Sustainable Fully Inkjet-Printed Humidity Sensor Based on Ionic Liquid and Hydroxypropyl Cellulose

The increasing number of sensors contributing to the Internet of Things (IoT) aggravates the e-waste generated globally. Thus, it is an urgent necessity to develop more sustainable sensors. This paper presents a fully inkjet-printed dual-response (electrical and visual) humidity sensor based on hydroxypropyl cellulose (HPC) and the ionic liquid bis(1-butyl-3-methylimidazolium) tetrachloronickelate ([Bmim]₂[NiCl₄]). The active layer was printed on interdigitated silver electrodes on a flexible cellulose acetate substrate. The optimized ink includes HPC, [Bmim]₂[NiCl₄], ethylene glycol, water, and Tergitol. HPC and the IL exhibit excellent compatibility, forming homogeneous films without phase separation even at high IL concentration. The printed sensor for an IL content of 50 wt % demonstrates a proportional response when varying the relative humidity (RH) from 30 to 90 RH%, with a high sensitivity of 163, comparable to that of a commercial reference sensor, a low hysteresis of 1.5 RH%, and a fast response time of 0.8 s. In addition, a visual response from colorless to cyan is observed upon dehydration. This color change is visible to the naked eye for a relative humidity below 30 RH% when a transmittance lower than 93% is obtained in the visible spectra. This dual-response humidity sensor, fabricated from sustainable materials and low-cost printing technology, has great potential for a variety of applications, including environmental monitoring, smart agriculture, fire safety, and quality control in the food industry.