pH Sensing Based on Ionic Current Rectification Using Triple-Barreled Glass Microelectrodes

Glass nanopores are known to provide proton selectivity in ionic current conditions in which silanol groups that fill glass surfaces support proton conduction along the surfaces in aqueous solutions. Negatively charged glass surfaces furthermore exclude anions from the glass nanopores. Therefore, glass electrodes are often used for the probes of pH sensors. We focused on the ion selectivity and ionic current rectification of glass nanopores. In this study, we propose triple-barreled glass microelectrodes that enable us to measure proton concentrations at local points with spatial resolution of the tip diameter. The glass microelectrodes consist of three capillaries for the working, counter, and reference electrodes filled with electrolyte and pH buffer solutions. A pH 1.68 buffer and KCl solution as a supporting electrolyte are maintained in a glass capillary for the counter electrode, and KCl solutions are in the working and reference electrodes. In galvanostatic current conditions, potential differences are measured in sample solutions. The potential difference is caused by the proton concentration gradient and proton-selective conduction, which is known as electro­diffusioosmosis. The ionic current is rectified due to the proton selectivity of the glass capillary, and as a result, the conductivity reflects the relative difference of pH values, ranging from 1.68 to 10.01 units. The temperature dependence of pH values is also investigated in the range from 303 to 333 K. The proposed glass microelectrodes achieve a pH resolution of 1.39 V/pH, which is much higher than the Nernst limit.