Breakthrough sensor measures glucose levels in saliva

The technique, which takes advantage of a convergence of nanotechnology and surface plasmonics, could offer people with diabetes a pain free and non-invasive way of checking their blood sugar levels. The engineers at Brown etched thousands of plasmonic interferometers onto a fingernail-sized biochip and measured the concentration of glucose molecules in water on it. Their results showed that the device could detect glucose levels similar to the levels found in human saliva, despite the fact that it is about 100 times less concentrated than in the blood. "This is proof of concept that plasmonic interferometers can be used to detect molecules in low concentrations, using a footprint that is ten times smaller than a human hair," said Domenico Pacifici, an assistant professor of engineering at Brown. "The technique can be used to detect other chemicals or substances, from anthrax to biological compounds, and to detect them all at once, in parallel, using the same chip." To create the sensor, the researchers carved a slit about 100nm wide and etched two 200nm wide grooves on either side of it. According to Pacifici, The slit captures incoming photons and confines them. The grooves, meanwhile, scatter the incoming photons, which interact with the free electrons bounding around on the sensor's metal surface. Those free electron-photon interactions create a surface plasmon polariton, a special wave with a wavelength that is narrower than a photon in free space. These surface plasmon waves move along the sensor's surface until they encounter the photons in the slit. This 'interference' between the two waves is said to determine maxima and minima in the light intensity transmitted through the slit. The presence of an analyte (the chemical being measured) on the sensor surface generates a change in the relative phase difference between the two surface plasmon waves, which in turns causes a change in light intensity, measured by the researchers in real time. "The slit is acts as a mixer for the three beams - the incident light and the surface plasmon waves," Pacifici explained. The engineers found they could vary the phase shift for an interferometer by changing the distance between the grooves and the slit, meaning they could tune the interference generated by the waves. The researchers were also able to tune the thousands of interferometers to establish baselines, which could then be used to accurately measure concentrations of glucose in water as low as 0.36milligrams per deciliter. The team is now planning to build sensors tailored for glucose and other substances to further test the devices. "The proposed approach will enable very high throughput detection of environmentally and biologically relevant analytes in an extremely compact design," conluded Pacifici. "We can do it with a sensitivity that rivals modern technologies."