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.