Indicator Dyes and Catalytic Nanoparticles for Irreversible Visual Hydrogen Sensing

Using ultraviolet–visible (UV–vis) absorption spectroscopy, we have tested the reactivity of various indicator molecules combined with catalytic bimetallic gold–palladium nanoparticles (Au–Pd NPs) in solution for an irreversible and visual response to H 2 . Our aim was to identify the most suitable indicator/Au–Pd NP system for the future development of a thin, wearable, and visual H 2 sensor for noninvasive monitoring of in vivo Mg-implant biodegradation in research and clinical settings with fast response time. The indicators studied were bromothymol blue, methyl red, and resazurin, and the reactions of each system with H 2 in the presence of Au–Pd NPs caused visual and irreversible color changes that were concluded to proceed via redox processes. The resazurin/Au–Pd NP system was deemed best-suited for our research objectives because (1) this system had the fastest color change response to H 2 at levels relevant to in vivo Mg-implant biodegradation compared to the other indicator/Au–Pd NP systems tested, (2) the observed redox chemistry with H 2 followed well-understood reaction pathways reported in the literature, and (3) the redox products were nontoxic and appropriate for medical applications. Studying the effects of the concentrations of H 2, Au–Pd NPs, and resazurin on the color change response time within the resazurin/Au–Pd NP system revealed that the H 2 -sensing elements can be optimized to achieve a faster or slower color change with H 2 by varying the relative amounts of resazurin and Au–Pd NPs in solution. The results from this study are significant for future optical H 2 sensor design.