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A new method for selective determination of Cr(VI) in environmental water samples was developed based on its quenching effect on the fluorescent N-doping graphene quantum dots (N-GQDs). The N-GQDs were synthesized by a simple one-step method using citric acid as the carbon source and ammonia as the nitrogen source with a 65% yield, showing that mass production of the N-GQDs is possible. The obtained N-GQDs with oxygen-rich functional groups exhibited a strong blue emission with a quantum yield of 18.6%, which was 7 times greater than that of graphene quantum dots (GQDs). Due to the selective coordination to Cr(VI), the N-GQDs can be used as a green and facile sensing platform for label-free sensitive and selective detection of Cr(VI) ions in aqueous solution and real water samples.Compared to GQDs, the N-GQDs as a fluorescent probe promises much improved selectivity for sensing of Cr(VI). The N-GQDs fluorescence probe shows a sensitive response to Cr(VI) in a wide concentration range of 0-140 mM with a detection limit of 40 nM. The N-GQDs-based fluorescence method was successfully used to selectively detect Cr(VI), and discriminate it and Cr(III) as well in aqueous samples.

The octahedral structure of the MIL-53(Fe) was facilely prepared by a microwave (MW)-assisted approach, and confirmed by scanning electron microscope (SEM) and X-ray diffraction (XRD). The MIL-53(Fe) MOFs were further characterized by thermo gravimetric (TG) analysis and fourier transform infrared (FTIR). It is found that the as-prepared MIL-53(Fe) exhibits intrinsic peroxidase-like activity, 10 and could catalytically oxidize 3,3',5,5'-tetramethylbenzidine (TMB), ABTS and OPD by H 2 O 2 to produce a typical colour reaction. The Michaelis-Menten behavior of the as-prepared MIL-53(Fe) was studied. The K m value of the as-prepared MIL-53(Fe) with H 2 O 2 as the substrate was 0.03 mM, which was at least seven times lower than that of Fe-MIL-88NH 2 and hemin@MIL-53(Al)-NH 2 . Interesting, the K m values of the as-prepared MIL-53(Fe) with H 2 O 2 and TMB as the substrates were both lower than those of 15 the MIL-53(Fe) obtained by conventional electric (CE) heating-based solvothermal method. This is probably attributed to the purely octahedral structure and small sized crystals of the MIL-53(Fe) obtained by MW-based synthesis method, confirming that MW-based synthesis method promised advantages of simplicity, fast crystallization and good phase selectivity. Results of electron spin resonance (ESR) experiments indicated that the as-prepared MIL-53(Fe) exhibited catalytic ability to H 2 O 2 decomposition 20 into •OH radicals. On this basis, a simple, sensitive and selective method for glucose detection was developed by coupling the oxidation of glucose catalyzed by glucose oxidase (GOx). As low as 0.25 µM glucose could be detected with a linear range from 0.25 −20 µM. The proposed method was successfully used to determine glucose in real human serum samples.

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