Pyrophosphate-Mediated On–Off–On Oxidase-Like Activity Switching of Nanosized MnFe2O4 for Alkaline Phosphatase Sensing

Developing reliable and facile approaches for alkaline phosphatase (ALP) sensing is important due to its role as a clinical biomarker for many diseases. In this study, we proposed a new and convenient colorimetric assay based on the pyrophosphate (PPi)-mediated oxidase-mimicking activity switching of nanosized MnFe2O4 for the detection of ALP. The synthesized MnFe2O4 exhibited high oxidase-like activity to catalyze the oxidation of colorless 3,3′,5,5′-tetramethylbenzidine (TMB) to its blue product TMBox in the presence of dissolved O2, leading to a color reaction rapidly and remarkably; PPi could significantly inhibit the activity of the MnFe2O4 nanozyme via the strong interaction between PPi and the Fe(III) species in MnFe2O4, resulting in the suppression of the TMB color reaction; when ALP was added, it hydrolyzed the PPi substrate to phosphate (Pi) that had no obvious effect on the MnFe2O4 activity, and such that the TMB color reaction catalyzed by the nanozyme could be observed again. With the above principle, linear colorimetric determination of ALP in the scope of 0.6–55 U L−1 was achieved, giving the limit of detection down to 0.27 U L−1. Besides, the developed assay could provide selective response toward ALP against other co-existing biological species. Furthermore, reliable detection of ALP in human serum samples was verified by our assay, revealing its great promise as an effective and facile tool for ALP monitoring in clinical practice.