Nanoparticles synthesis is done by two major routes, which include classical synthesis and green synthesis. The green synthesis techniques generally utilize relatively non-toxic chemicals non-toxic solvents, biological extracts and systems. Biological methods are considered safe and ecologically sound for the nanomaterial fabrication as an alternative to conventional physical and chemical methods.
Nanoparticles
Gold, silver, and copper have been used mostly for the synthesis of stable dispersions of nanoparticles, which are useful in areas of photography, catalysis, biological labeling, photonics, optoelectronics and surface-enhanced Raman scattering (SERS) detection.
Biological route
Biological routes to the synthesis of these particles have been proposed by exploiting microorganisms and by vascular plants. The functions of these materials depend on their composition and structure. Plants have been reported to be used for synthesis of metal nanoparticles of gold and silver and of a gold-silver-copper alloy. Of this colloidal silver is of particular interest because of its distinctive properties such as good conductivity, chemical stability, and catalytic and antibacterial activity.
Biomolecules
Researchers report that biomolecules like protein, phenols and flavonoids not only play a role in reducing the ions to the nanosize, but also play an important role in the capping of the nanoparticles. The reduction of Ag+ ions by combinations of biomolecules found in these extracts such as vitamins, enzymes/proteins, organic acids such as citrates, amino acids, and polysaccharides is environmentally benign, yet chemically complex.
Mechanism
The mechanism for the reduction of Ag ions to silver could be due to the presence of water-soluble antioxidative substances like ascorbate. This acid is present at high levels in all parts of plants. Ascorbic acid is a reducing agent and can reduce, and thereby neutralize, reactive oxygen species leading to the formation of ascorbate radical and an electron. This free electron reduces the Ag+ ion to Ag0.
It has been reported that ionic silver strongly interacts with thiol group of vital enzymes and inactivates them. Experimental evidence suggests that DNA loses its replication ability once the bacteria have been treated with silver ions. The antibacterial effect of nanoparticles can be attributed to their stability in the medium as a colloid, which modulates the phosphotyrosine profile of the bacterial proteins and arrests bacterial growth.
Synthesis from herb
Indian researchers at Patna University have biosynthesised silver nanoparticles from Desmodium triflorum. Desmodium triflorum is a wild much branched slender diffused herb with trifoliate leaves occurring as small under herb found in grasslands, fields, and agricultural lands forming a green turf on the ground. The dry plant was powdered, added with distilled water, heated and the extract was added to AgNO3 solutions. The bioreduction of Ag+ ions took place . The solution containing the signatory color of AgNPs (dark brown) was dryed in oven to get powders of silver nanoparticles. Thus stable and spherically shaped nanoparticles of average size ~10nm were synthesized using desmodium plant. The green synthesis of AgNPs fulfills all the three main steps, which must be evaluated based on green chemistry perspectives, including selection of solvent medium, selection of environmentally benign reducing agent and selection of nontoxic substances for the AgNPs stability. The study further showed that Ag nanoparticles presented good antibacterial performance against common pathogens. The nanoparticles when combined with the antibiotics show synergic effect in suppressing growth of antibiotics.