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 x 10(15) ions/cm(2), 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 (similar to 30 mJ/cm(2)) of the sp(3)-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 mu g/mL showed that the nanomaterial is non-cytotoxic and potentially useful for long-term PA bioimaging applications.