A ratiometric multicolor fluorescence biosensor for visual detection of alkaline phosphatase activity via a smartphone

Herein we designed a selective and smartphone-based strategy for visual detection of alkaline phosphatase (ALP) by utilizing the property of amino-functionalized copper (II)-based metal-organic frameworks (NH2-Cu-MOFs) with oxidase mimic property and fluorescence property. Surprisingly, the oxidase mimic property of NH2-Cu-MOFs can work well at a high pH value 8.0. Thus, a cascade reaction between ALP and NH2-Cu-MOFs was realized for the construction of a ratiometric multicolor sensing platform through the controllable catalytic activity of NH2-Cu-MOFs by pyrophosphate (PPi) and ALP. The catalytic activity of NH2-Cu-MOFs was greatly inhibited because of the binding ability of Cu2+ with PPi. When the ALP was added, the catalytic activity of NH2-Cu-MOFs was restored and then further catalyzed the o-phenylenediamine to form the 2, 3-diaminophenazine due to the hydrolysis function of ALP towards PPi into orthophosphates. RGB analysis of the fluorescent sample images was adopted for ALP quantitative analysis. Besides, a hydrogel test kit and mobile app for ALP detection were designed as conceptual products for point-of-care. The LODs of the fluorescence sensing platform was 0.078 mU mL−1 and 0.35 mU mL−1 by solution analysis and hydrogel test kit analysis, respectively. This fluorescent visual method was applied to ALP detection in serum samples with satisfying results, which opened a promising horizon for the diagnosis of other biomarkers in clinical serum samples based on ALP-mediated enzyme-linked immunosorbent assay for the development of biomedicine and clinical diagnosis.