Surfactant-Like π-Conjugated Axial Ligands Enable Single-Layer Zn-MOF Nanosheets for Fluorescence Sensing of Phenols

Abstract

Metal−organic framework (MOF) nanosheets have achieved great success in various fields; however, the preparation of monolayer MOF nanosheetsparticularly via ultrasonic exfoliationremains challenging. In this work, we report the synthesis of a Zn(II)-MOF, [Zn(HL)(L)0.5(bpy)0.5] (Zn-1, where H2L = 4,4’-[(2,5-dimethoxy-1,4-phenylene)di-2,1-ethenediyl]bis-benzoic acid and bpy = 4,4′-bipyridine), and its successful exfoliation into monolayer nanosheets (Zn-1-NS) by ultrasonic treatment. Single-crystal X-ray diffraction revealed that the coordination of Zn2+ with L2−/HL− and bpy generated two-dimensional (2D) networks, in which the partially deprotonated ligands were oriented vertically on both sides of the layers, resembling a surfactant-like arrangement. These 2D layers were further assembled into a three-dimensional (3D) supramolecular framework through C−H···π and π···π interactions. Ultrasonic exfoliation in isopropanol (IPA) yielded Zn-1-NS with a thickness of approximately 4 nm, consistent with the calculated thickness of a single coordination layer (4.19 nm), confirming the formation of monolayer MOF nanosheets. Compared with bulk Zn-1, Zn-1-NS exhibited enhanced fluorescence and superior sensing performance toward 2,4-dinitrophenol (2,4-DNP) in water, with high selectivity, sensitivity, anti-interference capability, fast response, good recyclability, and a low detection limit (0.14 μM) via a turn-off mechanism. Moreover, application tests in real water samples demonstrated excellent practicability and reliability.