Resolving the thermal dynamics of pulsed-laser excited gold nanorods in suspension

Photonic applications of metallic nanoparticles include the exploration of photo-thermal phenomena when the particle-loaded materials are subjected to intense short laser pulses. Nanoscale heat release and cavitation phenomena in aqueous suspension can be explored for so-called theranostics: thermal modification of tissue or nanobubbles as contrast agents.

Gold nanorods have obtained large attention in the last years due to the ability to tailor shapes and therefore optical response and a strong photonic resonance in the near-IR, the transparency window for biological tissue.

However, thermal reversible and nonreversible dynamics can be manifold and include laser-induced particle heating, bubble formation, particle reshaping or various kind of destructive laser interaction. We have employed time-resolved X-ray scattering on gold nanorod suspension in order to clarify all these phenomena with a sub-nanosecond time resolution. It turns out that reversible, strong reactions, such as bubble formation are not easily separable from destructive reactions in terms of laser power. The most prominent change is the spherical reshaping at rather low laser fluence. On the other hand, irreversible morphology changes might be useful for nanoscale structuring, e. g. of the optical response (color).

A dynamic “phase diagram” summarizes the observed reactions as function of delay time after laser excitation and as function of laser fluence (energy per cross sectional area).

Figure: Right: Electron microscopy, extinction and image of a gold nanorod suspension. Left: regions of the fluence – delay “phase diagram” of different transient relaxations with the dots marking the measured boundaries or onset of reactions.

We would like to acknowledged support by the chemistry lab, M. Wulff at the beamline ID09 at ESRF and Andreas Menzel at beamline cSAXS at SLS and the funding by a Heisenberg grant.

Publication:

A. Plech, S. Ibrahimkutty, S. Reich, G. Newby: Thermal dynamics of pulsed-laser excited gold nanorods in suspension, Nanoscale, 2017, DOI: 10.1039/C7NR06125K [link: http://pubs.rsc.org/en/content/articlelanding/2017/nr/c7nr06125k#!divAbstract ]