Diamond nanoparticles are known to be chemically stable and biocompatible. Moreover, nanodiamonds (< 100 nm) possess point crystal colour defects in their core lattice. One such defect is the Nitrogen-Vacancy (NV) centre which is responsible for unique spin-optical properties including photostable fluorescence, long lifetime and a quantum yield near unity. Together with easy surface functionalization, these properties make nanodiamonds a highly suitable fluorescent bio-label for single molecule tracking and imaging.
However, obtaining a monodisperse preparation of ultrasmall (< 10 nm) nanodiamonds remains one of the primary challenges for development of this material as a bio-label. One of the main issues in fact, is that smaller nanodiamonds (<20 nm) tend to aggregate. Here we present a purification and processing method for isolating nanodiamonds of various sizes from 4 nm to 100 nm. We also demonstrate functionalization of the nanodiamonds surface for targeting biological structures, specifically actin filaments. The specific labelling of the target biomolecule is shown using several independent techniques including confocal microscopy, transmission electron microscopy and fluorescence spectroscopy. Single molecule imaging is also performed using confocal fluorescent microscopy. These results open up many possibilities for using nanodiamonds as an imaging label in complex biological systems.