Insights into the Retention Mechanism of Small Neutral Compounds on Octylsiloxane-Bonded and Diisobutyloctadecylsiloxane-Bonded Silica Stationary Phases in Reversed-Phase Liquid Chromatography

The system constants of the solvation parameter model are used to prepare system maps for the retention of small neutral compounds on an octylsiloxane-bonded (Kinetex C8) and diisobutyloctadecylsiloxane-bonded (Kinetex XB-C18) superficially porous silica stationary phases for aqueous mobile phases containing 10–70% (v/v) methanol or acetonitrile. Electrostatic interactions (cation-exchange) are important for the retention of weak bases with acetonitrile–water but not for methanol–water mobile phases. Compared with an octadecylsiloxane-bonded silica stationary phase (Kinetex C18) retention is reduced due to a less favorable phase ratio for both the octylsiloxane-bonded and diisobutyloctadecylsiloxane-bonded silica stationary phases while selectivity differences are small and solvent dependent. Selectivity differences for neutral compounds are larger for methanol–water but significantly suppressed for acetonitrile–water mobile phases. The selectivity differences arise from small changes in all system constants with solute size and hydrogen-bond basicity being the most important due to their dominant contribution to the retention mechanism. Exchanging the octadecylsiloxane-bonded silica column for either the octylsiloxane-bonded or diisobutyloctadecylsiloxane-bonded silica column affords little scope for extending the selectivity space and is restricted to fine tuning of separations, and in some cases, to obtain faster separations due to a more favorable phase ratio. For weak bases larger differences in relative retention are expected with acetonitrile–water mobile phases on account of the additional cation exchange interactions possible that are absent for the octadecylsiloxane-bonded silica stationary phase.     

Multi-pesticide Residues Determination in Samples of Fruits and Vegetables Using Chemometrics Approach to QuEChERS-dSPE Coupled with Ionic Liquid-Based DLLME and LC–MS/MS

Pesticides play vital roles in agricultural practices, but the nature and mishandling of the chemicals have led to their accumulation in moist soil, vegetables and fruits. Thus, development of efficient methods for pesticides determination is one of the most important ways to address such challenges. Multivariate response surface methodology optimisation using Placket–Burman and Box–Behnken designs were respectively used for screening and optimisation of significant factors of quick, easy, cheap, effective, rugged and safe (QuEChERS) with dispersive solid-phase extraction (SPE). Consequently, the optimised QuEChERS-dSPE technique coupled with modified ionic liquid-based (IL-based) dispersive liquid–liquid microextraction was used for sample preparation before LC–MS/MS. The developed method was validated (SANTE 11831-2017) for multi-pesticide residues determination in samples of bananas, cabbages, tomatoes, oranges and onions. The precision results were satisfactory in terms of relative standard deviation (≤?20%) as recommended. The results of accuracy for relative recoveries (82–137%) were satisfactory because 92.5% of results were within the recommended range (70–120%). The matrix effects in all the samples were very weak (less effective) (≤???80%). The linearity of the evaluated results was 5–400 µg kg^?1 and regression coefficients (R²) were?>?0.99. The resulting limits of detection and quantitation were 0.02–0.32 and 0.07–1.06 µg kg^?1, respectively, and therefore satisfactory. Certifiably, the estimated measurement uncertainties range (1–16%) was acceptable (≤?50%). Thus, the developed method could be reliable and suitable for routine determination of multiple pesticide residues in various vegetable and fruit samples.     

An In situ Study on the Orderly Crystal Growth of Pluronic F127 Block Copolymer Blended with and without Ionic Liquid during Isothermal Crystallization

Isothermal crystallization behavior of Pluronic F127 blended with and without an ionic liquid (IL) was investigated by in situ polarized optical microscopy (POM) and Fourier transform infrared spectroscopy (FTIR). For the pure F127, the POM and FTIR results showed that the spherulite size and crystallinity of F127 increased with the melting temperature increasing to 60, 80, and 135°C. This could be explained by the flexibility of the polymer chain at high melting temperatures. For the F127 blended with IL, the POM results showed that the morphology of F127 evolved from spherulite to dendritic segregation and fibrous crystal with the increasing IL content. FTIR results indicated that hydrogen bonds were formed between F127 and IL, and the intensity of the hydrogen bonds became strengthened gradually with increasing IL content. The effect of hydrogen bonds on the morphology evolution of F127/IL is discussed.     

The Influence of Space Restriction on the Mechanical Properties of Isotactic Polypropylene/Reduced Graphene Oxide Nanocomposite Injection Bars

Isotactic polypropylene (iPP)/reduced graphene oxide (RGO) nanocomposite bars were prepared by injection molding and the effects of RGO on the iPP matrix were investigated via differential scanning calorimetry, two dimensional wide angle X-ray diffraction (2D WAXD) and small angle X-ray scattering (2D SAXS) techniques. It is demonstrated that RGO is an effective nucleation agent for iPP and the incorporation of RGO can enhance the degree of orientation of iPP crystals at flow direction. Furthermore, the orientation of iPP chains at flow direction intensifies with the increase of RGO contents, which can be attributed to that RGO obstructed the motion of orientated polymer chains. Although the enhancement effect is weaker than that of High density polyethylene (HDPE)/RGO nanocomposites with epitaxial crystallization, the mechanical properties of iPP/RGO nanocomposites can also be improved apparently.     

Highly Sensitive Nanostructured Electrochemical Sensor Based on Carbon Nanotubes-Pt Nanoparticles Paste Electrode for Simultaneous Determination of Levodopa and Tyramine

A multicomponent electrochemical sensor, with two nanometer-scale components in sensing matrix/electrode, was used to simultaneous determination of levodopa (LD) and tyramine (TR) in pharmaceutical and diet samples. Multiwall carbon nanotubes (MWCNTs) were used as carbonaceous materials in the electrode construction. 5-amino-3',4'-dimethoxy-biphenyl-2-ol (5ADMB) was used as electron mediator and Pt nanoparticles (nPt) as a catalyst. The 5ADMB catalyzes the oxidation of LD to the corresponding catecholamine, which is electrochemically reduced back to LD. Preparation of this electrode was very simple and modified electrode showed good properties at electrocatalytic oxidization of LD and TR. Using differential pulse voltammetry (DPV), a highly selective and simultaneous determination of LD and TR has been explored at the modified electrode. Differential pulse voltammetry peak currents of LD and TR increased linearly with their concentrations at the ranges of 0.50–100.0 μM and 0.60–100.0 μM, respectively. Also, the detection limits for LD and TR were 0.31 and 0.52 μM, respectively. The electrode exhibited an efficient catalytic response with good reproducibility and stability.     

Mechanism of the Enhanced Electrochemical Properties of LiNi<Subscript>1/3</Subscript>Co<Subscript>1/3</Subscript>Mn<Subscript>1/3</Subscript>O<Subscript>2</Subscript> Cathode Materials by a Pre-Sintering Process

α-NaFeO₂ layered LiNi_1/3Co_1/3Mn_1/3O₂ cathode materials were synthesized by mechanical milling accompanied by the solid phase sintering. The sample exhibited a good crystallinity and layered structure while sintered at 900°C, which can be further improved by adding a pre-sintering process at 500°C before high temperature sintering. The sample with a pre-sintering process presents an average particle size about 0.6 μm, and a hexagonal crystalline structure. The optimally fabricated sample showed a first charge capacity of 210.2 mA h/g, discharge capacity of 171.2 mA h/g with a current rate of 0.2 C within the voltage range of 2.7~4.5 V. With increasing the current rate to 1 C, the charge–discharge capacity faded quickly during the cycling process, which can be partially recovered while operated at a low current rate. However, the capacity fading at a current rate of 2 C was largely irreversible. The evolution of the surface chemical states was evaluated using X-ray photoelectron spectroscopy on the charged and discharged samples to understand the high rate capacity fading.     

Extraction and Determination of Two Antidepressant Drugs in Human Plasma by Dispersive Liquid–Liquid Microextraction‒HPLC

In this study, an effective method of ultrasound-assisted ionic liquid based dispersive liquid–liquid microextraction (UA?IL?DLLME) coupled with HPLC was applied for extraction and determination of two antidepressant drugs: venlafaxine hydrochloride and amitriptyline hydrochloride from human plasma samples. Three ionic liquids were studied: 1-butyl-3-methyl imidazolium hexafluorophosphate, 1-hexyl-3- methyl imidazolium hexa-fluoro-phosphate, and 1-octyl-3-methyl imidazolium hexafluorophosphate [C8MIM][PF6]. Various factors affect the stages and efficiency of extraction, some of which are pH of sample solution, type and volume of ionic liquid, the time of ultrasonication, centrifuging time and rate, and the ionic strength of solution. In this research, optimum conditions were obtained as 55 μL of [C8MIM][PF6] selected as ionic liquid, pH 11, 2% NaCl, 4 min ultrasonication and 5 min centrifuging at 3500 rpm. Under the optimized conditions, the linearity was obtained in the range of 0.2 to 250 μg/L. The limits of detection were 0.5 μg/L for venlafaxine and 0.8 μg/L for amitriptyline. Pre-concentration factors were 1.3 × 10³ for venlafaxine and 1.2 × 10³ for amitriptyline. The UA?IL?DLLME method coupled with HPLC was successfully used for the determination of venlafaxine and amitriptyline spiked into the real samples of human plasma.     

Graphene Quantum Dots Incorporated into β-cyclodextrin: a Novel Polymeric Nanocomposite for Non-Enzymatic Sensing of L-Tyrosine at Physiological pH

Graphene quantum dot-β-cyclodextrin modified glassy carbon electrode was used as a new nanosensor for determination of L-tyrosine (L-Tyr). It was found that graphene quantum dot-β-cyclodextrin has been stably electrodeposited on glassy carbon electrode modified by simple technique. The cyclic voltammograms of the modified electrode in an aqueous solution displayed a pair of well-defined, stable and irreversible reductive/oxidation redox systems. The apparent electron transfer rate constant (k_s) and transfer coefficient (α) determined by cyclic voltammetry were approximately equal to 8.0 s^–1 and 0.7, respectively. The modified electrode showed excellent catalytic activity towards the oxidation of L-Tyr at positive potential in buffer solution. The nanosensor also displayed fast response time, high sensitivity, low detection limit and a remarkably positive potential oxidation of L-Tyr that decreased the effect of interferences in analysis.     

Structure and function of a lignostilbene-α,β-dioxygenase orthologue from <Emphasis Type="Italic">Pseudomonas brassicacearum</Emphasis>

Background

Stilbene cleaving oxygenases (SCOs), also known as lignostilbene-α,β-dioxygenases (LSDs) mediate the oxidative cleavage of the olefinic double bonds of lignin-derived intermediate phenolic stilbenes, yielding small modified benzaldehyde compounds. SCOs represent one branch of the larger carotenoid cleavage oxygenases family. Here, we describe the structural and functional characterization of an SCO-like enzyme from the soil-born, bio-control agent Pseudomonas brassicacearum.

Methods

In vitro and in vivo assays relying on visual inspection, spectrophotometric quantification, as well as liquid-chormatographic and mass spectrometric characterization were applied for functional evaluation of the enzyme. X-ray crystallographic analyses and in silico modeling were applied for structural investigations.

Results

In vitro assays demonstrated preferential cleavage of resveratrol, while in vivo analyses detected putative cleavage of the straight chain carotenoid, lycopene. A high-resolution structure containing the seven-bladed β-propeller fold and conserved 4-His-Fe unit at the catalytic site, was obtained. Comparative structural alignments, as well as in silico modelling and docking, highlight potential molecular factors contributing to both the primary in vitro activity against resveratrol, as well as the putative subsidiary activities against carotenoids in vivo, for future validation.

Conclusions

The findings reported here provide validation of the SCO structure, and highlight enigmatic points with respect to the potential effect of the enzyme’s molecular environment on substrate specificities for future investigation.

     

A bioinspired strategy for assembling graphene oxide sheets into high-performance graphene films

正Since 2004, graphene has attracted intense attention from the scientific and industrial communities due to its extraordinary properties, such as ultrahigh strength and remarkable electrical conductivity. Mass production of high-quality graphene materials can be achieved by chemical vapor deposition and chemical exfoliation, and their applications have been widely explored [1–3]. The most common graphene derivative, graphene oxide (GO), can be synthesized in mass quantities by the oxidation and exfoliation of graphite, which has a low cost and is commercially available in market. One of the major applications for GO nanosheets is the construction of high-performance GO-based fibers,