Biological Materials Assisted Synthesis of Silver Nanoparticles and Potential Applications: A Review

 

Sanjay Basumatary^*, Nabajeet Changmai

Department of Chemistry, Bodoland University, Kokrajhar-783370, Assam, India

 

ABSTRACT:

Nanoparticle is termed as particle with a dimension in between 1–100 nm. Nanotechnology has potential prospects for research and development in modern science. Nanoparticles have wide applications in the field of electronics, biology, medicine, agriculture, cosmetics and many more. A bulk material has constant physical property but shows diverse physical, chemical and biological properties at the nano range. Silver nanoparticles (AgNPs) have gained a lot of interest in recent times due to their unique properties like electrical, optical, catalytic, antimicrobial and other biological properties. AgNPs can be produced by physical, chemical and biological processes. Although chemical synthesis produces large amount of AgNPs in a short period of time, the chemicals used are found to have many adverse effects on human health and the environment. Due to these adverse effects, the need for environment friendly and less harmful synthesis has become very essential. Hence, many researchers have developed several biological processes for the production of AgNPs. Biosynthesis of AgNPs includes synthesis using plants, fungi, algae, bacteria, yeast and other biological materials like fish scales, egg shells and crabs. Plant-based AgNPs synthesis is getting more and more attention among the researchers because this is nontoxic, cost-effective, relatively simple, and environmentally benign process. In this paper, an attempt has been made to review AgNPs synthesis using extracts of various biological materials along with their size, shape and applications such as antimicrobial, antifungal, antiviral, catalytic and anticancer activities.

 

 

 

1.                  INTRODUCTION:

With the advancement of time, miniaturization of materials became one of the main objectives of human society, and this led to the introduction of nanotechnology. Nanotechnology became the new scope for the development of modern science. The first nano-metal viz. silver and gold nanoparticle used by human back in the fifth century in Rome was found in Lycurgus chalice [1]. Nanotechnology has been proven to be a strong area in the prospect of research. Presently, it has been found to have wide applications in the field of electronics, biology, medicine, agriculture, cosmetics and many more [2–6].

 

The term ‘nano’ is adapted from the Greek word meaning ‘dwarf’ and a nanometer (nm) is one billionth of a meter. Hence the term nanoparticle can be termed as a particle with dimension less than 100 nm. Metal nanoparticles can be synthesized by physical and chemical processes. Two types of processes viz. top down and bottom up are associated with the synthesis. A bulk material has constant physical properties, but the property varies with their sizes at nano range i.e. 1–100 nm [7]. Due to the large surface to volume ratio and quantum confinement effect, nanoparticles exhibit unique and dramatically different physical, chemical and biological properties relative to bulk material [8]. Metal nanoparticles have been widely used in catalysis, biosensing, chemosensors, nanoparticle optics and surface enhanced Raman spectroscopy due to their unique electrical, optical and catalytic properties [9–13].

 

 

2. SILVER NANOPARTICLES:

In recent times, silver nanoparticles (AgNPs) have gained a lot of interest among all the metal nanoparticles. Development and uses of AgNPs have rapidly increased because of their unique properties like size and shape dependence, magnetic, electrical, optical, catalytic as well as antimicrobial properties. AgNPs can be prepared by chemical, physical and biological processes. Preparation of AgNPs using chemical reducing agents is one of the most effective and popular way. Various chemical reducing agents like sodium borohydride, ethylene glycol, hydrazine hydrate solution etc. are used for the synthesis [14–17]. However, besides these reducing agents; capping agents are also required for maintaining the size of nanoparticles in chemical synthesis. This chemical synthesis is effective in the production of AgNPs and the produced nanoparticles are found to have many medicinal applications like antibacterial activity [17]. Although chemical synthesis produces large amount of nanoparticles in a short period of time, the chemical reagents used are found to have many adverse effects on human health and the environment. Due to the toxicity of these reducing agents, the need for the environmental friendly synthesis became very essential. Hence, researchers have developed many biological processes for the production of AgNPs. Biosynthesis of AgNPs includes synthesis by using plants, fungi and algae [18]. Synthesis of AgNPs using fish scales and egg shells have also been reported [19, 20]. Plants provide a better platform for nanoparticle synthesis as they are free from toxic chemicals as well as provide natural capping agents and are cost effective. Researchers have synthesized silver nanoparticles using various parts of the plants viz. leaf, bark, roots, stems, and fruits [21]. Synthesis using plant extracts is the most adopted method of eco-friendly production of nanoparticles and also has special advantages that the plants are widely distributed, easily available, much safer to handle and act as a source of several metabolites. As few plants have medicinal qualities of their own, the medicinal quantity of the produced silver nanoparticles is enhanced.

 

Since ancient times, silver has been used extensively for many applications such as jewellery, metalcrafts, vessels, foils etc. Silver has also been a part of many medical applications throughout known history [22]. In the recent times, AgNPs are a vibrant part of nanotechnology because of growing demands in various fields like chemical and biological sensing due to surface-enhanced Raman scattering properties, electronics and optoelectronics applications owing to the highest electrical and thermal conductivity among all other metals, energy harvesting, catalysis, imaging, and biomedicine [23–30]. Hence, they serve as a tool in the development of new-generation electronic, optical and sensor devices. In recent years, the trend towards miniaturization and the necessity of modernization of technological processes led to the substantial increase in the number of researches devoted to the synthesis and properties of silver.

 

3. PROPERTIES OF AgNPs:

3.1 ANTIBACTERIAL PROPERTY OF AgNPs:

Antibacterial effect against anaerobic and aerobic bacteria of AgNPs is widely studied by different scientists and researchers. Small concentration of AgNPs is harmless for human cell, but deadly for majority of viruses and bacteria. AgNPs reduce toxicity of cell without affecting the antibacterial efficacy [31]. They show high antibacterial activity because of their finely honed surface and they can penetrate easily through the cell membrane to disturb the intracellular processes. AgNPs show higher antibacterial effect due to the formation of free radicals on its surface [32]. Several mechanism of the inhibitory effect of AgNPs on bacteria has been suggested. The high affinity of AgNPs towards phosphorus and sulphur present in the cell membrane is the main fact of the antibacterial effect. There are a lot of sulphur-having proteins on cell membrane. AgNPs react with sulphur-having amino acids in the cell membrane which destroys the cell viability of bacteria. It was also studied that Ag⁺ ions from the AgNPs react with phosphorous resulting in the deactivation of DNA replication or react with proteins containing sulphur inhibiting the enzyme functions. Generally, AgNPs with size less than 10 nm attack the proteins of bacteria containing sulphur and leads to penetrability of the cell membrane and eventually death of bacteria [33]. AgNPs with size less than 10 nm creates pores on the cell walls and changes the membrane structure of bacteria which cause cell death by releasing cytoplasmic content without altering extracellular and intracellular nucleic acids and proteins of bacterium [34–35]. AgNPs with addition to antibiotics show synergic effects against different microorganisms [36–37]. Modification of Ag sulphadiazine with addition of dendrimers improves the efficacy of anti-bacteria [38]. AgNPs also show anti-inflammatory properties [39].

 

3.2. OPTICAL PROPERTIES OF AgNPs:

AgNPs has specific strong interaction on electromagnetic radiation. The first application of AgNPs was to prepare pigments for fabrication of ceramics and glass. The individual particle size, shape, composition, environment and structure of adsorption layers determine the optical properties of AgNPs [40]. The distinctive feature of spectrum of scattering and absorption of AgNPs above 1 nm diameter is the appearance of a strong and wide-ranging band called surface plasmon resonance (SPR) in the visible range or near-UV or -IR ranges. Silver shows the highest SPR band intensity than any other metals like copper and gold. Light strikes on the surface of AgNPs and releases conduction electrons which leads to SPR band [41]. Oscillating dipoles are formed in AgNPs when the diameter of AgNPs is much less than the wavelength of incident light. The dipole formed near the surface is called the surface plasmon. Particles with different orientations give different bands with respect to incident wave. For this reason, cylindrical particles have two SPR band along and across its cylindrical axis. Dipoles with negative and positive charge regions cause polarization which decreases the frequency and amplitude of the oscillation of dipole and shifts the band of SPR to long wavelength [42]. The position and width of the SPR band depends on particle size and shape as dielectric permeability of Ag changes with the size of particles [43]. If the particle size becomes larger, additional band of SPR appears. Shift of absorption peak to longer wavelengths are observed when there is a change in the spherical size of the particles particularly due to the arrival of sharp corners. When the frequency of SPR band and the exciting radiation becomes equal, then plasmon field intensity reaches its peak as the probability of interaction of light and molecules through absorption, photoluminescence and scattering increases [44].

 

4. METHODS FOR SYNTHESIS OF AgNPs:

Various chemical, physical and biological processes have been reported in recent years for the synthesis of AgNPs (Fig. 1). It is known that various processes produce different shapes and sizes of AgNPs with varying stability. Chemical reduction process is one of the most important chemical processes for production of AgNPs. A large amount of AgNPs are produced by this process with different sizes. However, in this process a capping agent is required along with the reducing agent for the stability of the AgNPs. Moreover, due to the use of chemicals in this process the toxicity level goes high. Hence, various biological processes have been studied and reported successfully for the synthesis of AgNPs. The major advantage of biological process is because of its low toxicity and no requirement of extra capping agents.

 

 

Fig. 1: Various methods for synthesis of AgNPs

 

4.1. CHEMICAL PROCESS:

Chemical reduction is the most common method for the preparation of AgNPs as stable colloidal dispersions in water or organic solvents. Commonly used reducing agents are sodium borohydride, citrate, ascorbate and elemental hydrogen. The reduction of Ag⁺ in aqueous solution generally yields colloidal silver with particle diameters of several nanometers. Initially, the reducing agents reduce Ag⁺ ions to silver atoms (Ag^o), which follows agglomeration to form oligomeric clusters. These clusters eventually lead to the formation of colloidal Ag particles [45, 46]. Literature report [47] showed that use of a strong reductant such as borohydride resulted in production of small particles that were somewhat mono-dispersed, but the generation of larger particles was difficult to control, however use of a weaker reductant such as citrate resulted in a slower reduction rate, but the size distribution was far from narrow. In this process, protective agents are required to stabilize dispersive nanoparticles during the course of metal nanoparticle preparation; protective agents avoid the agglomeration of nanoparticles by binding or getting absorbed onto the nanoparticle surface. Polymers are often used as stabilizer and reducing reagent to prepare AgNPs. The most commonly used polymers are poly (vinyl pyrolidone), poly (ethylene glycol), poly (methacrylic acid), polymethyl methacrylate and so on [48–50]. Besides chemical reduction, other chemical processes for AgNPs synthesis include micro-emulsion technique [51], UV initiated photo reduction [52], photo-induced reduction [53], electrochemical synthetic method [54], microwave assisted synthesis [55], irradiation methods [56] and Tollens method [47].

 

4.2. PHYSICAL PROCESS:

Evaporation-condensation and laser ablation are widely used physical approaches for the synthesis of AgNPs. The advantage of physical methods over the chemical methods is the absence of hazardous chemicals and uniformity in the production of nanoparticles. Evaporation-condensation is done by using a tube furnace at atmospheric pressure. However, using a tube furnace has some disadvantages, like the requirement of large space, consumption of energy and time required for thermal stability [57–60]. Studies showed that AgNPs synthesized via small ceramic heater that has a local heating area has certain advantages over the use of tube furnace. High concentration of small nanoparticles can be synthesized by this process and the method might be suitable for various applications [61–62]. AgNPs can also be synthesized by laser ablation of metallic bulk materials in solution [63–67].

 

4.3. BIOLOGICAL PROCESS:

In recent years, emphasis have been given for the development of efficient green chemistry methods employing natural reducing and capping or stabilizing agents for the preparation of AgNPs with desired morphology and size. Major advantage of biological methods is the preparation of AgNPs without the use of any harsh, toxic and expensive chemical substances. The bioreduction of metal ions by combinations of biomolecules found in extracts of certain organisms like proteins, amino acids, polysaccharides and vitamins is environmentally benign synthesis. Many studies have reported successful synthesis of AgNPs using microorganisms also [68–70].

 

4.3.1. SYNTHESIS BY BACTERIA:

Synthesis of AgNPs using bacteria has been developed in the recent years for the quest of eco-friendly method. Culture supernatant of nonpathogenic bacterium Bacillus licheniformis is used for the bioreduction of aqueous silver ions, and the produced AgNPs (40 nm) are found to be highly stable [68]. Moreover, B. licheniformis can also be used for the synthesis of well dispersed silver nanocrystals [69]. Studies also showed formation of monodispersed AgNPs (5–50 nm) by using a combination of culture supernatant of B. subtilis and microwave irradiation in water [70]. It is thought that microwave irradiation provides uniform heating around the nanoparticles avoiding aggregation. Different compositions of silver nanocrystals were successfully synthesized by Pseudomonas stutzeri AG259 [71]. The crystals of silver formed ranged in size from few nm to 200 nm [71, 72]. Studies also showed that the silver resistant bacterial strain viz. P. stutzeri AG259 isolated from a silver mine had accumulation of AgNPs intracellularly along with some silver sulphide. The deposits were found to range from 35 to 46 nm [73]. Reduction to elemental silver is done by a detoxification mechanism in the periplasmic space of the bacterial strain. The capability of producing crystalline silver particles in nanometer scale with controlled morphology proved to be the basis of using this biological method in the field of material science [72]. Studies showed that culture supernatants of Klebsiella pneumonia, E. coli, and Enterobacter cloacae can be used for the rapid biosynthesis of AgNPs and it was believed that nitroreductase enzymes in the bacteria might be responsible for bioreduction of silver ions [74]. Biosynthesis of AgNPs can also be done by Lactobacillus strains within the bacterial cell [75, 76]. Spherical shaped AgNPs of size 25–50 nm (single) or 100 nm (in aggregates) were formed attached to the surface of biomass or inside and outside of the cells. It was confirmed by the study that the reduction of silver ions and the stabilization of the AgNPs was due to an enzymatic process [76].

 

4.3.2. SYNTHESIS BY FUNGI:

Many studies have been done in the recent years for the synthesis of AgNPs using fungi. It was reported that compared to bacteria, fungi have a better potential for higher production of nanoparticles. The reason being fungi are known to secrete much higher amount of proteins [77–78]. It was reported that the possible mechanism for the formation of AgNPs in the fungal biomass of Fusarium oxysporum could be the extracellular reduction of Ag⁺ ions in solution followed by precipitation into the cells [79]. Studies showed that the exposure of silver ions to F. oxysporum resulted in the release of nitrate reductase leading to the formation of highly stable AgNPs in solution [80]. It is also mentioned that high stability of AgNPs in solution was formed due to capping of particles by release of capping proteins by F. oxysporum. However, the stability of the capping protein was found to be pH dependent. Aspergillus flavus could be used to achieve highly stable AgNPs and it was reported that the produced NPs were found to be stable in water for more than 3 months because of the stabilizing materials secreted by the fungus [81]. Extracellular biosynthesis of AgNPs (5–25 nm) using Aspergillus fumigates has also been reported and the produced NPs were found to be spherical in nature with some triangular shapes [82]. Though the exact mechanism in the production of AgNPs by fungi is not fully confirmed, it is believed that the proteins might be the reason for the process. A major drawback of using microorganisms in comparison to the plant extract is the longer time required for synthesis of AgNPs.

 

 

 

4.3.3. SYNTHESIS BY PLANTS:

Among all the other biological methods, AgNPs synthesis using extracts of plant materials has more advantages because it does not require the maintenance of cell cultures and can be used for large scale synthesis [83]. Extracellular nanoparticle synthesis through plant extracts proves to be more inexpensive due to easier downstream processing. Due to easy availability of plants, they are found to be an important prospect for the production of AgNPs. The whole plant or extract of a particular plant part or the powder can be used for the production and the plant extract itself acts as capping agent. The availability of reducing agent is large in the extract than the whole plant and hence the extract is ideal for synthesis [84]. However, physical and chemical methods are highly expensive due to their energy intensive processes, and production of AgNPs by chemical reduction method may lead to absorption of harsh chemicals on the surfaces of nanoparticles raising the toxicity issues [85]. It has been supported by various reports that the bioreduction potential of plant extracts is comparatively higher than the microbial culture filtrate [85, 86]. Moreover, the waste products generated from the plant mediated synthesis processes are usually compatible with the environment as these particles are resulted from natural plant extracts. On the contrary, depending on the type of microbe used for the microbial synthesis methods, the waste products generated by the synthesis are likely to be dangerous to the environment. As far as the safety of biological synthesis procedures is concerned, the plant mediated approach has much reduced impact on the environment [87]. Apart from reducing the metal ions, the phytochemicals present in the plant extracts are also known to stabilize the synthesized nanoparticles [85, 88]. The main mechanism considered for the process is by bioreduction of Ag⁺ ions due to phytochemicals. Terpenoids, flavones, ketones, aldehydes, amides and carboxylic acids are the main phytochemicals involved. Flavones, organic acids and quinones are water soluble phytochemicals that are responsible for the immediate reduction of the ions. Apart from the leaf, other plant parts like seed, stem, fruit, essential oils etc. are also used for AgNPs preparation. A vast collection of secondary metabolites is originated in plants which have redox capacity for biosynthesis of AgNPs and thus AgNPs are formed from Ag⁺ ion by bio-reduction with the help of plant metabolites [84].

 

Environmentally benign synthesis of AgNPs is through biological materials and different materials produce different shapes and sizes of AgNPs with varying stability and property. Green synthesis of AgNPs by different researchers using extracts of various plant parts, different microorganisms and other biological materials, and their shapes, sizes and various activities like catalytic, photocatalytic, antibacterial, antifungal, antibiofouling, antimicrobial, acaricidal, larvicidal, antioxidant, antiparasitic, anti-proliferative, and anticancer activities are summarized in Table 1 and Table 2.

 

Table 1. Green synthesis of AgNPs using extracts of various plants

Name of plant	Plant part	Shape	Size (nm)	Significance of produced AgNPs	References
Lonicera japonica	Leaf	Spherical	~7.8	–	[89]
Cycas	Leaf	Spherical	2–6	–	[90]
Paederia foetida	Leaf	Spherical	4–15	Antibacterial activity	[91]
Azadirachta indica	Leaf	Spherical	~34	Antibacterial activity and toxicity	[92]
Ceratonia silique	Leaf	Spherical	5–40	Antibacterial activity	[93]
Manilkara zapota	Leaf	Spherical	70–140	Acaricidal activity	[94]
Euphorbia prostrata	Leaf	Rod shaped	25–80	Antiparasitic activity	[95]
Acalypha indica	Leaf		20–30	Antibacterial activity	[96]
Lippia citriodora	Leaf	Spherical crystalline	15–30 nm	–	[97]
Ocimum sanctum	Leaf	Circular	4–30	Antimicrobial activity	[98]
Murraya koenigii	Leaf	Spherical	40–80	Bactericidal activity	[99]
Mangifera indica	Leaf	Triangular, hexagonal	∼20	–	[100]
Hibiscus rosa sinensis	Leaf	Spherical	~13	–	[101]
Chenopodium album	Leaf	Spherical, triangular	10–30	–	[102]
Coleus amboinicus	Leaf	Triangle, spherical, decahedral, hexagonal	~17.6, ~30.6, ~35.8	–	[103]
Hibiscus cannabinus	Leaf	Spherical	~9	Antimicrobial activity	[104]
Prosopis juliflora	Leaf	Triangles, tetragons, pentagons, hexagons	35–60	Antimicrobial activity	[105]
Malva parviflora	Leaf	Face centered cubic (fcc)	19–25	–	[106]
Annona squamosal	Leaf	Spherical	20–100	Cytotoxicity against human breast cancer cell	[107]
Vitex negundo	Leaf	fcc	~20	Anti-proliferative effects	[108]
Mukia scabrella	Leaf	Spherical	18–21	Antimicrobial activity	[109]
Syzygium cumini	Leaf and seed		~30, ~29, ~92, ~73	–	[110]
Ocimum tenuiflorum	Leaf	fcc	25–40	Antibacterial activity	[111]
Vinca rosea	Leaf		27±2, 30±2	Antimicrobial activity	[112]
Citrus sinensis	Peel		35±2 (25°C), 10 (60°C)	Antibacterial activity	[113]
Citrus fruits (orange, lemon, lime)	Peel		7.36±8.06	–	[114]
Acacia	Gum	Cubic	1–20	Antimicrobial activity	[115]
Boswellia serrata	Gum	Spherical	7.5±3.8	Antibacterial activity	[116]
Cochlospermum gossypium	Gum	Spherical	~3	Antibacterial activity	[117]
Elaeocarpus granitrus	Seed	Spherical, irregular		Antimicrobial activity	[118]
Jatropha curcas	Seed	Spherical	15–50	–	[119]
Macrotyloma Uniflorum	Seed	fcc	∼12	–	[120]
Artocarpus heterophyllus	Seed	Irregular	~10.78	Antibacterial activity	[121]
Albizia adianthifolia	Leaf	Spherical	4–35	Anticancer activity	[122]
Phyllanthus amarus	Plant	Spherical	10–20	–	[123]
Cocos nucifera	Coir		23±2	Larvicidal activity	[124]
Breynia rhamnoides	Stem		~64	Catalytic activity	[125]
Tribulus terrestris	Fruit	Spherical	16–28	Antibacterial activity	[126]
Jatropha curcas	Latex	fcc	10–20	–	[127]
Euphorbia milii	Latex	Spherical	~10	–	[128]
Sesuvium portulacastrum	Callus and leaf	Spherical	5–20	Antimicrobial activity	[129]
Ocimum sanctum	Stem and root		10±2, 5±1.5	–	[130]
Cinnamon zeylanicum	Bark		31, 40	Antimicrobial activity	[131]
Areca catechu	Fruit	Spherical	80	Antitumor activity	[132]
Mentha piperita	Plant	Spherical	90	Antibacterial activity	[133]
Artemisia nilagirica	Plant		70–90	Antimicrobial activity	[134]
Cissus quadrangularis	Stem		~42.46	Anti-parasitic activities	[135]
Allium sativum	Bulb	Spherical	7.3±4.4	Antibacterial activity	[136]
Alstonia scholaris	Bark		~50	Antimicrobial activity	[137]
Ocimum tenuiflorum	Leaf		25–40	Antibacterial activity	[138]
Quercus infectoria	Fruit hull	Spherical	~40	Cytotoxicity effects against human breast cancer cells	[139]
Tagetes erecta	Flower	Spherical, hexagonal	10–90	Antimicrobial activity	[140]
Hypericum hookerianum	Hypericin-rich shoot culture		20–65	Antibacterial activity	[141]
Viburnum lantana	Leaf	Spherical	20–80	Antimicrobial activity	[142]
Psidium guajava	Leaf	Spherical	2–10	Cytotoxicity and antimicrobial activities	[143]
Terminalia chebula	Fruit		~25	Catalytic activity	[144]
Ficus panda	Leaf	Spherical	12–36	Catalytic activity	[145]
Tinospora crispa	Stems		40±2	Catalytic activity	[146]
Boswellia ovalifoliolata	Stem bark	Spherical	30–40	–	[147]
Callicarpa maingayi	Stem bark	fcc	12.4±3.2	–	[148]
Calotropis procera	Flower		35	–	[149]
Camellia sinensis	Leaf	Spherical	4	–	[150]
Catharanthus roseus	Leaf	Cubic	48–67	Antibacterial activity	[151]
Catharanthus roseus	Leaf	Spherical	35–55	–	[152]
Catharanthus roseus	Leaf	Spherical	27–50	Antibacterial activity	[153]
Chrysanthemum morifolium	Flower	Spherical	25–50	Antimicrobial activity	[154]
Coccinia grandis	Leaf	Spherical	20–30		[155]
Coleus amboinicus	Leaf	fcc	25.8±0.8	In-vitro antioxidant and anticancer activity against EAC cell line	[156]
Coleus aromaticus	Leaf	Spherical	40–50	Antibacterial activity	[157]
Solanum xanthocarpum	Fruit	Spherical	4–18	Antimicrobial and urease inhibitory activities	[158]
Cynodon dactylon	Leaf	Spherical	8–10	Antibacterial activity	[159]
Datura metel	Leaf	Quasilinear	16–40	–	[160]
Desmodium gangeticum	Leaf	Spherical	18–39	Antibacterial activity	[161]
Desmodium triflorum	Plant	Crystalline	5–20	Antibacterial activity	[162]
Dioscorea bulbifera	Tuber	Nanorods, triangles	8–20	Antibacterial activity	[163]
Euphorbia hirta	Leaf	Spherical	40–50	Antibacterial activity	[164]
Euphorbia nivulia	Latex	Spherical	10–20	Antibacterial and anticancer	[165]
Iresine herbstii	Leaf	Cubic	40–64	Antibacterial, in-vitro antioxidant, and anticancer activities	[166]
Melia azedarach	Leaf	Spherical	78	In-vitro and in-vivo cytotoxicity effects	[167]
Rhizophora apiculata	Leaf	Spherical	19–42	Antibacterial activity	[168]
Rhizophora mucronata	Leaf bud	Spherical	4–26	Antibacterial activity against aquatic pathogens	[169]
Trianthema decandra	Root	Spherical	36–74	Antimicrobial activity against bacterial and a yeast pathogens	[170]
Camelia sinensis	Leaf	Spherical	–	Antimicrobial activity	[171]
Erythrina indica	Root	Spherical	20–118	Antibacterial activity and cytotoxic effect on breast and lung cancer cell lines	[172]
Hydrocotyle rotundifolia	Leaf	Spherical, elliptical	7.39 ± 0.80	Antimicrobial activity	[173]
Centella asiatica	Leaf	Spherical	15.11±2.09	Antimicrobial composite film	[174]
Lippia javanica	Leaf	Spherical	17.5	Antibacterial activity	[175]
Paederia foetida	Leaf	Spherical	8.9 ± 3.6	Antibacterial activity	[176]
Ficus benghalensis	Leaf	fcc	16	Antibacterial activity	[177]
Eriobotrya japonica	Leaf	fcc	18	Antibacterial activity	[178]
Santalum album	Leaf	–	80–200	Antimicrobial activity	[179]
Chenopodium aristatum	Stem	Quasi-spherical	3–36	Catalytic and antibacterial Activities	[180]

 

Table 2. Synthesis of AgNPs by microorganisms and other biological materials

Microorganisms/ Other biological materials	Size (nm)	Shape	Significance of produced AgNPs	References
Algae
Caulerpa racemosa	5–25	Spherical, triangular	Antibacterial activity	[181]
Ulva fasciata	28–41	Spherical	Antibacterial activity	[182]
Gelidiella acerosa	~22	Spherical	Antifungal activity	[183]
Sargassum Wightii	8–27	Spherical	Antibacterial activity	[184]
Fungi
Rhizopus oryzae	~15	Spherical	Antimicrobial activity	[185]
Verticillium	25±12	–	–	[78]
Cladosporium cladosporioides	10–100	Spherical	–	[186]
Penicillium brevicompatum	23–105	–	–	[187]
Trichoderma viride	2–4 (10oC), 10–40 (27oC), 80–100 (40oC)	Spherical, plate like shape	–	[188]
Penicillum fellutanum	5–25	Spherical	–	[189]
Bacteria
Lactobacillus fermentum	~6	Spherical	Antibacterial and antibiofouling activities	[190]
Bacillus licheniformis	~40	–	–	[68]
Bacillus subtilis	5–60	Spherical	–	[70]
Lactobacillus sp.	25–50	Spherical	–	[76]
Bacillus cereus	60	Spherical	–	[191]
E. coli cells	~5	–	Antimicrobial activity	[192]
E. coli supernatant	10–90	Spherical	–	[193]
Other biological materials
Fish scales of Labeo rohita	~16.5, ~17.47	Spherical	Catalytic activity for reduction of aromatic nitro compounds	[19]
Egg shell	2.46	Spherical	Catalytic activity for reduction of nitro compounds	[20]
Egg shell	6–26	Spherical, Oval	Photocatalytic activity for degradation of hazardous dyes	[194]
Haemolymph of marine crabs	45–50	–	Antibacterial activity	[195]

 

5. MECHANISM BEHIND THE AgNPs SYNTHESIS:

AgNPs are produced from Ag⁺ ions. Silver salt provides the Ag⁺ ions in the solution. The reducing agent first reduces the Ag⁺ ions to atoms followed by nucleation in small clusters that grow into particles. The reduction of silver ions when mediated by plant extracts is known as bio-reduction. Size and shape of the nanoparticle formation depends on the availability of the atoms, which in turn depends on the silver salt to reducing agent concentration ratio. The reduction of Ag⁺ ions to produce a colored silver sol by green chemical methods proceeds through a one step process. Nucleation is observed due to the hyper reactivity of the produced nanoparticles. Hence there is a continuous increase in size. It is necessary to add capping agents to the reaction mixture as soon as possible to get nanoparticles of smaller size. The signatory feature of AgNPs formation is the appearance of intense color between red and black in reaction mixture during the reaction [196]. Although the exact mechanism of the AgNPs synthesis by reducing and capping agents has not been confirmed yet, various studies have proposed certain pathways or possible mechanisms for the synthesis. In one study, it has been reported that flavone and terpenoid in the leaf of Azadirachta indica were the surface active molecules stabilizing the nanoparticles and the formation of pure Au, Ag, and bimetallic Au core–Ag shell nanoparticles was facilitated by reducing sugars and/or terpenoids present in the plant extract [197]. Another study with Ananas comosus, the biomolecules responsible for the reduction and stabilization of AgNPs are believed to be antioxidants including phenols probably because of the presence of different types of antioxidant compounds in the extract [198]. The exact mechanism of reducing and capping of AgNPs by flavonoids is not well established. However, it has been believed that the various types of –OH groups present in flavonoids reduce the silver ion to silver metal and the capping of the metal is done by chelation of the nearby carbonyl and hydroxyl groups and also by the catechol moiety of flavonoids [199]. A DFT study showed that the bond dissociation energies of O-H bond of –OH groups of catechol moiety flavonoids are less than that of other –OH groups present in flavonoids [200]. Thus it can be inferred that –OH groups of catechol of flavonoids may take part in metal ion reduction. A study has shown a possible pathway of reduction of Ag⁺ and capping of the produced AgNPs by taking quercetin which is a flavonoid [201]. Currently, the mechanism for biosynthesis of AgNPs is still under exploitation. Only limited reports are published with proposed mechanism and most of them are hypotheses having no considerable experimental supports [201]. Some phytochemicals which have been reported to be responsible for the biosynthesis of AgNPs from AgNO₃ are ascorbic acid, citric acid, gallic acid, tannic acid, retinoic acid, catechin, apiin, cyclic peptide, ellagic acid, epicatechin gallate, euphol, gallagin, phyllanthin, pinocembrin, theaflavin, and quercetin [84, 201].

 

6. FACTORS AFFECTING THE SYNTHESIS OF AgNPs:

Various factors influence the size, shape and stability of AgNPs. The produced AgNPs was found to be influenced by the concentration of plant extract used for the synthesis. A study reported that the size of AgNPs and AuNPs decreased with the increasing concentration of tansy fruit extract from 1.8 to 4.8 mL [202]. Increase in the particle size of AgNPs and AuNPs was also observed with the increase in concentration of the metal ion solution [203–205]. Besides these, there are also other factors such as pH, temperature, and exposure time are also known to affect the size, shape, and stability of silver nanoparticles to some extent. The size and distribution of the nanoparticles are affected by the pH of the solution medium [206, 207]. Studies reported that highly stable AgNPs using plant extracts were found to be stable under wide range of pH due to their high zeta potential [208, 209]. Temperature is the basic physical factor that affects the synthesis of AgNPs. In most cases, silver nanoparticles were successfully synthesized only at room temperature. A report showed that a red shift appeared from 406 nm to 450 nm due to increase in temperature up to 50⁰C and further increase in temperature caused the broadening of the peak indicating the increased size of AgNPs [158].

 

7. APPLICATIONS OF AgNPs:

Silver nanoparticles due to their unique optical and antimicrobial properties have been widely used in many sectors. Silver nanoparticles are incorporated into products that range from photovoltaics to biological and chemical sensors due to their unique properties. In recent years, extensive researches on nanoparticles have been conducted for the purpose of therapy and diagnosis of cancer and HIV. Biogenic AgNPs are functionalized with biomolecules and hence are more biologically active as compared with chemically synthesized AgNPs. Besides application of biogenic AgNPs in cancer therapeutics, many studies have reported diverse applications of biogenic AgNPs such as antimicrobial, wound healing, antiviral, antifungal and many more [131–146].

 

7.1. ANTIBACTERIAL ACTIVITY OF AgNPs:

AgNPs have the ability to overcome the bacterial resistance against antibiotics and hence prove to be an alternative antibacterial agent to antibiotics. AgNPs due to their large surface to volume ratio and crystallographic surface structure seem to be a potential antibacterial agent. In a study it was reported that, E. coli cells treated with AgNPs showed antimicrobial activity by formation of ‘pits’ in the bacterial cell wall [35]. It is also reported that smaller particles showed more efficient antibacterial activity than larger particles when tested with E. coli strain [210]. AgNPs showed effective antimicrobial and antibacterial activities against gram-negative and gram-positive bacteria. A detailed study was carried out to investigate the efficiency of antimicrobial effects of AgNPs against yeast, E. coli and Staphylococcus aureus and it was reported that low concentration of AgNPs inhibited the growth of E. coli and yeast, whereas a mild effect was observed for S. aureus [171]. An interesting study showed that the healing time is reduced by an average of 3.35 days when wound dressing is loaded with AgNPs and showed increase in bacterial clearance from infected wounds compared to silver sulfadiazine [211]. However, it was also reported that healing of only superficial burn wounds were enhanced using AgNPs whereas no characteristic effects were observed for deep burn wounds [212]. Besides these, AgNPs are also used for dental applications for its antibacterial activity [213].

 

7.2. ANTIFUNGAL ACTIVITY OF AgNPs:

Various researches have been going on for the development of biocompatible, nontoxic and environment friendly antifungal agents. AgNPs play an important role in biomedical application as antifungal agents. In a study, it was reported that soda-lime glass containing AgNPs showed enhanced biocidal activity [214]. Fungus mediated biosynthesis of AgNPs with fluconazole showed antifungal activity [215]. The mechanisms of action of silver nanoparticles against fungi are thought to be similar to that of the antibacterial actions proposed for fungi [216]. Vivek et al. [183] synthesized AgNPs from Gelidiella acerosa extract and studied the antifungal effects against Humicola insolens, Fusarium dimerum, Mucor indicus and Trichoderma reesei which was found to have considerable antifungal activity in comparison with standard antifungal drug.

 

7.3. ANTIVIRAL ACTIVITY OF AgNPs:

Since viral mediated diseases are more prominent, developing AgNPs as antiviral agent has been an important prospect in recent times. Based on the size ranges and shapes, AgNPs show unique interactions with bacteria and viruses [217–219]. The first mechanistic study of anti-HIV activity of AgNPs was reported by Lara et al. [220]. Study of interaction of AgNPs with H1N1 influenza A virus has also been reported which shows that AgNPs exhibits anti H1N1 influenza A virus activity [221]. Elbeshehy et al. [222] reported that biosynthesized AgNPs showed a significant antiviral property against the Bean Yellow Mosaic Virus. Xiang et al. [223] reported that synthesized AgNPs can prevent H3N2 influenza virus infection both in vitro and in vivo, and showed that AgNPs can be used as potential therapeutics for inhibiting outbreaks of influenza. Gaikwad et al. [224] also reported that mycosynthesized AgNPs undergo a size-dependent interaction with herpes simplex virus types 1 and 2 and with human parainfluenza virus type 3 and showed smaller-sized nanoparticles can inhibit the infectivity of the viruses.

 

7.4. ANTIINFLAMMATORY ACTIVITY OF AgNPs:

AgNPs also show antiinflammatory activity although the reports on this are limited. Anti-inflammatory responses of AgNPs have been reported in mice [225]. Rapid healing and cosmetic appearance was observed in mice when treated with AgNPs. Reduction in wound inflammation along with antimicrobial properties was observed by the use of AgNPs [226]. Another study investigated the anti-inflammatory properties of AgNPs in which both in-vitro and in-vivo models were used and found that AgNPs could suppress inflammatory events in the early stages of wound healing [210].

 

7.5. ANTICANCER ACTIVITY OF AgNPs:

It is seen that one in every three people has the possibility to develop cancer. The chemotherapeutic agents currently used show enormous side effects and administration to patient is a tedious process [210]. Hence there are many researches in this prospect. Nanoparticles due to their unique properties than their bulk materials are used in various diagnosis and therapeutic applications. Nanoparticles have been developed as effective drug carriers and also found to have potential cytotoxic effect [227]. A study investigated the molecular mechanism of AgNPs towards cancer and non-cancer cells, and reported that programmed cell death was found to be concentration dependent [210]. In one study, AgNPs with a drug viz. 5-fluorouracil showed synergistic effect on apoptosis of uracil phosphoribosyltransferase (UPRT)-expressing cells and non-UPRT-expressing cells [228]. A study showed that biosynthesized AgNPs using Areca catechu showed anti-tumor activity against Dalton’s ascites lymphoma mice model [132]. In another study, it has been reported that cytotoxic activity of dispersed AgNPs in the Jaft extract was higher than AgNPs on MCF-7 breast cancer cell line and the cytotoxic activity increased with increasing concentration [139]. The AgNPs synthesized using guava extract showed significant efficacy against cervical carcinoma HeLa cell at a maximum dose of 1000 mg/mL along with antimicrobial property [143]. AgNPs obtained from aqueous extract of Coleus amboinicus showed cytotoxicity against the Ehrlich’s Ascite carcinoma cell line [156]. Green synthesized AgNPs from Iresine herbstii leaf showed cytotoxicity against HeLa cervical cell lines as well as strong antioxidant activity [166]. AgNPs obtained from Melia azedarach showed cytotoxicity against in vitro HeLa cell lines which exhibited a dose–response activity and also in vivo Dalton's ascites lymphoma mice model displayed significant increase in life span, induction of apoptosis was evidenced by acridine orange and ethidium bromide staining [167]. Rathi et al. [172] synthesized AgNPs from Erythrina indica aqueous root extract and reported excellent cytotoxic effect on breast and lung cancer cell lines.

 

7.6. CATALYTIC ACTIVITY OF AgNPs:

Due to the unique large surface-to-volume ratios and quantum sized effect, metal nanoparticles have attracted a lot of interest. Since industrial catalysts usually work on the surface of metals, metal nanoparticles due to much surface area per unit volume of metals have been considered as promising materials for catalysis than their bulk materials [229]. In recent years there have been many studies on various catalytic activities of AgNPs. AgNPs supported on silica spheres showed catalytic properties and successful catalytic reduction of dyes has been observed. The study also focused on the effects of electrolytes and surfactants on the catalytic activity of AgNPs. It was reported that surfactants decreased the catalytic activity and electrolytes increased the catalytic reaction rate as the reduction of dyes increased with increasing electrolyte concentration [230]. Another study showed that growing particles of AgNPs showed more catalytic activity than stable colloidal particles. Growing smaller AgNPs catalyzed borohydride reduction of several organic dyes in faster rate than stable and larger AgNPs. The catalytic activity of the AgNPs depends on their size, E1/2 of the dye, and the dye-particle interaction and on the selection of surfactants [231]. Good catalytic activity of AgNPs synthesized using leaf extract of Ficus panda was reported for the reduction of methylene blue dye [145]. The catalytic activity of AgNPs was reported in reduction of methylene blue by arsine in micellar medium [232]. Good catalytic activity of synthesized AgNPs using T. chebula fruit extract on the reduction of methylene blue was also reported [144]. Several studies also reported good catalytic properties of biosynthesized AgNPs [19, 20, 125, 180, 194].

 

7.7. OTHER APPLICATIONS OF AgNPs:

AgNPs has been used as silver-based biosensors for the clinical detection of serum p53 in head and neck squamous cell carcinoma [233]. The excellent optoelectronic and plasmonic properties of AgNPs showed potential applications ranging from sensing to biomedical applications [234]. Au–Ag nanorod aptamer-conjugate increases both the signal and binding strengths in cancer cell recognition [235]. Silver nanowires (AgNWs) in recent times have been used as conductive filler and thermal interfacial material in sophisticated nanodevices. AgNWs were developed and their applications in conducting polymer nanocomposites were also studied. They have high electrical conductivity and also useful for applications such as in touch screen, liquid crystal display and solar cell [236]. The electrical conductivity of silver nanowire/polymer nanocomposites were increased up to about 8 orders of magnitude by introducing silica nanoparticles with nanocomposites [237]. AgNPs are also used in textile industries [62]. The nanoparticles are incorporated into fiber or coated on fiber. AgNPs are used in T shirt, sporting clothes, underwear, socks etc. [238]. AgNPs are also used in food industries mainly due to their antibacterial properties [239]. AgNPs are used in food packaging. Sunriver industrial nano silver fresh food bag is an example of commercially available bag in which AgNPs are added [240]. A study reported the use of core shell magnet nanoparticles comprised of AgNPs as effective disinfectant in water purification system [241]. AgNPs are also used in the field of agriculture to inhibit the growth of plant pathogens. They showed inhibitory effects against the outbreak of Powdery Mildews Disease on cucumbers and pumpkins [242]. Various other reports on the control of Colletotrichum species and Pepper Anthracnose Disease in the field by using AgNPs have been reported in agricultural application [243].

 

8. TOXICITY OF AgNPs:

Due to antimicrobial and anti-inflammatory properties, AgNPs have been widely used for medicinal purposes [210]. However, AgNPs can cause adverse effects on human beings as well as the environment [244]. Silver is released into the environment through industrial wastes and it is believed that the toxicity of silver is mainly due to free silver ions in aqueous phase. Various effects of free silver ions on humans and on other living organisms include permanent bluish-gray discoloration of the skin (argyria) or the eyes (argyrosis). Moreover, exposure to soluble silver compounds may produce toxic effects like liver and kidney damage, irritations in eye, skin, respiratory and intestinal tract, and untoward changes in blood cells [245]. It was reported that AgNPs can kill microbes that are beneficial to living beings and/or ecological processes [246]. Oxidative stress and impaired mitochondrial function is observed in rat liver cells even on low level exposure of AgNPs [247]. In another study it was reported that aggregates of AgNPs are more cytotoxic than asbestos [248]. Research showed that AgNPs display severe toxic effects on the male reproductive system as AgNPs can cross the blood-testes barrier and be deposited in the testes where they adversely affect the sperm cells [249]. It was also reported that aged AgNPs are more toxic than freshly prepared AgNPs [250]. AgNPs showed toxic effects on aquatic animals by inhibiting osmoregulation in fish [251]. AgNPs also affects the aquatic ecosystem by its toxic effects on denitrifying bacteria and disrupts denitrification processes leading to ecosystem disruption. Environmental denitrification reduces plant productivity which in turn can result in eutrophication in rivers, lakes and marine ecosystems, and are a drinking water pollutant [244].

 

9. CONCLUSION:

The development of a simple, reliable, inexpensive, non-toxic, efficient and environmentally benign process for nanoparticle synthesis is an indispensable part of nanotechnology and is significantly increasing in recent times. AgNPs can be produced by physical, chemical and biological processes. AgNPs synthesis have gained a lot of interest in recent times due to their unique properties and wide applications in the field of electronics, catalysis, biology, medicine, agriculture, cosmetics and many more. Biosynthesis of AgNPs includes synthesis using plants, fungi, algae, bacteria, yeast and other biological materials like fish scales, egg shells and crabs. Plant-based synthesis of AgNPs is preferably getting more attention throughout the world as the raw material is readily available, nontoxic, inexpensive, relatively simple to handle and the process is environmentally benign. Moreover, biomolecules present in plants have the capability to act as reducing, chelating, and capping agents for AgNPs synthesis due to which produced particles are protected from further reactions and aggregation and also increases the stability and longevity of the particle. This review has summarized the green synthesis of AgNPs using extracts of different biological materials, sizes and shapes of produced AgNPs, and various properties like catalytic, antibacterial, antifungal, antibiofouling, antimicrobial, acaricidal, larvicidal, antioxidant, antiparasitic, anti-proliferative, and anticancer activities. The potential for silver and other metal nanoparticles synthesis using biological materials are yet to be completely explored due to their rich biodiversity in nature and there are commendable prospects for developing industrial scale production of nanoparticles for various applications.

 

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Research J. Pharm. and Tech 2018; 11(6): 2681-2694.