DNA prefers diamond

By Kimberly Patch, Technology Research News

Scientists have shown that DNA is an extremely useful molecule -- it can sense other substances and can automatically arrange itself into microscopic structures.

Tapping the talents of the largest known molecule to sense pathogens like those used in bioterrorism, however, means solving a couple of problems. In order to operate in the field, the delicate molecule needs a stable portable environment and a connection to electronics that will reveal immediately what the DNA has sensed.

Both challenges would be solved by physically and electronically attaching biological molecules to the silicon wafers computer chips are made from, but that has proved difficult.

A research team from the University of Wisconsin at Madison, Argonne National Laboratory and the Naval Research Laboratory has found that DNA prefers diamond.

What's more, there's electricity involved. The researchers found that when DNA is attached to diamond, it is possible to electronically detect the changes in DNA that mark its connection with another molecule.

The combination of a very stable diamond-DNA interface and direct electronic readout could lead to real-time biosensors, including devices that continuously monitor for pathogens, said Robert Hamers, a chemistry professor at the University of Wisconsin at Madison.

Diamond wasn't the obvious choice. The researchers first worked out, with limited success, chemistry for attaching DNA to silicon. "We always found that there was some slow degradation of the interfaces that we reasoned was intrinsic to the chemistry of silicon," said Hamers.

Today's gene chip technologies already commonly attach biological molecules to silicon dioxide, or glass, a method that works well in the lab, but is less appropriate for the field. "Glass is slightly soluble in water, and so when exposed to water for some time, the outermost layer of glass and the biological... layer that it is attached to simply fall off into solution," Hamers said.

Glass works well for biomedical applications because they tend to use molecules just once, said Hamers. "However, our target application is more along the lines of real-time, continuous monitoring," he said.

Gold is another common surface for attaching biological molecules, but "the molecules have sulfur atoms on the end that bond weakly to the gold surface. This makes nice layers, but again they're not stable for long periods of time," Hamers said.

Biological, or organic, molecules contain carbon, which is the sole atomic ingredient of diamond. "Since carbon-carbon bonds are stronger than silicon-carbon or silicon-silicon bonds we reasoned that diamond surfaces would be more stable," said Hamers. "We decided to see if the chemistry we developed for modifying silicon with DNA would work with diamond," he said. With some modifications, it worked.

The research group developed a method to chemically modify the diamond surface with organic chemical groups that served as good attachment points for biomolecules, said Hamers. "Once that was in place, the rest was reasonably straightforward," he said.

The researchers used ultraviolet light to chemically bond amine molecules to the diamond. Amine is similar to ammonia, and is a product of decomposing biological material. The researchers used a layer of cross-linker molecules to attach DNA to the amine layer.

The carbon-carbon bonds between biological molecules and diamond are extremely strong and are not subject to the degradation processes that plague other materials, said Hamers. The key was "simply recognizing the carbon-carbon bonds are very strong and very stable, and that if we could link biological materials to something like diamond, it would probably be incredibly stable," he said.

Diamond has a couple of other advantages for real-time field applications like biological sensors, said Hamers. Diamond, like silicon, is a semiconductor, meaning it conducts electricity in a way that can be controlled to make useful electronics. Diamond can also be grown in extremely thin layers, and can be easily integrated with silicon, said Hamers. "You could make an integrated microelectronic device with silicon and diamond," said Hamers.

The drawback to using diamond is that it takes longer to process than silicon, glass or gold; researchers have made recent progress, however, in efficiently producing very thin diamond films. The diamond surface the researchers used to attach DNA was 500 nanometers thick, which is 150 times thinner than a human hair. A nanometer is one millionth of a millimeter.

Just as important as the physical connection is the electronic one. "Using diamond, we have recently been able to sense DNA hybridization and protein binding entirely using electronic means," said Hamers. "We can make the diamond surface part of an electrical circuit, and when DNA binds to it we can sense that electrically."

The common alternative is optical sensing, which uses light to measure changes in DNA. Optical sensing is expensive and requires a lot of power, said Hamers.

In contrast, electrical sensing is inexpensive, requires little power, and integrates well with existing electronics, Hamers said. "By going to electronic detection, one has the possibility of leveraging all the power of microelectronics -- high degrees of parallelization, integration, small size, on-board signal amplification and processing, and [even] build-in communications," he said.

The method is ready now to apply to practical devices, said Hamers.

The researchers are working to make miniaturized versions of the system with all the electronics on the same chip, said Hamers. This would provide "a complete biosensor system on a chip [including] the sensor itself [and] all the fluid handling and signal processing," components, he said.

Eventually, the method could be used for a sort of bio-cell phone, Hamers said. Such a device could be "carried around or placed in high-traffic areas to continuously monitor for chemical or biological pathogens," he said. Because the sensing DNA can be read electronically, when the device encountered a pathogen it could immediately generate a warning signal, he said.

Because the diamond surfaces appear to be very stable they could be very useful, said Robert Corn, professor of chemistry at the University of Wisconsin at Madison.

Corn has teamed up with Hamers in a project to use the method to make better DNA arrays for biological and computing applications. "The robust surface chemistry will allow us to perform many enzymatic... reactions on the DNA monolayers without degradation," said Corn. "The electronic sensing abilities of DNA hybridization of diamond are also potentially very useful for the sensing of DNA in both [biological and computing] applications," he said.

Hamers' research colleagues were Wensha Yang, Wei Cai, Tanya Knickerbocker, Tami Lassiter and Lloyd M. Smith of the University of Wisconsin at Madison, Orlando Auciello, John A. Carlisle, Jennifer Gerbi and Dieter M. Gruen of Argonne National Laboratory, and James E. Butler and John N. Russell, Jr. of the Naval Research Laboratory.

They published the research in the November 24, 2002 issue of Nature Materials. The research was funded by the Office of Naval Research (ONR), the Wisconsin Alumni Research Foundation (WARF), and the National Science Foundation (NSF).

Timeline:   < 1 year
Funding:   Government; University
TRN Categories:   Biology; Biotechnology;Materials Science and Engineering; Nanotechnology
Story Type:   News
Related Elements:  Technical paper, "DNA-Modified Nanocrystalline Diamond Thin-Films As Stable, Biologically Active Substrates," Nature Materials, November 24, 2002.


December 11-25, 2002

Page One

DNA prefers diamond

Material soaks up the sun

Design links quantum bits

Microscopic mix strengthens magnet

Laser pulses could speed memory


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