DNA-GOLD NANOPARTICLES, employing the talent of DNA strands for recognizing and attaching to complementary strands and gold's electronic and optical properties, operate as a new kind of biosensor. Scientists at Northwestern University glue various "probe" DNA segments onto tiny gold particles (13 nm wide). When a "target" single-stranded DNA introduced into the solution happens to be complementary to DNA already stuck to the particles, the probe and target strands link up, creating a sort of polymer network whose color is different from that of the original solution. Thus recognition of the target DNA is signaled by a color change. The researchers can already use this approach to detect single-strand DNA samples in 10-femtomolar amounts. (Science, 22 August 1997.)