Near-Ultraviolet Fluorescent "ON-OFF-ON" Switching Sensors Based on Nitrogen-Enriched Dual-Color Single-Functional Polymer Carbon Nanosheets.

Graphene quantum-dot-like structures of highly enriched N-based dual-color single-functional polymer carbon nanosheets (GLNPCNs) with an average diameter as large as 170 nm (ca. five layers) are prepared through a facile hydrothermal method. Intermolecular C5 H5 N5 ⋅⋅⋅C5 H5 N5 and/or C2 H5 OH chemical polymerization interactions in the liquid state of the peripheral crown-ether-like "hole"-decorated compound GLNPCNs lead to a fascinating 2 D supramolecular polymer architecture, which exhibits near-ultraviolet (NUV) fluorescence, ultrahigh brightness, a narrow full-width at half-maximum, and excellent potential for practical applications. The unique NUV fluorescence of the GLNPCNs might be attributed to the "self-passivated" crown-ether-like layer on their surface; the GLNPCNs exhibit properties independent of the excitation wavelength, which is unexpected in the NUV region. Moreover, these GLNPCNs show highly selective and sensitive detection of hazardous and toxic silver (Ag+ ) ions, and discriminate other metal ions or anions in the range 1.5 nm to 50 μm through a fluorescence quenching response. The high sensing selectivity toward Ag+ ions could be attributable to the restricted rigid conformation of the peripheral crown-ether-like "hole", which binds exclusively to the Ag+ ion. In addition to Ag+ ion sensing, the GLNPCNs quenched by Ag+ ions exhibit high selectivity and sensitivity for cysteine (Cys) in the range 2.0 nm to 50 μm through a fluorescence recovery process, which could be attributed to the effective coordination/chelation interactions between Ag+ ions and the plentiful mercapto and amino groups of Cys. Our results suggest that the facile preparation, biocompatibility, outstanding photoluminescence, and physicochemical properties of these GLNPCNs make them potentially useful in numerous applications such as bioimaging, optical and electrochemical sensors, and energy devices.