Materials Potpourri

Although most Bostonians returning to work after the Thanksgiving holiday weren't aware of it, Boston Mayor Thomas M. Menino proclaimed Monday, Nov. 29, 2004--the first day of the Materials Research Society's annual fall meeting--to be the first-ever Materials Science Day in Boston.

As Menino noted in a proclamation marking the occasion, MRS "has met in Boston every fall for 27 years and draws more than 5,000 international attendees and exhibitors."

This year's MRS conferees braved the cold weather to absorb more than 2,500 talks and nearly 1,700 poster presentations. With five full days of symposia to occupy them, many attendees saw little need to desert the warmth of the Hynes Convention Center and its adjoining hotel and shopping mall complex, unless it was to attend the "Strange Matter" exhibit (C&EN, Jan. 12, 2004, page 40) held in conjunction with the meeting at the Boston Museum of Science.

Reflecting the growth and interdisciplinary nature of materials science, the meeting's offerings included presentations in areas as diverse as nanophotonics and art conservation. As the following highlights suggest, there were plenty of offerings for chemists, with talks about dendrimers, carbon nanotubes, and click chemistry.

Seeing Under The Sea

To create the next generation of optical devices, Joanna Aizenberg sought inspiration in an unlikely location--the ocean. Aizenberg, a scientist with Lucent Technologies in Murray Hill, N.J., teamed up with Gordon Hendler, head of invertebrate zoology at the Natural History Museum of Los Angeles County, to study the intricacies of the light-sensitive brittlestar Ophiocoma wendtii.

For decades, scientists have tried to reconcile the remarkable light sensitivity of O. wendtii with the fact that the organism seems to have no specialized eyes. Without eyes, they wondered, how could the creature be so adept at seeking shelter and finding coral crevices in which to hide from predators? Another mystery: Why did the creature have dark brown coloring during the day and a striking banded gray-and-black appearance at night?

The answer, Aizenberg and Hendler discovered, is that the creature does have eyes. In fact, it's covered with them. When Aizenberg closely examined O. wendtii's dorsal arm plates, which are on every joint of the brittlestar, she noted that they are covered by a thick, transparent material made up of hemispherical structures [J. Mater. Chem., 14, 2066 (2004)].

To Aizenberg, these calcium carbonate hemispheres looked a lot like an array of lenses. So, she used photosensitive material to see if the hemispheres focused light when illuminated. The experiment showed that the lenses focused the light 4–7 µm below the array--a distance consistent with the location of neural bundles that Hendler had previously identified.

Further study of the microlenses showed that they have many features that are superior to synthetic microlenses. Their ingenious design minimizes spherical aberration, has a crystallographic orientation that eliminates birefringence, and contains some type of organic-inorganic composite that prevents the brittle calcium carbonate from fracturing easily.

The scientists also learned that O. wendtii's diurnal to nocturnal color change is caused by pigment-filled tissue, which comes out of the pores surrounding each lens. This tissue, Aizenberg said, works like sunglasses for the brittlestar. When the pigment is bleached away, the brittlestars "have the same reaction that people do when going from a dark room into the sun," she said.

Aizenberg used the information she gleaned from the brittlestar's biological crystal growth techniques to build her own microlens array. "This new bio-inspired crystal engineering strategy made it possible, for the first time, to directly fabricate millimeter-size perforated single crystals with predetermined sub-10-µm pores and controlled crystallographic orientation," she noted.

The researchers also developed an alternative way to create the lenses using multibeam lithography in photoresist materials. "We don't have to take the same materials that nature does," Aizenberg remarked. "We can use materials that are easier to produce and easier to control."

Carbon Nanotube Electronics Powers Up

Electronics and semiconductor makers have been salivating over the potential of single-walled carbon nanotubes (SWNTs) since the earliest reports of the structures' electronic properties. However, there are still no SWNT-based electronic devices on the market.

That's about to change, according to Nanomix scientist Jean-Christophe P. Gabriel. The company, based in Emeryville, Calif., plans to begin marketing a SWNT-based hydrogen sensor early this year.

The main stumbling block to using SWNTs in a manufactured device has been that of SWNT chirality. The tubes are usually produced as a mixture of semiconducting and metallic SWNTs. While semiconducting tubes have the desirable electronic properties for making field-effect transistors, the metallic nanotubes do not.

Gabriel said that Nanomix was able to get around this problem of SWNT chirality by using a random network of nanotubes grown directly onto the transistor wafer, as opposed to just a few nanotubes specifically grown from patterned catalysts. This way, Gabriel explained, the chirality issue is moot so long as one can avoid metallic pathways between the electrodes by increasing the ratio of semiconducting SWNTs to metallic SWNTs. The chemical vapor deposition process that Nanomix uses to grow the SWNT network produces more than twice as many semiconducting nanotubes as it does metallic nanotubes.

"Increasing the number of nanotubes allows you to gain robustness and increase the signal-to-noise ratio without increasing size," Gabriel added.

The current wireless version of the hydrogen sensor that Nanomix plans to introduce is a little larger than a matchbook, although Gabriel reckoned that the device could be much smaller given that the battery and integrated circuit take up most of the space. The size of the hydrogen-sensing core, he noted, is in the submillimeter range, and the device requires only a few microwatts of power.

The mechanism of hydrogen sensing is different from hydrogen sensors that rely on the burning of hydrogen, Gabriel noted. Unlike those devices, the Nanomix sensor doesn't require high temperatures. Instead, a proprietary noble metal alloy in the sensor generates hydrogen radicals at room temperature. Gabriel said that the Nanomix scientists aren't precisely certain what the underlying molecular mechanism is, but they think that the hydrogen radicals react with oxygen that's adsorbed onto the SWNT surface. This reaction changes the conductance in the nanotubes, and that change, he explained, is what the device senses.

Gabriel also touted the system's versatility. It can be used in transparent, flexible substrates, he said [Nano Lett., 3, 1353 (2003)]. Furthermore, by functionalizing the nanotubes, it's possible to use the platform to sense for a range of different gases, liquids, and even light. The company recently used the SWNT transistor in a carbon dioxide monitor they hope to market as a portable, disposable breathing monitor for paramedics and first responders [Adv. Mater., 16, 2049 (2004)].

Georg S. Duesberg, a scientist with Infineon Technologies, in Munich, also presented a milestone in carbon nanotube electronics. Duesberg and his coworkers at Infineon have made the shortest operating carbon-nanotube-based transistor to date [Nano Lett., published online Nov. 17, 2004, http://dx.doi.org/10.1021/nl048312d].

The SWNT contacts the source and the drain of the field-effect transistor by spanning a channel only 18 nm wide. According to Duesberg, even without optimization, these transistors perform better in many respects than much larger state-of-the-art silicon-based devices.

Duesberg told C&EN that the research team wanted to see how small they could make a working transistor because the speed and performance of the transistor benefits from a short channel length, high mobility, and ballistic transport.

To build the short-channel transistors, the Infineon team began by growing SWNTs via chemical vapor deposition onto highly doped n-type silicon substrates covered with a thin layer of thermal oxide dielectric. The team went to great pains to ensure that only one SWNT bridged the 2-µm gap between catalyst islands.

To create the short channel, the researchers used an electron-beam lithography patterning method called negative tone resist. Finally, they layered on the source and drain contacts.

Duesberg also discussed the vertical SWNT nanotube transistor that Infineon and others are currently pursuing. "If you make a transistor, you want to make millions and billions of them," he explained. Unlike horizontal SWNT-based electronics, vertical nanotube transistors wouldn't need to be produced one by one. Vertical arrays of carbon nanotubes could be grown to a regular, specified height between the transistor's source and drain. The gate would then be built so that it surrounds the nanotube, improving the device's performance, Duesberg added.

While he and his group have seen some encouraging results, Duesberg admitted that they still need to overcome the major hurdle of nanotube chirality. "I think there's still a long way to go, but with respect to the achievements we have already seen, I'm confident that those issues will be solved soon." Duesberg said. "While transistors are still a ways off, sensors and field-emission displays are already on the way."

Dumplings And Donuts: The Shape Of Block Copolymers

Inspired by nature's ingenious packaging materials, such as lipoproteins and viruses, Karen L. Wooley's research team has been creating and studying nanoparticulate materials that could be used medically to carry imaging contrast agents, deliver drugs, or even scavenge for toxins.

"Our overall goal is to make nanostructures that have the same degree of sophistication as small-molecule natural products," said Wooley, a chemistry professor at Washington University in St. Louis. To that end, her group has spent the past few years developing the chemistry of core-shell nanoparticles made from block copolymers. In solution, these amphiphilic block copolymers form micellar structures with a hydrophobic core and hydrophilic shell. Then the researchers cross-link the shell portion of the micelle, creating a dumplinglike nanosphere.

Wooley demonstrated how this basic strategy can be modified to make nanospheres with a wide range of surface chemistries. For example, by adding to the reaction mixture block copolymers with mannose groups on the hydrophilic terminus, the researchers made nanospheres decorated with the sugar. The structures were designed to selectively interact with the receptors on gram-negative bacteria as a means of delivering deadly cocktails to the bacteria and thereby possibly combating antibiotic resistance [Biomacromolecules, 5, 903 (2004)].

Recently, in collaboration with Craig J. Hawker, a chemistry professor at the University of California, Santa Barbara, Wooley developed a new way to build nanostructures using click chemistry--the organic synthesis strategy wherein "spring loaded" functional groups react easily, irreversibly, and in high yield. The group functionalized the hydrophilic portions of the block copolymers with alkynes and azides. Wooley explained that these spring- loaded functional groups "click" to cross-link the shell portion of the nanosphere by reacting to form triazoles when they come into close proximity.

"We like the reaction because it works in water, and the chemistry is highly efficient," she said. Also, there are no by-products of the reaction, although Wooley noted that there is a catalyst that needs to be removed.

Wooley has also been looking into more complex nanostructures. While studying the conditions that lead to the formation of cylindrical micelles, her research team discovered that under the right conditions, triblock copolymers of poly(acrylic acid-b-methyl acrylate-b-styrene) formed donutlike toroids [Science, 306, 94 (2004)]. Wooley said that the result, which was discovered in collaboration with University of Delaware, Newark, materials science professor Darrin J. Pochan, came as a surprise.

Although they're not certain what causes the toroids to form, the scientists know that the copolymer composition and topology are critical. When they changed the order of the components in the triblock copolymer, the researchers saw entirely different structures form. Ultimately, Wooley hopes to be able to design the size, shape, chemistry, and function of these supramolecular assemblies. "Our goal is to control absolutely everything about the nanostructure," she said.

Regenerative Medicine Meets Nanotechnology

Nanomaterials built from a simple family of self-assembling molecules may offer hope for treating serious injuries such as stroke and spinal cord trauma, according to new results from Northwestern University chemistry professor Samuel I. Stupp and colleagues.

For several years, work in Stupp's lab has focused on creating materials that mimic extracellular matrices and can be used in regenerative medicine. In one example, the group builds these matrices using amphiphilic molecules that have a hydrocarbon chain on one end and a peptide epitope that's known to promote and direct the growth of dendrites and axons on the other end.

When Stupp's team added a suspension of neuroprogenitor cells to a dilute aqueous solution of these molecules, the amphiphiles assembled into cylindrical nanofibers. The hydrophilic epitopes form the surface of the cylinder, and the hydrocarbon chains form its core. These nanofibers then bundle together into a network, producing a gelatinous solid [Science, 303, 1352 (2004)].

The researchers found that the network can encapsulate the neuroprogenitor cells without damaging them. Furthermore, they observed that after only 24 hours in the nanofiber network, the cells began to differentiate and became neurons. Notably, Stupp said, the progenitor cells don't differentiate into astrocytes--neural cells that form scars around injury sites on the brain and spinal cord and prevent regeneration at those sites.

Working in collaboration with John A. Kessler, a neurology professor at Northwestern, Stupp's group decided to see if the nanofiber gels had a similar neurogenerative effect in vivo. To model human spinal cord injury, they compressed the spinal cords of laboratory animals. Twenty-four hours after the injury, the researchers injected the peptide amphiphile solution at the injury site.

Animals that received the injection of amphiphile solution had a low mortality rate, whereas a significant number of the injured, untreated animals died. "That in itself is very important," Stupp told C&EN. "That could translate as some sort of preventive therapy in humans."

Furthermore, ongoing studies by the group indicate that over time, animals treated with the solution recover some of the movement they lost because of the injury. And the team has seen some promising in vivo results that indicate the solution could be used to improve recovery from strokes.

High-Tech Tool Helps Conservators

When art conservators, museum scientists, and art historians want to learn about the layers of paint in a painting--as they frequently do to determine its condition, authenticity, and extent of previous restoration--they are almost always faced with the same problem: How can they get precise information about the nature of layers beneath the surface without actually removing a sample of the painting?

There are spectroscopic techniques that can noninvasively analyze subsurface paint, but they don't distinguish between different layers or give any information about the relative positions of the layers. Now, in collaboration with scientists at the Cornell High Energy Synchrotron Source (CHESS) and X-ray Optical Systems, East Greenbush, N.Y., Jennifer Mass, a chemist at Delaware's Winterthur Museum, has developed a technique that gives precise information about the composition and location of buried paint layers without the need to cut out a sample.

Mass found that she could use confocal X-ray fluorescence (XRF) spectroscopy, a technique that's used to study buried metal films, to get elemental data about different layers of paint. The technique gathers fluorescence data from a highly focused region, allowing Mass and her colleagues to create a detailed depth profile of the paint layers. For most of their experiments, Mass's group used the confocal XRF system at CHESS. However, they noted that portable XRF systems can be modified to do the same thing, "albeit with longer count times."

The researchers first tried the technique out on samples of four different paint layers, each about 20 µm thick. The two base layers were composed of lead white and malachite--two pigments found in paintings throughout antiquity--while the more modern pigments chromium oxide green and cadmium yellow made up the two upper layers. "That's typically what we see when we look at art restoration," Mass explained.

Mass reported that not only did the confocal XFR technique accurately analyze the different paint layers, it also revealed the presence of a zinc-based filler in the cadmium pigment that the researchers were previously unaware of.

To get a more realistic idea of the technique's utility, Mass's team used confocal XRF spectroscopy to analyze a 20th-century oil painting. Early in the experiment, the team members thought that they were sidetracked by a layer of lead-based pigment in the upper region of the painting. This layer absorbed the X-rays and prevented the researchers from gathering data on any deeper layers. However, they were able to get around the problem by simply flipping the painting over and gathering data through the back of the canvas.

The technique does have some limitations. It's not useful for studying paint layers thinner than 5 µm, although Mass noted that such thin layers of paint are atypical. Also, it only detects elements that fluoresce, so unpigmented layers appear as a "missing layer" in the confocal data. However, Mass said that these are minor problems, and she expects that confocal XRF spectroscopy will become "an invaluable research tool for conservation science."

Fighting Tumors With Gold Nanocomposites

When it comes to treating tumors, doctors often have to resort to aggressive measures like chemotherapy, surgery, and radiation. While these treatments can destroy tumors, they aren't particularly selective and often have serious side effects. Looking for an alternative way to selectively irradiate tumors, many scientists have turned to nanoparticles as potential drug-delivery devices.

"You don't need to deliver a high amount of radiation," explained Lajos P. Balogh, a professor at the University of Michigan, Ann Arbor, Center for Biologic Nanotechnology. "You need to deliver an exact amount of radiation at an exact location."

Balogh and his collaborator, Mohamed K. Khan, of Roswell Park Cancer Institute in Buffalo, have spearheaded an effort to use gold-dendrimer nanocomposites to selectively deliver radiation to tumors. Because the size, charge, and surface recognition characteristics of these dendrimer nanocomposites can be tuned, it is possible to modify them so that the structures specifically target a tumor's blood vessels, Balogh said. Furthermore, the structures also could be used to potentially image the tumor.

To create the dendrimer nanocomposites, Balogh's group uses poly(amidoamine) dendrimers. The highly branched molecules bind gold ions through their amine nitrogens, eventually encapsulating the metal as Au(0).

"The resulting 'soft' composite nanoparticles combine the properties of the encapsulated inorganic matter and the biofriendly macromolecule," Balogh noted. The team then uses a neutron beam to transform the gold in the nanocomposites into radioactive ¹⁹⁸Au.

The group's biological experiments showed that, in vivo, the dendritic nanocomposites that are 22 nm in diameter are retained better than smaller nanocomposites. Also, the group found that the particles' surface charge had a significant effect on their biodistribution.

Preliminary tests in mice showed that healthy animals can tolerate an injection that contains 400 µg of the dendrimer nanocomposites with no apparent toxicity. Although he cautions that there is still a substantial amount of work to be done, Balogh said that in the near future he plans to publish some early results that demonstrate the materials' ability to considerably slow tumor growth.