Scientists have invented plastic gels that, like high-tech litmus paper, change color after encountering a target chemical. The versatile gels, described in this week's issue of Nature, could lead to new tools for diagnosing illness, flagging environmental hazards, and monitoring industrial processes.
The new gels consist of tiny plastic beads arrayed, like atoms in a crystal, in a three-dimensional lattice that's surrounded by a water-saturated polymer. Because the beads are only about 100 nanometers across, slightly smaller than a wavelength of visible light, the collection diffracts light, making the gel shimmer with an iridescent glow. The glow's color depends on the spacing between spheres.
To make the gels sensitive to particular compounds, University of Pittsburgh chemists Sanford Asher and John Holtz equipped the polymer with chemical recognition groups, which are designed to bind to particular molecules. When a target molecule such as glucose binds to its recognition group (in this case an enzyme called glucose oxidase), the resulting pair carries a net electric charge. Water molecules try to distribute these charged pairs equally throughout the gel, but the pairs are stuck to the polymer; to compensate, the water molecules force their way into the polymer mesh, making the entire gel swell. With more space between neighboring beads, they now diffract longer wavelengths of light, changing the color, say, from a deep violet to a bright red.
"It's great stuff," says David Grier, a physicist at the University of Chicago. The technique, he says, is "sensitive, cheap, and very general." Asher says that a family of such gels could be developed to track everything from hazardous metals at Superfund sites to blood biochemistry changes that signal disease.