Whole-cell Escherichia coli-based bio-sensor assay for dual zinc oxide nanoparticle toxicity mechanisms

•Two toxicity mechanisms occur at the surface of zinc oxide nanoparticles.•Zinc oxide nanoparticles dissolve and release toxic zinc ions.•Zinc oxide nanoparticles produce photo-electrons in ultra violet light generating free radicals.•Both mechanisms can be detected using physical methods with limitations.•A whole cell biosensor using E. coli is an accurate and sensitive alternative.

A whole-cell biosensor assay for dual ZnO nanoparticle toxicity mechanisms has been developed based on the transcriptional response of Escherichia coli to: (1) Zn2+ from ZnO nanoparticle dissolution with genes zntA (Zn2+ efflux) and znuABC (Zn2+ uptake); and (2) redox stress from ZnO nanoparticle photo-electron production under ultraviolet light with genes soxS and katG. Both processes occur in a dispersion of ZnO nanoparticles leading to toxicity. ZnO nanoparticle dissolution was measured independently by ICP-MS and photo-radical generation was confirmed by the stochiometric reduction of the redox dye, 2, 6-dichloroindolphenol (DCPIP). The whole-cell biosensor can detect both toxicity mechanisms and is a species-specific assay capable of discriminating between ZnO nanoparticles and the Zn2+ dissolution product.