What's the best way to find out if an [--1--] contains a specific substance, like an environmental contaminant? You could use an expensive, bulky gas chromatograph—but Harvard researchers have developed an instrument you can carry in your pocket. They describe the device, called an inverse opal, in the Journal of the American Chemical Society.

An inverse opal is a chip of layered glass that includes [--2--]. This mini-labyrinth gets treated with chemicals to modify its inner surfaces. The resulting porous chip then sucks up only liquids that have specific surface tensions. When such a liquid enters the network, it changes the chip's color over a set area. An inverse opal designed to determine if ethanol is [--3--] methanol could thus reveal the letter M if the poison is present.

With this so-called Watermark Ink writing, the chips could also serve as secret code devices. For example, [--4--] a chip with water might make a decoy word like “hello” visible. While exposure to, say, benzene, could expose a secret message, such as “ditch your gas chromatograph.
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