Dendrimer-functionalized self-assembled monolayers as a surface plasmon resonance sensor surface

We report here a multistep route for the immobilization of DNA and proteins on chemically modified gold substrates using fourth-generation NH(2)-terminated poly(amidoamine) dendrimers supported by an underlying amino undecanethiol (AUT) self-assembled monolayer (SAM). Bioactive ultrathin organic films were prepared via layer-by-layer self-assembly methods and characterized by fluorescence microscopy, variable angle spectroscopic ellipsometry, atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and attenuated total internal reflection Fourier transform infrared spectroscopy (ATR-FTIR). The thickness of the AUT SAM base layer on the gold substrates was determined to be 1.3 nm from ellipsometry. Fluorescence microscopy and AFM measurements, in combination with analyses of the XPS/ATR-FTIR spectra, confirmed the presence of the dendrimer/biopolymer molecules on the multilayer sensor surfaces. Model proteins, including streptavidin and rabbit immunoglobulin proteins, were covalently attached to the dendrimer layer using linear cross-linking reagents. Through surface plasmon resonance measurements, we found that sensor surfaces containing a dendrimer layer displayed an increased protein immobilization capacity, compared to AUT SAM sensor surfaces without dendrimer molecules. Other SPR studies also revealed that the dendrimer-based surfaces are useful for the sensitive and specific detection of DNA-DNA interactions. Significantly, the multicomponent films displayed a high level of stability during repeated regeneration and hybridization cycles, and the kinetics of the DNA-DNA hybridization process did not appear to be influenced by surface mass transport limiting effects.     

Protein interactions with self-assembled monolayers presenting multimodal ligands: a surface plasmon resonance study

This paper describes the use of surface plasmon resonance (SPR) spectroscopy and self-assembled monolayers (SAMs) to understand the characteristics of surfaces that promote the adsorption of proteins at high ionic strengths (high-salt conditions). We synthesized SAMs presenting different multimodal ligands and determined the influence of surface composition, solution composition, and the nature of the protein on the extent of protein adsorption onto the SAMs. Our results confirm that hydrophobic interactions can contribute significantly to protein adsorption under high-salt conditions. In particular, the extent of protein adsorption under high-salt conditions increased with increasing surface hydrophobicity. The extent of protein adsorption was also influenced by the solution composition and decreased with an increase in the chaotropicity of the anion. The combination of SPR and SAMs is well-suited for studying the interaction of proteins with complex surfaces of relevance to chromatography.     

Multichannel surface plasmon resonance imaging and analysis of micropatterned self-assembled monolayers and protein affinity interactions

Multichannel images of 11-mercaptoundecanoic acid and 11-mercapto-1-undecanol self-assembled monolayers together with a biospecific interferon-gamma (IFN-gamma)/anti-IFN-gamma antibody immunoaffinity interaction were observed by the two-dimensional surface plasmon resonance (2D-SPR) imaging system. With the fabricated 2D-SPR imaging system, adopting a white light source in combination with a narrow band-pass filter, sharp images were resolved, minimizing the diffraction patterns on the resulting images. Micropatterning of self-assembled monolayers was acheived by exploiting the UV photolysis of thiol bonding, instead of conventional photolithography. The line profile calibration of the image contrast with ellipsometric analysis enabled us to discriminate the change in monolayer thickness within a sub-nanometer scale. For the protein interactions on the surface, the biospecific affinity recognition reaction of IFN-gamma antigen with surface-immobilized antibody was analyzed. Through the signal amplification strategy based on the enzyme-catalyzed precipitation reaction in a sandwich-type immunoassay, biospecific antigen binding was found detectable down to a concentration of 1 ng/mL.     

Electrochemical release of amine molecules from carbamate-based, electroactive self-assembled monolayers

In this paper, carbamate-based self-assembled monolayers (SAMs) of alkanethiolates on gold were suggested as a versatile platform for release of amine-bearing molecules in response to the electrical signal. The designed SAMs underwent the electrochemical oxidation on the gold surface with simultaneous release of the amine molecules. The synthesis of the thiol compounds was achieved by coupling isocyanate-containing compounds with hydroquinone. The electroactive thiol was mixed with 11-mercaptoundecanol [HS(CH(2))(11)OH] to form a mixed monolayer, and cyclic votammetry was used for the characterization of the release behaviors. The mixed SAMs showed a first oxidation peak at +540 mV (versus Ag/AgCl reference electrode), indicating the irreversible conversion from carbamate to hydroquinone groups with simultaneous release of the amine molecules. The analysis of ToF-SIMS further indicated that the electrochemical reaction on the gold surface successfully released amine molecules.     

Electrochemical behaviors of novel electroactive Au nanoparticles protected by self-assembled monolayers

There has been substantial recent interest in studying monolayer-protected gold clusters (MPCs) owing to their diverse applications. The present work is an electrochemical study of novel gold nanoparticles covered with a monolayer of mercapto-dodecanol ended chloro-dicyano-quinone (HS-C12O-CDQ), which was adsorbed on the electrode (CDQ-MPCs film). Our findings reveal a redox behavior for CDQ-MPCs film similar to the solution electrochemistry of dichloro-dicyano-quinone. Furthermore, a diffusion-like mechanism was found for electron transfer, which may have occurred due to proton diffusion towards or outwards the electrode through the film casted. Chronoamperometry confirmed diffusion behavior of the ET process. Finally, EIS was used to find the rate constant of ET process for the redox reaction that occurred and the contribution of MPCs in total interfacial capacitance.     

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.     

Adsorption of ethoxylated cationic surfactants on self-assembled monolayers of alkanethiols on gold using surface plasmon resonance detection

Adsorption of a series of ethoxylated cationic surfactants at model surfaces of alkanethiol self-assembled monolayers was studied by the surface plasmon resonance technique. Model surfaces were tailor-made by choosing alkanethiols or mixtures of alkanethiols with methyl, hydroxyl, carboxyl, and trimethylammonium groups in terminal position. The ethoxylated and quaternized cationic surfactants having from 2 to 18 oxyethylene units, showed a decrease in adsorbed amount with increasing oxyethylene chain length for both hydrophobic and hydrophilic surfaces. On a negatively charged surface, containing carboxylate groups, the surfactant with only two oxyethylene groups adsorbed strongly due to electrostatic attraction and the adsorption increased with increasing amount of surface carboxylate groups. This work shows the usefulness of self-assembled alkanethiols on gold as a tool for performing surfactant adsorption studies on surfaces with variable hydrophobicity and charge.     

Quantitative analysis of protein adsorption via atomic force microscopy and surface plasmon resonance

Surface properties have a significant influence on the performance of biomedical devices. The influence of surface chemistry on the amount and distribution of adsorbed proteins has been evaluated by a combination of atomic force microscopy (AFM) and surface plasmon resonance (SPR). Adsorption of albumin, fibrinogen, and fibronectin was analyzed under static and dynamic conditions, employing self-assembled monolayers (SAMs) as model surfaces. AFM was performed in tapping mode with antibody-modified tips. Phase-contrast images showed protein distribution on SAMs and phase-shift entity provided information on protein conformation. SPR analysis revealed substrate-specific dynamics in each system investigated. When multi-protein solutions and diluted human plasma interacted with SAMs, SPR data suggested that surface chemistry governs the equilibrium composition of the protein layer.     

Spectroelectrochemistry on electroactive self-assembled monolayers: Cyclic voltammetry coupled to spectrophotometry

We present a spectroelectrochemical bench involving an electrochemical/spectrophotometric coupling dedicated to probe electroactive self-assembled monolayers (SAMs). This bench is validated by the study of the oxidation of a 5,5′-disubstituted-2,2′-bithiophene immobilized on Au substrate, producing a reversible dimerization of the radical cation (i.e. EC_Dim process) leading to a dimer. A direct comparison between thin layer spectroelectrochemistry in solution and spectroelectrochemistry on SAMs shows that a SAM could be viewed as a highly concentrated solution.     

Electrical switching of DNA monolayers investigated by surface plasmon resonance

The switching of DNA monolayers between a "lying" and a "standing" state initiated by applying electric field, and the subsequent DNA hybridization at different states were investigated in a contactless, label-free mode by surface plasmon resonance (SPR) technique. The results showed that the strength of the electric field and surface coverage could influence the switching of DNA monolayers. In addition, it was found that DNA hybridization efficiency could be enhanced or decreased when DNA probes stood straight up or lay flat on the gold surface, depending on the potential of the gold substrate. The enhancement of DNA hybridization efficiency reached the maximum when surface coverage reached 5.87 x 10(12) molecules/cm(2) and the potential of gold substrate was more negative than -0.7 V (versus ITO-coated glass). The research may be helpful for the construction of sensitive biosensors, biochips, and nanoscale electronic devices.     

Electrochemical metallization of self-assembled porphyrin monolayers

Multifunctional sensor systems are becoming increasingly important in electroanalytical chemistry. Together with ongoing miniaturization there is a need for micro- and nanopatterning tools for thin electroactive layers (e.g. self-assembling monolayers). This paper documents a method for production of a micro-array of different metal-porphyrin monolayers with different sensor properties. A new method has been developed for the selective and local metallization of bare porphyrin monolayers by cathodic pulsing and sweeping. The metal-porphyrin monolayers obtained were characterized by cyclic voltammetry. It was shown that porphyrin monolayers can be metallized with manganese, iron, cobalt, and nickel by use of the new method. It is expected that all types of metal-porphyrin monolayers can be produced in the same manner.     

An in situ study of the adsorption behavior of functionalized particles on self-assembled monolayers via different chemical interactions

The formation of particle monolayers by convective assembly was studied in situ with three different kinds of particle-surface interactions: adsorption onto native surfaces, with additional electrostatic interactions, and with supramolecular host-guest interactions. In the first case carboxylate-functionalized polystyrene (PS-COOH) particles were assembled onto native silicon oxide surfaces, in the second PS-COOH onto protonated amino-functionalized (NH3+) self-assembled monolayers (SAMs), and in the third beta-CD-functionalized polystyrene (PS-CD) particles onto beta-CD SAMs with pre-adsorbed ferrocenyl-functionalized dendrimers. The adsorption and desorption behaviors of particles onto and from these surfaces were observed in situ on a horizontal deposition setup, and the packing density and order of the adsorbed particle lattices were compared. The desorption behavior of particles from surfaces was evaluated by reducing the temperature below the dew point, thus initiating water condensation. Particle lattices on native oxide surfaces formed the best hexagonal close packed (hcp) order and could be easily desorbed by reducing the temperature to below the dew point. The electrostatically modified assembly resulted in densely packed, but disordered particle lattices. The specificity and selectivity of the supramolecular assembly process were optimized by the use of ferrocenyl-functionalized dendrimers of low generation and by the introduction of competitive interaction by native beta-CD molecules during the assembly. The fine-tuned supramolecularly formed particle lattices were nearly hcp packed. Both electrostatically and supramolecularly formed lattices of particles were strongly attached to the surfaces and could not be removed by condensation.     

Immobilization of gold nanorods onto acid-terminated self-assembled monolayers via electrostatic interactions

We report the immobilization of gold nanorods onto self-assembled monolayers (SAMs) of 16-mercaptohexadecanoic acid (16-MHA). The simple two step protocol involves formation of a SAM of 16-MHA molecules onto gold-coated glass slides and subsequent immersion of these slides into the gold nanorod solution. The nanorods, formed by a seed-mediated, surfactant-assisted synthesis protocol, are stabilized in solution due to surface modification by the surfactant cetyltrimethylammonium bromide (CTAB). Attractive electrostatic interactions between the carboxylic acid group on the SAM and the positively charged CTAB molecules are likely responsible for the nanorod immobilization. UV-vis spectroscopy has been used to follow the kinetics of the nanorod immobilization. The nature of interaction between the gold nanorods and the 16-MHA SAM has been probed by Fourier transform infrared spectroscopy (FTIR). The surface morphology of the immobilized rods is studied by scanning electron microscopy (SEM) and atomic force microscopy (AFM) measurements. SEM was also used to determine the density of the immobilized nanorods as a function of the pH of immobilization. Control over the surface coverage of the immobilized gold nanorods has been demonstrated by simple pH variation. Such well-dispersed immobilized gold nanorods with control over the surface coverage could be interesting substrates for applications such as surface-enhanced Raman spectroscopy (SERS).     

Characterization of transparent conducting oxide surfaces using self-assembled electroactive monolayers

The electronic properties of various transparent conducting oxide (TCO) surfaces are probed electrochemically via self-assembled monolayers (SAMs). A novel graftable probe molecule having a tethered trichlorosilyl group and a redox-active ferrocenyl functionality (Fc(CH2) 4SiCl3) is synthesized for this purpose. This molecule can be self-assembled via covalent bonds to form monolayers on various TCO surfaces. On as-received ITO, saturation coverage of 6.6 x 10(-10) mol/cm2 by a close-packed monolayer and an electron-transfer rate of 6.65 s(-1) is achieved after 9 h of chemisorption, as determined by cyclic voltammetry (CV) and synchrotron X-ray reflectivity. With this molecular probe, it is found that O2 plasma-treated ITO has a significantly greater electroactive coverage of 7.9 x 10 (-10) mol/cm2 than as-received ITO. CV studies of this redox SAM on five different TCO surfaces reveal that MOCVD-derived CdO exhibits the greatest electroactive coverage (8.1 x 10(-10) mol/cm2) and MOCVD-derived ZITO (ZnIn2.0Sn1.5O) exhibits the highest electron transfer rate (7.12 s(-1)).     

Application of octadecanethiol self-assembled monolayer to cholesterol biosensor based on surface plasmon resonance technique

Octadecanethiol (ODT) self-assembled monolayer (SAM) prepared onto gold-coated glass plate has been modified by using nitrene reaction of 1-fluoro-2-nitro-4-azidobenzene (FNAB) that further covalently binds to cholesterol oxidase (ChOx) via thermal reaction. FNAB acts as a bridge (cross-linker) between SAM and ChOx. The ChOx/FNAB/ODT/Au electrode thus fabricated has been characterized using contact angle (CA) measurements, UV-vis spectroscopy, electrochemical techniques and X-ray photoelectron spectroscopy (XPS) technique, respectively. This ChOx/FNAB/ODT/Au bioelectrode has been utilized for estimation of cholesterol in solution using surface plasmon resonance (SPR) technique. This SPR based cholesterol biosensor has linearity from 50 to 500 mg/dl of cholesterol in solution with lower detection limit of 50 mg/dl and shelf life of about 2 months when stored at 4 °C.     

Electrochemical characteristics of ferrocenecarboxylate-coupled aminoundecylthiol self-assembled monolayers

The electrochemical characteristics of the modified electrodes with ferrocenecarboxylate-coupled aminoundecylthiol monolayers prepared in two sequential steps were studied. The self-assembled monolayer (SAM) was prepared through the covalent attachment of ferrocenecarboxylate in an activation solution containing N-(3-dimethylaminopropyl)-N'-ethylcarbodiimide coupling agent to aminoundecylthiol SAMs formed on a substrate. In the ferrocenecarboxylate-coupled aminoundecylthiol monolayers, the ferrocene moieties were expected to be packed regularly with enhanced ordering compared with those in the FcCOO(CH2)11SH monolayer. As the ferrocene coverage increases, the formal potential for the ferrocene-ferricenium (Fc/Fc+) couple shifts to the positive potential and the full width at half-maximum (deltaE(fwhm)) increases also. The maximum coverage is found to be about 3 x 10(-10) mol cm(-2), which is considered to be a value obtained from a well-ordered ferrocene-tethered SAM. As for the mass change, the increase in ferrocene coverage caused the enhancement in ion association between the ferricenium cations and perchlorate anions resulting in a mass increase upon oxidation; however, the mass change per mole electron decreases. The results obtained from the ferrocenecarboxylate-coupled aminoundecylthiol monolayers were explained to be due to the well-ordered packing with regular spacing compared with those of the FcCOO(CH2)11SH monolayer.     

UV polymerization of self-assembled monolayers of a novel diacetylene on silver: a spectroscopic analysis by surface plasmon resonance and surface enhanced Raman scattering

UV polymerization of self-assembled monolayers of a novel carbazolyl-diacetylene (CDS9) chemisorbed on silver films was demonstrated by surface plasmon resonance (SPR) and surface enhanced Raman scattering (SERS) experiments. SPR tests performed during UV exposure permitted one to observe the growth of the absorption coefficient, associated with the formation of the polymeric backbone. The Raman spectra of polymerized monolayers exhibited the bands associated with the C=C stretching modes of the conjugated backbone, typical of the blue and red polymeric phases usually present in polydiacetylenes, with a clear predominance of the red form. Moreover, the strong surface enhancement of the Raman band corresponding to the aromatic C=C stretching modes suggested that carbazolyl groups arrange nearly perpendicularly to the metal surface. In contrast, the absence of a SERS signal in the region of conjugated C[triple bond]C bond stretchings confirmed a polymerization scheme with conjugated triple bonds nearly parallel to the plane of the metal.     

Cholesterol biosensor based on amino-undecanethiol self-assembled monolayer using surface plasmon resonance technique

Cholesterol oxidase has been covalently immobilized onto 11-amino-1-undecanethiol hydrochloride (AUT) self-assembled monolayer (SAM) fabricated on gold (Au) substrates using glutaraldehyde as a cross-linker. These ChOx/AUT/Au bioelectrodes characterized using contact angle (CA) measurements; electrochemical technique and atomic force microscopy (AFM) have been utilized for the estimation of cholesterol in solution using the surface plasmon resonance (SPR) technique. These biosensing electrodes exhibiting linearity from 50 to 500 mg/dL of cholesterol in solution and sensitivity of 1.23 m0/(mg dL), can be used more than 20 times and have a shelf life of about 10 weeks when stored at 4 degrees C.     

Electrochemical study of self-assembled monolayer adsorption

The electrochemical behaviour of self-assembled monolayer (SAM) of aliphatic hexadecanethiol was studied by cyclic voltammetry (CV), elimination voltammetry with linear scan (EVLS) and crystal quartz microbalance (QCM). SAMs were electrochemically created on gold-coated QCM crystal through the sulphur in 1-hexadecanethiol molecule head group. The effect of thiol concentration and potential scan rate on the SAM formation was studied. Formation of SAM was confirmed by CV and QCM. EVLS results revealed the kinetically controlled process followed with electrode reaction in adsorbed state characteristic for SAM formation at lower concentration. The electrode reaction of a totally adsorbed electroactive species was indicated by means of a peak-counter peak signal at higher thiol concentration.