  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.
