Photoacoustic response optimization of gold nanorods in the near-infrared region

Photoacoustic imaging (PAI) combines the advantage of optical and ultrasonic imaging, which has a high signalto-noise ratio, and spatial resolution. To further improve the performance of PAI, gold nanorods can be utilized as exogenous contrast agents, and their size can be controlled. The size change of gold nanorods will change their absorption and heat transfer characteristics, and then affect their photoacoustic characteristics. Therefore, in the present work, the influences of absorption characteristics (absorption cross section and volume absorption coefficient) and heat transfer characteristics (specific surface area) of gold nanorods on photoacoustic response are studied by FDTD (FDTD Solutions, Lumerical) and FEM (Comsol Multiphysics). Results show that the increase of specific surface area will enhance the thermal coupling between gold nanorods and water, so as to improve the heat transfer and produce different photothermal responses without significantly affecting the photoacoustic quantum yield (As the specific surface area increases by about 10 times, the temperature increases by about 12 times, while the photoacoustic quantum yield increases by about 24%). On the contrary, absorption characteristics are decisive factors for both photothermal and photoacoustic responses. Therefore, in terms of improving photoacoustic quantum yield, when the size of gold nanorods changes, more attention should be paid to the improvement of the absorption characteristics rather than the specific surface area. In addition, increasing the specific surface area can significantly improve the photostability of gold nanorods, which is very important for practical applications.