Friction properties of black phosphorus: a first-principles study.

Based on the first-principle, the friction anisotropy, structural super-lubricity and oxidation induced ultra-low friction of black phosphorus at atomic scale under different loads have been studied. The results show that the interface friction of black phosphorus is anisotropic, that is, the friction along the armchair direction is greater than that along the zigzag direction. Moreover, the friction between the black phosphorus interfaces shows a structural superlubricity property, and the incommensurate interface friction is approximately one thousandth of the commensurate interface friction, which is mainly due to the less electronic charge and the smaller amplitude of electronic charge change between the incommensurate interfaces during the friction process. In addition, the oxidation of black phosphorus is beneficial for lubrication between interfaces.

Simultaneous detection of four pesticides in agricultural products by a modified QuEChERS method and LC-MS/MS.

A modified QuEChERS pretreatment method and LC-MS/MS technique were performed to simultaneously determine four pesticide (Hexachlorophene, Dinex, Dinosam, Dinoterb) residues in agricultural products. Through the optimization of LC-MS/MS detection parameters in SIM mode, the optimal instrument conditions are obtained. The modified QuEChERS method was used to pretreat the samples. Solid phase extractants PSA, C18 and GCB were used for sample purification. The research results showed that the correlation coefficients of the four pesticides were all greater than 0.991, which had a good linear relationship. The limits of quantitation (LOQ) of the four pesticides were 0.05-0.56 µg/kg. The recoveries were 70.51-113.20% with relative standard deviations (RSDS) of 1.6-11.2%. The developed method can provide reliable data support for the subsequent simultaneous detection of these four pesticides.

Establishment of an LC-MS/MS Method for the Determination of 45 Pesticide Residues in Fruits and Vegetables from Fujian, China.

Pesticide residues in food have become an important factor seriously threatening human health. Therefore, this study was conducted to determine the pesticide residues in fruits and vegetables commonly found in Fujian, China, with the aim of constructing a simple and rapid method for pesticide residue monitoring. We collected 5607 samples from local markets and analyzed them for the presence of 45 pesticide residues. A fast, easy, inexpensive, effective, robust, and safe (QuEChERS) multi-residue extraction method followed by liquid chromatography equipped with triple-quadrupole mass spectrometry (LC-MS/MS) was successfully established. This 12-min-long analytical method detects and quantifies pesticide residues with acceptable validation performance parameters in terms of sensitivity, selectivity, linearity, the limit of quantification, accuracy, and precision. The linear range of the calibration curves ranged from 5 to 200 mg/L, the limits of detection for all pesticides ranged from 0.02 to 1.90 µg/kg, and the limits of quantification for the pesticides were 10 µg/kg. The recovery rates for the three levels of fortification ranged from 72.0% to 118.0%, with precision values (expressed as RSD%) less than 20% for all of the investigated analytes. The results showed that 726 (12.95%) samples were contaminated with pesticide residues, 94 (1.68%) samples exceeded the maximum residue limit (MRL) of the national standard (GB 2763-2021, China), 632 (11.23%) samples were contaminated with residues below the MRL, and 4881 (87.05%) samples were pesticide residue-free. In addition, the highest number of multiple pesticide residues was observed in bananas and peppers, which were contaminated with acetamiprid, imidacloprid, pyraclostrobin, and thiacloprid.

Ab Initio Time-Domain Study of Charge Relaxation and Recombination in N-Doped Cu2O.

Cu2O is a good photoelectric material with excellent performance, and its crystal structure, electronic structure, and optical properties have been extensively studied. To further illustrate the charge distribution and the carrier transport in this system, the e-h recombination dynamics was studied. It is found that N doping induced a shallower impurity band above the VBM, leading to significant charge localization around the impurity atom. NAMD simulation reveals that the N doping system possesses a longer e-h nonradiative recombination time scale. Therefore, we demonstrate that the formation of the impurity band and charge localization play an essential role in suppressing e-h recombination in N doping systems. This work is conducive for understanding the carrier transport mechanism in N-doped Cu2O.

Study on Porosity of Thermal-Sprayed Commercially Pure Aluminum Coating.

Porosity is closely related to the corrosion and wear properties of a coating processed by thermal-spraying technology, and the quantitative characterization of porosity is a crucial part of the research on coating structures. The current image analysis method often uses the mechanical polishing method recommended by ISO to measure a coating porosity. This method has been proved to be an effective method for the characterization of oxide coatings. However, due to the significant differences in the physical and chemical properties between aluminum and oxides, this method may not be suitable for aluminum coatings, and a more appropriate approach needs to be explored. In this paper, the effects of three polishing technologies (mechanical polishing, argon-ion-beam polishing, and electrolytic polishing) on the porosity measurement of pure aluminum coatings were compared and studied. The research results showed that the commonly used mechanical polishing method and more advanced argon-ion-beam polishing method could not completely reveal the pore structure because SiC particles would be embedded in the pure aluminum coatings during mechanical polishing, filling large pores. Although electrolytic polishing technology had advantages in revealing the macroporous structure, it would introduce a microporous structure and oxides, which would affect the measurement of the coating porosity. The composite polishing technology (electrolytic polishing + argon-ion-beam polishing) could perfectly reveal the pore structure in the pure-aluminum coating, and the porosity of arc-sprayed aluminum coating was 9.9%, which was close to the macroscopic true value measured using the weighing method of 10.2%.

Interlayer Polarization Explains Slow Charge Recombination in Two-Dimensional Halide Perovskites by Nonadiabatic Molecular Dynamics Simulation.

Two-dimensional (2D) perovskites for applications in photovoltaics and optoelectronics are attracting a great deal of research interest. The nonradiative electron-hole (e-h) recombination is the major efficiency loss channel. Herein, we report a study of the thickness dependence of the e-h recombination dynamics in diamine-based 2D perovskite via ab initio NAMD. For multilayer structures, due to the emergence of spontaneous interlayer electric polarization, which is induced by the collective and correlated reorientation of methylammonium molecules, the electron and hole at the band edges are localized in different inorganic layers, suppressing the e-h recombination. Furthermore, a broad range of phonon excitation also inspired rapid pure dephasing related to the microscopic origin for longer recombination times. The combination of the two effects leads to the observation of a prolonged carrier lifetime in multilayer 2D perovskites, which is essential to understanding the nonradiative e-h recombination mechanism in such materials.

[Determination of 61 organophosphorous pesticide residues in fruits, vegetables, milk, vegetable oils and animal muscles by dispersive solid-phase extraction and ultra performance liquid chromatography-tandem mass spectrometry].

A dispersive solid-phase extraction coupled with ultra performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) method was developed for the determination of 61 organophosphorous pesticide residues in fruits, vegetables, milk, vegetable oils and animal muscles. The fruit, vegetable and milk samples were extracted with acetonitrile and separated with salting out method; vegetable oil samples were dissolved by n-hexane, and extracted with acetonitrile; animal muscle samples were extracted with acetonitrile-water assisted by n-hexane and separated with salting out method. And then the supernatants were purified using dispersive solid-phase extraction (C18 and primary secondary amine powder) prior to the UPLC-MS/MS analysis. The analytes were indentified in positive electrospray ionization (ESI+) and multiple reaction monitoring (MRM) mode. The matrix-matched external standard calibration curves were used for quantitative analysis. Under the optimal conditions, the detection limits (S/N > or = 10) of the method were 0.01 mg/kg. The recoveries were 62.8%-107%, and the relative standard deviations (RSDs) were in the range of 4.2%-19%. The method has the advantages of easy, fast, and more sensitive, and can meet the requirement of the determination of organophosphorous pesticide residues in the foods.

[Multi-residue determination of 289 pesticides in garlic by gas chromatography and gas chromatography/mass spectrometry].

A new idea of solvent transfer technique was developed and applied to determine 283 pesticide residues in garlic by gas chromatography-mass spectrometry (GC-MS) (method I), and the other method using normal phase silica/selective elution technique was applied to determine 6 pesticide residues with strongly polar in garlic by gas chromatography (method II). For the method I, the residues were extracted from homogenized tissue with acetonitrile-water, separated with liquid-liquid partition; the clear supernatant was purified by solvent transfer technique and solid phase extraction (Envi-18 and LC-NH2 columns), then was analyzed by GC-MS. For the method II, the residues were extracted from homogenized tissue using ethyl acetate and sodium sulfate assisted by ultrasonication. The supernatant was purified by solid phase extraction (primary secondary amine (PSA) powder and LC-Si column) prior to GC analysis. The determination was performed by using selected ion monitoring (SIM) mode in GC-MS method and flame photometric detector (FPD) in GC method, then external standard method was used in the quantification. Under the optimal conditions, the detection limits for the two methods (S/N > or = 10) of pesticides were 0.01-0.05 mg/kg, the recoveries carried out by the addition of standards of 0.01-0.20 mg/kg were 52%-163%, among which the recoveries for 88% pesticides were between 70% and 120%; the recoveries of the method II were 70%-111%; while the relative standard deviations were 2.4%-18% and 3.2%-9.3%, respectively. The model of solvent transfer technique and the sensitivity improvement of GC-MS was also studied. The methods are easy, fast, more sensitive, and can meet the requirement of the multiresidual analysis in garlic.

[Determination of dicofol residue in eel by gas chromatography-mass spectrometry].

A method was developed and applied to determine the residual dicofol in eel by gas chromatography-mass spectrometry (GC-MS). The residue was extracted from homogenized tissue with acetonitrile-water assisted by n-hexane, separated with liquid-liquid partition. A part of the supernatant was purified using solid phase extraction (Florisil column) prior to the GC-MS analysis. The determination was performed in selected ion monitoring (SIM) mode with the external calibration for quantitative analysis. Under the optimized conditions, the quantification limit of the method (S/N = 10) was lower than 0.01 mg/kg. The recovery of the method for the spiked standard at the concentration of 0.01-0.1 mg/kg was 91%-105% with the relative standard deviation (RSD) of 4.3%-6.1%. The method is easy, fast, and more sensitive. The method can meet the requirement of the determination of dicofol in eel.

[Two sample pretreatment methods and their applications in the determination of 111 pesticides and related chemicals in aquatic products].

Two sample pretreatment methods were developed and applied to determine 111 residues of pesticides in aquatic products. For Method I, the residues were extracted from homogenized tissue with acetone-ethylacetate-n-hexane (1 : 1 : 1, v/v/v), and then were redissolved in acetonitrile; for Method II, the residues were extracted from homogenized tissue with acetonitrile - water under ultrasonication, and then were separated by liquid-liquid partition. The supernatants were purified using solid phase extraction (Envi-18 and PSA columns) prior to the GC-MS analysis. The determination was performed in selected ion monitoring (SIM) mode with the external calibration for quantitative analysis. Under the optimal conditions, the detection limits (S/N = 3) for residues of pesticides were in the ranges of 0.001 -0.026 mg/kg. The recoveries of Method I for the spiked standards at concentration of 0.25 mg/kg were 72% - 113% with the relative standard deviations (RSDs) of 3.4% - 12.1%. The recoveries of Method II were 51% - 127%, among which the recoveries for 94% pesticides were between 70% and 120% with the RSDs of 3.2% - 13.8%.