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Quantitative Microscale Thermometry in Droplets Loaded with Gold Nanoparticles

Gold nanoparticles (AuNPs) are increasingly used for their thermoplasmonic properties, i.e. their ability to convert light into heat upon plasmon resonance. However, measuring temperature gradients generated at the microscale by assemblies of AuNPs remains challenging, especially when they are randomly distributed in three dimensions. Here, we introduce a label-free thermometry approach, combining optical wavefront microscopy and numerical simulations, to infer the heating power dissipated by a three-dimensional model system consisting in emulsion microdroplets loaded with AuNPs. This approach gives access to the temperature reached in the core of droplets upon irradiation without need of extrinsic calibration. These quantitative results are validated qualitatively via the observation of the phase transition of a thermoresponsive polymer added in the droplet as an in situ thermal probe. This versatile thermometry approach is promising for non-invasive temperature measurements in various three-dimensional microsystems involving AuNPs as colloidal heat sources, such as several light-responsive drug delivery systems.