Schematic shows glucose molecules dancing on the sensor surface illuminated by different colors of light. Changes in light intensity transmitted through the slit of each plasmonic interferometer yield information about the concentration of glucose molecules in solution. (Image by Domenico Pacifici)

Late last year, Medtech Pulse reported that researchers at the University of Michigan (Ann Arbor) are developing a method for monitoring glucose levels using tears, not blood. Now, another group of researchers—this time at Brown University (Providence, RI)—are tooting their horn about a biochip that measures glucose in saliva. Using plasmonic interferometers, the chip could perhaps eliminate the need for diabetics to draw blood to check their glucose levels and could also be used to measure a range of other biological and environmental substances.

Relying on both nanotechnology and surface plasmonics, which explores the interaction of electrons and photons, the Brown engineers etched thousands of plasmonic interferometers onto a fingernail-size biochip and then measured the concentration of glucose molecules in water on the chip. Their results showed that the chip could detect glucose levels similar to those found in human saliva. Glucose in human saliva is typically about 100 times less concentrated than it is 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, assistant professor of engineering and lead author of a paper published in Nano Letters.

To create the sensor, the researchers carved a slit about 100 nm wide and etched two 200-nm-wide grooves on either side of the slit. 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 determines 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 turn, causes a change in light intensity. This change is measured by the researchers in real time.

The engineers learned that they could vary the phase shift for an interferometer by changing the distance between the grooves and the slit. In addition, they can tune the thousands of interferometers to establish baselines, which could then be used to accurately measure concentrations of glucose in water as low as 0.36 mg per deciliter.

“It could be possible to use these biochips to carry out the screening of multiple biomarkers for individual patients, all at once and in parallel, with unprecedented sensitivity,” Pacifici remarks.

To further test the devices, the engineers next plan is to build sensors tailored for glucose and other substances. “The proposed approach will enable very high throughput detection of environmentally and biologically relevant analytes in an extremely compact design," Pacifici says. "We can do it with a sensitivity that rivals modern technologies.”