Metal ions-doped carbon dots: Synthesis, properties, and applications

Doping is an effective approach to regulate the physicochemical properties of carbon dots (CDs) and has received increasing attention in recent years. After CDs are doped with heteroatoms, their electronic structure, nano structure, and chemical composition will be changed due to the overlapping atomic orbitals of the heteroatoms and carbon atoms, as well as the push/pull electron effect of heteroatoms. As compared with the non-metallic atoms, metal ions have more electron and unoccupied orbitals, and larger atomic radius, doping CDs with metal ions may induce noticeable alterations of the optical, electronic, and magnetic properties by changing the electron density distribution and energy gap of the CDs. In this review, we summarize the synthesis methods of metal ions-doped CDs (M-CDs), discuss the novel physicochemical properties originating from the doping of metal ions, and provide examples of applications in sensing, imaging, phototherapy, optoelectronics, and catalysis. Furthermore, we describe challenges in preparing M-CDs and provide an outlook for their future development.