By combining nanofabrication techniques and biology, researchers at Cornell University (Ithaca, NY) have devised a silicon chip with a diffraction grating composed of antibodies that reveal the presence of potentially harmful bacteria. Harold Craighead, a professor of applied and engineering physics, and Carl Batt, professor of food science, used a microcontact printing process to stamp a pattern of antibodies onto the silicon, thus simplifying production.
The performance of the sensor was tested by capturing E. coli (Escherichia coli O157:H7) cells on grating lines that had been stamped with E. coli antibodies derived from a goat. Antibodies—proteins generated by the body to bond with and render harmless foreign substances (antigens)—are highly specific to each antigen. A diffraction pattern is established because the bacteria are bound only to the regions stamped with an antibody grating pattern. The antibody grating alone produces insignificant optical diffraction, but when the bacteria are captured by the antibodies, the optical phase change produces a diffraction pattern when illuminated with a laser.
To conduct the research, Craighead first generated by contact photolithography a silicon master of 10-µm lines with 30-µm spacing. A stamp was then cast into silicon, coated with the antibody solution, and brought into contact for 30 min with the native-oxide surface of a silicon wafer. Diffraction measurements were made with a 632.8-nm helium-neon laser focused to a 1-mm-diameter spot on a masked area of the grating. A silicon detector directly behind an aperture measured the signals in microwatts.
The antibodies stamped on the surface effectively and rapidly captured E. coli from solution with incubation times of less than 30 min. With fluorescence microscopy, the researchers could directly observe the acridine-orange nucleic-acid-stained cells attached to the antibodies (see photo). Measuring the diffraction intensity provides a direct means of measuring the bacteria bound on the antibody gratings. The pattern also was easily visible to the eye.
Microcontact printing proved a simple and effective method for generating micrometer-scale patterns on silicon surfaces, although there was variability in antibody coverage. Because optical diffraction measurements are not susceptible to small defects, the nonuniformities are not critical to the measurements.
As a result, the researchers note, this simple sensor may be robust enough to detect other bacteria or pathogens that have sufficient mass to generate a diffraction signal when bound to a silicon surface. Craighead says, "This is just one example of the possible use of nanofabrication technology for biological applications."
As a weapon against bacteria in particular, Batt sees applications in detecting hospital-borne infections, battlefield threats, and infectious diseases. With regard to the food-processing industry, he notes, "As history has shown us, if a small colony of bacteria gets into the system, the cost, both in health and economic terms, can be enormous."
