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.