Photoacoustic (PA) imaging is based on the detection of ultrasound signals emitted by physiological targets that underwent a pulsed laser irradiation. Gold nanoparticles are being currently studied by several research groups as potential molecular contrast agents for PA imaging. Aim of this paper was to test whether a highly biocompatible PEG (polyethylene glycol) coating can improve the stability of gold nanorods (GNRs) under laser irradiation and their effectiveness as contrast agents for PA imaging with respect to uncoated GNRs. Uncoated GNRs and PEG-coated GNRs were synthesized with the same size (48x7 nm) and very similar absorption spectra (main peak at 1055 nm). GNR stability was evaluated as a function of both laser fluence (range 40-100 mJ/cm2) and exposure duration (30-60 s), monitoring optical and morphological GNR changes. PA effectiveness was then tested using a custom-designed phantom which allowed laser irradiation of GNR solutions of variable concentration contained in a tissue-mimicking hydrogel and acquisition of the corresponding PA signals through a clinically-available ultrasound device. Obtained results showed that absorption spectrum of uncoated GNRs was significantly deteriorated after laser exposure already in the mildest adopted conditions (30-s exposure to 40-mJ/cm2 laser), while PEG-coated GNRs always resulted much more stable, with negligible peak intensity decrements in the mildest irradiation conditions. TEM analysis confirmed the higher morphological stability of PEG-coated GNRs, which also resulted more effective as PA contrast enhancers, since their PA signal intensity was always significantly higher than the corresponding value measured for uncoated GNRs.
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