Nitrogen/sulfur-doped dual-emission carbon dots with tunable fluorescence for ratiometric sensing of ferric ions and cell membrane imaging

Intrinsic dual-emission carbon dots (CDs) with long wavelength and self-calibration attributes are critical in fluorescence sensing and imaging, but their synthesis remains a great challenge. Here, nitrogen/sulfur-doped dual-emission CDs (NS-CDs) of different sizes were controllably synthesized by solvothermally treatment of sodium alginate (SA) and glutathione (GSH) in formamide. Interestingly, these as-prepared NS-CDs exhibit enchanting dual-emission behavior at 480 and 650 nm, tunable fluorescence, superior photostability, and high nitrogen level up to 19.57%. The decreasing mass ratio of SA/GSH for the synthesis can lead to larger size and lower graphitic nitrogen for NS-CDs, ultimately giving the enhanced photoluminescence (PL) and tunable fluorescence changing from cyan to white-blue. The selected dual-emission NS-CDs showed superior selectivity and sensitivity for ratiometric Fe3+ sensing with a detection limit as low as 0.56 μM because of the aggregation of NS-CDs induced by Fe3+, and achieved the Fe3+ detection in environment water. Moreover, the fluorescence-tunable NS-CDs possessing plenteous hydrophilic groups exhibit excellent biocompatibility and unprecedented localized capability for specific cell membrane imaging. This study designs a facile and economical strategy to prepare dual-emission CDs with long wavlength, tunable fluorescence and high nitrogen level, which present bright prospects in environmental monitoring and cell imaging.