Optimization of the tandem enzyme activity of V-MOF and its derivatives for highly sensitive nonenzymatic detection of cholesterol in living cells.

It remains a great challenge to properly design and synthesize single-component artificial tandem enzymes for specific substrates with high selectivity. Herein, V-MOF is synthesized by solvothermal method and its derivatives are constructed via pyrolyzing V-MOF in nitrogen atmosphere at different temperatures, which are denoted as V-MOF-y (y = 300, 400, 500, 700 and 800). V-MOF and V-MOF-y possess tandem enzyme-like activity, i.e. cholesterol oxidase-like and peroxidase-like activity. Among them, V-MOF-700 shows the strongest tandem enzyme activity for V-N bonds. Based on the cascade enzyme activity of V-MOF-700, the nonenzymatic detection platform for cholesterol by fluorescent assay can be established in the presence of o-phenylenediamine (OPD) for the first time. The detection mechanism is that V-MOF-700 catalyzes cholesterol to generate hydrogen peroxide and further form hydroxyl radical (•OH), which can oxidize OPD to obtain oxidized OPD (oxOPD) with yellow fluorescence. The linear detection of cholesterol ranges of 2-70 µM and 70-160 µM with a lower detection limit of 0.38 µM (S/N = 3) are obtained. This method is used to detect cholesterol in human serum successfully. Especially, it can be applied to the rough quantification of membrane cholesterol in living tumor cells, indicating that it has the potential for clinical application.

Theoretical and experimental research on two-phase flow image reconstruction and flow pattern recognition.

Two-phase flow is a kind of complex fluid flow state, and the flow pattern characteristics are very difficult to obtain accurately. First, the principle of two-phase flow pattern image reconstruction based on electrical resistance tomography technology and the complex flow pattern recognition method are developed. Next, the back propagation (BP), wavelet, and radial basis function (RBF) neural networks are applied to the two-phase flow pattern image identification process. The results show that the RBF neural network algorithm has higher fidelity and faster convergence speed than the BP and wavelet network algorithms, and the fidelity is more than 80%. Then, deep learning of the pattern recognition algorithm fusing the RBF network and convolution neural network is proposed to improve the precision of the flow pattern identification. Additionally, the recognition accuracy of the fusion recognition algorithm is more than 97%. Finally, a two-phase flow test system is constructed, the test is finished, and the correctness of the theoretical simulation model is verified. The research process and results provide important theoretical guidance for the accurate acquisition of two-phase flow patterns.

Fe-Ni metal-organic frameworks with prominent peroxidase-like activity for the colorimetric detection of Sn2+ ions.

Fe-Ni-MOFs with different amounts of Fe are synthesized through a two-step template etching method. Kinetic analysis indicates that Fe-Ni-MOF exhibits prominent intrinsic peroxidase-like activity, which could catalytically oxidize 3,3',5,5'-tetramethylbenzidine (TMB) to produce oxidized TMB (oxTMB) with a blue color. As a peroxidase-mimicking MOF, Fe-Ni-MOF with its efficient stable catalysis properties is demonstrated to allow a sensitive colorimetric assay for Sn2+ ions. The mechanism is explored, whereby Sn2+ ions could reduce the peroxidase-like activity of Fe-Ni-MOF based on a redox interaction, making the oxTMB color lighter and decreasing the absorbance intensity at 652 nm. The linear determination of the Sn2+ ion concentration using a UV-vis spectrometer ranges from 0.01 mM to 1.0 mM and from 1.0 mM to 4.0 mM. The presented Fe-Ni-MOF-based assay of Sn2+ ions was successfully applied to real water samples.

Nitrogen-doped carbon dots-V2O5 nanobelts sensing platform for sensitive detection of ascorbic acid and alkaline phosphatase activity.

In this work, the as-prepared V2O5 nanobelts can sensitively quench the fluorescence of nitrogen-doped carbon dots (N-CDs) based on the inner filter effect (IFE). In the presence of ascorbic acid (AA), the fluorescence of N-CDs can recover through the redox reaction between V2O5 nanobelts and AA. Meanwhile, in the presence of both alkaline phosphatase (ALP) and ascorbyl-2-phosphate (AAP), the fluorescence of N-CDs can also restore since AAP can be hydrolyzed into AA by ALP. Under optimum conditions, the linear range for AA is from 0.01 to 2.5 µM with a detection limit of 3 nM and that for ALP is from 0.1 to 30 U/L with a detection limit of 0.04 U/L (S/N = 3). Particularly, the proposed probe could be successfully used to detect AA and ALP in human serum samples. Furthermore, N-CDs can be applied in fluorescence imaging of Human breast cancer cells with satisfactory results.

A novel anodic electrochemiluminescence behavior of sulfur-doped carbon nitride nanosheets in the presence of nitrogen-doped carbon dots and its application for detecting folic acid.

The application of carbon dots as a coreactant for Ru(bpy)32+ (where bpy is 2,2'-bipyridine) electrochemiluminescence (ECL) has been widely studied. However, the high cost of Ru(bpy)32+ and its derivatives has prohibited its widespread use in ECL biosensors. Herein, a novel anodic ECL system based on sulfur-doped graphitic carbon nitride nanosheets (S-g-C3N4 NSs) and nitrogen-doped carbon dots (N-CDs) is presented. In this ECL system, N-CDs serve as a new ECL coreactant that can significantly enhance the anodic ECL signal of S-g-C3N4 NSs (approximately 83 times) under optimal conditions. The possible ECL response mechanism of the S-g-C3N4 NSs/N-CDs system is proposed in detail on the basis of cyclic voltammograms, ECL-time curves, and ECL spectra. Furthermore, the ECL signal of the S-g-C3N4 NSs/N-CDs system was quenched by folic acid (FA), which was chosen as a model analyte to study the potential application of the new ECL system. The ECL intensity decreased linearly with the concentration of FA in the range from 0.05 to 200 µM. The detection limit for FA measurement is 16 nM (signal-to-noise ratio of 3). The proposed new ECL system has many advantages over traditional approaches, such as low toxicity and excellent biocompatibility. Especially, the proposed approach can detect FA in diluted human serum samples with satisfactory recoveries, indicating promising application for bioanalysis. Graphical abstract.