A Eu-MOF-Based Fluorescent Sensing Probe for the Detection of Tryptophan and Cu2+ in Aqueous Solutions

Abnormal tryptophan (Trp) metabolism can be used as an important indicator of chronic hepatitis, paranoia, Parkinson's disease and other diseases. Deficiency or excessive accumulation of Cu2+ can cause diseases such as Wilson's disease and Alzheimer's disease. Eu-based metal-organic framework (Eu-MOF) was successfully prepared for fluorescence sensing of Trp and Cu2+ in an aqueous solution (pH = 7.4). Eu-MOF showed high selectivity and sensitivity for Trp and Cu2+ with detection limits of 0.22 µM and 0.09 µM and Ksv of 6.17 × 103 M- 1 and 2.37 × 104 M- 1 respectively. Trp and Cu2+ had overlapped UV absorption spectra with that of Eu-MOF and competed for the excitation light source. Trp also attenuated the antennae effect of organic ligands on Eu-MOF, thus quenching the red fluorescence of Eu-MOF. This study provides insights into the application of MOFs in bioanalysis and diagnostics.

Ratiometric fluorescence sensor based on europium-organic frameworks for selective and quantitative detection of cerium ions

BACKGROUND

Due to the unavoidable use of cerium in daily life, the accumulation of cerium in the environment increases health risks for humans. Therefore, it is crucial to develop a chemical sensing technology for the rapid, sensitive, and selective detection of cerium ions.

RESULTS

In this research work, a novel two-dimensional chain structure of a europium-based metal organic framework (Eu-MOF) [Eu2(tcpa)(Htcpa)2] was synthesized by using 3,4,5,6-tetrachloro-1,2-benzenedicarboxylic acid (H2TCPA) as the ligand and europium nitrate as the metal source. The results of powder X-ray diffraction and thermogravimetric analysis show that the synthesized Eu-MOF has excellent chemical and thermal stability. When the Eu-MOF suspension was excited by ultraviolet light at 292 nm, four fluorescence emissions were observed at 420, 595, 620 and 705 nm. It was particularly interesting that when cerium ions (Ce3+/Ce4+) were added to the Eu-MOF suspension, the fluorescence intensity at 420 nm was enhanced, while the fluorescence at 620 nm was quenched. On this basis, a ratiometric fluorescent sensor for detecting cerium ions was constructed, which has a good linear relationship in the range of 0.05-15 μM and a detection limit of 16 nM. The plausible mechanism of the change in the fluorescence characteristics of Eu-MOF caused by cerium ions was discussed in detail. Through the study of fluorescence lifetime and ultraviolet absorption, it was proven that the mechanism of Ce3+-quenching Eu-MOF fluorescence is the inner filter effect. Photoinduced electron transfer and internal filtering effects lead to fluorescence quenching at 620 nm, while redox reactions lead to fluorescence enhancement of the ligand at 420 nm.

SIGNIFICANCE

The proposed ratiometric fluorescence sensor was successfully employed for the detection of cerium ions in real water samples, confirming that it can be used as an alternative method for the detection of Ce3+ and Ce4+ in environmental samples.

A turn-off Eu-MOF@Fe2+ sensor for the selective and sensitive fluorescence detection of bromate in wheat flour

A new europium metal-organic framework (Eu-MOF) was prepared by simple hydrothermal method. The product exhibited intense red fluorescence, long fluorescence lifetime (0.454 ms) and excellent fluorescence stability. The fluorescence titration result showed that Fe³⁺ could completely quench the fluorescence of Eu-MOF, while the fluorescence quenching effect of Fe²⁺ or bromate was negligible. Considering the strong oxidizing property of bromate, a "turn off" Eu-MOF@Fe²⁺ sensor toward bromate was designed by generating Fe³⁺ due to the redox reaction. The results showed that the sensor displayed a wide linear range (0-0.2 mM), high sensitivity (LOD = 3.7 × 10^-6 mol/L), good selectivity and resistant to possible interferences in real four sample. Furthermore, the detection mechanism was investigated by PXRD, XPS and UV-Vis methods. More importantly, the Eu-MOF@Fe²⁺ sensor was further applied to detect bromate in wheat flour with satisfactory recovery (95.30%-104.38%) and accuracy (RSD < 2.85%). These results suggest that Eu-MOF@Fe²⁺ can be used as a potential sensor to detect bromate in food industry.

Synergistic Effects of Eu-MOF on Perovskite Solar Cells with Improved Stability

Enhancing device lifetime is one of the essential challenges in perovskite solar cells. The ultrathin Eu-MOF layer is introduced at the interface between the electron-transport layer and the perovskite absorber to improve the device stability. Both Eu ions and organic ligands in the MOF can reduce the defect concentration and improve carrier transport. Moreover, due to the Förster resonance energy transfer effect, Eu-MOF in perovskite films can improve light utilization and reduce the decomposition under ultraviolet light. Meanwhile, Eu-MOF also turns tensile strain to compressive strain in the perovskite films. As a result, the corresponding devices achieve a champion power conversion efficiency (PCE) of 22.16%. In addition, the devices retain 96% of their original PCE after 2000 h under the relative humidity of 30% and 91% of their original PCE after 1200 h after continuous 85 °C aging condition in N₂ .

A ratiometric fluorescence platform based on boric-acid-functional Eu-MOF for sensitive detection of H2O2 and glucose

A Eu-metal organic framework (Eu-MOF) probe with dual-emission was reported for the ratiometric fluorescence detection of H₂O₂ and glucose. Because of the special nucleophilic reaction between boric group and H₂O₂, Eu³⁺ and 5-boronobenzene-1,3-dicarboxylic acid (BBDC) were selected to synthesize the functional MOF probe via a simple one-pot solvothermal method. The Eu-MOF shows dual-emission at 370 and 623 nm with the single excitation at 270 nm due to the energy transfer efficiency change for antenna effect procedure. After addition of H₂O₂, the red emission of Eu-MOF weakened and the blue emission enhances immediately under 270 nm irradiation, so the ratiometric fluorescence detection is established. Moreover, the obvious color change for visual measuring of H₂O₂ and glucose is illustrated to reveal the merit of Eu-MOF probe. The proposed method was demonstrated to be highly sensitive and selective for H₂O₂ and glucose, with the low detection limits of 0.0335 and 0.0643 μM, respectively. The established boric-acid-functional Eu-MOF sensing platform was proved as the powerful probe for H₂O₂ and the metabolites involved in H₂O₂-generating reaction.