Interactions of Citrate-Coated Silver Nanoparticles in the Rhizosphere.

The increasing use of silver nanoparticles (AgNPs) in commercial antimicrobial products presents an opportunity for increased environmental exposures. However, while the behavior of silver nanoparticles in surface waters and even ideal bulk soil environments is becoming increasingly more understood, little research on the behavior of these, or any nanoparticles, is being done in the rhizosphere, which has been identified as the most active portion of the soil in terms of biogeochemical processes. Because of the rhizosphere’s importance, not only to biogeochemical cycling but also for the overall ecosystem health of natural and agricultural soils, it is necessary to better understand how silver nanoparticles behave in this unique environmental compartment. Complex plant-microbe interactions that originate in the rhizosphere may be disrupted by silver nanoparticles, resulting in an overall less healthy soil ecosystem. Using dynamic light scattering (DLS) and electrophoretic mobility, we measured the hydrodynamic diameter and zeta potential (two indicators of particle stability) of citrate-coated AgNPs in various environmental media. To further investigate stability, dissolution studies were carried out by filtering NP suspensions through ultrafine membranes and quantifying ionic Ag (Ag_aq) via inductively coupled plasma mass spectrometry (ICP-MS). Two bacterial species, B. subtilis and E. coli, and maize, Z. mays, were exposed to a range of AgNP concentrations to determine the extent of growth inhibition. Data show that the high ionic strength media promote aggregation of the NPs and that they do not release appreciable amounts of ionic Ag (Aq_aq). Both bacterial species of were significantly inhibited at the highest AgNP concentration, and we suspect that strong adhesion of the NPs to maize roots will result in similar levels of inhibition.