Single-Frequency Effective Capacitance and Membrane Resistance Readout for Solid-Contact Ion-Selective Electrodes.

Here, we propose new single-frequency effective capacitance and membrane resistance readout principle for solid-contact ion-selective electrodes (SCISEs). Conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) doped with polystyrenesulfonate (PSS), , PEDOT(PSS), as solid contact and valinomycin-based membrane were prepared for K-SCISEs. At high frequencies, the membrane resistance of K-SCISEs corresponding to impedance absolute value was recorded constantly as KCl aqueous solution diluted with water. The membrane resistance increases as the electrolyte concentration decreases. Under identical dilution steps, the linear slope of the logarithmic membrane resistance log log for K-SCISEs with the spin-coated membrane is larger than that of the electrode covered with the drop-cast membrane. As the K-SCISE resistance with the spin-coated membrane was reduced to hundreds of Ω, the log of K-SCISEs is linearly proportional to log in the range of -1 to -3.4, providing a possibility of utilizing membrane resistance as a calibration-free analytical signal for SCISEs. The effective capacitance of K-SCISEs with the spin-coated membrane was performed in 0.1 M KCl applied with single frequency ranging from 1 MHz and decreases by a factor of 10 to 10 mHz. The obtained of K-SCISEs with the spin-coated membrane is linearly proportional to login the range of 1 MHz to 10 Hz with a slope of -0.97, while at a low frequency ranging from 1 Hz to 10 mHz, the linear slope of log log is suppressed, where Warburg diffusion takes effect. Furthermore, the membrane resistance is independent of applied high frequencies, and the effective capacitance is independent of the excitation amplitude.