Self-Referenced Temperature Sensor Based on Conjugated Polyelectrolytes

Developing sensitive temperature sensors for nano- and microscale applications has become critical in many technologies and scientific fields. Such sensors enable the accurate measurement and control of temperature in extremely small domains, providing unparalleled insights into thermal behaviors and properties. In this study, we tune the photophysical properties of a short poly(phenylene ethynylene)-type (PPE-CO₂) conjugated polyelectrolyte (CPE) using poly(diallyldimethylammonium chloride) (PDDA), a positively charged polyelectrolyte, to develop a self-referenced fluorescence-based temperature sensor. In the presence of PDDA, PPE-CO₂ is initially quenched, but a small fraction of the disaggregated CPEs are believed to be stabilized. This in turn provides a thermally stable signal at 465 nm, which serves as an internal reference. The fluorescence intensity at 525 nm of the aggregated CPE maintained its thermal dependency which, when referenced with the 465 nm peak, created a sensitive and stable temperature sensor. The thermal response was further enhanced at low ionic strength. Specifically, without NaCl, individual polymers are less solubilized, minimizing fluctuations at the 465 nm peak and leading to higher thermal sensitivity and a wider linear range. The thermal response for PDDA/PPE-CO₂-108 was tested between 20.0 and 90.0 °C, with optimized sensitivities of 0.0028 and 0.0038 °C^1– with and without NaCl, respectively. Relative sensitivity (S_r) was 4.9% °C^1– at 20 °C for PDDA/PPE-CO₂-108 without NaCl.