Synergistic effect of fluorescence recovery and enhancement on ultrasensitive visual assay of cyanide anions based on N-Acetyl-L-Cysteine-Capped CdTe quantum dots and carbon dots

Developing a reliable fluorescent nanosensor for cyanide anions (CN − ), with the potential to enable visual discrimination by naked eye, is highly sought after yet is limited by low sensitivity and selectivity. Herein based on the synergistic effect of fluorescence recovery and enhancement, a specific dual-emission nano-system was designed for ultrasensitive detection of CN − , by employing carbon dots (peaking at 443 nm) as a reference and N-acetyl-L-cysteine (NALC)-capped CdTe quantum dots (QDs, peaking at 611 nm) as a reporter. The red fluorescence of CdTe QDs is first quenched by Cu 2+ primarily with electron transfer. Upon addition of CN − , CN − coordinates with Cu 2+ to form [Cu(CN) n ] (n−1)− complex, causing the Cu 2+ to detach from the QDs, and consequently fluorescence recovery. More importantly, the CN − can undergo a nucleophilic addition reaction with the carbonyl group of NALC ligands, inducing 1.3-fold fluorescence enhancement towards the original QDs while the blue fluorescence of carbon dots remains constant. This results the fluorescence intensity ratios ( I 611 / I 443 ) are proportional to the concentrations of CN − in the ranges of 0.02―10 and 15―80 μM, and an ultra-low detection limit down to 10.35 nM is achieved. By using both Cu 2+ -promoted complexation and addition reaction as recognition units, the present method also showed excellent selectivity for CN − over other coexisting anions. Especially, we have already demonstrated, by spiked tests, the practicability of monitoring the concentration changes of CN − in both environmental water and cassava samples, and further realized visual monitoring of CN − changes in aqueous solution and test paper.