Dual-mode visual fluorescent/colorimetric ratio sensing of alkaline phosphatase in milk based on porphyrinic MOF photonanozyme fibers.

The detection of alkaline phosphatase (ALP) in milk is crucial for determining the safety of dairy products, but single mode detection may result in false positives or false negatives. This work presents a dual-mode visual fluorescent/colorimetric ratio sensor for highly sensitive detection of ALP. Zinc meso-tetra(4-carboxyphenyl) porphyrin (ZnTCPP) metal-organic frameworks (MOFs) were grown on polyacrylonitrile with Al2O3 layers (Al-ZnTCPP@PAN). It exhibited enhanced photoresponsive oxidase-like activity, catalyzing 3,3',5,5'-tetramethylbenzidine (TMB) to amplify the colorimetric signal. ALP hydrolyzes l-Ascorbic acid 2-phosphate sesquimagnesium salt hydrate (AA2P) to produce phosphate ions (PO43-), which attack the Al node of Al-ZnTCPP@PAN and cause ZnTCPP to detach from the material surface. It results in weakened colorimetric signals of Al-ZnTCPP@PAN and enhanced fluorescent signals of the reaction solution. A smartphone-integrated platform enabled dual-mode visual fluorescent/colorimetric ratio detection of ALP with the limit of detection as low as 0.039 mU/mL, successfully applied to milk, highlighting its potential in food safety.