Measurement of ZnO Nanoparticles Using Diffusive Gradients in Thin Films: Binding and Diffusional Characteristics

Rapid growth in finding new applications for manufactured nanomaterials (MNM) has recently been accompanied by awareness about their related adverse toxicological and environmental impacts. Due to their intrinsic nature, measuring available concentrations of MNMs in the environment is a major challenge. This research is a launching point toward filling this gap, as it presents the potential of the well-established diffusive gradients in thin films (DGT) technique to determine MNMs concentrations in situ. Two binding layers commonly used in DGT devices were shown to be able to bind ZnO nanoparticles (ZnO NPs). The use of different types of diffusive layers demonstrated the critical role of their pore size for selective function of the DGT devices. The ZnO NPs can pass through the open pore diffusive layer used in standard DGT devices and be retained by the binding resin layer. However, the diffusion of ZnO NPs can be prevented when a 1000 MWCO (molecular weight cut off) dialysis membrane is placed in the front of the diffusive gel layer. A combination of two or more DGT devices with known diffusive layer properties should enable deduction of concentrations of available ZnO NPs in the environment. Unlike metal ions, determining diffusion coefficient values for ZnO NPs is challenging and greatly affected by shape, morphology, and solution-induced changes of the particles. Attenuated total reflection Fourier-transform infrared spectroscopy (ATR-FTIR) demonstrated that retention of ZnO NPs by Chelex and Metsorb binding layers occurs through chemisorption. The superior uptake kinetic for Chelex indicates that it is a better candidate for further development of DGT devices to measure ZnO NPs. These initial results are promising and important for further developing the DGT technique to measure available concentrations of manufactured nanomaterials in the different environmental media (waters, soils, and sediments). Further experiments investigating the effects of pH, ionic strength, and solution chemistry on the performance of DGT for measuring MNM concentrations are needed.