Salivary Glucose Levels Measured by Biochip Sensor

The device is sensitive enough to detect differences in glucose concentrations that amount to just a few thousand molecules in the sampled volume.

Scientists at Brown University (Providence, RI, USA) exploited the synergistic advantage of combining plasmonic interferometry with an enzyme-driven dye assay that yields an optical sensor capable of detecting glucose in saliva with high sensitivity and selectivity. The biochip is made from a 2.54 x 2.54-cm 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 nm wide and the slit is 100 nm wide.

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. The scientists added microfluidic channels to the chip to introduce two enzymes that react with glucose in a very specific way. The first enzyme, glucose oxidase, reacts with glucose to form a molecule of hydrogen peroxide. This molecule then reacts with the second enzyme, horseradish peroxidase, to generate a molecule called resorufin, which can absorb and emit red light, thus coloring the solution. The team could then tune the interferometers to look for the red resorufin molecules.

The team tested its combination of dye chemistry and plasmonic interferometry by looking for glucose in artificial saliva, a mixture of water, salts and enzymes that resembles the real human saliva. They found that they could detect resorufin in real time with great accuracy and specificity. They were able to detect changes in glucose concentration of 0.1 μM/L, which is 10 times the sensitivity that can be achieved by interferometers alone. The proposed device is highly sensitive and highly specific for glucose sensing in picoliter volumes, across the physiological range of glucose concentrations found in human saliva, which is 20 μM–240 μM.

Domenico Pacifici, PhD, an assistant professor of engineering, and who led the study, said, “We have demonstrated the sensitivity needed to measure glucose concentrations typical in saliva, which are typically 100 times lower than in blood. Now we are able to do this with extremely high specificity, which means that we can differentiate glucose from the background components of saliva.” The study was published in the June 2014 edition of the journal Nanophotonics.

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