Time-Resolved Shadowgraph Photography of Laser-Heating Plasmonic Gold Nanoparticles in Water

Fragmentation and the subsequent heat transfer from solvated gold nanoparticles with sizes 50, 100, 150, and 250 nm to water are studied by time-resolved shadowgraph photography. The measured fragmentation thresholds indicate that the particles heat up to temperatures between the boiling point and critical temperature of gold. At high laser energy densities complete fragmentation of the particles is observed, which is evidenced by the sudden breakdown of the thermally insulating Leidenfrost layer initially surrounding the laser-heated particles. The consequent heat transfer results in the production of water vapor, which expands through pressure-volume work to form a micrometer- sized bubble. The spatially resolved shadowgraph images of these bubbles show the exact locations of the parent nanoparticles in the solution. Furthermore, a simple thermodynamic model can be used to relate the observed maximum bubble diameters to the original nanoparticle sizes. Although the presented method for on-line analysis of plasmonic metal nanoparticles in the liquid phase is destructive, it can be implemented in practice to employ a single laser pulse with small beam size to achieve only minimal perturbation of the sample.