Size-dependent thresholds for melting and nanobubble generation using pulsed-laser irradiated gold nanoparticles

Abstract

The use of plasmonic nanoparticles (PNPs) in biophotonics has been steadily increasing, with a wide variety of emerging applications including detection and treatment of cancers. PNPs have unique, tunable optical properties arising from their surface plasmon resonance (SPR), and can be functionalized to achieve molecular targeting in vivo. Highly transient nanobubbles generated around PNPs due to pulsed laser-induced vaporization and cavitation may significantly impact diagnostic and therapeutic procedures. While much effort has been devoted to elucidating the mechanisms behind PNP nanobubble generation, there has not been a comprehensive study on the effect of nanoparticle size on nanobubble formation thresholds, and whether particle photodegradation occurs at these levels. Herein, we evaluate the melting and bubble generation thresholds of commercially available gold nanospheres in the 20-100 nm diameter range when exposed to nanosecond laser pulses. The 532-nm laser pulses are generated by a second-harmonic Q-switched Nd:YAG laser, providing output near the SPR of the gold nanospheres. Bubble generation is detected using a multimodality microscopy platform developed for simultaneous, nanosecond resolution pump-probe imaging, scattering response monitoring, and acoustic transient detection. PNP damage was further evaluated using transmission electron microscopy and spectrophotometry. The results of this study provide insights into damage thresholds as well as the concurrent and rapid optical, thermal, acoustic, and mechanical phenomena involved. This work will provide a foundation for improved understanding of a variety of PNPs and inform development of standard test methods for safety and efficacy evaluation of pulsed laser-PNP interactions in biophotonics.