LAUSR

Mobile integrated electrochemical sensors normally require a power supply for operation. Unfortunately, the practice of discarding batteries associated with these devices runs counter to our desire for a sustainable world. Self-powered sensing concepts that draw the energy directly from the measurement itself would overcome this limitation. Potentiometric sensors for the measurement of pH, many electrolytes, and gases are ubiquitous in analytical practice. However, in potentiometry, the voltage is acquired in the absence of current flow, making it seemingly impossible to draw power. Fortunately, it has been recently established that transient currents may be tolerated across potentiometric measurement cells to charge a capacitive or electrochromic element such as Prussian blue integrated in the measurement cell and whose absorbance then directly follows the potential changes in a reversible manner. We have shown here that commercial electronic paper (e-paper), widely used to make electronic ink and ebook readers, can directly be driven by a potentiometric measurement cell in a reversible manner at mild potentials of >100 mV typical for such sensors. The capacitance of the e-paper pixel studied here was found to be 0.53 μF mm-2, 30 times smaller than that of Prussian blue films. The colorimetric absorbance of the e-paper was also more stable (observed drift over 2 h corresponding to 0.76 mV h-1) and reproducible (corresponding to 1 mV standard deviation). The e-paper pixel was directly driven by a polymeric pH electrode as a model system. Choosing a basic inner solution (pH 12.9) behind the membrane gave sufficiently positive cell potentials for driving visible absorbance change in a sample pH range of 4-10, while a more acidic pH of 3.4 and alternating the connections to the e-paper were more suited for more basic samples of pH > 10. This convenient and cost-effective approach makes it possible to directly drive an optical display from the potentiometric measurement itself and should be suitable for moderate sensing membrane resistances of less than about 100 kΩ, depending on the area of the chosen pixel.