Gold and silver nanoparticles can be synthesised by plants, for example to recover metals from low levels in soil or mine tailings and to produce nanoparticles. Plants can even be used to produce catalysts of specific composition, perhaps even those difficult to synthesise by traditional methods. The nanoparticles may contain Au, Ag and Cu as an alloy.

Phytoextraction is a biological routes used by vascular plants to the synthesis of these particles. Many studies have looked at metal uptake by plants, particularly with regard to phytoremediation and hyperaccumulation. But few have distinguished between metal deposition and metal salt accumulation.

Hydroponically grown Brassica juncea has been reported to uptake of AgNO3, Na3Ag(S2O3)2, and Ag(NH3)2NO3 solutions and the conversion of these salts to silver metal nanoparticles of 2–35 nm. It is found that there is a limit on the amount of metal nanoparticles that may be deposited, of about 0.35 wt.% Ag on a dry plant basis, and that higher levels of silver are obtained only by the concentration of metal salts within the plant, not by deposition of metal.
This process is found to be controlled by the total reducing capacity of the plant for the reduction potential of the metal species and limited to reactions occurring at an electrochemical potential greater than zero volts as against the standard hydrogen electrode voltage.