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A Class of Microstructured Particles Through Colloidal Crystallization

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Science  24 Mar 2000:
Vol. 287, Issue 5461, pp. 2240-2243
DOI: 10.1126/science.287.5461.2240

Abstract

Microstructured particles were synthesized by growing colloidal crystals in aqueous droplets suspended on fluorinated oil. The droplets template highly ordered and smooth particle assemblies, which diffract light and have remarkable structural stability. The method allows control of particle size and shape from spheres through ellipsoids to toroids by varying the droplet composition. Cocrystallization in colloidal mixtures yields anisotropic particles of organic and inorganic materials that can, for example, be oriented and turned over by magnetic fields. The results open the way to controllable formation of a wide variety of microstructures.

The study of colloidal crystals has provided important fundamental insights into colloidal forces and self-organization (1, 2), and the crystals have promise as precursors for advanced materials. Closely packed crystals display a number of potentially usable characteristics such as light diffraction and photonic band gaps (2–4), high packing density and surface-to-volume ratio, maximal structural stability, and high catalytic/reactivity throughput. A major barrier to technological application of these structures is the lack of simple, easily controlled methods for mounting or shaping the crystals into usable solid objects.

Here we describe a method to template closely packed colloidal crystals into porous particles. Conventional methods for forming porous particles include stage-templated suspension polymerization (5), spray drying or drying in oil (6), sintering or dry impact blending (7), or aggregation in solution (8). Particles made by these methods are generally polydisperse or irregular, lack long-range ordering of the pores, and display a wide pore-size distribution that is not desirable for applications such as size-exclusion chromatography (5) and that decreases the mechanical stability of the particles. Nanoporous particles of narrow pore-size distribution have been synthesized by templating with self-assembled surfactant phases (9, 10), but this method has yet to be extended to larger pore sizes.

Few methods for forming structured particles by assembly of smaller particles produce aggregates with long-range order. Ordered shells have been assembled from monodisperse micrometer-sized latex particles by the use of emulsion droplets as templates (11), but only two-dimensional (2D) polydisperse assemblies form, and the scheme for their production is relatively complex. An alternative approach has been to use droplets for the assembly of larger objects by the use of capillary forces (12). This method, however, requires large, specially prefabricated objects, and the assemblies are not structured or porous on a submicrometer level.

In the method outlined here, suspended droplets were used as templates within which the colloidal particles were gradually concentrated by drying to form 3D crystals, yielding a variety of microstructured symmetric supraparticles. Aqueous droplets of colloidal suspension were placed on the surface of perfluoromethyldecalin, an inert, dense (1.95 g/cm3) liquid immiscible with water (Fig. 1). The droplets, of initial volume of a few microliters, contained 5 to 30 volume % (vol %) suspensions of negatively charged polystyrene latex microspheres 270 to 630 nm in diameter (13). To ensure the absence of strongly curved oil menisci at the walls of the container, which could attract and deform the droplets through capillary forces (14), we lined the inside of the Petri dish holding the oil with thin Teflon tape. The menisci formed in this way tended to attract the floating droplets gently to the walls, where they remained in contact with the surrounding Teflon tape but without noticeable deformation. The dish with the fluorinated oil was kept in a sealed chamber that contained a desiccating agent to drive removal of the water from the suspended droplets. The droplets shrank and ordered structures formed inside them within the first 2 to 4 hours of the incubation. After 12 hours, the crystalline porous particles were extracted and dried of oil.

Figure 1

Schematic of the assembly method. The template aqueous droplets are suspended on the surface of fluorinated oil and colloid crystals form during the drying process, as described in the text.

The initial experiments were carried out with 1- to 2-μl droplets of relatively high latex content (20 to 30 vol %). The assembly process yielded smooth spherical assemblies 100 to 500 μm in diameter and of very low density. Low-magnification optical microscopy shows brightly colored reflecting patches on the surface of these balls (Fig. 2). The colors depend on the size of the latex particles used: green and red for assemblies made of 270-nm microspheres, blue for 320-nm microspheres, and yellow-green for 630-nm microspheres. All porous supraparticles within a given batch displayed excellent homogeneity in size and appearance. The colors arise from light diffraction (opalescence) from the colloidal arrays, indicating long-range ordering of the latex particles, which can be visualized directly by scanning electron microscopy (SEM) (Fig. 3). Large domains of hexagonally packed particles are seen on the surface; each domain incorporates hundreds to thousands of microspheres in the first visible layer (Fig. 3A). The overall curvature of the porous balls is expressed in the dislocations between the different domains, but because of the large difference between the ball curvature and the latex diameter, the surface appears perfectly smooth on length scales comparable to the ball size. The size of the monocrystalline domains is comparable to those in the closely packed crystalline arrays obtained by slow evaporation or filtration of films from latex suspensions. However, the assemblies exhibit an almost complete absence of square arrays, which are commonly observed with planar crystals obtained by filtration or sedimentation [see, for example, (2,15)].

Figure 2

Optical micrographs of structured spherical assemblies obtained from (A) 270-nm and (B) 320-nm latex particles. Scale bars, 500 μm.

Figure 3

(A) A scanning electron micrograph of a typical area on the surface of the assemblies. (B) A scanning electron micrograph along the edge of a broken particle. The surface is seen at the top and the cut through the bulk is toward the bottom of the micrograph. Scale bars, 1 μm.

The structure of the latex particles inside the porous balls can be observed along the edges of broken particles (Fig. 3B). The long-ranged ordered structure on the surface (top half) continues through the bulk of the ball and usually down to its center. The types of phases seen—coexisting randomly stacked hexagonal closely packed planes and face-centered cubic lattices—are similar to those reported earlier for bulk colloidal crystals (1,2, 15). In contrast to crystals on solid surfaces, arrays of body-centered cubic packing are usually not observed. The absence of square arrays on the surface and in the bulk is probably due to the increased mobility of the particles adjacent to the nonsticky fluid drop surface, which allows better reorientation of the lattices during drying, and assembly into the more thermodynamically stable hexagonal phase. We showed that free rearrangement is a major factor favoring the formation of well-ordered crystals, both by addition of electrolyte, which decreases the quality of the arrays by decreasing the particle repulsion, and by gentle ultrasound treatment, which improves the structure by enhancing the rearrangement (16). The high packing density and order possibly contribute to the structural stability of the assemblies, which remain intact after mild to moderate mechanical manipulation and compression in both air and water. In contrast, the typical disordered latex aggregates obtained from suspension coagulation easily break into powder in air and spontaneously disassemble under water.

Our method allows control of the shape of the suspended template droplets by the interplay of droplet size, gravitational force, and interfacial tensions. The shape of a floating droplet (or lens) in general is given by the solution of the Laplace differential equation (17). The parameter that characterizes droplet deformation is the Bond number β = (Δρgb 2)/γ, where Δρ is the difference in the densities of the droplet and the surrounding fluid, gis the gravitational acceleration, b is the characteristic size of the droplet, and γ is the interfacial tension (17). The droplet is nearly spherical at values of β → 0 and flattens as the value of β increases. Different values of β at the lower and upper droplet interfaces can be set experimentally by varying the relative densities of the fluids or by adding appropriate surfactants. The interfacial tensions in our system in the absence of surfactant are γwater-air = 72 mN/m, γwater-oil = 53 mN/m, and γoil-air = 19 mN/m (18). As the tension at the water-air boundary is about equal to the sum of the tensions at the other two interfaces, the droplet will have a smooth boundary at the three-phase contact line rather than a sharp contact edge (17). For 1-μl template droplets of pure liquids in our system, β < 0.05, and thus the droplet shape will be essentially spherical. The water-fluorocarbon-air system provides a great degree of flexibility in independently controlling the interfacial tensions by the use of surfactants that adsorb exclusively on the different interfaces (19). For larger droplets in the presence of surfactants, values of β > 1 are accessible, and lenslike, discoidal, and ellipsoidal templates with various three-phase contact angles can be obtained. Detailed formulas for calculation of the shapes in all cases are available (17,20).

Major modifications of the particle shape also result from the coupling of hydrodynamics and particle accumulation during the drying/crystallization process. If the initial volume fraction of the microspheres in the droplets is close to the crystallization threshold of about 50 to 60 vol %, the assemblies obtained are close in size and shape to those of the original droplets. However, if the latex concentration is below about 20%, the particles accumulate at the droplet periphery during water evaporation, which leads to subsequent deviations from a spherical shape. The shape of the assemblies then evolves from spherical (for small droplets) to flattened disks or ellipsoids for larger (>3 μl) droplets. An example of how the aspect ratio of such assemblies can be varied by altering the latex volume fraction is shown in Fig. 4A.

Figure 4

Examples of shape control during assembly. (A) Assemblies from suspensions with latex concentrations of (1) 10%, (2) 6.7%, (3) 4%, and (4) 2.4%. Particle 1 is a sphere with flattened top and bottom, whereas all of the others are discs with different thicknesses. The focal plane is adjusted below the top of particle 1 and above the top of particle 4, showing the variation in the thickness. (B) Dimpled particles in the presence of fluorinated surfactant. (C and D) Toroidal, doughnutlike assemblies formed at higher surfactant concentration. Scale bars, 500 μm. Supplementary material with additional examples of variation of the shape and appearance of the particles is available atScience Online (25).

Further control of the shape can be achieved by adding surfactants to vary the interfacial tensions. Addition of a fluorinated surfactant, sodium perfluorooctanoate, led to formation of unexpectedly complex particles. Droplets containing 0.006 mM surfactant first flatten and then develop a dimple in the middle as they dry, ultimately yielding concave particles shaped like red blood cells (Fig. 4B). At higher surfactant concentrations (0.05 mM), the dimples between the two opposing surfaces bridge to form a hole. These dried assemblies are submillimeter toroids, or crystalline “doughnuts” (Fig. 4, C and D). The latex microspheres in the doughnuts are as well ordered as those in the spherical or the discoidal supraparticles, and the doughnuts also demonstrate the usual brightly colored diffraction patches on the surface (Fig. 4D). However, the lower interfacial tension caused by the surfactant allows some formation of steps between the bordering domains on the surface. The formation and the shape of the toroids are readily controllable; the outer diameter depends on the initial droplet size and particle concentration, whereas the size of the inner hole depends on the surfactant concentration.

Another important practical feature of the method is that complex composite assemblies can be fabricated when the original drop contains a mixture of two or more different types of particles. For example, the segregation of two types of colloids during assembly yields complex anisotropic particles. Such particles were formed from mixtures of 420-nm regular polystyrene beads (white in color) and 670-nm paramagnetic latex microspheres [encapsulating 20 weight % (wt %) magnetite, which are brown]. The heavier magnetic particles could be separated by gravity alone, so anisotropic particles can be produced merely by gravitational separation of the components in the droplets. However, a much quicker and cleaner separation results from application of a constant magnetic field during the drying/crystallization process (21). Complete separation of the microparticles into the two hemispheres of the dried assembly is seen in Fig. 5A. A valuable property of such anisotropic particles is that they can be manipulated by electric or magnetic fields. For example, a magnetic field immediately orders and orients the bicolored magnetic balls (Fig. 5B). The arrays then flip and change the displayed face from white to brown and back as the gradient of the field is reversed.

Figure 5

Examples of anisotropic particles. (A) Unoriented spherical assemblies incorporating regular (white) and magnetic (brown) latexes. (B) In the presence of a magnetic field originating from below, these particles immediately form an array with the white halves oriented up. If the magnetic field gradient is reversed, the particles flip to form an array with the brown side up. (C and D) Two assemblies of complex shape obtained in the presence of small gold particles, as viewed from opposite faces. A metallic ring is deposited only on the side of the particle that was originally exposed to air. Scale bars, 500 μm.

Finally, mixtures of two particles of substantially different sizes can be used both to create interesting anisotropic assemblies and to study the dynamics of the process. We studied suspensions containing 8% of 270-nm latex microspheres and 0.005 wt % of 40-nm colloidal gold particles. The much smaller gold particles are carried through the interstices of the latex by the retreating water during the drying process. These particles are ultimately deposited, and aggregated, at the surface from which the last traces of water evaporate. The dried gold colloids form blackish metallic rings only on the side of the assemblies in contact with air. The result is an unconventional “glazed doughnut” structure (Fig. 5, C and D). As the formation of such assemblies is also guided by the wetting of the particles, our method can be modified to make particles of anisotropic wettability, broadly similar to the “Janus” beads demonstrated earlier (22).

Crystalline assemblies such as those reported here display long-ranged ordering of the constituting small particles that ensures uniform porosity and maximum mechanical stability and allows distinctive optical properties. The most unconventional feature of our method is the control of particle shapes and compositions. For example, the doughnuts may offer advantages in chromatography over the usual spherical particles by virtue of their combination of reduced mass transfer length scale, reduced pressure drop, and uniform porosity. Particles with magnetic and color anisotropy such as those shown inFig. 5 can find application in switchable coatings, electronic paper (23), magnetooptical devices, or electrorheological or magnetorheological fluids. In addition to being anisotropic and magnetic, these particles are also of uniform porosity, monodisperse in size, and highly structured and can be produced in different shapes. All of these properties suggest areas for potential applications.

The variety of samples obtained warrants extension of the method to the formation of a wide range of particulate materials. Ideally, spherical particles can be assembled in droplets suspended in density-matched liquids (simulated microgravity conditions). The method can be modified for the assembly of inorganic particles from silica, TiO2, ZrO2, metals, or semiconductors. A wide variety of composites from organic, inorganic, and metallic particles are also possible. Anisotropic particles, responsive not only to magnetic but also to electric fields or light, can be created by such combinations. Another system of practical interest may be the combination of the droplet shape templating with colloidal crystal templating of structured lattices of pores (24). Thus, the structures reported here may be a prelude to a previously unidentified class of advanced particulate materials.

  • * To whom correspondence should be addressed. E-mail: velev{at}che.udel.edu

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