Energy-Absorbing and Local Plasmonic Nanodiamond/Gold Nanocomposites for Sustained and Enhanced Photoacoustic Imaging

Photoacoustic (PA) imaging is a laser-mediated optical ultrasound-based visualization that allows imaging of optical energy absorbers in deep tissue, offering higher spatial resolution, compared with that of NIR fluorescence. To enhance a gold nanoparticles-based PA agent, carbon crystalline nanodiamonds and gold nanocomposites (NDAuNPs) were synthesized by chemical reduction of a carboxylate nanodiamond and gold precursor. Reduced hydroxyl-terminated nano- diamonds have stable colloidal dispersion and provide a platform where AuNPs are localized on the ND surface with high density. NDAuNP agglutinates were 100 nm in size, and AuNPs with a size distribution of 5−20 nm were chemically conjugated on the ND surface. The surface- enhanced Raman scattering spectra showed enhanced intensity of NDAuNPs in a concentration-dependent manner. Energy-absorbing nanodiamonds facilitated energy transfer into AuNPs, inducing a local plasmonic effect. The PA signal of NDAuNPs was stronger than that of the AuNPs, as well as the signal maintenance during a prolonged period of laser irradiation. Tissue images of TEM showed that after 2 h irradiation NDAuNPs were maintained without gold degradation, while AuNPs were degraded. The local plasmonic and the energy-absorbing properties of NDAuNPs amplified the PA signal and impeded the degradation of gold without PA signal decay. The NDAuNP nanocomposites may serve as an imaging probe, providing high PA amplitudes.