Economic and Rapid Monitoring of Nanoparticle Size Changes during Nanosecond Laser Irradiation Based on Frequency Spectral Response of Photoacoustic Signals

In recent years, laser synthesis and processing of colloids (LSPC) has enabled the controlled preparation of nanoparticles of various materials; however, there are difficulties in real-time monitoring of nanoparticles in photothermal transient environments, such as size and structure, which limit further understanding and application of the technology. The photoacoustic (PA) signals are sensitive to the optical, thermal, and thermal expansion properties of the medium and contain information about the photothermal processes and size changes of the nanoparticles. In this paper, the change of product shape and size as well as the different responses to the corresponding PA signals are investigated under the same experimental conditions by taking the processes of melting after laser irradiation of TiO₂, evaporation after laser irradiation of CeO₂, and crushing after laser irradiation of CuI as examples. The relationship between the size of the nanoparticles and the optical absorption coefficient as well as the laser fluence was analyzed based on the Mie scattering theory (Mie) and the heated–melt–evaporation theory (HME). The relationship between the size of the nanoparticles and the intensity and frequency spectrum of the PA signal was analyzed based on the K-wave toolbox in MATLAB. It is found that the frequency spectrum of PA signals generated during laser irradiation of nanoparticles is related to the processes of laser melting, laser evaporation, and laser crushing and correlates with the variability of photothermal effects on nanoparticles. In addition, in a certain frequency range (1.0–7.0 MHz) the change in the nanoparticle size shows a certain regularity with the increase or decrease of the integral area of the frequency spectrum of PA signals, and this law can be widely applied to most of the nanoparticles, such as metals and oxides, and it can be used as a low-cost in situ detection technique for the change of the size of nanoparticles.