Synergistic effect of sandwich polyoxometalates and copper–imidazole complexes for enhancing the peroxidase-like activity

Two inorganic–organic hybrids based on copper(II)–imidazole complex modified sandwich-type tungstobismuthate or tungstoantimonite, Na4H2[Cu4(H4im)12(H3im)2][Cu3(H2O)3(XW9O33)2]·nH2O (H4im = imidazole, H3im = deprotonated imidazole, X = Bi (1), n = 28 or X = Sb (2), n = 25), have been synthesized and structurally characterized by single crystal X-ray diffraction and FTIR. The sandwich polyoxoanion [Cu3(H2O)3(XW9O33)2]12− is modified by two copper(II)–imidazole complexes to form 1 or 2, in which an anionic imidazole is rarely observed as a linker to coordinate to two neighboring copper ions. As compared to other Keggin-type polyoxometalate-based peroxidase-like mimics, 1 and 2 both demonstrate higher peroxidase-like activity using 3,3′,5,5′-tetramethylbenzidine (TMB) as a peroxidase substrate in the presence of H2O2 around physiological pH values in a heterogeneous phase. The maximized synergistic effects of sandwich-type POMs and copper(II)–imidazole complexes are responsible for the enhancement of the peroxidase-like activity. The negatively charged POMs and non-covalent interactions between imidazole ligands and TMB are beneficial to increase the affinity of TMB substrate, and thus lead to higher sensitivity towards H2O2. Considering the dependence of TMB colour on H2O2 concentrations, compound 1 can be used as an effective probe to detect H2O2 through a facile colorimetric method in a linear range from 1 μM to 50 μM with a low detection limit of 0.12 μM.