The secondary reduction will mean capturing one more electron by silver atom to become Ag- which is impossible because it cannot hold an extra electron into its orbit. There are some vascular plants which store crystal metal and are called metallophytes, for instance, Brassica juncea, Medicago sativa, etc. They accumulate metal up to 13.6% weight in 72 h when it is available for absorption in the form of salt, like AgNO3 [72]. It is quite obvious that reduction of AgNO3 is followed by absorption which means that the plant contains some compounds which reduce Ag+ to Ag nanoparticles

of approximately 50 nm size. It has been demonstrated that the metals thus stored in the plants as nanocrystals are analytically pure to the lowest limit of detection by any instrument BIBW2992 see more like AAS. The sequestering of metal by plant from a large heap of sand, sediments and non-essential non-metals is a process that saves time and manpower. If bacteria and small plants are grown in such mining areas where a large heap of nanocrystal of metal ions is available, they can be easily taken up by them and harvested. The extraction of metal by conventional method

is a tedious task as it takes a long span of time; even then, it is not as pure as sequestered by plants. It has been reported by Blaylock et al. [73] that the addition of a chelating agent like ethylene diamine tetraacetate (EDTA) to the soil increases the bioavailability of the metal. It is true that EDTA forms a soluble complex with metal ions available but not the metal. The EDTA therefore acts as a carrier, not as a reductant. Since EDTA is not a selective chelating agent, it may hook up all metal ions regardless of their useful/harmful effect. If 4-Aminobutyrate aminotransferase the metal remains bound to a chelating agent, it is not available even to the plants and hence may cause a deficiency of certain essential trace metals in them. Haverkamp and Marshall [74] have studied the uptake of AgNO3, Na3Ag(S2O3)2,

Ag(NH3)2NO3 and their reduction to nanoparticles by B. juncea. Quantitative determination of Ag by AAS and XANES has been done. The reduction of metal depends on the chemicals present in the plant and the concentration of metal salts in the solution. Gold [75–77], silver [78, 79], copper [80] and gold-silver-copper alloy [81] nanoparticles have been reported to be present in the plants. Besides the plants, some microorganisms also induce the metal ions which are accumulated and translocated in different parts of the plants. Ni, Cu, Cd, Pb and Cr have not been exclusively found to yield nanoparticles, perhaps these are also not common metals required by the plants for their growth. The uptake and distribution of metal ion/metal itself in the plant is a matter of debate. It is not clear whether nanocrystals are formed outside of the plants and then transported through the membrane into various parts or if the nanoparticles are formed within the plant by the reduction of the metal salt.