Preparation of multi-colored carbon dots via pH-controlled degradation of wheat bran/o-phenylenediamine for Fe3+ ion detection†

Multi-colored carbon dots (CDs) have attracted significant research interest due to their wide range of applications. However, the mechanisms underlying their luminescence and regulation still require further exploration. Herein, blue, green, and orange CDs with different quantum yields were successfully prepared, by changing the thermal degradation behavior of the waste wheat bran/o-phenylenediamine via adjusting the pH of the acidic hydrothermal conditions. The structure–photoluminescence property relationship of the prepared multi-colored CDs was thoroughly analyzed and discussed, aiming to provide new insights into the luminescence red-shift mechanisms of CDs. It was found that the larger sp² domains, higher graphitization of carbon cores, increased graphitic nitrogen, and elevated levels of CO functional groups contribute to the CDs' lower band gap, resulting in a red-shift of the emission fluorescence. Among them, the elevated levels of CO functional groups are normally neglected for their contribution in the red-shift of CDs. Additionally, the prepared CDs were explored for their application in monitoring Fe³⁺ content in aquatic environments. A detection limit of 378.76 nM was obtained for the prepared CDs.