Single-Frequency Effective Capacitance <i>C</i>_ec and Membrane Resistance <i>Z</i> Readout for Solid-Contact Ion-Selective Electrodes

Here, we propose new single-frequency effective capacitance <i>C</i> _ec and membrane resistance <i>Z</i> readout principle for solid-contact ion-selective electrodes (SCISEs). Conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) doped with polystyrenesulfonate (PSS^-), <i>i.e.</i>, 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 <i>Z</i> was recorded constantly as KCl aqueous solution diluted with water. The membrane resistance <i>Z</i> increases as the electrolyte concentration decreases. Under identical dilution steps, the linear slope of the logarithmic membrane resistance log<i>Z</i> <i>vs</i> log<i>a</i> _K+ 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<i>Z</i> of K⁺-SCISEs is linearly proportional to log<i>a</i> _K+ in the range of -1 to -3.4, providing a possibility of utilizing membrane resistance <i>Z</i> as a calibration-free analytical signal for SCISEs. The effective capacitance <i>C</i> _ec 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 <i>C</i> _ec of K⁺-SCISEs with the spin-coated membrane is linearly proportional to log<i>f</i>in the range of 1 MHz to 10 Hz with a slope of <i>ca.</i> -0.97, while at a low frequency ranging from 1 Hz to 10 mHz, the linear slope of log<i>C</i> _ec <i>vs</i> log<i>f</i> is suppressed, where Warburg diffusion takes effect. Furthermore, the membrane resistance <i>Z</i> is independent of applied high frequencies, and the effective capacitance <i>C</i> _ec is independent of the excitation amplitude.