Formation of a supported hybrid bilayer membrane on gold: A sterically enhanced hydrophobic effect

Adsorption of a monolayer of didecanoyl-L-alpha-phosphatidylcholine (DDPC) from dispersions of small unilamellar vesicles onto hydrophobic surfaces was investigated by mean of cyclic voltammetry and impedance spectroscopy. The hydrophobic surfaces were self-assembled monolayers of 2-mereapto-3-n-octylthiophene (MOT) on gold. One characteristic of the MOT monolayer is its permeability to organic molecules in aqueous solution, thus providing a more energetically favorable hydrophobic surface for the addition of phospholipid vesicles. The kinetics of the lipid monolayer formation were followed by measuring the time-dependent interfacial capacitance. Unusual values of thickness and capacitance of the MOT/ DDPC bilayers were observed. An interdigitating conformation of the bilayer structure was proposed to interpret the experimental results, The horseradish peroxidase reconstituted into the bilayer demonstrated the expected protein activity, showing practical use in research and in biosensor application.

Assembly of alternating polycation and DNA multilayer films by electrostatic layer-by-layer adsorption

The assembly of alternating DNA and positively charged poly(dimethyldiallylammonium chloride) (PDDA) multilayer films by electrostatic layer-by-layer adsorption has been studied. The real-time surface plasmon resonance (BIAcore) technique was used to characterize and monitor the formation of multilayer films in solution in real time continuously. Electrochemical impedance spectroscopy (EIS) and UV-vis absorbance measurements were also used to study the film assembly, and linear film growth was observed. All 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 the real-time BIAcore technique; the observed rate constant was calculated using a Langmuir model (k(obs) = (1.28 +/- 0.08) x 10(-2) s(-1).

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.

Secondary structure effects on DNA hybridization kinetics: a solution versus surface comparison

The hybridization kinetics for a series of designed 25mer probe�target pairs having varying degrees of secondary structure have been measured by UV absorbance and surface plasmon resonance (SPR) spectroscopy in solution and on the surface, respectively. Kinetic rate constants derived from the resultant data decrease with increasing probe and target secondary structure similarly in both solution and surface environments. Specifically, addition of three intramolecular base pairs in the probe and target structure slow hybridization by a factor of two. For individual strands containing four or more intramolecular base pairs, hybridization cannot be described by a traditional two-state model in solution-phase nor on the surface. Surface hybridization rates are also 20- to 40-fold slower than solution-phase rates for identical sequences and conditions. These quantitative findings may have implications for the design of better biosensors, particularly those using probes with deliberate secondary structure.

Measuring morphological features using light-scattering spectroscopy and Fourier-domain low-coherence interferometry.

We present measurements of morphological features in a thick turbid sample using light-scattering spectroscopy (LSS) and Fourier-domain low-coherence interferometry (fLCI) by processing with the dual-window (DW) method. A parallel frequency domain optical coherence tomography (OCT) system with a white-light source is used to image a two-layer phantom containing polystyrene beads of diameters 4.00 and 6.98 mum on the top and bottom layers, respectively. The DW method decomposes each OCT A-scan into a time-frequency distribution with simultaneously high spectral and spatial resolution. The spectral information from localized regions in the sample is used to determine scatterer structure. The results show that the two scatterer populations can be differentiated using LSS and fLCI.

Backside-surface imprinting as a new strategy to generate specific plastic antibody materials

A backside protein-surface imprinting process is presented herein as a novel way to generate specific synthetic antibody materials. The template is covalently bonded to a carboxylated-PVC supporting film previously cast on gold, let to interact with charged monomers and surrounded next by another thick polymer. This polymer is then covalently attached to a transducing element and the backside of this structure (supporting film plus template) is removed as a regular “tape”. The new sensing layer is exposed after the full template removal, showing a high density of re-binding positions, as evidenced by SEM. To ensure that the templates have been efficiently removed, this re-binding layer was cleaned further with a proteolytic enzyme and solution washout. The final material was named MAPS, as in the back-side reading of SPAM, because it acts as a back-side imprinting of this recent approach. It was able to generate, for the first time, a specific response to a complex biomolecule from a synthetic material. Non-imprinted materials (NIMs) were also produced as blank and were used as a control of the imprinting process. All chemical modifications were followed by electrochemical techniques. This was done on a supporting film and transducing element of both MAPS and NIM. Only the MAPS-based device responded to oxLDL and the sensing layer was insensitive to other serum proteins, such as myoglobin and haemoglobin. Linear behaviour between log(C, μg mL−1) versus charged tranfer resistance (RCT, Ω) was observed by electrochemical impedance spectroscopy (EIS). Calibrations made in Fetal Calf Serum (FCS) were linear from 2.5 to 12.5 μg mL−1 (RCT = 946.12 × log C + 1590.7) with an R-squared of 0.9966. Overall, these were promising results towards the design of materials acting close to the natural antibodies and applied to practical use of clinical interest.

Surface Imprinting Approach on Screen Printed Electrodes Coated with Carboxylated PVC for Myoglobin detection with Electrochemical Transduction

A novel surface molecularly-imprinted (MI) material to detect myoglobin (Myo) using gold screen printed electrodes (SPE) was developed. The sensitive detection was carry out by introducing a carboxylic polyvinyl chloride (PVC-COOH) layer on gold SPE surface. Myo was attached to the surface of gold SPE/PVC-COOH and the vacant spaces around it were filled by polymerizing acrylamide and N,N-methylenebisacrylamide (cross-linker). This polymerization was initiated by ammonium persulphate. After removing the template, the obtained material was able to rebind Myo and discriminate it among other interfering species. Various characterization techniques including electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) confirmed the surface modification. This sensor seemed a promising tool for screening Myo in point-of-care.

Propriétés optiques et analytiques des nanotrous : vers la conception de biocapteurs en résonance des plasmons de surface localisés

Les biocapteurs sont utilisés quotidiennement pour déterminer la présence de molécules biologiques dans une matrice complexe, comme l’urine pour les tests de grossesses ou le sang pour les glucomètres. Les techniques courantes pour la détection des autres maladies nécessitent fréquemment le marquage de l’analyte avec une autre molécule, ce qui est à éviter pour fin de simplicité d’analyse. Ces travaux ont pour but la maximisation de la sensibilité d’une surface d’or ou d’argent nanotrouée, afin de permettre la détection de la liaison de molécules biologiques par résonance des plasmons de surface localisés (LSPR), en utilisant la spectroscopie de transmission. Un biocapteur portable, rapide et sans marquage pour quantifier des analytes d’intérêt médical ou environnemental pourrait être construit à partir de ces travaux. Dans l’objectif d’étudier de nombreuses configurations pour maximiser la sensibilité, le temps et le coût des méthodes de fabrication de nanostructures habituelles auraient limité le nombre de surfaces nanotrouées pouvant être étudiées. Un autre objectif du projet consiste donc au développement d’une technique de fabrication rapide de réseaux de nanotrous, et à moindres coûts, basée sur la lithographie de nanosphères (NSL) et sur la gravure au plasma à l’oxygène (RIE). La sensibilité à la variation d’indice de réfraction associée aux liaisons de molécules sur la surface du métal noble et la longueur d’onde d’excitation du plasmon de surface sont influencées par les caractéristiques des réseaux de nanotrous. Dans les travaux rapportés ici, la nature du métal utilisé, le diamètre ainsi que la périodicité des trous sont variés pour étudier leur influence sur les bandes LSPR du spectre en transmission pour maximiser cette sensibilité, visant la fabrication d’un biocapteur. Les surfaces d’argent, ayant un diamètre de nanotrous inférieur à 200 nm pour une périodicité de 450 nm et les nanotrous d’une périodicité de 650 nm démontre un potentiel de sensibilité supérieur.

Fabrication des films microstructurés et leurs caractéristiques en spectroscopie de résonance des plasmons de surface

Cette thèse caractérise les propriétés optiques des matériaux plasmoniques microstructurés et procède à l’évaluation des paramètres analytiques afin de les employer comme plateforme de biodétection en spectroscopie de résonance des plasmons de surface (SPR). Aux dimensions micrométriques, les matériaux plasmoniques présentent des caractéristiques optiques propres aux nano- et macromatériaux. La cartographie physicooptiques en SPR de matériaux méso- et microscopiques s’est effectuée à l’aide de films structurés de motifs périodiques triangulaires et circulaires fabriqués par une technique modifiée de lithographie par nanosphères (nanosphere lithography, NSL). À partir de cette vue d’ensemble, quelques films structurés ont été sélectionné en fonction d’aspects analytiques tels que la sensibilité et la résolution face aux variations d’indice de réfraction (RI) pour déterminer le potentiel de ces matériaux comme plateforme de biodetection. Les propriétés optiques distinctes des films microstructurés proviennent d’interactions résonantes entre les modes de plasmons de surface (SP) localisé et délocalisé identifiés par la relation de dispersion en SPR ainsi que l’imagerie Raman. Les conditions de résonance des modes SP dépendant de paramètres expérimentaux (λ, θ, η) tel qu’observés numériquement par rigorous coupled wave analysis (RCWA) et empiriquement. Ces travaux démontrent la nature plasmonique distincte des micro-matériaux et leur potentiel d’intégration aux techniques analytiques SPR existantes. Les matériaux plasmoniques micrométriques furent également étudiés pour l’implémentation de la SPR à une pointe de microscopie à force atomique (atomic force microscopy, AFM) combinant ainsi la spectroscopie à l’imagerie topographique. Des travaux préliminaires se sont concentrés sur la signature spectroscopique de leviers en silicium (Si) et en nitrure de silicium (Si3N4), l’impact d’un revêtement d’or sur les pointes et l’influence de milieu environnant. Une image d’origine plasmonique a été obtenue avec des leviers en Si3N4 revêtus d’or en transmission dans un environnement aqueux, indiquant ainsi le potentiel de ces pointes comme micro-biocapteur SPR. Ces résultats préliminaires servent de fondement pour orienter les prochaines investigations dans ce projet.

Étude de l’oxydoréduction de monocouches électroactives de ferrocénylalcanethiolates par spectroscopie à résonance des plasmons de surface

L’oxydoréduction de monocouches auto-assemblées (SAMs) de ferrocénylalcanethiolates à la surface d’or (FcRSAu) a été étudiée en temps réel par la spectroscopie de résonance de plasmons de surface couplée avec l’électrochimie (E-SPR). La sensibilité de cette technique permet de déterminer des changements d’épaisseur de couche l’ordre de quelques angström résultant d’un changement de structure de la SAM. Plusieurs études antérieures ont proposé que l’oxydation électrochimique d’une SAM de FcRSAu induit une réorientation moléculaire. L’E-SPR est utilisé pour identifier l’origine de ce changement structurel. D’abord, une calibration du réfractomètre SPR utilisé a été effectuée afin de trouver une équation de conversion du signal SPR obtenu en pixel en angle d’incidence pour que l’on puisse calculer le changement d’épaisseur de monocouche à partir du changement d’angle de résonance avec le modèle de Fresnel. Par la suite, une caractérisation approfondie des SAMs de FcCnSAu (où n = 6, 8, 12, 14) en contact avec du NaClO4 acidifié a été réalisée par électrochimie, éllipsométrie, spectroscopie infrarouge et microscopie à force atomique. Les résultats obtenus montrent que l’augmentation de la longueur des chaînes alkyles donne des SAMs de ferrocènes plus épaisses et moins désordonnées. L’analyse par l’E-SPR de ces SAMs pures montre que le changement d’épaisseur induit par l’électro-oxydation dépend linéairement du nombre de méthylènes sur la chaîne alkyle. En appliquant la déconvolution mathématique aux voltampérogrammes cycliques enregistrés pour les SAM mixtes (FcC12SAu/C11SAu) de différentes compositions, on arrive à la conclusion qu’il y a un redressement des chaînes alkyles dans les domaines des ferrocènes agrégés mais la réorientation des têtes de ferrocène dans les domaines de ferrocènes agrégés ou dispersés ne peut pas être exclue. Enfin, l’effet de l’anion électrolytique sur le changement d’épaisseur de la SAM mesuré par l’E-SPR a été étudié. L’analyse électrochimique montre que la capacité de pairage d’anions avec les ferrocéniums décroit comme suit : PF6- > ClO4- > BF4- > NO3-. Tandis que l’épaisseur de la SAM donnée par le changement d’angle de résonance suit la tendance suivante : NO3- ≥ ClO4- > PF6- ≈ BF4-. Des études plus approfondies seront nécessaire pour clarifier cette tendance observée par E-SPR.

Development of impedimetric and optical calcium biosensor by using modified gold electrode with porcine S100A12 protein

We describe the development of a label free method to analyze the interactions between Ca(2+) and the porcine S100A12 protein immobilized on polyvinyl butyral (PVB). The modified gold electrodes were characterized using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM) and surface plasmon resonance (SPR) techniques. SEM analyses of PVB and PVB-S100A12 showed a heterogeneous distribution of PVB spherules on gold surface. EIS and CV measurements have shown that redox probe reactions on the modified gold electrodes were partially blocked due the adsorption of PVB-S100A12, and confirm the existence of a positive response of the immobilized S100Al2 to the presence of calcium ions. The biosensor exhibited a wide linear response to Ca(2+) concentrations ranging from 12.5 to 200 mM. The PVB-S100A12 seems to be bound to the gold electrode surface by physical adsorption: we observed an increase of 1184.32 m degrees in the SPR angle after the adsorption of the protein on the PVB surface (in an indication that 9.84 ng of S100A12 are adsorbed per mm(2) of the Au-PVB electrode), followed by a further increase of 581.66 m degrees after attachment of the Ca(2+) ions. In addition, no SPR response is obtained for non-specific ions. These studies might be useful as a platform for the design of new reusable and sensitive biosensing devices that could find use in the clinical applications. (C) 2010 Elsevier B.V. All rights reserved.

Changes on iron electrode surface during hydrogen permeation in borate buffer solution

Hydrogen interaction with oxide films grown on iron electrodes at open circuit potential (E-oc) and in the passive region (+0.30 V-ECS) was studied by chronopotentiometry, chronoamperometry and electrochemical impedance spectroscopy techniques. The results were obtained in deaerated 0.3 mol L-1 H3BO3 + 0.075 mol L-1 Na2B4O7 (BB, pH 8.4) solution before, during and after hydrogen permeation. The iron oxide film modification was also investigated by means of in situ X-ray absorption near-edge spectroscopy (XANES) and scanning electrochemical microscopy (SECM) before and during hydrogen permeation. The main conclusion was that the passive film is reduced during the hydrogen diffusion. The hydrogen permeation stabilizes the iron surface at a potential close to the thermodynamic water stability line where hydrogen evolution can occur. The stationary condition required for the determination of the permeation parameters cannot be easily attained on iron surface during hydrogen permeation. Moreover, additional attention must be paid when obtaining the transport parameters using the classical permeation cell. (c) 2007 Elsevier Ltd. All rights reserved.

The effect of potential bias on the formation of ionic liquid generated surface films on Mg alloys

An electrochemical approach to the formation of a protective surface film on Mg alloys immersed in the ionic liquid (IL), trihexyl(tetradecyl)phosphonium–bis 2,4,4-trimethylpentylphosphinate, was investigated in this work. Initially, cyclic voltammetry was used with the Mg alloy being cycled from OCP to more anodic potentials. EIS data indicate that, under these circumstances, an optimum level of protection was achieved at intermediate potentials (e.g., 0 or 0.25 V versus Ag/AgCl). In the second part of this paper, a small constant bias was applied to the Mg alloy immersed in the IL for extended periods using a novel cell design. This electrochemical cell allowed us to monitor in situ surface film formation on the metal surface as well as the subsequent corrosion behaviour of the metal in a corrosive medium. This apparatus was used to investigate the evolution of the surface film on an AZ31 magnesium alloy under a potential bias (between ±100 mV versus open circuit) applied for over 24 h, and the film evolution was monitored using electrochemical impedance spectroscopy (EIS). A film resistance was determined from the EIS data and it was shown that this increased substantially during the first few hours (independent of the bias potential used) with a subsequent decrease upon longer exposure of the surface to the IL. Preliminary characterization of the film formed on the Mg alloy surface using ToF-SIMS indicates that a multilayer surface exists with a phosphorous rich outer layer and a native oxide/hydroxide film underlying this. The corrosion performance of a treated AZ31 specimen when exposed to 0.1 M NaCl aqueous solution showed considerable improvement, consistent with electrochemical data.

An electrochemical impedance study of ionic liquid film formation and aqueous corrosion of magnesium alloy ZE41

Ionic liquid surface treatments are proposed as a method of controlling corrosion processes on magnesium alloys. An important magnesium alloy, ZE41 (nominally 4% Zn and 1% rare earth), was treated with the ionic liquid trihexyl(tetradecyl)phosphonium diphenylphosphate (P₆₆₆₁₄DPP). Impedance spectra were acquired at intervals during the treatment, indicating the development of a film and allowing a measure of the film formation process to be obtained over time. Mechanically polished and electro-polished surfaces were prepared; these surfaces, treated and untreated, were subsequently exposed to 0.1 M NaCl aqueous solutions. The corrosion behavior of the prepared surfaces were assessed using impedance spectroscopy and optical microscopy. The results indicated a significant role for the method of surface preparation used and, in both cases, the ionic liquid treatment produced a more corrosion-resistant surface.

Real-time, in-situ, extinction spectroscopy studies on silver-nanoseed formation

In this paper, real-time extinction spectroscopy was employed to analyze in situ the fast formation process of silver seeds. The influencing factors for silver-seed formation, including the concentration of the reducer (NaBH 4), the amount of stabilizer (citrate), and the addition procedure of NaBH4, were evaluated. The configuration of the surface plasmon resonance (SPR) spectra of silver nanoparticles, which reflected the morphology of the produced silver seeds, was found to be distinctly affected by the NaBH4 concentration. Homogeneous silver nanoparticles were obtained when the added NaBH4 was lower in concentration than 2.00 mM. In contrast, higher concentrations of reducing agent (>3.00 mM NaBH4) resulted in uneven silver nanoparticles. Mie theory was applied to clarify the correlation of the SPR and the size of the silver seeds. Repeated additions of a small amount of the reducing agent could increase the monodispersity and isotropy of silver seeds. Real-time extinction spectroscopy is a convenient technique that achieves the in-situ and nondestructive measurement of intermediates in the formation process of silver nanoseeds.