  Ensembles of negatively charged nitrogen vacancy centers (NV-) in diamond
have been proposed for sensing of magnetic fields and paramagnetic agents, and
as a source of spin-order for the hyperpolarization of nuclei in magnetic
resonance applications. To this end, strongly fluorescent nanodiamonds
represent promising materials, with large surface areas and dense ensembles of
NV-. However, surface effects tend to favor the less useful neutral form, the
NV0 centers. Here, we study the fluorescence properties and optically detected
magnetic resonance (ODMR) of NV- centers as a function of laser power in
strongly fluorescent bulk diamond and in nanodiamonds obtained by nanomilling
the native material. In bulk diamond, we find that increasing laser power
increases ODMR contrast, consistent with a power-dependent increase in
spin-polarization. Surprisingly, in nanodiamonds we observe a non-monotonic
behavior, with a decrease in ODMR contrast at higher laser power that can be
ascribed to more efficient NV- -> NV0 photoconversion in nanodiamonds compared
to bulk diamond, resulting in depletion of the NV- pool. We also studied this
phenomenon in cell cultures following internalization of NDs in macrophages.
Our findings show that surface effects in nanodiamonds substantially affect the
NV properties and provide indications for the adjustment of experimental
parameters.
