As reported in the Jan. 21 issue of Applied Physics Letters, researchers demonstrated that the electrochemically etched particles come in particular sizes and fluoresce in distinct colors. The smallest four sizes are blue, green, yellow and red luminescent particles.

"The availability of specific particle size and emission in the red, green and blue range makes the particles useful for electronic displays and flash memories," said Munir Nayfeh, a UI professor of physics and corresponding author of the APL paper. "The benign nature of silicon also makes the particles useful as ultra-bright fluorescent markers for tagging biologically sensitive materials."

Current medical and biological fluorescent imaging is limited by the use of dye markers, which are not photostable, Nayfeh said. The dyes can break down under photoexcitation, room light or higher temperatures.

Not only are the new silicon particles photostable, they are also bright. The light from a single nanoparticle can be readily detected.

To convert bulk silicon into nanoparticles, Nayfeh and his colleagues use an electrochemical treatment that involves gradually immersing a silicon wafer into an etchant bath of hydrofluoric acid and hydrogen peroxide while applying an electrical current.

The process erodes the surface layer of the material, leaving behind a delicate network of weakly interconnected nanostructures. The wafer is then removed from the etchant and immersed briefly in an ultrasound bath.

Under the ultrasound treatment, the fragile nanostructure network crumbles into individual particles, which may be easily separated into the different size groups.

"The availability of different colored markers is very important for biomedical applications," said Nayfeh, who also is a researcher at the UI's Beckman Institute for Advanced Science and Technology. "By placing particles of different colors in strategic locations, you could study such phenomena as growth factors in cancer cells or how proteins fold."

The silicon particles fluoresce when struck with ultraviolet light. They also can fluoresce when struck with two photons of infrared light -- a technique that can non-invasively penetrate human tissue.

In a separate paper, published in the Jan. 7 issue of Applied Physics Letters, the researchers also demonstrated laser oscillation in small aggregates of the silicon nanoparticles.

"At 6 microns in diameter, these clusters of particles are one of the smallest lasers in the world," said Sahraoui Chaieb, a UI professor of theoretical and applied mechanics and a co-author of both papers. "This microlasing is an important step towards the realization of a laser on a chip, which could ultimately replace wires with optical interconnects."

The emission was dominated by a deep-red color, said Chaieb, who also is a researcher at the Beckman Institute. The clusters are currently stimulated by green light from a mercury lamp. One of the researchers' goals is to excite them instead with electricity.

The research team included Nicholas Barry and Paul Braun at the UI and Lubos Mitas at North Carolina State University. Funding was provided by the National Science Foundation and the Illinois Department of Commerce and Community Affairs.

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