Two years ago, we first heard about how scientists at Rhode Island's Brown University were developing a biochip for detecting very low concentrations of glucose in saliva. Such a device could make life much easier for diabetics, as it would save them from having to perform fingerprick blood tests. At the time, it was limited to detecting glucose in water. Now, however, it's able to do so within a mixture of water, salts and select enzymes – also known as artificial saliva.
The biochip takes the form of a one-inch-square piece of quartz that's covered in a layer of silver. Etched into that silver are thousands of nanoscale interferometers. Each of these consists of one slit and two grooves; a 100-nanometer-wide slit that goes right through the silver, with 200-nm-wide grooves in the silver running parallel to the slit on either side.
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When light is shone through a liquid placed on one of these interferometers, some of the photons are captured within the slit, while other photons are scattered by the grooves and proceed to interact with the free electrons moving about on the silver's surface. These interactions result in oscillations known as surface plasmon polaritons.
Those move across the sensor's surface from the two grooves and, when they meet with the photons trapped in the slit, interference occurs. The amount of interference varies depending on the concentration of a given element present in the liquid sample and also determines how much light is ultimately able to shine through the slit. By measuring the light intensity coming through each slit, the sensor is thus able to determine the concentration of that element.
Previously, the scientists had used this plasmonic interferometry method to identify glucose in water at concentrations as low as 0.36 milligrams per deciliter – similar to levels found in human saliva. Unfortunately, though, the elements that make up the one percent of saliva that isn't water are enough to affect the response of the biochip.
To get around that problem, the researchers recently added dye chemistry to the process. More specifically, they added microfluidic channels that introduce two enzymes to the liquid being analyzed. One of those enzymes is glucose oxidase, which reacts with glucose to form a molecule of hydrogen peroxide. The second enzyme, horseradish peroxidase, reacts with each hydrogen peroxide molecule to form another molecule known as resorufin. That molecule absorbs and emits red light, coloring the liquid.
The nanoscale interferometers were tuned to detect the resorufin molecules (and thus the presence of glucose), and those molecules were then counted to determine the concentration of glucose. In lab tests, a combination of the plasmonic interferometry and dye chemistry methods were able to detect glucose in artificial saliva with 10 times more sensitivity than would be possible using only the interferometers.
The next step is to test the biochips on actual human saliva, with a small saliva-analyzing device for use by diabetics being the ultimate goal.
"We have demonstrated the sensitivity needed to measure glucose concentrations typical in saliva, which are typically 100 times lower than in blood," said project leader Domenico Pacifici, an assistant professor of engineering. "Now we are able to do this with extremely high specificity, which means that we can differentiate glucose from the background components of saliva."
A paper on the research was recently published in the journal Nanophotonics.
Source: Brown University