OPTICAL SENSING: Porous silicon biosensor provides high sensitivity

An optical sensor based on porous silicon can detect DNA, antibodies, and some other small organic molecules in pico- and femtomolar concentrations. The sensor, developed by researchers at The Scripps Research Institute (TSRI) and the University of California San Diego (UCSD; both La Jolla, CA), detected DNA concentrations as low as 9 fg/mm²; it is described in a paper by Victor S.-Y. Lin, Kianoush Moteshari, and others in Science (Oct. 31, 1997).

Porous silicon research at UCSD has been ongoing, carried out by professor Michael Sailor and graduate student Keiki-Pua S. Dancil. For these specific optical sensors, single-crystal boron-doped silicon wafers are etched to provide a 1- to 5-µm-thick layer of porous silicon. The layer has cavities as large as 200 nm in diameter and a large total surface area. A light source and charge-coupled-device (CCD) camera are set up above the sensor chip (see figure).

Refractive index changes

Reflected noncoherent white light from the top and bottom of the porous layer forms Fabry-Perot fringes, which are dependent on the thickness and refractive index of the layer. To make an antibody detector, for example, the porous silicon chips are immersed in a solution that contains another molecule that is known to link to the antibody to be detected. The "linker" molecules become embedded in the porous silicon during immersion. The chip is then dried under vacuum to remove the solvent.

Detection takes place when the chip is submerged in a solution containing the antibody. The antibody and linker molecules link within the porous silicon layer. The reaction causes the refractive index of the layer to change, thereby shifting the wavelength of the reflected interference pattern.

Exactly why the refractive index changes is still open to debate, although the researchers suggest two possibilities. Incorporating organic molecules (n = 1.45) in the layer should increase the refractive index because it is displacing water (n = 1.33). The overall change in the refractive index during detection, however, has been toward lower indices of refraction and shorter wavelength reflection. This may be due to the organic materials reducing the carrier density in the porous layer, which would reduce the refractive index.

Detecting DNA

The sensor can detect many classic biological reactions that involve the recognition and binding of one molecule to a partner molecule. The researchers demonstrated detection of small organic molecules, certain DNA sequences, and antibodies and other proteins. The sensor does not yet provide information on the concentration of the compound, however. This could be addressed by creating arrays of sensors, with differing concentrations of the linker molecules.

Such a sensor could be part of a relatively inexpensive, compact diagnostic device. Applications include screening of chemicals for drugs, some medical laboratory tests including checking for specific antigens, and DNA fingerprinting. "We have found nothing as simple or practical as this device with as much sensitivity," said coauthor M. Reza Ghadiri, associate professor at TSRI’s Skaggs Institute for Chemical Biology. Other methods for detecting DNA have lower sensitivity limits ranging from 300 fg/mm² for surface plasmon resonance to 5 pm/mm² for grating coupling. Ghadiri adds, "We are hopeful that we will see commercial applications within two to five years."