Looking like a liquid opal, submicroscopic balls of plastic--the same polystyrene used to make coffee cups--sit embedded in a water-based gel. These polymerized crystalline colloidal array (PCCA) can do amazing and useful things. For instance, University of Pittsburgh chemists John H. Holtz and Sanford A. Asher discovered they can use the PCCAs as chemical sensors to make chemical measurements. "Colloidal arrays have fascinating optical properties," says Asher. "Because of their electrical charge, they self organize into a cubic structure where all the plastic balls are equally spaced. Depending on this spacing, the colloidal array diffracts (or reflects) visible light, much in the same way that an opal does, and you get intense colors." The chemists have made PCCAs that are highly sensitive to particular chemical species or thermal changes. If exposed to certain chemicals, such as lead, the array swells, changing the spacing. That causes the PCCA to diffract light at new wavelengths, and it changes color. Asher's group has demonstrated that the arrays are effective at detecting lead concentrations in water, and that once the lead is cleaned from the array it can be reused without any loss of sensitivity. FAX Kevin Roark at (412) 624-4895.