Nonlinear optothermoacoustic phenomena in highly diluted suspensions of gold nanoparticles

Optoacoustic (OA) sensor was designed, fabricated and used to detect spherical gold nanoparticles (NPs) in suspensions at the level of up to a single particle. The sensor operating in the backward mode was designed to measure signals from microscopic volumes of nanoparticulate suspensions in water. Thermal nonlinearity was observed in the course of OA signal generation. The irradiation of the microvolume of gold nanoparticles gives rise to a multitude of thermomechanical processes, including heating of NPs above the vaporization temperature, thermal diffusion from nanoparticles to water and formation of vapor nanobubbles. As a result, a specific acoustic signal is produced exhibiting nonlinear behaviour with respect to the incident laser pulse energy. The optoacoustic profile of the laser-induced signal generated in a thin layer of highly diluted suspensions of gold nanospheres was examined thereby providing a basis for a method for detection of metal nanoparticles with high sensitivity.