Materials: All chemicals used in this work were of analytical grade and used as received without further purification. 3,3',5,5'-tetramethylbenzidine (TMB), horseradish peroxidase (EC1.11.1.17, 250-330 U/mg using pyrogallol, type VI-A) and glucose oxidase (GOx, from Aspergillus niger, ≥100 U/mg) were purchased from Sigma-Aldrich (St. Louis, American). H 2 O 2 , Co(COOH) 2 ·4H 2 O, 25% ammonia solution, ethanol and other regents were obtained from Sinopharm Chemical Reagent Co. (Shanghai,

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... Glucose, fructose, lactose and maltose were from Beijing Chemical Reagent Company (Beijing, China). Fe 3 O 4 nanoparticles (spherical, 20 nm) were purchased from Aladdin Regent Company (Shanghai, China). Preparation of Co 3 O 4 nanoparticles (NPs): The Co 3 O 4 NPs were prepared according to the method reported by Dong et al. 1 Briefly, 0.50 g of Co(CH 3 COO) 2 ·4H 2 O was dissolved in solution of 10 ml water and 15 ml ethanol, and 2.5 ml of 25% ammonia was added under vigorous stirring. The mixture was stirred in air for about 10 min to form a homogeneous fuscous slurry. Then the suspension was transferred into a 48 ml autoclave, sealed and maintained at 150 for 3 h. After this, the autoclave was cooled to room temperature naturally. The resulting black solid products were separated by centrifugation and washed with water three times, dried at 60 under vacuum for 4 h, and collected for characterization. The composition and phase of the product were identified by powder x-ray diffraction (XRD) on an D/max-rB x-ray diffractometer (Rigaku, Japan) using Cu Kα radiation (λ=1.5418 Å). The morphology and size of the product were examined by transmission electron microscopy (TEM) with a Hitachi H-800 transmission electron microscope (Hitachi, Japan) at an accelerating voltage of 200 kV. Kinetic analysis: Kinetic measurements were carried out in time-drive mode by monitoring the absorbance change at 652 nm on a Lambda 750 UV-Vis-NIR spectrophotometer (Perkin Elmer, American). Experiments were carried out using 10 μg mL -1 Co 3 O 4 NPs in a reaction volume of 3 ml buffer solution (100 mM acetate buffer, pH 5.0) with 0.3 mM TMB as substrate, and H 2 O 2 concentration was 100 mM, unless otherwise stated. 30 μg Co 3 O 4 NPs contain ~6.2×10 11 nanoparticles. 2 The Michaelis-Menten constant was calculated using Lineweaver-Burk plots of the double reciprocal of the Michaelis-Menten equation, 1/ν=(K m /V max )·(1/[S])+1/V max . Electrochemistry experiments of the Co 3 O 4 NPs modified electrodes: The Co 3 O 4 NPs (30 mg) was dispersed into distilled water (10 mL) to obtain a suspension of Co 3 O 4 NPs (3 mg mL -1 ). The glassy carbon electrodes (GCE, 3.0 mm in diameter) were firstly polished with 0.3 and 0.05 mm alumina slurry followed by thoroughly rinsing with water. The colloidal solution (5 μL) of the Co 3 O 4 NPs was then dropped on the pretreated GCE surface and allowed to dry under ambient conditions for 3 h to obtain Co 3 O 4 NPs modified electrodes. Cyclic voltammetric and amperometric measurements were performed on CHI 660D (Chenhua, China). A three-electrode system comprising a platinum wire as auxiliary, a saturated calomel electrode as reference and the Co 3 O 4 NPs-modified electrode as working electrodes was used for all electrochemical experiments. All experimental solutions were deoxygenated by bubbling highly pure nitrogen for at least 20 min and maintained under nitrogen atmosphere during the measurements. In amperometric experiments, the current-time data were recorded after a constant residual current had been established and successive additions of H 2 O 2 solution into the buffer were done. It was carried out by applying a potential of -0.6 V on a stirred cell at room temperature. ·OH radical measurement: 10 mM H 2 O 2 , 0.5 mM terephthalic acid and different concentrations of the Co 3 O 4 NPs were first incubated in 100 mM acetate buffer (pH 5.0) exposed to UV light at 365 nm for 20 min. After centrifugation, the solutions were used for fluorometric measurement by a LS55 fluorescence spectrometer (Perkin Elmer, American). Electronic Supplementary Material (ESI) for Chemical Communications This journal is