Rapid Constant Potential Capacitive Measurements with Solid-Contact Ion-Selective Electrodes Coupled to Electronic Capacitor

A constant potential capacitive readout of solid-contact ion-selective electrodes (SC-ISE) allows one to obtain easily identifiable current transients that can be integrated to obtain a charge vs logarithmic activity relationship. The resulting readout can therefore be much more sensitive than traditional open-circuit potentiometry. Unfortunately, however, comparatively long measurement times and significant baseline current drifts make it currently difficult to fully realize the promise of this technique. We show here that this challenge is overcome by placing the SC-ISE in series with an electronic capacitor, with pH probes as examples. Kirchhoff's law is shown to be useful to choose an adequate range of added capacitances so that it dominates the overall cell value. Two different ion-to-electron transducing materials, functionalized single-wall carbon nanotubes (f-SWCNTs) and poly(3-octylthiophene) (POT), were explored as solid-contact transducing layers. The established SC-ISE-based f-SWCNT transducer is found to be compatible with a wide range of external capacitances up to 100 μF, while POT layers require a narrower range of 1-4.7 μF. Importantly, the time for a charging transient to reach equilibrium was found to be less than 10 s, which is dramatically faster than without added electronic component. Owing to the ideal behavior of capacitor, the response current decays rapidly to zero, making the determination of the integrated charge practically applicable.