Photoluminescent Humidity Sensors Based on Droplet-Enabled Porous Composite Gels

Abstract

Humidity sensors are important components in healthcare monitoring and advanced tactile sensing systems. However, low sensitivity and poor mechanical properties limit their practical applications in integrated haptic platforms such as artificial skin. Here, photoluminescence (PL)-type humidity sensing materials with simultaneously high sensitivity, stretchability, and healability are reported based on poly(vinyl alcohol) (PVA) composite gels comprising Cd x Se 1−x Zn y S 1−y quantum dots (QDs). A droplet-assisted strategy is developed to achieve a uniform distribution of QDs throughout the polymer matrix and also to assist with, in combination with freeze−thaw and freeze−dry cycles, the formation of micropores during the gel preparation process that improves moisture adsorption. With further ligand optimization, the PVA/QD composite gels exhibit an excellent PL−humidity linearity (R 2 > 99%), a wide humidity sensing range (from 11% to 93%), short response/recovery time (∼40 s), and good recoverability and cyclic stability (over 100 cycles). The humidity sensing mechanism is attributed to surface state changes of the QDs that are induced by intermolecular interactions between QD ligands and water molecules, as revealed by molecular vibration studies and density function theory calculations. This work opens avenues for the development of high-performance humidity-sensitive materials that are promising for next-generation tactile sensors and artificial skin and provides fundamental insights into the sensing mechanism of PL-based humidity QD sensors.