A comparative potentiostatic study of the electrocrystallization of PbO2 on platinum and glassy carbon electrodes

The deposition of PbO₂ on platinum and glassy carbon electrodes was investigated experimentally. The comparison of the different behaviour on the two types of electrodes is presented. The effects of variation of temperature, potential, nucleation order and induction time-lags on the electrocrystallisation process are discussed.The present investigations have confirmed that electrocrystallisation of PbO₂ occurs through 3-D nucleation and growth on glassy carbon and most probably also on platinum. The data on platinum contains considerable noise from simultaneous chemical processes and is not amenable for analysis in a simple way. The steady state behaviour is unusual because it has generally been assumed that charge transfer is rate determining under most conditions. However, as shown here, if lattice incorporation is slow even at the steady state, it provides a method for separating the activation energies associated with the nucleation and growth rate constants. Finally, the dissolution of PbO₂ into the platinum substrate and its implication on transient rates merits more attention. In this context the Pt-PbO₂ system is probably not a good choice because of the large contribution of substrate oxidation to the measured transient rates.     

Conductimetric immunosensor based on poly(3,4-ethylenedioxythiophene)

A conductimetric reagentless immunosensor using the biospecific binding pair of goat antirabbit IgG and rabbit IgG has been designed and fabricated using poly (3,4-ethylenedioxythiophene) as the immobilization matrix-cumtransducer.     

Novel label-free DNA sensors based on poly(3,4-ethylenedioxythiophene)

We report on the design and development of a novel label-free DNA sensor based on conducting poly(3,4-ethylenedioxythiophene) for the direct detection and quantification of target ssDNA.     

Papuamides E and F, Cytotoxic Depsipeptides from the Marine Sponge Melophlus sp

Two known papuamides C (1) and D (2) together with two new depsipeptides, papuamides E (3) and F (4), were isolated from an undescribed sponge of the genus Melophlus collected in the Solomon Islands. The planar structures of the compounds were elucidated on the basis of spectroscopic studies. Papuamides C-F (1-4) showed cytotoxicity against brine shrimp with LD₅₀ values between 92 and 106 μg/mL.     

Molecular dynamics simulations of layer-by-layer assembly of polyelectrolytes at charged surfaces: effects of chain degree of polymerization and fraction of charged monomers

We performed molecular dynamics simulations of the electrostatic assembly of multilayers of flexible polyelectrolytes at a charged surface. The multilayer build-up was achieved through sequential adsorption of oppositely charged polymers in a layer-by-layer fashion from dilute polyelectrolyte solutions. The steady-state multilayer growth proceeds through a charge reversal of the adsorbed polymeric film which leads to a linear increase in the polymer surface coverage after completion of the first few deposition steps. Moreover, substantial intermixing between chains adsorbed during different deposition steps is observed. This intermixing is consistent with the observed requirement for several deposition steps to transpire for completion of a single layer. However, despite chain intermixing, there are almost perfect periodic oscillations of the density difference between monomers belonging to positively and negatively charged macromolecules in the adsorbed film. Weakly charged chains show higher polymer surface coverage than strongly charged ones.     

The nature of species adsorbed on platinum from SO2 solutions

The present studies have shown that at θ_S<0.9, the species adsorbed on platinum from SO₂ solutions are virtually identical with those obtained from H₂S solutions. The layer at θ_S>0.9 can be explained by the formation of a partial bilayer on top of the first layer wherein 70% of the sites are covered with S (2-site adsorption) and 30% with S (1-site adsorption). It has also been shown that the ‘difficultly reducible’ oxygen referred to by earlier workers is identical with the species formed by exposure of the electrode to SO₂ at potentials above 0.2 V. Evidence for the presence of two forms of ‘difficultly reducible’ oxygen, corresponding to the two forms of chemisorbed sulfur, is presented.     

Anticancer Properties of Highly Purified l-Asparaginase from Withania somnifera L. against Acute Lymphoblastic Leukemia

Withania somnifera L. has been traditionally used as a sedative and hypnotic. The present study was carried out for the purification, characterization, and in vitro cytotoxicity of l-asparaginase from W. somnifera L. l-Asparaginase was purified from the fruits of W. somnifera L. up to 95% through chromatography. The purified l-asparaginase was characterized by size exclusion chromatography, polyacrylamide gel electrophoresis (PAGE), and 2D PAGE. The antitumor and growth inhibition effect of the l-asparaginase was assessed using [3-(4, 5-dimethyl-thiazol-2yl)-2, 5-diphenyl-tetrazolium bromide] (MTT) colorimetric dye reduction method. The purified enzyme is a homodimer, with a molecular mass of 72 ± 0.5 kDa, and the pI value of the enzyme was around 5.1. This is the first report of the plant containing l-asparaginase with antitumor activity. Data obtained from the MTT assay showed a LD₅₀ value of 1.45 ± 0.05 IU/ml. W. somnifera L. proved to be an effective and a novel source of l-asparaginase. Furthermore, it shows a lot of similarity with bacterial l-asparaginases EC-2.     

Formic acid oxidation at platinized platinum electrodes: Part IV. Effect of preadsorbed sulfur and related studies

A study has been made of the effect of preadsorbed sulfur layer on the anodic oxidation of formic acid at a platinized platinum electrode. It is shown that, in the presence of the preadsorbed s sulfur layer, the oxidation process obeys the following rate expression,i=nFkc^αg(θ_s) exp(α_anF?r/RT)where α≈0.75 and α_an≈0.5. This is explained in terms of the following rate-determining step,

${\left(HCOOH\right)}_{ads}\to \underset{*}{C}OOH+H^{+}+e$

and involves the adsorption of formic acid on the sulfur-covered electrode surface. A strong catalytic effect of the sulfur layer is observed; the function g(θ_S) is greater than unity at all θ_S and exhibits two maxima at г_S values of 0.3 and 0.6. It is suggested that these effects arise from a coverage-dependent variation of bond strength between adsorbed sulfur and platinum. Evidence is adduced that the adsorption of sulfur from H₂S solutions involves 2-site adsorption at low coverages and 1-site adsorption at higher coverages. Possibility of multilayer adsorption under appropriate conditions is also discussed in the light of the results obtained.     

Two forms of chemisorbed sulfur on platinum and related studies

The effect of temperature on the potentiodynamic oxidation of adsorbed sulfur layers on platinum, obtained from H₂S or SO₂ was studied. The broad oxidation peak at 1.2–1.3 V observed at room temperature is resolved at 80°C. Two distinct peaks are observed at 80°C, oxidation peak I (at 0.97 V) corresponding to the weakly bound sulfur and oxidation peak II (at 1.10 V) corresponding to the strongly bound sulfur. Evidence is adduced to show that these two forms of chemisorbed sulfur are distinguished by the number of platinum sites they occupy. At elevated temperatures an extension of the hydrogen region was observed during cathodic charging in the presence of adsorbed sulfur. This phenomenon was found to be reversible with respect to temperature and does not correspond to a desorption of sulfur.     

Preparation,Spectroscopic and Structural Investigations of Spiro Derivatives of Hexachlorocyclotriphosphazatriene

Abstract

Mono-(N₃P₃Cl₄Y₂), bis-(N₃P₃Cl₂Y₄) and tris-spiro derivatives (N₃P₃Y₆) have been prepared with ethylene, 1, 3-propylene and 1,4-butylene glycols (Y₂ = glycol residue). The ¹H NMR spectra of mono- and tris-derivatives are relatively simple; those of the bis- very complex due to the intrinsic asymmetry of the methylene protons. This effect is made use of in studying the replacement pattern of N₃P₃Cl₄ [O (CH₂)₃O] with primary and secondary amines. Homonuclear ¹H decoupling simplifies the spectra and allows an unambiguous distinction to be made between the different isomeric possibilities of the bis amino derivatives N₃P₃Cl₂R₂ [O(CH₂)₃O] where R = amino residue. Primary amines give geminal, secondary amines nongeminal trans-derivatives. The trans-structure of the bis-pyrrolidino derivative has been confirmed by X-ray crystallography.