Size Affects the Stability of the Photoacoustic Conversion of Gold Nanorods

Abstract

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.

Optically induced microbubbles around gold nanorods: the influence of particle parameters and environment on cavitation threshold

“…We associate the first two to a sub-bubble regime, where the slope change is related to the phenomena of GNRs reshaping upon overheating. 29,[35][36][37][38][39][40][41][42] At fluences greater than 80 mJ/cm 2 , the increase of PA signal indicates the onset of the photocavitation regime.…”

“…Details of the experimental procedure are reported in Ref. 29. Briefly, we irradiated different points of the samples with 20 pulses at different nominal fluences with average values F exc .…”

“…A decrease of the PA signal at PA ref following irradiation at a given F exc is evidence for irreversible changes of the optical and structural properties of the samples. 29 We quantified this decrease by measuring the ratio R between PA ref after and before irradiation at each F exc . Figure 2 (full red circles) displays the trend of R as a function of F exc .…”

“…29, 30 for details). Briefly, the optical source was provided by an optical parametric oscillator pumped with the third harmonic of a Q-switched Nd:YAG laser (Continuum Surelite OPO plus, Santa Clara, USA, wavelength range 400-2500 nm, pulse duration 5 ns).…”

The influence of cellular uptake on gold nanorods photostability and photoacoustic conversion efficiency

“…[17][18][19][20] One of the main drawbacks for the application of GNRs when using short light pulses is their photo-instability under pulsed excitation. 21,22 In particular, when GNRs are irradiated with high-fluence nanosecond laser pulses in resonance with their plasmon oscillations, there may occur phenomena like reshaping into GNRs with shorter AR or even thermionic emission and fragmentation at higher optical fluences. These transformations result into dramatic modifications of their optical absorption bands that reduce their PA conversion efficiency.…”