Theoretical and experimental study of photoacoustic excitation of silica-coated gold nanospheres in water

TL;DR

This study combines theoretical modeling and experimental validation to analyze how silica coatings affect the photoacoustic signals generated by gold nanospheres in water, revealing that thicker coatings reduce signal amplitude.

Contribution

It provides a semi-analytical model for PA signal generation considering silica coatings and experimentally confirms the coating's impact on signal reduction.

Findings

Kapitza resistance has negligible effect at nanosecond pulses.

Silica coating reduces PA signal amplitude.

Thicker silica coatings lead to greater signal reduction.

Abstract

Silica-coated gold nanoparticles are commonly employed in biomedical photoacoustic (PA) imaging applications. We investigate theoretically and experimentally the PA signal generation by silica-coated gold nanospheres in water. Our theoretical model considers thermoelastic expansion in the long-pulse illumination regime, and the PA signals are determined based on a semi-analytical solution to the thermal diffusion equations and a finite-difference in time domain (FDTD) solution to the thermoelastic equations. Both the influence of interfacial thermal (Kapitza) resistance at the gold-water boundary and the influence of the silica coating on PA signal generation were investigated. Our results indicate that for the nanosecond pulses commonly employed in PA imaging, Kapitza resistance has a negligible effect on photoacoustic signal generation. Moreover, our model shows that the presence of a silica coating causes a reduction in the PA signal amplitude, with the level of signal reduction increasing with thicker silica coating. Our theoretical predictions are qualitatively consistent with our experimental results, where suspensions of in-house-synthesized and commercially available silica-coated gold nanosphere suspensions were excited with nanosecond-pulsed laser illumination at 532 nm. The PA signal amplitudes from silica-coated nanospheres were lower than the signal amplitudes for uncoated gold nanospheres of the same core gold diameter. The amount of reduction of the experimentally PA signal amplitude due to the silica coating was found to increase with thicker silica coating, in agreement with our theoretical predictions.