A 2D mixed-lanthanide metal‒organic framework as dual-emitting luminescent sensor for ratiometric detection of tetracycline and nitrophenols

Solvothermal treatment of Gd(III)/Eu(III) salts with 2,3,5,6-tetrachloroterephthalic acid (H2tClbdc) and 1,10-phenanthroline (phen) afforded three two-dimensional lanthanide metal‒organic frameworks (MOFs), [Ln(tClbdc)(phen)(H2O)(NO3)]n (1-Ln, Ln = Gd, Eu, Eu0.03Gd0.97). The dual-emitting characteristic along with the excellent solution and photophysical stability of the mixed-lanthanide MOF 1-Eu0.03Gd0.97 make it a highly sensitive luminescent sensor to ratiometrically detect tetracycline (TC), picric acid (PA) and 4-nitrophenol (4-NP), with the limits of detection of 0.38 μM, 0.75 μM and 1.49 μM for TC, PA and 4-NP, respectively. The significant luminescence quenching of the lanthanide-based sensor by these organic pollutants might be ascribed to inner filtration effect (IFE), fluorescence resonance energy transfer (FRET) and hydrogen-bonding and/or electrostatic interactions between the sensor and phenolic analytes. More significantly, 1-Eu0.03Gd0.97 filled mixed-matrix membranes could be employed as an easy-to-use sensory tool for visual recognition of TC by the turn-off red emission signal.