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Diagnosing Sugar Levels from Saliva

The standard means of checking those critical sugar levels in diabetic patients, is with a little pin-prick of blood extracted from a finger, and a hand held tester unit. This becomes routine for many if not most diabetics, but its invasive involving as it does the drawing of blood, and is neither sanitary enough nor practical for all situations.

Many different methods of glucose level testing have been tried, and continue to be developed, that use detection methods other than blood drawing, and are quicker or more accurate than the blood sugar test.

One that shows immense promise, is a biochip being developed by researchers at Brown University, which can detect a range of chemicals present in human saliva. The prototype is being tuned specifically to detect glucose levels, and is of a type where you just swab your mouth and test it – then rinse the sensor out and its ready for reuse.

It works using a sensor type called plasmonic interferometers, and has thousands of these simple sensors in each square milimetre of surface area. Each sensor consists of a simple slit flanked by two grooves etched in a silver metal film.

Here we see glucose molecules “dancing” on the sensor surface illuminated by light with different colours. Changes in light intensity transmitted through the slit of each plasmonic interferometer yield information about the concentration of glucose molecules in solution.

It is truly that simple. The chip can detect multiple chemicals in parallel, simply by shining different light wavelengths through the simple slits. All the complexity is on the detector end, not the swab end.

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 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.

The engineers next plan to build sensors tailored for glucose and for other substances to further test the devices. Glucose in human saliva is typically about 100 times less concentrated than in the blood.

The paper was published inNano Letters, a journal of the American Chemical Society. Lead author was Domenico Pacifici, assistant professor of engineering, with Tayhas Palmore, professor of engineering, as a contributing author on the paper.

Graduate students Jing Feng (engineering) and Vince Siu (biology), who designed the microfluidic channels and carried out the experiments, are listed as the first two authors on the paper. Other authors include Brown engineering graduate student Steve Rhieu and undergraduates Vihang Mehta, Alec Roelke.


Nanoscale Plasmonic Interferometers for Multispectral, High-Throughput Biochemical Sensing

Biochip measures glucose in saliva, not blood

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