Hexacoordinate germanate metal–organic frameworks for the detection of Pb2+ ions in aqueous solution

Abstract

Hexacoordinate silicate and germanate linkers supporting high negative charges have been prepared and applied as building blocks for anionic MOF frameworks with lanthanide metal-based nodes. The SiO₆/GeO₆ centred linkers [Et₃NH]₂[H₃L-Si/Ge] [H₃L-Si/Ge = tris(3,4-diolato-benzoic acid) silicate/germanate] were initially prepared and characterised, with the germanate ligand demonstrating excellent water stability. A series of rare earth MOFs (IMP-33-Sc and IMP-34-Ln, Ln = Y, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu) were then synthesised under hydrothermal conditions using H₃L-Ge. Both IMP-33-Sc and IMP-34-Ln exhibit (3,6)-connected networks with the same flu-3,6-C2/c topology, however, whereas IMP-33-Sc contains trimetallic nodes, IMP-34-Ln nodes are bimetallic, thus leading to a higher anionic network charge of 4− in IMP-34-Ln (vs. 1− in IMP-33-Sc). The terbium based MOF IMP-34-Tb exhibits characteristic luminescent properties and is shown to be selectively quenched by Pb²⁺ ions in aqueous solutions through a dynamic quenching process. A strong linear relationship between the fluorescence intensity and Pb²⁺ concentration in the range of 0.01–4 mM makes it a promising sensor for aqueous Pb²⁺ ions.