Porous Silicon Lights Way For New Analytical Devices

May 20, 1999

WEST LAFAYETTE, Ind. -- Porous silicon, a roughed-up version of the material that paved the way for the computer industry, is now smoothing the way for new types of chemical and medical analyses, including micro-laboratories designed to fit on a computer chip.

In the May 20 issue of the scientific journal Nature, Purdue University chemist Jillian Buriak with Jing Wei and Gary Siuzdak of the Scripps Research Institute describe how porous silicon can be combined with mass spectrometry, a method used to identify the chemical nature of a substance, to streamline and automate the analysis of biological molecules.

The technique provides new tools for pharmaceutical companies to analyze small molecules, and it may be incorporated into new technologies designed to put miniature laboratories on a computer chip, says Buriak, an assistant professor of chemistry.

"This is the first application that combines the unique properties of porous silicon with the capability of mass spectrometry for biochemical analyses," Buriak says. "Our technique can dramatically simplify many of these processes and will allow scientists to simultaneously test large numbers of compounds in a fraction of the time required by current methods."

Porous silicon is identical to the silicon used in many technological applications today, but its surface contains tiny openings, or pores, that can absorb and emit light when exposed to ultraviolet light. Both types of silicon are derived from silica, one of the most abundant compounds in the earth's crust. Porous silicon has been known to scientists since the 1950s, when they discovered that silicon could not always be polished smooth during manufacturing.

It wasn't until 1990 that this "rough" or porous silicon was found to have photoluminescent properties. In 1991, scientists discovered that it also emits light when electric current is applied, a finding that opened the door to coupling light and electronics to build computers and other devices.

Last year, Buriak developed a way to stabilize the surface of porous silicon, making it capable of withstanding the rigors of industrial use. Earlier this year, she developed ways to use white light to perform specific chemical reactions on porous silicon by exposing the surface to certain compounds.

While investigating the light-absorbing characteristics of porous silicon, Buriak teamed up with Siuzdak, who uses mass spectrometry to study biomolecular complexes. Mass spectrometry techniques often rely on light-absorbing substances, called matrices. When light is applied from a laser source, the matrix absorbs high levels of energy, sending the molecule that is being analyzed into the gas phase so its molecular mass can be characterized.

"A trick to using mass spectrometry techniques with biological molecules is to find a matrix that is both chemically compatible with biomolecules and capable of absorbing light at the wavelength produced by the laser," Buriak says. "The process often requires some trial-and-error efforts to find a matrix suitable to a specific molecule."

Buriak and Siuzdak found that porous silicon not only stood up to the high buffer conditions required by living molecules, but also could be used in place of traditional matrix substances for a wide range of biomolecules -- including sugars, peptides, drug molecules and other small molecules.

In addition, a porous silicon matrix did not generate the background ions, or "noise," associated with other matrix substances used with mass spectrometry techniques, Buriak says.

"We've shown that you can replace a number of matrices with just porous silicon for a wide range of compounds," she says. "This may allow researchers to automate some processes that now require human intervention."

The new technique, called desorption ionization on silicon, or DIOS, also will allow the use of matrix-assisted laser spectrometry for small-molecule analyses, such as those conducted by pharmaceutical companies to develop drugs.

"Drug companies often target drugs that have a relatively low mass," Buriak says. "The problem with matrix-assisted laser spectrometry is its limited use for low-molecular-weight molecules, because the matrix causes significant interference, making it difficult to distinguish the matrix from the sample."

Because most current computer technology is based on silicon, it will be easy to incorporate DIOS into manufacturing processes already in place, Buriak says.

Porous silicon might also be combined with new upcoming technologies, such as micro-fluidic devices, which are designed to pack dozens or hundreds of tiny "laboratories" on a single silicon chip.

"Porous silicon may be used to help downsize and automate the chemistry lab, just as silicon did for computers," Buriak says.
Jillian Buriak, 765-494-5302; buriak@purdue.edu
Gary Siudzak, 619-784-9415; siuzdak@scripps.edu

Desorption/Ionization Mass Spectrometry on Porous Silicon
Jing Wei, Jillian Buriak, and Gary Siuzdak
Desorption mass spectrometry has undergone significant improvements since the original experiments were performed over 90 years ago, the most dramatic change occurring in the early 1980s with the introduction of an organic matrix. A new desorption/ionization strategy for biomolecular mass spectrometry has been developed based on pulsed laser desorption/ionization from a porous silicon surface. Desorption/ionization on silicon (DIOS) uses porous silicon to trap analytes deposited on the surface and laser radiation to vaporize and ionize these molecules. DIOS is demonstrated for biomolecules at the femtomole and attomole level with little or no fragmentation, in contrast to what is typically observed with other direct desorption/ionization approaches. The ability to perform these measurements without a matrix also makes it more amenable to small molecule analysis. The ease of chemically and structurally modifying porous silicon has also been used to optimize the ionization characteristics of the surface for biomolecular applications. Overall, desorption/ionization on porous silicon permits analysis of a wide range of molecules with very good sensitivity and a demonstrated potential for automation, as well as compatibility with microfluidics and microchip technology on silicon.

Purdue University

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