Production of Metal-Free C, N Alternating Nanoplatelets and Their In Vivo Fluorescence Imaging Performance without Labeling

The use of luminescent probes with proper optical and morphological properties, high serum stability, low cytotoxicity, and good biocompatibility is a cost-effective method for bioimaging. In this work, a route is developed to produce a novel bioimaging probe framework. A C₃N₄ material (UCN-H) is produced by thermal condensation of urea under humidified air treatment. Chemical characterizations reveal that the UCN-H contains C₃N₄ networks with smaller grain sizes and more amine-based functionalities at the edges than UCN, which is separately produced without the humidified air treatment. Highly stable aqueous dispersions including fluorescent C₃N₄ nanoplatelets are generated by sonication of the UCN-H powder. The photoluminescence (PL), time resolved-PL, and 2D excitation-emission spectra of the dispersions show that the UCN-H has less-intra bandgap traps and longer PL lifetime than UCN. In confocal microscopic study using the nanoplatelets, clear fluorescent cell images are obtained without any cytosolic aggregation. In in vivo imaging studies with MDA-MB-231 tumor-bearing mice models, persistently strong fluorescence signals are successfully observed on tumor lesions without any interference of autofluorescence from live tissues after their accumulation by passive tumor targeting. Ex vivo biodistribution and histology results are well-matched with in vivo fluorescence imaging results.