Showing papers in "Langmuir in 1999"

Ultrahydrophobic and Ultralyophobic Surfaces: Some Comments and Examples

Abstract: The preparation of ultrahydrophobic and ultralyophobic surfaces using several techniques is described. Plasma polymerization of 2,2,3,3,4,4,4-heptafluorobutyl acrylate on poly(ethylene terephthalate) yields surfaces with water contact angles of θA/θR = 174°/173°. Argon plasma etching of polypropylene in the presence of poly(tetrafluoroethylene) renders surfaces with water contact angles as high as θA/θR = 172°/169°. Surfaces of compressed pellets of submicrometer, variable-diameter spherical particles of PTFE oligomers exhibit water contact angles of θA/θR = 177°/177°, methylene iodide contact angles of θA/θR = 140°/138°, and hexadecane contact angles of θA/θR = 140°/125°. We emphasize that contact angle hysteresis is more important in characterizing lyophobicity than is the maximum achievable contact angle. These surfaces are rough at the micrometer and submicrometer scales, and water drops roll easily on all of them. We make an intuitive argument that the topology of the roughness is important and contr...

The Lowest Surface Free Energy Based on −CF3 Alignment

Abstract: Free energy was measured for the surface of regular aligned closest hexagonal packed −CF3 groups. n-Perfluoroeicosane was vapor deposited onto glass, which gave epitaxially grown single-like crystallites with their molecular axes perpendicular to the glass surface. The dynamic contact angle of water on its surface was 119°, which corresponds to a surface free energy of 6.7 mJ/m2. This value is considered to be the lowest surface free energy of any solid, based on the hexagonal closed alignment of −CF3 groups on the surface.

Dynamics of Place-Exchange Reactions on Monolayer-Protected Gold Cluster Molecules

Abstract: Monolayer-protected gold clusters (Au MPCs) are stable, easily synthesized, organic solvent-soluble, nanoscale materials MPCs with protecting monolayers composed of alkanethiolate ligands (RS) can be functionalized (R‘S) by ligand place-exchange reactions, ie, x(R‘SH) + (RS)mMPC → x(RSH) + (R‘S)m(RS)m-xMPC, where x is the number of ligands place-exchanged (1 to 108) and m is the original number (ca 108) of alkanethiolate ligands per Au314 cluster The dynamics and mechanism of this reaction were probed by determining its kinetic order and final equilibrium position relative to incoming (R‘S) and initial (RS) protecting thiolate ligands The reactions were characterized by 1H NMR and IR spectroscopy, and the dispersity of place-exchange reaction products was preliminarily inspected by chromatography The results of these experiments show that ligand exchange is an associative reaction and that the displaced thiolate becomes a thiol solution product Disulfides and oxidized sulfur species are not involv

The Shape Transition of Gold Nanorods

Abstract: We report a revised synthetic procedure based on an electrochemical method for preparing an aqueous solution containing suspended Au nanorods. The mean aspect ratios of the Au nanorods can be experimentally adjusted between 1 and 7. The evolution of the longitudinal surface plasmon bands shows an eminently sensitive dependence on the aspect ratios of the nanorods. Their dependence is accordingly described by classical-electrostatic-model predictions. The shape transition of the nanorod particles has been studied by varying some key influencing factors such as the wavelength, the laser fluence, and matrix effects. The nanorods were exposed to laser lines at 532 and 1064 nm, frequencies which correspond closely to the short- and long-axis plasmon resonances, respectively. A photon-induced shape transition process was evidenced, and the corresponding rod-to-sphere conversion contributed by a photoannealing process was observed in both cases. Meanwhile, we observed a new type of “φ-shaped” Au nanostructure in...

Standard Nitrogen Adsorption Data for Characterization of Nanoporous Silicas

Abstract: The nitrogen adsorption isotherm measured at 77 K in the relative pressure range from about 5 × 10-7 to 0.988 for a macroporous silica is reported in a tabular form. The isotherm data are in good agreement in the high-pressure range with the reference data published by Gregg and Sing and by Kiselev and Aristov and provide an extension of the latter data to the low-pressure region, for which a reliable reference isotherm was not availalable in the literature. Application of the currently reported reference adsorption isotherm is discussed. The data can be used in comparative analysis (αs-plot or t-plot) to evaluate the external surface area, micropore volume, primary mesopore volume, and total surface area as well as to analyze surface properties of silica-based materials. The adsorption isotherm can also be applied as a statistical film thickness curve in the calculation of the mesopore size distribution.

Bilayer Surfactant Stabilized Magnetic Fluids: Synthesis and Interactions at Interfaces

Abstract: Aqueous magnetic fluids were synthesized by a sequential process involving the chemical coprecipitation of Fe(II) and Fe(III) salts with ammonium hydroxide (NH4OH) followed by resuspension of the ultrafine particles in water using fatty acids. This procedure produced Fe3O4 nanoparticles stabilized against agglomeration by bilayers of n-alkanoic acids with 9−13 carbons encapsulating the metal particles. The magnetic properties and particle size and size distributions of these magnetic fluids, characterized by transmission electron microscopy and superconducting quantum interference device, indicated the formation of single-domain nanoparticles of mean diameter ∼9.3 and ∼7.5 nm, respectively; the difference in values determined by the two methods implies the presence of a nonmagnetic layer on the particle surface. Thermogravimetric analysis measurements showed the existence of two distinct populations of surfactants on the particle surface, each having surfactant coverage of ∼21−24 A2/molecule, that was con...

Formation and Stabilization of Silver Nanoparticles through Reduction by N,N-Dimethylformamide

Abstract: The reduction of Ag+ ions in N,N-dimethylformamide (DMF) is shown to take place spontaneously at room temperature. When no other additives are present in the system, the slow reduction leads to silver deposition on the glass walls of the container, while in the presence of 3-aminopropyltrimethoxysilane (APS) stable dispersions of silver nanoparticles are obtained. Even though the reduction can be performed at room temperature, higher temperatures markedly increase the reaction rate and improve the monodispersity of the colloid. The reduction rate and morphology of the colloidal particles also depend on the ratio [Ag]/[APS]. The reduction takes place both with nitrate and perchlorate as counterions, which confirms that DMF plays the role of a reducing agent. Additionally, when excess APS is used, a thin, homogeneous silica shell is formed on the silver particle surface.

Role of hydrous ruthenium oxide in Pt-Ru direct methanol fuel cell anode electrocatalysts: The importance of mixed electron/proton conductivity

Abstract: Pt−Ru is the favored anode catalyst for the oxidation of methanol in direct methanol fuel cells (DMFCs). The nanoscale Pt−Ru blacks are accepted to be bimetallic alloys as based on their X-ray diffraction patterns. Our bulk and surface analyses show that although practical Pt−Ru blacks have diffraction patterns consistent with an alloy assignment, they are primarily a mix of Pt metal and Ru oxides plus some Pt oxides and only small amounts of Ru metal. Thermogravimetric analysis and X-ray photoelectron spectroscopy of as-received Pt−Ru electrocatalysts indicate that DMFC materials contain substantial amounts of hydrous ruthenium oxide (RuOxHy). A potential misidentification of nanoscale Pt−Ru blacks arises because RuOxHy is amorphous and cannot be discerned by X-ray diffraction. Hydrous ruthenium oxide is a mixed proton and electron conductor and innately expresses Ru−OH speciation. These properties are of key importance in the mechanism of methanol oxidation, in particular, Ru−OH is a critical component ...

Reversible electrowetting and trapping of charge : model and experiments

Abstract: We derive a model for voltage-induced wetting, so-called electrowetting, from the principle of virtual displacement. Our model includes the possibility that charge is trapped in or on the wetted su...

In situ assembly of colloidal particles into miniaturized biosensors

Abstract: We show how to create arrays of biosensors by in situ assembly of colloidal particles onto micropatterned electrodes. Latex microspheres from suspension are collected via dielectrophoresis in the micrometer-sized gaps between planar electrodes. The assembled particulate patches are fixed by changing the colloidal interactions to induce coagulation. Immuno-active sites on the latex surfaces bind the target molecules. A direct electric conductivity readout is accomplished after secondary tagging with colloidal gold and its enhancement by silver nucleation. The method holds promise for creating disposable on-chip arrays of highly sensitive miniature sensors for specific proteins, DNA fragments, or other biomolecules.

Evaluating Pore Sizes in Mesoporous Materials: A Simplified Standard Adsorption Method and a Simplified Broekhoff−de Boer Method

Abstract: The diameters of primary mesopores in materials with well-defined cylindrical pores can be accurately determined by the 4V/A method when the volume and surface area are determined by a standard adsorption method and 13.5 A2 is used as the molecular area of adsorbed nitrogen. A simple method for determining standard adsorption using the statistical thickness of the adsorbed gas layer defined by Frenkel−Halsey−Hill (FHH) theory is described. For materials in which the pores are not cylindrical or well-defined, a simplified Broekhoff−de Boer method can be used to determine pore dimensions. The use of Hill's approximation for the thickness of the adsorbed gas layer in the Broekhoff−de Boer method simplifies its use. The results of these methods on small pore MCM-41 materials, large pore SBA-15 materials, and spherical pore mesocellular foams are reported.

Thin Polymer Layers Formed by Polyelectrolyte Multilayer Techniques on Biological Surfaces

Abstract: Thin polymer films were formed on models of tissue surfaces using polyelectrolyte multilayer techniques, to evaluate the feasibility of using such techniques to build barrier materials onto the surfaces of tissues to improve postsurgical healing, or on the surfaces of tissue-engineered implants. By incubating heterogeneous surfaces with a polycation, followed by a polyanion, layers of polyelectrolyte were deposited onto the surfaces, as confirmed by ellipsometry and water contact angle measurement. Particularly favorable properties were found using the polyelectrolytes polylysine and alginate, which are capable of forming complex gels at physiologic pH; whereas others have demonstrated linear growth in film thickness, with this system, exponential growth was observed under certain conditions, which may be very useful in the coating of heterogeneous surfaces. Surfaces that were treated with multilayer techniques included gelatin, fibroblast extracellular matrix, and fibrillar type I collagen. All surfaces ...

Polymer Brushes that Resist Adsorption of Model Proteins: Design Parameters

Abstract: The adsorption of model proteins onto brush-coated surfaces can occur via two modes. Primary adsorption at the surface, where short range attraction is dominant, is important for small proteins and may be repressed by increasing the grafting density. Secondary adsorption, due to van der Waals attraction, occurs at the outer edge of the brush. Large rodlike proteins are likely to adsorb in this fashion. This mode may be repressed by increasing the brush thickness. The thermodynamics and kinetics of adsorption of model proteins are considered within a simple analytical theory distinguishing between the different adsorption mechanisms of small and big proteins.

Pore size distributions in porous glasses : A computer simulation study

Abstract: We have prepared a series of molecular models of porous glass using a recently developed procedure (Gelb, L. D.; Gubbins, K. E. Langmuir 1998, 14, 2097) that mimics the experimental processes that produce Vycor and controlled-pore glasses. We calculate nitrogen adsorption isotherms in these precisely characterized model glasses using Monte Carlo simulations. These isotherms are analyzed using the Barrett−Joyner−Halenda (BJH) method to yield pore size distributions, which are tested against exact pore size distributions directly measured from the pore structures. The BJH method yields overly sharp distributions that are systematically shifted (by about 1 nm) to lower pore sizes than those from our geometric method.

Metal-Nanocluster-Filled Carbon Nanotubes: Catalytic Properties and Possible Applications in Electrochemical Energy Storage and Production

Abstract: Ensembles of highly aligned and monodisperse graphitic carbon nanotubules have been prepared via the template method using chemical vapor deposition of carbon within the pores of alumina membranes. Tubules with diameters of 200 nm have been prepared, and smaller diameters are possible. Free-standing aligned carbon-tubule membranes are formed by this template method. These novel carbon tubule membranes can be filled with nanoparticles of electrocatalytic materials (i.e., Pt, Ru, Pt/Ru), which can then be used to electrocatalyze O2 reduction and methanol oxidation as well as the gas-phase catalysis of hydrocarbons. Hence, these membranes have potential applications in fuel cell development. Smaller, highly ordered graphitic-carbon tubules can also be prepared within the template-synthesized carbon tubules, using Fe nanoparticles as catalysts. In these novel tube-in-tube structures, both the outer and the inner tubules are electrochemically active for Li+ intercalation, suggesting possible applications such ...

Water-Soluble, Isolable Gold Clusters Protected by Tiopronin and Coenzyme A Monolayers

Abstract: Isolable, water-soluble gold clusters protected by monolayers of tiopronin (tiopronin−MPCs) or coenzyme A (CoA−MPCs) were synthesized by a procedure of comparable simplicity to the Brust synthesis for alkanethiolate monolayer-protected gold clusters. High-resolution transmission electron microscopy shows that, like their alkanethiolate−MPC counterparts, the average core diameters of tiopronin−MPCs can be systematically controlled by varying the tiopronin:Au mole ratio employed in the reaction. The UV−vis spectra of tiopronin−MPCs exhibit pH and core size dependency of the surface plasmon band position and intensity, respectively. Thermogravimetric analysis of the tiopronin−MPCs gave average numbers of tiopronin ligands per cluster; for example, tiopronin−MPCs with an average core size of 1.8 nm (∼Au201) are protected with an average of 85 tiopronin ligands. 1H NMR reveals a size-dependent evolution of spectral features interpreted as reflecting differences in attachment sites (terrace, defects) and/or res...

Adhesion Forces between E. coli Bacteria and Biomaterial Surfaces

Abstract: Bacterial infection of biomaterial surfaces is an important problem in the biomedical and health industries. The design of materials resistant to infections necessitates an understanding of the forces driving bacterial adhesion. Escherichia coli cells were immobilized onto the tip of a standard atomic force microscope (AFM) cantilever, and force measurements were performed by approaching the modified cantilever onto mica, hydrophilic glass, hydrophobic glass, polystyrene, and Teflon. Consistent with prior qualitative observations, we show that bacterial adhesion is indeed enhanced by the surface hydrophobicity of the substrate. The forces of interaction measured with the AFM are compared to those of model predictions based on an extended-DLVO approach. In this model, short-range acid−base and steric interactions are included with the conventional van der Waals attraction and electrostatic components. The theoretical predictions agree well with experimental data for E. coli D21f2, a strain whose outer surf...

Phosphate Adsorption onto TiO2 from Aqueous Solutions: An in Situ Internal Reflection Infrared Spectroscopic Study

Abstract: The adsorption of phosphate ions from aqueous solution onto thin films of colloidal TiO2 has been studied for the first time by in situ internal reflection infrared spectroscopy. Phosphate binds strongly to TiO2, as evidenced by the large changes in the PO stretching band structure in the infrared spectrum of the adsorbed species compared with the solution species. The Langmuir binding constant for phosphate onto TiO2 at pH = 2.3 is (3.8 ± 0.8) × 104 dm3 mol-1, which is similar to the binding constants onto TiO2 for bidentate ligand species such as oxalate and catechol. The strength of the binding is also apparent in the kinetics of adsorption, showing fast adsorption and much slower desorption, as expected for a strongly bound species. The kinetics data at pH = 8.3 have indicated coverage-dependent phosphate adsorption and desorption. Experiments with substituted phosphate species have confirmed the bidentate binding of phosphate to Ti(IV) ions at the TiO2 surface.

Correlation of Relative X-ray Photoelectron Spectroscopy Shake-up Intensity with CuO Particle Size

Abstract: X-ray photoelectron spectroscopy (XPS) analysis of four Cu/SiO{sub 2} catalyst systems of different particle sizes of CuO on the surface showed variation in the relative peak area intensities of the shake-up lines to main core levels of the Cu 2p orbitals. These differences were attributed to various degrees of XPS-induced reduction of CuO initially formed on the surface by spin coating copper(II) acetate {l_brace}Cu-(CH{sub 3}CO{sub 2}){sub 2}{center_dot}H{sub 2}O, Cu(ac){sub 2}{r_brace} solutions of varying concentration. Changes in CuL{sub 3}M{sub 4,5}M{sub 4,5}X-ray excited Auger (XAES) line shapes under time-dependent exposure to the soft Mg K{alpha} X-rays revealed that smaller particle sizes were more susceptible to reduction to Cu(+1) than larger ones. The degree of reduction of Cu(+2) to Cu(+1) correlated with measured atomic force microscopic (AFM) particle heights of CuO on these substrates prior to XPS.

Preparation and Properties of Magnetite and Polymer Magnetite Nanoparticles

Abstract: A novel approach to prepare magnetic polymeric nanoparticles by synthesis of the magnetite core and polymeric shell in a single inverse microemulsion is reported. Stable magnetic nanoparticles colloid dispersion with narrow size distribution can thus be produced. The microemulsion seed copolymerization of methacrylic acid, hydroxyethyl methacrylate, and cross-linker results in a stable hydrophilic polymeric shell of the nanoparticles. The preparation of the nanoparticles was carried out also by the two-stage microemulsion process and the seed precipitation polymerization. The particle size was varied in the range 80−320 nm by changing of the monomer concentration and water/surfactant ratio. The magnetic properties and the size distribution of the nanoparticles synthesized by these three methods were compared. The polymeric nanoparticles synthesized in single microemulsion have superparamagnetic properties and the narrowest size distribution.

Synthesis and Characterization of Carboxylate-Modified Gold Nanoparticle Powders Dispersible in Water

Abstract: Carboxylate-modified gold nanoparticles have been synthesized in a single-phase system based on the reduction of hydrogen tetrachloroaurate(III) by sodium borohydride in methanol using mercaptosuccinic acid (MSA) as the stabilizing thiol ligand. Five samples with diameters of 10.2, 10.8, 12.8, 19.4, and 33.6 A have been prepared as water-redispersible powders through decreasing the initial MSA/HAuCl4 molar ratio from 2.5 to 0.5. These samples were characterized by X-ray diffraction, transmission electron microscopy, elemental analysis, thermogravimetric analysis, ζ-potential measurement, Fourier transform infrared spectroscopy, and UV−vis spectroscopy. The results show that a large number of the particles are fcc single crystals with the polyhedral morphology of a truncated octahedral motif and that a self-assembled monolayer of thiolates has indeed formed through the adsorption of mercapto groups on the gold particle surface; the maximum packing density of the thiolates is 15.23 A2 per mercapto group. Th...

Dynamic wetting studied by molecular modeling simulations of droplet spreading

Abstract: The spontaneous spreading of liquid droplets is studied by molecular modeling simulations. By considering very large systems, we are able to study the influence of the solid−liquid interactions on the overall wetting dynamics. It is shown how microscopic parameters, such as the mobility of the molecules near the solid−liquid interface, can be measured. At the same time, relevant macroscopic parameters, such as dynamic contact angles and flow fields, are calculated from the simulations. The results strongly support the microscopic validity of the molecular-kinetic model of wetting.

Trialkylsilane Monolayers Covalently Attached to Silicon Surfaces: Wettability Studies Indicating that Molecular Topography Contributes to Contact Angle Hysteresis

Abstract: Chemically grafted monolayers of trialkylsilanes were prepared by reaction of (primarily) alkyldimethylchlorosilanes with silicon wafers under three conditions: in the vapor phase at elevated temperature (60−70 °C), in toluene in the presence of ethyldiisopropylamine (EDIPA) at room temperature, in toluene/EDIPA at 60−70 °C. It was determined that reactions at the solution−solid interface are very slow in the later stages of the reaction and that long reaction times are necessary to achieve maximum bonding density. The bonding density is determined and can be controlled by the reaction conditions. The highest carbon content on the surface (assessed by X-ray photoelectron spectroscopy) as well as the highest contact angles were obtained using vapor phase reactions. A series of nine H(CH2)nMe2Si− surfaces was prepared with n = 1, 2, 3, 4, 8, 10, 12, 18, and 22. Water contact angles (θA/θR = ∼105°/∼94°) are independent of chain length, indicating that these surfaces project disordered methyl groups toward t...

Supported Hybrid Bilayer Membranes as Rugged Cell Membrane Mimics

Abstract: Supported planar lipid bilayers based on alkanethiol-tethering chemistry are becoming increasingly important biomimetic materials Hybrid bilayers containing thiol-derivatized alkane moieties plus natural lipids provide a biomimetic matrix that permits the successful reconstitution of membrane protein activity The hybrid membrane is self-assembled and sufficiently rugged to be of practical use in research and in commercial sensing applications The coupling of the bilayer to a metal support allows a wide range of analytical techniques to be applied to this model membrane This article reviews some of the fundamental studies of the formation, structure, and response of hybrid bilayer membranes

Improvement of the Electrosynthesis and Physicochemical Properties of Poly(3,4-ethylenedioxythiophene) Using a Sodium Dodecyl Sulfate Micellar Aqueous Medium

Abstract: Electrosynthesis of poly(3,4-ethylenedioxythiophene) (PEDOT) films was performed in a micellar aqueous solution containing sodium dodecyl sulfate (SDS) by cyclic voltammetry, chronoamperometry, and chronopotentiometry on a platinum electrode. The electrocatalytic effect of SDS was characterized by a significant decrease of the EDOT oxidation potential (Ep) in the micellar medium relative to 0.1 M LiClO4 acetonitrile as well as aqueous solutions. Linear variation of Ep with SDS concentration indicated the formation of a pseudocomplex (Keq = 5.4 × 103 M-1). PEDOT films were characterized electrochemically and spectroscopically (UV−visible, X-ray photoelectron spectroscopy, IR, Raman spectra). Regular, well-ordered, and adherent films were obtained in SDS medium. The PEDOT film morphologies investigated by atomic force microscopy suggested the possible presence of columnar structures when the electrosynthesis is performed in the micellar medium.

An Interfacial Stress Rheometer To Study Rheological Transitions in Monolayers at the Air-Water Interface

Abstract: An interfacial stress rheometer has been constructed to study the rheology of Langmuir films subjected to time-dependent flows A magnetized rod resides at the air−water interface and is set into oscillation by applying a sinusoidal magnetic field gradient Analysis of the amplitude and phase of the resulting rod motion relative to the applied force allows the determination of the dynamic surface modulus, Gs*(ω), and measurement of the relative elastic and viscous contributions of the monolayer Measurements at 22 °C were conducted on eicosanol (C20) and mixtures of a rigid-rod polymer, phthalocyaninatopolysiloxane (PcPS), dispersed in eicosanol The surface pressure dependence of the rheology for eicosanol reveals the presence of a maximum in the loss modulus, Gs‘ ‘(ω), within the L2‘ phase at Π = 6 mN/m In the LSI phase at pressures above 15 mN/m, the monolayer is Newtonian and has a surface viscosity of 003 mN·s/m The mixtures of PcPS with eicosanol are known to have two-dimensional nematic behavior

Cation-Controlled Interfacial Charge Injection in Sensitized Nanocrystalline TiO2

Abstract: The photophysical and photoelectrochemical properties of Ru(deeb)(bpy)2(PF6)2, where bpy is 2,2‘-bipyridine and deeb is 4,4‘-(COOEt)2-2,2‘-bipyridine, anchored to nanocrystalline TiO2 (anatase) or ZrO2 films are reported. In neat acetonitrile (or 0.1 M tetrabutylammonium perchlorate) long-lived metal-to-ligand charge transfer (MLCT) excited states are observed on both TiO2 and ZrO2. Addition of LiClO4 results in a red shift in the MLCT absorption and photoluminescence, PL, spectra on both TiO2 and ZrO2, and a concentration-dependent quenching of the PL intensity on TiO2. The Li+-induced spectroscopic changes were found to be reversible by varying the electrolyte composition. Time-resolved absorption measurements demonstrate that the presence of lithium cations increases the quantum yield for interfacial charge separation with no discernible influence on the rate of charge recombination. A second-order kinetic model quantified charge recombination transients. A model is proposed wherein Li+ ion adsorption ...

Assembly of mesoscale particles over large areas and its application in fabricating tunable optical filters

Abstract: This paper describes a method for the crystallization of mesoscale particles over areas as large as ∼1 cm2. We injected an aqueous dispersion of spherical particles into a cell formed by two glass substrates and a square frame of photoresist that had been patterned on the surface of one of the substrates. One side of the frame had channels on its surface that could retain the particles while letting the solvent flow through. External gas pressure and sonication drove the particles into a cubic-close-packed (ccp) assembly with the (111) face parallel to the surfaces of the glass substrates. The smallest particles that have been crystallized by this method are polystyrene beads of ∼60 nm in diameter. The procedure presented here offers a number of attractive features: (i) It is relatively fast. For example, polystyrene beads of 0.48 μm in diameter can be crystallized into a 25 layer assembly over an area of ∼1 cm2 in ∼48 h. (ii) It has a tight control over the surface morphology and the number of layers of...

New Synthetic Routes to Alkyl Monolayers on the Si(111) Surface1

Abstract: Two new methods for the formation of Si−C monolayers from reactions with Si(111)−H are reported. Besides the photochemical method previously reported by Chidsey, Lewis acid-catalyzed hydrosilylation of alkenes and direct reaction of alkylmagnesium bromide produce a surface with similar chemical composition. These processes are demonstrated and compared using reactions of a C10 precursor. The surfaces are chemically stable and can be stored for several weeks without measurable deterioration. The availability of a variety of synthetic approaches leading to the same chemical product is key to the development of flexible surface synthetic strategies. It is expected that these approaches will underpin the development of stepwise solid-phase-like syntheses of more complex organic/bioorganic species on these surfaces.

Preparation and Optical Properties of Colloidal Gold Monolayers

Abstract: Colloidal gold adsorbed onto positively charged substrates forms a two-dimensional layer with a maximum coverage of 1 colloid/1000 nm2 as shown by optical absorption spectroscopy, small-angle X-ray scattering, and atomic force microscopy. Positively charged substrates are obtained by either chemisorption of end-group-functionalized silanes or by polycation adsorption. Bulk diffusion controls the gold adsorption; thus lower coverages can also be achieved. The optical spectra not only contain information on single colloids but also on their mutual interactions, as well as on the polymeric and solvent environments. Absorption spectra based on Mie and Maxwell−Garnett theory are calculated. The films remain stable on exchange of the external solvent. The local dielectric environment is shown to not only influence the position of the absorption band but also, more drastically, the intensity (by a factor of 2.5), in agreement with theoretical predictions (±15%). Particle aggregation induced by branched polycatio...