The construction of a ratiometric dual-emitting amorphous europium-organic frameworks for sensitive detection of water in white spirit

It is very crucial to develop a simple and fast-responsive sensor for the detection of water content in organic solvents. Herein, three different europium-organic frameworks (Eu-MOFs): amorphous Eu-2-aminoterephthalic acid (amEu-NH2BDC), crystalline Eu-2-aminoterephthalic acid (cryEu-NH2BDC) and amorphous Eu-1,4-benzenedicarboxylic acid (amEu-BDC) are synthesized, respectively. amEu-NH2BDC is employed as fluorescence probe for the determination of water in ethanol. Results indicate that amEu-NH2BDC exhibits good performance due to its dual-wavelength fluorescence (FL) with the ligand (NH2BDC)-centered blue luminescence at 430 nm and Eu-centered red luminescence at 615 nm. With the increase of water contents in ethanol, the emission intensity at 430 nm increases dramatically, while that at 615 nm decreases under a single excitation at 270 nm. By measuring the ratio of emission intensity at 430 nm and 615 nm (I430/I615), a wide concentration range from 1% to 99.5% water in ethanol is obtained with a lower detection limit value of 0.024% (S/N = 3). Besides, in the presence of water in ethanol, the fluorescence color of amEu-NH2BDC changes from red to blue under 365 nm UV light. Thus, the visual device for the detection of water is realized by using amEu-NH2BDC. Combined with the fluorescence lifetime and density functional theory, it is shown that red luminescence weakens due to dynamic quenching and non-radiative vibration energy transfer, and blue luminescence is enhanced due to the hydrogen bonding between water and carboxyl groups in the ligand, which enhances the intramolecular charge transfer process. The proposed method has characteristics of rapid response, excellent stability, high sensitivity and selectivity. Importantly, it can be successfully applied to monitor the content of water in white spirit samples.