Optical Gel Senses Chemicals

Kevin Robinson

PITTSBURGH -- By forming a functionalized gel around an array of colloidal particles, researchers have created a sensor that indicates the presence of specific chemicals by a shift in the gel's optical diffraction. Now they can use optical methods to quickly and reliably determine if a gel sensor has responded to a chemical.
Functionalized gel sensors are not new, but scientists have had a difficult time measuring gel response. According to Sanford Asher, whose work with John Holtz at the University of Pittsburgh led to this new method, the gels were not used as chemical sensors before because there was no good way of quantifying the response.
Previously the only methods for determining if a gel responded were either crude and inaccurate or extraordinarily expensive. The gels are made so that they are sensitive to particular chemicals; when in the presence of these chemicals they swell a little or a lot, depending on chemical concentrations and the gel makeup. Asher and Holtz came up with a way of forming the gel around an array of colloidal particles.
The colloidal particles assemble into a lattice that is very much like a Bragg grating. Then the gel is formed around the lattice. The spacing of the lattice depends on the volume of the gel. As the gel expands, the lattice constant changes, diffracting light to longer or shorter wavelengths, depending on how the gel is made.
To use this discovery, Asher and Holtz have created gels sensitive to chemicals such as lead, potassium and barium as well as glucose, and have attached them to fiber optic probes. They hooked the probes to a Perkin-Elmer spectrophotometer that detects the wavelength shifts when the probes respond to a chemical.
Asher said the group and the university are working out commercial licensing details. Because the amount of gel needed for a sensor is small, they may become a source of inexpensive, accurate chemical sensors that can be used in applications as varied as monitoring glucose for diabetics and testing drinking water . "We hope the system you will ultimately see will be much simpler and less expensive than the system we created here in the lab," said Asher.