Mechanism of Amplified Photoacoustic Effect for Silica-Coated Spherical Gold Nanoparticles.

Plasmonic nanomaterials are effective photoacoustic (PA) contrast agents with diverse biomedical applications. While silica coatings on gold nanoparticles (AuNPs) have been demonstrated to increase PA efficiency, the underlying mechanism remains elusive. Here, we systematically investigated the impact of silica coatings on PA generation under picosecond and nanosecond laser pulses. Experimentally, we demonstrated a record high PA amplification of up to 400% under noncavitation conditions with a thin silica coating and only under picosecond laser pulses. We provide a clear mechanism for the observed PA amplification that identifies two competing effects, including transient absorption, which reduces photon energy absorption, and electron-phonon energy transfer at the gold-silica interface, which partly reverses the transient absorption effect. This study provides the first evidence and mechanistic insight on the impact of nonlinear optical effects on the nanomaterial-property relationship in PA contrast agents and offers insights for designing highly efficient contrast agents for biomedical applications.