An ideal ratiometric fluorescent probe provided by the surface modification of carbon dots for the determination of Pb2+

Herein, carbon dots (Cdots) with a single green emission peak and no response to metal ions were converted to a high sensitive and selective ratiometric fluorescent probe for the detection of Pb2+ through a simple chemical surface modification with glutathione. The as-prepared modified Cdots show two emission peaks at a 170 nm distance (from 522 to 692 nm) that were excited with a single excitation wavelength (390 nm) and responded oppositely to Pb2+. This probe provided an ideal ratiometric fluorescent probe for the determination of Pb2+ in a linear concentration range of 10–700 nM with a detection limit of 2.7 nM. Theoretical calculations based on density functional theory (DFT) theory demonstrated that the surface modification of Cdots with glutathione forms new emitting centers on the surface of Cdots that were suppressed through a photo-induced electron transfer (PET) process from the sulfur lone pair electrons of the surface glutathione ligands. However, in the presence of Pb2+, chemical binding between the sulfide groups of surface glutathione ligands of neighboring Cdots and Pb2+ aggregated Cdots. Upon this complex formation, the involved PET process was stopped and subsequently a new emission peak at longer wavelengths appeared.