The selective detection of the anion pyrophosphate (PPi) has become a major research focus in past few decades. PPi is a biologically important target because it is the product of ATP hydrolysis under cellular conditions. It is also involved in DNA replication catalyzed by DNA polymerase; its detection is being investigated as a real-time DNA sequencing method. So far, binuclear Zn(II) or Cu(II)-dipicolylamine (DPA) complexes have been mostly used in devising fluorescent probes for PPi.
We synthesized a couple of mononuclear Zn(II) and Cu(II)-DPA complexes which give fluorescence enhancements (17 and 24 folds respectively) selectively in presence of PPi in aqueous medium. We also observed that in presence of PPi fluorescence increases with time. In these probes the PPi recognition site was installed onto a 2-hydroxy-6-cyanonaphthalene fluorophore. Coordination of the hydroxyl (or alkoxy) group from the fluorophore to the appended metal complex perturbs the electronic push-pull event of the fluorophore. Binding of PPi releases such hydroxyl (or alkoxy) linkage; thus restores the push-pull characteristics and cause a significant change in the fluorescence. Slow cleavage of such metal-oxygen linkage also accounts for the time-dependent nature in the sensing process. So far, only a couple of mononuclear Cu(II)-DPA complexes are reported to sense PPi selectively, but such time-dependency is not explicitly described in these publications.