Kevin M. Metz, Andrew N. Mangham, Matthew J. Bierman, Song Jin, Robert J. Hamers, and Joel A. Pedersen

Once released into the environment, engineered nanomaterials may be transformed by microbially mediated redox processes altering their toxicity and fate. Little information currently exists on engineered nanomaterial transformation under environmentally relevant conditions. We developed an in vitro biomimetic assay for investigation of nanomaterial transformation under simulated oxidative environmental conditions. The assay is based on the extracellular hydroquinone-driven Fenton’s reaction used by lignolytic fungi. We demonstrate the utility of the assay using CdSecore/ZnSshell quantum dots (QDs) functionalized with poly(ethylene glycol). QD transformation was assessed by UV-Visible spectroscopy, inductively-coupled plasma-optical emission spectroscopy, dynamic light scattering, transmission electron microscopy (TEM), and energy dispersive x-ray spectroscopy (EDX). QDs were readily degraded under simulated oxidative environmental conditions: the ZnS shell eroded and cadmium was released from the QD core (Fig. 1a,b). TEM, electron diffraction analysis and EDX of transformed QDs revealed formation of amorphous Se aggregates (Fig 1c). The biomimetic hydroquinone-driven Fenton’s reaction degraded QDs to a larger extent than did H2O2 and classical Fenton’s reagent (H2O2 + Fe2+). This assay provides a new method to characterize transformations of nanoscale materials expected to occur under oxidative environmental conditions.

Figure 1. (a) UV-visible absorption spectra of PEG5000-CdSecore/ZnSshell quantum dots (QDs) exposed to methoxyhydroquinone (MHQ)-driven Fenton’s reaction. All reactions were conducted in 10 mM acetate buffer (pH 4.1). (b) Concentrations of Cd from PEG5000-QDs exposed to H2O2 and (MHQ-driven) Fenton’s reagent, then filtered through a 10-kDa centrifugal concentrator (retaining particles with diameters ≥ 3 nm). (c) Transmission electron micrograph of nanoparticle aggregates obtained after overnight exposure of PEG5000-CdSecore/ZnSshell QDs to the MHQ-driven Fenton’s reaction. The dotted circle in (c) represents the area used in analysis by selected area electron diffraction (inset) and energy dispersive X-ray spectroscopy (not shown). Figure adapted from Metz et al. Environ. Sci. Technol.43, 1598-1604, 2009.