"Turn-on" fluorescence sensing for sensitively detecting Cr(<sc>vi</sc>) via a guest exchange process in Cu NCs@MIL-101 composites

Copper nanoclusters (Cu NCs) are a new fluorescent material that is often used for determining metal ions, but most sensing systems are based on the "turn-off" model. Here, a "turn-on" model of fluorescence sensing for the detection of Cr(vi) was developed based on Cu NCs@MIL-101 composites. The Cu NCs@MIL-101 composites were synthesized from a simple mixture of Cu NCs and MIL-101(Cr), in which the Cu NCs were uniformly distributed in MIL-101(Cr). Notably, the fluorescence intensity of Cu NCs@MIL-101 was significantly weakened due to the internal filtration effect (IFE) of MIL-101. When Cr(vi) was introduced, the fluorescence of Cu NCs@MIL-101 was recovered by the guest exchange process between Cr(vi) and the Cu NCs, which overcame the IFE of Cu NCs@MIL-101. Based on this, a "turn-on" fluorescence probe was successfully constructed for the quantitative detection of Cr(vi) with two linear ranges of 0.05-1 mu M and 1-20 mu M, and a low detection limit of 0.05 mu M. The proposed fluorescence probe possessed excellent selectivity and anti-interference ability, and was successfully applied for the detection of Cr(vi) in real water samples with satisfactory results. This study provides a new approach for the analytical application of Cu NCs.