Single Particle Nanothermometry Quantifies Heating in a Plasmonic Trap.

Plasmonic trapping enables nanoscale confinement of individual nanoparticles by exploiting localized electromagnetic field enhancement, an effect inherently accompanied by photothermal heating. Quantifying this local temperature increase is essential, as heating can affect trap stability through enhanced Brownian motion and the emergence of thermally induced forces such as thermophoresis. Here, we experimentally determine the local temperature experienced by a single upconverting nanoparticle optically trapped in the vicinity of a pyramid-like gold nanoantenna. By combining this measurement with numerical simulations of the heat distribution, we infer the temperature at the gold-water interface. While the trapped UCNP experiences a temperature increase of ∼40 °C at an average distance of ∼50 nm from the nanoantenna surface, thermal simulations predict that the nanoantenna surface may reach temperatures of up to ∼100 °C under the same excitation condition. Analysis of the nanoparticle's luminescence provides simultaneous determination of its local temperature and three-dimensional motion in the vicinity of the nanoantenna. The results reveal a linear increase in temperature with trapping power and demonstrate that the particle explores regions characterized by strong thermal gradients generated by the plasmonic structure. By correlating temperature and position, we estimate the relative magnitudes of optical and thermophoretic forces, providing a model-based explanation for the observed steady-state trapping position. These findings establish individual upconverting nanoparticles as probes for investigating thermoplasmonic effects at the single-particle level and provide insight into the interplay between optical confinement, thermal gradients, and Brownian motion in plasmonic trapping systems.