Fluorescence Origin of Nanodiamonds

Despite extensive work on the fluorescence behavior of graphite and graphene quantum dots, reports on the luminescence of nanodiamonds are so far much fewer. In fact, nanodiamonds are distinctly different from carbon quantum dots with nondiamond phases in both crystalline structure and electronic structure. Here, we report that fluorescent nanodiamond colloids exhibit strong visible fluorescence emissions and that their characteristics can be summarized as follows: (i) the fluorescence is unrelated to the size effect and (ii) obviously the excitation-dependent fluorescence, (iii) the maximum emission peak shows a giant red shift of 100 nm after heat treatment, and (iv) the red shift of fluorescence excited by a certain wavelength is out of sync with that of the strongest fluorescent peak. Based on these experimental observations above, the origin of nanodiamonds fluorescence is proposed to be the functional groups residing on the nanodiamonds, such as OH, ketone C=O, and ester C=O groups. These deductions are confirmed by the evolutions of the microscopic Fourier transform infrared spectrum. For a more specific aspect, the red shift and excitation dependence of the fluorescence in nanodiamonds is ascribed to the combined effect of the relative intensity changes of various types of oxygenous groups and low-lying effects resulting from n(OH) -> pi*(CO) interactions between the hydroxyl groups and carbonyl groups. Accordingly, both excitation-dependent fluorescence and excitation-independent fluorescence can be achieved by engineering the surface functional groups of nanodiamonds, which verifies the proposed mechanism. Overall, the essence of the fluorescence in nanodiamonds is the collaboration and competition of the functional groups. The collaboration is reflected in excitation-dependent fluorescence, whereas the competition is reflected in the change of the optimum excitation and emission wavelengths. Therefore, these results provide other guidelines for the design of carbon nanomaterials, not only nanodiamonds but also graphite and graphene quantum dots, for application in the field of biomedical labeling.