DNA nanotechnology builds 3D forms with complex curves (includes video)

Miniature architectural forms – some no larger than viruses – have been constructed through a revolutionary technique known as DNA origami. Now, Hao Yan, Yan Liu and their colleagues at ASU’s Biodesign Institute have expanded the capability of this method to construct arbitrary, two- and three-dimensional shapes, mimicking those commonly found in nature.

Such diminutive forms may ultimately find their way into a wide array of devices, from ultra-tiny computing components to nanomedical sentries used to target and destroy aberrant cells or deliver therapeutics at the cellular or even molecular level.

In today’s issue of Science [abstract], the Yan group describes an approach that capitalizes on (and extends) the architectural potential of DNA. The new method is an important step in the direction of building nanoscale structures with complex curvature – a feat that has eluded conventional DNA origami methods. …

“Our goal is to develop design principles that will allow researchers to model arbitrary 3-D shapes with control over the degree of surface curvature. In an escape from a rigid lattice model, our versatile strategy begins by defining the desired surface features of a target object with the scaffold, followed by manipulation of DNA conformation and shaping of crossover networks to achieve the design,” Liu said.

To achive this idea, Yan’s graduate student Dongran Han began by making simple 2-D concentric ring structures, each ring formed from a DNA double helix. The concentric rings are bound together by means of strategically placed crossover points. These are regions where one of the strands in a given double helix switches to an adjacent ring, bridging the gap between concentric helices. Such crossovers help maintain the structure of concentric rings, preventing the DNA from extending. …

Varying the number of nucleotides between crossover points and the placement of crossovers allows the designer to combine sharp and rounded elements in a single 2-D form …

The network of crossover points also can be designed in such a way as to produce combinations of in-plane and out-of-plane curvature, allowing for the design of curved 3D nanostructures. While this method shows considerable versatility, the range of curvature is still limited for standard B form DNA, which will not tolerate large deviations from its preferred configuration – 10.5 base pairs/turn. However, as Jeanette Nangreave, one of the paper’s co-authors, explains, “Hao recognized that if you could slightly over twist or under twist these helices, you could produce different bending angles.”

Combining the method of concentric helices with such non-B-form DNA (with 9-12 base pairs/turn), enabled the group to produce sophisticated forms, including spheres, hemispheres, ellipsoid shells and finally—as a tour de force of nanodesign – a round-bottomed nanoflask, which appears unmistakably in a series of startling transmission electron microscopy images …

Yan hopes to further expand the range of nanoforms possible through the new technique. Eventually, this will require longer lengths of single-stranded DNA able to provide necessary scaffolding for larger, more elaborate structures. He credits his brilliant student (and the paper’s first author) Dongran Han with a remarkable ability to conceptualize 2-D and 3-D nanoforms and to navigate the often-perplexing details of their design. Ultimately however, more sophisticated nanoarchitectures will require computer-aided design programs – an area the team is actively pursuing.

The successful construction of closed, 3-D nanoforms, such as the sphere, has opened the door to many exciting possibilities for the technology, particularly in the biomedical realm. Nanospheres could be introduced into living cells for example, releasing their contents under the influence of endonucleases or other digestive components. Another strategy might use such spheres as nanoreactors – sites where chemicals or functional groups could be brought together to accelerate reactions or carry out other chemical manipulations.

These 3D structures are large and complex compared with the 2.0 nm diameter of the DNA double helix. PowerPoint slides of several figures from the Science paper can be downloaded without a subscription. One of these shows the sphere to be 42 nm in diameter, the prolate ellipsoid to be 35 nm by 66 nm, and the round-bottomed nanoflask to be 40 nm wide and 70 nm tall.

NASA engineers develop 'blacker than black' nanotubes (w/ Video)

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Principal Investigator John Hagopian developed a new nanotech-based material that is 10 times more effective than black paint used by instrument developers to absorb stray light, which can contaminate scientific data. The sample on the left is black paint typically used to suppress errant light in instruments; the sample on the right is the new nanotube material. Credit: Chris Gunn/NASA

The nanotech-based material now being developed by a team of 10 technologists at the NASA Goddard Space Flight Center in Greenbelt, Md., is a thin coating of multi-walled carbon nanotubes — tiny hollow tubes made of pure carbon about 10,000 times thinner than a strand of human hair. Nanotubes have a multitude of potential uses, particularly in electronics and advanced materials due to their unique electrical properties and extraordinary strength. But in this application, NASA is interested in using the technology to help suppress errant light that has a funny way of ricocheting off instrument components and contaminating measurements.

Better than Paint

"This is a technology that offers a lot of payback," said engineer Leroy Sparr, who is assessing its effectiveness on the Ocean Radiometer for Carbon Assessment (ORCA), a next-generation instrument that is designed to measure marine photosynthesis. "It's about 10 times better than black paint" typically used by NASA instrument designers to suppress stray light, he said.

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Multi-walled carbon nanotubes are tiny hollow tubes made of pure carbon about 10,000 times thinner than a strand of human hair. NASA is investigating their use to help suppress errant light that ricochets off instrument components and contaminates measurements. Credit: NASA

The technology works because of its super-absorption abilities. The nanotubes themselves are packed vertically much like a shag rug. The tiny gaps between the tubes absorb 99.5 percent of the light that hits them. In other words, very few photons are reflected off the carbon-nanotube coating, which means that stray light cannot reflect off surfaces and interfere with the light that scientists actually want to measure. The human eye sees the material as black because only a small fraction of light reflects off the material.

The team began working on the technology in 2007. Unbeknownst to the group, the New York-based Rensselaer Polytechnic Institute also had initiated a similar effort and announced in 2008 that its researchers had developed the darkest carbon nanotube-based material ever made — more than three times darker than the previous record.

"Our material isn't quite as dark as theirs," said John Hagopian, the principal investigator leading the development team. "But what we're developing is 10 times blacker than current NASA paints that suppress system stray light. Furthermore, it will be robust for space applications," he said.

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NASA's Goddard Space Flight Center has a team of scientists testing micro and nanotechnology to use on spacecraft. The goal is to reduce the reflection off the surface of instruments satellites so that the data does not get polluted by the scattered light. The carbon nanotubes that the team grows have proven to be 10 times better than the NASA Z306 paint, currently used on spacecraft instruments.

That is an important distinction, said Carl Stahle, assistant chief of technology for Goddard's Instrument Systems and Technology Division. Not all technology can be used in space because of the harsh environmental conditions encountered there. "That's the real strength of this effort," Stahle said. "The group is finding ways to apply new technology and fly it on our instruments."

Big Breakthrough

The breakthrough was the discovery of a highly adhesive underlayer material upon which to grow the carbon nanotubes, which are just a few tens of nanometers in diameter. To grow carbon nanotubes, materials scientists typically apply a catalyst layer of iron to an underlayer on the silicon substrate. They then heat the material in an oven to about 750° C (1,382° F). While heating, the material is bathed in carbon-containing feedstock gas.

Stephanie Getty, the materials scientist on Hagopian's team, varied the underlayer as well as the thickness of the catalyst materials to create carbon nanotubes that not only absorb light, but also remain fixed to the material upon which they are grown. As a result, they are more durable and less likely to scratch off. The team also has grown durable nanotube coatings on titanium, a better structural material for space use. The team now is fine-tuning production techniques to assure consistent quality and light-suppression capabilities, Hagopian said.

New Capabilities Added

Should the team prove the material's suitability in space, the material would provide real benefits to instrument developers, Hagopian added.

Currently, instrument developers apply black paint to baffles and other components to reduce stray light. Because reflectance tests have shown the coating to be more effective than paint, instrument developers could grow the carbon nanotubes on the components themselves, thereby simplifying instrument designs because fewer baffles would be required. To accommodate larger components, the team now is installing a six-inch furnace to grow nanotubes on components measuring up to five inches in diameter. And under a NASA R&D award, the team also is developing a separate technique to create sheets of nanotubes that could be applied to larger, non-conforming surfaces.

In addition to simplifying instrument design, the technology would allow scientists to gather hard-to-obtain measurements because of limitations in existing light-suppression techniques or to gather information about objects in high-contrast areas, including planets in orbit around other stars, Hagopian said.

The ORCA team, which is fabricating and aligning an instrument prototype, is the first to actually apply and test the technology. The instrument is the front-runner for the proposed Aerosol/Cloud/Ecosystems (ACE) mission and requires robust light-suppression technologies because more than 90 percent of the light gathered by the instrument comes from the atmosphere. Therefore, the team is looking for a technique to suppress the light so that it doesn't contaminate the faint signal the team needs to retrieve.

"It's been an issue with all the (ocean sensors) we've flown so far," said ORCA Principal Investigator Chuck McClain.

Working with the ORCA team, Hagopian's group grew the coating on a slit, the conduit through which all light will pass on ORCA. "Having an efficient absorber is critical and the nanotubes could provide the solution," McClain said. "Right now, it looks promising," Sparr added. "If I can support them and they can continue advancing the technology so that it can be applied to other spacecraft components, it could be a very important development for NASA."

Goddard Chief Technologist Peter Hughes agrees, and, in fact, selected Hagopian and his team to receive his organization’s 2010 "Innovator of the Year" award. "Our job is to develop and advance new technology that will ultimately result in better scientific measurements. Goddard has a well-deserved reputation for creating technologies that enhance instrument performance because we are adept at quickly infusing emerging technology for specific spaceflight applications. John’s team demonstrated that key strength. And in doing so, he’s leading the way in NASA’s quest to bring about a new level of scientific discovery," Hughes said.

Light touch brightens nanotubes (w/ Video)

Single-walled carbon nanotubes treated with ozone incorporate oxygen atoms that shift and intensify the nanotubes' near-infrared fluorescence emission. The discovery by Rice University scientists should lead to new uses of nanotubes in biomedicine and materials science. (Credit: Bruce Weisman/Rice University)

The Rice lab of researcher Bruce Weisman, a pioneer in nanotube spectroscopy, found that adding tiny amounts of ozone to batches of single-walled carbon nanotubes and exposing them to light decorates all the nanotubes with oxygen atoms and systematically changes their near-infrared fluorescence.

Chemical reactions on nanotube surfaces generally kill their limited natural fluorescence, Weisman said. But the new process actually enhances the intensity and shifts the wavelength.

He expects the breakthrough, reported online in the journal Science, to expand opportunities for biological and material uses of nanotubes, from the ability to track them in single cells to novel lasers.

Best of all, the process of making these bright nanotubes is incredibly easy -- "simple enough for a physical chemist to do," said Weisman, a physical chemist himself.

He and primary author Saunab Ghosh, a graduate student in his lab, discovered that a light touch was key. "We're not the first people to study the effects of ozone reacting with nanotubes," Weisman said. "That's been done for a number of years.

"But all the prior researchers used a heavy hand, with a lot of ozone exposure. When you do that, you destroy the favorable optical characteristics of the nanotube. It basically turns off the fluorescence. In our work we only add about one oxygen atom for 2,000-3,000 carbon atoms, a very tiny fraction."

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Ghosh and Weisman started with a suspension of nanotubes in water and added small amounts of gaseous or dissolved ozone. Then they exposed the sample to light. Even light from a plain desk lamp would do, they reported.

Most sections of the doped nanotubes remain pristine and absorb infrared light normally, forming excitons, quasiparticles that tend to hop back and forth along the tube -- until they encounter oxygen.

"An exciton can explore tens of thousands of carbon atoms during its lifetime," Weisman said. "The idea is that it can hop around enough to find one of these doping sites, and when it does, it tends to stay there, because it's energetically stable. It becomes trapped and emits light at a longer (red-shifted) wavelength.

"Essentially, most of the nanotube is turning into an antenna that absorbs light energy and funnels it to the doping site. We can make nanotubes in which 80 to 90 percent of the emission comes from doped sites," he said.

Lab tests found the doped nanotubes' fluorescent properties to be stable for months.

Weisman said treated nanotubes could be detected without using visible light. "Why does that matter? In biological detection, any time you excite at visible wavelengths, there's a little bit of background emission from the cells and from the tissues. By exciting instead in the infrared, we get rid of that problem," he said.

The researchers tested their ability to view doped nanotubes in a biological environment by adding them to cultures of human uterine adenocarcinoma cells. Later, images of the cells excited in the near-infrared showed single nanotubes shining brightly, whereas the same sample excited with visible light displayed a background haze that made the tubes much more difficult to spot.

His lab is refining the process of doping nanotubes, and Weisman has no doubt about their research potential. "There are many interesting scientific avenues to pursue," he said. "And if you want to see a single tube inside a cell, this is the best way to do it. The doped tubes can also be used for biodistribution studies.

"The nice thing is, this isn't an expensive or elaborate process," Weisman said. "Some reactions require days of work in the lab and transform only a small fraction of your starting material. But with this process, you can convert an entire nanotube sample very quickly."

Single-crystal films could advance solar cells (w/ Video)

October 8, 2010 By Bill Steele

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Amorphous silicon, deposited on a porous template fills the empty spaces. Laser heating melts the deposit and the top few microns of the silicon substrate. In a few nanoseconds the melted silicon recrystallizes. The substrate acts as a seed crystal for the material above, causing it to crystallize with the same alignment. This makes it easier for electric charges to flow, making possible more efficient solar cells and batteries. Image: Wiesner lab

The "holy grail" for such applications has been to create on a silicon base, or substrate, a film with a 3-D structure at the nanoscale, with the crystal lattice of the film aligned in the same direction (epitaxially) as in the substrate. Doing so is the culmination of years of research by Uli Wiesner, professor of materials science and engineering, into using polymer chemistry to create nanoscale self-assembling structures.

He and his colleagues report the breakthrough in the Oct. 8 issue of the journal Science. They used the new method to create a film with a raised texture, made up of tiny pillars just a few nanometers across. "Just the ability to make a single-crystal nanostructure has a lot of promise," Wiesner said. "We combine that with the ability of organic polymer materials to self-assemble at the nanoscale into various structures that can be templated into the crystalline material."

Wiesner's research group previously used self-assembly techniques to create Gräetzel solar cells, which use an organic dye sandwiched between two conductors. Arranging the conductors in a complex 3-D pattern creates more surface area to collect light and allows more efficient charge transport, Wiesner said.

Performance improves the most when the conducting materials are single crystals, Wiesner said. Most techniques for creating such films produce polycrystalline material -- a collection of "grains" or small crystals bunched together at random -- and grain boundaries retard the movement of electric charges, he explained.

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Uli Wienser explains his research. Video by Bill Steele

Wiesner's method uses block co-polymers to create porous templates into which a new material can flow and crystallize. A polymer consists of organic molecules that link into long chains to form a solid. A block co-polymer is made by joining two different molecules at their ends. When they chain together and are mixed with metal oxides, one forms a nanoscale pattern of repeating geometric shapes, while the other fills the space in between. Burning the polymer away leaves a porous metal oxide nanostructure that can act as a template.

Wiesner's team created a template with hexagonal pores on a silicon single-crystal substrate and deposited films of amorphous silicon or nickel silicide over it. In collaboration with Mike Thompson, associate professor of materials science and engineering, they then heated the silicon surface with very short (nanosecond) laser pulses. This melts the newly deposited layer and the top few microns (millionths of a meter) of the silicon substrate. After only a few tens of nanoseconds the molten silicon recrystallizes with the single crystal silicon substrate acting as a seed crystal to trigger crystallization in the deposited material above it, causing that crystal to line up epitaxially with the seed.

The template is dissolved away, leaving an array of hexagonal pillars about 30 nm across. The team has made porous nanostructured films up to 100 nm thick with other complex shapes. In previous work Wiesner created lattices of cylinders, planes, spheres and complex "gyroids" by varying the composition of co-polymers.

Other materials could be deposited, the researchers said. The goal here, they said, was to demonstrate the formation of film with the same material as the substrate (officially known as homoepitaxy) and with a different material (heteroepitaxy).

In a further proof-of-concept experiment, the researchers showed that the structured thin film could be arranged in micron-scale patterns, as might be necessary in designing an electronic circuit, by laying a mask over the surface before applying laser heating.

"We have essentially gotten to the holy grail," Wiesner said. "It is not only a nanostructured single crystal, but it has an epitaxial relation to the substrate. There is no better control."

Provided by Cornell University (news : web)