Enlarge Image Catalysts are prized for their ability to speed chemical reactions by grabbing molecular building blocks and knitting them together. But most catalysts are either on or off—and there hasn't been much scientists could do to flip the switch. Now, however, researchers have created a sandwich-shaped scaffold for turning on and off nearly any catalyst at will. If developed further, the new design could allow researchers to detect minute amounts of a wide range of small molecules—from explosives such as TNT to neurotransmitters that carry messages in the nervous system. Unlike industrial catalysts, many enzymes—biological catalysts made from...