A water-stable zinc(ii)–organic framework as an “on–off–on” fluorescent sensor for detection of Fe3+ and reduced glutathione

Abstract

As an important biomarker, the level of reduced glutathione in serum is vital for human health. A novel 3D metal–organic framework with excellent photoluminescence properties, formulated as [Zn₃(TNB)(DPE)₂]·2H₂O·DMF (1) (H₃TNB = 4,4′,4′′-nitrilotribenzoic acid and DPE = 1,2-di(4-pyridyl)ethylene), was constructed by a solvothermal reaction. Complex 1 exhibits high water stability and pH stability in aqueous solution. It can detect Fe³⁺ and Pb²⁺ as a “turn-off” fluorescent probe in PBS solution with high sensitivity and selectivity. The detection limits for Fe³⁺ and Pb²⁺ are about 3.40 × 10⁻⁷ M and 4.59 × 10⁻⁷ M, respectively. Compared to Pb²⁺, Fe³⁺ has stronger oxidizability, and can be reduced to Fe²⁺ with the addition of a reductant, causing the fluorescence intensity of the Fe³⁺@1 system to significantly recover while that of the Pb²⁺@1 system has little change. Thus, the Fe³⁺@1 system exhibits a fluorescence “turn-on” signal after adding reduced glutathione (GSH) due to the reduction reaction, which indicates that complex 1 can be designed as an “on–off–on” fluorescent probe to detect Fe³⁺ ions and reduced GSH. The mechanism for fluorescence recovery is the elimination of the FRET process between Fe³⁺ and complex 1. In view of this reversible detection event, a two-input molecular logic gate was constructed. In addition, the “on–off–on” fluorescent sensor can be successfully applied to detection of reduced GSH in serum and fruits and vegetables with a high recovery range and low relative standard deviations (RSDs). To the best of our knowledge, this is the first example of a MOF-based probe for reduced GSH recognition by fluorescence “on–off–on” in PBS solution.