Dual photoacoustic monitoring in laser ablation synthesis of silver nanoparticles to find in situ their fluence threshold formation

Abstract

In this study, a dual pulsed photoacoustic technique was applied to monitor, in real time, the laser ablation synthesis in solution (LASiS) of silver nanoparticles. It was found that, at low optical fluence values, the ultrasound signal associated to the laser ablation process exhibited constant amplitude, independently of the ablation time. Further analysis confirmed that, under these conditions, the ablation process provided only suspended micro-sized Ag fragments. However, fluence values above 400 mJ cm−2 promoted the formation of nanoparticles, accompanied by amplitude changes in the photoacoustic signal. Determination of this threshold condition establishes a fingerprint in the photoacoustic signal that can be used to identify, in situ and in real time, the beginning of nanoparticle synthesis. Supplementary, the nanoparticle formation was also photoacoustically monitored during the ablation process introducing an additional laser pulse train of low fluence value propagating perpendicularly to the pulse train used for ablation. These laser pulses did not interfere with the nanoparticle LASiS process, but renders an ultrasound signal enhancement due to the surface plasmon resonance of metal nanoparticles. The characterization of the LASiS process through the photoacoustic technique can provide information about nanoparticle concentration and other synthesis features of interest. Keywords: Laser ablation, Laser-induced ultrasound, Metallic nanoparticles, Photoacoustics