Recent advances in fluorescence probes based on carbon dots for sensing and speciation of heavy metals

Heavy metal (HM) pollution is a major global concern. Carbon dots (CDs) have demonstrated unique properties as sensing platforms for HMs detection. This review summarizes the progress made in recent years in fluorescence methods to determine HMs and their species using CDs. First, the strategies to synthesize and purify CDs are reviewed. The photoluminescence principles of CDs and their sensing mechanisms as HMs sensors are then summarized. The binding strategies between CDs and HMs are proposed to provide salient principles to design desirable CD-based HMs sensors. The preparation and merits of "turn-on" and ratiometric CDs for HMs detection with higher accuracy are discussed compared with commonly used "turn-off" sensors. Subsequently, the progress on detecting single HM ions, multi-HMs, and different metal species in solution, and the development of gel/solid-state sensor platforms such as paper-based devices, sensor arrays, hydrogels, polymer films, and ion-imprinted polymers are critically accessed. Furthermore, the advances in the cell, bacterial, plant, and animal bioimaging of HMs with CDs as promising bioimaging reagents are presented. Finally, the challenges and prospects of CDs as HMs sensors in future investigations are discussed.