Heterostructured composite of NiFe-LDH nanosheets with Ti4O7 for oxygen evolution reaction

Developing oxygen evolution reaction (OER) electrocatalyst based on earth-abundant materials holds great promise for ascertaining water-splitting to surmount its deprived kinetics. In this regard, NiFe-LDH (layered double hydroxide) receives considerable attention owing to their layered structure. However, they still suffer from poor electronic conductivity and structural stability. We combined NiFe-LDH nanosheets with Magnéli phase Ti₄O₇ into a heterostructured composite. A series of analyses reveal that decorating Ti₄O₇ facilitates charge transfer to enhance the conductivity of NiFe-LDH-Ti₄O₇. During electrochemical measurement, Ni²⁺ is transformed to metastable Ni³⁺ (Ni (OH)→ NiOOH) before the OER onset potential. Thus, the presence of Ni³⁺ as the main active sites could improve the chemisorption of OH^? to facilitate OER. As a result, the NiFe-LDH-Ti₄O₇ catalyst delivers as low as onset potential (1.43 V). Combining the holey structure (NiFe-LDH and Ti₄O₇) and the defect engineering generated on NiFe-LDH-Ti₄O₇ as a synergistic effect improves the OER performance. The inclusion of Ti₄O₇ in the composite leads to more vacancy sites, as evidenced by the extended X-ray absorption fine structure (EXAFS) analysis. The obtained defective structure with a low coordination environment would improve the electronic conductivity and facilitate the adsorption process of H₂O onto metal cations, thereby increasing the intrinsic catalytic activity of NiOOH. The strong coupling of NiFe-LDH and Ti₄O₇ also increases the stability, and the heterostructured composite helps maintain the structural robustness of the LDH.     

Development and Validation of a Simple,Selective, and Accurate Reversed-Phase Liquid Chromatographic Method with Diode Array Detection (RP-HPLC/DAD) for the Simultaneous Analysis of 18 Free Amino Acids in Topical Formulations

Even though there are reported methods for the quantification of free amino acids (FAAs) in biological products, no work has been done on the analysis of these substances in formulations. Moreover, further research is required as the reported methods do not fulfill analytical method requirements. The objective of this study was, therefore, to develop and validate a rapid, reliable, and appropriate RP-HPLC/DAD method for the simultaneous determination of 18 FAAs (l-Ala, l-Arg, l-Asn, l-Asp, l-Gln, l-Glu, l-Gly, l-His, l-Ile, l-Lue, l-Lys, l-Met, l-Orn, l-Phe, l-Pro, l-Ser, l-Thr, and l-Val) in topical formulations. After appropriate method development, the technique was validated for selectivity, linearity and range, limit of detection, limit of quantification, precision, and accuracy. The samples were derivatized with 9-fluorenylmethyl chloroformate (Fmoc-Cl). Chromatographic separation was performed on InfinityLab Poroshell 120 E.C 18 (3?×?50) mm, 2.7 μm column at 25 °C. The mobile phase consisting of water and acetonitrile adjusted to appropriate pH was pumped in gradient mode at a flow rate of 0.7 mL/min. Ten microliters were injected and analyte detection was conducted using a DAD. The results indicate that the method was selective for these FAAs. It was linear over the concentration range of 5–80 µM with a correlation coefficient greater than 0.995. Moreover, it was sensitive, precise, accurate, and robust. All the reported drawbacks of RP-HPLC-based analysis of FAAs were resolved, and hence, this new method can be considered appropriate for the analysis of these FAAs in topical formulations.

     

Thermodynamic and kinetic study of chiral separations of coumarin-based anticoagulants on derivatized amylose stationary phase

Thermodynamic and kinetic studies are performed on amylose derivatized with tris-(3,5-dimethylphenyl carbamate) stationary phase for the chiral separation of coumarin-based anticoagulants. Polar-organic eluents that contain acetonitrile as bulk solvent with modifiers such as methanol, i-butanol, t-butanol, and tetrahydrofuran are used in the study. Temperature is varied from 5 to 45 °C at constant pressure of 1500 psi. In general, both retention and enantioselectivity decrease as the temperature increases and as hydrogen bond donating ability of the modifiers increases. The van’t Hoff plots are found to show both linear and non-linear behavior. The non-linear plots are believed to be the result of conformational changes in the derivatized amylose phase and are observed around room temperature. The retention behavior in acetonitrile mobile phase provides a linear enthalpy–entropy compensation plot, indicating that all coumarins may have a similar retention mechanism. In contrast, enthalpy–entropy compensation is not observed for warfarin and coumatetralyl enantiomers when separated with different organic modifiers in the mobile phase. The kinetic data indicate that the rate of sorption is always greater than the rate of desorption. An increase in the concentration of alcohol modifiers causes an increase in the desorption rate constant. In contrast, an increase in the concentration of tetrahydrofuran causes a decrease in the desorption rate constant. This effect is most significant for the second eluted enantiomer of coumatetralyl, for which the desorption rate is 36 times slower than the first eluted enantiomer.     

Analysis of Temocillin and Impurities by Reversed Phase Liquid Chromatography: Development and Validation of the Method

A robust, specific, precise and sensitive high-performance liquid chromatographic method has been described for purity control of temocillin. Chromatographic separation was achieved using a Symmetry C₁₈ (150 × 4.6 mm, 5 µm) column kept at 30 °C. The mobile phase consisted of a gradient mixture of mobile phases A (5 g/L solution of Na₂HPO₄·2H₂O, pH 7) and B (ACN-MeOH-H₂O, 50:10:40 v/v/v) pumped at a flow rate of 1.0 mL/min. UV detection was performed at 235 nm. The developed method was validated according to the ICH guidelines for its robustness, selectivity, sensitivity, precision and linearity. An experimental design was applied for the robustness study. Linearity was assessed both at impurity level in the range from LOQ to 10 % and assay level from 25 % to 150 % (0.6 mg/mL = 100 %). It is the first liquid chromatographic method described for the separation of temocillin and its potential impurities. It was possible to identify four degradation products from the forced degradation studies. The degradants do not interfere with the main peak and other known impurities showing that the method is specific and stability-indicating.     

Development and validation of a reversed-phase liquid chromatographic method for analysis of demeclocycline and related impurities

A simple, robust, and rapid reversedphase high-performance liquid chromatographic method for the analysis of demeclocycline and its impurities is described. Chromatographic separations were achieved on a Symmetry Shield RP8 (75 mm × 4.6 mm, 3.5 μm) column kept at 40°C. The mobile phase was a gradient mixture of acetonitrile, 0.06 M sodium edetate (pH 7.5), 0.06 M tetrapropylammonium hydrogen sulphate (pH 7.5) and water, A (2:35:35:28 v/v/v/v) and B (30:35:35:0 v/v/v/v) pumped at a flow rate of 1 mL/min. UV detection was performed at 280 nm. The developed method was validated according to the ICH guidelines for specificity, limit of detection, limit of quantification, linearity, precision, and robustness. An experimental design was applied for robustness study. Results show that the peak shape, chromatographic resolution between the impurities, and the total analysis time are satisfactory and better than previous methods. The method has been applied for the analysis of commercial demeclocycline bulk samples available on the market.     

A review of transition metal‐based bifunctional oxygen electrocatalysts

Electrochemical energy storage and conversion devices play a key role in the development of clean, sustainable, and efficient energy systems to meet the sustainable growth of our society. However, challenging issues including the sluggish kinetics of oxygen electrode reactions involving the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) are present, limiting the implementation of devices such as metal‐air batteries, water electrolyzers, and regenerative fuel cells. In this review, various monometallic and bimetallic transition metal oxides (TMOs) and hydroxides are summarized in terms of their application for ORR/OER, in which the merits and demerits of various precious metal and carbon‐based metal oxide materials are discussed, with requirements for better electrocatalysts and catalyst support being introduced as well. Following this, different approaches to improve catalytic activity such as the introduction of doping and defects, the manipulation of crystal facets, and the engineering of supports, compositions, and morphologies are summarized in which TMOs with improved ORR/OER catalytic activities can be synthesized, further improving the speed, stability, and polarization of electrochemical energy storage and conversion devices. Finally, perspectives into the improvement of performance and the better understanding of ORR/OER mechanisms for bifunctional electrocatalysts using in situ spectroscopic techniques and density functional theory calculations are also discussed.     

LC Assay for a HIV Tablet Containing Emtricitabine, Tenofovir Disoproxil Fumarate and Rilpivirine

A liquid chromatographic method with UV detection was developed for the assay of a tablet for HIV (human immunodeficiency virus) treatment containing three active components, which are emtricitabine, tenofovir disoproxil fumarate and rilpivirine. A Hypersil BDS-C18 column was used as stationary phase and the assay was performed with gradient elution using mobile phases containing acetonitrile, 0.2 M potassium dihydrogen phosphate and water. Dimethyl sulfoxide?Cdistilled water (1:1) was used as solvent for the active components. The method proved to be selective, linear, repeatable, sensitive and easy to perform.