Particles measuring approximately in the range 1 to 100 nm are called as nanoparticles. They can be obtained in a variety of sizes and shapes like cylindrical, rhombic or circular by controlling conditions of synthesis process. Historically, the synthesis of nanoparticles started by employing physical and chemical methods like reduction, sol-gel, ball milling, sputter deposition, laser ablation, self assembly like micelles and reverse micelles, lithography, ultrasonic technique, immobilization of particles in some matrix like glass, zeolites or polymer micelles and electrochemical techniques. However synthesis of nanoparticles using biological systems like microorganisms and plant extracts is novel approach. It is also cheap and ecofriendly. Physicochemical methods use toxic reactants and comparatively costly. The only disadvantage of biosynthesis is that it is very time taking; it may require seven days to form nanoparticles. Bacteria and fungi have been used for the synthesis of metal nanoparticles. Using plant extract is cheaper than microorganisms as it does not require culture preparation or maintenance of aseptic conditions. Nanoparticles find immense applications in Agriculture, Environment, Medicine, Biotechnology and Industry. They have been used in biosensors, biomedical devices, drug delivery systems, electronics, optics, optoelectronics, storage devices, reprography, solar batteries, photoelectrochemical devices, semiconductor devices, catalysis and to generate supermagnetism and superchilling conditions. Metal nanoparticles (selenium, tellurium, uranium, zirconium, zinc, palladium, silver, gold, copper, iron and nickel) have been extensively synthesized and studied because of their unique physicochemical properties and large number of applications. Nanoscale biosynthesis of two noble metals silver and gold is of particular interest and importance.

Biosynthesis process and characterization: In order to biosynthesize noble metal nanoparticles of particular shape, size and properties, specific methodologies have been formulated. Biosynthesis processes of nanoparticles of gold, silver and their alloy by bacteria, actinomycetes, fungi and yeasts have been developed. Before actual synthesis, the growth conditions of producing culture are physicochemically optimized. Knowledge regarding cellular, metabolic processes and genetics of the microbe if possible should be available. This information is useful for scaling up particular biosynthetic process. Isolated pure culture is inoculated in growth medium containing silver nitrate (AgNO3) solution. After defined incubation period silver nanoparticles are formed.

Nanoparticles can be observed and studied under scanning or transmission electron microscope. Microscopic study reveals location (periplasmic or cytoplasmic or cell wall) and shape of synthesized nanoparticles. Nanoparticles can be characterized further employing sophisticated techniques of Physics such as Photoluminescence spectra, X-ray diffraction or Atomic absorption spectroscopy. To increase the rate of reaction or to control size and monodispersity of nanoparticles formed; combinatorial approach can also be used. Approach can be heat treatment or cold storage or treating with microwave radiation or a genetic manipulation. Scientists have to try such troublesome efforts until nanoparticles of desired properties are obtained. The same methods are used in synthesis of gold nanoparticles except silver nitrate is replaced by aurium chloride (AuCl4). Bacteria like Pseudomonas, Klebsiella, Bacillus, Lactobacillus; fungi Verticillium, Fusarium, Actinomycete Thermomonospora; yeasts Torulopsis, Saccharomyces, Schizosaccharomyces have been used for synthesis of gold and silver nanoparticles.

Both gold and silver nanoparticles have also been prepared using plant (leaf) extracts from clove, onion and Aloe vera. Compared to microbial culture, the reduction time required is of few minutes, so the formations of nanoparticles do not take hours or days. This biosynthesis is very advantageous for rapid synthesis of nanoparticles.

Mechanism of biosynthesis: Microbes are since long times are known to produce extracellular or intracellular organic (metachromatic or PHB) and inorganic (magnetite, silicate or calcium carbonate crystals) compounds. By considering the microbial cell as a factory, we have modulated their metabolic activities at laboratory level for synthesis of nanomaterials. The principle of formation of nanoparticles is based on the microbial remediation of toxic chemicals in environment via reduction of metal ions. Some microbes produce extracellular enzymes with redox potential act as electron shuttle for metal reduction.

Applications of gold and silver nanoparticles:

Gold nanoparticles:
o In making transistors
o Photothermal agents
o As a catalyst to breakdown volatile organic compounds
o In therapeutics, for treatment of arthritis, Alzheimer's disease
o Efficient drug delivery systems
o For detection of cancerous tumors
o Immunostaining
o Biosensors

Silver nanoparticles:
o In manufacture of odor resistant fabrics
o Surgical instruments and dressings
o Catalysis
o Optoelectronics and home appliances
o Antifungal and antibacterial agent
o Water treatment
o Power cells and batteries

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