Radiation damage by high-energy particles is a common technique used for color enhancement of diamond in gem and jewelry industry. The treatment along with subsequent annealing generates a variety of crystallographic defects or color centers in diamond. Nearly all radiation-damaged diamonds show an absorption band associated with the neutral vacancy defect, V0, in their UV-Vis spectra. The vacancy, also known as the GR1 center, is one of the best characterized optical defects in diamond. It has a zero-phonon line (ZPL) at 741 nm, accompanied with a broad phonon sideband peaking at ~620 nm. When exposed to orange-red light, the center emits near-infrared fluorescence at ~898 nm but with a quantum yield of only ~1% at room temperature. Although GR1 is not a good chromophore for fluorescence imaging because of this low quantum yield, it is useful for photoacoustic (PA) imaging since most of the energy absorbed upon pulsed laser excitation will be released as heat, generating ultrasonic waves. Unlike other vacancy-related defects (such as nitrogen-vacancy centers), the GR1 centers can be generated in high density (>1000 ppm) by extensive ion irradiation, which makes it appealing for such applications.

PA imaging is a non-invasive imaging technique and has recently developed into a powerful diagnostic tool in biomedicine. This hybrid technique provides better spatial resolution for deep-tissue imaging than other optically based imaging modalities because acoustic waves are less scattered than optical waves in biological tissue. Gold nanorod is a popular PA contrast agent since the longitudinal surface plasmonic resonance band of this nanomaterial has an exceptionally large molar extinction coefficient in the near-infrared region. However, the application of the technique always suffers from the problem of photodamage due to shape deformation of the nanorods when illuminated with short and high-energy laser pulses. Extensively radiation-damaged or irradiated nanodiamonds (INDs) fabricated with high NIR absorbance offer an attractive alternative as the contrast agent for PA imaging. Side-by-side comparison of the PA signals of INDs and similarly dimensioned gold nanorods has indicated that the former can produce a ~70-fold higher intensity than the latter on molar basis. The finding was attributed to the excellent photostability of the sp3-carbon-based nanomaterials as well as the strong hydrogen bonding interaction between water and the oxidized IND surface that facilitates PA wave generation. These unique characteristics, together with its inherent biocompatibility and non-toxic nature, make IND a promising contrast agent for PA imaging both in vitro and in vivo.