With the increasing production and use of silver nanoparticles (AgNPs) in consumer products and for biomedical and research applications, it is inevitable that AgNPs will be released into natural aquatic systems.  Since the concentration of AgNPs in aquatic environments is likely to be much lower than that of naturally occurring colloids (NOCs), heteroaggregation between AgNPs and NOCs is expected to be the dominant aggregation process (compared to homoaggregation) that controls the fate and effects of AgNPs in these systems. In this study, the effects of heteroaggregation between citrate-coated AgNPs and hematite nanoparticles (HemNPs), a model NOC, on the antimicrobial activity of AgNPs toward Escherichia coli were investigated. Our toxicity experiments are conducted at an AgNP concentration at which the background dissolved Ag ion concentration is not high enough to inhibit the growth of E. coli cells. Our results show that the antimicrobial activity of AgNPs was decreased when heteroaggregation between AgNPs and HemNPs occurred.  Furthermore, a larger reduction in the AgNP antimicrobial activity was observed when the HemNP/AgNP number concentration ratio was increased. At the highest HemNP/AgNP ratio employed (= 13.0), most of the AgNPs were observed, through cryogenic transmission electron microscopy imaging, to be in direct contact with HemNPs.  Interestingly, some AgNPs were even surrounded by HemNPs to form �raspberry-like� heteroaggregates.  In light of our observations, we speculate that, when AgNPs are surrounded by HemNPs through heteroaggregation, the AgNPs are prevented from coming into direct contact or close proximity with the bacterial cells, which has been reported to enhance the cytotoxicity effects of AgNPs.

Heteroaggregation reduces antimicrobial activity of silver nanoparticles: Evidence for nanoparticle�cell proximity effects

248th National Meeting (2014)

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