Silica-Coated Gold Nanorods as Photoacoustic Signal Nanoamplifiers

TLDR

Photoacoustic signal generation by metal nanoparticles depends on efficient light‑to‑heat conversion, heat transfer to the environment, and the resulting pressure transients. The study demonstrates that silica coating on gold nanorods triples photoacoustic signal amplitude compared to uncoated rods of identical optical density. The authors varied silica thickness, gold/silica interface, and solvent to assess their impact on photoacoustic signal from coated nanorods. Silica coating does not alter optical absorption but reduces gold–solvent thermal resistance, leading to a three‑fold signal increase and strong contrast enhancement in phantoms, confirming the particles as high‑efficiency photoacoustic nanoamplifiers.

Abstract

Photoacoustic signal generation by metal nanoparticles relies on the efficient conversion of light to heat, its transfer to the environment, and the production of pressure transients. In this study we demonstrate that a dielectric shell has a strong influence on the amplitude of the generated photoacoustic signal and that silica-coated gold nanorods of the same optical density are capable of producing about 3-fold higher photoacoustic signals than nanorods without silica coating. Spectrophotometry measurements and finite difference time domain (FDTD) analysis of gold nanorods before and after silica coating showed only an insignificant change of the extinction and absorption cross sections, hence indicating that the enhancement is not attributable to changes in absorption cross section resulting from the silica coating. Several factors including the silica thickness, the gold/silica interface, and the surrounding solvent were varied to investigate their effect on the photoacoustic signal produced from silica-coated gold nanorods. The results suggest that the enhancement is caused by the reduction of the gold interfacial thermal resistance with the solvent due to the silica coating. The strong contrast enhancement in photoacoustic imaging, demonstrated using phantoms with silica-coated nanorods, shows that these hybrid particles acting as “photoacoustic nanoamplifiers” are high efficiency contrast agents for photoacoustic imaging or photoacoustic image-guided therapy.