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.     

Adsorption kinetics of p-nitrobenzenethiol self-assembled monolayers on a gold surface

The adsorption kinetics of octadecanethiol (ODT) and p-nitrobenzenethiol (NBT) from ethanol solutions has been studied by means of contact angle, optical ellipsometry, angle-resolved X-ray photoelectron spectroscopy (ARXPS), and grazing angle attenuated total reflection Fourier transform infrared (FTIR) measurements. ODT data were used as a reference for the kinetics studies of film growth. The growth of self-assembled monolayers from dilute solutions follows Langmuir isotherm adsorption kinetics. A saturated film is formed within 5 h after immersion in solutions of concentrations ranging from 0.0005 to 0.01 mM. The density of the monolayer depends on the concentration of the solution.     

Negative surface potential produced by self-assembled monolayers of helix peptides oriented vertically to a surface

Self-assembled monolayers (SAMs) of helix peptides oriented vertically to a gold surface were prepared and the surface potential measured using the Kelvin technique up to 140°C. Negative surface potentials of a few hundred millivolts were observed for the helix peptide SAMs, indicating the occurrence of the large dipole moment of the helices directing toward the surface. The longer the helix peptide, the larger was the negative surface potential obtained. The absolute value of the surface potential decreased with increase in temperature due to thermal perturbation in the helical structure. However, Fourier transform infrared reflection–absorption spectroscopy revealed that perturbation is not significant and the α-helical conformation is stable even at 140°C.     

Second-harmonic spectroscopy of surface immobilized gold nanospheres above a gold surface supported by self-assembled monolayers

We have investigated linear and nonlinear optical properties of surface immobilized gold nanospheres (SIGNs) above a gold surface with a gap distance of a few nanometers. The nanogap was supported by amine or merocyanine terminated self-assembled monolayers (SAMs) of alkanethiolates. A large second-harmonic generation (SHG) was observed from the SIGN systems at localized surface plasmon resonance condition. The maximum enhancement factor of SHG intensity was found to be 3 x 10(5) for the SIGN system of nanospheres 100 nm in diameter with a gap distance of 0.8 nm. The corresponding susceptibility was estimated to be chi((2))=750 pmV (1.8 x 10(-6) esu). In the SIGN system supported with the merocyanine terminated SAMs, the SHG response was also resonant to the merocyanine in the nanogap. It was found that the SHG response of the SIGN systems is strongly frequency dependent. This leads us to conclude that the large chi((2)) is caused by enhanced electric fields at the localized surface plasmon resonance condition and is not due to an increase of the surface susceptibility following from the presence of the gold nanospheres. The observed SHG was consistent with the theoretical calculations involving Fresnel correction factors, based on the quasistatic approximation.     

Maleimido-terminated self-assembled monolayers

Four approaches have been explored for the preparation of maleimido-functionalized self-assembled monolayers (SAMs) on silicon. SAMs prepared by self-assembly of maleimido-functionalized alkyltrichlorosilanes (11-maleimido-undecyl-trichlorosilane) on oxide-covered silicon yield higher signals from maleimido functionalities in ATR-IR (attenuated total reflection IR) spectroscopy and XPS (X-ray photoelectron spectroscopy) than the other three methods. The surface composition of maleimido groups was tailored further by the formation of mixed monolayers with nonfunctionalized alkyltrichlorosilanes (decyltrichlorosilane). The order of the alkyl chains within the monolayers only slightly depends on the composition of the mixed monolayers. We utilized the maleimido-terminated SAMs to bind various nucleophilic compounds, alkylamines, alkylthiols, and thiol-tagged DNA oligonucleotides by means of conjugate addition.     

Tetraferrocenylporphyrins as active components of self-assembled monolayers on gold surface

Novel tetraferrocenylporphyrins-containing self-assembled monolayers were prepared employing two different approaches. Self-assembled monolayers were characterized using UV-Vis spectroscopy and cyclic voltammetry (CV) whereas their photoelectrochemical properties were investigated by photocurrent generation (PG) experiments.     

Investigation into pH-responsive self-assembled monolayers of acylated anthranilate-terminated alkanethiol on a gold surface

This article describes the preparation of pH-responsive self-assembled monolayers (SAMs) of acylated anthranilate-terminated alkanethiol. These monolayers are formed by chemisorption of the alkanethiol molecules onto a gold surface, resulting in different wetting properties of the surfaces depending upon the pH. By using various characterization techniques (e.g., infrared spectroscopy, cyclic voltammetry, contact angle measurements, and surface energy analysis), we have found that the changes in the wetting properties originate from the different surface structures of the monolayers in different pH environments. From surface energy analysis, we found that the disperse components of the surface energy on such SAMs predominate after treatment with pH 1 water, whereas the polar components of the surface energy on such SAMs predominate after treatment with pH 13 water. It is greatly anticipated that this line of research will provide new insight into the mechanism behind pH-responsive properties, facilitating the design and synthesis of new surface-active molecules for the fabrication of pH-responsive functional surfaces.     

Chemical surface modification of self-assembled monolayers by radical nitroxide exchange reactions

This article describes the application of nitroxide exchange reactions of surface-bound alkoxyamines as a tool for reversible chemical modification of self-assembled monolayers (SAMs). This approach is based on radical chemistry, which allows for introduction of various functional groups and can be used to reversibly introduce functionalities at surfaces. To investigate the scope of this surface chemistry, alkoxyamines with different functionalities were synthesized and were then applied to the immobilization of, for example, dyes, sugars, or biotin. Surface analysis was carried out by contact angle, X-ray photoelectron spectroscopy, and fluorescence microscopy measurements. The results show that this reaction is highly efficient, reversible, and mild and allows for immobilization of various sensitive functional groups. In addition, Langmuir-Blodgett lithography was used to generate structured SAMs. Site-selective immobilization of a fluorescent dye could be achieved by nitroxide exchange reactions.     

Fabrication of self-assembled monolayers (SAMs) and inorganic micropattern on flexible polymer substrate

By grafting (aminopropyl)triethoxysilane (APTES) as the buffer layer on poly(ethylene terephthalate) (PET) surface, the SAMs ofoctadecyltrichlorosilane (OTS), phenyltrichlorosilane (PTCS), vinyltrichlorosilane (VTCS), andp-tolyltrichlorosilane (TTCS) were fabricated on the flexible polymer substrate. The properties of SAMs were accurately controlled by adjusting the immersing time of substrates in the solutions and the concentration of the solutions. The SAMs acted as templates, and TiO2 micropattern was successfully deposited on OTS, TTCS, and PTCS SAMs.     

Triphenylene silanes for direct surface anchoring in binary mixed self-assembled monolayers

New triphenylene-based silanes 2-(ω-(chlorodimethylsilyl)-n-alkyl)-3,6,7,10,11-penta-m-alkoxytriphenylene 4 (Tm-Cn) with n = 8 or 9 and m = 7, 8, 9, 10, or 11 were synthesized, and their self-assembly behavior in the liquid state and at glass and silicon oxide surfaces was investigated. The mesomorphic properties of triphenylene silanes 4 (Tm-Cn) and their precursors 3 (Tm-Cn) were determined by differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and X-ray diffraction. From the small-angle X-ray scattering (SAXS) regime, a preferential discotic lamellar mesophase can be deduced, and wide-angle X-ray scattering (WAXS) highlights the liquid-like characteristics of the alkyl side chains. To transfer these bulk structural properties to thin films, self-assembled monolayers (SAMs) were obtained by adsorption from solution and characterized by water contact angle measurements, null ellipsometry, and atomic force microscopy (AFM). Employing the concentration as an additional degree of freedom, binary SAMs of 2-(ω-(chlorodimethylsilyl)-undecyl)-3,6,7,10,11-penta-decyloxytriphenylene 4 (T10-C11) were coassembled with chlorodecyldimethylsilane or chlorodimethyloctadecylsilane, and their capability as model systems for organic templating was evaluated. The structure of the resulting binary mixed SAMs was analyzed by water contact angle measurements, null ellipsometry, and X-ray reflectivity (XRR) in combination with theoretical modeling by a multidimensional Parratt algorithm and AFM. The composition dependence of film thickness and roughness can be explained by a microscopic model including the steric hindrance of the respective molecular constituents.     

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.     

Electrochemistry of redox-active self-assembled monolayers

Redox-active self-assembled monolayers (SAMs) provide an excellent platform for investigating electron transfer kinetics. Using a well-defined bridge, a redox center can be positioned at a fixed distance from the electrode and electron transfer kinetics probed using a variety of electrochemical techniques. Cyclic voltammetry, AC voltammetry, electrochemical impedance spectroscopy, and chronoamperometry are most commonly used to determine the rate of electron transfer of redox-activated SAMs. A variety of redox species have been attached to SAMs, and include transition metal complexes (e.g., ferrocene, ruthenium pentaammine, osmium bisbipyridine, metal clusters) and organic molecules (e.g., galvinol, C₆₀). SAMs offer an ideal environment to study the outer-sphere interactions of redox species. The composition and integrity of the monolayer and the electrode material influence the electron transfer kinetics and can be investigated using electrochemical methods. Theoretical models have been developed for investigating SAM structure. This review discusses methods and monolayer compositions for electrochemical measurements of redox-active SAMs.     

Formation of gold-methanethiyl self-assembled monolayers

The energetics of formation of thiyl-gold self-assembled monolayers is investigated using density-functional theory simulations. It is found that the chemisorption of dimethyl disulfide on the reconstructed Au(111) (22 x radical3) surface is most favored at the fcc reconstruction stripe, with initial physisorption leading to disulfide dissociation, adatom/vacancy-pair formation, and then, at a coverage of 7.8% sulfur atoms per gold atom, surface reconstruction lifting. At higher coverages, monolayer formation proceeds similarly on the unreconstructed surface, leading to surface pitting. Formation of the analogous adatom/vacancy-pair bound dissociated adsorbate complex on exposure of the clean unreconstructed surface to methanethiol is shown to be endothermic, however.     

Anchoring of self-assembled monolayers of unsymmetrically-substituted chromophores with an oxoporphyrinogen surface clamp

Multichromophoric molecules of conjugated N-substituted oxoporphyrinogens have been assembled on the Au(111) surface using solution and sublimation techniques. The operation of the oxoporphyrinogen moiety as a surface anchoring unit in the formation of molecular monolayers has been demonstrated.     

Fabrication of vertically and unidirectionally oriented polypeptide assemblies on self-assembled monolayers by stepwise polymerization

A polypeptide assembly prepared by stepwise polymerization on a self-assembled monolayer consisting of amino-alkanethiol and dialkyl disulfide oriented vertically and unidirectionally to the surface.     

Effects of surface hydrophobization on the growth of self-assembled monolayers on silicon

The growth of self-assembled monolayers from octadecyltrichlorosilane (OTS) on modified silicon surfaces has been investigated. The influence of different immersion times in a deactivation reagent on the growth mechanism and the ordering of the films has been studied. Characterization of the films and the submonolayer coverage has been performed with tapping mode atomic force microscopy, ellipsometry, and infrared spectroscopy. We found that a deactivation of active sites led to a higher mobility of adsorbed molecules on the surface resulting in circular islands of highly ordered alkylsiloxane. However, upon prolonged immersion in OTS these ordered islands did not continue to grow and full monolayer coverage could not be obtained. Instead, an exchange reaction with the deactivation reagent leading to a disordered film between the ordered islands was observed. This was confirmed by external reflection infrared spectroscopy.     

Structures and dynamics of self-assembled surface monolayers observed by ultrafast electron crystallography

In this communication, we report our first study of self-assembled adsorbates on metal surfaces. Specifically, we studied single-crystal clean surfaces of Au(111) with and without a monolayer of reaction involving the assembly of 2-mercaptoacetic acid from 2,2'-dithiodiacetic acid. We also studied monolayers of iron hemes. With ultrafast electron crystallography, we are able to observe and isolate structural dynamics of the substrate (gold) and adsorbate(s) following an ultrafast temperature jump.     

Metallization of a thiol-terminated organic surface using chemical vapor deposition

The deposition and the subsequent decomposition of an organometallic precursor, (eta (3)-allyl)(eta (5)-cyclopentadienyl)palladium [Cp(allyl)Pd], on an organic surface exposed by self-assembled monolayers (SAM) was studied using X-ray photoelectron spectroscopy (XPS) and infrared reflection absorption spectroscopy (IRRAS). The interfacial chemical reactions of the vapor-deposited metal precursor with the pendant thiol group of the SAMs made from oligophenyldithiols, which are either prepared directly (terphenyldimethyldithiol, TPDMT) or by a deprotection route from SAMs formed by a monoacylated derivative of biphenyldimethyldithiol (dep. BPDMAc-1) have been studied in detail. When the TPDMT-SAMs were exposed to Cp(allyl)Pd vapor, a Pd (2+)/allyl-terminated SAM surface was obtained (to a lower extent this was also the case for dep. BPDMAc-1 SAMs), which was stable against exposure to H 2 gas. Reduction to Pd (0) by H 2 was only observed when small amounts of Pd (0) were already present, for example, after prolonged exposure to the precursor. The catalytic activity of the small Pd (0) particles also caused a decomposition of the SAMs upon exposure to air.     

Practical,high-yield synthesis of thiol-terminated diacetylenes for formation of conductive monolayers

Self-assembled monolayers of thiol terminated conjugated diacetylenes can be cross-linked using ultraviolet light to form highly conjugated polydiacetylenic conductive monolayers [1]; however, the reported syntheses of the diacetylene monomers present numerous problems that prevent the wide spread application of these in functional materials. We report a redesigned four-step synthesis that proceeds in 75–80% overall yields and allows gram scale production of an array of thiol terminated conjugated diacetylenes, thereby allowing examination and application of these low-dimensional conductive materials.