Multifunctional sensing activities toward heavy metals of three luminescent complexes: Effect of N-donor coligands and sensing medium

Abstract

The functional groups and flexibility of organic building blocks were crucial for regulating the structure and luminescence as well as detecting activity of MOFs. Three Complexes of a flexible carboxylate ligand with –CH2-O- groups, namely, [Cd3(L)2(DMF)4]n (1), {[Cd3(L)2(bpea)3(H2O)4]∙2H2O∙4DMF}n (2), and [Cd3(L)2(bpy)]n (3) (H3L = 3,5-bis[(3-carboxyphenyl)oxy]benzoic acid, bpea = bis(pyridyl)ethane, and bpy = 4,4′-bipyridine), were synthesized under solventhermal conditions. Both complexes 1 and 2 exhibit interesting 2-D trilayer structures with (3,6)- and (3,4,6)-connected topology, respectively. Complex 3 displays 3-D complicated framework, in which Cd2+ ions are connected by the carboxylate groups of L3− ligands into 3-D motifs and further linked by bpy ligands. The flexible carboxylate ligand adopts three different coordination modes with the aromatic rings forming diverse dihedral angles. Luminescence measurements reveal that both the frameworks and the solvents play an important role on the sensing activities. Three complexes can selectively and sensitively detect Fe3+ and Cr2O72− in DMF through luminescence quenching (turn-off) with the latter two showing high water stability and sensing activities toward Fe3+ and Cr(VI) (Cr2O72− and CrO42−). The detection limits are in the range of 0.21–2.05 μM, comparable or superior to those of previous luminescence sensors. Specially, complex 3 could also detect Pb2+ in H2O with detection limit of 1.43 μM (0.30 ppm). The building units, the structures, the sensing media all have an effect on the sensing activities. Possible quenching mechanisms for detection of these ions were carefully investigated and proposed based on luminescence inner filter effect (IFE) and weak interactions between the target ions and frameworks. Moreover, the recoveries of 96.7–103.4% for the detection of above ions in tap water samples are achieved, suggesting practical application.