Diffraction-based sensor  (Source: Birck Nanotechnology Center, Purdue University)
Despite its simple design, the sensor is highly-sensitive and precise

Cagri Savran, study leader and an associate professor of mechanical engineering at Purdue University, along with Babak Ziaie, a Purdue University professor of electrical and computer engineering and biomedical engineering, and a team of researchers, are developing a low-cost, highly-sensitive biological and chemical sensor that can detect changes in the pH level of an environment without needing several moving parts.

"Many sensors being developed today are brilliantly designed but are too expensive to produce, require highly-skilled operators and are not robust enough to be practical," said Savran. 

But this new sensor is inexpensive and easy to use while still remaining highly sensitive. The sensor, which is made of thin stripes of gelatinous hydrogel that expands and contracts based on the acidity of its environment, can expose information about different substances in liquid environments by measuring its pH.  

The diffraction-based sensor uses water-insoluble hydrogel to make a "diffraction grating," which is a series of raised stripes. These stripes are coated with gold, and expand and contract based on the pH level of the environment. The sensors can then analyze laser light, which reflects off of the cold coating. These reflections from the top of the stripes and the spaces in between them meddle with one another, which results in a diffraction pattern that changes based on the height of the stripes.  

The diffraction patterns show small changes in movement of the hydrogel stripes when they react to the environment, which ultimately measures pH changes. 

"By precise measurement of pH, the diffraction patterns can reveal a lot of information about the sample environment," said Savran. "This technology detects very small changes in the swelling of the diffraction grating, which makes them very sensitive." 

In laboratory tests, researchers found that the hydrogel sensor can detect changes less than one-1,000th on the pH scale. The pH scale typically measures a liquid on a scale of 0 to 14, where 0 is the most acidic and 14 is the most basic.  

"We know we can make them even more sensitive," said Savran. "By using different hydrogels, gratings responsive to stimuli other than pH can also be fabricated. 

"As with any novel platform, more development is needed, but the detection principle behind this technology is so simple that it wouldn't be difficult to commercialize."

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