Covalent modification of a glassy carbon surface by 4-aminobenzoic acid and its application in fabrication of a polyoxometalates-consisting monolayer and multilayer films

4-Aminobenzoic acid (4-ABA) was covalently grafted on a glassy carbon electrode (GCE) by amine cation radical formation in the electrooxidation process of the amino-containing compound. X-ray photoelectron spectroscopy measurement proves the presence of 4-carboxylphenylamine monolayer on the GCE. The redox responses of various electroactive probes were investigated on the 4-ABA-modified GCE. Electron transfer to Fe(CN)(6)(3-) in solutions of various pHs was studied by both cyclic voltammetry and electrochemical impedance analysis on the modified electrode. Changes in the solution pH value result in the variation of the terminal group charge state, based on which surface pK(a) values are estimated. The 4-ABA-modified GCE was used as a suitable charged substrate to fabricate polyoxometalates-consisting (POM-consisting) monolayer and multilayer films through layer-by-layer assembly based on electrostatic attraction. Cyclic voltammetry shows the uniform growth of these three-dimensional multilayer films. Taking K10H3[Pr-(SiMo7W4O39)(2)]. H2O (abbreviated as Pr(SiMo7W4)(2)), for example, the preparation and electrochemical behavior of its monolayer and multilayer film had been investigated in detail. This modification strategy is proven to be a general one suitable for anchoring many kinds of POMs on the 4-ABA-modified GCE.

Dynamic mechanical studies of the sulfonated butyl rubber ionomers and their blends

Dynamic mechanical properties of sulfonated butyl rubber ionomers neutralized with different amine or metallic ion (zinc or barium) and their blends with polypropylene (PP), high-density polyethylene (HDPE), or styrene-butadiene-styrene (SBS) triblock copolymer were studied using viscoelastometry. The results showed that glass transition temperatures of ion pair-containing matrix and ionic domains (T-g1 and T-g2, respectively) of amine-neutralized ionomers were lower than those of ionomers neutralized with metallic ions, and the temperature range of the rubbery plateau on the storage modulus plot for amine-neutralized ionomers was narrower. The modulus of the rubbery plateau for amine-neutralized ionomers was lower than that of ionomers neutralized with zinc or barium ion. With increasing size of the amine, the temperature range for the rubbery plateau decreased, and the height of the loss peak at higher temperature increased. Dynamic mechanical properties of blends of the zinc ionomer with PP or HDPE showed that, with decreasing ionomer content, the T-m of PP or HDPE increased and T-g1 decreased, whereas T-g2 or the upper loss peak temperature changed only slightly. The T-g1 for the blend with SBS also decreased with decreasing ionomer content. The decrease of T-g1 is attributed to the enhanced compatibilization of the matrix of the ionomer-containing ion pairs with amorphous regions of PP or HDPE or the continuous phase of SBS due to the formation of thermoplastic interpenetrating polymer networks by ionic domains and crystalline or glassy domains.

Self-assembled monolayer of polyoxometalate on gold surfaces: Quartz crystal microbalance, electrochemistry, and in-situ scanning tunneling microscopy study

A new kind of inorganic self-assembled monolayer (SAM) was prepared by spontaneous adsorption of polyoxometalate anion, AsMo11VO404-, onto a gold surface from acidic aqueous solution. The adsorption process, structure, and electrochemical properties of the AsMo11VO404- SAM were investigated by quartz crystal microbalance (QCM), electrochemistry, and scanning tunneling microscopy (STM). The QCM data suggested that the self-assembling process could be described in terms of the Langmuir adsorption model, providing the value of the free energy of adsorption at -20 KJ mol(-1). The maximum surface coverage of the AsMo11VO404- SAM on gold surface was determined from the QCM data to be 1.7 x 10(-10) mol cm(-2), corresponding to a close-packed monolayer of AsMo11VO404- anion. The analysis of the voltammograms of the AsMo11VO404- SAM on gold electrode showed three pairs of reversible peaks with an equal surface coverage of 1.78 x 10(-10) mol cm(-2) for each of the peaks, and the value was agreed well with the QCM data. In-situ STM image demonstrated that the AsMo11VO404- SAM was very uniform and no aggregates or multilayer could be observed. Furthermore, the high-resolution STM images revealed that the AsMo11VO404- SAM on Au(lll) surface was composed of square unit cells with a lattice space of 10-11 Angstrom at +0.7 V (vs Ag\AgCl). The value was quite close to the diameter of AsMo11VO404- anion obtained from X-ray crystallographic study. The surface coverage of the AsMo11VO404- SAM on gold electrode estimated from the STM image was around 1.8 x 10(-10) mol cm(-2), which was consistent with the QCM and electrochemical results.

Studies on the adduct reaction of disubstituted benzenes with ion system of acetone in gas phase

Ion - molecule reactions of disubstituted benzene with ion system of acetone and deuterium - labelled acetone under chemical ionization conditions were examined and the fragmentation reactions of the adduct ions formed by the ion - molecule reactions were studied using collision - induced dissociation (CID) technique. It was found that the electron - releasing groups favored the adduct reactions, whereas the electron - withdrawing groups did not. The position and properties of substituted groups affected the relative abundance of the adduct ions. The fragmentation reaction of the adduct ion formed by ortho - phenylenediamine with acetyl ion was similar to the reductive alkylation reaction of amine in condensed phase.

Determination of surface pK(a) of HS(CH2)(10)COOH SAMs using an electrochemical titration method

The self-assembled monolayer(SAM) of 11-mercaptoundecanoic acid [HS(CH2)(10)COOH] was formed on a gold electrode and the effect of the charge of end group on the electrochemical response of Fe(CN)(6)(3-) at the SAM modified electrode was studied by cyclic voltammetry. At high pH, when the -COOH groups are dissociated, the current of Fe(CN)(6)(3-) is suppressed; as the solution pH is lowered, the current of Fe(CN)(6)(3-) increases. The electrochemical titration curve was obtained by correlating the currents of Fe(CN)(6)(3-) to the different pH values of electrolyte, from which the surface pK(a) was obtained to be 3. 0+/-0. 2. Furthermore, the reason of small pK(a) value was explained using SAMs of different surface coverage.

Investigation of the electrocatalytic effect of polyaniline on the redox reaction of 2,5-dimercapto-1,3,4-thiadiazole by cyclic voltammetry

Electrochemical polymerized polyaniline(PAn) film electrode was used to investigate the electrocatalytic effect of PAn on the electrochemical redox reaction of 2,5-dimercapto-1,3,4-thiadiazole (DMcT), PAn film electrode was electrochemically treated or immersed in DMcT solution before it was scanned in 1.0 mol/L HCl electrolyte. The cyclic voltammograms of PAn film electrode in 1.0 mol/L HCl solution changed with the above treatment, implying the electrocatalytic effect of PAn on the redox reaction of DMcT, The formation of electron-donor-acceptor adducts through the interaction between thiol or disulfide groups of DMcT and amine or imine groups of PAn during the treatment was probably the reason of the catalysis, The electrochemical properties of the adduct were different from those of PAn and DMcT, The adduct possessed a higher electrochemical activity and a better electrochemical reversibility than DMcT or PAn used alone.

Self-assembled monolayer growth of phospholipids on hydrophobic surface toward mimetic biomembranes: Scanning probe microscopy study

Atomic force microscopy (AFM) and lateral force microscopy (LFM) were used simultaneously to analyze a model membrane bilayer structure consisting of a phospholipid outer monolayer deposited onto organosilane-derivatized mica surfaces, which were constructed by using painting and self-assembly methods. The phospholipid used as outer monolayer was dimyristoylphosphatidylcholine (DMPC). The hydrocarbon-covered substrate that formed the inner half bilayer was composed of a self-assembly monolayer (SAM) of octadecyltrichloroorganosilane (OTS) on mica. SAMs of DMPC were formed by exposing hydrophobic mica to a solution of DMPC in decane/isobutanol and subsequently immersing into pure water. AFM images of samples immersed in solution for varying exposure times showed that before forming a complete monolayer the molecules aggregated into dense islands (2.2-2.6 nm high) on the surface. The islands had a compact and rounded morphology. LFM, coupled with topographic data obtained with the atomic force mode, had made possible the distinction between DMPC and OTS. The rate constant of DMPC growth was calculated. This is the first systematic study of the SAM formation of DMPC by AFM and LFM imaging. It reveals more direct information about the film morphology than previous studies with conventional surface analytical techniques such as infrared spectroscopy, X-ray, or fluorescence microscopy.

Amplification of antigen-antibody interactions based on biotin labeled protein-streptavidin network complex using impedance spectroscopy

Antibody was covalently immobilized by amine coupling method to gold surfaces modified with a self-assembled monolayer of thioctic acid. The electrochemical measurements of cyclic voltammetry and impedance spectroscopy showed that the hexacyanoferrate redox reactions on the gold surface were blocked due to the procedures of self-assembly of thioctic acid and antibody immobilization. The binding of a specific antigen to antibody recognition layer could be detected by measurements of the impedance change. A new amplification strategy was introduced for improving the sensitivity of impedance measurements using biotin labeled protein- streptavidin network complex. This amplification strategy is based on the construction of a molecular complex between streptavidin and biotin labeled protein. This complex can be formed in a cross-linking network of molecules so that the amplification of response signal will be realized due to the big molecular size of complex. The results show that this amplification strategy causes dramatic improvement of the detection sensitivity of hIgG and has good correlation for detection of hIgG in the range of 2-10 mug/ml. (C) 2001 Elsevier Science B.V. All rights reserved.

Reactive blending of polyamide 6,6 and Vectra A

In this paper, blends of Nylon 6,6 with the liquid crystal polymer Vectra A950 are considered; specifically we focused our attention on Nylon 6,6 modifications by interchange reactions that can occur in the melt, as a function of mixing conditions and blend compositions. Two matrix samples have been used, characterised by a slightly different relative amount of amine and carboxylic end groups, being the latter predominant in both cases. The dried polymers Nylon 6,6/Vectra, combined in weight ratios between 95/5 and 50/50, were subjected to reactive blending with different methods (single-screw extruder, Brabender, pyrex reactor). Pure Nylon samples have been also investigated as reference materials. The soluble Nylon 6,6-rich fraction of each blend was separated from the insoluble Vectra-rich one and used for molecular and spectroscopic characterisations. Thermal and morphological analyses, as well as testing of tensile properties, were carried out on the blends. Evidences of the occurrence of interchange reactions are given and the most probable ones are suggested. (C) 2001 Elsevier Science Ltd. All rights reserved.