Precise near-wall pH measurement in pressure-driven and electrically-driven flows using nanoscale laser-induced fluorescence imaging

The present study developed a near-wall pH measurement technique based on nanoscale laser-induced fluorescence to evaluate the spatio-temporal distributions of pH in the vicinity of a glass-solution interface. Fluorescein sodium salt, a pH-sensitive dye, was dissolved in working fluids and excited by an evanescent wave generated by total internal reflection of a laser beam. Near-wall pH distributions were determined based on a ratiometric calibration using a high-pH buffer solution (pH 10) as a reference. The proposed technique was applied to both pressure-driven and electroosmotic flows to investigate the effects of interfacial potential and external electric field on proton concentration. The near-wall pH in pressure-driven flow was lower than the bulk pH by ∼0.2 due to the local excess of protons attracted towards the negatively charged glass wall. This pH shift was especially noticeable in solutions with low ionic strength, which indicates that the near-wall pH is closely related to the thickness of the electric double layer. The pH shift in electroosmotic flow was smaller than that in pressure-driven flow by ∼0.02 and decreased with electric field strength. In addition, a time-series measurement clearly visualized the temporal change in non-uniform pH distributions in electroosmotic flow at a spatial resolution of 5.2 × 5.2 μm. The measurement uncertainty was estimated to be 0.16 pH unit at 95% confidence level. These results demonstrated the feasibility of the present technique to evaluate spatio-temporal proton concentrations within the space 10-100 nm away from a solid-liquid interface. The proposed technique will therefore contribute to quantitative investigations into electrostatic interactions and ion transport in developing nanoscale electrochemical transport techniques and devices in, for example, biochemical analysis and water purification.