Synthesis and Photoluminescence Mechanism of Multicolored Nitrogen-Doped Carbon Nanodots and Their Application in Polymer Self-Assemblies

For applications in the fields of biological imaging and chemical sensing, fluorescent carbon nanodots (CNDs) have excellent possibilities because of their unique structure and performance. In this work, we synthesized a series of nitrogen-doped CNDs (N-CNDs) with different fluorescent emission, ranging from blue to yellow, via one-pot solvothermal strategy. Citric acid and urea were employed as the precursors. Formamide, ethanol and N, N -dimethylformamide were used as the solvent, respectively. The excitation-dependent emission and the optimal emission were measured with the ultraviolet–visible and photoluminescence spectra. The morphology and chemical structure of the prepared N-CNDs were characterized via high-resolution transmission electron microscope (HRTEM), atomic force microscope (AFM), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, carbon nuclear magnetic resonance ( 13 C NMR) spectroscopy, and so forth. The influence of reaction time, temperature, and type of solvent on the structure of N-CNDs were systematically investigated. The photoluminescence mechanism was discussed based on the chemical structure, the crystallinity and the sp 2 /sp 3 carbon ratio of the prepared N-CNDs. In addition, a biocompatible amphiphilic copolymer mPEG- b -PLLA was synthesized and self-assembled in an aqueous system. The prepared N-CNDs were introduced into the polymer self-assemblies as a fluorescent probe by an in situ grafting method. The fluorescence imaging performance of N-CNDs in the polymer self-assemblies was investigated by comparing with organic fluorescent dye coumarin-6.