A team of engineers at Brown University, Providence, RI, say that they have developed a new biochip sensor that that can selectively measure glucose concentrations in a complex fluid like saliva. This an important step toward a device that could enable diabetics to test their glucose levels without drawing blood.

The new chip uses a series of specific chemical reactions combined with plasmonic interferometry, which detects the chemical signature of compounds using light. They say that the device is sensitive enough to detect differences in glucose concentrations that amount to just a few thousand molecules in the sampled volume. This means that they can differentiate glucose from the background components of saliva.

The biochip is made from a one-inch-square piece of quartz coated with a thin layer of silver. Etched in the silver are thousands of nanoscale interferometers—tiny slits with a groove on each side. The grooves measure 200 nanometers wide, and the slit is 100 nanometers wide, about 1,000 times thinner than a human hair. When light is shined on the chip, the grooves cause a wave of free electrons in the silver, a surface plasmon polariton, to propagate toward the slit. Those waves interfere with light that passes through the slit. Sensitive detectors then measure the patterns of interference generated by the grooves and slits.

When a liquid is deposited on the chip, the light and the surface plasmon waves propagate through that liquid before they interfere with each other. That alters the interference patterns picked up by the detectors, depending on the chemical makeup of the liquid. By adjusting the distance between the grooves and the center slit, the interferometers can be calibrated to detect the signature of specific compounds or molecules, with high sensitivity in extremely small sample volumes.