A simple, sensitive, rapid fluorescence (FL) sensor has been developed to detect cyanide ions (CN−) based on the branched polyethylenimine-capped carbon quantum dots (BPEI-GQDs). The amino groups at the surfaces of BPEI-GQDs can selectively and sensitively capture copper ions (Cu2+) to form absorbent cupric amine, which can quench the FL of BPEI-GQDs through an inner filter effect. CN− can combine strongly with Cu2+ to [Cu(CN)x]n− species, preventing Cu2+ from being captured by the amino groups of BPEI-GQDs. As a result, Cu2+ can't quench the FL of BPEI-GQDs anymore in the presence of CN−. In other words, CN− can “turn on” the FL signal of BPEI-GQDs/Cu2+ system, producing a “recovered” FL signal. After optimizing experimental conditions, we demonstrated that this easy methodology could offer a sensitive, selective, simple and rapid method for detection of CN− with a detection limit of 0.65 μM (S/N = 3) and a linear response range of 2 to 200 μM.
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