A Method for in Situ Interfacial pH Detection

Functionalized silica-based surfaces are widely used across industries, from semiconductors to pharmaceuticals. Aminosilanes are commonly employed as coupling agents during surface functionalization to anchor diverse functional molecules. However, the surface modifications perturb interfacial physicochemical properties, resulting in a significant shift in interfacial pH compared to the bulk solution. This shift complicates direct measurement and accurate monitoring of interfacial conditions. To overcome this challenge, we functionalized glass surfaces with aminosilane-coupled pH-sensitive fluorescent dyes and utilized confocal microscopy to measure their fluorescence response to changes in bulk pH. Complementing these experiments, we developed a theoretical model describing equilibrium surface chemistry taking into account electrostatic interactions at aminosilane-functionalized glass interfaces. It revealed a linear relationship between interfacial and bulk pH, with the interfacial pH varying over a narrowed range compared to the bulk pH. Building upon these insights, we calibrate the fluorescence response of the grafted pH-sensitive dyes. This integrated approach enables precise and reliable in situ monitoring of interfacial pH under various conditions, demonstrating significant potential for environmental sensing and advanced material characterization.