Green Synthesis and Applications of Silver Nanoparticles: A Systematic Review

Abstract

Nanotechnology has become the foremost promising and rising field of analysis as a result of its applications in numerous fields. Development of consistent and greener ways for the synthesis of nanoparticles could be a dynamic step in the field of nanotechnology. To avoid the emergence of dangerous by-products, many attempts have been made in recent years to develop environment friendly methods. “Green” synthesis is a consistent, sustainable, and environment friendly method for the synthesis of an enormous range of nanoparticles. Green synthesis is seen as an important tool to reduce the harmful effects of traditional nanoparticle synthesis methods commonly used in laboratories and industries. Nanoparticles exhibit unique chemical and physical properties that are useful in various fields. Among metallic nanoparticles, silver nanoparticles have become a research hotspot due to their wide range of applications. Silver nanoparticles are important because of their exceptional chemical, physical, and biological properties. Because of these unique characteristics, silver nanoparticles have numerous applications and are used as antifungal, antiviral, and antibacterial agents. They have an excellent catalytic effect on dye degradation, are very good antioxidants, and can be used to treat various diseases and exhibit wound-healing activities. The current review complies with the database of green synthesis of silver nanoparticles using plant extracts, bacteria, and fungi, which have potential applications in fields of science, health, textiles, food packaging, agriculture, and environment. The review also highlights the application of silver nanoparticles as antimicrobial, antibacterial, antiviral, and antifungal agents. The knowledge on silver nanoparticle production conditions, properties, molecular mechanisms, and applications will be of great help for researchers to explore more applications of nanoparticles in fields that are still untouched.

Introduction

Nanotechnology is highly significant in the prevailing century as a result of its practicality for amending metals into their nano size, thereby significantly changing their physical, chemical, and optical properties. Nanotechnology refers to the technology of rearranging and processing atoms and molecules to create materials with nanospecifications such as a nanometer size. Nanotechnology will enable scientists and engineers to determine and manipulate matter at the molecular level atom by atom, to create new structures with fundamentally new molecular organization and to use new properties on this scale. Matter at the nanoscale is different from in its bulk form; its chemical, biological, magnetic, electrical, and other properties are different from the properties of macro matter, resulting in faster, better, and cheaper products. There have been large investments to promote research in nanotechnology in most countries to reveal the mysteries of nanomaterials and their applications, and to make them more useful for human comfort. ¹

Nanoparticles are the basic components of nanotechnology, and are usually 1 to 100 nm in size. In this century, the use of nanoparticles has become more and more important because they determine the optical, mechanical, and chemical properties. Metallic nanoparticles have potential uses in biomedicine, catalysis, photonics, optics, and antimicrobial activity.^2,3 Among numerous metals, silver nanoparticles (AgNPs) have significant antibacterial and local surface plasmon resonance (SPR) properties, which makes them unique, with applications such as broad-spectrum antibacterial agents,^4–6 improved surface enhanced Raman spectroscopy (SERS),^7,8 biomedical and chemical/biosensors materials,^9–11 biomarkers,^12–14 and so on. Therefore, scientists have used AgNPs in different fields such as catalysis, ¹⁵ food packaging, ¹⁶ sensors, ¹⁷ biomedical, ¹⁸ cosmetic industry, ¹⁹ textile industry, ²⁰ biological, coatings, ²¹ optoelectronics, ²² antimicrobial activities, ²³ plasmonics (SERS), ²⁴ DNA sequencing, ²⁵ SERS, ²⁶ weather change and contamination control, ²⁷ information storage, fresh water technology, ²⁸ and energy generation. AgNPs are utilized as tranquilizer conveyance agents and anti-infection agents. ²⁹ Furthermore, because of their high conductance, silver nanoparticles also have many applications in adhesives, pastes, inks, electronic devices, and so on. ³⁰

Green Synthesis of AgNPs

The synthesis of metal nanoparticles includes top-down and bottom-up approaches using physical, chemical, and biological methods. In the manufacture of nanoparticles, a piece of material is mechanically ground in a top-down manner. The bottom-up approach involves the controlled condensation of dissolved molecules that form during a chemical reaction. Restricting condensation or growth results in the formation of particles of the desired size and shape. ³¹ The green synthesis of silver nanoparticles belongs to a bottom-up approach (Figure S1 (in supporting material)). ³²

In the green synthesis of nanoparticles, plants or parts thereof, bacteria and fungi are used to biologically reduce metal ions in their elemental form. Using bacterial proteins or plant extracts as reducing agents, one can control the shape, size, and monodispersity of the nanoparticles. The availability of a wide range of biological resources is a benefit of green synthesis. Green synthesis is regarded as safer alternative to the usual physical and chemical methods. Synthesis of AgNPs using chemical methods includes contamination from precursor chemicals, the use of toxic solvents, and the generation of hazardous by-products. AgNPs synthesized using chemical methods are not of the expected purity because of the adsorption of chemically toxic substances, hindering their uses in medical applications. Compared to chemical methods, green synthesis makes it easier to regulate the shape, size, and distribution of produced nanoparticles by optimizing the synthesis procedures including the amount of precursors, temperature, pH, and the amount of reducing and stabilizing agents. ³³

The green method is more efficient, simpler, more economical, and can be easily expanded to larger companies. Compared with physical and chemical methods, this method does not require high pressure, energy, temperature, or hazardous chemicals. ³⁴ The main requirements for the green synthesis of AgNPs are silver nitrate solution and biological reducing agent. ³⁵ In most cases, reducing agents or other components present in cells act as stabilizers and capping agents, so no additional external capping agents and stabilizers are needed. ³⁶ Plants and their extracts, bacteria and fungi can undergo green synthesis.

AgNPs Synthesis from Plants

Plant-mediated AgNPs synthesis has advantages over the method using microorganisms because it is less harmful to organisms, can be easily improved, and does not require maintenance of cell culture.^37–41 Certain parts of plants such as bark, roots, stems, fruits, seeds, callus, pericarp, leaves, and flowers are used to synthesize silver nanoparticles. Due to the presence of phytochemicals, plant-assisted reduction is a key mechanism for plant-mediated AgNPs synthesis. The phytochemicals such as amides, ketones, flavonoids, terpenoids, aldehydes, carboxylic acids,^42,43 organic acids, flavonoids, and quinones, are responsible for the immediate reduction of silver ions in the reaction mixture. ⁴⁴

It is well known that the type of plant extract, pH, temperature, the concentration of metal salt, and contact time will affect the formation rate, quantity, and other characteristics of nanoparticles. ⁴⁵ One of the most important factors for nanoparticle formation is pH, which affects the shape and size of the nano particles. The formation of AgNPs at neutral and alkaline pH is instantaneous and an intense SPR band is observed due to the ionization of phenolic groups in the plant extract. At higher temperatures, the rate of reaction increases but is accompanied by peak broadening and a decrease in SPR intensity. As the concentration of silver ions increases, the size of nanoparticles increases due to secondary reduction of silver ions adsorbing onto the surface of constructed nuclei, resulting in larger nanoparticles (Figure S2, Table S1) (in supporting material). ⁴⁶ Nanoparticle size increases with increasing concentration of plant extract because silver ions are rapidly reduced, facilitating the subsequent growth of nanoparticles as described by the “Ostwald ripening” phenomenon. ⁴⁷

AgNPs Synthesis from Bacteria

AgNPs have been synthesized using extracellular or intracellular inorganic materials produced by bacteria. ⁹⁷ Biomolecules, including proteins, enzymes and biosurfactants found in microorganisms, act as reducing agents. As the name indicates, extracellular synthesis of nanoparticles takes place outside the bacterial cell. The extracellular synthesis of AgNPs captures metal ions on the outer surface of the cell and reduces them in the presence of enzymes or biomolecules, while the intracellular synthesis occurs from microbial cells. ⁹⁸ The synthesis of extracellular nanoparticles is considered cheap, conducive to large-scale production, and requires easier post-treatment (Figure S3, Table S2) (in supporting material). The extracellular method used to synthesize nanoparticles is superior to the intracellular method due to the easy recovery of nanoparticles from solution. ⁹⁹ The first evidence for the presence of bacteria that synthesize silver nanoparticles was obtained from the biomass of Pseudomonas stutzeri AG259 bacteria. ¹⁰⁰

Green Synthesis of AgNPs from Fungus

Due to its large-scale economic production, the use of fungi to synthesize AgNPs outside the cell is also a viable alternative method. ¹²⁸ Fungal strains were chosen over bacterial strains because of their greater resistance and ability to bioaccumulate metals (Figure S4, Table S3 (in supporting material)).

Various Uses of AgNPs

AgNPs have been used widely as antimicrobial, antibacterial, antiviral, and antifungal agents in the health industry, textile coatings, and food packaging, and have various ecological applications (Figure S5 (in supporting material)).

Antimicrobial Activity

Although the antibacterial effect of AgNPs is well known, the effect of AgNPs on microorganisms and antibacterial mechanisms has not been clearly elucidated. The mechanisms so far discovered are (also see Figure S6 (in supporting material)):

1. The electrostatic force of attraction between the negatively charged microbial cell membrane and the positively charged AgNPs.^158–160

2. Irregular pit formation on the outer membrane and changes in membrane permeability along with the possibility of degradation of lipid polysaccharide molecules and membrane proteins. ¹⁶¹

3. Free radical formation and subsequent free radical-induced membrane damage. ¹⁶²

4. AgNPs induce genotoxicity (DNA damage).

The stem- and leaf-synthesized AgNPs of P. nigrum exhibits excellent antibacterial activity against the agricultural phytopathogens Citrobacter freundii and Erwinia cacticida. The AgNP-binding antibiotic solution synthesized in the leaves represents 50 μL of 15.853 + 0.153 and 16.295 + 0.203 excellent inhibitory regions in Citrobacter freundii and Erwinia cacticida, respectively. Similarly, stem synthetic AgNPs-binding antibiotic solutions also show maximum inhibitory regions of 12.833 + 0.441 and 14.533 + 0.291 for Citrobacter freundii and Erwinia cacticida, respectively. ¹⁶³

The regions of maximum inhibition were recognized in aqueous extracts of Pithophora oedogonia support AgNPs against Pseudomonas aeruginosa (17.2 mm; MTCC 2581) and Escherichia coli (16.8 mm; MTCC 443). ¹⁶⁴ Using the disk diffusion method, the antibacterial activity of Datura extract was studied to reduce AgNPs in Staphylococcus aureus, Escherichia coli, and S. typhi. Sterile disks were immersed in AgNPs solution (20 μg/mL), placed on nutrient media plates, and incubated at 37°C for 24 h. ¹⁶⁵

The antimicrobial activity of AgNPs synthesized by C. Sinensis, S. cumini, C. asiatica, and S. tricobatum was studied against P. aeruginosa, S. aureus, K. pneumoniae, and E. coli, using the well diffusion method. The highest antimicrobial activity of AgNPs synthesized by C. asiatica and C. sinensis commercial plant powders was found for P. aeruginosa (16 mm). The lower antimicrobial activity of AgNPs synthesized by both C. asiatica and C. sinensis was found against E. coli (8 mm) and S. aureus (8 mm) and S. tricobatum against K. pneumoniae (8 mm). S. cumini and C. asiatica did not exhibit an inhibition zone for K. pneumoniae. ¹⁶⁶

As a result of evaluating the antimicrobial activity of AgNPs through an A. scholaris bark extract with a size of 50 nm in fungi, Gram-negative and Gram-positive bacteria (in vitro) using the disk diffusion method, it was found that the concentration of AgNPs was high (170 ppm) and showed significant antibacterial effect. The higher antimicrobial activity of AgNPs is due to their large surface area to volume ratio and surface activity.^167–169 In addition to the fact that the expression of ribosomal subunit proteins and several other cellular proteins and enzymes essential for adenosine triphosphate (ATP) production is inactivated, silver ion treatment results in the loss of the microbial DNA’s ability to replicate. ¹⁷⁰ In addition, AgNPs are more abrasive in nature than large amounts of AgNO₃, and therefore, contribute to more mechanical damage to cell membranes, which subsequently induces a stronger fungal effect. ¹⁷¹

There are two types of antibacterial activity exhibited by AgNPs: inhibitory action and biocidal action. A previous strategy merely stopped bacterial cell division, whereas a later strategy caused bacterial cells to die from AgNPs’ biocidal activity. The antibacterial activity of AgNPs obtained by different bacteria varies greatly. The reason for this may be due to the fact that AgNPs synthesized using different bacteria vary greatly in many aspects such as shape, size etc. AgNPs have different antibacterial properties against different strains of bacteria. Gram-negative bacteria are more susceptible to AgNPs than Gram-positive bacteria. There may be a difference in the thickness of the cell walls of the two kinds of bacteria. ¹⁷²

Size, shape, concentration, time, and charge may also influence AgNP antibacterial activity. In general, AgNPs have a pronounced antibacterial effect as particle size decreases. ¹⁷³ AgNPs show good antibacterial properties, especially when they are smaller than 10 nm. When AgNPs are treated for longer periods of time, their antibacterial effect can be significantly enhanced. ¹⁷⁴ The increase in bacterial mortality is thought to be caused by the accumulation of silver ions and AgNPs during the exposure period. In addition, the shape of AgNPs may have an effect on their antibacterial activity. When spherical, triangular, linear, and cubic AgNPs are compared for antibacterial activity, it is discovered that spherical shaped AgNPs have a greater antibacterial activity. This result showed that spherical shaped AgNPs have large surface-to-volume ratio, which corresponds to a high effective contact and a large reaction surface. ¹⁷⁵ Furthermore, the surface charge of AgNPs influences their antibacterial action. Bacterial membranes are largely loaded with negative charges due to the presence of lipopolysaccharide, peptidoglycan, and various groups, including carboxyl, amino, and phosphate groups.^176,177 Electrostatic attraction aids AgNPs adhesion to bacterial membranes when positive charges are present. As a result, adjusting the charge on the surface of AgNPs might help them have a better antibacterial activity. ¹⁷⁸

Antiviral Activity

AgNPs show antiviral activity against HIV-1, herpes simplex virus type 1 (HSV-1), herpes simplex virus (HSV-2), and respiratory syncytial virus (RSV). Cell adhesion and invasion of herpes simplex virus type 1 (HSV-1) are due to contact of heparan sulfate (HS) on the cell surface with the glycoprotein covering the virus. According to this mechanism, the capped AgNP virus in mercaptoethane sulfonate (Ag-MES) is targeted and through its sulfonate terminus binds to cellular HS, and blocks invasion into competing cells and prevents subsequent infection. ¹⁷⁹ AgNPs cause size-dependent interactions with HIV-1, so that only nanoparticles in the range of 1–10 nm attach to the virus. AgNPs inhibit HIV-1 viral infection in vitro by preferential binding to the gp120 subunit of the viral envelope glycoprotein. ¹⁸⁰ The binding of AgNPs to gp120 leads to the prevention of binding, infectivity and fusion of the CD4-dependent virion. Therefore, they play an effective validation role against cell-free as well as associated virus. Thus, they act as effective verifiers. They also inhibit the post-invasive phase of the HIV-1 life cycle. ¹⁸¹ In young children, the RSV is responsible for bronchiolitis and pneumonia. PVP-coated silver nanoparticles, which showed low cytotoxicity at low concentrations, inhibited RSV infection by 44%. ¹⁸²

Antibacterial Activity

AgNPs synthesized using Piper’s betel nut extract exhibited very strong antibacterial activity against Bacillus cereus (17 mm), Staphylococcus aureus (16 mm), Klebsiella pneumoniae (15 mm), and Escherichia coli (15 mm). ¹⁸³ AgNPs synthesized from extracts of Helicteres isora root showed excellent antibacterial activity against both Gram-negative and Gram-positive bacteria. However, they exhibited higher antimicrobial activity against S. typhi and P. aeruginosa (Gram-negative) than B. subtilis and M. luteus (Gram-positive). This is due to the difference in cell wall structure between Gram-positive and Gram-negative bacteria. ¹⁸⁴ The cell wall of Gram-positive bacteria is composed of a negatively charged peptidoglycan layer with a thickness of about 30 nm, whereas Gram-negative bacteria have a peptidoglycan layer of 3–4 nm.^185,186 The proliferation of Staphylococcus aureus and E. coli was highly inhibited by AgNPs synthesized using Zingiber officinale, suggesting that it had significant antibacterial activity compared to AgNPs standard antibacterial agents. ¹⁸⁷ Face-centered cubic silver nanoparticles with a size of 16 nm were synthesized using F. Bengal Bodhi tree leaf extract. These AgNPs exhibited efficient bactericidal properties in E. coli MTCC 1302 compared to silver salts. They have a very large surface area, so they have good contact with microorganisms. ¹⁸⁸ Circular AgNPs (1–4 mm) synthesized from coconut oil cake extract decreased the growth rate of various antibiotic-resistant bacteria such as Aeromonas sp., Acinetobacter sp., and Citrobacter sp. separated from livestock wastewater. ¹⁸⁹

Antifungal Activity

AgNPs exhibit a strong antifungal effect against fungi by disrupting the integrity of the plasma membrane. These important changes in the membrane are acknowledged by the formation of “pits” on its surface and eventually provide for the formation of pores and cell death. ¹⁹⁰ Fluconazole in combination with AgNPs showed the maximum suppression of C. albicans, followed by P. glomerata and Trichoderma sp. ¹⁹¹ Tineapedis (a common skin infection) and nail mycosis (a nail infection) are very widespread in adults, induced by the skin disease mold Trichophyton. ¹⁹² AgNPs showed strong activity against clinical isolates and ATCC strains of Trichophyton mentagrophytes and Candida species (IC80, 1–7 μg/mL). ¹⁹³ AgNPs exhibit bactericidal and antifungal activities against Candida albicans, ¹⁹⁴ Candida krusei, Candida species, Candida glabrata, Trichophyton mentagrophytes, and Candida tropicalis.

Electric Components

AgNPs are used to create sensors, electrodes, and integrated circuits.^195–197 Due to their electrochemical properties, AgNPs can be integrated into nanoscale sensors to provide faster response times and reduce detection limits. AgNPs are used for fluorescence SPR. SPR occurs when AgNPs are exposed to visible light, which induces free electron oscillations in the conduction band of nanoparticles. The width and position of the SPR peak depend primarily on the size and shape of the particles.^198,199 These properties allow AgNPs to be used in sensing detection such as peptide laser desorption/ionization mass spectrometry for detection of DNA sequences,^200,201 histidine colorimetric sensors, ²⁰² and measurement of human plasma fibrinogen. ²⁰³ These properties allow AgNPs to be used in detection applications such as DNA sequence detection, laser desorption/ionization mass spectrometry of histidine peptide colorimeter sensors, and Somuyusowon measurement of human plasma.

Food Packaging

Due to the antimicrobial activity of AgNPs, they are used in food packaging to prevent microbial contaminations^204–206 and prolong the lifespan. AgNPs can be integrated into non-degradable (polyvinyl chloride, polyethylene, vinyl alcohol) and biodegradable polymers (starch, cellulose, agarose, chitosan) to produce food packaging materials. ²⁰⁷ They offer superior physicochemical properties, reduced hydrophilicity and better biodegradability to these food packages. ²⁰⁸

Environmental Applications

The reverse osmosis membrane integrated into AgNPs exhibited excellent antifungal and antibacterial activity against pathogenic bacterial strains such as S. aureus, E. coli, K. pneumoniae, M. luteus, and P. aeruginosa, and fungal strains such as Candida tropicalis, C. glabrata, C. krusei, and C. albicans. This production reduces the need for pretreatment or cleaning with harmful chemicals, which are widely used in desalination plants, industrial wastewater treatment, agricultural farms and drinking water production applications. ²⁰⁹ AgNPs can be used as coating materials in PVC pipes for drinking water to control the microbial contamination of the water. Commercial garment (socks) silver wastewater treatment in water and its operation in wastewater treatment plants (WWTPs) using microscopy, physical separation, and selective electrodes ions (ISE) are analyzed. ²¹⁰

Cost-effective filter materials (zeolites, sand, fiberglass, anionic, and cationic resin substrates) are coated with AgNPs to remove pathogenic bacteria and disinfect groundwater. ²¹¹ The antibacterial effect of silver particles against bacterial contamination by coatings of activated carbon fibers (ACF) is being studied. ACF is used for air purification to remove toxic gas pollutants. However, because the ACF filter has a long surface area and high adsorption power, bacteria can grow on the ACF filter, so it may be a source of bioaerosols by itself. However, silver-film-deposited ACF filters are effective in removing bioaerosols by suppressing the survival of microorganisms. ²¹²

Medical Applications

AgNPs are used in drug delivery to reduce the dosage of drugs, improve specificity, and reduce toxicity.^213–215 They are used in imaging and wound healing by reducing the activity of these local matrix metalloproteinases (MMPs) and increasing intratraumatic neutrophil apoptosis.^216,217 AgNPs can suppress the activity of interferon gamma and tumor necrosis factor alpha, which are involved in inflammation. ²¹⁸ AgNPs is used in bone cement, which is used as an artificial joint replacement. Polymethylmethacrylate containing nanosilver is considered bone cement because nanosilver can have antibacterial activity. ²¹⁹ Nanosilver has the ability to be used for biosensing. ²²⁰ The nanosilver’s plasmonic properties make it an excellent candidate for bioimaging. ²²¹

AgNPs can be used as highly sensitive nanobacteria for targeting and imaging small molecules, DNA, proteins, tissues, and tumors.^222,223 They are used in cancer imaging and photothermal therapy to discover the location of cancer cells by absorbing light and destroying them with the photothermal effect. ²²⁴

Agriculture

AgNPs are used in the improvement of plant growth and the management of plant diseases. Nanosilver improves nutrient uptake in soil and improves seed germination in a variety of plants, thus acting as a nanofertilizer.^225,226 Nanosilver acts as an effective, non-toxic, safe pest control agent and is an innovative tool for pest control. To improve crop production, AgNPs can be given along with pesticides. ²²⁷

Textiles

AgNPs can greatly improve the coloration and durability of fabrics and are also used for antibacterial treatment.

Antibacterial Activity

All types of fabrics, such as synthetic or natural fibers, are not resistant to bacteria and pathogens. The growth of microorganisms in textiles during use and storage adversely affects not only the textile itself, but also the wearer. Using antibacterial silver nanoparticles, harmful effects can be reduced. Since silver nanoparticles can use a relatively large surface area, they are suitable for effective control of bacteria and fungi. The antibacterial treatment of textile fabrics is easily achieved by padding with nano-sized colloidal solutions of silver, and the antibacterial activity of the fabrics is maintained even after repeated silver washing. Silver nanoparticles synthesized using mold on fabrics show excellent antibacterial activity against Staphylococcus aureus. ²²⁸ The antibacterial activity is due to the silver ions released from the silver nanoparticles. These ions can break and/or cross cell membranes and bind to the –SH group of cellular enzymes. ²²⁹ This alters the metabolism of microorganisms, suppresses growth, and induces a severe decrease in enzyme activity that can lead to cell death. ²³⁰ Silver nanoparticles can also catalyze the production of oxygen radicals and lead to oxidation of the molecular structure of microorganisms. Radicals formed when silver ions bind to cell walls or enzyme proteins also have the potential to inhibit many processes in living cells. ²³¹ Silver nanoparticles showed excellent antibacterial activity against Escherichia coli and Staphylococcus aureus when attached to the surface of nylon, polyester, and cotton by ultrasonic irradiation. ²³²

Coloration and Fastness Properties

Silver nanoparticles have received particular attention as an emerging textile dye. Silver nanoparticles exhibit vibrant colors due to their localized SPR (LSPR) properties. The properties of LSPR are demonstrated by the shape and size of the nanoparticles. ²³³ There are three methodologies available for staining fabrics using silver nanoparticles:

1. Fabric impregnated with a solution of colloidal nanoparticles;

2. Formation of fabric/particle nanocomposites during spinning;

3. In situ synthesis of nanoparticles on fabric. ²³⁴

AgNPs enhance the applications of synthetic fabrics, especially polyester fabrics in different areas.^235,236 Optimizing the processing conditions is essential for obtaining the most intense colors in fabrics as well as making them durable and washable. The pH, temperature, and silver ion concentration have an effect on the color and color fastness of fabrics. ²³⁷ AgNPs are different from traditional dyes in that they are not the chromophores of traditional dyes but rather the shape and size of the nanoparticles determine the color of the fabric. Synthetic and natural dyes have poor resistance to washing and ultraviolet (UV) light, causing discoloration. ²³⁸ Compared to traditional synthetic dyes, the color produced using AgNPs is stable to UV light and unchanged provided there is no change in particle size by growth or reduction of discrete nanoparticles.^239,240 The SPR of AgNPs is used to greatly improve the coloration and fastness properties of the fabric. For example, silver nanoparticles synthesized using an aqueous extract of lemon leaves (Citrus limon) were used for a durable textile finish on cotton and silk fabrics. Lemon leaves act as a reducing agent and encapsulating cage. ²⁴¹

Other Applications

AgNPs also develop AgNP’s business unit, used in a variety of consumer products such as water filters and sterilization systems, deodorants, soaps, socks, food storage, paints, sunscreen cosmetics, and indoor sprays.^242–245 AgNPs’ large surface area provides high surface energy and promotes surface reactions such as adsorption and catalysis. AgNPs and their composites have high catalytic activity by improving the reduction kinetics of NaBH₄ in dye reduction,^246–248 the oxidation of CO, ²⁴⁹ benzene oxidation of phenol, ²⁴⁹ the photolysis of acetaldehydes, ²⁵⁰ and the reduction of p-nitrophenol to p-aminophenol.^251,252

Conclusion

The above review covers green synthesis and applications of silver nanoparticles. The green synthesis of silver nanoparticles is cheap, easily extended, and environmentally friendly. The green approach takes precedence over traditional methodologies for the synthesis of AgNPs. Existing methods of synthesizing AgNPs require enormous amounts of energy and toxic chemicals, which can lead to the formation of harmful products. Phytochemicals extracted from plants, bacteria, and molds under ambient conditions can lead to sustainable synthesis of uniformly sized silver nanoparticles. Despite the advantages of biological green synthesis, controlling the polydispersity of nanoparticles is a key challenge. To address this problem, the reaction conditions can be improved by adjusting the pH, temperature, incubation period, irradiation, and salt concentration. In the case of plants, pH variations can also affect the size of nanoparticles, since they result in changes in the charge of the phytochemicals, which in turn affects the reduction and binding of silver during the synthesis process. In the case of microorganisms, the enzyme responsible for nanoparticles synthesis is more active at high temperatures, so it is recommended to grow the microorganisms at the highest temperature possible. Green synthesis of nanoparticles is time consuming but provides better control over size, shape, and crystallinity. The synthesis of AgNPs using plant extract is an ecofriendly and biocompatible process, but it may require additional stabilizing agents to prevent the agglomeration of nanoparticles. During green synthesis of AgNPs, many critical aspects need to be considered; for instance, the nature of microorganisms, suitable conditions for cell growth and enzyme activity, optimum reaction conditions such as pH and temperature, and the concentration of silver ion to obtain the desired stable nanoparticles.

AgNPs have a wide range of potential applications in medicine, environment, cosmetics, textiles, food packaging, sensors, and so on, but their most preferred function is anti-inflammatory and antibacterial activity. This is being applied in various ways in the medical field. Although the uses of AgNPs are expected to expand in the future, their accumulation in the environment and long-term effects on humans and animals are still to be understood.

Supplemental Material

sj-pdf-1-aat-10.1177_24723444221119847 – Supplemental material for Green Synthesis and Applications of Silver Nanoparticles: A Systematic Review

Supplemental material, sj-pdf-1-aat-10.1177_24723444221119847 for Green Synthesis and Applications of Silver Nanoparticles: A Systematic Review by Monika Moond, Sushila Singh, Seema Sangwan, Ritu Devi and Rajita Beniwal in AATCC Journal of Research

Supplemental Material

sj-pptx-2-aat-10.1177_24723444221119847 – Supplemental material for Green Synthesis and Applications of Silver Nanoparticles: A Systematic Review

Supplemental material, sj-pptx-2-aat-10.1177_24723444221119847 for Green Synthesis and Applications of Silver Nanoparticles: A Systematic Review by Monika Moond, Sushila Singh, Seema Sangwan, Ritu Devi and Rajita Beniwal in AATCC Journal of Research