Abstract
Potentiometric sensors induce a spontaneous voltage that indicates ion activity in real time. We present here an advanced self-powered potentiometric sensor with memory. Specifically, the approach allows for one to record a deviation from the analyte's original concentration or determine whether the analyte concentration has surpassed a threshold in a predefined time interval. The sensor achieves this by harvesting energy in a capacitor and preserving it with the help of a diode. While the analyte concentration is allowed to return to an original value following a perturbation over time, this may not influence the sensor readout. To achieve the diode function, the sensor utilizes an additional pair of driving electrodes to move the potentiometric signal to a sufficiently high base voltage that is required for operating the diode placed in series with the capacitor and between the sensing probes. A single voltage measurement across the capacitor at the end of a chosen time interval is sufficient to reveal any altered ion activity occurring during that period. We demonstrate the applicability of the sensor to identify incurred pH changes in a river water sample during an interval of 2 h. This approach is promising for achieving deployable sensors to monitor ion activity relative to a defined threshold during a time interval with minimal electronic components in a self-powered design.
Highlights
Potentiometric sensors induce a spontaneous voltage that indicates ion activity in real time
Chemical substrates such as polymer films and electronic capacitors proved to be useful in realizing electric and optical readout of ions using this transduction principle.[17−20] Based on these developments, we recently demonstrated a self-powered version of a potentiometric sensor by placing a capacitor directly between the indicator and reference probes.[21]
The capacitor serves as a memory element and the existence of a transient current in the circuit is due to the nature of the element and is not the parameter of interest
Summary
The approach adopted to detect a deviation occurring in activity of an analyte during a predefined time interval without requiring continuous monitoring is illustrated in Scheme 1. Nernstian response for pH with a potential shift of ∼1.065 V relative to the potentiometric measurement with a double junction Ag/AgCl (3 M KCl/1 M LiOAc) electrode was observed (Figure 3b) This shift is due to the asymmetry induced by the Zn electrode E3, which acts in some analogy to the voltage applied with an external source meter. Upon acquiring a single capacitor voltage measurement at the end of 2 h with the help of a simple hand-held multimeter, the sensor revealed the introduced pH perturbations in river water despite returning the sample to the original pH (Figure 4e) In this manner, the capacitor voltage in combination with the diode served as a memory element to recognize that the pH had been altered within the measurement period. Develop sensors capable of detecting both upper and lower threshold limits
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