Graphitic carbon nitride(g-C3N4),as a graphene-like carbon-based material,has received a lot of attention due to its special structure and properties.1 In recent year,nanoscale g-C3N4 materials,such as fluorescent g-C3N4 nanosheets and g-C3N4 nanodots,have drawn great attention in sensing because of their excellent optical and physicochemical features.2 In this work,we produced two kinds of fluorescent graphitic carbon nitride nanomaterials and their sensing applications.G-C3N4 nanosheets within 200 nm were prepared using the chemically oxidization and liquid exfoliation method.The g-C3N4 nanosheets exhibited high fluorescence and showed excellent sensitivity and selectivity for the determination of 2,4,6-trinitrophenol(TNP).The strong inner filter effect and molecular interactions(electrostatic,π-π and hydrogen bonding interactions)between TNP and the g-C3N4 nanosheets led to the fluorescence quenching of the g-C3N4 nanosheets.Under optimal conditions,a detection limit of 8.2 nM could be found for TNP.The prepared g-C3N4 nanosheets can transfer onto a test paper and detect TNP with a visual detection limit of 0.5 μM.3 Highly fluorescent phosphorus,oxygen-doped g-C3N4 nanodots(P,O-g-C3N4 nanodots)were synthesized using chemical oxidation and hydrothermal etching of bulk P-g-C3N4 obtained via pyrolysis of phytic acid and melamine.The P,O-g-C3N4 nanodots emitted strong blue fluorescence with a high quantum yield of 90.2%,and displayed high resistance to photobleaching and high ionic strength.A small amount of Cu2+ can quench the fluorescence of the P,O-g-C3N4 nanodots through the static fluorescence quenching and photoinduced electron transfer.Under optimal conditions,a rapid detection of Cu2+ could be completed in 5 min with a detection limit of 2 nM,and a linearity ranging from 0 to 1 μM.Using acetylthiocholine(ATCh)as the substrate,the fluorescence of the P,O-g-C3N4 nanodots-Cu2+ system could be sensitively turned on in the presence of acetylcholinesterase(AChE)through the reaction between Cu2+ and thiocholine,the hydrolysis product of ATCh by AChE.A linearity ranging from 0.01 to 3 mU/mL could be obtained with a detection limit of 0.01 mU/mL.In addition,the proposed approach showed potential application for the detection of Cu2+ in natural water samples and AChE activity in human plasma.