Photothermal transformation of Au–Ag nanocages under pulsed laser irradiation

Abstract

Pulsed laser irradiation has emerged as an effective means to photothermally transform plasmonic nanostructures after their use in different biomedical applications. However, the ability to predict the products after photothermal transformation requires extensive ex situ studies. Here, we report a systematic study of the photothermal transformation of Au–Ag nanocages with a localized surface plasmon resonance at ca. 750 nm under pulsed laser irradiation at different fluences and a pulse duration of 5 ns. At biologically relevant laser energies, the pulsed laser transforms Au–Ag nanocages into pseudo-spherical, solid nanoparticles. The solid nanoparticles contained similar numbers of Au and Ag atoms to the parent Au–Ag nanocages. At increased laser fluences (>16 mJ cm⁻²) and number of pulses (>150), the average diameter of the resulting pseudo-spherical particles increased due to the involvement of Ostwald ripening and/or attachment-based growth. The changes in optical properties as a result of the transformation were validated using simulations based on the discrete dipole approximation method, where the spectral profiles and peak positions of the initial and final states matched well with the experimentally derived data. The results may have implications for the future use of Au–Ag nanocages in biomedicine, catalysis, and sensing.