A comparative interlaboratory study on photocatalytic activity of commercial ZnO and CeO₂ nanoparticles

Photocatalytic activity (PCA) was one of a number of physicochemical end points identified by the Organisation for Economic Cooperation and Development (OECD) as relevant to environmental safety and human health as part of their Sponsorship Programme for the Testing of Manufactured Nanomaterial. Photoactive surfaces can produce reactive oxygen species including free radicals which have the potential to cause oxidative stress in tissue or even oxidative damage to DNA. Here we report a study that involved three laboratories in Australia that independently characterised the PCA of commercially available ZnO and CeO₂ NPs provided by the OECD programme. This inter-laboratory comparison found that PCA is a stable characteristic of NPs which was insensitive to variations in interlaboratory protocols and with ZnO NPs being more photoactive (by an order of magnitude) than CeO₂ NPs. Comparisons were made between NPs of different sizes and the effect of the presence or absence of a surface coating on PCA. Because the competition between surface and volume effects determined PCA, a critical particle size was found for CeO₂ NPs to achieve maximum PCA. The presence of a surface coating appeared to significantly mitigate, but not eliminate PCA.