Three nanoparticle suspensions were obtained from US Research Nanomaterials, Inc. (Houston, TX): ZnO (30e40 nm), CuO
(25e55 nm), and CeO2 (30e50 nm). All suspensions were at least 99% purity. Aliquots were withdrawn from the nanoparticle stock suspension and diluted in deionized water to a total volume of 2 L.