Monolayer Of 11-mercaptoundecanoic Acid Research Articles

Interfacial charge-transfer resistance at ionizable thiol monolayer-modified gold electrodes as studied by impedance spectroscopy

This paper examines the influences of ionizable thiol monolayers on the interfacial charge-transfer rates for redox-active ions by electrochemical impedance spectroscopy. Whereas short-chain thiols such as 4-aminothiophenol hardly affected the charge-transfer resistance for Ru(NH 3) 6 3+ having a high standard heterogeneous rate constant, the 11-mercaptoundecanoic acid monolayer raised this resistance to 3×10 4 times that observed at a bare electrode in acidic solutions. As the terminal carboxyl group ionized, the charge-transfer resistance decreased by a factor of 300 because of a double-layer effect. When 3-mercaptopropionic acid was mixed with 1-hexadecanethiol, the charge-transfer resistance varied over four orders of magnitude with the composition of the two-component monolayer. The apparent rate constant for indigotetrasulfonate at 4-aminothiophenol-modified electrodes decreased by a factor of 25–30 as the solution pH varied from 4 to 2. This decrease was opposite to the change expected from protonation of the terminal amino group. It can be ascribed to a strong binding interaction between the redox anion and the positively charged monolayer.

Regulation of anti-LDL immobilization on self-assembled protein G layer using CHAPS and its application to immunosensor

In antibody-based immunoassay, it is important to fabricate a structured base plate with high activity for antibody immobilization. To prevent the reduction of anti-LDL binding capacity due to protein G aggregation, detergent was introduced to control the size of protein G aggregate. The self-assembled monolayer of 11-mercaptoundecanoic acid (11-(MUA)) and hexanethiol mixture was fabricated to form the stable protein G layer. 3-[(cholamidopropyl)dimethyl-ammonio]-1-propane sulfonate (CHAPS) was used for the regulation of protein G aggregate immobilized on the 11-(MUA) surface. The effect of various CHAPS concentrations on the amount of anti-low density lipoprotein (LDL) immobilized on protein G layer was investigated. Twelve millimolars of CHAPS were determined as the optimal concentration to maximize the binding capacity of anti-LDL due to the control of protein G density on the surface. The anti-LDL layer on self-assembled protein G using CHAPS was applied to surface plasmon resonance immunosensor for detection of LDL and its detection limit was 100 pM.

Patterning of photosensitive polyimide LB film and its application in the fabrication of biomolecular microphotodiode array

Ultra thin film of photosensitive polyimide having benzene and sulfonyloxyimide moieties in the main chain was prepared using a Langmuir–Blodgett (LB) technique, and then micro array pattern of the polyimide LB film on a gold substrate was obtained by deep UV lithographic technique. In order to array cytochrome c molecules along the micro-patterned gold substrate, the well-characterized monolayer of cytochrome c was immobilized with a mixed monolayer of 11-mercaptoundecanoic acid (11-MUDA) and decanethiol. The redox activity and electron transfer between cytochrome c molecular center and gold electrode interface for the self-assembled cytochrome c monolayer were investigated by cyclic voltammetry measurement. Biomolecular photodiode consisting of cytochrome c and green fluorescent protein (GFP) onto the patterned gold substrate was fabricated by self-assembly process. The integration and morphology of cytochrome c and GFP were studied from the measurements of atomic force microscopy (AFM) and fluorescence emission. Especially, current–voltage characteristics of the protein multilayers were investigated by scanning tunneling microscopy (STM) and its application in biomolecular photodiode was also examined.

Reductive desorption of 11-mercaptoundecanoic acid monolayers modified by covalent attachment of 1,3- and 1,4-phenylenediamine

1,3- and 1,4-phenylenediamine were covalently attached to a self-assembled monolayer of 11-mercaptoundecanoic acid by formation of an amide bond. Bonding of the phenylenediamine isomers was confirmed by the presence of infrared features attributable to ring vibrational modes, the amide I vibration, and the amide II vibration in the reflection–absorption infrared spectrum of the modified interface. Reductive desorption peak potentials were shifted by 120 mV for the monolayer modified by 1,4-phenylenediamine and by 90 mV for the monolayer modified by 1,3-phenylenediamine relative to the desorption potential of the unmodified 11-mercaptoundecanoic acid monolayer. These potential shifts represent 23 and 17 kJ/mol stabilization of the monolayers modified by 1,4- and 1,3-phenylenediamine, respectively. Some of the additional stabilization energy is attributed to lateral hydrogen bonding between the amide groups and unreacted carboxylic acid groups in the monolayer. The difference between the stabilization energies for the interfaces modified with the different phenylenediamine isomers is attributed to structural differences between the isomeric monolayers influencing the molecular packing of the modified interface.

Characterization of oxidoreductase–redox polymer electrostatic film assembly on gold by surface plasmon resonance spectroscopy and Fourier transform infrared–external reflection spectroscopy

The electrostatic assembly of nanocomposite thin films consisting of alternating layers of an organometallic redox polymer (RP) and oxidoreductase enzymes, glucose oxidase (GOX), lactate oxidase (LOX) and pyruvate oxidase (PYX), was investigated. Multilayer nanostructures were fabricated on gold surfaces by the deposition of an anionic self-assembled monolayer of 11-mercaptoundecanoic acid, followed by the electrostatic attachment of a cationic RP, poly(vinylpyridine Os(bis-bipyridine) 2Cl-co-allylamine) (PVP-Os-AA), and anionic oxidoreductase enzymes. Surface plasmon resonance (SPR) spectroscopy, Fourier transform infrared external reflection spectroscopy (FT-IR–ERS) and electrochemistry were employed to characterize the assembly of these nanocomposite films. The surface concentration of GOX was found to be 2.4 ng/mm 2 for the first enzyme layer and 1.96 ng/mm 2 for the second enzyme layer, while values of 10.7 and 1.3 ng/mm 2 were obtained for PYX and LOX, respectively. The apparent affinity constant for GOX adsorption was found to be 8×10 7 M −1. FT-IR–ERS was used to verify the incorporation of GOX and its conformational stability inside of these nanocomposite thin films. An SPR instrument with a flow-through cell was modified by additions of Ag/AgCl reference and Pt counter electrodes, with the gold-coated SPR surface film serving as the working electrode. This enabled real-time observation of the assembly of sensing components and immediate, in situ electrochemical verification of substrate-dependent current upon the addition of enzyme to the multilayer structure. A glucose-dependant amperometric response with sensitivity of 0.197 μA/cm 2/mM for a linear range of 1–10 mM of glucose was obtained. The SPR and FT-IR–ERS studies also showed no desorption of polymer or enzyme from the nanocomposite RP–GOX structure when stored in aqueous environment occurred over the period of 3 weeks, suggesting that decreasing substrate sensitivity with time was due to loss of enzymatic activity rather than loss of film compounds from the nanostructure.

Mass Transfer in Amperometric Biosensors Based on Nanocomposite Thin Films of Redox Polymers and Oxidoreductases

Abstract : Mass transfer in nanocomposite hydrogel thin films consisting of alternating layers of an organometallic redox polymer (RP) and oxidoreductase enzymes was investigated. Multilayer nanostructures were fabricated on gold surfaces by the deposition of an anionic self-assembled monolayer of 11-mercaptoundecanoic acid, followed by the electrostatic binding of a cationic redox polymer, poly[vinylpyridine Os( bis -bipyridine) 2 Cl-co-allylamine], and an anionic oxidoreductase. Surface plasmon resonance spectroscopy, Fourier transform infrared external reflection spectroscopy (FTIR-ERS), ellipsometry and electrochemistry were employed to characterize the assembly of these nanocomposite films. Simultaneous SPR/electrochemistry enabled real time observation of the assembly of sensing components, changes in film structure with electrode potential, and the immediate, in situ electrochemical verification of substrate-dependent current upon the addition of enzyme to the multilayer structure. SPR and FTIR-ERS studies also showed no desorption of polymer or enzyme from the nanocomposite structure when stored in aqueous environment occurred over the period of three weeks, suggesting that decreasing in substrate sensitivity were due to loss of enzymatic activity rather than loss of film compounds from the nanostructure.

Glucose sensing based on interdigitated array microelectrode.

A micro glucose sensor consisting of an interdigitated array gold microelectrode was developed. The interdigitated array structure, which has 10 microns band width and 10 microns band gap, was fabricated in a small region (2.5 x 5 mm2) on a quartz substrate. Glucose oxidase was chemically fixed onto the electrode surface through self-assembled monolayer of 11-mercaptoundecanoic acid; ferroceneacetic acid was used as electron mediator. Electrochemical properties of the glucose oxidase-immobilized microelectrode were investigated by cyclic voltammogram measurements. Results confirmed that the reductive ferroceneacetic acid generated at counter electrode diffuses through a narrow band gap (10 microns) and can reach the working electrode surface.

Microstructuring of solid-supported lipid layers using SAM pattern generation by scanning electrochemical microscopy and the chemical lens

To prepare patterns of adsorption sites for alkanethiols with high lateral resolution, we used the scanning electrochemical microscopy (SECM) to etch masks into uniform layers of nickel coated on gold surfaces. The patterning of the nickel mask was accomplished in aqueous solutions by electrogenerating nitric acid out of nitrite at an ultramicroelectrode. Due to the sluggish kinetics of nickel etching in acidic media, the pattern generated by a 10-μm tip was about 50-μm wide, depending on the duration of the etching. As an alternative, applying the principle of the chemical lens by adding potassium hydroxide as a scavenger, the size of the adsorption sites had been reduced to 4 μm, independent of the duration of etching. In a follow-up step, monolayers of 11-mercaptoundecanoic acid were formed on the exposed gold areas of the surface by self-assembly. Fluorescent liposomes containing tetramethylrhodamine-labeled phospholipids were used to create solid-supported lipid layers (SSLLs). These fluorescent liposomes showed a selective binding affinity to the self-assembled monolayers (SAMs) modified areas, but not to the nickel surface. The patterns generated were imaged by the SECM itself, as well as by optical and fluorescence microscopy.

Phosphatidylserine/Cholesterol Bilayers Supported on a Polycation/Alkylthiol Layer Pair

1-Stearoyl-2-oleoyl phosphatidylserine (SOPS)/cholesterol bilayers, supported on a polycation/alkylthiol layer pair on a gold surface, were investigated by surface plasmon resonance (SPR) and fluorescence recovery after photobleaching. The substrate was formed by electrostatic adsorbance of a hydrated poly(diallyldimethylammonium chloride) (PDDA) layer on the negatively charged surface of a self-assembled monolayer of 11-mercaptoundecanoic acid (MUA) on gold. Lipid membranes with different SOPS/cholesterol compositions were deposited on the PDDA/MUA layer pair by vesicle fusion. When the cholesterol content was below 20%, single bilayers were deposited. Fluorescence recovery after the bleaching experiments revealed that the SOPS/cholesterol bilayers were mobile at room temperature; lateral diffusion coefficients of a fluorescence probe were approximately 1×10−9 cm2/s. The kinetics of the addition of the ion-channel-forming peptide gramicidin to the supported bilayers was detected by SPR.

Quartz crystal microbalance for l-leucine sensing based on growth of l-leucine crystals immobilized on a monolayer of 11-mercaptoundecanoic acid

A l-leucine-seed-modified quartz crystal microbalance (QCM) oscillator showed selectively chiral discrimination in situ for leucine enantiomers. The obtained results exhibited strong QCM response to l-leucine, but the response to d-leucine was negligible. This selectivity was attributed to preferential crystallization of l-leucine at the solid-solution interface.

Mobile Phospholipid Bilayers Supported on a Polyion/Alkylthiol Layer Pair

The formation of phospholipid bilayers on a polymer/alkylthiol layer pair was investigated by surface plasmon resonance (SPR). The organic layer pair between the lipid membrane and the solid gold surface consisted of a self-assembled monolayer of 11-mercaptoundecanoic acid (MUA) on a gold surface followed by a thin layer of hydrated cationic poly(diallyldimethylammonium chloride) (PDDA). The lipid layers were formed by vesicular fusion of small unilamellar vesicles of an anionic lipid 1-stearoyl-2-oleoylphosphatidylserine (SOPS), a zwitterionic lipid 1-palmitoyl-2-oleoylphosphatidylcholine (POPC), and mixtures of these two lipids with different compositions. In the case of pure SOPS and lipid mixtures with a POPC composition below 25%, the lipid layer thickness was approximately that of a single bilayer, while multilayers tended to build up with higher concentrations of POPC. The electrostatic interaction of the cationic PDDA with the negatively charged lipid membrane is most probably the driving force for the adsorption of a single bilayer. Fluorescence recovery after photobleaching (FRAP) experiments showed that single bilayers supported on the PDDA/alkylthiol layer pair were mobile at room temperature with lateral diffusivities of approximately (1−2) × 10-9 cm2/s. A membrane fusion peptide wt-20 of the influenza virus was bound to the supported bilayers as detected by SPR. This suggests the potential of this model membrane system for use as a biosensor.

Azurin immobilisation on thiol covered Au(111)

Azurin is a blue single copper protein involved in the respiratory chain of denitrifying bacteria. The structural gene for azurin from Pseudomonas aeruginosa was cloned in an Escherichia coli recombinant strain. The protein overexpressed in the bacterial periplasmic space was subsequently purified. Two strategies were followed to anchor azurin to gold surfaces. First, the protein was immobilised on bare gold. Azurin adsorbs on gold ia its disulfide group. Scanning tunnelling microscopy (STM) inspection of the azurin–Au(111) interface revealed the formation of a closely packed protein monolayer and allowed individual azurin molecules to be resolved. In order to uncouple the protein layer from the metal, the gold surfaces were then covered with self-assembled monolayers of 11-mercaptoundecanoic acid. The changes in the sample morphology due to the protein adsorption have been investigated by atomic force microscopy (AFM). A fairly uniform distribution of protein molecules covers the surface. Owing to the tip broadening effect, an average protein diameter of about 20 nm was measured. An upper limit of 1 nN for the non-disruptive imaging force in the contact mode was found.

STM Imaging of Cyanine Dye J-Aggregates Formed on Carboxyl-Terminated Self-Assembled Monolayers

We report on the preparation and STM imaging of J-aggregates of 1,1‘-diethyl-2,2‘-cyanine (PIC) dye formed on a self-assembled monolayer of 11-mercaptoundecanoic acid (HS−(CH2)10−COOH). The formation of J-aggregates on the COOH surface was verified by reflection absorption measurements; however, the molecular assemblies were not bound strongly enough to the SAM to withstand the lateral forces applied during STM imaging. We found that pretreating the COOH surface with a solution of 1 mM AgNO3 before application of the PIC dye solution greatly improved the stability of the J-aggregates. We suggest that the reason for this is the formation of a silver chelate with the carboxyl head groups of the SAM which permits the subsequent adsorption of bromide counterions from the dye solution. The formation of a counterion monolayer has been shown to be important in the formation of stable J-aggregates. STM images of these samples revealed long, essentially linear aggregates (with a packing structure similar to the pr...

An Indirect Laser-Induced Temperature Jump Determination of the Surface pKa of 11-Mercaptoundecanoic Acid Monolayers Self-Assembled on Gold

The indirect laser-induced temperature jump (ILIT) method is used to determine the acidity (pKa) of monolayers composed of 11-mercaptoundecanoic acid (MUA) self-assembled on vapor-deposited gold film electrodes in contact with either 0.10 or 1.0 M ionic strength NaClO4 electrolyte solutions. The ILIT technique may be used to measure the pKa of a surface-attached acid because the magnitude of the ILIT response is related to the potential gradient at the electrode/electrolyte interface which is, in turn, related to the extent of ionization of the acid. The relevant data are the potentials of zero ILIT response versus pH. A simple two-layer model of the acid-modified electrode/electrolyte interface and Gouy−Chapman double layer theory (modified for the onset of dielectric saturation effected by the large concentration of charge present at this interface when a substantial fraction of the carboxylic acid moieties are ionized) were employed to analyze these data. This analysis gives pKa = 5.7 ± 0.2 at 0.10 M ionic strength and pKa = 4.4 ± 0.2 at 1.0 M ionic strength while the total concentration of MUA comprising the self-assembled monolayer is determined to be essentially the same at both ionic strengths, averaging (5.4 ± 0.3) × 10-10 mol/cm2. Reasons for the observed variation of pKa with ionic strength are discussed.

Characterization of poly-L-lysine adsorption onto alkanethiol-modified gold surfaces with polarization-modulation Fourier transform infrared spectroscopy and surface plasmon resonance measurements

The electrostatic adsorption of poly-L-lysine molecules onto a vapor-deposited gold film modified with a carboxylic acid-terminated alkanethiol monolayer is monitored with the spectroscopic techniques of polarization-modulation Fourier transform infrared (PM-FTIR) spectroscopy and surface plasmon resonance (SPR). The PM-ITIR spectrum of a monolayer of poly-L-lysine (PL) adsorbed onto a self-assembled monolayer of 11-mercaptoundecanoic acid (MUA) indicates that the lysine residues and the MUA carboxylic acid moieties form ammonium-carboxylate ion pairs which electrostatically bind the polypeptide to the surface. The PL molecules can be desorbed from the surface by rinsing with a buffer solution at a pH that destroys the ion pairing (pH 12). Measurements of the shift in the SPR angle upon adsorption were used to determine the thicknesses of the adsorbed monolayers; the MUA and PL monolayers were found to be 17.0 and 10.5 A thick, respectively. These thickness results suggest that the poly-L-lysine monolayer adsorbs onto the packed MUA surface in an extended conformation with the PL backbone lying parallel to the surface. Subsequent exposure ofthe PL monolayer to a solution ofiron phthalocyanine tetracarboxylic acid (FePc) resulted in the adsorption of a third layer onto the surface. The ability of the adsorbed PL molecules to interact with FePc indicates the presence of free lysine residues available for interaction with molecules other than the MUA monolayer.