New insight into naturally formed nanosilver particles: role of plant root exudates

Manufactured silver nanoparticles (nAg) have been extensively studied because of questions and concerns on their unique properties, behavior and associated impacts on environmental health and safety. In contrast, naturally formed nAg, as one long-existing source of nanoparticles, have been less noticed and investigated. In this study, we evaluated the ability of natural ligands in plant root zone – root exudates (RE) – in transforming silver ions (Ag+) to nAg and investigated the associated mechanism. We found that wheat RE had high reducing ability to convert Ag+ to nAg under light exposure. A further test on molecular weight (MW)-based RE fractions showed that the photo-induced reduction of Ag+ to nAg in pristine RE was mainly attributed to the 0–3 kDa fraction. Quantification of the silver species change over time suggested that Cl− played an important role in photoconversion of Ag+ to nAg through the formation and redox cycling of photoreactive AgCl. The accelerative function of AgCl as a photocatalyst was further demonstrated by the dramatically decreased generation of nAg in the dark. Potential electron donors for the photoreduction of Ag+ were identified to be reducing sugars and organic acids of low MW. Meanwhile, the stability of the formed particles was controlled by both low (0–3 kDa) and high (>3 kDa) MW molecules. This study provides new insight into the formation mechanism of metal nanoparticles mediated by RE, which may further our understanding of the biogeochemical cycling and toxicity of heavy metal ions in agricultural and environmental systems.