Experimental and Theoretical Studies of Light-to-Heat Conversion and Collective Heating Effects in Metal Nanoparticle Solutions

TLDR

These studies are crucial for understanding photothermal effects in nanoparticles and their potential applications in nano‑ and biotechnologies. The study combines experimental photoheating of gold nanoparticles in water droplets with theoretical modeling of thermal effects at nano‑ and millimeter scales. Photocalorimetric measurements show near‑unity light‑to‑heat conversion efficiency and convection‑dominated heat transfer, while theory indicates collective photoheating as the main mechanism, yielding a ~7 °C millimeter‑scale temperature rise at high nanoparticle concentrations with low laser intensity (but only ~0.02 °C at the nanoparticle surface), and conversely a ~0.1 °C millimeter rise with strong ~3 °C nanoscale spikes at low concentrations and high intensity.

Abstract

We perform a set of experiments on photoheating in a water droplet containing gold nanoparticles (NPs). Using photocalorimetric methods, we determine efficiency of light-to-heat conversion (η) which turns out to be remarkably close to 1, (0.97 < η < 1.03). Detailed studies reveal a complex character of heat transfer in an optically stimulated droplet. The main mechanism of equilibration is due to convectional flow. Theoretical modeling is performed to describe thermal effects at both nano- and millimeter scales. Theory shows that the collective photoheating is the main mechanism. For a large concentration of NPs and small laser intensity, an averaged temperature increase (at the millimeter scale) is significant (∼7 °C), whereas on the nanometer scale the temperature increase at the surface of a single NP is small (∼0.02 °C). In the opposite regime, that is, a small NP concentration and intense laser irradiation, we find an opposite picture: a temperature increase at the millimeter scale is small (∼0.1 °C) but a local, nanoscale temperature has strong local spikes at the surfaces of NPs (∼3 °C). These studies are crucial for the understanding of photothermal effects in NPs and for their potential and current applications in nano- and biotechnologies.