We have previously demonstrated (2010) that coatings on ENP are a key determinate of the interactions between bacterial cells and ENP as well as the expression of toxicity by the ENP. We showed that the toxicity of nano Fe0 (NZVI), which is only observed during cell-ENP contact, was prevented by both engineered and natural organic matter (NOM) coatings on NZVI. TEM microscopy revealed that the coatings prevented direct contact between NZVI and the cells. The implication is that the coatings prevent ENP-cell contact through electrosteric and/or electrostatic mechanisms.
While both natural and engineered coatings decreased the toxicity of NZVI towards cells, in the case of TiO2 ENP, polymer coatings had little effect on the toxic behavior while NOM disrupted toxicity (Li et al, in review). The mechanism of this coating-specific disruption of toxicity was linked to scavenging of reactive oxygen species by NOM and is likely the result of aromatic groups in NOM that are absent in the engineered polymer coatings. These findings together highlighted the importance of understanding the mechanisms of toxicity as well as the mechanisms by which coatings interfere with the toxic expression of ENP.
We recently addressed the impact of coatings on the toxicity of AgNP, an ENP that, unlike NZVI which is toxic through direct attachment to cells and TiO2 which produce (photochemically) toxic, reactive oxygen species, produce a toxic silver ion through dissolution in aqueous media. Surprisingly, disruption of AgNP cytotoxicity was only observed when NOM was added in the aqueous phase, independent of AgNP addition and not when present as a coating on AgNP. AgNP were toxic towards E. coli with polymer regardless of whether it was added as a coating or in the aqueous phase. Studies with silver ion alone show that NOM in solution reduced its cytotoxicity by reducing the concentration of Ag+. Addition of NOM with higher sulfur content increased the protective effect of NOM. This finding suggests that NOM is protective of cells from AgNP toxicity by scavenging toxic silver ion on sulfur groups associated with the NOM. Taken together, our work with the coatings show that the type of macromolecule, and not merely its presence) is important for determining the toxicity of ENP. Moreover, the results show that understanding the mechanisms of toxicity expression and the disruptive mechanisms of the coatings will be important for determining the risks associated with ENP in the environment as well as the engineering of relatively benign particles.