Nanozymes　have　attracted　great　interest　in　the　field　of　biotechnology,　(bio)sensing,　and　environmental　monitoring.　Herein,　a　co-doping　strategy　is　proposed　to　enhance　the　peroxidase-like　activity　of　graphitic　carbon　nitride　(g-C3N4).　Cu2+　and　triazole　group　co-doped　graphitic　carbon　nitride　(g-C3N5-Cu2+)　was　derived　from　thermal　polymerization　of　3-amino-1,2,4-triazole　and　then　coordination　with　Cu2+.　g-C3N5Cu2+　nanosheets　(NSs)　could　catalyze　the　oxidation　reaction　of　3,3′,5,5′-tetramethylbenzidine　(TMB)　by　H2O2　under　neutral　conditions,　yielding　10.9-fold　better　catalytic　activity　than　g-C3N4　NSs.　Reactive　oxygen　species　scavenging　experiments　indicated　that　·OH　played　a　crucial　role　during　the　catalytic　process.　When　glutathione　(GSH)　was　introduced,　the　absorbance　of　the　system　decreased　because　of　the　reduction　of　the　oxidation　product　of　TMB　by　GSH.　The　colorimetric　method　demonstrated　a　sensitive　response　for　GSH　with　a　linear　range　of　0.8–33.3　μmol　L-1　and　a　limit　of　detection　of　0.3　μmol　L-1.　The　sensitivity　is　higher　than　most　other　peroxidase　mimetic-based　colorimetric　methods　for　GSH.　Finally,　the　feasibility　for　the　detection　of　GSH　in　HeLa　cells　was　demonstrated.　©　2023　The　Author(s).