Ratiometric fluorescence and colorimetry dual-mode assay based on manganese dioxide nanosheets for visual detection of alkaline phosphatase activity

•A MnO2-based dual-mode assay for visualizing ALP at greatly different activities.•One mode of ratiometric fluorescence for high-activity ALP in livestock serum.•Another mode of colorimetry for ultralow-activity ALP in pasteurized milk.•Both modes provided ALP-dependent (fluorescence) color changes for visualization.•Potential for sensitive and selective detection of other reductant-related analytes.