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Colloid science & engineering

• AC Electrokinetic Technique for Particle Collection on Patterned Electrodes
• Convective Assembly of Antireflective Silica Coatings
• Dielectrophoretic Assembly of On-chip Nanoparticle Wires
• Controlled Deposition and Modification of Structured Colloidal Coatings
• Synthesis and Self-assembly of Particles with Anisotropic Shape, and/or Dipolar Charge
• Manipulation of droplets with suspended particles on a dielectrophoretic liquid chip

Biomaterials & sensors

• Making new SERS materials
• Controlled deposition of live cell coatings
• Silver-Enhanced Immunoassays for Analyte Detection
• Controlled dielectrophoretic assembly of biomaterials from live cells and colloidal particles
• Developing quantitative micro-bioassays needing only microliter volumes of analyte for biomoleculer detection

AC Electrokinetic Technique for Particle Collection on Patterned Electrodes

Ketan Bhatt

Micro-total analysis systems are now being developed for various chemical and biological applications, including biosensors, bio-warfare agent detection and gene separation and analysis. The first step in using any of these systems involves separating and collecting the analytes of interest from the bulk fluid medium. The use of alternating electric fields allows the separation of different particles and further handling and analyzing of the particles of interest. We have developed a new type of AC electrokinetic technique that collects and concentrates colloidal particles from the bulk liquid medium using a combination of AC electrohydrodynamics and dielectrophoresis.

A conductive silicon wafer covered by a layer of photoresist is patterned by lithography to expose square patches of the underlying silicon. The patterned silicon substrate acts as the bottom electrode. An ITO-coated glass slide acts as the opposing electrode in 0.75 mm thick chambers. Dilute suspensions (0.001-0.01% solids) of polystyrene microspheres of diameters 0.2 - 5 µm are enclosed in the chamber. When AC electric fields of 5-15 V/mm are applied to the electrodes the latex particles entrained by the electrohydrodynamic flow begin collecting in the center of the conductive regions. The collected particles create patterns with four-fold symmetry inside the "corrals". With time, all latex particles are collected and entrapped on the bottom of the corrals. The combination of phenomena that make the system work has not been described in the literature and is of both fundamental and applied interest.

We will show that the collection and concentration of particles in the patterned "corrals" is a result of the combined action of positive dielectrophoretic force and the AC electrohydrodynamic flow in the chamber. The collection speed and efficiency of the chips is dependent on a number of process parameters, including pattern geometry, particle and electrolyte concentration and field intensity and frequency. The experimental results are interpreted by means of electrostatic calculations and a model is proposed to describe the various forces acting on the particles under these conditions taking into account the electrohydrodynamic flow.

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Convective Assembly of Antireflective Silica Coatings

Yeon Hwang
http://plaza.snut.ac.kr/~mse/new/index.htm

One route to AR coatings is through porous materials in which the void space is readily occupied by the ambient surrounding media (air). A wide array of top down electrochemical etching techniques have been employed to create porous surfaces. Alternatively, effective AR materials have been made via the bottom up approaches of soft lithography, vacuum deposition techniques (sputtering, reactive ion etching, and laser ablation…), sol-gel dip coating or spin coating, polyelectrolyte multilayer, colloidal particle deposition from liquid suspension, and electrostatic multilayer assemblies of polymers and colloids. Convective assembly at high volume fraction provides an economic and facile alternative to AR coating deposition.

In this study, colloidal silica coatings were deposited onto glass and silicon substrates via convective assembly to investigate their antireflective properties. Control over the film structure provided by the deposition technique allowed the reflectance to be reduced to 1% (over a range of wavelengths) for a single glass/air surface, which respectively translate to 70% relative reductions per surface. On silicon, the coatings provided up to a 51% reflectance reduction. Using binary mixtures of SiO 2 particles, the AR ability of the coatings on glass was further optimized to 0.5 % for a single surface, an 88% relative reduction in the glass single surface reflectance, with and average 67% relative reduction over the visible spectrum. The long range uniformity of the coatings allowed them to be well modeled by classical optics calculations, which provided additional insight and analysis of their properties.

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Prevo, Hwang, and Velev, Submitted (2005)

Manipulation of droplets with suspended particles on a dielectrophoretic liquid chip

Suk Tai Chang

This project began as an exploration of a platform for manipulation of freely suspended micro- and nanoliter droplets as a "liquid-liquid" alternative approach to microfluidics instead of using microchannels or tubes which are present in conventional microfluidic devices. Droplets of water or hydrocarbons floating on the surface of a dense, inert perfluorinated liquid are driven by alternating or constant electric fields created by addressable arrays of electrodes immersed in the oil. The ability to manipulate droplets from suspensions, and to observe the internal dynamics using tracer particles has revealed charge and polarization effects, as well as Marangoni flow within the droplets. At present, focus is on evaporation-induced microseparations inside the freely floating droplets. When the microdroplets with suspended colloidal particles inside are floating on fluorinated oil, the particles are vertically redistributed at the top of the microdroplets due to evaporation, which induces complex flow patterns within the droplets. Particles settle on the bottom in the absence of evaporation. By controlling the microseparation, we can make advanced anisotropic particles, perform microassays and study biological objects. By fundamental understanding of the microseparation mechanisms inside droplets, we will develop a novel microfluidic system for highly sensitive and high-throughput chemical/biological microassays.

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Velev, Prevo, and Bhatt, Nature 426, 515-516 (2003). [PDF]
Millman, Bhatt, Prevo, and Velev, Nature Materials 4, 98-102 (2005). [PDF]

Control and Modeling of the Dielectrophoretic Assembly of On-chip Nanoparticle Wires

Ketan Bhatt

Suspensions of metallic nanoparticles in water could be assembled via the action of alternating electric field (dielectrophoresis) into wires of micrometer thickness. We are developing tools to control the assembly process in order to use these structures in electrical microcircuits. Two modes of microwire assembly, one through the bulk of the suspension, and one as half-cylinders on the surface between the electrodes were identified. We show the effects responsible for these two assembly modes and model the growth of the microwires using finite element electrostatics calculations. The control of the process parameters allows making, for example, straight single connectors, or massively parallel arrays on the surface of the chip, which can be extracted in dry form. The microwire growth can be guided by introducing conductive islands or particles in the liquid and by switching the field to different electrodes. 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.

Controlled Deposition and Modification of Structured Colloidal Coatings

Brian Prevo

Our research is essentially devoted to fundamentally simple and technologically feasible means for manipulating and/or depositing functional colloidal materials. This project seeks to investigate the effects of deposition conditions and process parameters on the resultant coating structures using a variety of solvents and colloidal materials. The governing mechanism for deposition is convective assembly, whereby the evaporation of a thin liquid film of particle suspension contacting the substrate induces a replacement flux of solvent from the bulk which carries particles to the substrate. We have developed a technique for making thin films of structured micro- and nanoparticle particle layers by dragging a meniscus with constant velocity. Working at high volume fractions permits faster deposition speeds based on the principles of convective assembly. The size range of materials studied covers the range from 10 nm - 1 micrometer. Ordered coatings of larger than few square centimeters are deposited in minutes from aqueous suspension volumes of approximately 10 microliters. The advantages of this technique are improved process speed and efficiency and reduced material consumption relative to standard dip coating techniques. We have applied this deposition technique to a wide variety of different particle systems, which are discussed at more length in the publications listed below.

Silver-Enhanced Immunoassays for Analyte Detection

Shalini Gupta

Our research focuses on developing principles of bioassays and biosensors for low antigen or antibody concentrations in blood, plasma, serum etc. We developed and characterized sandwich immunoassays using simple silver-enhancement techniques. Sandwich immunoassays are relatively new methods of detecting the presence of disease-causing bacteria and viruses, using the fundamentals of specific antibody-antigen interactions. In our model experiments goat-anti-mouse immunoglobulin (GAM IgG) is immobilized on a hydrophobized glass surface to selectively detect the presence of mouse (M) IgG in the analyte. This spot is further incubated with gold conjugated GAM (GAMg) leading to the formation of GAM-M-GAMg sandwich assembly. Silver ions are then added to enhance the bound gold colloids, which form nucleation sites for silver ion reduction. The darkness of the spot is measured by densitometry. All experiments take place in a 30 µL flow chamber and the detection sensitivity limit of this experiment was found to be as low as 0.1 µg/ml. The detection time is approximately 1 hr and 30 mins. Negative and positive control experiments have been performed for this process to prove the specificity of detection. The results point out that only sandwich assays possess high selectivity, while false positives can occur in direct assays. Data for various concentrations of M IgG and different gold incubation times were collected and plotted against the corresponding silver enhanced intensities in order to characterize the role of mass transfer in this assay. Using these data, we will model our system with a diffusion rate limited process. We prove that silver enhanced immunoassays provide a simple, low-cost and effective way of detecting antigens in diluted solutions.

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Making new SERS materials

Daniel Kuncicky

We are interested in developing a new class of advanced substrates for chemical detection by surface-enhanced Raman scattering (SERS). Aqueous suspensions of Au and Ag nanoparticles are good precursors for SERS materials since the intrinsic size of the metallic particles leads to electric field enhancement. When the particles are used as building blocks to fabricate larger structures (self assembled fractal aggregates, convectively assembled aggregates and templated aggregates are examples), coupling between the electric fields of adjacent nanoparticles allows for even higher enhancement. We make highly active SERS films by depositing these colloidal suspensions onto solid substrates by way of convective assembly. Additional performance is achieved, if during the convective assembly process the metal colloid is ordered into a 3D array by templating the Au nanoparticles around colloidal crystals made from micron-sized latex spheres (allows for a detection limit of 2 ppb for aqueous NaCN). We are systematically studying the effect of film structure on Raman enhancement in order to understand how the high performance is achieved with 3D ordering of the porous structure, and to optimize the SERS films for chemical sensor applications. To complement these results with other types of data we plan to characterize the microphotonic properties of the films and identify possible "hot spot" regions. We are also studying routes towards modification of the SERS films in order to tune their selectivity for specific chemical agents. The results will be used to design millifluidic chip sensors that target different chemical and biological agents and allow for continuous monitoring of water or gas fluxes.

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Kuncicky, Christesen and Velev, Applied Spectroscopy, 59, 401-409 (2005). [PDF]
Kuncicky, Prevo, and Velev, Journal of Materials Chemistry 16, 1207-1211 (2006). [PDF]

Synthesis and Self-assembly of Particles with
Anisotropic Shape, and/or Dipolar Charge

Rossitza Alargova

A major problem in the colloidal assembly to date is that the particles commonly used as building blocks interact with each other mainly by non-directional (DLVO or non-DLVO) interactions. For that reason they may assemble in many possible configurations leaving no large space for architecture of structures with predefined geometry. The fabrication of particles with directional interactions could introduce a qualitatively new level of complexity and precision in the fabrication of micro- and nanostructures. Examples of such particles are the ones with anisotropic shape or permanent dipole charge. In general, little or almost nothing has been done so far to fabricate polymer anisotropic or dipolar particles in the sub- and micrometer range.Our project aims at developing convenient procedures for large scale synthesis of polymer particles with anisotropic shape, and/or dipolar charge and studying their self-organization into structures predetermined by their directional interactions. So far, we have developed a method for synthesis of polymer microrods which is simple and readily scalable to produce large amounts of particles. It allows fabrication of polymer rods with large aspect ratio, different content and surface properties. At the present we are focusing on understanding the fundamentals of the process and finding the best way to control the rods sizes and surface properties. The polymer rods can find an immediate application as model anisotropic particles for the needs of the fundamental and applied research. In addition to their possible use as liquid crystals, they may also serve as basic units for designing new paints and materials.

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Alargova, Bhatt, Paunov, and Velev, Advanced Materials 16, 1653-1657 (2004). [PDF]
Alargova, Warhadpande, Paunov and Velev, Langmuir 20, 10371-10374 (2004). [PDF]

Controlled Dielectrophoretic Assembly of
Biomaterials from Live Cells and Colloidal Particles

Shalini Gupta

The main focus of this project is applying dielectrophoresis to live cells and to colloidal particles in order to yield new class of biomaterials and sensor elements. We illustrate novel techniques of synthesizing permanent cell chains & arrays, co-assembled irreversible cell-particle microstructures and 2D membranes of cells using functionalized nano- and microparticles as biocolloidal binders. These particles attach to cells via biospecific lectin-polysaccharide interactions. The presence of synthetic particles between cells adds a new dimension to the class of biocomposite materials wherein the functionality of the particles gets combined with the functionality of the organized structure. Such architectures can find potential applications in areas such as biosensors, biomedical applications and biocomposite materials.

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Controlled deposition of live cell coatings

Lindsey Jerrim

This project is focused on extending the colloidal deposition techniques developed in our group to making uniform coatings of live cells on substrates and subsequently characterizing the coatings in view of the cell function. While the cell-coatings are deposited via convective assembly (from droplets of cell suspensions), the concurrent sedimentation of the relatively large yeast cells (~5 microns) affects the way they deposit on the substrate. Changing the deposition angle (from the horizontal) on the coating device helps to alleviate the sedimentation issue, and relatively large monolayers of yeast cells can be deposited. Two models have been developed, for complete mixing and no mixing within the droplet of cell suspension, to help interpret the experimental results.

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Developing quantitative micro-bioassays
which need only microliter volumes of analyte for biomoleculer detection

Vinayak Rastogi

Latex agglutination assays have been used so far as the tools for immunodetection. The disadvantages of these tests are that they require large amounts of sample, have ineffective optical detection, are hard to miniaturize and do not give simple electronic readouts. We use the novel microfluidic system developed by our group to manipulate microliter volume droplets to overcome the disadvantages of current immunodetection techniques. The liquid droplets float on the surface of high density oil and can be moved around using dielectrophoretic force. The evaporation rate of the droplets is controlled using a small humidity chamber. Due to evaporation, convective flow is established within the droplets and the dispersed colloidal particles collect on the surface of droplet exposed to the air. Depending upon the aggregation state of the biomolecules in the droplet, different visible patterns are obtained.

Using these bioassays multiple immunodetection tests can be carried out on a single chip using only nano-liter volumes of the sample. We are currently developing a automatic microfluidic system where a large droplet containing the analyte can be broken into smaller droplets of controlled volume and then moved onto the oil surface for immunodetection with multiple detectors.

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Last updated May 05, 2006