Surface Plasmonic Biochip to Detect Glucose Levels in Saliva

Saliva has long been known to be a candidate in which glucose is measured as an indicator for how much is present in blood, but reliable detection of the small amounts of glucose in saliva has been difficult. Researchers at Brown University are reporting in Nano Letters that they developed a sensor based on surface plasmonics that is capable of detecting glucose levels in water similar to those found in saliva.

To create the sensor, the researchers carved a slit about 100 nanometers wide and etched two 200 nanometer-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, much like two ocean waves coming from different directions and colliding with each other. 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 turns causes a change in light intensity, measured by the researchers in real time.

"The slit is acting as a mixer for the three beams -- the incident light and the surface plasmon waves," Pacifici said.

The engineers learned 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 could 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 milligrams per deciliter.

Brown press release: Biochip measures glucose in saliva, not blood

Abstract in Nano Letters: Nanoscale Plasmonic Interferometers for Multispectral, High-Throughput Biochemical Sensing