Self-Powered Potentiometric Sensor with Relational Operation Function to Capture Concentration Excursions

Abstract

Self-powered potentiometric sensors spontaneously respond to activity changes of target species without the need for an external power source. Here, a self-powered potentiometric sensing approach is described that may store concentration perturbations that occur before the sensor readout through a combination of capacitors and diodes. Two channels, termed "more than" and "less than" operators, are utilized as memory modules in the sensor circuit to record positive and negative concentration excursions, respectively. Each channel is constructed with a capacitor-diode pair in which each diode is connected to a capacitor in the opposite direction to prevent unwanted capacitor discharge. With this design, only potential variations that agree with the polarity of the diode may pass and be stored in the capacitor. A limitation of the principle is that the conductivity of the diode is very small if the voltage across it diminishes over time as it approaches the equilibrium value. To address this, the forward voltage is increased by about 1 V by switching from an initial Ag/AgCl reference electrode (RE) to a Zn/Zn 2+ element. The device may be used to monitor whether a concentration excursion has occurred in the time leading up to the signal readout in a semiquantitative manner. The approach also differentiates pH excursions of different durations (20, 40, 60 min). As an example, four different pH excursions of 20 min duration were successfully distinguished in river water samples with amplitudes of 1 to 4 pH units relative to the case without pH perturbation.