A wearable pH sensor with high sensitivity based on a flexible charge-coupled device

Flexible chemical sensors could be used to create wearable devices that continuously monitor a variety of health conditions through the analysis of a person's sweat. Current wearable pH sensors for sweat monitoring have a limited pH detection sensitivity at room temperature of about 59 millivolts per pH unit, which is determined by the Nernst equation. Here we show that a sensitive pH sensor can be created using a flexible charge-coupled device (CCD) and integrated with a temperature sensor. Our CCD-based pH sensor can, through accumulation cycles of electron charge transfer, achieve a sensitivity of around 240 millivolts per pH unit, which is roughly four times larger than the Nernst theoretical limit. Furthermore, the integrated flexible temperature sensor can be simultaneously used to compensate for the temperature dependence of the pH detection and to monitor skin temperature. As a proof of concept, we demonstrate that our CCD-based chemical sensor can be used to monitor the sweat pH and skin temperature of a person in real time.