非等温热分析法研究白藜芦醇及其苷的熔点、热稳定性及分解动力学

首次采用差示扫描量热法(Differential scanning calorimetry,DSC)和热重法(Thermogravimetry,TG)在氮气气氛下对白藜芦醇和白藜芦醇苷进行非等温热分析,采用Van't Hoff方程求得其纯度和熔点,并使用积分Coats-Redfern法、微分Achar法以及Malek法3种热分析动力学方法对热重实验数据进行分析,推断两种天然产物快速热分解阶段的最概然机理函数,并求得相应的动力学参数——表观活化能Ea和指前因子A。研究表明,白藜芦醇及其苷的纯度分别为99.76%和98.90%,熔点分别为257.09℃和198.79℃;白藜芦醇的热分解发生在220~468℃之间,失重率为46.69%;白藜芦醇苷在198~369℃之间发生分解,主要是糖苷键断裂引起的分解失重,失重率为37.47%;白藜芦醇的热分解为化学反应控制机制,符合反应级数方程,反应级数n=2;白藜芦醇苷的热分解为三维扩散控制机制,符合Z.-L.-T.方程;根据白藜芦醇及其苷的热分解动力学参数,推断二者在室温(25℃)下的贮存期分别为3年和4~5年,糖苷键的引入使白藜芦醇苷比白藜芦醇有更长久的贮存期。     

Influence of Temperature on the Formation of Silver Nanoparticles by using a Seed‐Free Photochemical Method under Sodium‐Lamp Irradiation

Silver nanoparticles can be prepared by using a seed‐free photo‐assisted citrate reduction method under the irradiation of a sodium lamp. Under the same irradiation intensity, bath temperatures are crucial in influencing the reaction rate, morphologies of final products, and shape evolution of the silver nanostructures. For example, when the bath temperature is 80 °C, the product yields of silver nanoplates, nanorods, and nanodecahedra are 38±6 %, 35±10 %, and 12±8 %, respectively. However, when the bath temperature is 30 °C, the product yields of silver nanoplates, nanorods, and nanodecahedra are 6±3 %, 0 %, and 83±16 %, respectively. Time‐dependent UV/Vis spectra and TEM images show that silver nanoplates were formed at the earlier reaction stage and greatly decreased in amount at the later stage when the bath temperatures are less than or equal to 40 °C. This indicates that the silver nanoplates, which can be regarded as intermediates, are kinetically favored products. They are not thermodynamically favored products at these relatively low bath temperatures. The SERS spectra of crystal violet (CV) show that all the silver colloids synthesized at various temperatures exhibit good enhancement factors and that the colloids prepared at lower bath temperatures have a higher enhancement factor.     

Size separation of graphene oxide using preparative free‐flow electrophoresis

Graphene oxide nanosheets often bear a wide size distribution. However, it is critical to have nanosheets with narrow size distribution for their unique size‐dependent physiochemical properties, and nanosheets with a narrow size distribution are the cornerstones for application. Therefore, efficient separation methods of graphene nanosheets have been given considerable attention in many scientific areas recently. Free‐flow electrophoresis is extensively used in the separation and purification of biological molecules with continuous flow separation. The charged graphene oxide nanosheets to some extent are very close in size to biological molecules and share similarity in motion behavior in an electric field. Thus, in the present work, we present a new and simple means to separate graphene oxide nanosheets into more mono‐dispersed size groups by using the free‐flow electrophoresis technique. By optimizing the separation conditions, we were able to obtain graphene oxide sheets with narrow size distribution. The separated samples were characterized by atomic force microscopy, and the size measurements were made by using the software “Image Pro Plus.” In addition, a brief discussion is also given into the theoretic background of the separation of graphene oxide according to the size by the technique of preparative free‐flow electrophoresis.     

Recent advances of green pretreatment techniques for quality control of natural products

A few advancing technologies for natural product analysis have been widely proposed, which focus on decreasing energy consumption and developing an environmentally sustainable manner. These green sample pretreatment and analysis methods following the green Analytical Chemistry (GAC) criteria have the advantage of improving the strategy of chemical analyses, promoting sustainable development to analytical laboratories, and reducing the negative effects of analysis experiments on the environment. A few minimized extraction methodologies have been proposed for replacing the traditional methods in the quality evaluation of natural products, mainly including solid-phase microextraction (SPME) and liquid phase microextraction (LPME). These procedures not only have no need for large numbers of samples and toxic reagent, but also spend a small amount of extraction and analytical time. This overview aims to list out the main green strategies on the application of quality evaluation and control for natural products in the past 3 years.     

The effect of chiral dopant on the rewriting speed of optically driving liquid crystal display

ABSTRACT

Mixing liquid crystal with chiral dopant for increasing the response speed of optical driving liquid crystal display was investigated in this paper. It is demonstrated that by adding chiral dopant, rewriting time could be decreased from 30 s to 20 s with good repeatability. Without adding other steps in the process of making cells, this improvement is due to the stronger elastic energymaking LC molecules twist more easily.     

邻苯二酚-四乙烯五胺改性超高分子量聚乙烯纤维

应用Cat-TEPA改性超高分子量聚乙烯(UHMWPE)纤维,在难黏附的纤维表面形成均匀涂层.采用透射电子显微镜(TEM)、红外光谱(FTIR)、X射线光电子能谱(XPS)、X射线衍射(XRD)、示差扫描量热(DSC)、热重分析(TGA)和静态接触角测试等手段对其结构和性能进行了表征,并通过单丝拔出实验研究改性前后纤维与环氧树脂之间的界面剪切强度(IFSS),探索了反应物配比、反应时间对表面性能的影响,并确定最佳改性条件.结果表明Cat-TEPA共沉积改性未影响纤维的结晶和热稳定性,改性后纤维表面浸润性得到改善,且适当增加反应时间和TEPA含量能够提高纤维和树脂之间的IFSS,当Cat-TEPA摩尔比为1:4,反应时间为24 h时效果最佳,与未改性纤维相比,界面剪切强度提升约44%.     

High-Efficiency Oxygen Reduction to Hydrogen Peroxide Catalyzed by Nickel Single-Atom Catalysts with Tetradentate N2O2 Coordination in a Three-Phase Flow Cell

Carbon-supported Ni^II single-atom catalysts with a tetradentate Ni-N₂O₂ coordination formed by a Schiff base ligand-mediated pyrolysis strategy are presented. A Ni^II complex of the Schiff base ligand (R,R)-(−)-N,N′-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexanediamine was adsorbed onto a carbon black support, followed by pyrolysis of the modified carbon material at 300 °C in Ar. The Ni-N₂O₂/C catalyst showed excellent performance for the electrocatalytic reduction of O₂ to H₂O₂ through a two-electron transfer process in alkaline conditions, with a H₂O₂ selectivity of 96 %. At a current density of 70 mA cm⁻², a H₂O₂ production rate of 5.9 mol g_cat.⁻¹ h⁻¹ was achieved using a three-phase flow cell, with good catalyst stability maintained over 8 h of testing. The Ni-N₂O₂/C catalyst could electrocatalytically reduce O₂ in air to H₂O₂ at a high current density, still affording a high H₂O₂ selectivity (>90 %). A precise Ni-N₂O₂ coordination was key to the performance.     

Anti-NLRP3 inflammasome abietane diterpenoids from Callicarpa bodinieri and their structure elucidation

Callica rpabodinieri is a Chinese traditional medicine herb with anti-inflammatory activity in clinic.Herein,we report two new 9,10-seco and etherified abietane diterpenoids bodinieric acids J and K(1 and 2) and one known compound(3) isolated from the leaves and twigs of this plant.Their chemical structures were elucidated by detailed spectrometry data analysis and DP4+NMR calculation methods.Hypothetical biosynthetic pathways of 1-3 were preliminarily speculated.Compound 3 inhibited inflammasome activation and exhibited blockage of NLRP3 inflammasome activation at non-cytotoxic concentrations in vitro.     

A new car-following model with consideration of inter-vehicle communication

In this paper, we construct a new car-following model with inter-vehicle communication (IVC) to study the driving behavior under an accident. The numerical results show that the proposed model can qualitatively describe the effects of IVC on each vehicle’s speed, acceleration, movement trail, and headway under an accident and that the new model can overcome the full velocity difference (FVD) model’s shortcoming that collisions occur under an accident, which illustrates that the new model can better describe the driving behavior under an accident than the FVD model.