Monitoring Gold Nanoparticle Formation by <i>In Situ</i> Transient Absorption Spectroscopy

Performing ultrafast spectroscopy during nanoparticle synthesis can provide new insights into the kinetics and mechanism of nanoparticle formation. In these experiments, we thoroughly studied the ultrafast decay of the transient absorption signal from reference gold nanoparticles (with well-defined diameters of 5, 20, and 40 nm) and from synthesized gold nanoparticles with 1:1 and 1:4 gold-to-citrate molar ratio. The application of the high repetition rate laser (100 kHz) with short pump pulse duration (10 fs, 470–570 nm) allows the rapid in situ TAS monitoring of the particle growth. Following excitation with femtosecond pulses, the acoustic vibrations of the particles produced periodic oscillations in the transient absorption signal. The decay traces at the selected wavelengths were fitted, and the Fast Fourier Transform (FFT) analysis was used for the determination of the characteristic frequencies of the acoustic vibrations. The frequency of the periodic oscillations decreases with an increasing size. We demonstrate that the formation of monodisperse gold nanoparticles at 75 °C can be monitored during the synthesis, providing direct evidence about their quality and particle size in the 12–55 nm particle diameter range. We concluded that the average primary size of the monodisperse gold nanoparticles increased in the 19–23 min synthesis time range as the frequency of the acoustic vibrations decreased for the Au-Cit 1:4 sample and remained constant in the last phase of the synthesis.