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

Abstract

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