Comparative studies on the adsorption and association of 3-methylxanthine and 7-methylxanthine at a charged interface

Summary A systematic comparative study of the adsorption and association of 3-methylxanthine (3MXan) and 7-methylxanthine (7MXan) at mercury-solution interfaces in acidic, neutral, and alkaline buffer solutions was undertaken by ac voltammetry. At bulk concentrations of3MXan above a threshold value, the stacking interactions between vertically oriented molecules lead to a slow reorientation aroundE _ecm, and the molecules adopt a perpendicular orientation. The association of the adsorbed molecules of7MXan is hindered and is not effective in promoting base-base stacking interactions. The enhanced surface activity of3MXan compared to7MXan results from the different position of the methyl group in the purine moiety. The effect of some divalent metal ions on adsorption stages and association of the investigated compounds has been studied. The results indicate that the complexation of methylated xanthine enhances the stacking interactions and hence would be expected to facilitate the formation of perpendicularly stacked layers ofM(II)-MXan complexes on the electrode surface. The adsorption parameters of the investigated compounds have been computed in absence and presence of Cu(II) at differentpH values. The results are compared with the behaviour of xanthine.

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Vergleichende Untersuchungen zur Adsorption und Assoziation von 3-Methylxanthin und 7-Methylxanthin an einer geladenen Grenzfläche

Zusammenfassung Eine systematische vergleichende Untersuchung zur Adsorption und Assoziation von 3-Methylxanthin (3MXan) und 7-Methylxanthin (7MXan) an Quecksilber-Lösungs-Grenzflächen wurde mittels AC-Voltammetrie in sauren, neutralen und basischen Pufferlösungen durchgeführt. Oberhalb einer bestimmten Grenzkonzentration kommt es durch Wechselwirkungen zwischen vertikal angeordneten3MXan-Molekülen zu einer langsamen Reorientierung umE _ecm, was zu einer senkrechten Anordnung führt. Die Assoziation von7MXan ist gehindert und erlaubt daher keine ausgeprägten Basen-Basen-Wechselwirkungen. Die erhöhte Oberflächenaktivität von3MXan gegenüber7MXan resultiert aus der unterschiedlichen Stellung der Methylgruppe am Purinrest. Der Effekt einiger zweiwertiger Metallionen auf die Adsorption und Assoziation von3MXan und7MXan wurde untersucht. Die Ergebnisse weisen auf eine Begünstigung der Wechselwirkungen hin; die Bildung von vertikal angeordneten Grenzschichten an der Elektrodenoberfläche sollte daher im Fall vonM(II)-Mxan-Komplexen erleichtert sein. Die Adsorptionsparameter der untersuchten Verbindungen wurden in Abwesenheit und in Gegenwart von Cu(II) bei verschiedenenpH-Werten berechnet. Die Ergebnisse werden mit dem Verhalten von Xanthin verglichen.

     

Recent Advances in Nanomaterial‐Modified Pencil Graphite Electrodes for Electroanalysis

Pencil graphite electrodes (PGEs) have several advantages over other carbon‐based or commercial metal electrodes, including widespread availability, very low cost, and ease of modification. To make the best use of PGEs in electroanalysis, significant recent advances in the development of different nanomaterial‐PGEs have been observed. The literature published up to mid‐2015 is summarized in the present review, with a focus on the various methodologies used to readily modify graphite pencil electrodes using nanomaterials. This review also touches on the surface characterization of these electrodes and their potential applications in a variety of electrochemical detection applications. The review outlines the scope for further research in this area and discusses the importance of surface modifications of conventional PGE electrodes using nanomaterials or a combination of nanomaterials and electroactive polymers.     

Unsteady MHD double-diffusive convection boundary-layer flow past a radiate hot vertical surface in porous media in the presence of chemical reaction and heat sink

This paper presents an analytical study of the unsteady MHD free convective heat and mass transfer flow of a viscous, incompressible, gray, absorbing-emitting but non-scattering, optically-thick and electrically conducting fluid occupying a semi-infinite porous regime adjacent to an infinite moving hot vertical plate with constant velocity. We employ a Darcian viscous flow model for the porous medium the Rosseland diffusion approximation is used to describe the radiative heat flux in the energy equation. The homogeneous chemical reaction of first order is accounted in mass diffusion equation. The governing equations are solved in closed form by Laplace-transform technique. A parametric study of all involved parameters is conducted and representative set of numerical results for the velocity, temperature, concentration, shear stress function $\frac{\partial u}{\partial y} \vert_{y=0}$ , temperature gradient $\frac{\partial \theta }{ \partial y}\vert_{y=0}$ , and concentration gradient $\frac{ \partial \phi }{\partial y}\vert_{y=0}$ is illustrated graphically and physical aspects of the problem are discussed.     

Amplified label-free electrical detection of DNA hybridization

A new protocol is described for amplifying label-free electrochemical measurements of DNA hybridization based on the enhanced accumulation of purine nucleobases in the presence of copper ions . Such electrical DNA assays involve hybridization of the target to inosine-substituted oligonucleotide probes (captured on magnetic beads), acidic dipurinization of the hybrid DNA, and adsorptive chronopotentiometric stripping measurements of the free nucleobases in the presence of copper ions. Both amplified adenine and guanine peaks can be used for detecting the DNA hybridization. The dramatic signal amplification advantage of this type of detection has been combined with efficient magnetic removal of non-complementary DNA, use of microliter sample volumes and disposable transducers. Factors influencing the signal enhancement were assessed and optimized. A detection limit of 40 fmol (250 pg) was obtained with 10 min hybridization and 5 min adsorptive-accumulation times. The advantages of this procedure were demonstrated by its application in the detection of DNA segments related to the BRCA1 breast cancer gene. The copper enhancement holds great promise not only for the detection of DNA hybridization, but also for trace measurement of nucleic acids.     

Renewable pencil electrodes for highly sensitive stripping potentiometric measurements of DNA and RNA

Renewable graphite pencil electrodes are demonstrated to be excellent materials for adsorptive stripping measurements of trace nucleic acids. While displaying an attractive stripping performance, comparable to that of conventional carbon paste electrodes, the pencil electrode offers a convenient (mechanical) renewal, with each stripping potentiogram recorded at a fresh surface. Various pencil lead materials and lengths have been examined and experimental variables of the pretreatment and measurement procedures have been explored and optimized. The extremely low detection limits (e.g., 3 micrograms l-1 tRNA with 10 min accumulation) are coupled to a good surface-to-surface reproducibility (RSD of 6.4% for 14 repetitive measurements of 1 mg l-1 ssDNA).     

Synthesis,Spectroscopic, and Antimicrobial Activity Studies of Novel 10-Substituted Camptothecin Phosphorothioate Analogs

Abstract

A series of title compounds 2 and 3 were efficiently synthesized via the condensation of 10–hydroxycamptothecin with various symmetric (O,O′-monoaryl)-thiophosphoryl chlorides and asymmetric (O-ethyl-O′-aryl)-thiophosphoryl chlorides in sodium hydroxide powder and acetonitrile system. The structures of title compounds 2 and 3 were confirmed by elemental analysis, IR, ¹H NMR, ¹³C NMR, ³¹P[¹H] NMR,and mass spectral data. These symmetric [(O,O′-monoaryl)-thiophosphoryl)]-(20S)-camptothecin (2a–f) and asymmetric [(O-ethyl-O′-aryl)-thiophosphoryl)]-(20S)-camptothecin (3a–f) compounds were also tested for their in vitro antimicrobial activities against some bacterial strains, namely, S. aureus, B. Simplex, E. acetylicum, E. coli, P. aeruginosa, S. flexenari, S. aureus, S. typhi, and some fungal strains Aspergillus niger, Aspergillus flavus (molds), S. cerevisiae, C. albicans, T. longifucus, A. flavus, M. canis, F. solani, and C. glaberata (yeasts).

Supplemental materials are available for this article. Go to the publisher's online edition of Phosphorus, Sulfur, and Silicon and the related elements to view the free supplemental file.     

Studies on the Chelation of Cyclodiphosph(V)azane Complexes of Co(II), Ni(II), Cu(II), and Pd(II): Preparation,Characterization, Thermal,Solid State Electrical Conductivity,and Biological Activity Studies

Cyclodiphosph(V)azane of chromene, (1,3-diphenyl-2,4-bis(3-amino-9-methoxy1-tolyl-3H-benzo[f]chromene-2-carbonitrile)-2,2,4,4-tetrachlorocyclodiphosph(V)azane (III), reacts with stoichiometric amounts of transition metal salts such as Co(II), Ni(II), Cu(II), and Pd(II) to afford colored complexes in a moderate to high yield. The structure of the isolated complexes was suggested based on elemental analyses, IR, molar conductance, UV-Vis, ¹H, ¹³C, and ³¹P-NMR, magnetic susceptibility measurements, and dark electrical conductivity of solid state from room temperature up to 450 K. The complexes have been investigated in solution by spectrophotometric molar ratio and conductometric methods. Kinetic and thermodynamic parameters were computed from the thermal decomposition data using the Coats and Redfern method. The prepared complexes showed high to moderate bactericidal activity compared with the ligand.     

Nanomolar amperometric sensing of hydrogen peroxide using a graphite pencil electrode modified with palladium nanoparticles

We report on a simple and rapid method for the preparation of a disposable palladium nanoparticle-modified graphite pencil electrode (PdNP-GPE) for sensing hydrogen peroxide (H₂O₂). The bare and PdNP-modified GPEs were characterized by cyclic voltammetry and SEM. The two electrodes displayed distinct electrocatalytic activities in response to the electrochemical reduction of H₂O₂. The amperometric detection limits were 45 nM and 0.58 mM, respectively, for the PdNP-GPE and bare-GPE, at an S/N of 3. The electrodes can be prepared simply and at low cost, and represent a promising tool for sensing H₂O₂.

An insight into the KOH activation mechanism through the production of microporous activated carbon for the removal of Pb cations

Chemical activation was used through direct mixing of KOH with maize stalks in the smallest amount of water to provide KOH-to-stalk ratios of 33, 50, 66 and 75% by weight. The KOH-treated stalks were carbonized at 700 °C to produce a series of four activated carbons, besides a non-activated sample that was prepared and carbonized at 550 °C. The porous properties of these carbons were characterized by the Langmuir, BET and Dubinin-Radushkevich linear equations as well as both α_s (alpha-s) and t methods based on nitrogen adsorption isotherms. The chemical reactions involved during the impregnation and the carbonization processes for these hydroxide/lignocellulose mixtures have been proposed. Deep insight has been obtained concerning the possible reactions mechanism. The results showed that the KOH ratio was found to be the basic indicator of micoporosity development. The increase in the concentration of KOH much increased the S^α values of the resulting carbons reaching a maximum limit at 66 wt% KOH with S^α of 1684 m²/g and micropore ratio of ∼85% displaying an inverse correlation thereafter. The thermal behaviour and the surface microstructure in addition to the surface functional groups of the maize stalks and their prepared carbons were investigated by TGA, SEM and FTIR. The investigated carbons took up significant amounts of Pb²⁺ ions from aqueous solutions, which are ascribed to both the porosity and surface chemical nature of the adsorbents.     

Impact of Mo and Ce on growth of single-walled carbon nanotubes by chemical vapour deposition using MgO-supported Fe catalysts

A series of nine catalysts containing Ce/Fe and Mo/Fe at various loadings on MgO supports have been studied as catalysts for chemical vapour deposition (CVD) of single-walled carbon nanotubes (SWCNTs) using a methane carbon source. Our results show that the Ce/Fe system is very suitable as a catalyst that favours SWCNT growth, and we question the special importance that has been attributed to Mo as an additive to Fe-based catalysts for SWCNT growth, as it appears that Ce is equally effective. Our results indicate that dehydroaromatization (DHA) is not a defining step for the growth mechanism, as has been suggested for Mo/Fe systems previously, and show that Ce and Mo do not seriously perturb the well-known Fe/MgO system for growth of high quality SWCNT. Using Raman spectroscopy, we have shown that the Ce/Fe/MgO catalyst system favours growth of SWCNTs with a different distribution of chiralities compared to the analogous Mo/Fe/MgO system.