Biosensors respond with colored light.

When the color of autumn leaves changes from green to red, orange, and gold, it's easy to tell that winter is afoot. Similarly, sensors that change color provide a clear way to discern the presence of chemical substances. Recently, two research groups have demonstrated new schemes for such devices.

One design consists of a specially prepared silicon wafer that signals the presence of organic molecules, DNA, and proteins through subtle changes in color. The device is "exquisitely sensitive," says Michael J. Sailor of the University of California, San Diego in La Jolla, detecting concentrations one-hundredth the size of those currently observable. Sailor, M. Reza Ghadiri of the Scripps Research Institute in La Jolla, and their colleagues describe the sensor in the Oct. 31 Science.

Etching produces a forest of tiny vertical channels in the thin silicon wafer, making the surface layer porous. This changes the optical properties of the normally silver-colored silicon, giving it an iridescent sheen. The specific colors produced depend on the thickness of the porous layer and the geometry of the channels.

The researchers then attach to the porous silicon a number of molecules that recognize and bind to the substance they want to detect. For example, to detect a particular sequence of DNA,, the team attaches many complementary single strands of DNA to the wafer. As a solution containing the target sequence diffuses into the pores and the matching DNA strands bind to each other, the color of the silicon changes. By monitoring that color change with a spectrometer, the researchers can directly confirm the presence of the DNA.

Another sensor, developed by researchers at the University of Pittsburgh, takes advantage of the properties of colloidal crystals, or arrays of tiny polymer spheres (SN: 4/12/97, p. 224). Colloidal crystals diffract visible light and produce different colors, depending on the spacing of the spheres.

John H. Holtz and Sanford A. Asher embed a colloidal crystal in a gel that can be made to swell when it encounters substances such as glucose or metal ions. As the gel increases in volume, it spreads the polymer spheres apart, thus changing the color of the diffracted light. The team reports its findings in the Oct. 23 Nature.