Green preparation of versatile nitrogen-doped carbon quantum dots from watermelon juice for cell imaging, detection of Fe3+ ions and cysteine, and optical thermometry

Abstract

Photoluminescent nitrogen-doped carbon quantum dots (N-CQDs) hold great promise for numerous applications because of their attractive attributes. However, the development of N-CQDs with bright fluorescence and unique optical responsibility is still in its infancy. Herein we present a facile and green approach to prepare versatile N-CQDs via hydrothermal carbonization of watermelon juice. The resulted N-CQDs with high fluorescence quantum yield (10.6%) showed superior water dispersibility, stable photoluminescence, and low cytotoxicity. In addition, the N-CQDs exhibited selective and sensitive fluorescence quenching behavior towards Fe3+ ions and the limit of detection (LOD) can reach 0.16 μM. It was also demonstrated that the N-CQDs/Fe3+ system can be employed to selectively sense cysteine (LOD = 0.27 μM) based on the fluorescence turn on effect. Moreover, the N-CQDs also showed interesting temperature-dependent fluorescence behavior and can be utilized as a nanothermometer, as demonstrated by determining temperature in cells in this study.