Benchtop Fluorination of Fluorescent Nanodiamonds on a Preparative Scale: Toward Unusually Hydrophilic Bright Particles

Abstract

Fluorination of diamonds modulates their optical and electromagnetic proper- ties and creates surfaces with increased hydrophobicity. In addition, fluorina- tion of diamonds and nanodiamonds has been recently shown to stabilize fluorescent nitrogen-vacancy centers, which can serve as extremely sensitive single atomic defects in a vast range of sensing applications from quantum physics to high-resolution biological imaging. Traditionally, fluorination of carbon nanomaterials has been achieved using harsh and complex experi- mental conditions, creating hydrophobic interfaces with difficult dispersibility in aqueous environments. Here, a mild benchtop approach to nanodiamond fluorination is described using selective Ag+-catalyzed radical substitution of surface carboxyls for fluorine. In contrast to other approaches, this high- yielding procedure does not etch diamond carbons and produces a highly hydrophilic interface with mixed C−F and C−OH termination. This dual func- tionalization of nanodiamonds suppresses detrimental hydrophobic interac- tions that would lead to colloidal destabilization of nanodiamonds. It is also demonstrated that even a relatively low surface density of fluorine contributes to stabilization of negatively charged nitrogen-vacancy centers and boosts their fluorescence. The simultaneous control of the surface hydrophilicity and the fluorescence of nitrogen-vacancy centers is an important issue enabling direct application of fluorescent nanodiamonds as nanosensors for quantum optical and magnetometry measurements operated in biological environment.