Laser Pulse Duration Optimization for Photothermal Therapy with Gold Nanostars

Photothermal therapy (PTT), which utilizes light radiation to create localized heating effect in the targeted areas, is a promising solution for highly specific yet minimally invasive cancer therapy. PTT uses photothermal agents, which are usually nanoparticles that absorb strongly in the near-infrared optical window where minimal tissue absorption occurs. Photothermal agents are also highly functionalized to target at specific tumor sites. Gold nanostar is an ideal candidate for photothermal agents, because it not only has a Surface Plasmon Resonance in the near-infrared, but also can be easily produced and purified, and is extremely versatile in the drug delivery process. In order to achieve maximum amount of localized heating, pulse lasers are usually used in laser ablation processes like photothermal therapy. However, intensive laser radiation can cause damage to regular tissues as well the nanostructures themselves. Therefore, identifying the optimal pulse duration to effectively generate localized heating in the tumorous tissues while keeping the normal tissues and the nanostructures intact is important to achieving optimal photo-therapeutic results. This manuscript provides a numerical calculation method with Comsol Multiphysics to optimize the pulse condition of the gold nanostars under photothermal therapy settings. Based on results, gold nanostar displays significant temperature heterogeneity under femtosecond and picosecond laser radiation, while nanosecond laser only induces rather uniform heating effects across the entire gold nanostar particle. This finding indicates that femtosecond laser, which is the most common type of laser used for ablation, is likely to melt the tip of the gold nanostar before the nanostar body reaches a reasonably high temperature. Picosecond and nanosecond lasers are much less likely to induce such dramatic morphology change. This study offers important insight into finding the optimal condition for photothermal therapy with maximal efficacy and minimal damage.