od dependent fl uorescence from nitrogen-vacancy ( NV ) centers of nano-diamonds †

Fluorescent nanodiamonds (FNDs) with high photo stability at a subwavelength scale are highly desirable for nano-photonics and bio-imaging applications. Nanodiamonds (NDs) with embedded fluorescent color centers made by ion-implantation need to be purified to remove the sp layer on their surfaces which significantly degrades the optical properties. In this work, we discuss the structural and photo physical properties of NDs containing nitrogen-vacancy (NV) centers prepared by two different purification methods; chemical etching (H2SO4:HNO3) and air oxidation (450 C). Chemically etched NDs show better uniformity in their shape, de-aggregation and higher dispersibility in water as compared to air oxidized ones. On the other hand it is observed that air oxidation is more effective in removing the sp layer and allows a higher fluorescence photon flux. Therefore, we suggest that air oxidation is more appropriate for bright fluorescent sources, and chemical etching is more appropriate for fluorescent markers in bio-imaging applications with high uniformity in shape and good dispersibility.