Optimising gold nanorods for photoacoustic imaging in vitro

Abstract

Gold nanorods (AuNRs) can be synthesised with different sizes but similar aspect ratios and therefore similar surface plasmon resonances (SPRs). Their strong optical absorbance governed by their SPRs facilitates their ability to be used as molecular-targeted contrast agents for photoacoustic (PA) imaging. The size of AuNRs has an effect on the PA conversion efficiency, melting threshold, and cytotoxicity, indicating that size can have a significant impact on overall biomedical efficacy. We investigated these factors for four different AuNRs (widths of 10, 25, 40 and 50 nm) all with SPRs of 815 AE 26 nm. A size-dependent linear relationship between fluence and PA amplitude was observed, along with particle melting. Reshaping was confirmed via transmission electron microscopy and spectrophotometry at a laser fluence of 11 AE 1.7 mJ cm À2 , 20 AE 2.2 mJ cm À2 , and 40 AE 2.6 mJ cm À2 . Cytotoxicity was tested on lung cancer cells (A549) via a colourimetric assay at a maximum concentration of 3 Â 10 10 NP ml À1 . Results demonstrate the 40 nm and 50 nm AuNRs produced the highest signal for equivalent particle numbers, but displayed the highest toxicity. Conversely, the 10 nm AuNRs were the most efficient photoacoustic converters, at equivalent total mass. This study demonstrates the importance of AuNR size and concentration on selection of AuNRs for their eventual clinical use.

Photostability of Contrast Agents for Photoacoustics: The Case of Gold Nanorods

“…The analysis of such losses allows for a quantitative definition of a damage threshold [ 123 , 149 ], but relevant values are hardly comparable from experiment to experiment. For instance, the onset of damage may depend on whether the same sample is reused for exposure at different conditions [ 151 ], or instead refreshed after each train of pulses at a given fluence [ 123 ]. This protocol has been mainly applied with liquid samples, and its implementation requires laser sources of high energy, in order to homogeneously shine large areas of cuvettes or multiwell plates containing the dispersion of plasmonic particles for investigation with standard spectrophotometers.…”

“…Another method that exploits the dependence of the photoacoustic response on the optical absorbance of the gold nanorods [ 14 ] is based on the dynamic analysis of the trend of the signal amplitude with the extant fluence of the optical excitation [ 151 , 153 , 154 , 155 ]. For each condition, the signal amplitude is averaged over a given time or number of pulses, and then added to a scatter plot as a function of optical fluence.…”

Microbubble Resonators for All-Optical Photoacoustics of Flowing Contrast Agents

“…Finally, to demonstrate a quantitative reconstruction of the PA intensity through the height of the main peak at 5.75 MHz, we plotted this quantity against pump laser fluence in Figure 4. The typical photostability trend of GNRs can be recognized: a linear regime (up to about 5 mJ/cm 2 ), when GNRs are stable under excitation and the PA response is linear with their absorbance; a sub-linear regime (5-10 mJ/cm 2 ) associated with the onset of GNR reshaping [42,43]; a third regime (10-30 mJ/cm 2 ), associated with the onset of cavitation regime, where shockwaves are produced through water evaporation in proximity of the GNRs [44]. Additionally, we challenged the MBR system by reducing the GNR concentration, obtaining an overall linear scaling of the photostability curve and therefore demonstrating sensitivity towards the concentration of the contrast agent (see Figure S2…”

Improving Plasmonic Photothermal Therapy of Lung Cancer Cells with Anti-EGFR Targeted Gold Nanorods

“…The untargeted S-Au40-849s displayed little cellular uptake across all of the time points, which was surprising since it might be expected that after 24 incubation, much higher levels of uptake would be seen compared to that which was observed here. In comparison with the dark-field images taken of the citrate-capped AuNRs used in [ 13 , 17 ], there was considerably less cellular uptake of the untargeted S-Au40-849s. This was likely due to the streptavidin ligands that were already conjugated to the surface of the AuNRs, limiting the penetration of the S-Au40-849s into the lung cancer cells, since streptavidin has a relatively large molecular weight (approximately 60 kDa) and has been shown to restrict cellular uptake [ 18 ].…”