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

## TL;DR

Researchers used real-time spectroscopy to study how gold-silver-gold nanoparticles grow, showing how their shape and optical properties change during synthesis.

## Contribution

A novel in situ method combining SHG and extinction spectroscopy to monitor nanoparticle growth dynamics in real time.

## Key findings

- The surface morphology of nanoparticles becomes smoother with each gold shell addition.
- Plasmon extinction spectra blue shift and narrow as nanoparticle size and smoothness increase.
- SHG signal peaks during the first addition, then decreases as the surface becomes smoother.

## Abstract

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.