Flexible Ligands\nConstructed Metal–Organic\nFrameworks as Visual Test Paper for Fluorescent Detection

In this paper, four new metal–organic frameworks\n(MOFs)\nwere synthesized by the solvothermal method based on two imidazolidyl\nflexible ligands, named, CUST-662–665, [Zn(pbbm)(HBTC)]·3H<sub>2</sub>O (CUST-662), [Cd<sub>2</sub>(pbbm)<sub>2</sub>(BTC)(NO<sub>3</sub>)·H<sub>2</sub>O]·2H<sub>2</sub>O (CUST-663), [Cd<sub>3</sub>(bbbm)<sub>3</sub>(BTC)<sub>2</sub>·4H<sub>2</sub>O]·3H<sub>2</sub>O (CUST-664), and [Zn<sub>2</sub>(bbbm)<sub>2</sub>(BTC)(HCOO)]·3H<sub>2</sub>O (CUST-665). All four MOFs with blue fluorescence show excellent\nperformance in the detection of pollutants including Fe<sup>3+</sup>, Cr<sub>2</sub>O<sub>7</sub><sup>2–</sup>, trinitrophenol\n(TNP), and tetracycline antibiotics. Furthermore, the color of CUST-664\nand CUST-665 changes from blue to yellow when detecting tetracycline\n(TC) and oxytetracycline (OTC), which can be further developed as\nsimple and rapid field visual test paper as antibiotic detection devices.\nThe detection limits of TNP, OTC, and TC can reach 0.36, 0.90, and\n1.39 ppb, respectively, which are better than that of most of the\nreported fluorescence sensing materials. Moreover, CUST-662–665\ndisplay high selectivity, sensitivity, stability, and recycling efficiency.\nThe competitive absorption between the crystals and pollutants was\nan important factor affecting fluorescence quenching. This work provides\nan experimental basis for subsequent fluorescence visualization field\ndetection.