Stability of the gold/silica thin film interface: electrochemical and surface plasmon resonance studies

This article reports chemical stability studies of a gold film electrode coated with thin silicon oxide (SiOx) layers using electrochemical, surface plasmon resonance (SPR) and atomic force microscopy (AFM) techniques. Silica films with different thicknesses (d = 6.4, 9.7, 14.5, and 18.5 nm) were deposited using a plasma-enhanced chemical vapor deposition technique (PECVD). For SiOx films with d >/= 18.5 nm, the electrochemical behavior is characteristic of a highly efficient barrier for a redox probe. SiOx films with thicknesses between 9.5 and 14.5 nm were found to be less efficient barriers for electron transfer. The Au/SiOx interface with 6.4 nm of SiOx, however, showed an enhanced steady-state current compared to that of the other films. The stability of this interface in solutions of different pH was investigated. Whereas a strongly basic solution led to a continuous dissolution of the SiOx interface, acidic treatment produced a more reticulated SiOx film and improved electrochemical behavior. The electrochemical results were corroborated by SPR measurements in real time and AFM studies.     

Detection of adrenaline on poly(3-aminobenzylamine) ultrathin film by electrochemical-surface plasmon resonance spectroscopy

In this Article, we present a novel method to detect adrenaline on poly(3-aminobenzylamine) (PABA) ultrathin films by electrochemical-surface plasmon resonance (EC-SPR) spectroscopy. We prepared a PABA film, which specifically reacts with adrenaline, on a gold electrode by electropolymerization of 3-aminobenzylamine. The specific reaction of benzylamine within the PABA structure with adrenaline was studied by XPS, UV-vis spectroscopy, and EC-SPR techniques. Adrenaline was detected in real time by EC-SPR spectroscopy, which provides simultaneous monitoring of both optical SPR reflectivity and electrochemical current responses upon injecting adrenaline into the PABA thin film. The number of changes in both current and SPR reflectivity on the injection of adrenaline exhibited the linear relation to the concentration, and the detection limit was 100 pM. The responses were distinctive to those for uric acid and ascorbic acid, which are major interferences of adrenaline.     

Recent progress in the studies of electrochemical interfaces by surface plasmon resonance spectroscopy and microscopy

Surface plasmon resonance (SPR) is a label-free spectroscopic technique that is highly sensitive to various surface reactions. Incorporating SPR into electrochemical measurements has emerged as a powerful method to study both faradaic and non-faradaic processes. SPR microscopy (SPRM) integrates an optical microscope into SPR detection, which further offers high throughput detection and spatially resolved information at an electrode surface and thus, has attracted attention especially in single entity electrochemical studies. In this review, the progress in the studies of electrochemical interfaces by SPR and SPRM during the past two years will be discussed.     

Application of surface plasmon resonance spectroscopy for adsorption studies of different types of components on poly(dimethylsiloxane)

Surface plasmon resonance (SPR) spectroscopy is utilized to study in real-time and, by label-free means, the reversible and quasi-irreversible adsorption of small ionic or neutral molecules, pharmaceuticals, and proteins on poly(dimethylsiloxane) (PDMS) surfaces. The SPR sensor is covered with 0.2% (w/v) PDMS in octane. During the timescale of a typical lab-on-a-chip analysis or an electrophoretic separation, it was found that small neutral components containing a hydrophobic part do not adsorb or absorb onto PDMS, while larger, water-soluble polymer-like materials like proteins generally irreversibly adsorb to PDMS. The technique can be used to monitor the kinetics of adsorption and desorption of the molecules. For the non-specific adsorption of teicoplanin to PDMS, a Langmuir-like adsorption isotherm was obtained (K_d = 32 ± 2 μmol L⁻¹).     

In situ Wilhelmy balance surface energy determination of poly(3-hexylthiophene) and poly(3,4-ethylenedioxythiophene) during electrochemical doping-dedoping

Changes in the contact angle between conjugated polymers surface poly(3-hexylthiophene) [P3HT] and poly(3,4-ethylenedioxythiophene) (PEDOT) upon electrochemical doping-dedoping in aqueous electrolyte were determined in situ using a Wilhelmy plate tensiometer in an electrochemical cell. The hydrophobic P3HT was less hydrophobic in the oxidized state than in the neutral state; the more hydrophilic PEDOT was less hydrophilic in the oxidized state than when neutral. The tensiometry results were in good agreement with those measured by contact angle goniometry, and further corroborated by the capillary rise upon doping in a fluid cell with two parallel polymer coated plates, another in situ dynamic determination method. The contact angle changes depend on doping potential, electrolyte type, and concentration. We also deconvoluted the surface energy into components of van der Waals and acid-base interactions, using three probe liquids on the polymer surfaces, ex situ the electrochemical cell. The methods and the obtained results are relevant for the science and technology areas of printed electronics and electrochemical devices and for the understanding of surface energy modification by electrochemical doping.     

Conformational dynamics of poly(acrylic acid). A study using surface plasmon resonance spectroscopy

The conformational dynamics of poly(acrylic acid) induced by pH change is reported here. Poly(acrylic acid) immobilized on gold surface was exposed to pH changes, and the conformational changes thus induced were followed in real time using surface plasmon resonance spectroscopy. The temporal profile of the stretching-coiling phenomenon showed a minimum point, which was proposed to be arising due to the contradictory behavior of two different property changes in the polymeric system. Normally surface plasmon resonance (SPR) response would be a convoluted effect of the thickness and refractive index changes, but the behavior observed here, where the SPR response is predominantly governed by either one of the two, is unique and to the author's knowledge is a feature that is observed for the first time. Analysis of the kinetics of the angle change revealed that it takes longer for the polymer to stretch than it takes for it to collapse, with the kinetic rate constants varying by at least an order of magnitude. The SPR angle change as well as the kinetic constants increased linearly with molecular weight. Effect of Ca2+ was studied, and it was found that the polymer was locked in its conformation due to the binding of the multivalent cations.     

In situ sensing of metal ion adsorption to a thiolated surface using surface plasmon resonance spectroscopy

The kinetics of the adsorption of metal ions onto a thiolated surface and the selective and quantitative sensing of metal ions were explored using surface plasmon resonance (SPR) spectroscopy. The target metal ion was an aqueous solution of Pt2+ and a thin-gold-film-coated glass substrate was modified with 1,6-hexanedithiol (HDT) as a selective sensing layer. SPR spectroscopy was used to examine the kinetics of metal ion adsorption by means of the change in SPR angle. The selectivity of the thiolated surface for Pt2+ over other divalent metal ions such as Cu2+, Ni2+, and Cd2+ was evident by the time-resolved SPR measurement. SPR angle shift, deltatheta(SPR), was found to increase logarithmically with increasing concentration of Pt2+ in the range of 1.0 x 10(-5)-1.0 mM. The rate of Pt2+ adsorption on HDT observed at both 0.1 and 1 mM Pt2+ accelerates until the surface coverage reaches approximately 17%, after which the adsorption profile follows Langmuirian behavior with the surface coverage. The experimental data indicated that heavy metal ions were adsorbed to the hydrophobic thiolated surface by a cooperative mechanism. A mixed self-assembled monolayer (SAM) composed of HDT and 11-mercaptoundecanoic acid was used to reduce the hydrophobicity of the thiol-functionalized surface. The addition of hydrophilic groups to the surface enhanced the rate of adsorption of Pt2+ onto the surface. The findings show that the adsorption of metal ions is strongly dependent upon the hydrophilicity/hydrophobicity of the surface and that the technique represents an easy method for analyzing the adsorption of metal ions to a functionalized surface by combining SPR spectroscopy with a SAM modification.     

Stability of an ultrathin plasma polymerized film in aqueous solution: in situ detection by surface plasmon resonance

Recently, surface plasmon resonance (SPR) has been successfully implemented to characterize the film stability in real time: a radio frequency (RF) plasma polymerized film was monitored by swelling or extracting behaviors in aqueous solution. The high-resolution SPR revealed that film stability strongly depended on the monomers, plasma mode, and substrate locations as well as plasma polymerization conditions: incident power and working pressure. By Fourier transform infrared (FTIR), the possible reasons of film stabilization affected by plasma conditions and sample locations are explained. It is recommended that as an adhesive layer for future applications the plasma polymerized polymer shall be prepared in low working pressure pulsed plasma.     

A comparison between dual polarization interferometry (DPI) and surface plasmon resonance (SPR) for protein adsorption studies

This work was performed with the aim of comparing protein adsorption results obtained from the recently developed dual polarization interferometry (DPI) with the well-established surface plasmon resonance (SPR) technique. Both techniques use an evanescent field as the sensing element but completely different methods to calculate the adsorbed mass. As a test system we used adsorption of the lipase from Thermomyces lanuginosus (TLL) on C18 surfaces. The adsorbed amount calculated with both techniques is in good agreement, with both adsorption isotherms saturating at 1.30–1.35 mg/m² at TLL concentrations of 1000 nM and above. Therefore, this supports the use of both SPR and DPI as tools for studying protein adsorption, which is very important when comparing adsorption data obtained from the use different techniques. Due to the spot sensing in SPR, this technique is recommended for initial kinetic studies, whereas DPI is more accurate when the refractive index and thickness of the adsorbed layer is of more interest.     

Characterization of micropatterned lipid membranes on a gold surface by surface plasmon resonance imaging and electrochemical signaling of a pore-forming protein

We report the fabrication and characterization of a micropatterned membrane electrode for electrochemical signaling of a bacterial pore-forming toxin, Streptolysin O (SLO) from S. pyogenes. Microcontact printing of an alkylthiol monolayer was used to fabricate an array template, onto which cholesterol-containing DMPC vesicles were fused to form lipid layer structures. The construction of the supported membranes, including pattern transfer and vesicle fusion, was characterized by in-situ surface plasmon resonance (SPR) imaging and electrochemistry. Quantitative analysis of the resulting membrane by using SPR angular shift measurements indicates that the membranes in the hydrophilic pockets have an average thickness of 8.2 +/- 0.4 nm. Together with fluorescence microscopy studies, the results suggest that this could be a mixed lipid assembly that may consist of a bilayer, vesicle fragments, and lipid junctions. The voltammetric response of the redox probe ferrocene carboxylic acid (FCA) was measured to quantify the toxin action on the supported membrane. The electrochemical measurements indicate that fusion of vesicles on the template blocked the access of FCA, whereas the injection of SLO toxin restored the redox response. The anodic peak current of FCA was found to increase with toxin concentration until a plateau was reached at 40 HU/mL. The method is highly sensitive such that 0.1 HU/mL of SLO (1.25 pM) can yield a well-defined response. In addition, it eliminates the need for a highly insulating layer in membrane sensing, which opens up new avenues in developing novel sensing interfaces for membrane-targeting proteins and peptides.     

Waveguide surface plasmon resonance studies of surface reactions on gold electrodes

We describe the fabrication and characterisation of gold-coated graded index channel waveguide sensors designed for simultaneous electrochemical and surface plasmon resonance studies. The active sensing electrode area is a thin gold film between 0.5 and 5 mm in length and 200 microm wide deposited on top of a 3 microm wide waveguide which forms one arm of a Y-junction while the other arm of the Y-junction serves as a reference. Using these devices we have measured simultaneously the changes in transmittance through the device whilst carrying out cyclic voltammetry in either sulfuric or perchloric acid solution or during the deposition of an UPD layer of copper at the gold surface. In all cases we obtain stable and reproducible results which demonstrate the very high sensitivity of the devices to sub-monolayer changes occurring at the gold electrode surface. The response of these integrated optoelectrochemical devices is discussed in terms of a numerical model for the propagation of light within the waveguide structure.     

In situ self-assembly synthesis of gold nanoparticle arrays on polystyrene microspheres and their surface plasmon resonance

Gold (Au) nanoparticle arrays with tunable morphology and optical characteristics were synthesized by in-situ self-assembly process that occurred on the surface of aniline-modified polystyrene (PS) microspheres. The method can be used to control the growth of both single and aggregated Au nanoparticle arrays on PS microsphere surface. This method could also be adapted for synthesis of other noble metals hybrid materials, which opens exciting opportunities for their practical applications.     

Use of surface plasmon resonance to study the adsorption of detergents on poly(dimethylsiloxane) surfaces

This paper demonstrates the use of surface plasmon resonance to study adsorption (either reversible or irreversible) of detergents on PDMS surfaces in real time. The surface plasmon resonance measurements can directly provide information about the adsorption/desorption processes of detergents on the surface revealing the durability of the adsorbed layer and the anticipated degree of the EOF. Hydroxypropyl methylcellulose very strongly adsorbs onto PDMS and can be considered both a semipermanent layer and stable semipermanent coating. Adsorbed SDS or CTAB layers were stable for several minutes upon rinsing the surface with solution not containing the detergent. It was shown that SDS coated onto PDMS in microchips has the potential to afford similar separations in PDMS as found in conventional fused silica capillaries.     

In situ surface plasmon resonance measurements of self-assembled monolayers of ferrocenylalkylthiols under constant potentials.

A commercial system for surface plasmon resonance (SPR) possessing a batch-type flow channel has been simply modified so as to conduct in situ SPR measurements under polarization of an Au sensor chip at constant potentials. The modified instrument can monitor electrochemical reactions of monolayer materials with high stability and high reproducibility. The redox reaction of a self-assembled monolayer (SAM) of 6-ferrocenyl-1-hexanethiol (FcHT) induced the resonance angle shifts, the magnitudes of which accorded with the Nernst equation. The measurements in electrolyte solutions containing different electrolyte anions revealed that the SPR measurements detected ion pairing of electrolyte anions with oxidized FcHT. In cases of measurements in alkylsulfonic acid solution, simulation of the results based on the N-layer model has clarified that alkylsulfonate anions make an assembled layer on the FcHT SAM.     

Detection of carbohydrate-binding proteins by oligosaccharide-modified polypyrrole interfaces using electrochemical surface plasmon resonance

This paper reports on the use of electrochemical surface plasmon resonance (E-SPR) for the detection of carbohydrate-binding proteins. The generation of an SPR sensor specific to lectins Arachis hypogaea (PNA) and Maackia amurensis (MAA) is based on the electrochemical polymerization of oligosaccharide derivatives functionalized by pyrrole groups. The resulting thin conducting polymer films were characterized using E-SPR and atomic force microscopy (AFM). The specific binding of PNA to polypyrrole-lactosyl and of MAA to polypyrrole-3'-sialyllactosyl films was investigated using SPR. The detection limit was 41 nM for PNA and 83 nM for MAA. Through Scatchard analysis and linear transformation of the SPR sensorgram data, association (k(ass)) and dissociation rate constants (k(diss)) could be determined.     

In situ electrochemical fluorescence studies of PPV

Conjugated phenylene-vinylene polymers are widely used in organic light-emitting and photovoltaic devices. The comprehension of the optical properties upon charge injection is of crucial importance for the improvement of such organoelectronic devices. The processes of electrochemical doping, electrolyte diffusion, and degradation have been studied by cyclic voltammetry and chronoamperometric methods. Kinetic studies by in situ fluorescence spectroscopy have been used for the determination of the mobility of charge carriers in the polymer making used of electrochemical Stern-Volmer analysis. The mobility of holes for MDMO-PPV measured by this method was 2.5 x 10(-7) cm2 V s(-1). Non-Faradic variations of the fluorescence after doping-dedoping cycles have been related to morphological changes in the polymeric layer. The evolution of the fluorescence obeys a first-order kinetics law, similarly to the trend of the variation of volume during gel shrinking.     

Modeling of vapor sorption in polymeric film studied by surface plasmon resonance spectroscopy

Sorption process by surface plasmon resonance (SPR) was studied by exposing polymeric film made from anthracene labeled poly(methyl methacrylate) (An-PMMA) chains to various concentrations of saturated chloroform vapor. It was observed that the reflectivity changes were fast and reversible. The changes in reflectivity implied the swelling behavior of polymeric film during adsorbtion and can be explained by capturing of chloroform molecules. When clean air is introduced into gas cell similar behavior is observed but this time in the opposite direction as a result of desorption. Fick's law for diffusion was used to quantify real time SPR data for the swelling and desorption processes. It was observed that diffusion coefficients (D(s)) for swelling obeyed the t(1/2) law and found to be correlated with the amount of chloroform content in the cell. Diffusion coefficients (D(d)) during desorption were also measured and found to be increased as the saturated chloroform vapor content is increased in the cell.     

In situ electrochemical SERS studies on electrodeposition of aniline on 4-ATP/Au surface

The electrochemical polymerization of 0.01 M aniline in 1 M H₂SO₄ aqueous solution on roughened Au surface modified with a self-assembled monolayer (SAM) of 4-aminothiophenol (4-ATP) has been investigated by in situ electrochemical surface-enhanced Raman scattering spectroscopy (SERS). The repeat units and possible structures of the electrodeposited polyaniline (PANI) film were proposed; i.e., aniline monomer is coupled in head-to-tail predominately at the C₄ of aniline and amine of 4-ATP, and the thin PANI film is orientated vertically to substrate surface. Simultaneous Raman spectra during potential scanning indicate clearly that the ultrathin PANI film (in initial growth of the film) consists of semiquinone radical cation (IP⁺), para-disubstituted benzene (IP and IP⁺) and quinine diimine (NP) while it is oxidized, and without quinine diimine and semiquinone radical cation while reduced. Meanwhile, the results confirm that 4-ATP monolayer shows a strong promotion on the electrodeposition of aniline monomer, and a possible polymerization mechanism was proposed.     

Interactions of doxorubicin with self-assembled monolayer-modified electrodes: electrochemical, surface plasmon resonance (SPR), and gravimetric studies

We present the results on the partitioning of doxorubicin (DOX), a potent anticancer drug, through the model membrane system, self-assembled monolayers (SAMs) on gold electrodes. The monolayers were formed from alkanethiols of comparable length with different ω-terminal groups facing the aqueous electrolyte: the hydrophobic -CH(3) groups for the case of dodecanethiol SAMs or hydrophilic -OH groups of mercaptoundecanol SAMs. The electrochemical experiments combined with the surface plasmon resonance (SPR) and gravimetric studies show that doxorubicin is likely adsorbed onto the surface of hydrophilic monolayer, while for the case of the hydrophobic one the drug mostly penetrates the monolayer moiety. The adsorption of the drug hinders further penetration of doxorubicin into the monolayer moiety.