Size Affects the Stability of the Photoacoustic Conversion of Gold Nanorods

Gold nanorods exhibit intense optical
absorption bands in the near-infrared
region of principal interest for applications in biomedical optics,
which originate from sharp plasmon resonances. This high absorbance,
combined with the biochemical inertness and targetability of gold
nanoparticles, makes these materials excellent candidates to provide
contrast in photoacoustic imaging and for other applications such
as the selective hyperthermia of cancer. One issue demoting the potential
of gold nanorods as contrast agents in photoacoustic applications
is their limited photostability, which falls below relevant permissible
exposure limits. In particular, when gold nanorods are resonantly
excited by laser pulses in the nanosecond duration regime, there may
occur phenomena like reshaping into rounder nanoparticles as well
as fragmentation and sublimation, which modify their optical absorption
bands and hinder their efficiency of photoacoustic conversion. Here
we investigate the influence of nanoparticle size on the photostability
and reproducibility of photoacoustic conversion of gold nanorods embedded
in biomimetic phantoms. We compare samples containing gold nanorods
with different sizes but the same shapes and overall optical densities.
We demonstrate clear size effects as the thresholds of optical fluences
for nanoparticle deformation improve from below 2 to above 6 mJ/cm2 with nanoparticle miniaturization from 22 to 5 nm effective
radii. We interpret these results in terms of a better thermal coupling
and faster heat dissipation from smaller nanoparticles to their environment,
originating from their larger specific surface area.