Sensitive and selective colorimetric detection of alkaline phosphatase activity based on phosphate anion-quenched oxidase-mimicking activity of Ce(Ⅳ) ions.

As alkaline phosphatase (ALP) plays crucial roles in disease warning and dephosphorylation-related cellular regulation, it is widely recognized as an important biomarker for clinical diagnosis. In this work, we proposed a facile colorimetric assay based on phosphate anion-quenched oxidase-mimicking activity of Ce(Ⅳ) ions for sensitive and selective detection of ALP activity. Free Ce(Ⅳ) ions exhibited a strong oxidase-like capability (providing a 40-fold catalytic turnover number compared with CeO2) to catalyze the oxidation of colorless 3,3',5,5'-tetramethylbenzidine (TMB) into its blue product TMBox mediated by dissolved O2 at neutral pH, thus triggering a remarkable color reaction visually. When PO43- was added, its strong affinity to Ce(Ⅳ) ions rapidly precipitated the free Ce(Ⅳ) ions, resulting in the quenching of their enzymatic ability. Given that ALP catalyzed the hydrolysis of adenosine triphosphate to produce PO43-, determination of ALP activity could be achieved using the colorimetric assay with no need of complicated instrumentation and protocol. As demonstrated, our assay offered a highly sensitive readout for ALP activity in two linear scopes of 0-50 U L-1 and 50-250 U L-1, providing a detection limit down to 2.3 U L-1. Besides, it could provide specific response toward ALP against other enzymes and biological species. Furthermore, the developed assay was successfully applied to evaluate ALP activity in human plasma accurately and reliably, indicating its great promise as a powerful and convenient tool for monitoring of ALP activity in clinical practice.