Synthesis,CMC determination and influence of the micelles, β-cyclodextrin,ionic liquids and liposome(dipalmitoylphosphatidylcholine) vesicles on the kinetics of an outer-sphere electron transfer reaction

The surfactant–cobalt(III) complex, cis-[Co(trien)(4AMP)(DA)](ClO₄)₃, trien = triethylenetetramine, 4AMP = 4-aminopyridine, DA = dodecylamine was synthesized and characterized by various spectroscopic and physico-chemical techniques. The critical micelle concentration (CMC) value of this surfactant–cobalt(III) complex in aqueous solution was found out from conductance measurements. The conductivity data (at 303, 308, 313, 318 and 323 K) were used for the evaluation of the temperature-dependent CMC and the thermodynamics of micellization (ΔG _m ^° , ΔH_m and ΔS _m ^° ). Also the kinetics of reduction of this surfactant–cobalt(III) complex by hexacyanoferrate(II) ion in micelles, β-cyclodextrin, ionic liquids (ILs) and in liposome vesicles (DPPC) media were studied at different temperature. The rate constant for the electron transfer reaction in micelles was found to increase with increase in the initial concentration of the surfactant–cobalt(III) complex. This peculiar behaviour of dependence of second-order rate constant on the initial concentration of one of the reactants has been attributed to the presence of various concentration of micelles under different initial concentration of the surfactant–cobalt(III) complex in the reaction medium. Inclusion of the long aliphatic chain of the surfactant complex ion into β-cyclodextrin leads to decrease in the rate constant. Below the phase transition temperature of DPPC, the rate decreased with increasing concentration of DPPC, while above the phase transition temperature the rate increased with increasing concentration of DPPC. It is concluded that below the phase transition temperature, there is an accumulation of surfactant–cobalt(III) complex at the interior of the vesicle membrane through hydrophobic effects, and above the phase transition temperature the surfactant–cobalt(III) complex is released from the interior to the exterior surface of the vesicle. In the presence of ionic liquid medium the second order rate constant for this electron transfer reaction for the same complex was found to increase with increasing concentration of ILs has also been studied. An outer-sphere mechanism is proposed for all these reactions and the results have been explained based on the hydrophobicity of the ligand and the reactants with opposite charges.     

Multiobjective Optimization of Fabrication Parameters of Jute Fiber/Polyester Composites with Egg Shell Powder and Nanoclay Filler

In the present study, a model is presented to optimize the fabrication parameters of natural fiber reinforced polyester matrix composites with dual fillers. In particular, jute fiber mat was chosen as reinforcement and eggshell powder (ESP) and montmorillonite nanoclay (NC) were selected as fillers. The weight per square meter (GSM) of the fiber, the weight percentage of ESP and NC have been chosen as independent variables and the influence of these variables on tensile, flexural and impact strength of the composite has been inspected. The permutations of the different combinations of factors are intended to accomplish higher interfacial strength with the lowest possible number of tested specimens. The experiments were designed by the Taguchi strategy and a novel multi-objective optimization technique named COPRAS (COmplex PRoportional ASsessment of alternatives) was used to determine the optimal parameter combinations. Affirmation tests were performed with the optimal parameter settings and the mechanical properties were evaluated and compared. Experimental results show that fiber GSM and eggshell powder content are significant variables that improve mechanical strength, while the nanoclay appears less important.     

Studies on outer-sphere electron transfer reactions of surfactant–cobalt(III) complexes with iron(II) in liposome (dipalmitoylphosphotidylcholine) vesicles

The effect of unilamellar vesicles of dipalmitoylphosphotidylcholine (DPPC), both below and above the phase transfer region, on the second-order rate constants for outer-sphere electron transfer between Fe²⁺ and the surfactant?Ccobalt(III) complexes, cis-[Co(en)₂(C₁₂H₂₅NH₂)₂]³⁺ and cis-[Co(trien)(C₁₂H₂₅NH₂)₂]³⁺ (en?=?ethylenediamine, trien?=?triethylenetetramine, C₁₂H₂₅NH₂?=?dodecylamine) was studied by UV?CVis absorption spectroscopy. Below the phase transition temperature of DPPC, the rate decreased with increasing concentration of DPPC, while above the phase transition temperature the rate increased with increasing concentration of DPPC. It is concluded that below the phase transition temperature, there is an accumulation of surfactant?Ccobalt(III) complexes at the interior of the vesicle membrane through hydrophobic effects, and above the phase transition temperature the surfactant?Ccobalt(III) complex is released from the interior to the exterior surface of the vesicle. Through isokinetic plots, we have established that the mechanism of the reaction does not alter during the phase transition of DPPC.     

Direct electrochemistry of glucose oxidase and sensing glucose using a screen-printed carbon electrode modified with graphite nanosheets and zinc oxide nanoparticles

We have studied the direct electrochemistry of glucose oxidase (GOx) immobilized on electrochemically fabricated graphite nanosheets (GNs) and zinc oxide nanoparticles (ZnO) that were deposited on a screen printed carbon electrode (SPCE). The GNs/ZnO composite was characterized by using scanning electron microscopy and elemental analysis. The GOx immobilized on the modified electrode shows a well-defined redox couple at a formal potential of −0.4 V. The enhanced direct electrochemistry of GOx (compared to electrodes without ZnO or without GNs) indicates a fast electron transfer at this kind of electrode, with a heterogeneous electron transfer rate constant (K_s) of 3.75 s⁻¹. The fast electron transfer is attributed to the high conductivity and large edge plane defects of GNs and good conductivity of ZnO-NPs. The modified electrode displays a linear response to glucose in concentrations from 0.3 to 4.5 mM, and the sensitivity is 30.07 μA mM⁻¹ cm⁻². The sensor exhibits a high selectivity, good repeatability and reproducibility, and long term stability.

Graphical representation for the fabrication of GNs/ZnO composite modified SPCE and the immobilization of GOx

     

Selective and Simultaneous Determination of Dihydroxybenzene Isomers Based on Green Synthesized Gold Nanoparticles Decorated Reduced Graphene Oxide

In the present study, we report the simultaneous electrochemical determination of hydroquinone (HQ), catechol (CC) and resorcinol (RC) at gold nanoparticles (Au‐NPs) decorated reduced graphene oxide (RGO) modified electrode. An enhanced and well defined peak current response with a better peak separation of HQ, CC and RC is observed at RGO/Au‐NPs composite than that of RGO and Au‐NPs modified electrodes. The fabricated modified electrode shows a wide linear response in the concentration range of 3–90 µM, 3–300 µM and 15–150 µM for HQ, CC and RC, respectively. The detection limit of HQ, CC and RC is found as 0.15 µM, 0.12 µM and 0.78 µM, respectively.     

Voltammetric determination of the anti-cancer drug nilutamide using a screen-printed carbon electrode modified with a composite prepared from β-cyclodextrin,gold nanoparticles and graphene oxide

The authors describe an electrochemical method for the determination of the anti-cancer drug nilutamide. The method is based on the use of a composite prepared from β-cyclodextrin, gold nanoparticles and graphene oxide (β-CD-AuNP/GO). An alkaline solution of glucose was used as a reducing agent to reduce the gold ions, rather than citric acid and a harmful reducing agent such as hydrazine and sodium borohydride. The structure and surface morphology of the β-CD-AuNP/GO composite was characterized by Raman spectroscopy, transmission electron microscopy and energy-dispersive X-ray spectroscopy. A screen printed carbon electrode was modified with the nanocomposite, and the resulting electrode used as a disposable sensor for the determination of nilutamide by differential pulse voltammetry. Best operated at a working voltage of 0.43 V (vs Ag/AgCl), it exhibits excellent electrocatalytic activity and a detection limit as low as 0.4 nM. The sensor was applied to the determination of nilutamide in (spiked) human serum, as well as in a tablet, where it displays good recovery and accuracy. The sensor is repeatable, reproducible, stable and selective even in the presence of other aromatic nitro compounds.

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Graphical abstract An electrochemical method for the determination of the anti-cancer drug nilutamide is described. A screen printed carbon electrode is modified with the nanocomposite prepared from β-cyclodextrin, gold nanoparticles and graphene oxide (β-CD-AuNP/GO). Best operated at a working voltage of 0.43 V (vs Ag/AgCl), it exhibits excellent electrocatalytic activity and a detection limit as low as 0.4 nM. The sensor was applied to the determination of nilutamide in (spiked) human serum and a tablet where it displays good recovery and accuracy.

     

Diproline templates as folding nuclei in designed peptides. Conformational analysis of synthetic peptide helices containing amino terminal Pro-Pro segments

The effect of N-terminal diproline segments in nucleating helical folding in designed peptides has been studied in two model sequences Piv-Pro-Pro-Aib-Leu-Aib-Phe-OMe (1) and Boc-Aib-Pro-Pro-Aib-Val-Ala-Phe-OMe (2). The structure of 1 in crystals, determined by X-ray diffraction, reveals a helical (alphaR) conformation for the segment residues 2 to 5, stabilized by one 4-->1 hydrogen bond and two 5-->1 interactions. The N-terminus residue, Pro(1) adopts a polyproline II (P(II)) conformation. NMR studies in three different solvent systems support a conformation similar to that observed in crystals. In the apolar solvent CDCl3, NOE data favor the population of both completely helical and partially unfolded structures. In the former, the Pro-Pro segment adopts an alphaR-alphaR conformation, whereas in the latter, a P(II)-alphaR structure is established. The conformational equilibrium shifts in favor of the P(II)-alphaR structure in solvents like methanol and DMSO. A significant population of the Pro(1)-Pro(2) cis conformer is also observed. The NMR results are consistent with the population of at least three conformational states about Pro-Pro segment: trans alphaR-alphaR, trans P(II)-alphaR and cis P(II)-alphaR. Of these, the two trans conformers are in rapid dynamic exchange on the NMR time scale, whereas the interconversion between cis and trans form is slow. Similar results are obtained with peptide 2. Analysis of 462 diproline segments in protein crystal structures reveals 25 examples of the alphaR-alphaR conformation followed by a helix. Modeling and energy minimization studies suggest that both P(II)-alphaR and alphaR-alphaR conformations have very similar energies in the model hexapeptide 1.     

Surface stress generation during formation of alkanethiol self-assembled monolayer (SAM)

A high resolution curvature interferometer [Wang, J., Shrotriya, P., Kim, K.S., 2006. Surface residual stress measurement using curvature interferometry. Experimental Mechanics 46 (1), 39–46] is utilized to measure surface stress development associated with formation of self-assembled monolayers (SAM) of octadecanethiols on macroscale domains (25 mm × 25 mm). Atomistic simulations are performed to investigate surface stress generation associated with SAM formation. The results of the molecular simulations are incorporated into the multiscale framework to understand the surface stress generation and curvature change observed during experiments at continuum scale.     

A Highly Sensitive and Selective Enzymatic Biosensor Based on Direct Electrochemistry of Hemoglobin at Zinc Oxide Nanoparticles Modified Activated Screen Printed Carbon Electrode

A sensitive enzymatic biosensor has been developed for the detection of hydrogen peroxide (H₂O₂), nitrite ( ) and trichloroacetic acid (TCA) by using hemoglobin (Hb) immobilized on activated screen printed carbon electrode (ASPCE) and zinc oxide (ZnO) composite. A pair of well defined redox peaks is observed with a heterogeneous electron transfer rate constant (K_s) of 5.27 s^?1 for Hb at ASPCE/ZnO. The biosensor exhibits the detection of H₂O₂, TCA and in the concentration range of 0.5–129.5 µmol L^?1, 2.5–72.5 mmol L^?1 and 0.2–674 µmol L^?1 with the detection limit of 0.083 µmol L^?1, 0.12 mmol L^?1 and 0.069 µmol L^?1, respectively.     

Chitosan-based tyrosinase optical phenol biosensor employing hybrid nafion/sol-gel silicate for MBTH immobilization

The development of an optical biosensor based on immobilization of 3-methyl-2-benzothiazolinone hydrazone (MBTH) in hybrid nafion/sol-gel silicate film and tyrosinase in chitosan film for the detection of phenolic compounds has been described. Tyrosinase was immobilized in chitosan film deposited on the hybrid nafion/sol-gel silicate film containing MBTH. The enzymatic oxidation product of phenolic compounds were stabilized through formation of adduct with MBTH to produce a maroon color adduct. The color intensity of adduct was found to increase proportionally with the increase of the substrate concentrations after 5 min exposure. The linearity of the biosensor towards phenol, catechol and m-cresol were in the respective concentration range of 0.5-7.0, 0.5-10.0 and 1.0-13.0 mg/L with detection limit of 0.18, 0.23 and 0.43 mg/L, respectively. The biosensor shows a good stability for at least 3 months.