Measuring Proton Transport Diffusion in Silica Nanoparticles: Counterion Effects and Water-Regulated Transport Pathways

Abstract

Proton transport is a fundamental and ubiquitous chemical process. While self-diffusion can be measured under equilibrium conditions, quantifying transport diffusion under a concentration gradient remains experimentally challenging because of the ultrafast nature of proton motion. Here, we measure proton transport diffusion within colloidal silica nanoparticles by incorporating the proton-sensitive molecular rotor 4-(4-dimethylaminostyryl)-1-methylpyridinium (DSM). Upon stopped-flow mixing (<1 ms) with acid solutions, proton-associated species diffuse into the silica micropores and induce fluorescence changes recorded at 12.5 μs intervals. This method enables determination of apparent proton transport diffusion coefficients, D, despite rapid equilibration (0.1–1 s). Comparison among six acids (HCl, HBr, HI, HNO3, H2SO4, and HClO4) reveals pronounced counterion dependence, with D decreasing as the counterion size increases, even at elevated water mole fraction (x(H2O) = 0.179). H2O/D2O isotope experiments support a transition from vehicle-type motion of proton-associated species to Grotthuss-type hopping at low x(H2O) (0.023–0.131). The diffusivity then plateaus in a regime where the local microenvironment favors Grotthuss-type transport, but declines once x(H2O) exceeds 0.224. Even when Grotthuss-type transport dominates, the counterion effect remains evident, with an apparent activation energy of 0.361 eV. Hence, nanometer-range proton transport encounters additional kinetic barriers, in contrast to localized self-diffusion. This study provides a direct method to investigate proton transport diffusion, revealing under-recognized mechanisms affecting nanoscale proton transport.