L.C.C. Elliott, S.M. Stavis
National Institute of Standards and Technology, University of Maryland,
Keywords: environmental, water, contamination, materials characterization, microspectroscopy, nanotechnology, microdeposition
Summary:We seek measurement solutions to an open problem with potentially serious consequences for both water purity and commercial nanotechnology. Manufacturers are increasingly integrating nanoparticles into consumer products, which eventually disintegrate and release the nanoparticles into environmental and drinking water. However, methods are lacking to quantitatively, sensitively, and routinely measure the size and composition of nanoparticle contaminants in complex aqueous media. We explore the limits of optical microspectroscopy to do so, with the goal of developing measurement methods for environmental monitoring, waste treatment, and point-of-use purification of water. To prepare test samples, we disperse metal and metal oxide nanoparticles of varying sizes into an aqueous suspension. Deposition and evaporation of microdroplets of the suspension on imaging substrates, as Figure 1 shows, enables measurement of nanoparticle concentration and correlative optical and electron microspectroscopy of nanoparticle size and composition. Nanoparticle properties, substrate–suspension interactions, and drying parameters influence the microdeposition process and resulting nanoparticle patterns. Darkfield and hyperspectral darkfield optical microscopy reveal differences in the scattering of light from nanoparticles of varying size and material composition. Imaging measurements using a basic darkfield optical microscope with a halogen lamp and color camera can distinguish between gold and silver nanoparticles with diameters of approximately 40 nm and approximately 100 nm, as Figure 2 shows. Hyperspectral darkfield optical microscopy with dispersed wavelength detection shows the potential to discriminate between some metal oxide particles and resolves size differences of tens of nanometers in noble metals, in a format that is suitable for environmental monitoring and waste-water treatment with higher throughput than is currently possible, as Figure 3 shows. Correlative scanning electron microscopy and energy-dispersive X-ray spectroscopy validate the optical measurements and define their capabilities and limits, as Figure 3 shows. These results advance the development of practical and quantitative optical measurements of nanoparticles as water contaminants. References 1. Gottschalk, F. & Nowack, B. The release of engineered nanomaterials to the environment. J. Environ. Monit. 13, 1145–1155 (2011). 2. Gottschalk, F., Sun, T. & Nowack, B. Environmental concentrations of engineered nanomaterials: Review of modeling and analytical studies. Environ. Pollut. 181, 287–300 (2013).