Thermometry at the sub-microscale is a highly desired goal for the study of nanostructures and microbiological systems. Rare-earth doped upconversion nanoparticles constitute ideal elements to act as optical probes for the ratiometric measurement of the local temperature. In this work, we combine the ability of upconversion nanoparticles to operate as luminescent thermometers with hyperspectral microscopy to construct thermal images based on the heat dissipation of a percolating network of silver nanowires containing the nanoparticles under controlled electrical current flow. We quantify the electrothermal action by analyzing the hyperspectral data and constructing 2D maps for the emission intensity, the signal-to-noise ratio, and temperature, concluding that no significant thermal gradients were identified. The thermal evolution is clearly sensed by the upconversion nanoparticles, validating the use of this method for studying slow-dynamical thermal processes. We finally present a discussion about the accuracy of the thermal readings and the systematic limitations of the proposed method.

This project has received funding from the European Union's Horizon 2020 FET Open programme under grant agreement No 801305 (NanoTBTech).