Low-frequency capacitance and mass-change of un-conditioned Nafion-coated electrodes - towards calibration-free aAg(I) ion sensing in pure water

Here, we propose a method for chemical sensing of Ag+ ions in pure water by utilizing single-use and un-conditioned Nafion-coated Au electrodes together with capacitive and mass transduction principles. Ag+ ions were accumulated and simultaneously Ag was deposited in the Nafion membrane at an applied potential of −0.6 V vs. Ag/AgCl, followed by electrochemical impedance spectroscopy (EIS) to obtain the low-frequency capacitance. After Ag deposition, the resulting change in the low-frequency capacitance at 10 mHz of the Nafion-coated Au was determined with respect to the activity of Ag+ ion (aAg+) in solution in the range of logaAg+ = −11 to −2. The precision of the low-frequency capacitance was improved by utilizing spin-coated thin-layer Nafion membrane with applied single frequency at 1 Hz. Under identical experimental conditions, the capacitive response of the Nafion/Au electrode was larger than that of a Nafion-coated glassy carbon (Nafion/GC) electrode, indicating better electrocatalytic activity of Au than GC towards Ag+. Based on quartz crystal microbalance (QCM) measurements, the mass change due to Ag deposition at Nafion/Au was linearly proportional to logaAg+ in the range of −5 to −2. Both EIS and QCM methods show a response that correlates with the concentration or activity of Ag+ ions in pure water, which might give a possibility for calibration-free determination of Ag+ without any electrode conditioning needed.