Development Of Novel Colorimetric Methods Based On Nanoceria

Enzyme, as one kind of biological catalysts, can quickly and efficiently catalyze a variety of physiological and biochemical reactions. Because of that, enzyme plays an indispensable role on food industry such as food processing, food composition analysis and detection and many other aspects. For instance, enzyme can improve the quality of food production such as texture, flavor and taste etc. In addition, enzyme biosensors play an important role on freshness analysis and food additives detection. However, narrow resource, high cost and unstable activity are the limits for the wider application of enzymes. Nanozymes, with the characteristics of wide resource, low cost and stable, can make up the defects of enzymes. Therefore, the development and application of nanozymes in establishing new methods food analyzing is of great value and potential. Nanoceria, as one kind of nanoenzymes, exhibits excellent catalytic activities, Nanoceria can convert the mutation of the oxidation state between Ce3+ and Ce4+quickly and expediently. But traditional nanoceria has poor dispersivity and serious agglomeration, and nanoceria is less concerned in food science. This thesis aims to modify the solubility of nanoceria via dextran coating. This new material shows good dispersivity, stability and excellent catalytic activity. Based on the capacity of catalyst, this thesis has built a new method for glucose detection. As well, a novel method in evaluating the capacity of antioxidants in food industry has been studied and explored. The results of the researches in detail are as follows:Firstly, dextran-coated nanoceria with good solubility were synthesized by hydrothermal method. Initially, UV-Vis spectroscopy was used to observe whether nanoceria were successfully prepared via the absorbance peak. In addition, X-ray photoelectron spectroscopy（XPS） and Fourier transform infrared spectrometer（FT-IR） are employed to confirm whether nanoceria were successfully wrapped with dextran. Meanwhile, high resolution transmission electron microscopy（HRTEM） was used to characterize nanoceria. The morphology and the microstructure are stable and steady. The average size of nanoceria is 3 to 6 nm, which is corresponded with the results got by dynamic light scattering（DLS）. Furthermore, the stability of nanoceria was investigated by Thermal Gravity Analysis（TG）.Secondly, a series of experiments were performed to investigate the oxidase-like activity of the as-prepared CeO₂ NPs. CeO₂ NPs can immediately catalyze the oxidation of 3,3’,5,5’-Tetramethylbenzidine（TMB） as peroxidase substrate in the absence of peroxide（H2O₂）. At the same time, CeO₂ NPs were demonstrated to play a dual role on the oxidation both as a promoter and an inhibitor. Lots of optimizations were completed to ensure the best environment for the reaction and further application. The optimum p H value is 4, temperature 45℃ and the concentration of CeO₂ NPs is 48 μg/mL. To explore the process, the kinetics of the reaction was studied, confirming the reaction fitted to Michaelis-Menten equation As well, the material’s pH and thermal stability were compared with horse radish peroxidase（HRP） which is a kind of natural enzymes. The result showed that CeO₂ NPs exhibited much more advantages over HRP.Thirdly, a creative colorimetric method to detect H2O₂ and glucose was established. Linear range of the detection of hydrogen peroxide is from 5 μM to 45 μM. Linear range of the detection for glucose is from 6 μM to 75 μM, and limit of detection is 2.5 μM. The concentration of glucose in serum was detected. The recovery is from 99.7% to 101.2%, and RSD value is between 1.5% and 3.3%, confirming the method is reasonable.In addition, an innovation to evaluate the antioxidant capacity of antioxidants in food industry based on CeO₂ NPs to detect radical-scavenging capacity was established. CeO₂ NPs can catalyse the oxidation of ABTS（2,2’-azinobis-（3-ethylbenzothiazoline-6-sulfonic acid）） to generate radicals which produce blue color and have absorbance peak at 405 nm. Besides, the antioxidants can scavenge the radicals resulting in color change and smaller value of absorbance peak at 405 nm. A simple, time-saving, stable and sensitive method to appraise the capacity of antioxidants was found in food industry.