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N-Doped Fluorescent Carbon Nanosheets as a Label-Free Platform for Sensing Bisphenol Derivatives

In this paper, we report the synthesis of fluorescent carbon nanosheets from carbon nanoparticles produced from the burning of oil. This has been achieved by nitric acid oxidation, which initiates the sheet formation. We performed a detailed study of the formation mechanism, which revealed that this oxidation technique stitches the small carbon nanoparticles into a large sheet via nitrogen defect incorporation, which, in turn, introduces strain in the nanosheet through pyridinic or pyrazolic ring formation. The synthesis technique also introduces several oxygenated surface functional groups, which provide excellent colloidal stability to the nanosheets. Nitrogen incorporation also assists in generating strong greenish-yellow fluorescence with ∼15% quantum yield. A comprehensive study of the fluorescence origin reveals that this emission has two different origins: one originating from the excitation wavelength-dependent conjugation of sp2 clusters in the sp3 backbone and the other originating from the fixed n−π* transition of oxygenated and nitrogenated defect states, which is excitation wavelength-independent. These nanosheets are several microns in length and ∼1 to 2 nm in thickness. We explored the application of these nanosheets for label-free sensing of bisphenol derivatives as organic molecules and found that they can interact with the π-ring structures of the nanosheets. Interestingly, bisphenol derivatives interact selectively with the nanosheets, creating a blue shift of the emission spectra. In addition to the high selectivity for bisphenol detection, the detection limit was found to be as low as a few nanomolar (limit of detection (LOD) ∼0.3 nM).