New sensors, materials with silicon

PURDUE UNIVERSITY researchers have discovered how to harness the light- emitting properties of porous silicon to stabilise the material's surface and direct it to respond to specific chemical environments or cues.

The development may allow scientists to tap unique photo emissive qualities of porous silicon to create new drug-delivery systems, or biological and chemical sensors capable of performing real- time measurements in medicine and manufacturing, says Jillian Buriak, associate professor in Purdue's Department of Chemistry.``We've shown, for example, that we can tailor the surface of a porous silicon wafer to survive within simulated in- vivo conditions, such as those found in blood plasma,'' Buriak says.

``Untreated porous silicon dissolves too quickly to be useful in such environments. By functionalising the surface, we may be able to develop sensors for use in diagnosing and treating disease.''

The study was published in the Journal of the American Chemical Society. It details how Buriak, working with Michael P. Stewart, used the light emitting properties of porous silicon to carry out an unprecedented chemical reaction on material's surface.

It also illustrates how nanocrystalline silicon - a form of porous silicon made up of crystals measuring just billionths of a meter in diameter - works to emit light. Nanocrystalline is one- billionth of a meter. Though porous silicon is identical in makeup to the silicon used in many microelectronic and computing applications today, its surface contains tiny openings, or pores.

In 1990 scientists discovered that some forms of porous silicon absorb and emit light. . ``Oxygen and water molecules in the air interact with the surface of porous silicon to create a glass- like coating that disrupts its photo luminescent properties.'

In 1998 Buriak's research group developed two new routes to protecting the surface from oxidation. The first involved treatment with a Lewis acid and a class of organic molecules.

The second outlined preliminary results concerning this white light promoted reaction. In the latest study details a chain of events that occurs when photons of light interact with nanocrystalline silicon, causing electrons to jump to a higher energy level.

Energy is then emitted in the form of light as the particles move back to their former state. In the process of moving to a higher energy level, the electrons leave a positively charged hole where a second electron might react, creating highly reactive molecules called``excitons.''

This type of reaction, , also occurs in nanocrystalline particles of some other materials such as titanium dioxide, which is used in solar cell applications. Using white light of moderate intensity from a tungsten source, Buriak and her team created excitons in the laboratory by exposing wafers made of nanocrystalline silicon to the light for 30 to 60 minutes in the presence of alkenes or alkynes, chemicals compounds that contain hydrogen and carbon. ``In this highly reactive state, nanocrystalline silicon reacts with the compounds to create a carbon-silicon bond that produces a stabilising coat,''Buriak says.

``This is a clean, practical reaction that allows us to stabilize the surface without the need for special equipment.'' An additional benefit is that by using a mask between porous silicon and light, the reaction can be photo patterned, since it takes place in illuminated surface.

This step allowed researchers to tailor the material in specific areas, directing the light- emitting properties of porous silicon to respond with chemicals where it was required.

``This method can be exploited to develop new types of sensing devices for use in medicine or industry,'' Buriak says. ``In drug delivery, for example, you need a device that releases a drug molecule in a specific way.

By functionalising the surface of silicon in this way, we can develop a sensing device capable of bonding to specific sites or detecting specific molecules.''

The study also demonstrates how the material can be divided into defined arrays to prompt chemical reactions on different parts of the material.

Buriak says this feature can be used in opto- electronics, bio analysis or integrating light-emitting devices with silicon chips.

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Section  : Science & Tech
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