Application of Luminescent Diamond Nanocrystals to Intra-Cellular Optical Imaging

Nanodiamond is capable of filling a niche in the demand for new optical labelling technologies. This is due to the potential to encapsulate photo-stable luminescent nitrogen-vacancy defects into diamond crystals which are biologically compatible and have dimensions smaller than 10 nanometres. The focus of this thesis is the development of diamond as a near-ideal optical label for intracellular imaging. Facets of this work include producing, enhancing and detecting luminescence in single-digit nanodiamonds and interfacing this material with an intracellular environment. In this thesis the first observation of nitrogen-vacancy luminescence in weakly bound clusters of 5-nm diamond is described. These luminescent defect centres are produced in crystals with a diameter of only 29 carbon atoms. Interesting properties are then characterised, such as the scaling of luminescence intensity with crystal size and the distribution of nitrogen in single-digit nanodiamonds. Enhancement of this luminescence is found to be possible by surface oxidation treatment which removes intrinsic amorphous carbon shells. These shells surround sub-10 nm diamonds and as our results show, quench nitrogen-vacancy luminescence. By applying this technique, previously observed background luminescence is also ameliorated. A technique for background–free luminescence detection is demonstrated, despite the low absorption cross-section of nitrogen-vacancy centres. This is achieved by exploiting the long excited-state lifetime of luminescent nanodiamonds with time-gated detection. The high frame-rate desired for particle tracking in cells is maintained by implementing a novel wide-field arrangement. Finally, the first demonstration of nanodiamond as a scattering optical label in a biological environment is presented, showing that nanodiamonds can be biologically compatible and well contrasted from the cell background. Together these results represent a step towards resolving extended intracellular biological processes, which require sub-10 nm, photostable optical labelling. For other applications such as high-resolution magnetometry and near-field optical microscopy, the discovery of luminescence in 5-nm crystals and the reduction of short-lived luminescence presented in this work is a critical step forward