A small angle neutron scattering study of the interface between solids and oil-continuous emulsions and oil-based microemulsions

We have measured the small angle neutron scattering (SANS) from slurries of powder in contact with surfactant solutions and emulsions to determine the fluid/solid interfacial structure. The slurry solids consisted either of graphite or pyrites particles; and the fluids were hexadecane containing the robust commercial polyisobutylenesuccinamide (PIBSA) surfactant, or a high internal phase emulsion of aqueous ammonium nitrate in hexadecane stabilised by PIBSA. To resolve the interfacial structure for both systems, combinations of deuterated and protonated materials were used.At low concentration in hexadecane, PIBSA forms a complete monolayer on graphite with a footprint per molecule of 103 Å² and a layer thickness of 19 Å. At higher concentrations, the complete monolayer of footprint is 61 Å² and 30 Å thick indicating compression of the PIBSA chain coil structure. Geometric exclusion effects caused by the stacking of the graphite particles also results in an excess of oil for ca. 160 Å above the surfactant monolayer.For pyrites in contact with surfactant in hexadecane, the oxidised surface layer, while smooth at the oil interface, is diffuse and/or rough at the interface with the bulk sulphide below. There is again a complete monolayer of surfactant adsorbed at the oxide surface, in a relatively compressed state with a footprint of 70 Å², more tightly bound than on graphite. The excess of oil phase above the adsorbed surfactant monolayer is observed for samples with larger pyrites particle sizes but not for a sample with smaller particles. This suggests that the oil excess does arise from purely geometric solid particle packing, but that the local particle surface curvatures are significantly higher than the overall particle size would suggest.The scattering from the pyrites/emulsion interface was modelled by a 30 Å thick monolayer of surfactant coating an oxide surface with a molecular footprint of 123 Å². For the larger particle size samples, there is a 30 Å thick layer of oil above the pyrites particle surface before a bulk emulsion/pyrites mixture is reached.These results extend previous reflectometry experiments on the silicon/emulsion interface, indicating that for stable emulsions the structures are qualitatively similar for three dissimilar solid surfaces. They show that useful results on surfactant structure and emulsion layering at the solid/emulsion and other solid/fluid interfaces can be simply obtained by SANS on powder samples variously contrasted by deuteration. SANS can be applied to a much greater range of solid interfaces than reflectometry since large neutron-transparent single crystals are not required, although the variety of faces in a powdered material degrades the quality of the information.     

Formation of a polymer particle monolayer by continuous self-assembly from a colloidal solution

The preparation of two-dimensional monolayers of polymer particles over a large area was demonstrated via a facile solution process. Polymer microspheres were continuously self-assembled into a close-packed monolayer from a colloidal solution confined between two plates such that the top plate was carefully dragged at a constant velocity in the direction opposite that of the monolayer growth. In situ direct observation of the particle movement during the coating process confirmed that particle transport was directed toward the contact line of the solution meniscus by evaporation-induced convective flow. Sliding of the top plate apparently effectively counterbalanced the convective flow to provide the particles with a contact line for growth of a monolayer particle array. The influence of particle concentration, sliding speed of the top plate, and surface wettability of the bottom substrate were investigated and optimized. Monolayer particle arrays were successfully demonstrated as a template for the preparation of ZnO films with ordered hollow hemispherical structures. This approach is applicable to the fabrication of ordered structures of monodispersed particles composed of various materials over large areas.     

Self-assembly of two- and three-dimensional particle arrays by manipulating the hydrophobicity of silica nanospheres

The surface hydrophobicity of colloidal silica (SiO2) nanospheres is manipulated by a chemical graft of alkyl chains with silane coupling agents or by physical adsorption of a cationic surfactant. The surface-modified SiO2 spheres can be transferred from the aqueous phase to organic solvents and readily self-assemble at the water-air interface to form two-dimensional (2D) particle arrays. Closely packed particle monolayers are obtained by adjusting the hydrophilic/hydrophobic balance of the synthesized SiO2 spheres and may further be transferred onto solid substrates layer by layer to form three-dimensional (3D) ordered particle arrays with a hexagonal close-packed (hcp) crystalline structure. The 2D monolayer and 3D multilayer SiO2 films exhibit photonic crystal properties, which were determined by the UV-visible spectroscopic analysis in transmission mode. In the multilayer films, the Bragg diffraction maxima increased with an increase in thickness of the particle layers. The experimentally observed diffraction positions are in good agreement with those that were theoretically calculated.     

Probing Innovative Microfabricated Substrates for their Reproducible SERS Activity

New types of microfabricated surface‐enhanced Raman spectroscopy (SERS) active substrates produced by electron beam lithography and ion beam etching are introduced. In order to achieve large enhancement factors by using the lightning rod effect, we prepare arrays consisting of sharp‐edged nanostructures instead of the commonly used dots. Two experimental methods are used for fabrication: a one‐stage process, leading to gold nanostar arrays and a two‐stage process, leading to gold nanodiamond arrays. Our preparation process guarantees high reproducibility. The substrates contain a number of arrays for practical applications, each 200×200 μm² in size. To test the SERS activity of these nanostar and nanodiamond arrays, a monolayer of the dye crystal violet is used. Enhancement factors are estimated to be at least 130 for the nanodiamond and 310 for the nanostar arrays.     

Colloid Particle Adsorption on Partially Covered (Random) Surfaces

The random sequential adsorption (RSA) approach was used to model irreversible adsorption of colloid particles at surfaces precovered with smaller particles having the same sign of surface charge. Numerical simulations were performed to determine the initial flux of larger particles as a function of surface coverage of smaller particles θ(s) at various size ratios lambda=a(l)/a(s). These numerical results were described by an analytical formula derived from scaled particle theory. Simulations of the long-time adsorption kinetics of larger particles have also been performed. This allowed one to determine upon extrapolation the jamming coverage θ(l)(infinity) as a function of the lambda parameter at fixed smaller particle coverage θ(s). It was found that the jamming coverage θ(l)(infinity) was very sensitive to particle size ratios exceeding 4. Besides yielding θ(l)(infinity), the numerical simulations allowed one to determine the structure of large particle monolayers at the jamming state which deviated significantly from that observed for monodisperse systems. The theoretical predictions suggested that surface heterogeneity, e.g., the presence of smaller sized contaminants or smaller particles invisible under microscope, can be quantitatively characterized by studying larger colloid particle adsorption kinetics and structure of the monolayer. Copyright 2001 Academic Press.     

Fabrication of hydrophobic surfaces by coupling of Langmuir-Blodgett deposition and a self-assembled monolayer

A novel method coupling the Langmuir-Blodgett (LB) deposition of silica particles and the formation of a self-assembled monolayer (SAM) of alkylsilane is proposed for fabricating hydrophobic surfaces. The LB deposition and the SAM are supposed to confer the substrate surface roughness and low surface energy, respectively. By controlling the hydrophobic-hydrophilic balance of the silica particle surface through the adsorption of surfactant molecules, deposition of monolayers consisting of hexagonally close-packed arrays of particles on a glass substrate can then be successfully conducted in a Langmuir trough. LB particulate films with a particle layer number up to 5 were thereby prepared. A sintered and hydrophobically finished particulate film with roughness factor of 1.9 was finally fabricated by sintering and surface silanization. Effects of particle size and particle layer number on the wetting behavior of the particulate films were systematically studied by measuring static and dynamic water contact angles. The experimental results revealed that a static contact angle of about 130 degrees resulted from the particulate films regardless of the particle size and particle layer number. This is consistent with the predictions of both the Wenzel model and the Cassie and Baxter model in that roughness of a hydrophobic surface can increase its hydrophobicity and a switching of the dominant mode from Wenzel's to Cassie and Baxter's. In general, an advancing contact angle of about 150 degrees , a receding contact angle of about 110 degrees , and a contact angle hysteresis of about 40 degrees were exhibited by the particulate films fabricated.     

2D assembly of gold-PNIPAM core-shell nanocrystals

2D arrays of Au-PNIPAM core-shell nanocrystals were fabricated using convective deposition and spin-coating. The particle density and ordering were studied by AFM. Annealing at 700 °C removes the polymer shell, while retaining a monolayer of well-separated gold nanoparticles. The surface plasmon modes of the colloid monolayers could be measured by spectroscopic ellipsometry.     

Bimetallic gold–silver nanoplate array as a highly active SERS substrate for detection of streptavidin/biotin assemblies

The silver-modified gold nanoplate arrays as bimetallic surface-enhanced Raman scattering (SERS) substrates were optimized for the surface-enhanced Raman detection of streptavidin/biotin monolayer assemblies. The bimetallic gold–silver nanoplate arrays were fabricated by coating silver nanoparticles uniformly on the gold nanoplate arrays. Depending on silver nanoparticle coating, the localized surface plasmon resonance (LSPR) peak of the bimetallic gold–silver nanoplate arrays blue-shifted and broadened significantly. The common probe molecule, Niel Blue A sulfate (NBA) was used for testing the SERS activity of the bimetallic gold–silver nanoplate arrays. The SERS intensity increased with the silver nanoparticle coating, due to a large number of hot spots and nanoparticle interfaces. The platforms were tested against a monolayer of streptavidin functionalized over the bimetallic gold–silver nanoplate arrays showing that good quality spectra could be acquired with a short acquisition time. The supramolecular interaction between streptavidin (strep) and biotin showed subsequent modification of Raman spectra that implied a change of the secondary structure of the host biomolecule. And the detection concentration for biotin by this method was as low as 1.0 nM. The enhanced SERS performance of such bimetallic gold–silver nanoplate arrays could spur further interest in the integration of highly sensitive biosensors for rapid, nondestructive, and quantitative bioanalysis, particularly in microfluidics.     

Two-dimensional arrays of human C-reactive protein formed on a dense aqueoys subphase underneath a lipid layer

A recently developed protein spreading technique, using a subphase of higher density than that of the injected protein solution, was combined with the lipid monolayer approach for two-dimensional crystallization experiments of human C-reactive protein (hCRP) at the air-water interface. Densely packed two-dimensional arrays of hCRP adsorbed on a stearylamine/egg yolk phosphatidylcholine lipid layer have been obtained in the pH range from 5.4 to 5.8. Correct choices of the lipid mixture and of the pH of the subphase were important for the formation of large arrays. Image analysis of transmission electron micrographs showed that the densely packed arrays do not possess crystalline order. The orientational homogeneity of the protein molecules adsorbed on the lipid layer could be used, however, to facilitate single particle averaging techniques. At a resolution of about 1.5 nm a clear handedness of the five-fold symmetric molecule becomes visible, and an asymmetric mass distribution of each subunit is revealed.     

Size-controlled synthesis of near-monodisperse gold nanoparticles in the 1-4 nm range using polymeric stabilizers

We report here a simple one-step protocol for the preparation of near-monodisperse gold hydrosols in the small size regime (<5 nm). The particle size can be controlled by varying the concentration of the stabilizing polymer, which can be readily displaced by thiol ligands to yield monolayer protected clusters of the usual type.     

Molecularly-tunable nanoelectrode arrays created by harnessing intermolecular interactions

Intermolecular interactions play a critical role in the binding strength of molecular assemblies on surfaces. The ability to harness them enables molecularly-tunable interfacial structures and properties. Herein we report the tuning of the intermolecular interactions in monolayer assemblies derived from organothiols of different structures for the creation of nanoelectrode arrays or ensembles with effective mass transport by a molecular-level perforation strategy. The homo- and hetero-intermolecular interactions can be fully controlled, which is demonstrated not only by thermodynamic analysis of the fractional coverage but also by surface infrared reflection absorption and X-ray photoelectron spectroscopic characterizations. This understanding enables controllable electrochemical perforation for the creation of ensembles or arrays of channels across the monolayer thickness with molecular and nanoscale dimensions. Redox reactions on the nanoelectrode array display molecular tunability with a radial diffusion characteristic in good agreement with theoretical simulation results. These findings have implications for designing membrane-type ion-gating, electrochemical sensing, and electrochemical energy storage devices with molecular level tunability.

Intermolecular interactions in monolayer assembly are harnessed for creating molecularly-tunable nanoelectrode arrays or ensembles.     

Deposition of gallium oxide nanodots prepared from metal‐assembling dendrimer molecules isolated by the spacing of the nonmetal‐assembling dendrimer molecules in the two‐dendrimers mixture monolayer

Phenylazomethine dendrimers (DPA) can precisely incorporate metal chlorides onto the imine sites in a stepwise fashion. Such precise dendrimer–metal complexes allow the preparation of size‐controlled subnanometer metal particles. We now propose a novel approach for the fabrication of size‐controlled subnanometer metal oxide dots isolated on a substrate using two different‐type dendrimers. One is a fourth‐generation phenylazomethine dendrimer (DPAG4) and the other is a benzylether dendrimer (BzEG3) with a zinc porphyrin core. Even though the diameter of BzEG3 corresponds to that of DPAG4, BzEG3 has no metal‐complexing site. Upon dip coating on a highly oriented pyrolytic graphite substrate by the mixed solution of the metal chloride‐assembling DPAG4 molecules and BzEG3 molecules, the dendrimer monolayer was immobilized on the substrate. The concentration of the dendrimer mixture was determined in order to separate each DPAG4–metal chloride complex molecule by BzEG3. Monodispersed metaloxide nanodot arrays could be obtained from the dendrimer monolayer in which DPAG4–metal chloride complex molecule is well isolated by the BzEG3 as a spacer after the hydrolysis of metal chlorides followed by the complete removal of dendrimers. Copyright © 2013 John Wiley & Sons, Ltd.     

Close-packed hemispherical microlens array from two-dimensional ordered polymeric microspheres

This paper describes a facile, reproducible soft-lithography-based method for fabricating hexagonally close-packed microlens arrays by templating the surface of a colloidal monolayer, which is formed by spin-casting monodisperse polystyrene microspheres. The relief structure of colloidal monolayers has successfully generated PDMS elastomers with hexagonal arrays of hemispherical air voids. Closely packed hemispherical microlens arrays were imprinted on ultraviolet-curable photopolymers which are bound on glass substrates. Atomic force microscopy measurements showed that each spherical hole of the PDMS molds is 103 nm deep and the replicated microlens is 95 nm in height with narrow size distribution and good reproducibility. Without a multistep engineering process, this method might provide a reliable route to fabricate embossed thin layers ranging from nanometer to micrometers by controlling the size of polymer microspheres over a centimeter scale area.     

Deposition of silica thin films formed by sol–gel method

Deposition of silica thin films on silicon wafer was investigated by in situ mass measurements with a microbalance configured for dip coating. Mass change was recorded with respect to deposition time when the substrate was fully immersed in the silica sol. Mass gain during deposition was higher than predicted from monolayer coverage of silica nano particles. This implied that deposition was facilitated by gelling of the nanoparticles on the substrate. The rate of deposition was enhanced by increasing the particle concentration in the sol and by decreasing the particle size from 12 to 5 nm. Increasing the salt concentration of the silica sol at constant pH enhanced the deposition of the silica particles. Reducing the pH of the sol from 10 to 6 decreased the deposition rate due to aggregation of the primary silica particles.     

Transferable ordered ni hollow sphere arrays induced by electrodeposition on colloidal monolayer

We report an electrochemical synthesis of two-dimensionally ordered porous Ni arrays based on polystyrene sphere (PS) colloidal monolayer. The morphology can be controlled from bowl-like to hollow sphere-like structure by changing deposition time under a constant current. Importantly, such ordered Ni arrays on a conducting substrate can be transferred integrally to any other desired substrates, especially onto an insulting substrate or curved surface. The magnetic measurements of the two-dimensional hollow sphere array show the coercivity values of 104 Oe for the applied field parallel to the film, and 87 Oe for the applied field perpendicular to the film, which is larger than those of bulk Ni and hollow Ni submicrometer-sized spheres. The formation of hollow sphere arrays is attributed to preferential nucleation on the interstitial sites between PS in the colloidal monolayer and substrate, and growth along PSs' surface. The transferability of the arrays originates from partial contact between the Ni hollow spheres and substrate. Such novel Ni ordered nanostructured arrays with transferability and high magnetic properties should be useful in applications such as data storage, catalysis, and magnetics.     

Self-assembly of monolayer-thick alumina particle-epoxy composite films

Monolayer-thick composite films composed of alpha-alumina and Spurr's epoxy were prepared via a self-assembly process known as fluid forming. The process makes use of a high-spreading-tension fluid composed of volatile and nonvolatile components to propel particles across the air-water interface within a water bath. Continuous addition of the particle suspension builds a 2D particle film at the air-water interface. The spreading fluid compresses the film into a densely packed array against a submerged substrate. The assembled monolayer is deposited onto the substrate by removing the substrate from the bath. A dispersion containing a narrow size distribution, 10 microm alpha-alumina particles, light mineral oil, and 2-propanol was spread at the air-water interface and the alumina particles were assembled into densely packed arrays with an aerial packing fraction (APF) of 0.88. However, when mineral oil was replaced by Spurr's epoxy nonuniform films with low packing density resulted. It was found that replacing 2-propanol with a mixture of 2-propanol and 1-butanol with a volume ratio of 4:1 produced uniform, densely packed alumina/epoxy composite films. The role of the solvent mixture will be discussed.     

Morphological and compositional evolution of polymeric colloidal monolayer during UV irradiation

In this study, we investigated the morphological and compositional evolution polymeric colloidal monolayer during UV irradiation. A PS colloidal monolayer with interparticle bridges was prepared and exposed to the UV light. As a consequence of photochemical reactions containing chain-scission, UV irradiation induced morphological changes in the monolayer surface including changes in the size, shape, and packing structure of PS particles. By manipulating the UV irradiation time, fine tuning of size and shape of the interstice in the monolayer was achieved. In these procedures, the interparticle bridges play an important role. The UV irradiation induced the formation of polar groups in the PS particle surface and thus the particle surface became highly hydrophilic.     

Fabrication of nanoscale latex arrays based on hydroxylated poly(butyl methacrylate-<Emphasis Type="Italic">b</Emphasis>-glycidyl methacrylate)

We succeeded in fabricating nanoscale arrays of polymers based on hydroxylated poly(butyl methacrylate-b-glycidyl methacrylate) which was prepared via a novel atom-transfer radical polymerization technique. Nanosized latex particles of the copolymer were obtained in the tetrahydrofuran/toluene solvent system. By evaporation of the latex solution on a substrate, the ordered self-organization monolayer was formed. Rather regular, two-dimensional arrays of nanopaticles with diameters down to approximately 12 nm were observed by means of transmission electron microscopy. The regular nature of the arrays can be controlled well by depositing the monolayer at a lower temperature.     

Spontaneous formation of cluster array of gold particles by convective self-assembly

Cluster arrays composed of metal nanoparticles are promising for application in sensing devices because of their interesting surface plasmon characteristics. Herein, we report the spontaneous formation of cluster arrays of gold colloids on flat substrates by vertical-deposition convective self-assembly. In this technique, under controlled temperature, a hydrophilic substrate is vertically immersed in a colloid suspension. Cluster arrays form when the particle concentration is extremely low (in the order of 10(-6)-10(-8) v/v). These arrays are arranged in a hierarchically ordered structure, where the particles form clusters that are deposited at a certain separation distance from each other, to form "dotted" lines that are in turn aligned with a constant spacing. The size of the cluster can be controlled by varying the particle concentration and temperature while an equal separation distance is maintained between the lines formed by the clusters. Our technique thus demonstrates a one-step, template-free fabrication method for cluster arrays. In addition, through the direct observation of the assembly process, the spacing between the dotted lines is found to result from the "stick-and-slip" behavior of the meniscus tip, which is entirely different from the formation processes observed for the striped patterns, which we reported previously at higher particle concentrations. The difference in the meniscus behavior possibly comes from the difference in colloidal morphology at the meniscus tip. These results demonstrate the self-regulating characteristics of the convective self-assembly process to produce colloidal patterns, whose structure depends on particle concentration and temperature.