Tryptophan-containing proteins as label-free nanothermometers

There remains a need for techniques to monitor thermal processes at high spatiotemporal resolution, with myriad potential applications in chemistry, biology, and engineering. Measurement of temperature from nanoscale molecular phenomena are particularly promising due to their general compatibility with biological systems. Among these phenomena, fluorescence polarization anisotropy is particularly attractive due to its high sensitivity to thermal and photophysical information, and its general utility with a wide spectrum of fluorophores. In this work, we propose that the measurement of the intrinsic fluorescence polarization anisotropy of tryptophan can be used to measure label-free temperature at the nanoscale in a variety of tryptophan-containing proteins. We present a theoretical model of the temperature sensitivity of fluorescence polarization in free tryptophan and tryptophan-containing proteins, empirically explore these properties, and discuss the potential development of proteins as thermal memory sensors.