FTIR External Reflectance Spectroscopy Research Articles

In situ step-scan time-resolved microscope FTIR spectroscopy applied in irreversible electrochemical reactions

We demonstrated in the present paper, for the first time, the application of step-scan time-resolved FTIR external reflection spectroscopy in studies of irreversible electrochemical reactions. Through combination of a particularly designed flow thin layer IR cell and a nanostructured microelectrode, the solution in the thin layer between IR window and microelectrode as well as adsorbates on electrode surface can be rapidly renewed, which allows irreversible reactions to be repeated quickly. Electrooxidation of CO adsorbed on a nanostructured platinum microelectrode (nm-PtME) in H 2SO 4 solutions was employed as a test reaction to illustrate the performance of the newly developed technology. It has confirmed that to regenerate a CO adlayer on the nm-PtME after stripping completely CO adsorbates may take about 6 s in CO-saturated solution (CO concentration ∼1.0 mM) at a flow rate of 20 ml min −1, which satisfies the requirement of the repeatability in step-scan FTIR data collection. Time-resolved FTIR spectra for CO irreversible oxidation have been recorded at a time resolution of 250 μs. It has been revealed that the IR band center of CO slightly decreases with time within initial 4 ms and then maintains a constant value in the evolvement of the oxidation. Such variation demonstrates that CO oxidation on nm-PtME is mainly occurred in through a nucleation and growth mechanism. The present study has thrown light on investigation of fast dynamic processes and kinetics of irreversible electrochemical reactions by using in situ step-scan time-resolved FTIR spectroscopy.

Electrochemically Derived Gradients of the Extracellular Matrix Protein Fibronectin on Gold

Surface composition gradients of extracellular matrix proteins show great promise as tools in understanding cellular adhesion and motility. The formation of a surface coverage gradient of the protein fibronectin (FN) is described. FN is covalently bound to a counterpropagating two-component gradient formed from thiols with carboxylic acid and hydroxyl terminal functional groups using carbodiimide coupling chemistry. Areas of the surface with initial −COOH termination react with amine groups on the FN, while protein adsorption is resisted on the −OH terminated regions. The −COOH/−OH gradient is generated by applying a linear potential profile to a thin Au gold film on which an 11-mercaptoundecanol (MUD) monolayer is present. A gradient in surface coverage of MUD results, which is then backfilled with 11-mercaptoundecanoic acid (MUA). The −COOH/−OH gradient is characterized by tagging with 200 nm diameter, fluorescently doped, amine-modified nanospheres using the same carbodiimide coupling chemistry and imaging with fluorescence microscopy. The surface coverage of FN on MUA determined by surface plasmon resonance reflectometry is 1.08 ng/mm2, with 0.08 ng/mm2 of physisorbed protein present on MUD. FN gradients are mapped with FTIR external reflection spectroscopy and imaged with fluorescence microscopy by tagging with anti-FN and FITC−IgG. The gradients exhibit a sigmoid shape, with the width of the gradient region occupying ∼10% of the total film length. When bovine serum albumin is used to block nonspecific cellular adhesion, the adhesion of the 3T3 fibroblasts to these gradients follows the spatial composition profile of FN.

Raman and FTIR spectroscopic characterization of electrochemically synthesized poly(triphenylamine), PTPA

Triphenylamine was electrochemically polymerized in a mixture of toluene and acetonitrile with different electrolyte salts. The poly(triphenylamine) (PTPA) films are insoluble in polar solvents and show high stability and no degradation or loss in electrochemical properties when stored in a laboratory atmosphere. The PTPA films were characterized in situ by FTIR external reflection spectroscopy and by Raman spectroscopy.

Controlled and Reversible Formation of Nanoparticle Aggregates and Films Using Cu2+−Carboxylate Chemistry

The controlled and reversible formation of transiently soluble alkanethiolate- and tiopronin-monolayer-protected cluster (MPC) aggregates is demonstrated by using Cu2+−carboxylate chemistry to form cluster−Cu2+−cluster linkages. The aggregation process and its reversibility were monitored using UV−vis and FTIR spectroscopy. The amount of Cu2+ required to induce aggregation of dissolved tiopronin-MPCs is a strong function of pH, increasing at lower pH. Cu2+−carboxylate chemistry can also be utilized to prepare surface-attached films of tiopronin-MPCs in a manner analogous to alkanethiolate-MPCs, monitoring the film formation using FTIR external reflectance spectroscopy. Experiments exploring the diffusional and capacitive properties of tiopronin-MPCs are also reported.

Exchange of self-assembled thiol monolayers on gold: characterization by FT-IR external reflection spectroscopy

Abstract The exchange behavior of various thiol monolayers self-assembled on a gold surface is described. FT-IR external reflection spectroscopy was used for monitoring of the self-exchange of unsubstituted alkanethiol with the aid of deuterated thiol. Intensity changes of the absorption peaks for a specific functionality were used for the quantization of thiols in the monolayers. Exchange of compact monolayers such as eicosanethiol with the same type of alkanethiol in ethanol solution was found to be very slow. We used n-eicosanethiol [CH3(CH2)19SH] and partially deuterated eicosanethiol [CD3(CH2)19SH] in order to avoid the chain length difference effect of adsorption and maintain exactly the same chain length of molecules. The exchange of the non-compact ferrocene terminated alkanethiol monolayer with dodecanethiol was faster than the self-exchange of the compact unsubstituted alkanethiol monolayer. The result shows that the compactness of the monolayer is critical to the exchange kinetics of self-assembled monolayers.

Corrosion Passivation of Gold by n-Alkanethiol Self-Assembled Monolayers: Effect of Chain Length and End Group

Electrochemistry, FTIR-external reflection spectroscopy (FTIR-ERS), and scanning tunneling microscopy (STM) were used to study the ability of n-alkanethiol self-assembled monolayers (SAMs) to protect Au from corrosion in aqueous Br- solutions. The thickness and terminal functional group of the SAMs were varied to determine which factors lead to the greatest corrosion passivation. SAMs prepared on Au from the following molecules were studied: HS(CH2)15CH3, HS(CH2)11CH3, HS(CH2)10COOH, and HS(CH2)11OH. The data reveal that, prior to corrosion, the SAMs are crystalline and highly ordered but that afterward they are disordered and oriented parallel to the surface plane. The results show that, for SAMs containing the same terminal functional group, corrosion resistance increases as the SAM thickness increases. For SAMs that are equal in thickness but contain different terminal functional groups, the end groups resulting in the most corrosion resistance follow the order OH > COOH > CH3. The hydrophilic SAM-mod...

Spectroscopic, Voltammetric, and Electrochemical Scanning Tunneling Microscopic Study of Underpotentially Deposited Cu Corrosion and Passivation with Self-Assembled Organomercaptan Monolayers

FTIR−external reflectance spectroscopy (FTIR-ERS), X-ray photoelectron spectroscopy (XPS), electrochemistry, and electrochemical scanning tunneling microscopy (ECSTM) were used to study the effect of aromatic and linear-chain thiol adsorbates on the oxidation of underpotentially deposited Cu on Au (Au/Cu-UPD) in HClO4-containing electrolyte solutions. The morphology of the corroding Cu layer and its stripping potential are influenced by the presence of the organomercaptan self-assembled monolayers (SAMs). For n-alkanethiol SAMs, the Cu-UPD stripping potential shifts positive as the SAM thickness increases. Aromatic SAMs were found to passivate the Cu−UPD surface more effectively than linear-chain SAMs of equal thickness. Furthermore, a methyl-terminated aromatic SAM shifts the potential for Cu−UPD oxidation more positive than a hydroxy-terminated aromatic SAM. For longer chain n-alkanethiol SAMs, the presence of the Cu−UPD layer markedly improves the stability of the SAM compared to when it is adsorbed di...

Interactions between Organized, Surface-Confined Monolayers and Vapor-Phase Probe Molecules. 12. Two New Methods for Surface-Immobilization and Functionalization of Chemically Sensitive Dendrimer Surfaces

Abstract : In this report we demonstrate two new methods for covalently linking dendrimers to surfaces. In the first method (Method 1) a poly(iminopropane-1,3,-diyl) dendrimer with 64 terminal-amine groups is first attached to a mixed mercaptoundecanoic-acid (MUA)/mercaptopentane (MP) SAM, and then the unreacted terminal-amine groups of the dendrimer are converted to amide-linked functional groups by condensation with acid chlorides. The second method (Method 2) involves bulk-phase coupling of suitable functional groups with the primary-amine-terminated dendrimer followed by reaction of the few unfunctionalized primary amines with the MUA component of the SAM to yield amide linkages. Five different dendrimer terminal groups are considered: primary amine, benzamide, 4- (trifluoromethyl)benzamide butanamide, and triphenylacetamide. FTIR external reflection spectroscopy, ellipsometry, variable take-off angle XPS, and surface acoustic wave (SAW) device-based gravimetry reveal that these two approaches result in very different types of dendrimer monolayers. When the dendrimers are prepared by Method 2, their surface concentration is lower than when the functionalization is done after attachment. However, the density of surface functionalities on each dendrimer is higher when dendrimer modification is performed prior to surface attachment. When the benzamido-terminated dendrimer surfaces are dosed with a variety of volatile organic compounds (VOCs), we find that the surface prepared by Method 2 is more sensitive and that there is enhanced selectivity for the VOCs having pi electrons. This result is interpreted in terms of pi-stacking interactions with the aromatic groups on the dendrimer surfaces.

Consecutive Selective Adsorption of Pentamidine and Phosphate Biomolecules on a Self-Assembled Layer: Reversible Formation of a Chemically Selective Coating

In situ ellipsometric film thickness measurements, FT-IR external reflectance spectroscopy, and potentiometric measurements indicated that the amphiphile pentamidine (PAM), a bisbenzamidine, associated by self-assembly with a preformed self-assembled monolayer of a mercaptoalkanoic acid on gold. The structural properties of PAM and the nature of the substrate were ideal for the formation of a densely packed monolayer. This process was fully reversible, as demonstrated by changing the pH of the surrounding medium. Thus, disassembly−reassembly occurred when the pH was cycled between 8.7 and 3. The bilayer structure, featuring a positively charged amidinium surface, was subsequently used for selective adsorption of phosphate biomolecules. Thus, selective binding of adenosine and inositol phosphates (Ka = 5 × 104 M-1 for ATP) to the surface, as well as strong binding of DNA−oligonucleotides, was monitored by in situ ellipsometry. This system suggests a new approach to chemical sensing, based on the reversible formation of a chemically selective coating.

ChemInform Abstract: The Characterization of Organic Monolayers by FTIR External Reflectance Spectroscopy

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Molecular interactions between organized, surface-confined monolayers and vapor-phase probe molecules. 5. Acid-base interactions

Interactions between self-assembled monolayers containing carboxylic or sulfonic acid terminal groups, and vapor-phase n-alkylamine probe molecules, have been studied using FTIR-external reflectance spectroscopy, mass-sensitive surface acoustic wave devices, and ellipsometry. The results indicate that proton transfer from the surface-acid group to the amine-functionalized probe is controlled by both the proton affinity and the degree of structural ordering of the surface-acid groups. In contrast to bilayers formed by hydrogen bonding between carboxylic acid-terminated monolayer, the bilayer structures that result from proton transfer are stable for periods of many hours. 19 refs., 6 figs.