Development of a flexible film sensor for surface temperature measurement based on fluorescence polarization

We developed a flexible temperature-sensitive film sensor based on fluorescence polarization and demonstrated its applicability to surface temperature measurements. The sensor consists of two transparent polydimethylsiloxane (PDMS) films with a fluorescent dye solution sealed between them. The solution temperature is measured by detecting the degree of fluorescence depolarization, which reflects the rotational Brownian motion of the dye molecules. The film sensor was designed by considering the temperature sensitivity of the polarization degree, optical transmittance, heat transfer characteristics, and solvent evaporation through PDMS. Theoretical and experimental investigations revealed that optimizing the solution viscosity within the target temperature range significantly improves the temperature sensitivity of the degree of polarization. Calibration experiments using 80 wt% glycerol-water solution containing uranine demonstrated that the temperature sensitivity increased more than tenfold, and the random error in the measured temperature was reduced to 0.2 K compared to that when using aqueous uranine solution without glycerol in the 293–313 K range. The relationship between the film sensor thickness and the thermal response time was investigated, revealing that it can be well explained by a one-dimensional heat transfer model. Furthermore, comparison of the water- and oil-based solutions confirmed that the oil-based solution, which was less susceptible to evaporation, exhibited higher temporal stability when used in gas-permeable PDMS film sensors. The sensor filled with an olive oil-based Nile red solution successfully visualized two-dimensional surface temperature distributions, which agreed with the thermocouple measurements within 0.8 K. These findings provide fundamental design guidelines for polarization-based film sensors and serve as a basis for optimizing their performance for various future applications.