Alkaline　phosphatase　(ALP)　as　an　essential　enzyme　plays　an　important　role　in　clinical　diagnoses　and　biomedical　researches.　Hence,　the　development　of　convenient　and　sensitivity　assay　for　monitoring　ALP　is　extremely　important.　In　this　work,　on　the　basis　of　chemical　redox　strategy　to　modulate　the　fluorescence　of　nitrogen-doped　graphene　quantum　dots　(NGQDs),　a　novel　label-free　fluorescent　sensing　system　for　the　detection　of　alkaline　phosphatase　(ALP)　activity　has　been　developed.　The　fluorescence　of　NGQDs　is　firstly　quenched　by　ultrathin　cobalt　oxyhydroxide　(CoOOH)　nanosheets,　and　then　restored　by　ascorbic　acid　(AA),　which　can　reduce　CoOOH　to　Co2+,　thus　the　ALP　can　be　monitored　based　on　the　enzymatic　hydrolysis　of　L-ascorbic　acid-2-phosphate　(AAP)　by　ALP　to　generate　AA.　Quantitative　evaluation　of　ALP　activity　in　a　range　from　0.1　to　5　U/L　with　the　detection　limit　of　0.07　U/L　can　be　realized　in　this　sensing　system.　Endowed　with　high　sensitivity　and　selectivity,　the　proposed　assay　is　capable　of　detecting　ALP　in　biological　system　with　satisfactory　results.　Meanwhile,　this　sensing　system　can　be　easily　extended　to　the　detection　of　various　AA-involved　analytes.　©　2017　Elsevier　B.V.