Abstract:
This disclosure describes a self-assembly templating of a cationic surfactant in the presence of a silica precursor that is free of an excessive variability of the assembled shapes and has a yield approaching one hundred percent. This disclosure describes a self-assembly process that includes cooling and keeping a resultant solution at cold temperatures during the synthesis.

Description:
CROSS REFERENCES 
     This application is related to Provisional Patent Application 60/631,224 filed on Nov. 29, 2004 entitled “Self-Assembly of Nanoporous Silica Fibers of Uniform Shape” and is hereby incorporated by reference. 
    
    
     FIELD OF INVENTION 
     The present invention is related to the self-assembly of nanoporous silica fibers of uniform shape, in particular a method having an improved yield and improved size distribution of the fibers. 
     BACKGROUND OF THE INVENTION 
     Existing Technologies and Problem Description 
     With the discovery of the liquid crystal templating of hexagonal, cubic and lamellar meso(nano)structured silica, materials chemistry has moved into the realm of design and synthesis of inorganics with complex form. It becomes possible to synthesize inorganics with structural features of a few nanometers and architectures over such large length scales, up to hundreds of microns. This nanochemistry is inspiring research in materials science, solid state chemistry, semiconductor physics, biomimetics and biomaterials. 
     Manipulation of surfactant packing parameter, headgroup charge, co-surfactants, solvents, co-solvents and organic additives have been used to template particular nanostructures. Dimension of the pores can be tuned with angstrom precision over the size range of 20-100 Å. 
     It has been shown that the use of cationic surfactants in the presence of a silica precursor can result in synthesizing a variety of well-defined nanoporous silica shapes. A cationic molecule can exchange its counteranion, say chloride, with a mineralizable inorganic anion, protonated silicate. Synthesis of mesoporous thin films, spheres, curved shaped solids, tubes, rods and fibers, membranes, and other monoliths has been reported recently. See references in Y. Kievsky and I. Sokolov, “Self-Assembly of Uniform Nanoporous Silica,  IEEE Transactions on Nanotechnology , v. 4 (5), pgs. 490-494, the entire article is hereby incorporated by reference. Despite demonstrated success of this approach, there are two problems that prevent broader use of the assembled porous shapes:
         1. Self-assembly process brings an excessive variety of the assembled shapes; and   2. The yield of the desired shapes is far from one hundred percent.       

     These factors make it difficult to extract, and subsequently, to use the desired shapes. 
     SUMMARY 
     This disclosure describes a self-assembly templating of a cationic surfactant in the presence of a silica precursor that is free of an excessive variety of the assembled shapes and has a yield approaching one hundred percent. This disclosure describes a self-assembly process that includes cooling and keeping a resultant solution at temperatures that are less than typically used for the synthesis, in a particular example of 4° C. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where: 
         FIG. 1   a  illustrates SEM images of the fibers assembled at the cold temperature (magnification a the horizontal bar is 11 μm); 
         FIG. 1   a ′ illustrates SEM images of the fibers assembled at the cold temperature (magnification a′—the horizontal bar is 11 μm); 
         FIG. 1   b  illustrates SEM images of the fibers assembled at room temperature (magnification b—the horizontal bar is 11 μm); 
         FIG. 1   b ′ illustrates SEM images of the fibers assembled at room temperature (magnification b′—the horizontal bar is 11 μm); 
         FIG. 2   a  illustrates a “Zoo” of shapes assembled at the room temperature (magnification a); 
         FIG. 2   a ′ illustrates a “Zoo” of shapes assembled at the room temperature (magnification a′); 
         FIG. 2   b  illustrates a “Zoo” of shapes assembled at the room temperature (magnification b); 
         FIG. 2   b ′ illustrates a “Zoo” of shapes assembled at room temperature (magnification b′); 
         FIG. 3  illustrates higher magnification SEM images of fibers assembled in the cold synthesis—the horizontal bar is 5 μm; 
         FIG. 4  illustrates the distribution of the fiber diameters, lengths and length-diameter ratio; 
         FIG. 5  illustrates distribution of the effective particle radii; 
         FIG. 6  illustrates SEM and TEM images of parallel nanotubes; and 
         FIG. 7  illustrates low-angle powder XRD pattern of the synthesized fibers. 
     
    
    
     DESCRIPTION 
     The Method of Self-Assembly 
     The object of the invention is the self-assembly templating of a cationic surfactant in presence of a silica precursor that is free of both of the problems mentioned above. 
     An acidic synthesis of nanoporous silica fibers was described previously by H. Yang and G. A. Ozin, “Morphogenesis of shapes and surface patterns in mesoporous silica,” Nature, vol. 386, pp. 692-695, 1997 [Yang et al], hereby incorporated by reference. Yang et al synthesized mesoporous silica bodies under quiescent aqueous acidic conditions using cetyltrimethylammonium chloride (CTACl) as surfactant template and tetraethylorthosilicate (TEOS) as silica precursor. Here we use the same ideas of liquid crystal templating and condensation of silica precursor, (for example, tetraethylorthosilicate or tetramethyorthosilicate (TMOS), “silica precursor” hereafter). Either CTACl or CTAB is used as an exemplary cationic surfactant template. The required acidity is created by means of hydrochloric, sulfuric, nitric, phosphoric or other strong acids. All chemicals are standard (used as purchased). The surfactant, acid, and either ultrapure or deionized water are mixed, stirred first at room temperature and the mixture then is cooled down to a temperature that is well below the room temperature (4° C. is used as an exemplary value) in a refrigerator for a reasonable time that can be judged by one skilled in art (15 minutes is an exemplary value). Cooled to the same cold temperature, a silica precursor is added to the acidic solution of surfactant and stirred for a reasonable time that can be judged by one skilled in art (30 seconds is used an exemplary value). The final molar ratio of the reactants is 100 H 2 O:X HCl:Y cationic surfactant:0.13 silica precursor. Here part X ranges from approximately 7 to 11 (9 is taken an exemplary value), Y ranges from 0.05 and higher (0.22 for CTACl is taken as an exemplary value). The resulting solution is kept under then same cold temperature for a period from 1 to 24 hours, with 3 hours as an exemplary time. The material is collected by either centrifugation or filtration. The collected powder washed with pure water, dried in ambient conditions. For a number of applications it may be beneficiary to remove the surfactant from the pores. This can be done though either using various organic solvents or the process of calcination at elevated temperatures (both processes are known to one skilled in art). To amplify, the 4° C. temperature is an exemplary value, but the process will produce the expected results over range from a freezing solution temperature (depending on specific composition of the synthesizing solution) to +10° C. The 3 hour time at the cold temperature is an exemplary value, but the process will produce the described particles if cold from 1 hour to 24 hours. 
     To demonstrate the importance of the self-assembly in low temperatures, the synthesis process was repeated as described above, but done in room temperature (specifically 24° C.). 
     DESCRIPTION OF THE RESULTS 
     The disclosed synthesis produced:
         1. A surprisingly high yield (virtually 100%) of nanoporous fibers; and   2. Synthesized fibers having a very narrow size-distribution.       

     A scanning electron microscope (SEM, JEOL JSM-6300) was used to characterize morphology of the synthesized particles. A thin layer of gold was spattered on the particle surface to improve the SEM contrast. The pore periodicity was found by using low-angle powder X-ray diffraction (XRD) technique (Ordela 1050X). The particle size distribution was measured by using a light-scattering technique (ALV-NIBS High Performance Particle Sizer). Gas absorption was done on a Quantochrome apparatus. Transmission Electron Microscopy (TEM, JOEL) images were collected on the particle edges; the particles were calcined in nitrogen to keep carbon inside the pores, and consequently, improve the TEM contrast. 
     SEM images are used to measure statistical distribution of the fiber diameters (“diameter” for a hexagonal cylinder is defined as the diameter of circumference inside the hexagonal cross-section) and lengths. To estimate the diameters of the fibers and their length, about 80 fibers were measured in the SEM images. 
     Using the SCM illustrates that the low-temperature synthesis results in a surprising high, virtually 100% yield, of hexagonal fibers (“yield: denotes the volume percentage of well defined shapes in the collected batch versus shapeless “junk”).  FIGS. 1(   a ) and  1 ( a ′″) illustrate a representative SEM image of the fibers obtained in the cold synthesis, while  FIGS. 1(   b ) and  1 ( b ′) illustrates the best part (closest to the straight fibers) of the batch synthesized at room temperature. To clearly see the difference,  FIGS. 1(   a ′) and  1 ( b ′) show a higher resolution image of the particles assembled at cold and room (best part) temperatures respectfully. The horizontal bar in  FIGS. 1(   a ),  1 ( a ′),  1 ( b ), and  1 ( b ′) represents a space of 11 μm. One can see that the fibers in  FIGS. 1(   b ) and  1 ( b ′) are not as uniform as those assembled in a cold environment as shown if  FIGS. 1(   a ) and  1 ( a ′). The room temperature results have a number of round shapes, discoids, and the fibers are bent. In contrast, the cold synthesis results in the shapes of only one type, almost straight fibers of a hexagonal cross section. Furthermore, the yield of the fibers synthesized at room temperature is hard to estimate due to the high variability of the shapes.  FIGS. 2   a ,  2   b ,  2   c  and  2   d  illustrate a typical “zoo” of such shapes obtained at room temperature. One can see a large variety of fibers of different size including some fibers somewhat close to “haired” fibers (see  FIG. 2   d ), which are hard to distinguish from shapeless “junk.” 
     It needs to be stressed that there is no variation of the fibers assembled in the cold synthesis.  FIG. 3  illustrates a higher magnification SEM image of fibers assembled in the cold synthesis. The hexagonal cross section of the fiber is clearly seen. The horizontal bar in  FIG. 3  is 5 μm. 
     The SEM images in  FIGS. 1(   a ) and  1 ( a ′) represent all of the fibers in the batch. This makes this material, an article of manufacture, very attractive for various applications as will be discussed below. 
       FIG. 4  illustrates histograms of distributions of the fiber diameter, length, and the length-diameter aspect ratio. The average diameter of the fibers is 2.0 μm (standard deviation is 0.3 μm), the average length is 4.8 μm (standard deviation is 0.5 μm), and the average aspect ratio is 2.4 (standard deviation is 0.4 μm). Based on these numbers the dispersion of the diameter, length, and the ratio are 16%, 11%, and 16%, respectively. 
     To demonstrate that the above statistics are robust, light scattering measurements were done.  FIG. 5  illustrates the distribution of the effective particle radii measure by means of the light scattering. The simulated distribution (the histogram) is based on the SEM statistics. The measured distribution with the help of a light-scattering setup is shown by a solid line. The length distribution is the most important consideration for practical applications. The size distribution of the fibers was confirmed for a macroscopical number of fibers with a light scattering technique, as illustrated in  FIG. 5 . One can see that the distribution simulated from  FIG. 4  matches closely the light scattering distribution of macroscopical number of fibers. 
       FIG. 6  illustrates a SEM image of parallel nanotubes in fiber like arrays of silica nanotubes (ASNT) of uniform shape. A large area (left bar size is 22 μm) and zoom to a few ANST (right bar size is 5 μm) are illustrated. In addition,  FIG. 6  illustrates a schematic showing the arrangement of nanotubes and a TEM image of near the fiber edge showing the periodicity of about 3 nm. The microscopical structure of the product is illustrated in the TEM of  FIG. 6 . 
     Statistically, the size of the periodicity can be characterized by low angle powder X-ray (SAXS) as illustrated in  FIG. 7  and gas absorption (not shown) techniques. The periodicity that corresponds to maximum in  FIG. 7  is 3.8 nm. From gas absorption measurements, the pore size is 3.0 nm leaving 0.8 nm for the wall thickness. 
     The assembled product contains fibers that have a hexagonal cross-section of ca. 2 μm and the length of ca. 4.8 μm. The highly uniform cylindrical pores with diameter of 3 nm (the wall between the pores is ˜0.8 nm) are unidirectional along the fiber. The pore size can be varied by changing the length of the templating molecule. The synthesized particles can be called arrays of (closely packed) silica nanotubes (ASNT). 
     With their lower cost and high yield, the nanoporous silica fibers of a uniform shape produced by this cold temperature technique, create many possible applications. Uniformity in distribution means uniformity of properties. Therefore, the use of the assembled shapes will have advantages in any areas in which such properties are desirable. The area where the uniformity is required includes, but is not limited to:
         1. Filtering applications for chromatography;   2. More non-trivial use of these shapes would be in drug delivery; Well controlled shapes will allow reliably controlled drug release by diffusion out through the pores; When taking the inside of the organism, silica has a serious advantage by being a chemically resistive and biocompatible for oral application;   3. Uniform silica shapes will allow assembly of various types of nanowires in their pores of uniform length; This characteristic can be used for magnetic and gas sensors;   4. 3D catalysts; some chemical reaction can be done in the confined space inside the channels;   5. Storage of biomolecules for extension of their life time (can also be used in biosensors); and   6. A 3D matrix for quantum dots.
           These synthesized particles have significant broader applications as well. The problem of how to “package” nanodots, carbon nanotubes, fullerenes, and other popular nano objects into larger scale devices is one of the most important problems of modern nanotechnology. The self-assembly of nano objects into larger functional shaped devices is a very attractive way of addressing this problem because of its intrinsic simplicity and low-cost. An ultimate example of such hierarchical self-assembly exists already in the world of biological objects. While rational design of bio objects is definitely not a readily feasible task at the present, an example of such a hierarchical self-assembly is already well-known. This hierarchical self-assembly is the co-assembly of organic liquid crystals with an inorganic precursor of some metal oxides, in particular, silica. In some specific conditions such a synthesis can result in the assembly of complex shapes, which resemble the shapes typically found in the biological world. These materials are in a prime position to be used in a broad variety of applications. The major obstacle here is a broad polydispersity of the synthesized arrays-shapes.   The method of self-assembly of nanoporous silica fibers of uniform shape presented here is the first indication that hierarchical self-assembly can be used for controllable “mass-manufacturing” of larger nanostructured objects.   The illustrative embodiments and modifications thereto described hereinabove are merely exemplary. It is understood that other modifications to the illustrative embodiments will readily occur to persons of ordinary skill in the art. All such modifications and variations are deemed to be within the scope and spirit of the present invention as will be defined by the accompanying claims.