Regulating the overlap between the absorption spectrum of metal ion-chromogenic agent and the emission spectrum of carbon-based dots to improve the sensing performance for metal ions

Carbon-based dots (CDs) exhibiting excellent fluorescence (FL) properties have been well applied in sensing. In particular, many FL sensors based CDs have been developed for various metal ions. However, the sensors based on the CDs without special functionalization usually show poor selectivity and sensitivity. In this work, cobalt(II) ion (Co2+) is taken as the model to introduce a novel, sensitive and selective FL sensing mode for metal ions based on CDs without special functionalization. A kind of highly luminescent nitrogen and sulfur co-doped CDs (N,S-CDs) are used as the probes. The FL signal of N,S-CDs is insensitive to Co2+ itself, but is quenched sensitively and selectively by Co2+ in the presence of cysteamine. Actually, cysteamine reacts with Co2+ to form a positively charged and absorbent complex (Co(cys)(3)(2+)), whose absorption band covers completely the emission spectrum of N,S-CDs. Then, a FL resonance energy transfer (FRET) system is formed, in which N,S-CDs act as the donor and Co(cys)(3)(2+) acts as the acceptor. Furthermore, the electrostatic interaction between the positivity charged Co(cys)(3)(2+) and the negatively charged N,S-CDs increases the collision, enhancing the FRET effects. As a result, the FL signal of N,S-CDs can be dynamically quenched by Co(cys)(3)(2+). Finally, a sensitive and selective FL sensor is developed for Co2+. More importantly, this paper provides CDs a novel FL sensing mode, which may be applied in many other metal ions. (C) 2016 Published by Elsevier B.V.