BIOSYNTHESIS OF NANOPARTICLES

BIOSYNTHESIS OF NANOPARTICLES

K. Vithiya and S. Sen*

Industrial Biotechnology Division, School of Bio Sciences and Technology, VIT University, Vellore- 632014, Tamil Nadu, India

ABSTRACT

Biosynthesis of nanoparticles is reviewed in detail in this study. Comparison of different synthesis methods, namely physical, chemical and green methods giving emphasis to biological synthesis is documented here. This study also details limitations of the present techniques and envisages the future scope of nanoparticle biosynthesis. Important applications of nanoparticles are also discussed briefly in the present report

Nanoparticles

INTRODUCTION: Nanotechnology has started leaving conforms of laboratory and conquering the new application to change our lives. The increased surfaces of these nanoparticles are responsible for their different chemical, optical, mechanical, megnatic properties as compared to large bulk materials ¹. Nanoparticles were used by the artisans of Mesopotamia to generate glittering effects to pots. The properties of nanoparticles were proved in 1857 in Faraday’s famous paper “Experimental relations of gold (and other metals) to light” ².

Due to incredible properties nanoparticles have become significant in many fields in the recent years, such as energy, health care, environment, agriculture, etc. The preparation of nanoparticles are carried out either by (i) Nanoparticles synthesis or by (ii) Processing of nanomaterials into nanostructure particles ³. The silver nanoparticles are prepared by using physical, chemical and biological methods ⁴. The physical and chemical methods are very expensive ⁵.  Biological methods of nanoparticles synthesis would help to remove harsh processing conditions by enabling the synthesis at physiological pH, temperature, pressure, and at the same time at lower cost. Large number of micro organisms have been found capable of synthesizing inorganic nanoparticles composite either intra or extracellularly ⁶.

In view of all these, a detail survey of available literature for various biosynthesis methods, their corresponding merits and demerits, numerous applications of silver nanoparticles and future aspects of synthesis and applications of nanoparticle are presented in this review.

Bio production of Nanoparticles: There are numerous methods available using various approaches including chemical, physical, and biological protocols for the synthesis of nanoparticles. Chemical method of synthesis is advantageous as it takes short period of time for synthesis of large quantity of nanoparticles. However, in this method capping agents are required for size stabilization of the nanoparticles. Moreover, chemicals reagents used generally for nanoparticles synthesis and stabilization are toxic and lead to byproducts that are not environment benign. The need for eco-friendly non-toxic methods for nanoparticles synthesis is developing interest in biological approaches which are free from the use of toxic chemicals as byproducts. A variety of natural sources are there for metal nanoparticle synthesis including plants, fungi, yeast, actinomycetes, bacteria etc. The unicellular and multicultural organisms can produce intracellular and extra cellular inorganic nanoparticles. An overview of biological synthesis of nanoparticles is focused in Table 1.

TABLE 1: BIOLOGICAL SYNTHESIS OF NANOPARTICLES

 Plant in Nanoparticles Synthesis: Plants offer a better option for synthesis of nanoparticle as the protocols involving plant sources are free from toxic chemicals; moreover, natural capping agents are readily supplied by the plants. Sastry et al., reported the synthesis of gold and silver nanoparticles using geranium extracts ²¹. Further, gold nanotriangles and silver nanoparticles were synthesized using Aloe Vera plant extracts ²². Most reports available on the synthesis of silver or gold nanoparticles use broths resulting from boiling fresh plant leaves.

However, Huang et al., synthesized silver and gold nanoparticles using the sundried Cinnamomum camphora leaf extract ²³. A simple green synthesis method for production of well-defined silver nanowires was reported recently by Lin et al²⁴. The method involves reduction of silver nitrate with the broth of sundried Cassia fistula leaf at room temperature without using any additive. Various plants/plant tissues, nanoparticles synthesized using them, their shape and size and the references are listed in Table 1.

Bacteria in Nanoparticles Synthesis: Bacteria have been most extensively researched for synthesis of nanoparticles because of their fast growth and relative ease of genetic manipulation. Slawson et al., found the Silver producing bacteria isolated from the silver mines Pseudomonas stutzeri AG259 where the silver nanoparticles were accumulated in the periplasmic space but the particles size ranges from 35 to 46 nm ²⁵.

The Lactobacillus strains present in the milk were exposed to larger concentration of nanoparticles to produced silver, gold, and alloy crystals of defined morphology ²⁶. Bacteria have also been used to synthesize gold nanoparticles²⁷. The PH was an important factory in controlling the morphology of bacteriogenic nanoparticles and location of the deposition. These nanoparticles were used in many application e.g. direct electrochemistry of proteins²⁸. The most important application of bacterium would be in industrial silver recovery.

Yeast in Nanoparticle Synthesis: Kowshik et al., have demonstrated that MKY3, a silver-tolerant yeast species, when challenged with soluble silver in the log phase of growth, majority of silver precipitate extracellularly as elemental nanoparticles. When challenged with soluble silver in the log phase of growth, based on differential thawing of the sample, for separation of the Metallic nanoparticles from the medium ²⁹.

Fungi in Nanoparticle Synthesis: The fungi taking the center stage of studies on biological generation of nanopartilces because of the tolerance and bioaccumulation ³⁰. The advantages of using fungi in their scale up process (e.g., using a thin solid substrate fermentation method). Fungi are efficient secretor of extra cellular enzymes it can easily obtain large scale production of enzymes. Further advantages of using fungal mediated green approach for synthesis of metallic nanoparticles include economic viability and ease in handling biomass.

The main drawback of biosynthesis of nanoparticles synthesis in eukaryotic organisms lies in the problem of genetic manipulation of the organism as a mean to over express the enzymes which is relatively much more difficult in eukaryotes than that in prokaryotes. Mukherjee et al demonstrated “green synthesis” of highly stabilized nanocrystalline silver particles by a nonpathogenic and agriculturally important fungus, Trichoderma asperellum ³¹.

Algae in Nanoparticles Synthesis: Algae are similar to yeast for biosynthesis of nanoparticle, still very few reports used algae as a “Biofactory” for the nanoparticles synthesis ³². The marine algae used for the biosynthesizing highly stable extracellular gold nanoparticles in a relatively short time period compare to other biosynthesizing process ³². Palladium and platinum nanoparticles starting with their corresponding metallic chloride- containing salts have been investigated ³².

Actinomycetes in Nanoparticles Synthesis: The monodispersity of the silver and gold nanoparticles produced either intracellular or extracellularly is of interest to scientists. But it was not very high and was far inferior to that obtained by conventional methods. Most of the actnimycetes especially the thermophilic actinimycetes, Thermomonospora sp was exposed to gold and silver irons and the metals got reduced extracellularly ³³. These micro-organisms have developed numerous special adaptations to survive in such extreme habitats which include new mechanism of enzyme transduction, regulating metabolism, maintaining the structure and function of the membrane. Actinomycetes are micro organisms that can share important characteristics of fungi and bacteria ³⁴. Even though the Actinomycetes have close relation with the mycobacterium and coryneforms, they were originally designated with ‘ray fungi’ (Strahlenpilze). These actinomycetes are able to produced secondary metabolites. The nanoparticles were also found to be non-toxic to the cells which continued to multiply even after the formation of the nanoparticles.

Viruses in Nanoparticle Synthesis: Biological synthesis of nanoparticles has been extended to biological particles like viruses, proteins, peptides and enzymes. Cowpea chlorotic mottle virus and cowpea mosaic virus have been used for the mineralization of inorganic materials ³⁵. Tobacco mosaic virus has shown to direct successfully the mineralization of sulphide and crystalline nanowires. One step further, peptides capable of nucleating Nano crystal growth have been identified from combinatorial screens and displayed on the surface of M13 bacteriophage ³⁶.

Applications: Silver nanoparticles have been used extensively in anti bacterial agents in the health industry, food storages, textile coating and number of environmental applications. Important to note, despite and decades of use, the evidence of toxicity of silver is not still clear ³⁷. The antibacterial property of silver nanoparticles has allowed its wide range of application from disinfecting devices and home appliances to water treatment ³⁷. Catalytic activities of nanoparticles differ from chemical properties.

Silver Dressing: Dressings play a major part in the management of wounds ³⁸. The antibacterial properties and the toxicity of silver to micro-organisms is well known, thus, now a days, silver is used in different kinds of formulations like surface coating agents, wound dressing, etc ³⁹.  The nanocrystalline silver dressings, creams, gel effectively reduce bacterial infections in chronic wounds.

Silver Toxicity: The toxicity from the silver is observed in the form of argyria when there is a large open wound and large amount of silver ions are used for dressing. There are no regular reports of silver allergy⁴⁰. Important to note despite and decades of use the evidence of toxicity of silver is not still clear. Wide applications of nanotechnology are possible in the fields like electronics, engineering, medicine, etc., but the possible side effects of nanoparticles have not been much studied. Hence, detailed study needs to carry out before the introduction of products related to nano medicine in the market ⁴¹.

Environmental Applications: The silver nanoparticles can be used for the treatment of water to avoid contamination of the environment ⁴². Environmental-friendly antimicrobial nanopaint can be developed ⁴². Inorganic composites are used as preservatives in various products ⁴³. Silver nanoparticles can be used for water filtration ⁴⁴. The silver nanoparticles used as a Anti bacterial agents and wide range of application from disinfecting devices and home appliances to water treatment ⁴⁵.

Future Prospects: Nanoparticles are widely used to improve the various catalytic reactions due to their novel physico-chemical properties as compared to there bulk size components. Because of its their wide applications, there is need to produce nanoparticles in industries. As stated above the physical and chemical methods of nanoparticle synthesis are having many disadvantages. Biological synthesis would be a preferred method because of its environmental friendly approach ⁴⁶.

However, very limited studies have been reported on the factors affecting or responsible for biosynthesis of metal nanoparticles. Synthesis of nanoparticles by using microorganisms has been developed over the last decades. It is known that the synthesis of nanoparticles using microorganisms is a quite slow process compared to physical and chemical approaches. Synthesis using microorganism is still on the laboratory scale. Efforts should be made to investigate the practical application of microorganism in nanoparticles production ⁴⁷.

The use of nanoparticles already established for some medical applications like wound infections, dressing, and treatment of preclinical stages. Recent research has revealed exciting new biological properties of NS that could be translated into new therapeutic and pharmacological treatments ⁴⁸. The full potential of this technology has yet to be discovered. The antibacterial, antifungal and antiviral properties of silver ions, silver compounds and silver nanoparticles have been extensively studied.

Silver is also found to be non-toxic to humans in minute concentrations. The microorganisms are unlikely to develop resistance against silver as compared to antibiotics as silver attacks a broad range of targets in the microbes ⁴⁹. Experimental trials are needed to understand the toxicity. There are some questions, which need to be addressed, such as, the exact mechanism of interaction of silver nanoparticles with the bacterial cells, how the surface area of nanoparticles influence its killing activity, use of animal models and clinical studies to get a better understanding of the antimicrobial efficiency of silver dressings, the toxicity if any of the silver dressings, etc.

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