Dual-Responsive Fluorescent Polymer Films for Wide-Range Temperature and Humidity Sensing

Polymer films are widely used in packaging, electronics, and biomedical technologies. Preparing thin polymer films with temperature and humidity sensing capabilities can enable the improvement of device performance, durability, and functionality. In the present work, we report on thin fluorescent polymer films that can detect small temperature changes with excellent sensitivity over a wide temperature range from 20 to 60 °C. The probe was prepared using poly(phenylene ethylene) (PPE-CO2-108) in complexation with a polymer mixture of polyvinylpyrrolidone (PVP) and poly(1-vinylpyrrolidone-co-vinyl acetate) (PVP-VA). The macromolecule mixture resulted in clear and colorless films. Upon heating, we observed up to an 11-fold increase in the fluorescence intensity, which was recorded using an unmodified and commercially available camera. The thermal response profile of these films could be tuned by altering the polymer composition and ratio. The best-performing films had an absolute sensitivity of 1.51 °C–1. The enhanced fluorescence signal was preserved even after several days of heat exposure; however, it would revert to its original intensity when exposed to humidity. As such, these prepared films can act as an on–off temperature sensor and as an on–off humidity sensor. ATR-FTIR measurements revealed that the actuating mechanism of the polymer films is through water adsorption–desorption in the polymer film. Fluorescence confocal imaging of the films before and after heating revealed a significant transformation in their morphology. Initially uniform, the films became highly porous upon heating, forming a distinct network-like structure. As a proof of concept, we demonstrated that these thermally sensitive films could serve as a valuable tool for investigating localized heating effects, such as the hyperthermia induced by magnetic nanoparticles embedded in thin polymer matrices.