Single Laser Pulse Effects on Suspended-Au-Nanoparticle Size Distributions and Morphology

Samples of suspended gold nanoparticles in the diameter range 10 to 100 nm were subjected to a single 7 ns pulse from a 532 nm laser to determine the effect of laser power on particle size distribution, mean size, and morphology. The experimental techniques used were dynamic light scattering (DLS), depolarized dynamic light scattering (DDLS), electrospray-differential mobility analysis (ES-DMA), ultraviolet visible absorption spectroscopy, and transmission electron microscopy (TEM). For 60 nm particles, a laser pulse of fluence 10 mJ/cm(2) was sufficient to produce observable changes. In the range 10-72 mJ/cm(2), DLS indicated little change in mean particle size but a more than three-fold reduction in the polydispersity index (significantly tightened distribution) and a decrease in scattering intensity. TEM showed that the particles became highly spherical and that there was a growing population of particles <10 nm in size that could not be detected by DLS and ES-DMA. Fused dimers were also observed, which suggest that heated particles can interact prior to cooling. DDLS showed a decrease in scattering due to shape anisotropy with a 20 mJ/cm(2) pulse and a decrease in the diffusion time constant. At higher power, the mean particle size decreased until all particles were <10 nm in size. The threshold for observable changes decreased with increasing particle size in the range 10 to 60 nm but increased for 100 nm particles. These results will be useful for potential therapeutic applications for pulse-heated nanoparticles and demonstrate the use of a simple laser treatment for modifying and improving nanoparticle properties.