Formation and characterization of self-organized TiO2 nanotube arrays by pulse anodization

This paper describes TiO2 nanotube arrays prepared by anodic oxidation of Ti substrates using pulse voltage waveforms. Voltages were pulsed between 20 and -4 V or between 20 and 0 V with varying durations from 2 to 16 s at the lower limit of the pulse waveform. Ammonium fluoride or sodium fluoride (and mixtures of both) was used as the electrolyte with or without added medium modifier (glycerol, ethylene glycol, or poly (ethylene glycol) (PEG 400)) in these experiments. The pulse waveform was optimized to electrochemically grow TiO2 nanotubes and chemically etch their walls during its cathodic current flow regime. The resultant TiO2 nanotube arrays showed a higher quality of nanotube array morphology and photoresponse than samples grown via the conventional continuous anodization method. Films grown with a 20 V/-4 V pulse sequence and pulse duration of 2 s at its negative voltage limit afforded a superior photoresponse compared to other pulse durations. Specifically, the negative voltage limit of the pulse (-4 V) and its duration promote the adsorption of NH4+ species that in turn inhibits chemical attack of the growing oxide nanoarchitecture by the electrolyte F- species. The longer the period of the pulse at the negative voltage limit, the thicker the nanotube walls and the shorter the nanotube length. At variance, with 0 V as the low voltage limit, the longer the pulse duration, the thinner the oxide nanotube wall, suggesting that chemical attack by fluoride ions is not counterbalanced by NH3/NH4+ species adsorption, unlike the interfacial situation prevailing at -4 V. Finally, the results from this study provide useful evidence in support of existing mechanistic models for anodic growth and self-assembly of oxide nanotube arrays on the parent metal surface.     

Effects of abiotic stressors on synthesis of the mycosporine-like amino acid shinorine in the Cyanobacterium Anabaena variabilis PCC 7937

In the present investigation we show that the cyanobacterium Anabaena variabilis PCC 7937 produces a single mycosporine-like amino acid (MAA), shinorine (retention time = 2.3 min and absorption maximum at 334 nm) when isolated and purified by HPLC. Although there was significant induction of MAA synthesis from its initial value under 395 or 320 nm cutoff filters, MAA induction was significantly more pronounced in samples covered with 295 nm cutoff filters after 72 h of exposure. Heat as a stress factor had no effect on MAA induction with or without UV radiation. In contrast, salt and ammonium treatment had synergistic effects with UV stress. MAA synthesis was also induced by salt and ammonium in a concentration-dependent manner without UV stress in samples covered with 395 nm cutoff filters. The results indicate that MAAs may have other functions in addition to photoprotection in this organism.     

Electrodeposition of Silver Vanadate Films: A Tale of Two Polymorphs

Two polymorphs of AgVO₃, namely the α- and β- forms, were prepared and their physical, structural, optical, electrochemical, and photoelectrochemical characteristics were compared using a battery of experimental and theoretical tools. A two-step method, previously developed in the our laboratory for the electrodeposition of inorganic semiconductor films, was applied to the electrosynthesis of silver vanadate (AgVO₃) films on transparent, conducting oxide surfaces. In the first step, silver was cathodically deposited from a non-aqueous bath containing silver nitrate. In the second step, the silver film was anodically stripped in an aqueous medium containing ammonium metavanadate. The anodically generated silver ions at the interface underwent a precipitation reaction with the vanadate species to generate the desired product in situ. Each of these steps were mechanistically corroborated via the use of electrochemical quartz crystal microgravimetry, used in conjunction with voltammetry and coulometry. As-deposited films were crystalline and showed p-type semiconductor behavior. Theoretical insights are provided for the electronic origin of the α→β phase transformation in AgVO₃ and the disparate optical band gaps of the two polymorphs. Finally, implications for the application of this material in solar cells are provided.     

Detection of Free Thiols and Fluorescence Response of Phycoerythrin Chromophore after Ultraviolet-B Radiation Stress

The chemistry of thiol-chromophore linkage plays a central role in the nature of fluorescence of phycoerythrin (PE). Interaction of thiol and chromophore is crucial for the energy transfer, redox signal and inhibition of oxidative damage. In the present investigation the effects of ultraviolet-B radiation on an emission fluorescence intensity and wavelength shift in PE due to interaction between thiol and chromophore by remarkable strategy of detection technique was studied. Purification of PE was done by using a gel permeation and ion exchange chromatography that yielded a quite high purity index (6.40) in a monomeric (αβ) form. UV-B radiation accelerated the quenching efficiency (24.9 ± 1.52%) by reducing fluorescence emission intensity of thiol linked chromophore after 240 min of UV-B exposure. However, after blocking of transiently released free thiol by N-ethylmaleimide, quenching efficiency was increased (36.8 ± 2.80%) with marked emission wavelength shift towards shorter wavelengths up to 562 nm as compared to 575 nm in control. Emission fluorescence of free thiol was at maximum after 240 min that was detected specifically by monobromobimane (mBrB) molecular probe. The association/dissociation of bilin chromophore was analyzed by SDS- and Native-PAGE that also indicated a complete reduction in emission fluorescence. Our work clearly shows an early detection of free thiols and relative interaction with chromophore after UV-B radiation which might play a significant role in structural and functional integrity of terminal PE.     

Kinetics and mechanism of quinolinium dichromate mediated oxidation of sugar alcohols in Bronsted acid media

Bronsted acid catalyzed oxidation of certain sugar alcohols (polyols) has been studied by quinolinium dichromate (QDC) using aqueous sulfuric, perchloric, and hydrochloric acids at different temperatures. At constant acidity, reaction kinetics revealed the second-order kinetics with a first order in [Alcohol] and [QDC]. Zucker-Hammett, Bunnett, and Bunnett-Olsen criteria were used to analyze acid-dependent rate accelerations. Bunnett-Olsen plots of (log k + H_ν) versus (H_ν + log [H⁺]), and (log k) versus (H_ν + log [H⁺]) afforded slope values (ϕ and ϕ^*, respectively) > 0.47, suggesting that a water molecule acts as a prton transfer agent in the slow step of the mechanism in the oxidation of alcohols by QDC in the presence of aqueous sulfuric, perchloric, and hydrochloric acids.     

Effect of prior mechanical treatment on the thermal decomposition of alkali metal perchlorates

The kinetics of the thermal decomposition of sodium, potassium, rubidium and caesium perchlorates show a pronounced dependence on the past mechanical history of the materials. The variation in the decomposition rate with strain energy and lattice distortion shows that crystal defects are important factors in the thermal reactivity of alkali metal perehlorates.     

Unexpected Synthesis of Highly Substituted Indole Derivatives

An unexpected synthesis of highly substituted indole derivatives was provided by treating 2-[2-(4-fluorophenyl)-2-oxo-1-phenylethyl]-4-methyl-3-oxo-pentanoic acid phenylamide with aryl hydrazines in the presence of trifluoroacetic acid in tetrahydrofuran.     

Recent Advances in Copper Intercalators as Anticancer Agents

Copper is a part of various enzymes and helps them to function properly. It can be effectively used to produce promising anticancer drugs and presently, many studies are being pursued worldwide on the development of copper-based complexes as potential anticancer drugs. Herein, we briefly discuss the importance of reactive oxygen species in biological applications and copper(II) complexes as anticancer drugs. The anti-angiogenic properties of mono-nuclear copper(II) complexes have been demonstrated by in vivo chick embryo angiogenesis analysis. The plausible mechanism behind anticancer activity of these complexes is by the formation of excessive intracellular Reactive Oxygen Species (ROS). ROS is a composite term used for oxygen derivative non-radicals and free radicals of highly reactive components, that enhances the killing response of immune cells to microbial invasion. Previous reports have shown that ROS plays an important role as a messenger in cell cycling and normal cell signal transduction.

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Graphical Abstract The generation of singlet oxygen and healing the tumor cells with singlet oxygen in presence of UV-light.

     

CuCN catalyzed one pot synthesis of γ-keto diesters: domino double Michael addition followed by Nef reaction

An efficient one pot synthesis of γ-keto diesters through double Michael addition coupled with Nef reaction in the presence of CuCN catalyst and Cs₂CO₃ base is described. The reaction proceeds rapidly in DCM to give the products in good yields. A plausible mechanism is proposed.