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.

本研究开发了一种基于纳米级激光诱导荧光的近壁pH测量技术，以评估玻璃 - 溶液界面附近pH的时空分布。荧光素钠盐（一种对pH敏感的染料）溶解在工作流体中，并由激光束全内反射产生的消逝波激发。以高pH缓冲溶液（pH⩾10）作为参考，通过比率校准确定近壁pH分布。所提出的技术应用于压力驱动流和电渗流，以研究界面电位和外部电场对质子浓度的影响。在压力驱动流中，由于质子局部过量地被吸引到带负电的玻璃壁上，近壁pH比本体pH低约0.2。这种pH偏移在低离子强度的溶液中尤为明显，这表明近壁pH与双电层厚度密切相关。电渗流中的pH偏移比压力驱动流中的小约0.02，并且随着电场强度的增加而减小。此外，时间序列测量以5.2×5.2 µm的空间分辨率清晰地显示了电渗流中不均匀pH分布的时间变化。在95%置信水平下，测量不确定度估计为0.16个pH单位。这些结果证明了本技术用于评估距固 - 液界面10 - 100 nm空间内质子时空浓度的可行性。因此，所提出的技术将有助于在例如生化分析和水净化中开发纳米级电化学传输技术和设备时对静电相互作用和离子传输进行定量研究。