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

Abstract

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.

Micro- and Nanofluidic pH Sensors Based on Electrodiffusioosmosis

“…We have also investigated ICR effects on ion selectivity in micro- and nanofluidic channels, which are applied to EHD flows through ion-exchange membranes [ 31 , 32 ] and proton concentration measurements [ 33 , 34 ]. The results suggested that EOF and EHD flows could be induced with quite low ion concentrations [ 13 ].…”

“…The results suggested that EOF and EHD flows could be induced with quite low ion concentrations [ 13 ]. The ion selectivity is possibly improved as the ion concentration decreases [ 13 , 33 , 34 ] because EDLs in nanochannels tend to overlap between opposite surfaces at quite low concentrations with electrical charge carriers limited to surface charges [ 11 ]. Furthermore, ion concentration gradients in a nanochannel cause diffusion in ionic currents along the channel [ 8 , 34 ].…”

“…The ion selectivity is possibly improved as the ion concentration decreases [ 13 , 33 , 34 ] because EDLs in nanochannels tend to overlap between opposite surfaces at quite low concentrations with electrical charge carriers limited to surface charges [ 11 ]. Furthermore, ion concentration gradients in a nanochannel cause diffusion in ionic currents along the channel [ 8 , 34 ]. That is, the resistivity increases when both the electrophoresis and electroosmosis of ions are in the counter direction to the concentration diffusion direction, an effect that is termed electrodiffusioosmosis [ 8 , 34 ].…”

“…Furthermore, ion concentration gradients in a nanochannel cause diffusion in ionic currents along the channel [ 8 , 34 ]. That is, the resistivity increases when both the electrophoresis and electroosmosis of ions are in the counter direction to the concentration diffusion direction, an effect that is termed electrodiffusioosmosis [ 8 , 34 ]. Glass pipettes or glass films are usually used for electrodes for pH measurements because of the proton selectivity and high conductivity of glass nanopores [ 35 ] due to a unique proton transport mechanism in water [ 36 , 37 ].…”

“…Our goal is schematically shown in Figure 1 . Previously demonstrated glass microelectrodes, which were portable and easy to move to the measurement points, were fabricated based on the same principle [ 33 , 34 ]. On the other hand, in the present method, the electrodes are stationary and can be embedded in micro- and nanochannels.…”