Lipids and Minerals Form Novel Composite Microstructures

RON DAGAN ... They mix a galactocerebroside (a sugar-based lipid found in brain tissue) with a small amount of an anionic sulfated lipid derivative in...
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SCIENCE/TECHNOLOGY

lipids and Minerals Form Novel Composite Microstructures • Variety of shapes and compositions possible by varying organic, inorganic ingredients and reaction conditions hemists at the University of Bath, England, have created unusual organic-inorganic microstructures by crystallizing iron oxides on the surfaces of lipid tubules. Although only a laboratory curiosity

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at this point, these fascinating lipid-mineral composites could find use one day in a range of chemical and materials science applications, including catalysis, separations, and molecular electronics. The research also might shed light on biomineralization, the process by which inorganic crystals are incorporated, in an organized fashion, into living organisms. In the laboratory process developed by chemists Douglas D. Archibald and Stephen Mann, the first step is to prepare the lipid tubules. They mix a galactocerebroside (a sugar-based lipid found in brain tissue) with a small amount of an anionic sulfated lipid derivative in

ethylene glycol. Heating and sonicating this mixture and then letting it cool to room temperature causes multilayer lipid tubules to form spontaneously. The tubules typically are on the order of 25 |im in length and 100 nm in diameter. When the tubule suspension is then treated with an acidic ferric chloride solution, the tubules become covered with shards of the mineral lepidocrocite (iron oxyhydroxide, or y-FeOOH) and a small amount of ferrihydrite (Fe 2 0 3 • 7?H20). Further chemical treatment of this composite yields tubules encrusted with clusters of a magnetic iron oxide, possibly magnetite (Fe304).

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Reprinted with permission from Nature [ 3 6 4 , 4 3 0 (1993)]; © 1993 Macmillan Magazines Ltd.

General scheme for producing microstructures in ethylene glycol suspension involves three related lipids: Lipid A is a-hydroxy fatty acid galactocerebroside, lipid B is nonhydroxy fatty acid galactocerebroside, and lipid C is a mixture of anionic sulfated galactocerebrosides. Changing the relative proportions of these molecules variously leads to disks (I), multilayer tubules (II), a viscoelastic gel of fibrous single-layer tubules (III), or other structures. In synthetic route A, the tubules (II) become coated with lepidocrocite (y-FeOOH) after incubation in acidic Fe(III) solution. Route B illustrates that the y-FeOOH on these tubules can be chemically converted in situ to a ferrimagnetic iron oxide (magnetite or maghemite). If route C is followed [tubules are suspended in acidic solution before addition of Fe(III)], the resulting microstructure consists of thin plates of y-FeOOH crystals intercalated between lipid disks. AUGUST 9,1993 C&EN 19

SCIENCE/TECHNOLOGY When the lipid tubules are instead acidified before exposure to ferric ions, the final product—after a period of weeks—includes a different structure: stacks of disklike lepidocrocite crystals intercalated between lipid disks. Archibald explains that under these altered reaction conditions, the lipid molecules forming the multilayer tubules partially reorganize to form stacked disks, and these disks confine the growth of the sheetlike lepidocrocite crystals between them. Since the disks are about 6.5 run thick, the researchers point out, the mineralized structure is a nanocomposite of inorganic and organic layers. The results were reported in the July 29 issue of Nature [364, 430 (1993)]. The mechanisms behind the formation of these structures aren't yet understood in detail. But it appears that the role of the sulfated lipid molecules is to provide anionic sites where iron oxides can bind and then nucleate crystalline phases. Similar mechanisms may also be at work in living organisms that incorporate iron oxides into their tissues. Lepidocro-

cite, ferrihydrite, and magnetite, for example, have all been found to occur in the teeth of certain marine mollusks. A few groups previously have reported using lipid tubules as templates for depositing inorganic materials such as metals or silica. The results reported by Archibald and Mann are particularly impressive, says one scientist, because their system is so flexible: Using three related lipids, they can mix any two in different proportions to produce a variety of lipid microstructures—platelike disks, singlelayer tubules, helical multilayer ribbons and tubules, and multilayer cylinders that look like a jelly roll. All of these could potentially serve as templates for the formation of mineralized structures. Furthermore, "the inorganic portion need not be inorganic oxides," says Mark E. Davis, a professor of chemical engineering at California Institute of Technology, Pasadena, in a related article in the July 29 Nature. "Clever choices of inorganic precursors should

allow a broad spectrum of inorganic materials to be formed." One of the challenges still facing Mann's group is to mineralize the interior of the lipid cylinders, a cavity about 10 to 30 ran across. "Thaf s a trickier problem," Archibald remarks, because ideally one would want to mineralize the cavity without mineralizing the outer surface of the tubule. Accomplishing that would provide a route to nanometer-scale inorganic filaments coated with organic material. Caltech's Davis points out that Archibald and Mann's work is in the same general vein as that of other groups that have used the "intricate patterns" of self-assembled organic molecules to create various types of organic-silica composites. "Once the mechanisms that govern the self-assembly of [such] organic-inorganic composites have been worked out," he says, "the possibility of designing new materials will really be with us." Ron Dagani

Systems produce ultrapure chemicals on site

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AUGUST 9,1993 C&EN

Integrated point-of-use chemical production systems that will achieve parts-pertrillion purity levels are being developed jointly by MEGA Systems & Chemicals and Chemical Suppliers Inc. (CSI). The two companies made the announcement at the recent Semicon West semiconductor trade show in San Francisco, saying that the technology would be offered to customers at little or no capital cost. CSI president R. Scot Clark says that his company's Chem Supplier system for ammonium hydroxide, installed in January at a Texas Instruments plant in Dallas, is the first such on-site production system used in the U.S. Analyses of production lots of the ammonium hydroxide produced at the plant are consistently below 100 ppt for all 35 standard elements specified, he says. Loren Sutherland, vice president for sales and marketing at MEGA, headquartered in Phoenix, explains that the ability of chemical manufacturers to improve the purity of delivered chemicals in containers is nearing its limit. Container cleanliness, shipping and handling, storage, container shedding, and contamination during drum hookup are beyond the manufacturer's control. CSI, based in Fallbrook, Calif., is a joint venture of Startec Inc. and France's Air Liquide. For its Chem Supplier, the corn-

pany uses a proprietary gas purification technology from Air Liquide to remove contaminants from the crude gas feed stream. In the case of ammonium hydroxide production, Chem Supplier produces ultrapure ammonia gas that is then combined with deionized water. Clark says CSI is developing systems to produce ultrapure hydrofluoric acid, ammonium fluoride, hydrochloric acid, and nitric acid. The MEGA-CSI alliance marries MEGA's advanced chemical-filtration and distribution technology with CSI's purification expertise to create three system configurations, Clark says. One is a fully integrated system to generate ultrapure chemicals and distribute them directly to the point of use. Another is a core distribution system, designed for installation immediately outside the semiconductor clean room. And the last is a bulk-chemical dispensing system. In each case, Clark says, the customer pays only for the chemicals produced. For example, a MEGA-CSI unit might have a capacity of 1000 gal per week. In such a case, the typical agreement would be a five-year take-or-pay contract for 600 gal per week, with the customer having the option of purchasing any amount required above that base level. Rudy Baum