A　novel　fluorescence　and　colorimetric　dual　probe　based　upon　the　inner　filter　effect　(IFE)　is　reported　to　selectively　and　sensitively　determine　alkaline　phosphatase　(ALP).　First,　up-conversion　nanoparticles　(UCNPs)　were　prepared　via　a　facile　hydrothermal　method　and　characterized　by　transmission　electron　microscopy　(TEM),　X-ray　diffractometry　(XRD),　Fourier　transform　infrared　spectroscopy　(FT-IR),　and　zeta　potential.　Under　near　infrared　(NIR)　excitation　at　980　nm,　the　prepared　nanoparticles　emit　green　light　at　541　nm.　The　absorption　band　of　p-phenylenediamine　(PPD)　overlaps　the　emission　of　UCNPs,　which　effectively　quenches　the　fluorescence　of　UCNPs.　However,　ascorbic　acid　(AA)　generated　from　the　reaction　of　ascorbic　acid　2-phosphate　(AAP)　and　ALP　impedes　the　oxidation　of　PPD　and　the　fluorescence　is　restored.　Based　on　this　change,　a　“turn-on”　sensor　was　used　to　determine　AA　and　ALP　with　the　limits　of　detection　of　2.49　μM　and　0.018　U/L,　respectively.　The　constructed　assay　was　applied　to　determine　ALP　in　serum　and　the　recoveries　were　from　93.3%　to　101.3%　with　a　relative　standard　deviation　from　1.1%　to　3.3%.　A　facile　and　efficient　approach　is　reported　for　the　determination　ALP　and　offers　a　theoretical　basis　for　the　related　diseases.　©　2023　Taylor　&　Francis　Group,　LLC.