Amperometric NADH sensor based on a carbon ceramic electrode modified with the natural carotenoid crocin and multi-walled carbon nanotubes

A carbon ceramic electrode (CCE) was fabricated from a composite consisting of sol-gel, ceramic graphite, multi-walled carbon nanotubes and the natural carotenoid crocin. The resulting sensor is shown to allow for the determination of NADH at a rather low working potential of 0.22 V (vs. Ag/AgCl). The heterogeneous electron transfer rate constant (k_s) and the surface coverage of the modified electrode are 16.8 s^?1 and 22 pmol·cm^?2, respectively. The sensor shows excellent and linear response in solutions of pH 7.0 over the 0.5 to 100 μM NADH concentration range, a 0.1 μM detection limit, and a sensitivity of 251.3 nA·μM^?1·cm^?2.

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Graphical abstract Schematic of the preparation of a carbon ceramic electrode modified with electropolymerized crocin on multi-walled carbon nanotubes. This sensor has a strongly decreased oxidation overpotential for NADH.

     

Convenient and Efficient Method for the Synthesis of N-Heteroaryl Aminonaphthols under Solvent-Free Conditions

A convenient, efficient and green synthesis of N‐heteroaryl aminonaphthols has been developed by one‐pot, three‐component condensation of β‐naphthol, heteroaryl amines and substituted benzaldehydes under solvent‐free conditions at elevated temperature. The advantages of these reactions are simplicity of the reaction procedure, short reaction times, simple work‐up, catalyst‐free conditions and pure products in good to excellent yields.     

Determination of triazole pesticide residues in edible oils using air‐assisted liquid–liquid microextraction followed by gas chromatography with flame ionization detection

In the present study, a rapid, simple, and highly efficient sample preparation method based on air‐assisted liquid–liquid microextraction followed by gas chromatography with flame ionization detection was developed for the extraction, preconcentration, and determination of five triazole pesticides (penconazole, hexaconazole, diniconazole, tebuconazole, and triticonazole) in edible oils. Initially, the oil samples were diluted with hexane and a few microliter of a less soluble organic solvent (extraction solvent) in hexane was added. To form fine and dispersed extraction solvent droplets, the mixture of oil sample solution and extraction solvent is repeatedly aspirated and dispersed with a syringe. Under the optimum extraction conditions, the method showed low limits of detection and quantification between 2.2–6.1 and 7.3–20 μg/L, respectively. Enrichment factors and extraction recoveries were in the ranges of 71–96 and 71–96%, respectively. The relative standard deviations for the extraction of 100 and 250 μg/L of each pesticide were less than 5% for intraday (n = 6) and interday (n = 3) precisions. Finally edible oil samples were successfully analyzed using the proposed method, and hexaconazole was found in grape seed oil.     

Development of a dispersive liquid–liquid microextraction method with a new sequence of steps and its comparison with a conventional method

In this paper, a dispersive liquid–liquid microextraction method with a new sequence of steps from the view of salt addition has been developed for the extraction and preconcentration of some organophosphorous pesticides from aqueous samples before analysis by gas chromatography with flame ionization detection. In this method, an appropriate mixture of extraction and disperser solvents is rapidly injected by a syringe into the aqueous sample. Then, sodium chloride is added into the solution to increase its ionic strength. The obtained results by the proposed method are compared with those of the conventional dispersive liquid–liquid microextraction in which the salt is added into the aqueous phase before dispersion of the extraction solvent. Some effective parameters on the method efficiency including type and volume of extraction and disperser solvents, type and percent of salt, etc. are investigated. Under the optimal conditions, limits of detection and quantification of the proposed method compared to conventional one were improved by a factor between 1.4–2.2 and 1.3–2.3, respectively. Extraction recoveries and enrichment factors of the proposed method with respect to conventional one enhanced from 43–60 to 72–99% and 1433–2000 to 2404–3285, respectively.     

Development of organic solvents‐free mode of solidification of floating organic droplet–based dispersive liquid–liquid microextraction for the extraction of polycyclic aromatic hydrocarbons from honey samples before their determination by gas chromatography–mass spectrometry

In this study, a green mode of solidification of floating organic droplet – based dispersive liquid–liquid microextraction has been developed for the extraction of 16 polycyclic aromatic hydrocarbons from honey samples before their determination by gas chromatography–mass spectrometry. In this method, an appropriate volume of menthol:decanoic acid deep eutectic solvent (as an extraction solvent) is added on a sugar cube (as a disperser agent). In the following, the cube is released into the diluted honey sample placed in a tube. After manual shaking a cloudy state is obtained as a result of dispersing the extraction solvent droplets throughout the sample solution and the analytes are extracted into them. After placing the tube in an ice bath, the droplet of the extractant is solidified on the top of the solution. This drop is taken and after dissolving in acetonitrile, an aliquot of the solution is injected into the separation system. Under optimum conditions, the suggested approach had high extraction recoveries (76–93%) and enrichment factors (380–465), low limits of detection (14–52 ng/kg) and quantification (47–173 ng/kg), and satisfactory repeatability (relative standard deviation ≤ 9%).     

Disjoint weighing matrices

Journal of Algebraic Combinatorics - The notion of disjoint weighing matrices is introduced as a generalization of orthogonal designs. A recursive construction along with a computer search leads to...     

Hadamard Matrices of Order 32

Two Hadamard matrices are considered equivalent if one is obtained from the other by a sequence of operations involving row or column permutations or negations. We complete the classification of Hadamard matrices of order 32. It turns out that there are exactly 13,710,027 such matrices up to equivalence.     

Electro-oxidation of paracetamol in the presence of malononitrile: Application for green,efficient, none-catalyst,simple and one-pot electro-synthesis of new paracetamols

Electrochemical oxidation of paracetamol has been studied in the presence of malononitrile as a nucleophile in a phosphate buffer solution (0.15 mol/L, pH 7), using cyclic voltammetric techniques. The results indicated that the N-acetyl-p-benzoquinone-imine derived from paracetamol participates in a 1,4-Michael-type addition reaction with the malononitrile to form the corresponding paracetamol derivatives (6a, 7a). The present study has led to the development of a simple, green, non-catalyst and one-pot electrochemical method with high atom economy under mild conditions.