Photothermal reshaping and cavity formation in silica-coated gold nanorods using nanosecond pulsed lasers.

The aspect ratio of Au nanorods can be precisely engineered using near-infrared nanosecond-pulsed laser irradiation, which enables ultrafast, confined energy deposition inaccessible under continuous-wave irradiation. In this study, we demonstrate a novel method for reshaping Au nanorods encapsulated within thin (<10 nm) silica shells. By leveraging the silica shell as a rigid nanocrucible, nanosecond laser irradiation induces rapid, end-selective shortening of the Au nanorod core, creating terminal cavities of a controllable size. Transmission electron microscopy confirms that while hexadecyltrimethylammonium bromide-coated Au nanorods convert into spherical and ϕ-shaped nanoparticles, the silica shell constrains the laser induced reshaping process, preserving the rod-like morphology while systematically reducing the aspect ratio. Consequently, the distinct longitudinal and transverse plasmon resonances are retained post-irradiation. The reshaping can be precisely controlled by adjusting laser fluence, resulting in a fine-tuned aspect ratio and a significantly narrowed longitudinal resonance, an outcome typically associated with femtosecond laser systems. 4D Scanning transmission electron microscopy reveals that the Au nanorods transform from single crystal to polycrystalline structures upon irradiation, providing direct evidence that nanosecond pulsed irradiation induces complete Au core melting and rapid recrystallization within the shell - with multiple nucleation sites. Despite the polycrystalline structure the resonance peak was narrower than that of the starting nanorod. This method for fabricating Au nanorods with integrated cavities within the offers significant potential for applications in triggered drug delivery, biosensing, and photoacoustic imaging.