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Pyrophosphate ion-triggered competitive displacement of ssDNA from a metal-organic framework and its application in fluorescent sensing of alkaline phosphatase.

Nanomaterial/aptamer assembly has been extensively explored in the detection of various targets, but some limitations still exist in its practical applications, especially time consumption and low-efficient detachment of the aptamer from the nanomaterial surface. In this work, we demonstrated the ligand role of pyrophosphate ion (PPi) in the competitive displacement of ssDNA from the nanoscaffold surface. For this purpose, a fluorescein-labeled ssDNA (F-DNA) and a mixed valence state cerium (Ce3+/Ce4+)-based MOF (MVCM) were employed as the signal response unit and nanoscaffold, respectively. Benefiting from the existence of Ce4+, the MVCM exhibits an ultrahigh quenching efficiency (more than 90%) to F-DNA fluorescence, which is 3-fold higher than that of the MOF with Ce3+ only. However, it was found that PPi can effectively suppress the quenching effect of the MVCM by competitive coordination with the MVCM to displace F-DNA. Different from the conventional target-induced conformational change of aptamers, the PPi-triggered displacement assay is independent of the ssDNA sequence and can be rapidly completed in just 2 min. The displacement assay is also highly sensitive, even at a PPi concentration as low as 55 nM. In contrast to PPi, however, the phosphate ions and other anions cannot displace F-DNA from the MVCM surface to switch on the F-DNA fluorescence. Inspired by this fact, the PPi-triggered displacement assay was further applied in the detection of alkaline phosphatase (ALP). The detection limit toward ALP was obtained at 0.18 mU mL-1. Moreover, the accurate determination of ALP concentration in serum samples indicates the applicability of this sensing system in detecting real samples.