Photothermal and Photoacoustic Response of a VO2@Au Nanoshell Irradiated by a Nanosecond Laser Pulse

Abstract

Plasmonic nanoparticles (PNPs) are considered as a proper mediator in photothermal therapy due to their capability to efficiently convert the absorbed light energy into localized heat. As the localized heat diffuses during therapy processes, it is crucial to control and detect the temperature in a non-invasive way. Synthesizing a new generation of PNPs that utilize optical phase-change materials leads to a tunable photothermal response without geometrical variation. This tunability originates from the prompt variation of optical and thermal properties during the phase-change transition. In this paper, we numerically study the photothermal response of a VO2@Au nanoshell in an aqueous medium when irradiated by a nanosecond pulsed laser (5 ns). For the temperature profiles, we simultaneously solve a self-consistent multiphysics problem consisting of electromagnetism and thermodynamics. We do our calculations for two wavelengths of 658 and 737 nm where the absorption spectrum of the nanoshell has two extrema during the VO2 core’s phase-change transition. Finally, for the two selected wavelengths, by coupling the structural dynamic physics to the problem, we calculate the acoustic pressure signals generated by the photothermal expansion of both the nanoshell and its surrounding medium. This photoacoustic signal could be considered as a non-invasive method to measure the local temperature in deep tissues accurately.