Er³⁺-Doped Upconversion Nanoparticle Coatings for Thermometric Microscanning: Tackling Experimental Factors

The generation and flow of heat in the microscale are involved in a great variety of physical, chemical, and biological processes. Accurate measurement of local temperatures provides essential information for the complete characterization of nano/microdevices and the comprehension of local thermal effects. Using Er³⁺-doped upconversion nanoparticles (UCNPs) as thermal probes deposited on different thermally active microstructures, we assembled a motorized platform for scanning the upconversion luminescence (UCL) and retrieved temperature readings. We discuss critical aspects of the technique, such as calibration procedures, self-heating of substrate materials, the role of power density of the excitation light, and the uniformity of the coatings. A temperature resolution of 0.24(2) K, a spatial resolution of 0.24(7) μm, and a temporal resolution of 0.034(6) s were achieved. The platform was used for the characterization of two thermally active systems of interest: a microheater device and a photothermal nanoparticle coating immersed in a buffered solution. We emphasize potential applications for this method in the fields of cell biology and explore potential improvements with the use of UCNPs with increased thermal sensitivity, uniform self-assembled monolayer covering, and the integration of specific optical elements in the setup.