Deposition of poly(styrene/alpha-tert-butoxy-omega-vinyl-benzyl-polyglycidol) microspheres on mica plates crossing the liquid-air interface: formation of stripe pattern

Formation of stripelike assemblies of poly(styrene/alpha-tert-butoxy-omega-vinyl-benzyl-polyglycidol) microspheres adsorbed on nonpatterned mica plates moving perpendicularly from suspension of particles through the water-air interface has been observed. It was found that ordered assemblies were formed by capillary forces acting on particles crossing the water-air boundary. At sufficiently high rates of plate movement (i.e., at appropriate dynamic loading conditions) the adsorbed microspheres approaching the water surface begin sliding on the plate, due to capillary forces, in the direction opposite to the plate movement and are kept below the water surface. Plate movement brings new adsorbed particles to the water-air interface, where particles are assembled into aggregates. When particle aggregates are large, the capillary forces cannot overcome shearing forces and the particle assemblies are withdrawn on the plate above the water surface. This process repeated during continuous movement of the plate results in the formation of the quite regularly distributed stripes of adsorbed microspheres. Formation of the regularly distributed particle assemblies depends on concentration of microspheres in suspension.     

One-, two-, and three-dimensional organization of colloidal particles using nonuniform alternating current electric fields

We demonstrate here the use of nonuniform alternating current (AC) electric fields, generated by planar electrodes, for the organization of num-sized particles into one-, two-, and three-dimensional assemblies. The electrodes, with separations that vary from 35 to 300 num, are made of gold deposited on glass substrata. Latex, silica and graphite particles have been examined inside organic or aqueous media in order to illustrate the general applicability of the technique. Theoretical predictions of the particle response under the electric fields are experimentally confirmed for all the above particle/media combinations and can thus be used as a valuable design tool. The size and shape of the final structures are mainly dependent on the electrode shape and dimensions, but are also subject to the particle type and operating conditions. Particle organization in one dimension (strings) is achieved under conditions of positive or negative dielectrophoresis in the space between two energized electrodes. Two-dimensional particle organization (ordered, planar particles assemblies) was observed under conditions of negative dielectrophoresis, when quadrupole electrodes were employed. Moreover, when negative dielectrophoresis and stronger electric fields are applied (of the order of 50 kV(rms) m(-1)), three-dimensional, pyramid-like structures with a vertical dimension 1000-fold higher than that of the corresponding (planar) electrodes can be assembled. These 3-D structures can grow as free-standing assemblies, or inside templates etched in the substratum. The dielectrophoresis (DEP)-organized particle assemblies can subsequently be rendered permanent via the in situ fixing (cross-linking) of the individual particles.     

Digital microfluidics and delivery of molecular payloads with magnetic porous silicon chaperones

Digital microfluidics involves the manipulation of molecules and materials in discrete packages. This paper reviews our work using amphiphilic magnetic microparticles constructed from porous silicon. An individual porous particle can be used to carry a nanomole or smaller quantities of a reagent, and assemblies of the particles can encapsulate and transport microliter droplets of liquid containing inorganic, organic, or biological molecules. The tracking and identification of each particle can be accomplished with spectral labels that are encoded into the particles during their synthesis. When used to chaperone liquid droplets, the labels can identify the separate droplets prior to mixing and also the combined droplets after mixing. Magnetic iron oxide nanoparticles encapsulated in the porous matrix allow the manipulation of the particles or whole droplet assemblies with a magnetic field, and they also allow heating of the particle's payload by means of an externally applied RF field. Examples of organic, inorganic, and biomolecular addition reactions, catalytic reactions, and thermolysis reactions are described.     

Assemblies of poly(styrene/⇌-tert-butoxy-ω-vinyl-benzyl-poryglycidol) Microspheres with different diameters deposited on mica plates

Formation of poly(styrene/α-tert-butoxy-ω-vinyl-benzyl-polyglycidol) microsphere assemblies on mica plates modified with 3-aminopropyltriethoxysilane was investigated. Microsphere assemblies contained two types of particles similar with respect of their chemical structure but with different diameters (D _n = 1000 and 350 nm). Methods of particle deposition included: deposition from water suspension of a mixture of small and large particles on mica plates placed at the bottom of suspension container, deposition of particles from a drop of ethanol suspension (containing large and small microspheres) placed on the mica substrate, deposition of microspheres on modified mica plates crossing the liquid-air interface-sequential deposition of large and small particles, and one-batch deposition from a mixed water suspension of large and small microspheres. Deposition from water suspension containing large and small microspheres on plates placed on the bottom of suspension container yielded assemblies with large particles randomly distributed among the small ones. Fraction of large particles in adsorbed particle assembly was smaller than fraction of large particles in suspension. Particle assemblies prepared by placement of ethanol suspension of large and small microspheres on mica were composed of quite regularly distributed large particles among the small ones. A two step process consisting of withdrawal of mica plate from water suspension of large particles and then on using this plate as substrate in a second step during which the plate was withdrawn from suspension of small particles yielded particle assemblies containing aggregates of large particles randomly distributed among the small ones. Withdrawal of mica plates from water suspension of large and small microspheres resulted in particle assemblies composed of regularly distributed stripes of large and small microspheres. Formation of the described above microsphere assemblies is possible only in case of reversible adsorption of particles.     

Self-assembly patterning of silica colloidal crystals

We developed a self-assembly process of silica particles to fabricate desired patterns of colloidal crystals having high feature edge acuity and high regularity. A micropattern of colloidal methanol prepared on a self-assembled monolayer in hexane was used as a mold for particle patterning, and slow dissolution of methanol into hexane caused shrinkage of molds to form micropatterns of close-packed SiO2 particle assemblies. This result is a step toward the realization ofnano/micro periodic structures for next-generation photonic devices by a self-assembly process.     

Particle Network Self-Assembly of Similar Size Sub-Micron Calcium Alginate and Polystyrene Particles Atop Glass

Particle-mediated self-assembly, such as nanocomposites, microstructure formation in materials, and core-shell coating of biological particles, offers precise control over the properties of biological materials for applications in drug delivery, tissue engineering, and biosensing. The assembly of similar-sized calcium alginate (CAG) and polystyrene sub-micron particles is studied in an aqueous sodium nitrate solution as a model for particle-mediated self-assembly of biological and synthetic mixed particle species. The objective is to reinforce biological matrices by incorporating synthetic particles to form hybrid particulate networks with tailored properties. By varying the ionic strength of the suspension, the authors alter the energy barriers for particle attachment to each other and to a glass substrate that result from colloidal surface forces. The particles do not show monotonic adsorption trend to glass with ionic strength. Hence, apart from DLVO theory—van der Waals and electrostatic interactions—the authors further consider solvation and bridging interactions in the analysis of the particulate adsorption-coagulation system. CAG particles, which support lower energy barriers to attachment relative to their counterpart polystyrene particles, accumulate as dense aggregates on the glass substrate. Polystyrene particles adsorb simultaneously as detached particles. At high electrolyte concentrations, where electrostatic repulsion is largely screened, the mixture of particles covers most of the glass substrate; the CAG particles form a continuous network throughout the glass substrate with pockets of polystyrene particles. The particulate structure is correlated with the adjustable energy barriers for particle attachment in the suspension.     

Microparticle sampling by electrowetting-actuated droplet sweeping

This paper describes a new microparticle sampler where particles can be efficiently swept from a solid surface and sampled into a liquid medium using moving droplets actuated by the electrowetting principle. We successfully demonstrate that super hydrophilic (2 microm and 7.9 microm diameter glass beads of about 14 degrees contact angle), intermediate hydrophilic (7.5 microm diameter polystyrene beads of about 70 degrees contact angle), and super hydrophobic (7.9 microm diameter Teflon-coated glass beads and 3 microm size PTFE particles of over 110 degrees contact angles) particles on a solid surface are picked up by electrowetting-actuated moving droplets. For the glass beads as well as the polystyrene beads, the sampling efficiencies are over 93%, in particular over 98% for the 7.9 microm glass beads. For the PTFE particles, however, the sampling efficiency is measured at around 70%, relatively lower than that of the glass and polystyrene beads. This is due mainly to the non-uniformity in particle size and the particle hydrophobicity. In this case, the collected particles staying (adsorbing) on the air-to-water interface hinder the droplet from advancing. This particle sampler requires an extremely small amount of liquid volume (about 500 nanoliters) and will thus be highly compatible and easily integrated with lab-on-a-chip systems for follow-up biological/chemical analyses.     

SURFACE TENSION,STICKINESS AND ENGULFMENT

The thermodynamic conditions for the engulfment of one set of particles by another has been given in terms of interfacial energies. Experimentally, it has been shown that a polymer with a high glass transition temperature can be engulfed by a particle of low glass transition temperature; also, that Inorganic particles can be engulfed by polymer particles. As a precursor to the engulfment stage heterocoagulation can be used for bringing the particles together in a ‘sticking’ mode. This appears to be a general process which is applicable to a number of scientific areas, e.g. in biology, phagocytosis, and in material science for the preparation of composite particles.     

Stabilization of oil-in-water emulsions by colloidal particles modified with short amphiphiles

Emulsions stabilized through the adsorption of colloidal particles at the liquid-liquid interface have long been used and investigated in a number of different applications. The interfacial adsorption of particles can be induced by adjusting the particle wetting behavior in the liquid media. Here, we report a new approach to prepare stable oil-in-water emulsions by tailoring the wetting behavior of colloidal particles in water using short amphiphilic molecules. We illustrate the method using hydrophilic metal oxide particles initially dispersed in the aqueous phase. The wettability of such particles in water is reduced by an in situ surface hydrophobization that induces particle adsorption at oil-water interfaces. We evaluate the conditions required for particle adsorption at the liquid-liquid interface and discuss the effect of the emulsion initial composition on the final microstructure of oil-water mixtures containing high concentrations of alumina particles modified with short carboxylic acids. This new approach for emulsion preparation can be easily applied to a variety of other metal oxide particles.     

Self-assembled multilayers of gold nanoparticles: nitrate-induced rectification of quantized capacitance charging and effects of alkaline (earth) ions in aqueous solutions

Gold nanoparticle multilayers were self-assembled onto an electrode surface by using a dipping method. The particle assemblies exhibited quantized capacitance charging characteristics in aqueous media that were rectified by hydrophobic anions such as PF6-, BF4- and ClO4-, similar to the behavior with the monolayer counterparts. More interestingly, even in the presence of less hydrophobic anions such as NO3-, very well-defined single electron transfers were observed voltammetrically with these particle multilayers, a response unseen previously with particle monolayers. This was ascribed, in part, to the enhanced interactions between the particle multilayers and the electrolyte anions as well as the minimization of the structural defects within the particle thin films as compared to the monolayer counterparts. Further studies showed that with particles functionalized with oligo(ethylene oxide) moieties, the particle charge transfer properties were also found to be affected by electrolyte cations, reflected by the variation of the particle molecular capacitance and formal potentials with the nature of the alkaline (earth) metal ions.     

Dielectrophoretic assembly of metallodielectric Janus particles in AC electric fields

"Janus" particles with two hemispheres of different polarizability or charge demonstrate a multitude of interesting effects in external electric fields. We reported earlier how particles with one metallic hemisphere and one dielectric hemisphere self-propel in low-frequency alternating current (AC) electric fields. Here, we demonstrate the assembly of such Janus particles driven by AC electric fields at frequencies above 10 kHz. We investigated the relation between field-induced dielectrophoretic force, field distribution, and structure of the assemblies. The phase space for electric field intensity and frequency was explored for particle concentrations large enough to form a monolayer on a glass surface between two gold electrodes. A rich variety of metallodielectric particle structures and dynamics were uncovered, which are very different from those obtained from directed assembly of plain dielectric or plain conductive particles under the action of fields of similar frequency and intensity. The metallodielectric particles assemble into new types of chain structures, where the metallized halves of neighboring particles align into lanes along the direction of the electric field, while the dielectric halves face in alternating direction. The staggered chains may assemble in various orientations to form different types of two-dimensional metallodielectric crystals. The experimental results on the formation of staggered chains are interpreted by means of numerical simulations of the electric energy of the system. The assembly of Janus metallodielectric particles may find applications in liquid-borne microcircuits and materials with directional electric and heat transfer.     

Controlled particle placement through convective and capillary assembly

A wide variety of methods are now available for the synthesis of colloidal particle having controlled shapes, structures, and dimensions. One of the main challenges in the development of devices that utilize micro- and nanoparticles is still particle placement and integration on surfaces. Required are engineering approaches to control the assembly of these building blocks at accurate positions and at high yield. Here, we investigate two complementary methods to create particle assemblies ranging from full layers to sparse arrays of single particles starting from colloidal suspensions of gold and polystyrene particles. Convective assembly was performed on hydrophilic substrates to create crystalline mono- or multilayers using the convective flow of nanoparticles induced by the evaporation of solvent at the three-phase contact line of a solution. On hydrophobic surfaces, capillary assembly was investigated to create sparse arrays and complex three-dimensional structures using capillary forces to trap and organize particles in the recessed regions of a template. In both methods, the hydrodynamic drag exerted on the particle in the suspension plays a key role in the assembly process. We demonstrate for the first time that the velocity and direction of particles in the suspension can be controlled to perform assembly or disassembly of particles. This is achieved by setting the temperature of the colloidal suspension above or below the dew point. The influence of other parameters, such as substrate velocity, wetting properties, and pattern geometry, is also investigated. For the particular case of capillary assembly, we propose a mechanism that takes into account the relative influences of these parameters on the motion of particles and that describes the influence of temperature on the assembly efficiency.     

A Photochemical Approach to Integrated Optics

Highly resolved refractive index patterns or patterns of iron oxide are obtained by photolysis of (CH₃)₃SnI or Fe(CO)₅ absorbed onto Coming's code 7930 porous Vycor glass followed by consolidation to a nonporous glass. The photochemistries of the molecules on the glass surface, as well as the distribution and relative sizes of the photodeposited metal oxides, are described. Rutherford backscattering, small-angle x-ray scattering, and scanning electron microscopy show that the glass consolidates about the iron oxide particle but not about the tin oxide particle. Tin oxide chemically modifies the glass surface so that it does not consolidate at temperatures as high as 1200°C.     

Size selective assembly of colloidal particles on a template by directed self-assembly technique

We report a simple and effective approach to organize micron- and submicron-sized particles in a size selective manner. This approach utilizes the template assisted directed self-assembly technique. A topographically patterned photoresist surface is fabricated and used to create an ordered array of colloidal particles from their aqueous suspensions. Assembly of particles on this template is then achieved by using a conventional spin coating technique. Feasibility of this technique to form a large area of patterned particle assemblies has been investigated. To arrange the particles on the template, the physical confinement offered by the surface topography must overcome a joint effect of centrifugal force and the hydrophobic nature of the photoresist surface. This concept has been extended to the size selective sorting of colloidal particles. The capability of this technique for sorting and organizing colloidal particles of a particular diameter from a mixture of microspheres is demonstrated.     

Magnetic Fe2O3-polystyrene/PPy core/shell particles: bioreactivity and self-assembly

This paper describes the synthesis of new magnetic, reactive polystyrene/polypyrrole core/shell latex particles. The core consists of a polystyrene microsphere containing gamma-Fe2O3 superparamagnetic nanoparticles (PSmag), and the shell is made of reactive N-carboxylic acid-functionalized polypyrrole (PPyCOOH). These PSmag-PPyCOOH latex particles, average diameter 220 nm, were prepared by copolymerization of pyrrole (Py) and the active carboxyl-functionalized pyrrole (PyCOOH) in the presence of PSmag particles. PNVP was used as a steric stabilizer. The functionalized polypyrrole-coated PSmag particles were characterized in terms of their particle size, surface morphology, chemical composition, and electrochemical and magnetic properties using transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), cyclic voltammetry, and SQUID magnetometry. Activation of the particle surface carboxyl groups was achieved using 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide (EDC) and N-hydroxysuccinimide (NHS), which helps transform the carboxyl groups into activated ester groups (NSE). The activated particles, PSmag-PPyNSE, were further evaluated as bioadsorbents of biotin used as a model biomolecule. It was shown that biotin was immobilized at the surface of the PSmag-PPyNSE particles by forming interfacial amide groups. The assemblies of PSmag-PPyCOOH particles on glass plates were further investigated. When no magnetic field is applied, the particles assemble into 3D colloidal crystals. In contrast, under a magnetic field, one-particle-thick chains gathered in hedgehog-like architectures are obtained. Furthermore, PSmag-PPyCOOH coated ITO electrodes were shown to be electroactive and electrochemically stable, thus offering potentialities for creating novel high-specific-area materials for biosensing devices where the conducting polymer component would act as the transducer through its conductive properties.     

Influence of adsorbed polymers on the removal of mineral particles from a planar surface

 Experimental results on the role of adsorbed polymers on the particle adhesion are presented. Both Brownian (silica particles) and non-Brownian (glass beads) particles were used. The particles were deposited onto the internal surface of a glass parallelepiped cell, and then submitted to increasing laminar flow rates. The pH and the ionic strength of the electrolytes were fixed. The adhesive force was related to the hydrodynamic force required to dislodge 50% of the initially attached beads. We found that high molecular weight PEO had little effect on the adhesion of small silica beads due to the low affinity of the polymer for silica or glass surfaces. On the contrary, PEO greatly enhanced the adhesion of bigger glass beads forced to deposit on the capillary surface because of gravity. The increase was all the more pronounced as the molecular weight of the polymer was high. The effect of high molecular weight cationic copolymers on the adhesion of silica particles was drastic. The maximal force (1500 pN) applied by the device could not enable any particle detachment even using polymers of low cationicity rate (5%), showing the efficiency of electrostatic attractions. When copolymers were adsorbed on both surfaces (particles and plane), the adhesive force exhibited a maximum at intermediate coverage of particles. This optimum was related to the optimum flocculation concentration classically observed in flocculation of suspensions by polymers. Received: 16 February 1996 Accepted: 10 September 1996     

Enhanced electrocatalytic oxygen reduction through electrostatic assembly of Pt nanoparticles onto porous carbon supports from SnCl2-stabilized suspensions

Monodisperse Pt nanoparticles with atomic structures that span the cluster to crystal transition have recently been synthesized in electrostatically stabilized, aqueous-based suspensions. In the present study, the anionic charge from the stabilizing SnCl(2) sheath adsorbed on the surface of these particles is used for the first time to assemble Pt directly onto porous carbon supports via electrostatic assembly. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) reveals that these assemblies have substantially higher Pt-C dispersions than obtained from precipitation methods commonly used for commercial electrocatalyst systems. Energy dispersive spectroscopy (EDS) and inductively coupled plasma-mass spectrometry (ICP-MS) are used to determine that loadings of 10-30% by weight Pt (particle packing fractions from 0.05 to 0.25) are obtained through a single electrostatic application of these particles on Vulcan carbon, depending on particle size. The highest average oxygen reduction reaction (ORR) mass activity obtained using this approach is 90.4 A/g(Pt) at 0.9 V vs RHE in 0.1 M perchloric acid is with 1-2 nm particles that exhibit a transitional atomic structure. This activity compares to an average value of 74.0 A/g(Pt) obtained from densely packed electrostatic layer-by-layer (LbL) assemblies of unsupported particles and 36.7 A/g(Pt) commercial Vulcan electrocatalyst from Tanaka Kikinzoku Kogyo (TKK). Enhanced activity is observed with electrostatic assembly of any particle size on Vulcan relative to unsupported or commercial electrocatalyst with comparable durability. Such enhanced activity is attributed to improved reactant accessibility to the catalyst surface due to the increase in particle dispersion. An extinction coefficient of 7.41 m(2)/g at 352 nm is obtained across the entire cluster to crystal transition from 20 atom clusters to 2.9 nm single crystal nanoparticles, indicating that observed variation in ORR activity with particle size may be associated primarily with changes in atomic surface structure as opposed to the metallic character of the nanoparticles as assessed by UV-vis spectroscopy.     

Thermal expansion of epoxy-resin/particle composites—a size effect

The thermal expansion of epoxy-resin (Epikote 828)/particle composites has been measured in the range 77 to 450 K. The fillers used were Cu spheres (seven sizes from 5 to 150 μm diameter) and glass ballotini spheres (three sizes from 3.5 to 200 μm diameter). The volume concentrations used were 0.3 and 0.5 for Cu and 0.3 for glass. The experiments show that the addition of filler raises the glass transition temperature T_g, especially for fine particles. Below the normal value of T_g the thermal expansion is independent of particle size while above T_g the expansion is considerably smaller for samples containing the smaller particles. The effect is more pronounced for Cu than for glass filler. In addition a rapid heating rate reduces the expansion for specimens containing smaller particles but it does not effect the expansion for those containing large particles. The results, which are discussed in the light of the work of other authors, suggest that the addition of particles increases T_g by changing the nature of the polymer not only immediately at the particle surface but also for a considerable distance into the polymer itself. This probably occurs because the epoxy bonds strongly to the particles and this inhibits segmental rotations of the polymer even at considerable distances from the particle surface.     

Self-organization of polymer particles on hydrophobic solid substrates in aqueous media. I. Self-organization of cationic polymer particles on alkylated glass plates

Monodisperse, cationic polymer particles bearing quaternary ammonium groups effectively self-organized on hydrophobic solid substrates such as alkylated glass plates and polymer films to form particle monolayers. With an increase of the particle surface charge density, the surface coverage decreased and the morphology of particle monolayers changed from aggregated type to dispersed type. The dispersed type of particle monolayers having a relatively regular particle distance was formed at higher temperature. The self-organization behaviors on alkylated glass plates were different from those on unmodified glass plates through electrostatic interaction. The formation of particle monolayers on alkylated glass plates occurred only over a certain latex concentration range in contrast with that on unmodified glass plate. The adhesive strength of particle monolayers was enhanced by annealing at temperatures above the glass transition temperature (T _g) of the particles. Lens-shaped particle monolayers were fabricated by annealing the dispersed type of particle monolayers.     

Negative dielectrophoretic patterning with colloidal particles and encapsulation into a hydrogel

Microparticle patterns have been fabricated on a nonconductive glass substrate and a conductive indium tin oxide (ITO) substrate using negative dielectrophoresis (n-DEP). The patterned microparticles on the substrate were immobilized by covalent bonding or embedded into polymer sheets or strings. The patterning device consisted of an ITO interdigitated microband array (IDA) electrode as the template, a glass or ITO substrate, and a polyester film (10-microm thickness) as the spacer. A suspension of 2-microm-diameter polystyrene particles was introduced into the device between the upper IDA and the bottom glass or ITO support. An ac electrical signal (typically 20 Vpp, 3 MHz) was then applied to the IDA, resulting in the formation of line patterns with low electric field gradient regions on the bottom support. When the glass substrate was used as the bottom support, the particles aligned under the microband electrodes of the IDA within 5 s because the aligned areas on the support were regions with the weakest electric field; however, for the ITO support, the particles were directed to the regions under the electrode gap and aligned on the support because these regions had the weakest electric field. The width of the particle lines could be roughly controlled by regulating the initial concentration of the suspended particles. The particles forming the line and grid patterns with single-particle widths were immobilized by using a cross-linking reaction between the amino groups on the aligned particles and N-hydroxysuccinimide-activated ester on the glass substrate activated by succinimidyl 4-(p-maleimidophenyl)-butyrate (SMPB). The patterned particles were also embedded in a photoreactive hydrogel polymer. A prepolymer solution of poly(ethylene glycol) diacrylate (PEG-DA) was used as the suspension medium to maintain the particle patterns in the polymerized hydrogel sheet and string following photopolymerization. The hydrogel sheets with particle patterns were fabricated by ultraviolet (UV) irradiation through the ITO-IDA template for 120 s. Hydrogel strings with the aligned particles were fabricated by using a conductive ITO support and a Pt-IDA template. Pt-IDA was used as a template as well as a photomask to block UV transmission. The present procedure affords extremely simple, rapid, and highly reproducible fabrication of particle arrays. The reusability of the template IDA electrode is also a substantial advantage over previous methods.