Dry Powder Assay Rapidly Detects Metallic Nanoparticles in Water by Measuring Surface Catalytic Reactivity

We designed the "catalytic reactivity to nanoparticle" assay (CRNP), which uses a dry powder containing methylene blue (MB) and sodium borohydride (NaBH₄) to rapidly (2 min) detect metallic nanoparticles in water. Tested with gold (Au) NPs in water, the CRNP response was linearly and reproducibly correlated to the NP surface-area concentration and has a detection limit of 0.3 m²/m³ as the equivalent surface area of Au NPs. We described the heterogeneous catalytic mechanisms on the NP surface by treating the NPs as electrodes, which store and transfer electrons, and comprehensively simulated the kinetics of borohydride hydrolysis, MB reduction, and leuco methylene blue (LMB) oxidation. CRNP was able to assess the catalytic reactivity of multiple engineered NP species in water, including Au, silver, palladium, platinum, and copper oxide (CuO), and quantify them with pre-established calibration curves. In water samples containing known or unknown NP species, CRNP can be reported as an equivalent surface area of gold NPs per volume of solution and directly quantifies NP reactivity in response to electron mediated stimuli, which may become relevant to the environmental fate or safety of nanomaterials.