Preparation of Fluorescent Diamond Nanoparticles Stably Dispersed under a Physiological Environment through Multistep Organic Transformations

Imparting multiple functions to nanoparticles through organic functionalization has been attracting significant interest, particularly in terms of biomedical applications. Among them, diamond nanoparticles—the so-called nanodiamond (ND)—have been recognized as one of the best platforms, because of its nontoxic or low-toxicity properties, as well as its organic characteristics that enable covalent bonding with introduced functionalities. Here, we show multistep organic transformations on the ND surface that amass the requisite functions layer by layer through covalent bonds. The functionalities introduced onto the ND surface were well-characterized by solution-phase 1 H NMR and 13 C NMR spectroscopies, recently developed by us and other groups, as well as conventional infrared (IR), fluorescence microscopy and spectroscopy, and elemental analysis. As a result, hydrophilic and fluorescent characteristics were incorporated onto the ND surface by adding polyethylene glycol (PEG) and fluorescein segments, providing fluorescent ND stably dispersed under a physiological environment. ND has been confirmed to be viable for use as a cellular imaging agent by introducing them into HeLa cells.