Ultraviolet　B　(UVB)　irradiation　induces　the　generation　of　reactive　oxygen　species　(ROS)　and　causes　damages　to　human　skin.　The　depletion　of　glutathione　(GSH)　to　scavenge　ROS　can　protect　skin　cells　from　oxidative　damage.　However,　little　is　known　about　the　concentration　level　changes　of　cysteine　(Cys),　a　precursor　of　GSH　in　skin　cells　after　exposure　to　UVB　irradiation.　Herein,　phenyl　doped　graphitic　carbon　nitride　nanosheets-Au　nanoparticles　nanocomposite　was　prepared　by　in　situ　deposition　of　Au　nanoparticles　on　the　surface　of　phenyl　doped　graphitic　carbon　nitride　nanosheets　and　showed　a　turn-on　fluorescence　response　toward　Cys　over　homocysteine,　glutathione　under　physiological　conditions.　In　the　presence　of　Cys,　remarkable　enhancement　of　green　fluorescence　was　observed.　This　nanocomposite　was　successfully　applied　for　fluorescence　imaging　of　Cys　in　human　skin　epidermal　cells　and　monitoring　the　changes　of　Cys　concentration　level　under　the　oxidative　stress　upon　exposure　to　UVB　irradiation　in　keratinocytes.　It　was　found　that　the　concentration　of　Cys　was　increased　in　the　initial　period　after　exposure　to　UVB　irradiation　and　then　gradually　decreased　to　the　normal　level　for　the　synthesis　of　GSH　to　defense　the　oxidative　stress.　Our　result　helps　to　understand　the　physiological　function　of　Cys　in　human　skin　cells　under　UVB　exposure.　©　2019　Elsevier　B.V.