Pulsed optoacoustic interaction in suspension of gold nanoparticles: detection sensitivity based on laser-induced nanobubbles

Abstract

The sensitivity of optoacoustic (OA) detection in diluted suspensions of gold nanoparticles under irradiation with nanosecond laser pulses was studied as a function of incident laser fluence. The range of moderate values of the laser fluence from 20 mJ/cm² to 2 J/cm² was studied theoretically and experimentally. Under these laser fluences, the usual thermoelastic mechanism of OA generation faces competition from laser-induced cavitation, a statistical process, which leads to considerable fluctuations of the acoustic response from one laser pulse to another. Analytical expressions for the statistical characteristics of the acoustic signal were obtained. A simulation of the statistical characteristics of the cavitation contribution to the signal was performed using the method of Monte Carlo. The experiment utilized the second harmonic pulses (532 nm) of an Nd:YAG laser to irradiate samples of water suspensions of spherical gold nanoparticles (NPs). A series of laser pulses each having from 100 to 2000 pulses were used to iradiate the samples. The statistical rank distributions of the magnitudes of optoacoustic signals recorded by a wide band ultrasonic transducer attached to the measurement cell were used as a tool for sensitive detection of a low concentration of the gold nanoparticles in water.