Plasmonic Nanoparticles\nfor Photothermal Therapy:\nBenchmarking of Photothermal Properties and Modeling of Heating at\nDepth in Human Tissues

Many different types\nof nanoparticles have been developed for photothermal\ntherapy (PTT), but directly comparing their efficacy as heaters and\ndetermining how they will perform when localized at depth in tissue\nremains complex. To choose the optimal nanoparticle for a desired\nhyperthermic therapy, it is vital to understand how efficiently different\nnanoparticles extinguish laser light and convert that energy to heat.\nIn this paper, we apply photothermal mass conversion efficiency (η<i><sub>m</sub></i>) as a metric to compare nanoparticles of different\nshapes, sizes, and conversion efficiencies. We selected silica-gold\nnanoshells (AuNShells), gold nanorods (AuNRs), and gold nanostars\n(AuNStars) as three archetypal nanoparticles for PTT and measured\nthe η<i><sub>m</sub></i> of each to demonstrate the\nimportance of considering both photothermal efficiency and extinction\ncross section when comparing nanoparticles. By utilizing a Monte Carlo\nmodel, we further applied η<i><sub>m</sub></i> to\nmodel how AuNRs performed when located at tissue depths of 0–30\nmm by simulating the depth penetration of near-infrared (NIR) laser\nlight. These results show how nanoparticle concentration, laser power,\nand tissue depth influence the ramp time to a hyperthermic temperature\nof 43 °C. The methodology outlined in this paper creates a framework\nto benchmark the heating efficacy of different nanoparticle types\nand a means of estimating the feasibility of nanoparticle-mediated\nPTT at depth in the NIR window. These are key considerations when\npredicting the potential clinical impact in the early stages of nanoparticle\ndesign.