In Situ Second Harmonic Generation and Extinction Spectroscopy for Studying Colloidal Gold–Silver–Gold Core–Shell–Shell Nanoparticle Growth Dynamics

Time-dependent in situ second harmonic generation (SHG) spectroscopy coupled with extinction spectroscopy is used to monitor the growth dynamics involved in the synthesis of colloidal gold-silver-gold core-shell-shell (Au-Ag-Au CSS) nanoparticles in real time. A stepwise seed-mediated method is applied to grow an outer gold shell onto gold-silver core-shell nanoparticles in aqueous solution, using four sequential additions of chloroauric acid and reducing agents. The first addition results in Au-Ag-Au CSS nanoparticles with a bumpy, urchin-like morphology. With each subsequent addition, the outer gold shell thickness increases, while the nanoparticle surface morphology becomes smoother and more uniform. Transmission electron microscopy (TEM) is also utilized to determine the nanoparticle size distribution and surface morphology after each addition. As the size and surface smoothness of the CSS nanoparticles increase, the plasmon extinction spectra blue shift with spectral narrowing and increasing extinction intensity. Comparison with corresponding Mie theory extinction spectra calculations shows general agreement after the fourth addition, demonstrating a spherical concentric nanoarchitecture with a smooth nanoparticle surface. The surface-sensitive SHG signal increases dramatically during the first addition, corresponding to the urchin-like surface morphology, and then decreases as the surface becomes smoother with each subsequent addition. In situ monitoring of the two-photon fluorescence (TPF) signal provides complementary information for comparison to the extinction and SHG results. This combined approach of in situ SHG and extinction spectroscopy with Mie theory simulations and TEM imaging provides a detailed analysis of the synthesis of Au-Ag-Au CSS nanoparticles for investigating complex colloidal nanoparticle growth dynamics occurring at the nanoscale.