The study of nanomaterial has been emerging dramatically throughout the world in the 21st century due to their incredible applications in all spheres of human life (). It has opened several arms in the development of new nanomaterials and examining their properties by tuning the particle size, shape and distribution (,). Metal nanoparticles have been extensively studied due to their specific characteristics such as catalytic activity, optical properties, electronic properties, antimicrobial properties and magnetic properties (). Traditionally UV irradiation, aerosol technologies, lithography, laser ablation, ultrasonic fields, and photochemical reduction techniques have been used successfully to produce various metal nanoparticles such as gold, silver, platinum and palladium. However, considering the fast growth in the usage of nanomaterials in diverse fields, there is an urgent need to develop clean, nontoxic, simple and eco-friendly procedures for their synthesis.
Synthesis of noble metal nanoparticles, in particular silver nanoparticles (AgNPs) synthesis using natural organism has become a major research area in the field of nanotechnology. This may due to their simplicity of procedures, stability of nanoparticles, and their potential applications in chemical sensing, biological imaging, antimicrobial, gene silencing, drug delivery (). Recently, several studies have reported natural polymers such as chitosan, starch and tannic acid as reducing agents for the synthesis of silver and gold nanoparticles (,). A vast array of biological resources including plants, algae, fungi, yeast, bacteria, and viruses has been studied so far for the intra and extracellular synthesis of silver, gold, platinum and titanium nanoparticles in different sizes and shapes were tabulated in . The major drawback of metal nanoparticles synthesis using plant extracts as reducing and stabilizing agent. This differs due to significant variation of biochemical compositions present in the plant extract of the same species differ from other part of the world. Therefore, identifying the biomolecules responsible for mediating the nanoparticles synthesis is a problem to overcome ().
Human cells are heterotrophic in nutrition. They need to be provided with energy for their survival. Human cancer cells and non-cancerous cells intracellularly produced some metal nanoparticles in vitro conditions that mimic their natural cellular environment. With an incubation of 1 mM of tetrachloroaurate solution, human cancer cells like SiHa (malignant cervical epithelial cells), SKNSH (human neuroblastoma) and HeLa (malignant cervical epithelial cells), and non-cancer cells like HEK-293 (non-malignant human embryonic kidney cells) synthesized gold nanoparticles in the size range of 20~100 nm. These nanoparticles were located in the cytoplasm and in the nucleus of the cells. The dimensions of these particles were smaller in nucleus compared to the cytoplasmic particles (,).
Indeed, a number of bacteria, fungi and yeast have been well-known for the synthesis of non-toxic noble nanoparticles. However the microbial mediated synthesis of nanoparticles is not industrially feasible as it requires expensive medium and maintenance of highly aseptic conditions. Hence, exploration of the plant systems as the potential bio-factories has gained heightened interest in the biological synthesis of nanoparticles. Hence, exploration into plant systems has been considered to be a potential bioreactor for synthesis of metal nanoparticles without using toxic chemicals.
Algae are eukaryotic aquatic oxygenic photoautotrophs, which produce its food through photosynthesis using sunlight producing oxygen as their by-products. Their photosynthesis machinery has been evolved from cyanobacteria via endosymbiosis. They are predominant primary producers in many aquatic environments. Among various algae, sp. was found to accumulate various heavy metals such as cadmium, uranium, copper, and nickel. is a single-celled green algae belonging to phylum , and the extracts of showed anti-tumor properties (). The dried algal cells were found to have a strong binding ability towards tetrachloroaurate (III) ions to form algal-bound gold, which was subsequently reduced to form Au(0). Nearly 88% of algal-bound gold attained metallic state and the crystals of gold were accumulated in the interior and exterior of cell surfaces with tetrahedral, decahedral and icosahedra structures. Though chemical synthesis produces nanoparticles more rapidly with well-controlled shape, size and dispersity, the use of toxic and expensive chemicals as reducing and capping agents restricts its use in biomedical applications.
Viruses are unicellular organisms that hijack the replication machinery of the host cell and suspend most endogenous cellular activity. Their structure consists of nucleic acid, either DNA or RNA, which is surrounded by a protein shell that may or may not contain a lipid envelope. Viral genomes can be non-segmented, consisting of a single nucleic acid molecule, or segmented, consisting of more than one nucleic acid molecule. The nucleic acid molecules of a virus can be contained within a single virus or separated into multiple viruses. Viruses do not express their own ribosomal RNA. Viruses hold great promise in assembling and interconnecting novel nanosized components, allowing developing organized nanoparticle assemblies. Due to their size, monodispersity, and variety of chemical groups available for modification, they make a good scaffold for molecular assembly into nanoscale devices. Virus based nanocomposites are useful as an engineering material for the construction of smart nano-objects because of their ability to associate into desired structures including a number of morphologies. Viruses exhibit the characteristics of an ideal template for the formation of nano-conjugates with noble metal nanoparticles.
In the recent years, noble nanoparticles have attracted and emerged in the field of biology, medicine and electronics due to their incredible applications. There were several methods have been used for synthesis of nanoparticles such as toxic chemicals and high energy physical procedures. To overcome these, biological method has been used for the synthesis of various metal nanoparticles. Among the nanoparticles, silver nanoparticles (AgNPs) have received much attention in various fields, such as antimicrobial activity, therapeutics, bio-molecular detection, silver nanocoated medical devices and optical receptor. Moreover, the biological approach, in particular the usage of natural organisms has offered a reliable, simple, nontoxic and environmental friendly method. Hence, the current article is focused on the biological synthesis of silver nanoparticles and their application in the biomedical field.
Soil is an extensively explored ecological niche for sources of microorganisms that are involved in various interactions. Among these, Metal-microbe interactions have important roles with fascinating applications such as bioremediation, biomineralization, bioleaching and microbial corrosion. However, recently that microorganisms have been explored as potential biofactory for synthesis of metallic nanoparticles such as cadmium, gold and silver (,). Among the microbes, the use of bacteria, like in this study, is rapidly gaining importance due to its growing success, ease of handling and genetic modification. Klaus . demonstrated that the AG259, isolated from a silver mine, produced silver nanoparticles of well-defined size and distinct morphology within the periplasmic space of the bacteria (). In recent study various bacterial strains such as , , and could effectively induce the synthesis of silver nanoparticles (,). Biosynthetic methods can be categorized into intracellular and extracellular synthesis according to the place where nanoparticles are formed. Of which, the extracellular synthesis of nanoparticles is still continually emerging in order to understand the mechanisms of synthesis, easy downstream processing and rapid scale-up processing. For these reasons, a bacterial system could prove to be a potential source for the extracellular synthesis of metal nanoparticles instead of physical and chemical procedures.
The synthesis of metal nanoparticles is the current research trend because they exhibit different physical and chemical properties compared to their bulk metals (Gratzel, 2001 and Xia, et al., 2003). This is due to the large surface area obtained in the nanoparticles, where the chemical properties of the metals are intensified (Ray, et al., 2011). In the past decade there has been a tremendous amount of research interest in nano-materials with respect to its production, properties and applications (Narayanan & Sakthivel, 2010 and Singh, et al., 2011). Artificially made metal-NPs are typically produced on a small laboratory scale using methods such as chemical vapor deposition, irradiation or chemical reduction of metal salts. However, most of these processes give rise to harmful byproducts (Mansoori, 2005). Therefore, stress is laid on benign biosynthesis process which results in environment friendly nanoparticles of biological origin. The use of microorganisms as nano-factories enables us to use simple large scale production of nanomaterials, which does not give rise to toxic waste products. Microbial assisted biosynthesis of NPs is therefore a rapidly progressing area of nano-biotechnology (Jaidev & Narasimha, 2010).
Cell mass or extracellular components from fungi, such as , , , and (,) have been utilized for the reduction of silver ions to AgNPs. Filamentous fungi possess some distinctive advantages over bacteria due to high metals tolerance, wall binding capacity, and intracellular metal uptake capabilities (). Previously, Vigneshwaran . () also showed that the use of resulted in the accumulation of silver nanoparticles on the surface of its cell wall when incubated with silver nitrate solution for 72 hr. The average particle size was found to be 8.92 nm. The intracellular synthesis of gold nanoparticles produced by () showed morphologies of spherical, hexagonal and rods in the size range of 8.92~25 nm. Fungi are more advantageous compared to other microorganisms in many ways. Fungal mycelial mesh can withstand flow, pressure, agitation and other conditions in bioreactors or other chambers compared to plant materials and bacteria. These are fastidious to grow, easy to handle and easy for fabrication. The extracellular secretions of reductive proteins are more and can be easily handled in downstream processing. Since the nanoparticles precipitated outside the cell is devoid of unnecessary cellular components, it can be directly used in various applications.