Biogenic Synthesis of Au Nanoparticles Using Vascular Plants

Abstract

The known metallophytes Brassica juncea (B. juncea) and Medicago sativa (M. sativa) were investigated for their ability to accumulate and sequester gold (Au) from aqueous solutions of KAuCl4. Once sequestered, some of the metal was stored as nanoparticles, throughout the epidermis, cortex, and vascular tissue for both species, but predominantly located in the xylem parenchyma cells. Nanoparticle size distribution within the plant tissues was determined. In general, particle sizes ranged between 2 nm to 2 μm in B. juncea and 2 nm to 1 μm in M. sativa and was location dependent; root located nanoparticles had similar size distributions in both species, whereas the distribution within above ground tissues differed between M. sativa and B. juncea, with B. juncea showing a much broader range of particle sizes. Au(0) nanoparticles were also formed ex vivo following contact between root exudates and an aqueous solution of KAuCl4 resulting in the reduction of Au(III) to Au(0). The largest proportion of particles was in the range 5−10 nm (B. juncea) and 10−20 nm (M. sativa). The mechanism of growth of Au(0) nanoparticles using live plants, both in vivo and ex vivo is consistent with Turkevich [Gold Bull. 1985, 18 (3), 86−91], who suggested that the process of Au(0) particle formation involved the interplay of crystal nucleation, growth, and coagulation.