Preparation and characterization of ion-irradiated nanodiamonds as photoacoustic contrast agents

Highly radiation-damaged or irradiated nanodiamonds (INDs) are a new type of nanomaterial developed recently as a potential photoacoustic (PA) contrast agent for deep-tissue imaging. This work characterized in detail the photophysical properties of these materials prepared by ion irradiation of natural diamond powders using various spectroscopic methods. For 40-nm NDs irradiated with 40-keV He⁺ at a dose of 3 × 10¹⁵ ions/cm², an average molar extinction coefficient of 4.2 M^-1 cm^-1 per carbon atom was measured at 1064 nm. Compared with gold nanorods of similar dimensions (10 nm x 67 nm), the INDs have a substantially smaller (by > 4 orders of magnitude) molar extinction coefficient per particle. However, the deficit is readily compensated by the much higher thermal stability, stronger hydrophilic interaction with water, and a lower nanobubble formation threshold (∼30 mJ/cm²) of the sp³-carbon-based nanomaterial. No sign of photodamage was detected after high-energy (> 100 mJ/cm²) illumination of the INDs for hours. Cell viability assays at the IND concentration of up to 100 μg/mL showed that the nanomaterial is non-cytotoxic and potentially useful for long-term PA bioimaging applications.