Layer-by-layer self-assembly xylenol orange functionalized CdSe/CdS quantum dots as a turn-on fluorescence lead ion sensor

A new turn-on fluorescence sensor based on xylenol orange (XO) functionalized CdSe/CdS quantum dots (QDs) is developed for the determination of lead ion. CdSe/CdS QDs were first modified by mercaptoacetic acid (MAA). The MAA-modified QDs were then capped with the natural biopolymer chitosan and the negatively charged XO. The XO-functionalized QDs were formed via the layer-by-layer self-assembly reaction. The fluorescence of the QDs was quenched by electron transfer mechanism after XO was bound to the QDs. Upon the addition of Pb2+, a dramatic enhancement of the fluorescence intensity was observed, which resulted from the coordination between Pb2+ and XO on QDs surface and the disruption of the electron transfer mechanism. Hence the fluorescence of the QDs was recovered. The recovery of the fluorescence intensity showed a good linear relationship with the concentration of Pb2+ added from 0.05 to 6 μmol L−1. A detection limit of 0.02 μmol L−1 was achieved. This method was successfully applied to the determination of lead in real samples with satisfactory results.