Colloids are small particles with at least one of their dimensions in the range of a few nanomete... more Colloids are small particles with at least one of their dimensions in the range of a few nanometers to one micrometer, where Brownian motion plays a critical role. 1–3 They are analogous to giant molecules in some respects, and behave in fair agreement with ...
This paper describes a sol−gel approach for the coating of superparamagnetic iron oxide nanoparti... more This paper describes a sol−gel approach for the coating of superparamagnetic iron oxide nanoparticles with uniform shells of amorphous silica. The coating process has been successfully applied to particles contained in a commercial ferrofluid (e.g., the EMG 304 of Ferrofluidics) and those synthesized through a wet chemical process. The thickness of silica coating could be conveniently controlled in the range of 2−100 nm by changing the concentration of the sol−gel solution. Fluorescent dyes, for example, 7-(dimethylamino)-4-methylcoumarin-3-isothiocyanate (DACITC) and tetramethylrhodamine-5-isothiocyanate (5-TRITC), have also been incorporated into the silica shells by covalently coupling these organic compounds with the sol−gel precursor. These multifunctional nanoparticles are potentially useful in a number of areas because they can be simultaneously manipulated with an externally applied magnetic field and characterized in situ using conventional fluorescence microscopy.
This paper describes a simple and convenient approach that allowed for the facial synthesis of si... more This paper describes a simple and convenient approach that allowed for the facial synthesis of silver/silica coaxial nanocables with wellcontrolled sheath thicknesses in the range of 2−100 nm. The lengths of these nanocables could be up to ∼50 µm. Such nanocables were prepared by directly coating bicrystalline silver nanowires with conformal sheaths of silica through a sol−gel process. The silver nanowires were, in turn, synthesized using a polyol method that involved the reduction of silver nitrate with ethylene glycol in the presence of poly(vinyl pyrrolidone). The thickness of silica coating could be controlled by changing the concentration of the precursor solution and/or the reaction time. Selective removal of the silver cores led to the formation of silica nanotubes having well-controlled dimensions and wall structures.
The Stöber method has been adopted to prepare hybrid core-shell particles by coating the surfaces... more The Stöber method has been adopted to prepare hybrid core-shell particles by coating the surfaces of monodisperse polystyrene beads with uniform silica shells. Polystyrene beads with diameters in the range of 0.1-1.0 microm have been successfully demonstrated for use with this process, and the thickness of the silica coating could be controlled in the range of 50-150 nm by adjusting the concentration of tetraethoxysilane, the deposition time, or both. The morphology and surface smoothness of the deposited silica were found to strongly depend on a number of parameters such as the surface functional groups on the polymer beads, the pH value of the medium, and the deposition time. Hollow spheres made of silica could be obtained by selectively removing the polymer cores via calcination in air at an elevated temperature or by wet etching with toluene. These core-shell colloids were also explored as building blocks to fabricate long-range ordered lattices (or colloidal crystals) that exhibited stop bands different from those assembled from spherical colloids purely made of either polystyrene or silica.
This paper describes a convenient approach to the fabrication of fluidic cells to be used for cry... more This paper describes a convenient approach to the fabrication of fluidic cells to be used for crystallizing spherical colloids into three-dimensionally periodic lattices over large areas. The major component of the fluidic cell was a rectangular gasket sandwiched between two glass substrates. Here we demonstrate that these gaskets could be simply cut out of commercial Mylar films. Three non-photolithographic methods were also demonstrated to create shadow channels between the Mylar film and two glass substrates: (i) by wiping (along one single direction) both sides of the Mylar film with a piece of soft paper (Kimwipes EX-L); (ii) by coating both surfaces of the Mylar film with polymer beads whose size was smaller than those to be packed in the cell; and (iii) by patterning the surface of the bottom glass substrate with an array of gold channels using a combination of microcontact printing and selective etching. When an aqueous dispersion of monodispersed spherical colloids was injected into this packing cell, a crystalline lattice nucleated and grew from the edge(s) of the cell as a result of solvent depletion through the channels between the Mylar film and the glass substrates. The capability and feasibility of this new approach have been demonstrated by the fabrication of uniform opaline lattices of polystyrene beads and silica colloids over areas as large as several square centimeters. Because Mylar films with thicknesses in the range 20-100 µm are commercially available in large quantities and at reasonably low costs, the present approach offers a flexible tool to those who want to explore the use of large crystals of spherical colloids but have no access to clean room facilities.
This paper describes a strategy that combines physical templating and capillary forces to assembl... more This paper describes a strategy that combines physical templating and capillary forces to assemble monodispersed spherical colloids into uniform aggregates with well-controlled sizes, shapes, and structures. When an aqueous dispersion of colloidal particles was allowed to dewet from a solid surface that had been patterned with appropriate relief structures, the particles were trapped by the recessed regions and assembled into aggregates whose structures were determined by the geometric confinement provided by the templates. We have demonstrated the capability and feasibility of this approach by assembling polystyrene beads and silica colloids (g150 nm in diameter) into complex aggregates that include polygonal or polyhedral clusters, linear or zigzag chains, and circular rings. We have also been able to generate hybrid aggregates in the shape of HF or H 2 O molecules that are composed of polymer beads having different diameters, polymer beads labeled with different organic dyes, and a combination of polymeric and inorganic beads. These colloidal aggregates can serve as a useful model system to investigate the hydrodynamic and optical scattering properties of colloidal particles having nonspherical morphologies. They should also find use as the building blocks to generate hierarchically self-assembled systems that may exhibit interesting properties highly valuable to areas ranging from photonics to condensed matter physics.
This paper describes a procedure for generating asymmetric dimers from two types of monodispersed... more This paper describes a procedure for generating asymmetric dimers from two types of monodispersed, spherical colloids that could be different in size, chemical composition, surface functionality, density or sign of surface charges, bulk properties, or a combination of characteristics. The availability of colloidal particles that are uniform in size, shape, composition, and surface or bulk properties has played an important role in elucidating and understanding the optical, rheological, and electrokinetic behaviors of these materials. 1 Spherical colloids have been the predominant subject of research for many years due to their ease of production as monodispersed samples. 2,3 They may also represent the simplest form of building blocks that could be readily self-assembled into three-dimensionally ordered structures: colloidal crystals or opaline arrays. 4 The capability to crystallize spherical colloids into highly ordered, 3D structures has allowed one to obtain interesting and useful functionality not only from the constituent materials but also from the long-range, 3D order (or periodicity) that characterizes these crystalline lattices. 4,5 Despite of their predominant roles in colloid science, spherical colloids are not necessarily the best option for all fundamental studies or real-world applications that are associated with colloidal particles. They cannot, for example, model the behaviors of highly irregular colloids that are more commonly found in industrial products. 1 Theoretical studies have also indicated that they are not well-suited as building blocks in generating 3D photonic crystals with complete band gaps because of a degeneracy in the photonic band structure as caused by the spherical symmetry of the lattice points. 6 Nonspherical particles offer some immediate advantages over their spherical counterparts in applications that require lattices with lower symmetries and higher complexities. Although a variety of chemical methods have been developed for synthesizing spherical colloids (e.g., polymer latexes or silica beads) as monodispersed systems, only a few methods are available for generating nonspherical colloids as truly monodispersed samples, in which the shape, size, and charge chemically fixed on the surface are all identical to within 2%. 7-11 Here we describe a general approach that uses geometrical confinement 10.
The monolayer of silica colloids was prepared by placing a drop of the colloidal dispersion on a ... more The monolayer of silica colloids was prepared by placing a drop of the colloidal dispersion on a flat substrate (such as silicon wafer or glass slide) and letting the solvent (in this case, water) to
This article presents an overview of current research activities that center on monodispersed col... more This article presents an overview of current research activities that center on monodispersed colloidal spheres whose diameter falls anywhere in the range of 10 nm to 1 mm. It is organized into three parts: The first part briefly discusses several useful methods that have been developed for producing monodispersed colloidal spheres with tightly controlled sizes and well-defined properties (both surface and bulk). The second part surveys some techniques that have been demonstrated for organizing these colloidal spheres into two-and three-dimensionally ordered lattices. The third part highlights a number of unique applications of these crystalline assemblies, such as their uses as photonic bandgap (PBG) crystals; as removable templates to fabricate macroporous materials with highly ordered and three-dimensionally interconnected porous structures; as physical masks in lithographic patterning; and as diffractive elements to fabricate new types of optical sensors. Finally, we conclude with some personal perspectives on the directions towards which future research in this area might be directed.
Colloids are small particles with at least one of their dimensions in the range of a few nanomete... more Colloids are small particles with at least one of their dimensions in the range of a few nanometers to one micrometer, where Brownian motion plays a critical role. 1–3 They are analogous to giant molecules in some respects, and behave in fair agreement with ...
This paper describes a sol−gel approach for the coating of superparamagnetic iron oxide nanoparti... more This paper describes a sol−gel approach for the coating of superparamagnetic iron oxide nanoparticles with uniform shells of amorphous silica. The coating process has been successfully applied to particles contained in a commercial ferrofluid (e.g., the EMG 304 of Ferrofluidics) and those synthesized through a wet chemical process. The thickness of silica coating could be conveniently controlled in the range of 2−100 nm by changing the concentration of the sol−gel solution. Fluorescent dyes, for example, 7-(dimethylamino)-4-methylcoumarin-3-isothiocyanate (DACITC) and tetramethylrhodamine-5-isothiocyanate (5-TRITC), have also been incorporated into the silica shells by covalently coupling these organic compounds with the sol−gel precursor. These multifunctional nanoparticles are potentially useful in a number of areas because they can be simultaneously manipulated with an externally applied magnetic field and characterized in situ using conventional fluorescence microscopy.
This paper describes a simple and convenient approach that allowed for the facial synthesis of si... more This paper describes a simple and convenient approach that allowed for the facial synthesis of silver/silica coaxial nanocables with wellcontrolled sheath thicknesses in the range of 2−100 nm. The lengths of these nanocables could be up to ∼50 µm. Such nanocables were prepared by directly coating bicrystalline silver nanowires with conformal sheaths of silica through a sol−gel process. The silver nanowires were, in turn, synthesized using a polyol method that involved the reduction of silver nitrate with ethylene glycol in the presence of poly(vinyl pyrrolidone). The thickness of silica coating could be controlled by changing the concentration of the precursor solution and/or the reaction time. Selective removal of the silver cores led to the formation of silica nanotubes having well-controlled dimensions and wall structures.
The Stöber method has been adopted to prepare hybrid core-shell particles by coating the surfaces... more The Stöber method has been adopted to prepare hybrid core-shell particles by coating the surfaces of monodisperse polystyrene beads with uniform silica shells. Polystyrene beads with diameters in the range of 0.1-1.0 microm have been successfully demonstrated for use with this process, and the thickness of the silica coating could be controlled in the range of 50-150 nm by adjusting the concentration of tetraethoxysilane, the deposition time, or both. The morphology and surface smoothness of the deposited silica were found to strongly depend on a number of parameters such as the surface functional groups on the polymer beads, the pH value of the medium, and the deposition time. Hollow spheres made of silica could be obtained by selectively removing the polymer cores via calcination in air at an elevated temperature or by wet etching with toluene. These core-shell colloids were also explored as building blocks to fabricate long-range ordered lattices (or colloidal crystals) that exhibited stop bands different from those assembled from spherical colloids purely made of either polystyrene or silica.
This paper describes a convenient approach to the fabrication of fluidic cells to be used for cry... more This paper describes a convenient approach to the fabrication of fluidic cells to be used for crystallizing spherical colloids into three-dimensionally periodic lattices over large areas. The major component of the fluidic cell was a rectangular gasket sandwiched between two glass substrates. Here we demonstrate that these gaskets could be simply cut out of commercial Mylar films. Three non-photolithographic methods were also demonstrated to create shadow channels between the Mylar film and two glass substrates: (i) by wiping (along one single direction) both sides of the Mylar film with a piece of soft paper (Kimwipes EX-L); (ii) by coating both surfaces of the Mylar film with polymer beads whose size was smaller than those to be packed in the cell; and (iii) by patterning the surface of the bottom glass substrate with an array of gold channels using a combination of microcontact printing and selective etching. When an aqueous dispersion of monodispersed spherical colloids was injected into this packing cell, a crystalline lattice nucleated and grew from the edge(s) of the cell as a result of solvent depletion through the channels between the Mylar film and the glass substrates. The capability and feasibility of this new approach have been demonstrated by the fabrication of uniform opaline lattices of polystyrene beads and silica colloids over areas as large as several square centimeters. Because Mylar films with thicknesses in the range 20-100 µm are commercially available in large quantities and at reasonably low costs, the present approach offers a flexible tool to those who want to explore the use of large crystals of spherical colloids but have no access to clean room facilities.
This paper describes a strategy that combines physical templating and capillary forces to assembl... more This paper describes a strategy that combines physical templating and capillary forces to assemble monodispersed spherical colloids into uniform aggregates with well-controlled sizes, shapes, and structures. When an aqueous dispersion of colloidal particles was allowed to dewet from a solid surface that had been patterned with appropriate relief structures, the particles were trapped by the recessed regions and assembled into aggregates whose structures were determined by the geometric confinement provided by the templates. We have demonstrated the capability and feasibility of this approach by assembling polystyrene beads and silica colloids (g150 nm in diameter) into complex aggregates that include polygonal or polyhedral clusters, linear or zigzag chains, and circular rings. We have also been able to generate hybrid aggregates in the shape of HF or H 2 O molecules that are composed of polymer beads having different diameters, polymer beads labeled with different organic dyes, and a combination of polymeric and inorganic beads. These colloidal aggregates can serve as a useful model system to investigate the hydrodynamic and optical scattering properties of colloidal particles having nonspherical morphologies. They should also find use as the building blocks to generate hierarchically self-assembled systems that may exhibit interesting properties highly valuable to areas ranging from photonics to condensed matter physics.
This paper describes a procedure for generating asymmetric dimers from two types of monodispersed... more This paper describes a procedure for generating asymmetric dimers from two types of monodispersed, spherical colloids that could be different in size, chemical composition, surface functionality, density or sign of surface charges, bulk properties, or a combination of characteristics. The availability of colloidal particles that are uniform in size, shape, composition, and surface or bulk properties has played an important role in elucidating and understanding the optical, rheological, and electrokinetic behaviors of these materials. 1 Spherical colloids have been the predominant subject of research for many years due to their ease of production as monodispersed samples. 2,3 They may also represent the simplest form of building blocks that could be readily self-assembled into three-dimensionally ordered structures: colloidal crystals or opaline arrays. 4 The capability to crystallize spherical colloids into highly ordered, 3D structures has allowed one to obtain interesting and useful functionality not only from the constituent materials but also from the long-range, 3D order (or periodicity) that characterizes these crystalline lattices. 4,5 Despite of their predominant roles in colloid science, spherical colloids are not necessarily the best option for all fundamental studies or real-world applications that are associated with colloidal particles. They cannot, for example, model the behaviors of highly irregular colloids that are more commonly found in industrial products. 1 Theoretical studies have also indicated that they are not well-suited as building blocks in generating 3D photonic crystals with complete band gaps because of a degeneracy in the photonic band structure as caused by the spherical symmetry of the lattice points. 6 Nonspherical particles offer some immediate advantages over their spherical counterparts in applications that require lattices with lower symmetries and higher complexities. Although a variety of chemical methods have been developed for synthesizing spherical colloids (e.g., polymer latexes or silica beads) as monodispersed systems, only a few methods are available for generating nonspherical colloids as truly monodispersed samples, in which the shape, size, and charge chemically fixed on the surface are all identical to within 2%. 7-11 Here we describe a general approach that uses geometrical confinement 10.
The monolayer of silica colloids was prepared by placing a drop of the colloidal dispersion on a ... more The monolayer of silica colloids was prepared by placing a drop of the colloidal dispersion on a flat substrate (such as silicon wafer or glass slide) and letting the solvent (in this case, water) to
This article presents an overview of current research activities that center on monodispersed col... more This article presents an overview of current research activities that center on monodispersed colloidal spheres whose diameter falls anywhere in the range of 10 nm to 1 mm. It is organized into three parts: The first part briefly discusses several useful methods that have been developed for producing monodispersed colloidal spheres with tightly controlled sizes and well-defined properties (both surface and bulk). The second part surveys some techniques that have been demonstrated for organizing these colloidal spheres into two-and three-dimensionally ordered lattices. The third part highlights a number of unique applications of these crystalline assemblies, such as their uses as photonic bandgap (PBG) crystals; as removable templates to fabricate macroporous materials with highly ordered and three-dimensionally interconnected porous structures; as physical masks in lithographic patterning; and as diffractive elements to fabricate new types of optical sensors. Finally, we conclude with some personal perspectives on the directions towards which future research in this area might be directed.
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