“Turn-on” fluorescent detection of cyanide based on polyamine-functionalized carbon quantum dots

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.