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Tunable gel senses surroundings

06 Nov 2007

Colorimetric sensors could benefit from research into photonic gels that can be tuned across the entire visible spectrum.

Researchers at the Massachusetts Institute of Technology (MIT) have unveiled a material that reflects a wide range of wavelengths in response to a variety of stimuli. Based on a photonic-crystal structure, the gel responds to changes in salt concentration, humidity, pressure, and temperature and could have useful applications as colorimetric sensors. (Nature Materials published online doi:10.1038/nmat2032)

"Through changes in the environment around the gel, we have been able to shift the wavelength of reflected light by over 575 % from the ultraviolet to the near infrared," Edwin Thomas, a researcher at MIT, told optics.org. "Also, the ability of the gel to respond to such a wide variety of stimuli is an important improvement over prior research in the field."

Practical applications include measuring the level of chlorine in a pool and for color changing clothing, such as bathing suits. "These suits would change color depending on whether you were swimming in a pool, river or the ocean," explained Thomas. "Each of these environments contains different concentrations and types of salts and thus the suit would take on different colors in each environment."

The key to the design is a polyelectrolyte layer that changes thickness according to its environment. "The polymer is composed of two constituent polymers that self assemble into a structure that can interact with light," explained Thomas. "One of the polymers (polystyrene) is relatively inert and will not respond to changes in the environment. The other is a polyelectrolyte gel and is exceedingly sensitive to certain stimuli."

Immersing the material in various solvents changes the distance between repeating polymer layers and in turn the wavelength of the reflected light. "We can tune from about 1600 nm to 300 nm depending on the length of the polymer chain used," explained Thomas. "The structure of the polymer diffracts light such that certain wavelengths are forbidden to propagate in the material and are instead reflected."

Humidity in the air, temperature and pressure also affect the thickness of this polyelectrolyte layer. "In order for the system to be tunable it doesn't always have to be in contact with a salt solution, but it has to be in contact with something, including vapors that causes the layer to swell," explained Thomas.

The team is now is working on a gel that changes color in response to an applied electric field as well as other gel forming materials, with the aim of increasing the strength of the polymer.

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