Copper　(Cu)　is　a　vital　metal　element　for　humans　and　animals.　Monitoring　and　evaluating　the　concentration　level　of　Cu2+　in　a　biological　body　is　an　effective　way　to　prevent　a　variety　of　diseases.　In　this　work,　phenyl　doped　graphitic　carbon　nitride　(PDCN)　nanosheets　with　strong　green　fluorescence　exhibited　a　sensitive　and　selective　detection　for　Cu2+　with　a　linear　range　from　0.1-2.0　µmol　L-1.　Furthermore,　fluorescent　imaging　was　applied　to　semiquantitatively　detect　Cu2+　in　HeLa　cells　using　PDCN　nanosheets　as　the　probe,　which　can　avoid　the　interference　of　background　autofluorescence.　This　work　provided　a　low-cost　and　biologically　friendly　fluorescent　probe　to　monitor　the　concentration　level　of　Cu2+　in　living　cells.　©　2020　The　Royal　Society　of　Chemistry.