Fabrication of silica-on-titania and titania-on-silica nanoparticle assemblies

Structures of silica particles on a titania surface and titania particles on a silica surface were formed by deposition of SiO₂ or TiO₂ nanoparticles on pre-patterned substrates. Photolithography was used to create a matrix for the selective deposition of nanoparticles by immersion in a colloidal suspension. Atomic force microscopy was used to investigate the topography of these inorganic assemblies. Whereas two-dimensional colloidal patches of TiO₂ nanoparticles are obtained on silica surfaces, SiO₂ nanoparticles form three-dimensional, U-shaped channels on titania surfaces.The influence of electrostatic forces on assembly structure is vital. The isoelectric points of the particles, the pre-patterned matrix and the photo-resist are key parameters and may be manipulated to achieve various microstructures. The 2D nanoparticle arrays of titania on silica and 3D channels (built of silica nanoparticles) on flat titania surfaces are of potential interest in lab-on-a-chip applications.     

Self-assembly of poly(N-isopropylacrylamide)-carrying microspheres into two-dimensional colloidal arrays

Droplets containing polymer particles were deposited on a substrate. Poly(N-isopropylacrylamide) (PNIPAM) hydrogel and particles with PNIPAM graft chains on the surface self-assembled into a two-dimensional (2-D) superlattice when their dilute dispersions were dried on substrates. The capillary force between the particles induced ordered array formation during water evaporation. The presence of a PNIPAM layer on the particle surface gave the particles steric stability during ordered array formation. By grafting PNIPAM chains on particle surfaces by living radical polymerization, we successfully controlled the structural patterns of the colloidal arrays. These, controllable, 2-D colloidal arrays were generated on various substrates upon air-drying.     

Pyramidal assemblies of colloidal particles by micromolding underneath top-gathering pillar arrays

Assemblies of colloidal particles are known to have special photonic and optical properties. Periodic pyramidal assemblies of SiO2 particles with diameters of 0.5 and 1 microm were fabricated using top-gathering pillar arrays as a new template. These top-gathering pillar arrays consisted of four pillars gathered at the top, and were fabricated by photolithography of an organic-inorganic hybrid material. The top-gathering units were obtained by controlling both the capillary and restoring forces. When a colloidal water suspension was spread on the template and the water was allowed to evaporate, the SiO2 particles were molded under the top-gathering pillars according to particle size, resulting in pyramidal assembly arrays of the particles. From in situ observation during the evaporation of water, it was found that the particles were molded underneath the top-gathering pillars by flux generated by the evaporation and by the capillary force among the particles.     

Collodial cluster arrays by electrohydrodynamic printing

A "stable" electrohydrodynamic jet is used to print arrays of colloidal suspensions on hydrophobic surfaces. Printed lines break up into sessile drops, and capillary forces guide the self-assembly of colloidal particles during the evaporation of the liquid, resulting in arrays of colloidal single particles or particle clusters depending on the concentration of the suspensions. The clusters differ from those formed in the absence of a substrate when the number of particles is larger than three. Multiple structures are found for the same number of particles.     

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.     

Fabrication of colloidal grid network by two-step convective self-assembly

We explored a "template-free" approach to arranging colloidal particles into a network pattern by a convective self-assembly technique. In this approach, which we call "two-step convective self-assembly," a stripe pattern of colloidal particles is first prepared on a substrate by immersing it in a suspension. The substrate with the stripes is then rotated by 90° and again immersed in the suspension to produce stripes perpendicular to the first ones, resulting in a grid-pattern network of colloidal arrays. The width of the colloidal grid lines can be controlled by changing the particle concentration while maintaining an almost constant spacing between the lines. On the basis of these results, we propose a mechanism for grid pattern formation. Our method is applicable to various types of particles. In addition, the wide applicability of this method was employed to create a hybrid grid pattern.     

Fast, high-throughput creation of size-tunable micro/nanoparticle clusters via evaporative self-assembly in picoliter-scale droplets of particle suspension

We report a fast, high-throughput method to create size-tunable micro/nanoparticle clusters via evaporative assembly in picoliter-scale droplets of particle suspension. Mediated by gravity force and surface tension force of a contacting surface, picoliter-scale droplets of the suspension are generated from a nanofabricated printing head. Rapid evaporative self-assembly of the particles on a hydrophobic surface leads to fast clustering of micro/nanoparticles and forms particle clusters of tunable sizes and controlled spacing. The evaporating behavior of the droplet is observed in real-time, and the clustering characteristics of the particles are understood based on the physics of evaporative-assembly. With this method, multiplex printing of various particle clusters with accurate positioning and alignment are demonstrated. Also, size-unifomity of the cluster arrays is thoroughly analyzed by examining the metallic nanoparticle cluster-arrays based on surface-enhanced Raman spectroscopy (SERS).     

Assembly of colloidal particles by evaporation on surfaces with patterned hydrophobicity

Drops containing suspended particles are placed on surfaces of patterned wettability created using soft lithography; the drop diameter is large compared to the dimensions of the patterns on the substrate. As the three-phase contact line of the drop recedes, spontaneous dewetting of the hydrophobic domains and flow into the hydrophilic domains create discrete fluid elements with peripheries that can mimic the underlying surface topography. Suspended particles are carried with the fluid into the wetted regions and deposit there as the discrete fluid domains evaporate. If particle volume fractions are sufficiently high, the entire wetted domain can be covered with colloidal crystals. At lower volume fractions, flow within the evaporating fluid element can direct the deposition of colloidal particles at the peripheries of the domains. High-resolution arrays of particles were obtained with a variety of features depending upon the relative size of the wetting regions to the particles. When the wetting region is larger than the particles, three-dimensional and two-dimensional arrays of ordered particles mimicking the shape of the wetting pattern form, depending on the particle volume fraction. For lower volume fractions, one-dimensional (1-D) arrays along the wet/non-wet boundaries form. When the particle size is similar to the height of fluid on the wetted domain, zero-dimensional distributions of single particles centered in the wet regions can form for wetted squares or 1-D distributions (stripes) form along the axis of striped domains. Finally, when the wetting region is smaller than the particle size, the particles do not deposit within the features but are drawn backward with the receding drop. These results indicate that evaporation on surfaces of patterned wetting provides a highly parallelizable means of tailoring the geometry of particle distributions to create patterned media.     

Assembly of nanoparticles at the contact line of a drying droplet under the influence of a dipped tip

The manipulation of colloidal nanoparticles (NPs) in a drying droplet has critical importance not only for several industrial applications but also their assembly into patterns on surfaces. The influence of a tip with hydrophilic or hydrophobic surfaces dipped into a drying droplet on hydrophilic or hydrophobic surfaces on the behavior of 98 nm latex NPs was investigated. The formation of concentric rings on hydrophilic glass surfaces regardless of the surface chemistry of the dipped tip was observed. On the other hand, no pattern formation on hydrophobic surfaces was observed with the insertion of the tip. With a hydrophilic tip, the concentric rings were formed due to stick-slip motion of the solvent contact line resulting from competition between pinning and capillary forces while the capillary effect was not effective until the surface of the tip was changed by adherent NPs making the tip surface available for water adherence with a hydrophobic tip, which results in the pulling of droplet towards the tip. It is also found that the tip thickness and suspension concentration significantly influences the formation of concentric rings on surfaces. This simple procedure can be used to influence the distribution or assembly of NPs in the droplet area.     

Non-Lithography Hydrodynamic Printing of Micro/Nanostructures on Curved Surfaces

A key issue of micro/nano devices is how to integrate micro/nanostructures with specified chemical components onto various curved surfaces. Hydrodynamic printing of micro/nanostructures on three-dimensional curved surfaces is achieved with a strategy that combines template-induced hydrodynamic printing and self-assembly of nanoparticles (NPs). Non-lithography flexible wall-shaped templates are replicated with microscale features by dicing a trench-shaped silicon wafer. Arising from the capillary pumped function between the template and curved substrates, NPs in the colloidal suspension self-assemble into close-packed micro/nanostructures without a gravity effect. Theoretical analysis with the lattice Boltzmann model reveals the fundamental principles of the hydrodynamic assembly process. Spiral linear structures achieved by two kinds of fluorescent NPs show non-interfering photoluminescence properties, while the waveguide and photoluminescence are confirmed in 3D curved space. The printed multiconstituent micro/nanostructures with single-NP resolution may serve as a general platform for optoelectronics beyond flat surfaces.     

How to impose stick boundary conditions in coarse-grained hydrodynamics of Brownian colloids and semi-flexible fiber rheology

Long-range hydrodynamics between colloidal particles or fibers is modelled by the fluid particle model. Two methods are considered to impose the fluid boundary conditions at colloidal surfaces. In the first method radial and transverse friction forces between particle and solvent are applied such that the correct friction and torque follows for moving or rotating particles. The force coefficients are calculated analytically and checked by numerical simulation. In the second method a collision rule is used between colloidal particle and solvent particle that imposes the stick boundary conditions exactly. The collision rule comprises a generalisation of the Lowe-Anderson thermostat to radial and transverse velocity differences.     

悬浮液气-液界面二元胶体颗粒的漂浮组装机理

提出一种在悬浮液气-液界面漂浮组装亚微米单分散聚苯乙烯(PS)微球和纳米SiO2颗粒二元胶粒晶体的新方法, 并系统研究了漂浮组装机理. 研究表明, 聚苯乙烯微球和二氧化硅两种胶体颗粒在悬浮液气-液界面的漂浮组装是以PS微球的组装为主导的. 在一定PS微球相浓度范围内, 悬浮液中PS 微球与SiO2颗粒的初始体积配比基本不影响PS微球有序组装的形成. PS微球粒径在150-500 nm时易于形成有序排列, 较小或较大粒径的PS微球难以形成有序排列. SiO2颗粒的组装是一种以PS微球为“基底”的沉积过程. 二元胶粒晶体中SiO2颗粒的体积分数由其在混合悬浮液中的相浓度所决定.     

Non‐Lithography Hydrodynamic Printing of Micro/Nanostructures on Curved Surfaces

A key issue of micro/nano devices is how to integrate micro/nanostructures with specified chemical components onto various curved surfaces. Hydrodynamic printing of micro/nanostructures on three‐dimensional curved surfaces is achieved with a strategy that combines template‐induced hydrodynamic printing and self‐assembly of nanoparticles (NPs). Non‐lithography flexible wall‐shaped templates are replicated with microscale features by dicing a trench‐shaped silicon wafer. Arising from the capillary pumped function between the template and curved substrates, NPs in the colloidal suspension self‐assemble into close‐packed micro/nanostructures without a gravity effect. Theoretical analysis with the lattice Boltzmann model reveals the fundamental principles of the hydrodynamic assembly process. Spiral linear structures achieved by two kinds of fluorescent NPs show non‐interfering photoluminescence properties, while the waveguide and photoluminescence are confirmed in 3D curved space. The printed multiconstituent micro/nanostructures with single‐NP resolution may serve as a general platform for optoelectronics beyond flat surfaces.     

Biotin-doped porous polypyrrole films for electrically controlled nanoparticle release

A novel method for the preparation of biotin-doped porous conductive surfaces has been suggested for a variety of applications, especially for an electrically controlled release system. Well-ordered and three-dimensional porous conductive structures have been obtained by the electrochemical deposition of the aqueous biotin-pyrrole monomer mixture into particle arrays, followed by subsequent removal of the colloidal particles. Advantageously, direct incorporation of biotin molecules enhances the versatility by modifying surfaces through site-directed conjugate formation, thus facilitating further reactions. In addition, the porosity of the surfaces provides a significant impact on enhanced immobilization and efficient release of streptavidin-tagged gold nanoparticles. Biotinylated porous polypyrrole (Ppy) films were characterized by several techniques: (1) scanning electron microscopy (SEM) to evaluate surface topography, (2) X-ray photoelectron spectroscopy (XPS) to assess the potential-dependent chemical composition of the films, (3) four-point probe evaluation to measure the conductivity, cyclic voltammetry to observe surface eletroactivity, and contact angle measurement to evaluate the surface wettability, and (4) fluorescence microscopy to image and quantify the adsorption and release of gold nanoparticles. Overall, our results demonstrate that these biotinylated porous Ppy films, combined with electrical stimulation, permit a programmable release of gold nanoparticles by altering the chemical strength of the Ppy-biotin interaction.     

Fabrication, Characterization, and Surface-Enhanced Raman Activity Study of Silver Coated Gold Nanoparticulate Films

This paper reports a study on the preparation of Ag-clad Au colloidal monolayer films by a combination of colloid self-assembly and liquid phase microwave high-pressure technique. Firstly, monodisperse Au nanoparticles prepared by microwave heating method were assembled onto a quartz slide. Then, these Au colloidal particles on the quartz surface acted as seeds for growing the Ag-clad Au composite particulate films. The obtained particulate films were characterized by UV-Vis spectra and atomic force microscopy. It was found that the thickness of the shell and thus the size of particles in the composite colloidal films could be controlled by deposition of Ag on the preformed Au colloidal particle film in the microwave reaction system, and such films significantly increased the surface-enhanced Raman scattering enhancement （SERS） ability compared with Au colloidal particle films. Their strong enhancement ability may mainly stem from relatively large particle consisting of Ag cladding as well as effective coupling among particles in the Ag-clad Au particle ftlms.     

Controlled, rapid deposition of structured coatings from micro- and nanoparticle suspensions

The objective of the study was to develop the operational basis for rapid and controlled deposition of crystal coatings from particles of a wide size range. We deposited such structured coatings by dragging with constant velocity a small volume of liquid confined in a meniscus between two plates. Two types of structured coatings were characterized: latex colloidal crystals and thin layers from metallic nanoparticles. The crystal deposition was sped up by use of preconcentrated suspensions. Crystal coatings larger than a few square centimeters were deposited in minutes from aqueous suspension volumes of approximately 10 microL. The governing mechanism of crystal deposition is convective assembly at high volume fractions. The two major process parameters that allow control over the coating thickness and structure were the deposition speed and particle volume fraction. The evaporation rate was not found to affect the process to a large extent. A volumetric flux balance was used to relate the deposition parameters to coating structure and properties. Operational "phase" diagrams were constructed, relating the crystal layer thickness and packing symmetry to the process parameters. These diagrams could be instrumental in transforming the convective colloidal deposition into a robust scaleable technology.     

Effect of growth conditions on the structure of two-dimensional latex crystals: modeling

Essential experimental features of the nucleation and growth of a 2D colloidal crystal on a solid substrate are modeled. The crystal, composed of sub-micron-sized latex spheres, is grown by the evaporation of water from the particle suspension in a circular cell. The calculation of the meniscus profile in the cell allows the prediction of the particle volume fraction in the suspension surrounding the crystal as a function of time. This quantity enters into a convective-diffusion model for the crystal growth which calculates the crystal radius as a function of time. Comparison with experimental data for 2D latex particle crystals shows predominant convective growth over a wide range of evaporation rates set by varying the humidity of the air. Microscopic parameters of the particle assembly can also be estimated such as the particle velocity, diffusivity, characteristic time constants, Peclet number, etc. The nucleation is simulated by simultaneously solving the equations of motion for the ensemble of particles trapped in a thin liquid film using the discrete-element method. These equations account for the forces which are physically important in the system: contact particle–particle friction, increased viscous resistance during the particle motion in a wetting film, long-range capillary attraction between two particles screened by the rest of particles. The final result of the simulation is a particle cluster of hexagonal packing, whose structure resembles very much the monolayer nucleus of latex particles observed experimentally. The models proposed by us could also be implemented for the aggregation of species in a variety of practical processes such as coating, texturing, crystal growth from a melt or liquid solution, or a biological array. Received: 10 May 1999 Accepted in revised form: 6 July 1999     

In situ study of colloid crystallization in constrained geometry

We visualized in real time electrodeposition-driven colloid crystal growth on patterned conductive surfaces. The electrode was patterned with dielectric ribs and conductive grooves; the groove width was commensurate or incommensurate with a two-dimensional colloid crystal lattice. Electrodeposition was carried out against gravity to decouple sedimentation and electrodeposition of colloid particles. Our experiments reveal the following: (i) Colloid crystal growth occurs under the action of electrohydrodynamic forces, in contrast with colloid assembly under the action of capillary forces. (ii) Confinement of the colloid arrays reduces the size of particle clusters. Small clusters easily undergo structural rearrangements to produce close-packed crystals when the groove width is commensurate or nearly commensurate with the 2D lattice. (iii) Incommensurability between the two-dimensional crystalline lattice and the groove width exceeding ca. 15% leads to the formation of non-close-packed structures and the distortion of colloid arrays.     

The influence of gravity on the distribution of the deposit formed onto a substrate by sessile, hanging, and sandwiched hanging drop evaporation

The focus of the present article is the study of the influence of gravity on the particle deposition profiles on a solid substrate during the evaporation of sessile, hanging and sandwiched hanging drops of colloidal particle suspensions. For concentrations of nanoparticles in the colloidal solutions in the range 0.0001-1 wt.%, highly diluted suspensions will preferentially form rings while concentrated suspensions will preferentially form spots in both sessile and hanging drop evaporation. For intermediary concentrations, the particle deposition profiles will depend on the nanoparticle aggregation dynamics in the suspension during the evaporation process, gravity and on the detailed evaporation geometry. The evaporation of a drop of toluene/carbon nanoparticle suspension hanging from a pendant water drop will leave on the substrate a circular spot with no visible external ring. By contrast, a clear external ring is formed on the substrate by the sessile evaporation of a similar drop of suspension sandwiched between a water drop and the substrate. From the application viewpoint, these processes can be used to create preferential electrical conductive carbon networks and contacts for arrays of self-assembled nanostructures fabricated on solid substrates as well as on flexible polymeric substrates.     

Control and modeling of the dielectrophoretic assembly of on-chip nanoparticle wires

Suspensions of metallic nanoparticles in water were assembled via the action of an alternating electric field (dielectrophoresis) into wires of micrometer thickness. Two modes of microwire assembly, one through the bulk of the suspension and one as half-cylinders on the glass surface between the electrodes, were identified. The operating conditions responsible for the two assembly modes were recognized. The control of the process parameters allows making, for example, straight single connectors or massively parallel arrays of microwires on the surface of the chip, which can be extracted in dry form. The microwire assembly process was modeled using finite element electrostatic calculations. The direction of growth can be guided by introducing conductive islands or particles in the suspension. The experiments, supported by electrostatic calculations, show that the wires grow in the direction of highest field intensity, "automatically" making electrical connections to the objects between the electrodes. The results point the way to controlled dielectrophoretic assembly of nanoparticles into on-chip electrical connectors, switches, and networks.