  The fluorescent nitrogen-vacancy (NV) defect in diamond has remarkable
photophysical properties, including high photostability, which allows stable
fluorescence emission for hours; as a result, there has been much interest in
using nanodiamonds (NDs) for quantum optics and biological imaging. Such
applications have been limited by the complexity and heterogeneity of ND
photophysics. Photophysics of the NV center in NDs have been studied before,
but the lack of a sensitive and high-throughput method has limited the
characterization of NDs. Here is reported a systematic analysis of NDs using
two-color wide-field epifluorescence imaging coupled to high-throughput
single-particle detection of single NVs in NDs with sizes down to 5-10 nm. By
using fluorescence intensity ratios, the charge conversion of single NV center
(NV- or NV0) is observed and the lifetimes of different NV charge states in NDs
is measured. The discovery of reversible manipulation of NV charge states by pH
is also presented. In addition to provide another mechanism to control the NV
charge state in nanodiamonds, our findings open the possibility to perform pH
nanosensing with a non-photobleachable probe.
