Sub-5 nm Nanodiamonds Fabricated by Plasma Immersion Ion Implantation as Fluorescent Probes

Fluorescent nanodiamonds (FNDs) contain nitrogen vacancy (NV) centers that emit photostable fluorescence in the wavelength range between 500 and 800 nm, which does not overlap with cellular autofluorescence. Also, the FNDs possess excellent biocompatibility. For this reason, they are promising fluorescent probes for background-free imaging for biomedical applications. The FNDs used in vivo, however, should be <10 nm for renal clearance. In this study, we introduce sub-5 nm nanodiamonds fabricated by mesh-assisted plasma immersion ion implantation as fluorescent probes for biomedical purposes. The FNDs generated with this process emitted bright fluorescence with their photoluminescence (PL) peaks centered at 620 nm. Atomic force microscopy, transmission electron microscopy, and X-ray diffraction (XRD) measurements confirmed that the average diameter of the treated sample was <5 nm. Moreover, the electron paramagnetic resonance analysis results demonstrated that the 7.5 min ion-implanted FND sample contained twice as much NV- center concentration as the annealed, oxidized, and acid-cleaned detonation nanodiamonds (DNDs). Finally, the purity of samples was confirmed using X-ray photoelectron spectroscopy (XPS) and XRD data. This study opens an avenue toward sub-5 nm FNDs with promising applications as fluorescent probes in biomedicine.