Silane Monolayers Research Articles

Grafting Control of Mainstay Terpyridine Self-Assembled Monolayers for the Preparation of Planar Silicon Surfaces with Variable Catalytic Loadings

Monolayers of terpyridine-derivatized silanes were self-assembled, with accurately controlled grafting densities, on single-crystal silicon surfaces. Complexation of the resulting terpyridine monolayers with Pd(OAc)(2) afforded a series of catalytic surfaces covering a full range of Pd loadings (0.14-0.85 nmol.cm(-2)) in the aim to explore their impact on catalysis methodically. X-ray reflectivity (XRR), X-ray photoelectron spectroscopy (XPS), and inductively coupled plasma mass spectrometry (ICP-MS) were combined to afford a precise picture of the grafting density, chemical composition, and catalyst loadings of the surfaces investigated here. We report that the control of the terpyridine density and thus the control of catalytic loadings can be achieved through a fine modification of silanization concentrations, which affords surfaces with tunable catalytic activity.

Polymer blend lithography: A versatile method to fabricate nanopatterned self-assembled monolayers

A rapid and cost-effective lithographic method, polymer blend lithography (PBL), is reported to produce patterned self-assembled monolayers (SAM) on solid substrates featuring two or three different chemical functionalities. For the pattern generation we use the phase separation of two immiscible polymers in a blend solution during a spin-coating process. By controlling the spin-coating parameters and conditions, including the ambient atmosphere (humidity), the molar mass of the polystyrene (PS) and poly(methyl methacrylate) (PMMA), and the mass ratio between the two polymers in the blend solution, the formation of a purely lateral morphology (PS islands standing on the substrate while isolated in the PMMA matrix) can be reproducibly induced. Either of the formed phases (PS or PMMA) can be selectively dissolved afterwards, and the remaining phase can be used as a lift-off mask for the formation of a nanopatterned functional silane monolayer. This “monolayer copy” of the polymer phase morphology has a topographic contrast of about 1.3 nm. A demonstration of tuning of the PS island diameter is given by changing the molar mass of PS. Moreover, polymer blend lithography can provide the possibility of fabricating a surface with three different chemical components: This is demonstrated by inducing breath figures (evaporated condensed entity) at higher humidity during the spin-coating process. Here we demonstrate the formation of a lateral pattern consisting of regions covered with 1H,1H,2H,2H-perfluorodecyltrichlorosilane (FDTS) and (3-aminopropyl)triethoxysilane (APTES), and at the same time featuring regions of bare SiOx. The patterning process could be applied even on meter-sized substrates with various functional SAM molecules, making this process suitable for the rapid preparation of quasi two-dimensional nanopatterned functional substrates, e.g., for the template-controlled growth of ZnO nanostructures [1].

Fabrication of dense, uniform aminosilane monolayers: A platform for protein or ligand immobilization

Designing biointerfaces with enhanced biomolecule binding abilities, along with their distribution and presentation on the surface, largely requires control over underlying surface chemistries. In an effort to correlate experimental parameters to surface properties, ligand distribution and biomolecule binding, the influence of (3-aminopropyl)triethoxysilane (APTES) concentration (1, 2 and 4vol%) and duration of the silanization reaction (5–60min) on the number of available NH2 groups were determined and correlated with the amount of surface bound human immunoglobulin G (HIgG). Surfaces silanized with 2vol% APTES for 30min yielded a densely populated silane monolayer (1.0–1.2nm) where the average molecular orientation was 38±2° with respect to the surface normal. A surface density of NH2 moieties of ∼1015/cm2 was obtained and was significantly higher compared to other conditions evaluated. The combined data from ellipsometry and atomic force microscopy (AFM) analyses supports our findings. Coupling of HIgG to surfaces silanized with 2vol% APTES for 30min yielded ∼1013IgG/cm2, which was significantly higher than values obtained at 1vol% APTES for 30min. The distribution of immobilized HIgG was noted to be dense and uniform on surfaces silanized with 2vol% APTES when visualized using Nanogold® antibody conjugates. This study highlights the critical role of the experimental parameters that impact the biomolecule immobilization process on functionalized surfaces.

The Behavior of Galvanized Steel Coated with a Double and Triple Layer of Hybrid Films

The aim of this work is to characterize by electrochemical, physicochemical and morphological analysis the hybrid films applied on galvanized steel by dip-coating process. Preliminaries studies showed that a monolayer of silane is not enough to increase the corrosion resistance protection of the substrate. In this context, the galvanized steel was coated with a double and triple layer of hybrid film obtained from a sol constituted by silane precursors 3 - (trimetoxisililpropil) methacrylate (TMSPMA) and tetraethoxysilane (TEOS) with addition of cerium nitrate at a concentration of 0.01M. The hybrid films were cured for 20 minutes at 60°C in each application of the film. The results showed that a double and triple layer of hybrid film increase the barrier effect contributing to corrosion protection of galvanized steel.

Preparation of polypyrrole films on insulating substrates by self-assembled monolayers

The objective of this study is to develop an effective method for the preparation of polypyrrole (PPy) films on insulating substrates modified by self-assembled silane monolayers (SAMs). The morphology of silane SAMs and PPy films was investigated using scanning electron microscopy (SEM). The SEM pattern of SAMs showed that silane coupling agent molecules were arranged uniformly, orderly and tightly on the surface of substrate. The as-prepared PPy films exhibited the compact and uniform morphology when the substrates were dipped in silane coupling agent for about 10min. The hydrolysis of silane and deposition process scheme of SAMs and PPy films were described. Chemical analysis of PPy films was performed by X-ray photoelectron spectroscopy (XPS). The studied PPy films showed potential applications in electromagnetic interference shielding and solid electrolytic capacitor.

Immobilization of Amine-modified Oligonucleotides on Bifunctional Polymer Brushes Synthesized by Surface-initiated Polymerization

Polymer brushes on solid substrates formed by surfaceinitiated polymerization can tailor the substrates' surface properties, allowing for example, the functionalization of glass surfaces for the construction of DNA microarrays used in the diagnosis of diseases and the study of genomics. Glass surfaces are often functionalized by the formation of self-assembled monolayers (SAMs) of silanes containing functional groups, such as amine, alcohol, or carboxylic acid, at their terminal ends. However, silanization is difficult to control for the formation of high-quality SAMs, resulting in poorly reproducible surface functionalization. Therefore polymer brushes, which can be grown as highdensity brushes in a controllable manner, have been studied for the surface functionalization of glass substrates. For example, polymer brushes based on polyethylene glycol methacrylate (PEGMA) have been used for surface coatings because they can limit the nonspecific adsorption of biomolecules onto the surfaces. Here, we report the synthesis of a bifunctional polymer brush by surface-initiated polymerization and its application in the immobilization of oligonucleotides. The polymer brush possessed two functional moieties, bio-inert polyethylene glycol (PEG) and bio-reactive aldehyde groups, which respectively provided effective resistance to biofouling and high binding ability for the specific immobilization of amine-modified oligonucleotides. The surface-initiated polymerization of the bifunctional polymer brush is outlined in Scheme 1. 2-Bromo-2-methylN-(3-triethoxysilylpropyl)-propionamide (BTPAm) was synthesized by a synthetic procedure reported previously (Supporting Information, Figure S1) and a SAM of BTPAm was formed by immersing a freshly cleaned and O2 plasmatreated glass substrate in a toluene solution of BTPAm (0.3 w/v %) with subsequent curing at different temperatures. The resulting SAM was confirmed by the appearance of peaks characteristic of NH-CO at 1650 cm, N-H bending at 1530 cm, and Si-O stretching at 1112-1026 cm in the ATR-FTIR spectrum of the BTPAm initiator film (Figure 1, spectrum b) compared with the spectrum of bare surface (Figure 1, spectrum a). The surface-tethered BTPAm presented a terminal bromoisobutyrate moiety, which allowed surface-initiated atom transfer radical polymerization (SIATRP) on the glass substrates. SI-ATRP can provide control over chain length and surface density of polymers under ambient conditions. The surface-initiated polymerization was then conducted in an oxygen-free environment using CuBr/Tris[2-(dimethylamino)ethyl]amine (Me6TREN) cata-

Surface chemistry at the nanometer scale influences insulin aggregation

We synthesized surfaces with different hydrophobicities and roughness by forming self-assembled monolayers (SAMs) of mixed amine and octyl silanes. Insulin aggregation kinetics in the presence of the above surfaces is characterized by a typical lag phase and growth rate. We show that the lag time but not the growth rate varies as a function of the amine fraction on the surface. The amount of adsorbed protein and the adsorption rate during the aggregation process also vary with the amine fraction on the surface and are maximal for equal parts of amine and octyl groups. For all surfaces, the growth phase starts for identical amounts of adsorbed insulin. The initial surface roughness determines the rate at which protein adsorption occurs and hence the time to accumulate enough protein to form aggregation nuclei. In addition, the surface chemistry and topography influence the morphology of aggregates adsorbed on the material surface and the secondary structures of final aggregates released in solution.

Electrochemical characterization of Self-assembled Monolayers (SAMs) of silanes on indium tin oxide (ITO) electrodes – Tuning electron transfer behaviour across electrode–electrolyte interface

In this work, we have systematically investigated the formation and characterization of Self-assembled Monolayer (SAM) films of several silanes on indium tin oxide (ITO) surfaces. Silane molecules having different domains namely substrate binding domain (siloxanes), electron transport region (aliphatic and aromatic spacer) and terminal functional groups (–SH, –CH3 groups) are employed for the study in order to tune the electron transfer (ET) behaviour across SAM modified electrode–electrolyte interface. Structural characterization of these monolayer films is carried out using X-ray photoelectron spectroscopy (XPS) studies. Wettability (hydrophilic and hydrophobic nature) of such modified electrodes is evaluated using contact angle measurements. ET behaviour of these modified electrodes is investigated by electrochemical techniques namely cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) using K4FeII(CN)6|K3FeIII(CN)6 redox couple as a probe. Disappearance of redox peaks in the CV measurements and formation of semicircle having a higher charge transfer resistance (Rct) values during EIS studies suggest that the resultant monolayer films are compact, highly ordered with very low defects and posses good blocking property with less pinholes. The heterogeneous ET rate constant (k) values are determined from EIS by fitting them to an appropriate equivalent circuit model. Based on our results, we comment on tuning the ET behaviour across the interface by a proper choice of spacer region.

Light-directed nanosynthesis: near-field optical approaches to integration of the top-down and bottom-up fabrication paradigms

The integration of top-down (lithographic) and bottom-up (synthetic chemical) methodologies remains a major goal in nanoscience. At larger length scales, light-directed chemical synthesis, first reported two decades ago, provides a model for this integration, by combining the spatial selectivity of photolithography with the synthetic utility of photochemistry. This review describes attempts to realise a similar integration at the nanoscale, by employing near-field optical probes to initiate selective chemical transformations in regions a few tens of nm in size. A combination of near-field exposure and an ultra-thin resist yields exceptional performance: in self-assembled monolayers, an ultimate resolution of 9 nm (ca. λ/30) has been achieved. A wide range of methodologies, based on monolayers of thiols, silanes and phosphonic acids, and thin films of nanoparticles and polymers, have been developed for use on metal and oxide surfaces, enabling the fabrication of metal nanowires, nanostructured polymers and nanopatterned oligonucleotides and proteins. Recently parallel lithography approaches have demonstrated the capacity to pattern macroscopic areas, and the ability to function under fluid, suggesting exciting possibilities for surface chemistry at the nanoscale.

High-Pressure Total Internal Reflection Fluorescence Apparatus

Important interfacial processes in disciplines ranging from medicine to the separations sciences occur over a wide range of pressures, temperatures, and time scales. In this paper we report a new high-pressure total internal reflection fluorescence (HP-TIRF) apparatus that allows rapid fluorescence measurements of sub-monolayers in contact with liquids and supercritical fluids between 293 K and 353 K and up to 250 bar with picosecond time resolution. We use the HP-TIRF system to study the in-plane rotational reorientation dynamics of the fluorescent probe BODIPY 494/503 (C(2v) symmetry) covalently attached to silica surfaces that have been silanized with n-propyltrimethoxysilane (C(3)-TMOS) or 3,3,3-trifluoropropyltrimethoxysilane (CF(3)-TMOS) when the interface is subjected to pure supercritical carbon dioxide (scCO(2)). The in-plane BODIPY 494/503 rotational reorientation dynamics are assessed by using the Debye-Stokes-Einstein expression. As the scCO(2) density increases the local microviscosity surrounding the tethered BODIPY 494/503 molecule decreases. The terminal group (CH(3) versus CF(3)) within the silane monolayer governs the onset and absolute magnitude of the observed viscosity changes. The results are explained in terms of the wellknown solubility of fluorine-containing species in scCO(2).

High-Resolution X-ray Photoelectron Spectroscopy of Mixed Silane Monolayers for DNA Attachment

The amine density of 3-aminopropyldimethylethoxysilane (APDMES) films on silica is controlled to determine its effect on DNA probe density and subsequent DNA hybridization. The amine density is tailored by controlling the surface reaction time of (1) APDMES, or (2) n-propyldimethylchlorosilane (PDMCS, which is not amine terminated) and then reacting it with APDMES to form a mixed monolayer. High-resolution X-ray photoelectron spectroscopy (XPS) is used to quantify silane surface coverage of both pure and mixed monolayers on silica; the XPS data demonstrate control of amine density in both pure APDMES and PDMCS/APDMES mixed monolayers. A linear correlation between the atomic concentration of N atoms from the amine and Si atoms from the APDMES in pure APDMES films allows us to calculate the PDMCS/APDMES ratio in the mixed monolayers. Fluorescence from attached DNA probes and from hybridized DNA decreases as the percentage of APDMES in the mixed monolayer is decreased by dilution with PDMCS.

Enhanced Sensing of Nonpolar Volatile Organic Compounds by Silicon Nanowire Field Effect Transistors

Silicon nanowire field effect transistors (Si NW FETs) are emerging as powerful sensors for direct detection of biological and chemical species. However, the low sensitivity of the Si NW FET sensors toward nonpolar volatile organic compounds (VOCs) is problematic for many applications. In this study, we show that modifying Si NW FETs with a silane monolayer having a low fraction of Si-O-Si bonds between the adjacent molecules greatly enhances the sensitivity toward nonpolar VOCs. This can be explained in terms of an indirect sensor-VOC interaction, whereby the nonpolar VOC molecules induce conformational changes in the organic monolayer, affecting (i) the dielectric constant and/or effective dipole moment of the organic monolayer and/or (ii) the density of charged surface states at the SiO(2)/monolayer interface. In contrast, polar VOCs are sensed directly via VOC-induced changes in the Si NW charge carriers, most probably due to electrostatic interaction between the Si NW and polar VOCs. A semiempirical model for the VOC-induced conductivity changes in the Si NW FETs is presented and discussed.

Structural Characterization of Individual Vesicles using Fluorescence Microscopy

Extrusion of hydrated lipid suspensions is frequently employed to produce vesicles of uniform size, and the resulting vesicles are often reported to be unilamellar. We describe a method for the quantitative fluorescence image analysis of individual vesicles to obtain information on the size, lamellarity, and structure of vesicles produced by extrusion. In contrast to methods for bulk analysis, fluorescence microscopy provides information about individual vesicles, rather than averaged results, and heterogeneities in vesicle populations can be characterized. Phosphatidylcholine vesicles containing small fractions of biotin-modified phospholipid and fluorescently labeled 7-nitro-2,1,3-benzoxadiazol-4-yl (NBD) phospholipid were immobilized through biotin-avidin-biotin binding to the surface of a biotin-modified glass coverslip. Biotin was attached to the surface in a mixed cyano-terminated silane monolayer. Initial fluorescence intensities for each immobilized vesicle were recorded, and a solution of membrane impermeable quencher was passed through the flow cell to quench the fluorescence of the outer layer. Fluorescence from individual vesicles was measured by fitting the spots to 2-dimensional Gaussian functions. The integrated signals under the peaks yielded a pre- and postquench intensity. From the fractional loss of intensity, the number and structure of the bilayers in individual vesicles could be quantified; the results showed that extruded vesicles exhibit a distribution of size, lamellarity, and structure.

Tribological Durability of Silane Monolayers on Silicon

We report the frictional performance and long-term tribological stability of various alkyl silane monolayer films on silicon by using pin-on-disk tribometry at ambient conditions. We show that the durability of monolayers derived from n-alkyltrichlorosilanes on silicon increases exponentially with the chain length of the silane precursor, which we relate to the cohesive energy of these monolayers through molecular dynamics simulations. X-ray photoelectron spectroscopy (XPS) was used to show that tribological damage consisted of the loss of molecular components that could be partially replaced upon exposure to a solution containing perfluorinated silane precursors. For monolayers derived from n-octadecyltrichlorosilane, a critical load was identified to be approximately 250 mN (200 MPa), above which failure of films occurred within 100 cycles of testing. Monolayers with hydroxyl surfaces exhibited reduced stabilities due to stronger tip-surface interactions. Monolayers with the capability for cross-linking exhibited much greater stabilities than monolayers where cross-linking was limited or prevented. Collectively, these results demonstrate that the mechanical durability of monolayers when subjected to a tribological load is greatly enhanced by maximizing dispersional interactions and cross-linking and minimizing tip-surface interactions.

Nanostructured Polymer Brushes by UV‐Assisted Imprint Lithography and Surface‐Initiated Polymerization for Biological Functions

AbstractFunctional polymer brush nanostructures are obtained by combining step‐and‐flash imprint lithography (SFIL) with controlled, surface‐initiated polymerization (CSIP). Patterning is achieved at length scales such that the smallest elements have dimensions in the sub‐100 nm range. The patterns exhibit different shapes, including lines and pillars, over large surface areas. The platforms obtained are used to selectively immobilize functional biomacromolecules. Acrylate‐based polymer resist films patterned by SFIL are first used for the selective immobilization of ATRP silane‐based initiators, which are coupled to unprotected domains of silicon substrates. These selectively deposited initiators are then utilized in the controlled radical SIP of poly(ethylene glycol)methacrylates (PEGMA). Nanostructured brush surfaces are then obtained by removal of the resist material. The areas previously protected by the SFIL resist are passivated by inert, PEG‐based silane monolayers following resist removal. PEGMA brush nanostructures are finally functionalized with biotin units in order to provide selective attachment points for streptavidin proteins. Atomic force microscopy and fluorescence spectroscopy confirm the successful immobilization of streptavidin molecules on the polymer grafts. Finally, it is demontrated that this fabrication method allows the immobilization of a few tens of protein chains attached selectively to brush nanostructures, which are surrounded by nonfouling PEG‐functionalized areas.

Site-selective metal-coordination-based patterning of silane monolayers

Herein, we report a method to pattern a derivatized nifedipine silane monolayer that aromatizes under UV irradiation. Using a functionalized SCS-Pd(II) pincer complex, we demonstrate that a strong metal-coordination complex between the aromatized nifedipine derivative and the pincer complex is formed. This methodology integrates SAM photolithography (top-down) and molecular recognition directed self-assembly (bottom-up) strategies to create simple and rapid synthesizable functional patterned surfaces.

Analysis of the interaction of vinyl and carbonyl silanes with carbon nanofiber surfaces

Carbon nanofibers (CNFs) have been treated with vinyltryethoxy silane (VTS) and 3-metacryloxypropyltrimethoxy silane (MPS). CNF–silane interactions have been analyzed by means of TGA, FTIR-ATR, TEM, HRTEM, SEM and nitrogen adsorption. For similar silane concentration solutions TG analysis has shown that VTS and MPS form one and three silane monolayers, respectively. This has also been corroborated by the presence of an FTIR-ATR band at 1250 cm −1 assigned to Si–O–Si bonds of silica layers. For low silane concentrations, the vinyl group of VTS is bonded to the graphene CNF surface mainly through π–π interactions. However, MPS interacts through the carbonyl group with hydroxyl groups of graphene defect sheets existing probably in micropores. Silanol–CNF hydroxyl interactions are also expected at these silane concentrations. For high silane concentration, when the silica layer is formed, both silanes present vinyl free and carbonyl free groups, as observed by the 1370 and 1686 cm −1 FTIR-ATR bands, respectively. Nitrogen adsorption has shown that while VTS is adsorbed mainly on the defect free graphene surface, MPS is adsorbed on the micropores and, therefore on the hydroxyl defect graphene sites. These results are finally correlated with the dispersion stability of both silanes on water and styrene solutions.

Protein microarray assay for the screening of SH3 domain interactions

Analysis of cellular signal transduction processes increasingly focuses on the systematic characterization of complete protein interaction networks. Understanding the interplay of signaling components enables insight into the molecular basis of diverse diseases such as cancer. This paves the way for the rational design of specific therapeutics. Protein interactions are often mediated by conserved modular domains, e.g., SH3-domains, which recognize proline-rich sequences in their cognate ligands. In the course of this study, different microarray formats (reactive silane monolayers and nitrocellulose on glass slides) and assay work flows were evaluated to develop a microarray based screening assay that permits the reliable identification of interactions between certain target proteins with a set of SH3 domains. Nine representative SH3 domains which were produced and purified as GST-fusion proteins were spotted on the microarray substrates and probed with two well-characterized ligands, the Nef protein from HIV-1 and the human protein Sam68. The best results from these low-density model arrays were obtained with nitrocellulose slides. We show that a straightforward and highly robust detection of ligand binding is achieved by staining with a fluorescently labeled antibody directed against the N-terminal His-tag attached to these proteins. The optimized assay protocol reported here allows for the identification of SH3-interactions with high reproducibility and adequate signal-to-background and signal-to-noise ratios, as well as the quantitative determination of relative binding affinities.

Multi-purpose optical biosensors for real-time detection of bacteria, viruses and toxins

A universal optical microchip sensing platform demonstrating real-time, label-free detection of a wide range of biological agents is presented. SpectroSens™ chips containing high-precision planar Bragg gratings are exploited as low-cost, robust refractive index sensors. Sensitivity to biological agents is conferred by functionalising the sensing surface with antibodies selected against targets of interest. Several methods for immobilisation of bio-molecules on the metal oxide-coated sensing surface have been investigated. In this study, surfaces were modified with an amino-terminated silane monolayer and activated by glutaraldehyde cross-linking for covalent attachment of recombinant Protein A/G, to which agent-specific antibodies were immobilised. Binding of target antigens (introduced under flow) to the surface-immobilised antibodies results in localised changes in refractive index; upon laser-induced interrogation of the sensing region via optical fibres, these antibody–antigen interactions manifest as increases in wavelength of light reflected from the Bragg grating. Detection of biological targets including proteins (ovalbumin < 10 nm), viruses (MS2 < 100 nm), bacterial cells ( Escherichia coli > 1 μm) and spores ( Bacillus atrophaeus > 1 μm) in real-time has been demonstrated. The large size range of detection targets is attributed to a large penetration depth of the sensing light of >1 μm into the sample liquid using these sensors. This multi-analyte capability enables utilisation of this sensing technology in applications ranging from bio-threat detection for defence and homeland security to point-of-care clinical diagnostics.

Ion-scattering analysis of self-assembled monolayers of silanes on organic semiconductors

We describe new Rutherford Backscattering Spectroscopy (RBS) measurements to explore the surface chemistry associated with the growth of self-assembled monolayer (SAM) molecules on conducting organic films. The report includes a description of the optimization of both substrates and RBS scattering parameters to appropriately analyze these novel and damage susceptible structures. Our RBS measurements reveal that the final surface stoichiometry is consistent with a specific model of hydrolyzed and crosslinked trichlorosilanes that form a dense two-dimensional network (a monolayer) at the surface of small-molecule organic semiconductors and a bulk SAM network in the case of conjugated polymer films. Organic semiconductors used in this study are thin films of rubrene (a small molecule semiconductor (C42H28)) and poly(3-hexyl)thiophene (P3HT) (a conjugated polymer (C10H18S)n). As a substrate we used a thick (1 μm) film of parylene (a non-conjugated polymer (C8H8)n) deposited on Si (1 0 0) wafers. The SAM molecules used to functionalize the organic semiconductor films are fluoroalkyl trichlorosilane (FTS) (C8(H4F13)SiCl3) and octyltrichlorosilane (OTS) (C8H17SiCl3). Quantitative detection of medium and small-mass elements, such as O, F, Si, S and residual Cl is demonstrated and used to elucidate the surface chemistry in these novel organic systems.