Deep tissue optoacoustic monitoring of photothermal treatments in the NIR-II assisted with silica-coated gold nanorods.

Gold nanoparticles (AuNPs) absorbing light in the near-infrared (NIR) range offer unparalleled benefits for both optoacoustic (OA) imaging and photothermal therapy (PTT), stemming from their ability to transform optical energy into heat. These unique theranostic capabilities are further complemented by the high sensitivity of OA signals to temperature variations. However, AuNPs typically experience rapid photodegradation when exposed to high laser intensities, which hinders their efficient monitoring with OA. To address this critical limitation, we synthesized silica-coated gold nanorods (AuNRs) featuring enhanced photostability and an absorption peak in the second NIR window (NIR-II, 1064 nm) for optimal tissue penetration. Their comprehensive evaluation under exposure to nanosecond-pulsed and continuous-wave (CW) radiation revealed that the synthesized AuNRs are photostable under laser energy densities required for efficient therapy under OA imaging guidance, which was confirmed with electron microscopy images. Real-time volumetric OA mapping of PTT-induced temperature variations was verified using simultaneous thermal camera readings, whilst post-mortem experiments in mice corroborated the viability of this theranostic approach in deep biological tissues.