Study on adsorption and oxidation of calf thymus DNA at glassy carbon electrode

The oxidation and adsorption of the temperature-denatured DNA at GC electrode are studied by differential pulse voltammetry and in situ FTIR spectroelectrochemistry. The temperature-denatured DNA is adsorbed and formed a DNA multilayer at electrode surface. The temperature-denatured DNA showing partly reversible process was first observed based on the reduction peaks appearing at negative scans and the reversible spectral change. The oxidation product of the temperature-denatured DNA can not diffuse away from the electrode surface easily due to the impediment of the DNA multilayer, so it can be partly reduced.

Oriented polyoxometalate-polycation multilayers on a carbon substrate

In this paper, a new method of fabricating multilayers on a carbon substrate is presented. First, a uniformly charged carbon surface was prepared through molecular design. Then an ultrathin film consisting of layer-pairs of oppositely charged polymeric cationic poly(diallyldimethylammonium chloride) (PDDA) and silicotungstate, SiW12O404- (SiW12), was grown layer-by-layer onto the grafted carbon substrate using a molecular self-assembly technique and an electrochemical method. The technique allows one to prepare highly adherent, dense and smooth films of polyoxometalates with special properties. By combining cyclic voltammetry (CV) and X-ray (XR) reflectometry, it was determined that the average surface density of SiW12 was 2.10 x 10(-10) mol cm(-2), and the thickness increase per adsorption of PDDA-SiW12 was 1.7 +/- 0.2 nm, indicating that the amount of SiW12 anion per one layer adsorption corresponded to a monolayer coverage. Atomic force microscopy (AFM) was also used to examine the surface morphology and determine the grain size distribution and roughness for multilayer films. An increase in root-mean-square (RMS) surface roughness from 7 to 9 Angstrom was observed as the number of layer-pairs in the film increased from 2 to 6. FTIR results showed that the good stability of the multilayer films was due to Coulomb interactions between the SiW12 anion and the polymeric cations PDDA. Moreover, the multilayer films, in acidic aqueous solution, showed good electrocatalytic activity toward the reduction of NO2-, and the catalytic currents increased with increasing the layer numbers of SiW12 adsorption. These characteristics of the multilayer films might find potential applications in the field of sensors and microelectronics devices.

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.

Investigation into self-assembled monolayers of a polyether dendron thiol: Chemisorption, kinetics, and patterned surface

Different sizes of Frechet-type dendrons with a thiol group at the focal point were synthesized, well characterized, and used as building blocks for the preparation of self-assembled monolayers (SAMs) on metal surfaces. From the studies of the kinetic process of dendron thiol self-assembling on gold, it is shown that the dendron thiol assembling proceeds with different adsorption rates depending on the assembly time. In contrast to normal alkanethiols forming highly molecular structures on metal surfaces, the SAMs of polyether dendron form patterned surfaces with nanometer-sized features and in long-range order. It is found that the patterned stripes are closely related to the size of the dendron, and the patterned stripes can be improved by thermal annealing.

Self-organization of BaF2 single crystal film under a compressed Langmuir monolayer

Self-organization of BaF2 single crystal film under a compressed monolayer of behenic acid (BA) has been investigated by using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The experimental results indicated the (100)-oriented single crystal film of BaF2 was formed under the BA monolayer. The relation between the BaF2 single crystal and the monolayer was discussed.

Studies on electrostatic adsorption of proteins on modified surface by real-time surface plasmon resonance technique

The elucidation of key influence factors for electrostatic adsorption is very important to control protein nonspecific adsorption on modified surfaces. In this study, real-time surface plasmon resonance technique is used to characterize the electrostatic adsorption of two proteins (mouse IgG and protein A) on carboxymethyldextran-modified surface. The results show that protein solution pH and ionic strength are key influence factors for efficient electrostatic adsorption. The influence of protein, solution pH on the amount of electrostatic adsorption depends on the type of the charge and the charge density of both protein and modified matrix on the surface. The electrostatic adsorption process involves a competition between the positively charged protein and other positively charged species in the buffer solution. A decrease of ionic strength leads to an increasing electrostatic adsorption. The kinetic adsorption constants of protein A at different pH values were also calculated and compared.

Controlled nucleation and growth of surface-confined gold nanoparticles on a (3-aminopropyl)trimethoxysilane-modified class slide: A strategy for SPR substrates

The thickness of the gold film and its morphology, including the surface roughness, are very important for getting a good, reproducible response in the SPR technique. Here, we report a novel alternative approach for preparing SPR-active substrates that is completely solution-based. Our strategy is based on self-assembly of the gold colloid monolayer on a (3-aminopropyl)trimethoxysilane-modified glass slide, followed by electroless gold plating. Using this method, the thickness of films can be easily controlled at the nanometer scale by setting the plating time in the same conditions. Surface roughness and morphology of gold films can be modified by both tuning the size of gold nanoparticles and agitation during the plating. Surface evolution of the Au film was followed in real time by UV-vis spectroscopy and in situ SPRS. To assess the surface roughness and electrochemical stability of the Au films, atomic force microscopy and cyclic voltammetry were used. In addition, the stability of the gold adhesion is demonstrated by three methods. The as-prepared Au films on substrates are reproducible and stable, which allows them to be used as electrodes for electrochemical experiments and as platforms for studying SAMs.

In situ infrared spectroelectrochemical studies on adsorption and oxidation of nucleic acids at glassy carbon electrode

The adsorption and oxidation of yeast RNA and herring sperm DNA (HS DNA) at glass carbon (GC) electrode are studied by differential pulse voltammetry (DPV) and in situ FTIR spectroelectrochemistry. Two oxidation peaks of yeast RNA are obtained by DPV, whose peak potentials shift negatively with increasing pH. The peak currents decrease gradually in successive scans and no corresponding reduction peaks occur, thus indicating that the oxidation process of yeast RNA is completely irreversible. The IR bands in the 1200-1800 cm-l range, attributed to the stretching and ring vibrations of nucleic acid bases, show the main spectral changes when the potential is shifted positively, which gives evidence that the oxidation process takes place in the base residues. The oxidation process of HS DNA is similar to that of yeast RNA. The results both from DPV and in situ FTIR spectroelectrochemistry confirm that the guanine and adenine residues can be oxidized at the electrode surface, which is consistent with the oxidation mechanism of nucleic acids proposed previously. (C) 2001 Elsevier Science B.V. All rights reserved.

Electrostatic layer-by-layer assembly of polycation and DNA multilayer films by real-time surface plasmon resonance technique

The assembly of alternating DNA and positively charged poly(dimethyldiallylammonium chloride) (PDDA) multilayer films by electrostatic layer-by-layer adsorption has been studied. Real time surface plasmon resonance (BIAcore) technique was used to characterize and monitor the formation of multilayer films in solution in real time continuously. The results indicate that the uniform multilayer can be obtained on the poly(ethylenimine) (PEI) coated substrate surface. The kinetics of the adsorption of DNA on PDDA surface was also studied by real-time BIAcore technique, and the observed rate constant was calculated using a Langmuir model (k(obs) = (1.28 +/- 0.08) x 10(-2) s(-1)).

Selective crystallization of BaF2 under a compressed Langmuir monolayer of behenic acid

Selective crystallization of BaF2 crystals under a compressed Langmuir monolayer of behenic acid [CH3(CH2)(20)COOH] has been studied by using X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, and energy-dispersive X-ray analysis. It was found that, in the absence of a monolayer, three kinds of crystals (Ba2ClF3, BaClF, and BaF2) can be obtained by mixing BaCl2 with a NH4F solution. However, in the presence of the monolayer of behenic acid, only BaF2 crystals appear at the monolayer-subphase interface and crystals have a special crystal face (100). During this process of crystallization, the monolayer plays a very important role and acts as a template that can preferentially select a special crystal and a special crystal face. The above results can be explained in terms of a specific molecular interaction between ions and the headgroups of the monolayer and specific electrostatic, geometric, and stereochemical interactions at the organic-inorganic interface.

In-situ FTIR study on adsorption and oxidation of native and thermally denatured calf thymus DNA at glassy carbon electrodes

In-situ Fourier transform infra-red (FTIR) spectra of native and thermally denatured calf thymus DNA (CT DNA) adsorbed and/or oxidized at a glassy carbon (GC) electrode surface are reported. The adsorption of native DNA occurs throughout the potential range (-0.2 similar to 1.3 V) studied, and the adsorbing state of DNA at electrode surface is changed from through the C=O band of bases and pyrimidine rings to through the C=O of cytosine and imidazole rings while the potential shifts negatively from 1.3 V to -0.2 V. An in-situ FTIR spectrum of native CT DNA adsorbed at GC electrode surface is similar to that of the dissolved DNA, indicating that the structure of CT DNA is not distorted while it is adsorbed at the GC electrode surface. In the potential range of -0.2 similar to 1.30 V, the temperature-denatured CT DNA is adsorbed at the electrode surface first, then undergoes electrochemical oxidation reaction and following that, diffuses away from the electrode surface. (C) 2001 Elsevier Science B.V. All rights reserved.

Superlattice structure of gold nanoparticles film deposited by Langmuir-Blodgett technique

Dodecanethiol-capped gold nanoparticles could spread on water subphase and be transferred by LB technique. The superlattice structure of gold nanoparticles multilayer was discussed.

Langmuir-Blodgett film formation from surfactant-capped zinc sulfide nanoparticles spread on water surface

The surfactant-capped ZnS nanoparticulate multilayer film has been fabricated by Langmuir-Blodgett(LB) technique. ZnS LB firm was investigated by the small-angle x-ray diffraction(XRD), atomic force microscopy(AFM) and transmission electron microscopy(TEM). The results indicate that ZnS nanoparticulate LB film is one-dimensional superlattice.

A double logarithmic method for studying the adsorption effects in an irreversible electrochemical process

The adsorption of an electroinactive product greatly influences an irreversible electrochemical reaction in three ways, including self-block, self-inhibition, and self-acceleration, and changes not only the heterogeneous electron-transfer rate constant but also the modified formal potential and electron-transfer coefficient of the electrochemical reaction. In order to study these adsorption effects, a double logarithmic method was suggested to be used in processing the potential-controlled thin layer spectroelectrochemical data. The result shows three types of double logarithmic plots for three kinds of adsorption effects. These double logarithmic plots can be a diagnostic criterion of the adsorption effects and enable us to determine some thermodynamic and kinetic parameters. The combination of nonlinear regression with double logarithmic method is a convenient way to examine the suggested mechanism and to extract more information from the limited experimental data. Some examples are given to test the theoretical results. (C) 1999 The Electrochemical Society. S0013-4651(98)05-012-5. All rights reserved.

Potential-dependent adsorption/desorption of organic adsorbate at HOPG electrode and accompanying delamination of graphite surface

In situ electrochemical scanning tunneling microscopy, alternating current voltammetry, and electrochemical quartz crystal microbalance have been employed to follow the potential-dependent adsorption/desorption processes of nucleic acid bases on highly oriented pyrolytic graphite (HOPG) electrode. The results show that (i) potential-dependent adsorption/desorption of nucleic acid bases on HOPG electrode was accompanied by delamination of the HOPG surface, and the delamination initiates from steps or kinks on the electrode surface, which provide highly active sites for adsorption; (ii) the delamination usually occurred when the electrode potential was changed or when the electrode was at potentials where the phase transition of adsorbate occurred. These results suggest that the surface stress resulting from the interaction between the substrate and adsorbate, as well as the interaction due to potential-induced surface charge distribution and the hysteresis of charge equilibrium are the main factors resulting in HOPG delamination. (C) 1999 The Electrochemical Society. S0013-4651(97)12-013-4. All rights reserved.