefin microporous membrane, suffers from poor electrolyte uptake and poor thermal stability [ 284 ]. Due to the aerogel structure of some NPs (such as ZnO NPs), they are an ideal choice for separator plates in batteries [ 284 ]. This makes the batteries store a significantly higher amount of energy compared to traditional batteries. For lithium-air batteries, using Pt-Au bimetallic NPs strongly enhances oxygen reduction and oxygen evolution reactions [ 285 ]. Moreover, batteries made of nanocrystalline Ni and metal hydrides last longer and require less charging [ 23 ]. In addition to battery applications, several NPs such as CdS and ZnSe are also used in light-emitting diodes (LED) of modern displays to get higher brightness and bigger screens [ 23 , 286 ]. Other NPs such as CdTe NPs are also used in liquid crystal displays (LCDs) [ 287 ]. The addition of a NP layer to LED and LCD enables them to generate more light using the same amount of energy and enhances their lifetime.

Applications in agriculture

NPs have potential to benefit the agriculture field by providing new solutions to current agricultural and environmental problems [ 288 ]. NPs are mainly used in two forms in agriculture, as nanofertilizers and nanopesticides. Chemical fertilizers have poor efficiency due to leaching and volatilization. In these cases, the farmers usually react by using excessive amounts of fertilizers, which increases crops productivity but has an environmental cost [ 288 ]. In contrast, nanofertilizers are compounds that are applied in smaller amounts than regular chemical fertilizers but yet have better efficiencies [ 289 ]. The difference in efficiency comes from the fact that they are able to release the nutrients just when and where they are required by the plants. In that way, they limit the conversion of excess amounts of fertilizer to gaseous forms or from leaking into the ground water [ 290 ]. Several NPs have been employed in the development of fertilizers, including SiO 2 , ZnO, CuO, Fe, and Mg NPs [ 291 , 292 , 293 ]. These nanofertilizers provide the plants with increased nitrogen fixation, improved seed germination, amelioration to drought stress, increased seed weight, and increased photosynthesis ability [ 291 , 292 , 293 ]. The large surface area and small size of these NPs are the main reasons for the better efficiencies of nanofertilizers over conventional fertilizers [ 294 ].

Several NPs have proven antimicrobial, insecticidal, and nematicidal activities, which makes them a promising alternative to chemical pesticides and a potentially cheaper alternative to biopesticides [ 294 ]. For instance, the photocatalytic activity of TiO 2 NPs gives them a potent antimicrobial activity against Xanthomonas perforans , the causing agent of tomato spot disease [ 295 ]. CuO NPs show insecticidal activity against Spodoptera littoralis , known as African cotton leafworm [ 296 ]. Ag NPs show nematicidal activity against Meloidogyne spp. , root-knot nematodes [ 297 ].

Applications in the food industry

NPs, despite toxological concerns, have impactful applications in several food industry-related process such as food production, preservation, and packaging. TiO 2 NPs are a major promising player in this industry. Their photocatalytic antimicrobial activity makes them an interesting material for food packaging [ 298 ]. In addition, they are also used in sensors to detect volatile organic compounds [ 299 ]. Ag NPs are also promising in food packaging due to their antimicrobial activity. They play an important role in reducing the risk of pathogens and extending food shelf-life [ 294 ]. The efficiency of doping Ag and ZnO NPs to degradable and non-degradable packaging materials for meat, bread, fruit, and dairy products was tested against several yeast, molds, aerobic, and anaerobic bacteria [ 300 ]. For instance, polyvinyl chloride doped with Ag NPs was evaluated for packing minced meet at refrigerator temperature (4 °C); the results showed that Ag NPs significantly helped to slow down bacterial growth, increasing the shelf-life of minced meet from 2 to 7 days [ 301 ].

Effects of NPs on biological systems

Although the use of NPs is exponentially growing, their possible toxicological and hazardous impacts to human health and environment cannot be ignored. NPs may get released to the environment during production stages, usage, recycling, or disposal. These NPs may persist in air, soil, water, or biological systems [ 302 ]. NPs can enter the human or animal body though the skin, orally, or via the respiratory tract, and afterwards move to other parts of the body. The exposure to NPs was found to activate proinflammatory cytokines and chemokines with recruitment of inflammatory cells, which impacts the immune system homeostasis and can lead to autoimmune, allergic, or neoplastic diseases [ 302 ]. Moreover, the exposure to ultrafine particles can cause pulmonary, cardiac, and central nervous system diseases [ 303 , 304 , 305 ]. Similarly, NPs can enter plants cells and cause harmful effects [ 306 ]. For instance, the exposure of ZnO and Al NPs was found to cause root growth inhibition in plants [ 307 , 308 ].

Nanoscience and nanotechnology are inherently transdisciplinary fields of science. With new bio-based approaches, there is a need for biologists to understand not only the basic principles of nanoscience, but also the technologies and methods traditionally employed to characterize nanomaterials. We hope that this review can help to inspire new collaborations across different scientific disciplines, by helping biologists to identify the best technologies—and partners—to characterize their nanomaterials. At the same time, we recommend to take potential biological risks of these new materials into careful consideration already during the planning phase of such experiments.

Availability of data and materials

Not applicable.

https://www.etymonline.com/word/nano .

[SOURCE: ISO/TS 80,004‑2:2015, 4.4].

Abbreviations

Atomic force microscopy

Brunauer–Emmett–Teller

Barrett–Joyner–Halenda

Cyclic voltammetry

Dynamic light scattering

Derjaguin–Landau–Verwey–Overbeek

Dynamic mechanical analysis

Derjaguin–Muller–Toporov

UV–vis diffuse reflectance spectroscopy

Differential scanning calorimetry

Differential thermal analysis

Energy-dispersive X-ray spectroscopy

Electron microscopy

Electron paramagnetic resonance spectroscopy

Electron spin resonance spectroscopy

Fourier-transform infrared spectroscopy

High-angle annular dark-field imaging

International Organization for Standardization

Johnson–Kendall–Roberts

Liquid crystal display

Light-emitting diode

Localized surface plasmon resonance

Magnetic force microscopy

Magnetic resonance imaging

Nanoparticles

Nanoparticle tracking analysis

Photoluminescence spectroscopy

Critical radius

Threshold radius for superparamagnetism

Selected area electron diffraction

Scanning electron microscopy

Surface-enhanced Raman spectroscopy

Surface plasmon resonance

Superconducting quantum interference device

Scanning transmission electron microscopy

Scanning tunneling microscopy

Transmission electron microscopy

Thermogravimetry/differential thermal analysis

Thermogravimetric analysis

Transient hot wire

Universal testing machine

Ultraviolet

Ultraviolet–visible spectroscopy

Vibrating-sample magnetometry

X-ray photoelectron spectroscopy

X-ray diffraction analysis

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This work was supported by the Research Council of Norway, Grant 294605 (Center for Digital Life) to DL.

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Joudeh, N., Linke, D. Nanoparticle classification, physicochemical properties, characterization, and applications: a comprehensive review for biologists. J Nanobiotechnol 20 , 262 (2022). https://doi.org/10.1186/s12951-022-01477-8

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Synthesis and catalytic application of nanostructured metal oxides and phosphates

First published on 16th September 2024

The design and development of new high-performance catalysts is one of the most important and challenging issues to achieve sustainable chemical and energy production. This Feature Article describes the synthesis of nanostructured metal oxides and phosphates mainly based on earth-abundant metals and their thermocatalytic application to selective oxidation and acid–base reactions. A simple and versatile methodology for the control of nanostructures based on crystalline complex oxides and phosphates with diverse structures and compositions is proposed as another approach to catalyst design. Herein, two unique and verstile methods for the synthesis of metal oxide and phosphate nanostructures are introduced; an amino acid-aided method for metal oxides and phosphates and a precursor crystallization method for porous manganese oxides. Nanomaterials based on perovskite oxides, manganese oxides, and metal phosphates can function as effective heterogeneous catalysts for selective aerobic oxidation, biomass conversion, direct methane conversion, one-pot synthesis, acid–base reactions, and water electrolysis. Furthermore, the structure–activity relationship is clarified based on experimental and computational approaches, and the influence of oxygen vacancy formation, concerted activation of molecules, and the redox/acid–base properties of the outermost surface are discussed. The proposed methodology for nanostructure control would be useful not only for the design and understanding of the complexity of metal oxide catalysts, but also for the development of innovative catalysts.

Introduction

Metal oxide-based catalysts have redox and/or acid–base active sites that promote various types of chemical reactions such as oxidation, 15–18 hydrogenation, 19,20 dehydrogenation, 21 acid–base reactions, 22–24 biomass conversion, 25–27 environmental protection, 28,29 photocatalysis, 30,31 and electrocatalysis 32,33 because their physicochemical properties can be controlled according to the crystal structure, morphology, composition, defects, and doping. In the field of condensed matter science, the unique structures and electronic states of crystalline complex oxides have been widely studied for application as novel functional materials with optoelectronic, conductive, magnetic, and superconductive properties; 34–38 however, these complex oxides are typically synthesized by the conventional solid-state method to obtain highly pure materials from metal precursors without impurities, which limits the catalytic application and overall performance (especially, in liquid-phase reactions) of the bulk catalyst materials. Versatile, reproducible, scalable, and high-yield synthesis of nanostructure-controlled crystalline complex oxide materials with diverse compositions and crystal structures is essential to more effectively utilize the complexity of such materials as catalysts and realize efficient catalytic systems under mild reaction conditions. 38 Although complexity has made it difficult to elucidate mechanistic aspects, recent advances in the synthesis of high-quality materials has gradually enabled the clarification of catalyst local structures and structure–activity correlations. For example, the roles of promoters in industrial multicomponent molybdenum oxide-based catalysts for the selective oxidation of propylene to acrylic acid can be discussed at the atomic scale. 39,40 With this research background, we have developed simple but versatile methods for the synthesis of nanostructure controlled crystalline complex oxides with diverse compositions and structures, 41–44 and these methods have contributed to the discovery of new catalytic properties ( e.g. , aerobic oxidation, 42–50 direct methane conversion, 51–54 concerted acid–base catalysis, 55–58 one-pot synthesis, 59,60 biomass conversion, 43,57,61,62 water electrolysis 63–67 ) of nanomaterials that have not received much attention until now. Herein, we first introduce methods used for nanostructural control of crystalline metal oxides and phosphates mainly based on earth-abundant metals, and then touch upon the development of catalytic materials and reactions with a focus on thermocatalysis ( Fig. 1 ).

Synthesis of nanostructured complex oxides

The co-precipitation method is one of the most useful techniques for the facile synthesis of complex oxides. The addition of precipitants ( e.g. , NaOH, NH 3 , amine) to an aqueous solution containing two or more metal species components gives homogeneous precursors, and the precursors recovered after washing the collected precipitates with solvents are calcined at an appropriate temperature to give the product. 81 Control of the chemical composition of multicomponent complex oxides is difficult because specific soluble metal species are formed during the precipitation and/or the washing steps. Another useful technique is the sol–gel method, whereby complex oxides with relatively high surface areas can be synthesized. The solution sol containing the metal sources and additives are gradually changed to a solid-phase gel by heating, pH control, and drying. Calcination of the dried gel precursors gives the desired and homogeneous complex oxides with controlled chemical compositions. Frequently used sol–gel methods include the hydroxyacid-aided method using citric acid (CA), 82 the Pechini method using ethylenediaminetetraacetic acid–ethylene glycol (EG) complexation, 83 and the polymerized complex (PC) method using CA–EG complexation. 84 The solution combustion method involves heat treatment of a mixture at lower temperatures than those used for conventional synthesis routes because the heat required for phase formation is supplied by the self-sustaining exothermic chemical reactions of metal nitrates with suitable organic fuels. 85 This method has been applied to the rapid synthesis of various multicomponent complex oxides, although consistent control of the process parameters and the final product quality is limited. Soft/hard templating methods are effective for the synthesis of porous complex oxides using polymeric materials and porous silicates as soft and hard templates, respectively, in contrast to non-porous bulk metal oxides. 7 Although the templating methods offer high surface areas of more than 100 m 2 g −1 and ordered pore structures, their applicability is limited due to the need for expensive templates and their subsequent removal, which involves multi-step complicated processes.

Synthesis of crystalline complex oxide nanoparticles using the amino acid-aided method

The formation of an amorphous precursor in which various constituent metal species are highly dispersed would be required to synthesize highly pure crystalline complex oxide materials, and the ligand exchange between the metal sources and additive reagents affects the structure of the precursors. 41,44–49,53,55,56,63–67 First, we investigated the effect of carboxylic acids and metal sources on the synthesis of hexagonal SrMnO 3 as a benchmark material because of its unique catalysis properties for aerobic oxidation of various types of organic substrates. 44 The precursor formation process influences the purity of SrMnO 3 and the resultant surface areas. When using metal nitrates ( i.e. , Sr(NO 3 ) 2 and Mn(NO 3 ) 2 ) as starting materials in the presence of malic acid, only X-ray diffraction (XRD) peaks assignable to Sr(NO 3 ) 2 and complex mixed phases were observed, despite pH adjustment (pH 3.0–9.0) during precursor preparation and/or the calcination of the precursor at high temperatures (∼950 °C). On the other hand, the precursor prepared from metal acetates ( i.e. , Sr(OAc) 2 and Mn(OAc) 2 ) was completely amorphous and calcination at 550 °C gave single-phase SrMnO 3 ( Fig. 4 ). Formation of the amorphous precursor through ligand exchange reaction between the metal nitrates and malic acid is likely prevented by the presence of NO 3 − in the starting materials, and the weaker acidity of acetic acid (p K a = 4.76) than malic acid (p K a1 = 3.4) would accelerate the ligand exchange reactions. On the basis of infrared (IR) spectroscopy and electrospray ionization time-of-flight mass spectrometry (ESI-TOF-MS) measurements, the precursor is an amorphous malate salt with the metal cations linked by malate anions in a bridging bidentate manner.

The effect of various carboxylic acid additives on precursor formation was examined, and only aspartic acid and malic acid gave single phase SrMnO 3 . In particular, the specific surface area ( S BET ) for SrMnO 3 prepared using aspartic acid was up to 47 m 2 g −1 and much larger than those (2–25 m 2 g −1 ) for SrMnO 3 synthesized by conventional methods such as the polymerized complex, acetate, sol–gel combustion, cellulose templating, and citrate methods. 50,86–88 The amorphous precursors with relatively low carbon contents are readily decomposed; therefore, the present method using aspartic acid ( i.e. , the amino acid-aided method) yields pure SrMnO 3 at lower calcination temperatures (by 100–400 K). The high pH of the reaction solution containing metal acetates and aspartic acid containing amino groups (pH = 4.23) resulted in acceleration of the ligand exchange reaction without pH adjustment. Scanning electron microscopy (SEM) observation of SrMnO 3 prepared using aspartic acid showed that the nanoparticles have a sphere-like morphology and the particle size was estimated to be 10–40 nm. On the other hand, sintering of SrMnO 3 nanoparticles was observed in the case of the polymerized complex method because a higher calcination temperature is required.

Various crystalline complex oxides can be synthesized by calcination of precursors prepared with different metal sources and molar ratios at an appropriate temperature using the amino acid-aided method. In particular, this method has advantages in the synthesis of hexagonal perovskite nanoparticles that contain alkaline earth metals, which tend to segregate as carbonates with the conventional method including carbonaceous precursors ( Fig. 3(a) ). In contrast to the cubic perovskite structure (3C structure) in which typical BO 6 octahedra are connected by corner-sharing, the hexagonal perovskite contains unique face-sharing octahedral units involving high-valency metal species ( e.g. , Mn 4+ , Ru 4+ , Fe 4+ ) that are similar to the active sites of oxidases. 89 We have found for the first time that hexagonal perovskite nanoparticles (4H-SrMnO 3 , 9R-BaRuO 3 , 6H-BaFeO 3− δ ) function as effective solid catalysts for the liquid-phase selective oxidation of various organic substrates using O 2 as the sole oxidant. 41,44,45,47–50 Although this sol–gel method is mainly limited to the synthesis of perovskite oxides with B-site metal cations of group 7–10 elements due to the requirement of metal acetates as starting materials, control of the solution states of d 0 -transition metals (Ti 4+ , Zr 4+ , and Nb 5+ ) in the presence of dicarboxylic acids enables the synthesis of high-surface-area perovskite oxides containing d 0 -transition metals. 55,56 In addition, the specific surface area can be increased by a simple procedure of changing the atmosphere from N 2 to air during calcination of the amorphous precursor ( e.g. , S BET of SrTiO 3 : 46 m 2 g −1 from 30 m 2 g −1 ) because the decomposition processes of the precursor ( i.e. , combustion and pyrolysis under oxidative and inert atmospheres, respectively) likely affects the growth and aggregation of SrTiO 3 nanoparticles ( Fig. 5 ). This sol–gel method can be applied not only to the perovskite nanostructure of ABO 3 , but also to other crystalline metal oxides such as simple oxides, 48 metal phosphates, 51,53 and murdochite-type Mg 6 MnO 8 nanostructures. 46

Synthesis of porous manganese oxide nanoparticles by the precursor crystallization method

The morphology and porous structure of the manganese oxides are dependent on the pH conditions during precipitation of the Mn 4+ precursors, and plate-like and sphere-like nanoparticles are formed under acidic and weakly acidic conditions, which results in slit-like and ink-bottle shaped mesopores, respectively ( Fig. 6 ). Non-rigid aggregates of plate-like particles with slit-like pores ( β-MnO 2 -1 and -2 ) were obtained from combinations of NaMnO 4 /MnSO 4 and NaMnO 4 /Mn(NO 3 ) 2 , respectively. On the other hand, sphere-like particles with ink-bottle shaped pores ( β-MnO 2 -3 ) were formed in NaMnO 4 /Mn(OAc) 2 with pH adjustment (pH 0.8). The specific surface areas for β-MnO 2 -1 , -2 , and -3 were 106, 100, and 122 m 2 g −1 , respectively, which are comparable to those (68–195 m 2 g −1 ) for the mesoporous β-MnO 2 materials synthesized by the soft- and hard-template methods, 107,108,113,114 and much higher than that (14 m 2 g −1 ) for non-porous β-MnO 2 nanorods synthesized using the typical hydrothermal method ( β-MnO 2 -HT ). In the case of β-MnO 2 -1 , the number of pores with a size of ∼3 nm in the precursor gradually decreased (≤300 °C), while the number of pores with a size of ∼7 nm increased at 400 °C. Similarly, the number of pores with a size of ∼6 nm in the precursor of β-MnO 2 -3 did not change significantly at ≤300 °C, but the number of pores with a size of ∼9 nm increased at 400 °C.

The oxidation state of Mn species in manganese oxides plays an important role in materials and catalytic science because the distortion due to the Jahn–Teller disproportionation effect of Mn 3+ is related to various applications such as the synthesis of nanosheets by exfoliation, 115 activity in oxidations of water and organic compounds, 50,59–62,116 and improving the durability of battery materials. 117 On the other hand, there was no significant difference among β-MnO 2 -1 , -3 , and -HT in the bulk and surface Mn valence states and the amount of adsorbed oxygen species ( i.e. , surface defects) determined using iodometry and Mn 2p and O 1s XPS, which indicates that the difference in synthesis conditions does not significantly affect the bulk and surface oxidation states of manganese and defects. Despite the similar electronic structures for β-MnO 2 -1 , -3 , and -HT as mentioned above, the catalytic activity of β-MnO 2 -1 and -3 for the aerobic oxidation of various substrates was much higher than that of β-MnO 2 -HT , which suggests that the increase in the catalytic reactivity of β-MnO 2 nanoparticles would be mainly caused by their high surface area.

Focus on the ion exchange reaction of a K + -containing precursor with H 3 O + under acidic conditions has realized the synthesis of β-MnO 2 nanoparticles using inexpensive and readily available KMnO 4 . The K + -containing precursor obtained from KMnO 4 and Mn(NO 3 ) 2 is isolated followed by treatment in 0.1 M aqueous HNO 3 solution to give the proton-exchanged precursor. In contrast to the K + -containing precursor, 43 a single phase of β-MnO 2 ( β-MnO 2 -4 ) is formed by calcination of the proton-exchanged precursor. 60 The specific surface area of β-MnO 2 -4 was 124 m 2 g −1 and comparable to those (100–122 m 2 g −1 ) of β-MnO 2 -1 , -2 , -3 . SEM and transmission electron microscopy (TEM) measurements also showed the presence of flower-like spherical particles composed of small nanoplates (∼10–80 nm).

This method was applied to synthesize the todorokite-type manganese oxide octahedral molecular sieve (OMS-1) nanoparticle catalyst with larger tunnel structures than α-MnO 2 . 42 OMS-1 has 6.9 × 6.9 Å (3 × 3) edge-sharing MnO 6 octahedra chains that are corner-connected to form a one-dimensional tunnel structure, 100,101 and its related materials have been studied as electrodes and/or catalysts. 116,117 The insertion/desertion of various metal cations or the sorption of organic compounds in the tunnel structure is possible, so that metal-modified OMS-1 can act as a catalyst for the oxidation of inert molecules, and for the decomposition of ozone. 118–120 A multi-step process that includes ion exchange and crystallization under hydrothermal or reflux conditions has been typically utilized to synthesize OMS-1. 100,118,121–124 However, this can sometimes cause a particle size increase and/or a decrease in the specific surface area, which are detrimental to improvement of the catalytic performance. 125,126 Although ultrasmall nanocrystalline OMS-1 catalysts have been synthesized by the reduction of MnO 4 − in an organic solvent, 127 the synthesis and catalytic application of OMS-1 nanoparticles are still limited in sharp contrast to the widely investigated cryptomelane-type manganese oxide (OMS-2) with a 2 × 2 tunnel structure.

Mesoporous OMS-1 nanoparticles with high specific surface areas were synthesized in one step by solid-state transformation of a precursor formed by redox reaction between Mg(MnO 4 ) 2 and Mn 2+ reagents. 42 Similar to the synthesis of β-MnO 2 , the conditions for preparation of the precursors and the calcination temperature of the precursors affect the formation of OMS-1. Low-crystallinity buserite precursors were prepared from Mg(MnO 4 ) 2 , MnX (X = 2OAc − , 2NO 3 − , and SO 4 2− ), and MgX with various molar ratios (0.30, 0.40, 0.50, and 0.67) of MnO 4 − /Mn 2+ . The pH conditions during precipitation of the precursors have a strong effect on the morphology, particle size, and specific surface area of OMS-1 ( Fig. 7 ). Aggregates of small nanoparticles were formed under weakly acidic conditions when using acetate salts, while the acidic conditions when using nitrates and sulfates led to the formation of flowerlike particles that contained relatively large nanoplates. The present solid-state transformation of a layered buserite to OMS-1 with 3 × 3 tunnel structures requires precursors of nanosized Mg-rich Mn 3+ -containing layered manganese oxides with orthogonal layer symmetry because the layer-to-tunnel transition would be initiated by the release of layer distortion and strain caused by locally concentrated Jahn–Teller [Mn 3+ O 6 ] octahedra. The crystal structure of the final manganese oxides is dependent on the calcination temperature of the buserite precursors. The calcination of low-crystallinity buserite at 200 °C gives mesoporous OMS-1 nanoparticles, whereas the collapse of the OMS-1 structures to other phases such as Mn 2 O 3 and MgMn 2 O 4 gradually occurs at higher temperatures. From the viewpoints of both inorganic synthesis and catalytic application, ultrasmall OMS-1 nanoparticles synthesized by heat treatment of a precursor prepared using acetate salts (MnO 4 − /Mn 2+ = 0.40) at 200 °C were the most effective catalyst.

The specific surface area of the optimal OMS-1 ultrasmall nanoparticles was 249 m 2 g −1 , which was higher than those of OMS-1 synthesized by a hydrothermal method (35 m 2 g −1 ) and previously reported Mg-OMS-1 prepared by the reduction of MnO 4 − in an organic solvent (155 m 2 g −1 ), 127 by reaction under hydrothermal or reflux conditions (13–185 m 2 g −1 ), 118,120,128–130 and by microwave irradiation (13–48 m 2 g −1 ). 130 N 2 adsorption/desorption isotherms for the optimal OMS-1 ultrasmall nanoparticles showed a type IV pattern with hysteresis loops, which indicates a mesoporous structure. The shapes of the loops were similar to type H3, which is observed in non-rigid agglomerates of plate-like particles with slit-like pores. According to the pore size distribution, the optimal OMS-1 ultrasmall nanoparticles showed a broad peak around 7 nm, which corresponds to ink-bottle type pores, and the total pore volume was up to 0.46 cm 3 g −1 .

Catalysis of crystalline oxide nanomaterials

Liquid-phase aerobic oxidation catalyzed by mesoporous manganese oxides.

In addition, β-MnO 2 -1 could efficiently catalyze the one-pot aerobic oxidative sulfonamidation of various aromatic and heteroaromatic thiols (12 examples) without the need for any additives. 59 The present system was reusable and could be applied to the large-scale sulfonamidation of p -toluenethiol to yield the industrially important sulfonamide. The formation of disulfide and thiosulfonate as intermediates during sulfoamidation was proposed on the basis of mechanistic control experiments, and the reaction mechanism is shown in Fig. 9 . DFT calculations confirmed that the reaction pathway is downhill and thermodynamically favourable. The Lewis acidic Mn sites on β-MnO 2 -1 promote the amination of a thiosulfonane intermediate through the interaction between sulfone and Mn; therefore, such dual-functionality of β-MnO 2 -1 results in highly efficient one-pot aerobic sulfonamidation.

Liquid-phase aerobic oxidation catalyzed by complex oxides

Methane oxidation catalyzed by metal phosphate

Despite the fewer reports than those for supported catalysts, MOFs, and zeolites, several metal phosphates have also attracted much attention as catalysts for the selective oxidation of CH 4 . Low-crystallinity FePO 4 with isolated tetrahedrally coordinated redox-active iron sites was an efficient catalyst for the selective oxidation of CH 4 into CH 3 OH and HCHO with O 2 . 150 Although Gomonaj and Toulhoat reported that the M–O bond strength and the acidity of various crystalline metal phosphates were correlated with the selectivity of CH 4 oxidation toward HCHO using a flow circulation reactor with reaction mixture quenching, 151 the effect of the metal phosphate crystal structures and the reaction mechanism on the direct oxidation of CH 4 is still unclear. Therefore, various crystalline metal phosphate nanoparticles were systematically investigated for the oxidation of CH 4 into HCHO with O 2 as the sole oxidant.

Among four crystalline iron-based catalysts (trigonal FePO 4 , trigonal Fe 3 O 3 (PO 4 ), monoclinic Fe 4 (P 2 O 7 ) 3 and trigonal Fe 2 O 3 ), FePO 4 exhibited the highest HCHO yield (0.19%) at 500 °C. 53 CH 4 conversion tended to increase with the Fe/P ratio, while the selectivity to HCHO decreased. FePO 4 also exhibited durability without significant change in the HCHO, CO, and CO 2 yields at 500 °C for 24 h time-on-stream. Kinetic studies and CH 4 pulse reaction experiments suggested that the structure of bulk FePO 4 was preserved during CH 4 oxidation, and that lattice oxygen on the FePO 4 surface was responsible for the selective oxidation of CH 4 into HCHO with structural preservation of the bulk FePO 4 ( Fig. 13 ). IR spectroscopy measurements were conducted using adsorbed pyridine, chloroform, and methanol as probe molecules to investigate the detailed surface structures. Both FePO 4 and Fe 2 O 3 possess Lewis acid sites and base sites; however, the basicity of FePO 4 was much lower than that of Fe 2 O 3 . Heterolytic cleavage of the C–H bond of CH 4 could proceed using strong acidity via a proton-coupled electron transfer (PCET) mechanism; 152 therefore, the redox-active Lewis acidic Fe 3+ species likely play an important role in CH 4 activation. Methanol-adsorbed IR spectroscopy measurements indicated that FePO 4 without significant base sites had only a weak interaction with methanol, in sharp contrast to basic Fe 2 O 3 that forms methoxide species. The temperature-programmed reaction of adsorbed methanol indicated that the molecularly adsorbed methanol on FePO 4 easily desorbed as methanol without significant formation of CO 2 , but that methanol adsorbed by methoxy species caused only desorption of CO 2 and CO. Therefore, this study suggests the importance of the effect of phosphate units as weak base sites to suppress the overoxidation of C1 products.

BiPO 4 nanoparticles synthesized in a mixed solvent of diethylene glycol and water exhibited a higher HCHO yield than that of FePO 4 above 550 °C, while FePO 4 showed a higher HCHO yield than BiPO 4 below 550 °C. 52 The lack of correlation between the catalytic activity and oxidizability estimated from H 2 -TPR measurements and the absence of any oxidation products in the CH 4 pulse reaction experiments suggested that CH 4 oxidation did not proceed with lattice oxygen supplied from the BiPO 4 solid. Therefore, the reactive oxygen species for CH 4 activation over BiPO 4 is different from that over FePO 4 . Mechanistic studies including the catalyst effect, kinetics, and near-ambient pressure XPS (NAP-XPS) suggest that the oxidation of CH 4 may proceed at surface adsorbed oxygen species generated on the surface Bi atoms of BiPO 4 ( Fig. 14(a) ). Two pathways for H-abstraction from CH 4 using surface O atoms on Bi or lattice O atoms on PO 4 units were then compared by DFT calculations to further investigate the possible involvement of surface oxygen species on BiPO 4 ( Fig. 14(b) and (c) ). H-abstraction using surface O atoms on Bi (Δ E = −1.65 eV) was more favourable than H-abstraction by lattice O atoms on PO 4 units (Δ E = 0.94 eV). Such an O 2 activation mechanism is similar to that proposed for BO x -based benchmark catalysts with relatively high HCHO yields, 153 which suggests that such an O 2 activation mode for BiPO 4 would result in the highly selective formation of HCHO in sharp contrast to FePO 4 nanoparticles with the redox mechanism.

Inspired by the different catalytic properties of FePO 4 and BiPO 4 , various crystalline metal phosphates (37 examples including Cu, Fe, Bi, Ga, Ti, V, Cr, Mn, Co, Ni, Zn, In, K, Ba, B, La, Ce, Nb, Mo, Sb, and Sn) were surveyed, which revealed that Cu is the most active metal species for the direct oxidation of CH 4 to HCHO. 51 To further elucidate the relationship between structure and activity, four types of copper phosphates (monoclinic Cu 2 P 2 O 7 , monoclinic Cu 2 (P 4 O 12 ), triclinic Cu 3 (PO 4 ) 2 , and triclinic Cu 4 O(PO 4 ) 2 ) were investigated ( Fig. 15 ). The CH 4 conversion rate increased with Cu/P ratio, while the selectivity toward formaldehyde decreased. Based on H 2 -TPR and IR spectroscopy measurements of adsorbed probe molecules, the trends for conversion and selectivity could be explained by the changes in the base properties and oxidizability associated with variations in the Cu/P ratio as with that for FePO 4 . Furthermore, the HCHO yield at 550 °C was increased to 0.79% from 0.42% when the starting material was changed from Cu(OAc) 2 to Cu(NO 3 ) 2 . TEM measurements, IR spectral analysis with pyridine adsorption, and H 2 -TPR analysis revealed that the enhancement of surface crystallinity led to an increase in Lewis acidic Cu 2+ sites, which facilitated CH 4 activation and thereby enhanced oxidative ability. Therefore, the catalytic performance for the direct oxidation of CH 4 is highly sensitive to the surface nanostructures as well as the bulk structure. Kinetic studies, isotope experiments using CD 4 or 18 O 2 and the pulse reaction on Cu 2 P 2 O 7 showed that CH 4 was oxidized by surface lattice oxygen and that the C–H activation of CH 4 is most likely the rate-determining step in a similar manner to that with FePO 4 . DFT calculations revealed that the oxygen vacancy formation energy for Cu 2 P 2 O 7 decreases upon phase transition from α-Cu 2 P 2 O 7 to β-Cu 2 P 2 O 7 during the catalysis, which contributes to the high catalytic performance of Cu 2 P 2 O 7 .

The oxidation reactions discussed in this article can be classified into three main mechanism depending on the types of catalyst and reaction based on the experimental studies using spectroscopy, kinetics, and isotopes: (i) Mars–van Krevelen mechanism, 42,43,49,51,53,60–62 (ii) mechanism involving O 2 activation on surface redox-active Lewis acidic metals, 44,46,50,52,54 and (iii) radical-mediated oxidation mechanism induced by high-valent metal oxygen species. 45,47,48 In particular, the oxygen vacancy formation energy determined from DFT calculations is a good descriptor for both liquid- and gas-phase oxidations in which Mars–van Krevelen mechanism has been proposed. We have shown that structurally unique metal–oxygen sites, such as face-sharing octahedra in BaRuO 3 , 49 planar oxygen sites in β-MnO 2 , 61 and Cu–O–P sites in β-Cu 2 P 2 O 7 formed by the phase transition of α-Cu 2 P 2 O 7 under the catalytic conditions, 51 are likely possible active sites.

Acid–base catalysis

Based on the results of these investigations, Fig. 16(e) shows a proposed reaction mechanism for the CePO 4 -catalyzed acetalization of HMF with methanol. First, the activation of both HMF and methanol by adjacent acid–base pairs on CePO 4 facilitates the nucleophilic attack of the OH group in methanol on the carbon atom of the carbonyl group in HMF to give the corresponding hemiacetal derivative. Further reaction of the hemiacetal with methanol then occurs, most likely with the assistance of the catalyst, to give the corresponding acetal derivative. Aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) observations confirmed that the dominant facets exposed on the CePO 4 nanorods are (110) and (100), with (110) being particularly dominant. 54 The surface density (1.6 nm −2 ) of Lewis acidic Ce 3+ cations at the (110) facet was in good agreement with the experimental value (1.6 nm −2 ) calculated from S BET and pyridine-IR results, which supports the presence of uniform surface Ce species on the CePO 4 nanorods.

The acid–base catalysis of various perovskite oxides 165–167 has been reported since the 1990s, and is typically estimated by titration using Hammett indicators, spectroscopy measurements such as IR and electron spin resonance (ESR) using probe molecules, and simple test reactions ( e.g. , 2-propanol conversion, esterification, and transfer hydrogenation). 168,169 Metal cations generally act as Lewis acid sites and oxygen anions as base sites, and the catalytic performance is strongly dependent on the bulk and surface configurations. Foo and co-workers systematically investigated the acidity/basicity of Ti and Zr-based perovskite oxides such as SrTiO 3 , BaTiO 3 , SrZrO 3 , and BaZrO 3 with a focus on the conversion of 2-propanol to propylene and acetone mainly promoted by acid and base sites, respectively. 170 However, the catalytic application of perovskite oxides to liquid-phase organic reactions has been limited.

The acid–base properties of SrTiO 3_ N 2 -air were investigated using pyridine-, chloroform-, and acetophenone-adsorbed IR and CO 2 -TPD measurements, which showed the presence of both Lewis acid and base sites in the catalyst and that acetophenone was activated by Lewis acid Ti sites. Next, reactions were conducted by the addition of pyridine and acetic acid (AcOH) to poison the acidic and basic active sites on SrTiO 3_ N 2 -air, respectively ( Fig. 17(a) ). The reaction rate decreased with increasing amounts of both poisoning molecules, which suggested that both the acid and base sites on the catalyst surface contribute to the cyanosilylation, although the poisoning effect with AcOH was much stronger than that with pyridine. These results suggest that the reaction is mainly promoted by the base sites with possible cooperative action of the acid sites on the catalyst.

Fig. 17(b) shows a possible mechanism for cyanosilylation over the SrTiO 3 _N 2 -air catalyst. First, TMSCN is activated by surface oxygens that act as base sites. Similarly, carbonyl compounds are adsorbed and activated by Lewis acid sites on the catalyst surface. Such cooperative activation facilitates the nucleophilic attack of a CN − anion at the carbon atom of the carbonyl groups, followed by desorption of the corresponding products. In this case, the distance between the acid and base sites may also be important for cooperative acid–base catalysis, and the presence of adjacent acid–base sites was confirmed by CHCl 3 -adsorbed IR spectroscopy measurements.

To investigate the effect of the calcination atmosphere, the surface atomic ratio of Sr to Ti (Sr/Ti) was estimated from the Sr 3d and Ti 2p peaks in the XPS spectra of the different catalysts. The Sr/Ti value for SrTiO 3 _N 2 -air was 1.51 and higher than that for SrTiO 3 _air (1.37), which suggested that the enrichment of Sr–O termination at the surface of SrTiO 3 led to an increase in the amount of base sites. The effect of the outermost surface structure of SrTiO 3 (100) facets of SrTiO 3 for the activation of TMSCN, was investigated using DFT calculations ( Fig. 17(c) and (d) ). The adsorption energy of chemisorbed TMSCN on the Sr–O terminated (100) facet was calculated to be −2.01 eV and lower than that (−1.01 eV) on the Ti-terminated (100) facet, which suggests that TMSCN is strongly activated on the SrO-rich surface. These results are consistent with the basicity and catalytic activity differences between the SrTiO 3 samples treated in different atmospheres. Such a significant effect of the surface composition of perovskite oxides on the catalysis has been reported by Polo-Garzon and co-workers; thermal treatment of commercially available SrTiO 3 in oxygen improved the selectivity toward acetone in 2-propanol conversion compared with that for bare SrTiO 3 . 173 In contrast, SrTiO 3 treated with HNO 3 showed high selectivity toward propylene produced from 2-propanol by dehydration on acid sites. Various techniques, including calculation of the surface Sr/Ti ratio using XPS and low-energy ion scattering (LEIS) analysis, which is a more surface-sensitive measurement, direct observation of catalyst surfaces including HAADF-STEM analysis and DFT calculations revealed that the basicity of the Sr-rich surface was enriched by thermal treatment of SrTiO 3 and the Ti-rich surface with acidic characteristics likely appeared due to chemical etching of SrTiO 3 by HNO 3 . Other approaches based on the morphology/facets of perovskite oxides 170 and the engineering of oxygen vacancies on the catalyst surface 174–176 have been reported to be effective for controlling the surface composition.

Summary and outlook

The unique structures and valences of hexagonal perovskite oxides and β-MnO 2 contribute to promote the selective oxidation of alkanes, alcohols, and sulfides with O 2 as the sole oxidant under milder reaction conditions than previously reported catalysts. In the case of the Mars–van Krevelen mechanism where surface lattice oxygen is involved in the reaction, the oxygen vacancy formation energy becomes an important descriptor that enables the design of catalysts with desirable oxidation properties. The concerted action of redox and acid–base properties is also an effective method for the activation of C–H bonds using Murdochite-type oxides. In addition, the combination of Lewis acidic metal species and base units makes it possible to develop acid–base catalysts based on a variety of metal phosphates and perovskites. The use of weakly basic phosphate units is effective for the conversion of unstable biomass-based substrates and the direct selective oxidation of CH 4 through the inhibition effect of successive oxidation. The synergistic and cooperative activation of molecules on acid and base sites results in high catalytic performance when using more basic oxo units.

Further elemental complexity is expected to be an important factor in the development of high-performance catalysts in the future, as exemplified by the recent developments of multimetal-doped 29,38,40,179 and/or high-entropy metal oxides 180,181 as catalysts for selective oxidation, NO reduction, and electrochemical oxygen evolution reaction and energy storage materials. Control of electronic structures ( e.g. , spin-state, magnetic ordering) 179,182 and design of catalytically active interfaces based on a synergy effect of binary oxide composites 183,184 would be a promising approach to achieve highly active and durable catalysts. There is also an urgent need to design and develop catalysts in a shorter time through limited experiments. Therefore, in addition to the present approaches based on precise experiments and first-principle calculations, these fields will be developed by the incorporation of new methods such as high-throughput synthesis and machine learning. Such a continuous approach to catalyst design is likely to revolutionize the entire process of catalyst development to achieve potentially difficult reactions required to build a sustainable society.

Author contributions

Data availability, conflicts of interest, acknowledgements.

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Ordered versus non-ordered mesoporous ceo 2 -based systems for the direct synthesis of dimethyl carbonate from co 2.

Graphical Abstract

1. Introduction

2. materials and methods, 3. results and discussion, 4. conclusions, supplementary materials, author contributions, data availability statement, acknowledgments, conflicts of interest.

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Rusta, N.; Secci, F.; Mameli, V.; Cannas, C. Ordered versus Non-Ordered Mesoporous CeO 2 -Based Systems for the Direct Synthesis of Dimethyl Carbonate from CO 2 . Nanomaterials 2024 , 14 , 1490. https://doi.org/10.3390/nano14181490

Rusta N, Secci F, Mameli V, Cannas C. Ordered versus Non-Ordered Mesoporous CeO 2 -Based Systems for the Direct Synthesis of Dimethyl Carbonate from CO 2 . Nanomaterials . 2024; 14(18):1490. https://doi.org/10.3390/nano14181490

Rusta, Nicoletta, Fausto Secci, Valentina Mameli, and Carla Cannas. 2024. "Ordered versus Non-Ordered Mesoporous CeO 2 -Based Systems for the Direct Synthesis of Dimethyl Carbonate from CO 2 " Nanomaterials 14, no. 18: 1490. https://doi.org/10.3390/nano14181490

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Gold Nanoparticles: Synthesis Methods, Functionalization and Biological Applications

• Review Paper
• Published: 04 June 2022
• Volume 34 , pages 705–725, ( 2023 )

Cite this article

• Tejaswini Patil 1 ,
• Rutuja Gambhir 1 ,
• Anuja Vibhute 1 &
• Arpita Pandey Tiwari   ORCID: orcid.org/0000-0002-2692-0011 1  

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Nanotechnology has vast applications in medicine and biomedical engineering like tissue engineering, diagnosis, and therapy. Nowadays incorporation of functionalized nanostructures in various biomedical applications has generated considerable research interest. Gold nanoparticles (AuNPs) are one of the most stable metal nanoparticles with unique physicochemical properties and are reflected as a promising candidate for widespread biological applications. Among different synthesis methods, biological synthesis methods are advantageous as it reduces the need for toxic chemicals for reduction purpose. Surface functionalization provides colloidal stability to gold nanoparticles which are achieved by using various materials. This review mainly focuses on the biological applications of AuNPs such as bioimaging, biosensing, anticancer therapy, drug delivery, hyperthermia, and antimicrobial activity. The surface plasmon resonance (SPR) related optical properties are used for biosensing and bioimaging applications for diagnosis to detect pathogens as well as biomarkers. Biomolecules and drug functionalized AuNPs are effectively used to treat various cancer and other diseases. Thus, the study of gold nanoparticles opens a new percept in the biological field for varieties of applications.

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The work has been supported by University funded project (No. DYPES/DU/R&D/2021/274).

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Tejaswini Patil, Rutuja Gambhir, Anuja Vibhute & Arpita Pandey Tiwari

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Patil, T., Gambhir, R., Vibhute, A. et al. Gold Nanoparticles: Synthesis Methods, Functionalization and Biological Applications. J Clust Sci 34 , 705–725 (2023). https://doi.org/10.1007/s10876-022-02287-6

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DOI : https://doi.org/10.1007/s10876-022-02287-6

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