A　hybrid　material　composed　of　graphitic　carbon　nitride　(g-C3N4)　and　platinum　nanoparticles　(PtNPs)　with　peroxidase　mimicking　activity　was　used　to　design　a　rapid,　sensitive　and　low-cost　colorimetric　method　for　the　determination　of　Ag(I).　The　g-C3N4-PtNPs　hybrid　was　synthesized　by　reduction　of　chloroplatinic　acid　using　sodium　borohydride　under　ultrasonication　and　in　the　presence　of　g-C3N4.　The　hybrid　can　catalyze　the　oxidation　3,3′,5,5′-tetramethylbenzidine　(TMB)　to　produce　a　blue-colored　product　with　an　absorption　maximum　at　652 nm.　On　addition　of　Ag(I)　and　in　the　presence　of　citric　acid,　it　will　be　reduced　to　form　Ag(0)　under　the　catalytic　action　of　PtNPs.　Ag(0)　is　then　adsorbed　on　the　surface　of　the　g-C3N4-PtNPs.　This　results　in　the　inhibition　of　the　enzyme　mimetic　activity　of　the　hybrid.　Hence,　less　blue　product　will　be　formed　from　TMB.　Under　optimum　conditions,　Ag(I)　can　be　quantified　in　the　0.05–5.0 nM　concentration　range　with　a　22　pM　detection　limit.　This　assay　is　rapid　and　reliable　and　was　applied　to　the　determination　of　Ag(I)　in　spiked　real　water　samples.　©　2018,　Springer-Verlag　GmbH　Austria,　part　of　Springer　Nature.