Functionalized Graphene Nanomaterials As Highly-Efficient Peroxidase Mimics And Their Applications

Natural enzymes, known as macromolecular biological catalysts, can effectively catalyze various chemical reactions in biological processes with high selectivity, and have been applied in many fields. However, natural enzymes suffer from some serious disadvantages such as high cost, inherent instability due to denaturation and digestion, and rigorous reaction conditions, which have inevitably restricted their widespread applications. Therefore, construction and discovery of novel enzyme mimic is under intensive investigation.Grapheme oxide（GO） has been evaluated as peroxidase mimics, whereas, oxygen-containing functional groups on the surface of GO are found to destroy π-π conjugated structure of graphene and thus weak its electronic conduction ability. Thus, graphene is possibly endued with more excellent mimetic enzyme properties than GO. However, graphene is hydrophobic and tends to agglomerate in water owing to van der Waals interactions and π-π stacking, limiting their further applications. This problem can be initially resolved through polymeride functionalization, the graphene functionalized with polymeride exhibits good water dispersibility as well as excellent stability. Moreover, due to special property of the functional molecules, it is easy to combine graphene with other nanomaterils, and thus to gain higher catalytic activity.In the thesis, poly（sodium styrene sulfonate）-functionalized graphene（PSS-Gr） and PSS and Pt bifunctionalized graphene（PSS-Gr-Pt） were for the first time proposed as artificial enzyme. Based on their outstanding peroxidase-like activity, we developed some colorimetric methods for the determination of H₂O₂, glucose, ascorbic acid（AA） and glutathione（GSH）, and the sensing systems were used in practical samples. The contents are described as follows:1. PSS-Gr as peroxidase mimic and its applications in colorimetric sensingThe kinetic behavior and catalytic mechanism of PSS-Gr were further investigated, indicated that the catalysis is in accordance with typical Michaelis-Menten kinetics and followed a ping-pong mechanism. PSS-Gr is more favorable to enrich TMB onto its surface under acidic conditions due to stronger electronegativity, and thus PSS-Gr showed much higher mimic enzyme catalytic activity than graphene and GO. A colorimetric method was developed for detection of H₂O₂, as low as 1.50×10-7 mol·L^-1 H₂O₂ could be detected with a linear range from 5.00×10-6 to 1.00×10-3 mol·L^-1. H₂O₂ is the main by-product of glucose oxidase（GOx）-catalyzed reaction. Thus, a colorimetric sensor for glucose detection was realized when coupled with GOx. The linear response was from 6.00×10-6 to 4.00×10-4 mol·L^-1 with a detection limit of 2.80×10-7 mol·L^-1. Moreover, this method was also used for the detection of glucose in diluted serum, and the result agreed well with that obtained from the glucose kit. AA has some reducibility and can consume H₂O₂, and thus suppress the PSS-Gr-catalyzed oxidation of TMB. On the basis of the finding, a novel colorimetric method for detection of AA was developed, as low as 1.50×10-7 mol·L^-1 AA was detected with a linear range from 8.00×10-7 to 6.00×10-5 mol·L^-1, and it was further used to determine the concentration of AA in vitamin C tablets and fruit juice with satisfying results.2. PSS-Gr-Pt as peroxidase mimic and its applications in colorimetric sensingPSS-Gr-Pt showed considerable mimetic enzyme catalytic activity that could quickly catalyze the oxidation of TMB at room temperature, which may be attributed to the promotion of the electron transfer between the substrate and graphene-based carbon materials, and also the synergistic effect in simulated enzyme between metal nanoparticles and functionalized graphene. Kinetic analysis indicated that the catalysis was in accordance with the character of enzyme catalytic reaction and followed a ping-pong mechanism. Efficient colorimetric methods were developed for highly sensitive detection of H₂O₂ and glucose with the detection limit of 5.19×10-9 mol·L^-1 and 9.28×10-8 mol·L^-1, respectively, which was 29 times and 3 times lower than that of previous working. Based on the reaction between GSH and H₂O₂ or oxidized TMB（ox TMB）, an efficient colorimetric method for GSH was developed, the linear response was from 4.00×10-7 to 6.00×10-5 mol·L^-1 with a detection limit of 2.90×10-7 mol·L^-1. Moreover, the feasibility of this proposed method was examined by determining the concentration of GSH in eye drops.