faster/' And that also makes computers run faster. The UCSD work also has implications for holographic technology. Washington's face on the dollar note is drawn against a background of cross-hatched lines that form a grid. The image of this grid, when reduced and imprinted on silicon, essentially becomes a threedimensional diffraction grating. That's because the etch has produced a porosity gradient in the top micrometer of the silicon surface, and the refractive index varies according to the density (or porosity) of the etched area. Thus, when the grid is illuminated with a visible laser, the beam diffracts in two directions, resulting in a simple hologram. This suggests that holographic information can be stored on such a silicon wafer, Sailor says. However, silicon's slow oxidation may detract from its potential as a holographic storage medium. Using interference patterns or holographic diffraction patterns for the archival storage of images is attractive, Sailor explains, because the colors arise from the physical properties of the system rather than from pigments. Thus, ideally, the colors won't change over time. This certainly is not the first time silicon has been patterned. That's been done before using photoresist and other techniques. But this is the first demonstration that gray-scale images can be imprinted on luminescent silicon using this photoelectrochemical process, Sailor points out. With photoresists, he adds, "you're limited to black and white"— storing data as yes or no, one or zero. With his technique, the gradations of gray make possible several intermediate states, "and that allows you to store more information in a given area." Thus far, Sailor's silicon-etched images are still too grainy to be practical for image storage, compared with the current generation of optical discs. "The state-of-the-art resolution now with optical discs is about a factor of 100 times smaller than we can now obtain with silicon," he says. But if this lower resolution can be achieved with silicon, he believes, the greater information storage density possible with silicon might make it more competitive. Earlier this year, Sailor and his coworkers reported that the light given off by porous silicon can be modified by exposing the material to different compounds [/. Am. Chem. Soc, 114, 1911 (1992)]. This suggests that the material
could be used as a chemical sensor. Sailor envisions using his patterning technique to put, say, 15 different microsensor pads onto a fingernail-size silicon chip that also carries detector and amplifier circuits. Such a device could be used to sniff out 15 different air pollutants, for instance. Whether any of these potential applications actually see the light of day depends on a number of unresolved issues, caution other scientists. For example, the nature of porous silicon itself is a matter of controversy. The prevailing view has been that the etching sculpts pure silicon into a forest of nanometer-size pillars. Electrons confined in this silicon forest shed their energy as light—"a manifestation of the ethereal world of quantum effects," in the words of one writer. An opposing view is that the glow comes not from silicon but from a compound of silicon, oxygen, and hydrogen known as siloxene. Siloxene is not particularly stable. So if it is indeed the lightemitting species, it would have to be stabilized to be useful. Electrochemist Allen J. Bard of the University of Texas, Austin, who has been studying porous silicon (and favors the siloxene hypothesis), says, "Maybe you could build a technology on siloxene. [But] I think it's going to be a lot more difficult than if it were pure silicon." Sailor, though, is not convinced that siloxene is the cause of silicon's glow. "The mechanism is still up in the air," he remarks. And even though his most recent findings have technological implications, his underlying goal—like that of most researchers in this field—is to figure out what makes silicon shine. Ron Dagani
Cell-surface receptors confer antigenicity Mannose-specific cell-surface receptors on bacteria, which are not themselves antigenic, have been exploited by chemists at the University of California, Berkeley, to target antibodies to the bacteria, and thus, to activate immune system responses against the bacteria. The research suggests an approach for overcoming the genetic variation many pathogens exploit to avoid elimination by a host's immune system. The research was carried out by
Bertozzi: methodology devised in 1980s Mark D. Bednarski, an assistant chemistry professor at Berkeley, and Carolyn R. Bertozzi, a graduate student in Bednarski's lab. The Berkeley chemists reported their results in a series of three papers that have appeared this year [Carbohydrate Research, 223, 243 (1992); /. Am. Chem. Soc, 114,2242,5543 (1992)]. Bednarski and Bertozzi point out that antibody binding to bacterial cells is necessary to activate subsequent immune system defense mechanisms. These include activation of complement, an immune system mechanism for lysing antibody-coated cells, and recognition by macrophages, which are phagocytic immune system cells that bind to antibodies already bound to a pathogen. Many viral and bacterial pathogens possess surface receptors for proteins or carbohydrates that are on the surfaces of cells susceptible to pathogen infection. The interaction between these pathogen receptors and a cell-surface protein or carbohydrate is often directly involved in infectivity. Cell-surface protein receptors for carbohydrates are known as lectins. Although viruses and bacteria often undergo rapid genetic changes to evade the host's immune system, the biological function, and thus the binding specificity, of surface receptors must be conserved. For reasons that are not completely understood, however, the genetically conserved regions of receptors like JULY 6,1992 C&EN
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SCIENCE/TECHNOLOGY lectins seem to be, in many cases, "invisible" to the Glycoside binds to bacterial hosfs immune system. mannose receptor Several strains of enterobacteria, such as Escheri chia, Klebsiella, Shigella, and Salmonella, have proteinaceous appendages called pili on their surfaces. Some of these pili contain lectins specific for terminal Biotinylated α-C-glycoside of mannose (BCM) alpha-linked mannosides. These pili mediate the ad hesion of the bacteria to host cells. In their research, which Antiavidin was supported by the Na antibody tional Institutes of Health, Mannose receptor Bednarski and Bertozzi targeted these mannose r e ceptors. Previous research, Bertozzi demonstrated that such cells trigger an points out, had shown that a protein re immune response that involves activa ceptor—specifically the human immun tion of both complement and macroph odeficiency virus (HIV) envelope glyco ages, leading to bacterial cell death. protein, gpl20—could be targeted for Bertozzi points out that the method immune system attack by engineering ology she used to make the C-glycothe Τ cell surface protein, CD4, to which sides was developed in the early 1980s. gpl20 binds when HIV infects a T cell. "Carbohydrate chemistry is, in some The Berkeley chemists first synthe ways, the oldest and newest branch of sized a series of C-mannopyranosyl de organic chemistry," Bertozzi says. Emil rivatives and assessed their binding to Fischer's research on carbohydrates in the mannose-specific lectin of a patho the late 19th century is one of the sem genic strain of Escherichia coli. In these inal events in natural products chemis compounds, a carbon atom replaces the try, she notes. And yet it is only in the natural oxygen atom in the glycosidic past two decades that the ubiquitous position—which is where monosaccha carbohydrate molecules that are at rides are alpha- or beta-linked to other tached to proteins and lipids and that monosaccharides to form oligosaccha coat the surfaces of cells have come to rides. This change makes the com be studied as biochemically important pounds resistant to enzymatic degrada in their own right. tion. The research demonstrated that a Bertozzi acknowledges that the BCMnumber of the compounds bind to the avidin complex itself is unlikely to be lectin with affinities comparable or su therapeutically useful because it in volves a protein. The strategy employed, perior to a-D-mannopyranoside. Bertozzi also synthesized a biotiny- however, may well prove to be useful in lated version of one of these deriva developing therapeutic agents. Scientists tives and showed that it, too, bound to at the University of Pennsylvania report the lectin. Biotin, a vitamin, is known ed last year, for example, that HIV gpl20 to bind with high affinity to avidin, a binds to a glycolipid molecule found on protein found in egg white. Avidin is a cells in the central nervous system. They tetramer; thus, each protein molecule speculated that this glycolipid might binds four biotins. Bertozzi demon provide an alternative to CD4 for HIV to strated that the multivalent complex of bind to and subsequently infect nervous the biotinylated oc-C-glycoside of man- system cells that do not produce CD4. Bednarski and Bertozzi are collaborat nose (BCM) with avidin binds to the bacterial lectin with the highest affinity ing with the University of Pennsylvania group to produce C-glycosidic analogs of all compounds tested. Because avidin is antigenic, it can tar of the glycolipid to assess their binding get antibodies to a cell to which it is to gpl20. If these analogs bind gpl20, it bound. Bertozzi showed first that anti might be possible to further modify the bodies do bind to E. coli that are exposed glycolipid to make it antigenic. Rudy Baum to the BCM-avidin complex. She then
