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 H₂. Our aim was to identify the most suitable indicator/Au-Pd NP system for the future development of a thin, wearable, and visual H₂ 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 H₂ 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 H₂ 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 H₂ 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 H₂, Au-Pd NPs, and resazurin on the color change response time within the resazurin/Au-Pd NP system revealed that the H₂-sensing elements can be optimized to achieve a faster or slower color change with H₂ by varying the relative amounts of resazurin and Au-Pd NPs in solution. The results from this study are significant for future optical H₂ sensor design.