Heteroaggregation between silver nanoparticles (AgNPs) and naturally occurring colloids is a critical process that controls the environmental impacts of AgNPs. In this study, the effect of heteroaggregation between citrate-coated AgNPs and hematite nanoparticles (HemNPs) on the antimicrobial activity of AgNPs toward Escherichia coli is investigated. Our experiments are conducted at a AgNP concentration of 2.2 mg/L, at which the dissolved silver concentration of 19 g/L in the bulk solution is not high enough to inhibit E. coli growth. At these AgNP and dissolved silver concentrations, the bacteria grew in samples containing AgNP-HemNP heteroaggregates while growth was completely inhibited in samples containing only AgNPs, indicating that heteroaggregation can reduce the antimicrobial activity of AgNPs. Furthermore, the degree of reduction in the AgNP antimicrobial activity is elevated when the HemNP/AgNP number concentration ratio is raised from 0.4 to 13.0. At a high HemNP/AgNP ratio (13.0), most AgNPs are observed through cryogenic transmission electron microscopy to attach to HemNPs, and some are surrounded by multiple HemNPs. These observations suggest that heteroaggregation can decrease AgNP antimicrobial activity by physically preventing AgNPs from coming into direct contact or close proximity with E. coli cells.

Enviro. Sci. Technol. Lett. 2014, 1, 361-366