A new sensor measures blood sugar levels by determining glucose concentrations in saliva, which could mean that diabetics no longer have to draw their own blood. Drawing blood through finger-sticks or other means is invasive and at least minimally painful — but for the 26 million Americans with diabetes, it is the most prevalent and effective method of checking glucose levels on a daily basis. The new technique, developed at Brown University, combines nanotechnology and surface plasmonics. The Brown engineers etched thousands of plasmonic interferometers onto a fingernail-size biochip and measured the concentration of glucose molecules in water on the chip. Their results showed that the specially designed biochip can detect glucose levels similar to those found in human saliva, where it typically is about 100 times less concentrated than in blood. The technique also could be used to detect chemicals and substances such as anthrax in biological compounds. The researchers created the sensor by carving a 100-nm-wide slit and etching two 200-nm-wide grooves on either side. The slit captures incoming photons and confines them, while the grooves scatter the incoming photons, which interact with the free electrons bounding around on the sensor's metal surface. The free electron-photon interactions create a surface plasmon polariton. These surface plasmon waves move along the sensor's surface until they encounter the photons in the slit. Each plasmonic interferometer — thousands of them per square millimeter — consists of a slit flanked by two grooves etched in a silver metal film. The schematic shows glucose molecules "dancing" on the sensor surface, illuminated by light with different colors. Changes in light intensity transmitted through the slit of each plasmonic interferometer yield information about the concentration of glucose molecules in solution. Courtesy of Domenico Pacifici. The interference between the two waves determines maxima and minima in the light intensity transmitted through the slit. The presence of an analyte on the sensor surface generates a change in the relative phase difference between the two surface plasmon waves, which in turn causes a change in light intensity, which the researchers measure in real time. "The slit is acting as a mixer for the three beams — the incident light and the surface plasmon waves," said Domenico Pacifici, assistant professor of engineering. The scientists discovered that they could vary the phase shift for an interferometer by changing the distance between the grooves and the slit, meaning that they can tune the wave-generated interference. They tuned thousands of interferometers to establish baselines, which then could be used to accurately measure concentrations of glucose in water as low as 0.36 mg/dl. "It could be possible to use these bio-chips to carry out the screening of multiple biomarkers for individual patients, all at once and in parallel, with unprecedented sensitivity," Pacifici said. The research was published in Nano Letters (doi: 10.1021/nl203325s) and was funded by the National Science Foundation and Brown (through a Richard B. Salomon Faculty Research Award).