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Precise sensor made of hydrogels

Photonics Spectra
Apr 2011
WEST LAFAYETTE, Ind. – A new type of biological and chemical sensor made of thin stripes of hydrogel, a gelatinous material that expands and contracts depending upon the acidity of its environment, has been developed by researchers at Purdue University.

The sensors, which have few moving parts, are highly sensitive, sturdy and long-lasting, and can be used to precisely determine pH – how basic or acidic a liquid is – to reveal information about substances within liquid environments. They comprise flexible, water-insoluble hydrogels formed into a series of raised stripes that form a diffraction grating, which is coated with gold on both sides of, and in between, the stripe surfaces. Depending on the pH level of the environment, the stripes expand or contract.

Laser light reflected off the gold coating is analyzed by the sensor to determine changes in pH. The reflections from the stripes and spaces in between interfere with one another, creating a diffraction pattern that varies according to the height of the stripes. Minute changes in the movement of hydrogel stripes in response to the environment are indicated in the diffraction pattern, measuring the changes in pH.

The researchers can record the pH of a liquid on a scale from 0 to 14, with 0 being the most acidic and 14 being the most basic. Their findings showed that the device can detect changes smaller than one-1000th on the pH scale, measuring swelling of only a few nanometers.

Because the detection principle behind the technology is simple, the scientists do not foresee any difficulties commercializing the device. Because of their sensitivity, the sensors are suitable for use in chemical and biological applications such as environmental monitoring in waterways and glucose monitoring in blood.

They are confident that even more precise sensors can be developed and that gratings responsive to stimuli other than pH can be fabricated by using other types of hydrogels. Their work is ongoing.

Findings were presented in a paper during the IEEE Sensors 2010 Conference in November 2010. They were published in the conference proceedings (ISBN: 978-1-4244-8168-2/10).



GLOSSARY
diffraction grating
A glass substrate carrying a layer of deposited aluminum that has been pressure-ruled with a large number of fine equidistant grooves, using a diamond edge as a tool. Light falling on such a grating is dispersed into a series of spectra on both sides of the incident beam, the angular dispersion being inversely proportional to the line spacing. By proper shaping of the diamond edge, however, the grooves can be formed in such a way as to concentrate most of the energy into a single spectral...
diffraction pattern
The interference pattern formed by light waves diffracted at the edges of an object as seen on a screen placed in their path.
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