Monodisperse Five-Nanometer-Sized Detonation Nanodiamonds Enriched in Nitrogen-Vacancy Centers

Nanodiamonds containing negatively charged nitrogen-vacancy (NV–) centers are versatile nanosensors thanks to their optical and spin properties. While currently most fluorescent nanodiamonds in use have at least a size of a few tens of nanometers, the challenge lies in engineering the smallest nanodiamonds containing a single NV– defect. Such a tiny nanocrystal with a single NV– center is an “optical spin label” for biomolecules, which can be detected in a fluorescence microscope. In this paper, we address two key issues toward this goal using detonation nanodiamonds (DNDs) of 4–5 nm in size. The DND samples are treated first with electron irradiation to create more vacancies. With the aid of electron paramagnetic resonance (EPR) spectroscopy, we confirm a steady increase of negatively charged NV– centers with higher fluence. This leads to a 4 times higher concentration in NV– defects after irradiation with 2 MeV electrons at a fluence of 5 × 1018 e–/cm2. Interestingly, we observe that the annealing of DND does not increase the number of NV– centers, which is in contrast to bulk diamond and larger nanodiamonds. Since DNDs are strongly aggregated after the irradiation process, we apply a boiling acid treatment as a second step to fabricate monodisperse DNDs enriched in NV– centers. These are two important steps toward “optical spin labels” having a single-digit nanometer range size that could be used for bioimaging and nanosensing.