Under short-pulsed laser irradiation, plasmonic metallic nanoparticles convert light energy into thermal energy and heat up the surrounding medium. This sudden heating leads to the thermal expansion of the medium and generates pressure transients, known as the photoacoustic effect. PA signal has been developed as imaging tools for noninvasive diagnosis of disease. Compared with conventional PA agents such as organic dyes, plasmonic nanoparticles possess tunable plasmon resonance and good photostability. These unique properties make plasmonic nanoparticles promising contrast agents for PA imaging. Furthermore, the surface engineering of nanoparticles can improve their biocompatibility, thermal and colloidal stability, and PA signal generation. The surface coating, such as a thin silica layer on the gold nanoparticle surface, can significantly enhance the photoacoustic generation. One hypothesis is that silica coatings can change the heat transfer process on the interface of nanoparticles and medium resulting in PA improvement. However, a comprehensive understanding of how the silica coating enhances the photoacoustic generation is far from complete. Here, we use experimental methods and numerical simulations to investigate two effects on photoacoustic enhancement of silica coating on gold nanosphere, namely nanoparticle structure and laser duration. Firstly, from numerical simulation studies, we found that besides the phononic heat transfer between gold and silica, the energy will directly be transported from gold electrons to silica phonons. This additional transport mechanism enhances the heating rate of adjacent medium under the picosecond laser. Secondly, it was experimentally observed that silica coating can enhance photoacoustic signal by 2 folds under picosecond laser while no enhancement was observed under nanosecond laser. Furthermore, the silica thickness range for the signal amplification was wider as the size of the core particle increased. These experiment results agree with our theoretical model and demonstrate that the energy transport channel between gold electrons and silica phonons contributes to PA enhancement under picosecond laser.

Mechanism of enhanced thermal transport and photoacoustic effect for silica-coated gold nanoparticles

ACS Spring Meeting 2021

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