An　enzyme-catalyzed　fluorescence　＂switch＂type　sensor　was　constructed　for　the　determination　of　alkaline　phosphatase　(ALP)　activity　by　combining　the　fluorescence　quenching　effect　of　Ag+　on　ultrathin　g-C3N4　nanosheets　(CNNSs)　with　the　simple　redox　reaction　of　AA　and　Ag+.　Briefly,　Ag+　exhibits　a　significant　quenching　effect　on　the　fluorescence　of　CNNSs.　Thus　the　fluorescence　signal　of　the　CNNS-Ag+　system　is　extremely　weak　even　in　the　presence　of　l-ascorbic　acid-2-phosphate　(AAP)　(＂off＂state).　When　ALP　coexists　in　the　system,　the　enzyme　can　specifically　catalyze　the　hydrolysis　of　AAP　to　form　ascorbic　acid　(AA),　which　reduces　Ag+　to　Ag0.　In　this　case,　the　fluorescence　signal　of　the　system　is　recovered　(＂on＂state).　Based　on　this　principle,　a　signal-enhanced　CNNS　fluorescence　sensor　was　developed　to　determine　the　activity　of　alkaline　phosphatase.　The　experimental　results　show　that　the　detection　range　of　alkaline　phosphatase　is　0.5-20　U　L-1,　and　the　detection　limit　is　0.05　U　L-1　(S/N　=　3).　Meanwhile,　this　method　was　used　to　assay　ALP　in　serum　samples.　This　journal　is　©　The　Royal　Society　of　Chemistry.