Gold Nanorods for Photoacoustic Imaging and Microsurgery

Recent advances in Photoacoustic (PA) techniques has provided momentum for the development of innovative imaging and therapeutic applications. The PA effect rests on the interaction of short optical pulses with tissues, which triggers a cascade of photothermal and thermoelastic processes, leading to the emission of ultrasound and even the generation of vapor microbubbles. Ultrasound emission is exploited for imaging, while microbubbles may be exploited to damage malignant cells, with the advantage, when compared with the standard use of optical hyperthermia, of a far better localization. On the other hand, when compared with ultrasound imaging or acoustic cavitation, an optical excitation is compatible with the use of exogenous contrast agents that may permeate tumors and target specific markers, thus enabling the use of the same contrast agents and the same optical source for theranostic purposes. In this context, Gold NanoRods (GNRs) are achieving resounding success as contrast agents for PA imaging and microsurgery. The high absorbance in the near infrared region combined with the inertness of gold make these particles excellent candidates to improve the contrast in PA imaging and to trigger optical cavitation for the selective damage of malignant cells. However, GNRs still suffer from limitations. Their cylindrical profile is unstable against overheating and tends to revert into more spherical shapes without optical absorbance in the near infrared window. We developed an experimental set-up that allows us to investigate the photoacoustic efficiency and photostability of GNRs and the generation of vapor microbubbles. This quantitative approach enables a systematic optimization of relevant parameters, such as size, shape, coating and particles environment. We expect these results will provide useful indications to the development of GNRs as contrast agents for innovative theranostics techniques based on light and ultrasound interaction.