Nanomaterials-based electrochemical sensors for the detection of natural antioxidants in food and biological samples: research progress.

Antioxidants are healthy substances that are beneficial to the human body and exist mainly in natural and synthetic forms. Among many kinds of antioxidants, the natural antioxidants have great applications in many fields such as food chemistry, medical care, and clinical application. In recent years, many efforts have been made for the determination of natural antioxidants. Nano-electrochemical sensors combining electrochemistry and nanotechnology have been widely used in the determination of natural antioxidants due to their unique advantages. Therefore, a large number of nanomaterials such as metal oxide, carbon materials, and conducting polymer have attracted much attention in the field of electrochemical sensors due to their good catalytic effect and stable performance. This review mainly introduces the construction of electrochemical sensors based on different nanomaterials, such as metallic nanomaterials, metal oxide nanomaterials, carbon nanomaterials, metal-organic frameworks, polymer nanomaterials, and other nanocomposites, and their application to the detection of natural antioxidants, including ascorbic acid, phenolic acids, flavonoid, tryptophan, citric acid, and other natural antioxidants. In the end, the limitations of the existing nano-sensing technology, the latest development trend, and the application prospect for various natural antioxidant substances are summarized and analyzed. We expect that this review will be helpful to researchers engaged in electrochemical sensors.

Nitrogen Doped Graphene Supported Pt Nanoflowers as Electrocatalysts for Oxidation of Formaldehyde.

A facile Pt nanoflowers/nitrogen-doped graphene (PtNFs/NG) electrocatalyst was prepared via depositing Pt nanoflowers (PtNFs) onto the nitrogen-doped graphene (NG) matrix with urea as the nitrogen source and PtNFs/NG modified glassy carbon electrode (GCE) was prepared by electro-chemical method. Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), Raman spectroscope, X-ray photoelectron spectroscopy (XPS) and Scanning electron microscope (SEM) were used to characterize the resulting composites. Also oxidation of formaldehyde on the resulting PtNFs/NG modified electrode was investigated. The influence of deposition time, electrodeposition potential and formaldehyde concentration on electrooxidation of formaldehyde was detected, the experimental results indicate the high performance of PtNFs/NG catalyst for formaldehyde oxidation is at electrodeposition time of 300 s with the applied potential of −0.3 V. Electrochemical process, electrocatalytic stability and chronoamperometry were also inspected, it was indicated that formalde-hyde oxidation reaction on the PtNFs/NG electrode is diffusion-controlled and PtNFs/NG exhibits a high catalytic activity, stability as well as excellent poisoning-tolerance towards formaldehyde oxidation, which is attributed to the synergistic effect of PtNFs and NG. It turns out that PtNFs/NG can be used in direct liquid-feed fuel cells as a promising alternative catalyst.

Effect of A, B-site cation on the catalytic activity of La1-xAxMn1-yByO3 (A = Ce, B = Ni) perovskite-type oxides for toluene oxidation.

ABO3-type perovskites (A = La, Ce; B = Mn, Ni) were prepared by sol-gel method, and applied for catalytic oxidation of toluene. The activity test results show that the activity of LaMnO3 can be improved when a small amount of Ce and Ni are doped into the A and B sites of LaMnO3, respectively. The effects of different calcination temperatures and different calcination time on the preparation of La-based perovskites were also investigated. The results illustrate that the toluene conversion of La0.8Ce0.2Mn0.8Ni0.2O3 is the highest when the calcination temperature is 700 °C and the calcination time is 4 h in La1-xCexMn1-yNiyO3 perovskites, and it requires lower reaction temperature when the conversion rate of toluene reaches 100% as compared to other catalysts, the T90 is 295 °C (T90, the temperature corresponding to the 90% of toluene conversion). Importantly, the mechanism of catalytic oxidation was also discussed. Therefore, the catalyst has potential prospects in the volatile organic compounds (VOCs) degradation.

CeO2 supported MOFs derived LaBxFeyO3 (B=Mn, Co) perovskite catalysts for degradation of toluene.

In this work, LaCoO3 and LaMnO3 perovskites with the higher specific surface area were synthesized using MOFs as precursor, then, the composite catalysts CeO2-LaCoxFeyO3 and CeO2-LaMnxFeyO3 were prepared by using CeO2 as support and Fe element doping LaCoO3 and LaMnO3, respectively. The as-prepared samples were characterized by XRD, SEM, XPS, H2-TPR, and N2 physisorption techniques. Subsequently, toluene was used as the probe molecule for volatile organic compounds (VOCs) to test the catalytic activity of these as-prepared catalysts. The results show that the initial reaction temperature for toluene oxidation on supported perovskite catalysts is lower. Among which, CeO2-LaCo0.25Fe0.75O3 (T90=215 °C, T90: the temperature corresponding to 90% conversion of toluene) and CeO2-LaMn0.25Fe0.75O3 (T90=205 °C) catalysts show the best catalytic performance. Therefore, the supported perovskite prepared in this study has the advantages of high specific surface area, abundant oxygen vacancies, and excellent oxygen mobility, which makes it exhibit better performance in VOCs catalytic oxidation.

Three-dimensional g-C3N4/MWNTs/GO hybrid electrode as electrochemical sensor for simultaneous determination of ascorbic acid, dopamine and uric acid.

A three-dimensional g-C3N4/MWNTs/GO hybrid modified electrode was constructed as an electrochemical sensor for the simultaneous determination of ascorbic acid (AA), dopamine (DA) and uric acid (UA). Due to the high conductivity of MWCNTs and the strong synergy between g-C3N4 and GO, the combination of the three effectively improved the electrocatalytic activity of the modified electrode for the oxidation of AA, DA, and UA, and solved the problems such as overlapping anodic peaks. The electrochemical performance of the as-constructed sensor was investigated and optimized by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). The linear response range of AA, DA, and UA in the optimal condition was 0.2-7.5 mM, 2-100 µM, and 4-200 µM, respectively. The detection limits (S/N = 3) of AA, DA, and UA were 96, 0.22, and 1.36 µM, respectively. The recoveries of AA, DA and UA in serum samples from three groups were 92.82-106.50%, and the relative standard deviations were less than 2%. The results show that the as-constructed g-C3N4/MWNTs/GO modified electrode has the advantages of simplicity, high sensitivity and good selectivity, and can simultaneously determine AA, DA, and UA.

A novel high-performance CeO2-CuMn2O4 catalyst for toluene degradation.

A series of spinel CuM2O4 (M = Mn, Fe, and Al) was used as the catalyst to investigate the effective degradation of toluene, and then CuMn2O4 with better catalytic activity was selected as the research object to study its activity at different ratios of Cu and Mn. Meanwhile, CeO2 was introduced to modify CuMn bimetallic oxide to improve its catalytic performance. The structure, morphology, and valence states of surface elements of as-prepared catalysts were characterized by XRD, TEM, SEM, N2 adsorption-desorption, XPS, and H2-TPR. Using toluene as a probe molecule, the catalytic activity of the catalyst was tested and the results showed that the conversion rate of toluene catalyzed by CeO2-CuMn2O4 catalyst can reach 90% at 200 °C (T90) and 100% at 240 °C (T100). The CO2 yield can also reach 100% at 248 °C. Moreover, the possible catalytic mechanism for toluene by the CeO2-CuMn2O4 was briefly explored. The catalytic oxidation of toluene over the oxide follows the Mars-van Krevelen mechanism.

Direct Z-scheme La1-xCexMnO3 catalyst for photothermal degradation of toluene.

A series of Ce-doped LaMnO3 (La1-xCexMnO3) were prepared and were tested for gaseous toluene oxidation in order to investigate the effect of cerium doping in LaMnO3 on activity under photothermal conditions. It was found that the activity and CO2 yield of the catalyst can be effectively increased when x = 0.25. A group of characterization is used to characterize the morphology, composition, and physical properties of the as-prepared catalysts. Results show that the Ce-doped LaMnO3 can form coexistence of La1-xCexMnO3 and CeO2, the reaction of CeO2/La1-xCexMnO3 under photothermal conditions follows the Mars-van Krevelen redox cycle mechanism, and the prepared CeO2/La1-xCexMnO3 can form a highly efficient Z-scheme heterojunction, which can enhance the electrons transfer speed of the catalyst. Moreover, in the photothermal catalytic degradation, lattice oxygen is the most important active substance, a small amount of cerium doping can increase the lattice oxygen content of perovskite and increase the activity of the reaction.