A dual-mode paper-based humidity sensor: Optical and electrical dual-signal sensing platform for multifunctional applications

Developing dual-signal sensing system that enables simultaneous optical and electrical humidity detection holds significant application foreground in the realm of real-time environmental humidity monitoring. In this work, utilizing cobalt chloride (CoCl₂) hybridized with polyvinyl alcohol (PVA) as sensitive materials, and cellulose paper with pencil-drawn graphite electrodes as the substrate, a paper-based humidity sensor capable of dual-signal sensing with colorimetric visualization and electrical readout is proposed for the monitoring of environment humidity across multiple application scenarios. This designed dual-mode humidity sensor exhibits superior electrical sensing properties, with a three-order-of-magnitude change in impedance over a relative humidity range from 11 % to 95 %, and a desired maximum hysteresis of only 3.5 %. Additionally, the sensor reveals remarkable electrical long-term stability and reliability, with a response variation of merely 5.11 % after one month of operation. Furthermore, this designed dual-mode humidity sensor also manifests outstanding optical sensing performance with reversible naked-eye readable color change in different humidity environments, without significant degradation in colorimetric performance even after 30 days. The designed dual-signal humidity sensor is further explored as wearable devices, demonstrating its potential practicality in multi-scenario fields such as respiration monitoring, non-contact sensing, and smart diapers. The proposed dual-mode humidity sensor allows for the simultaneous visualization and precise electrical quantification of humidity changes, offering a significant contribution to the field of environmental sensing and opening new avenues for the development of multifunctional sensing technologies.