A technique which works well at room temperature to detect micromolar quantities of cocaine in a matter of minutes using gold nanoparticles and DNA "aptamers" has been developed by researchers in China and Singapore. An aptamer is a nucleic acid macromolecule that binds tightly to a specific molecular target. Like all nucleic acids, a particular aptamer may be described by a linear sequence of nucleotides. Aptamer exhibits remarkable specificity and generally bind their targets with very high affinity. Chunhai Fan of the Shanghai Institute of Applied Physics and colleagues made an aptamer consisting of two pieces of random, coil-like single-stranded DNA that has a high affinity for cocaine molecules. When cocaine binds to the aptamer, it alters the affinity and affects the stability of the gold nanoparticles in salt solution. In the absence of cocaine, the aptamer is attached to the gold nanoparticles as a complex which is stable and red in colour. When there is cocaine present, it binds to the aptamer, so liberating the aptamer from the nanoparticles. This leads to the aggregation of nanoparticles in solution, turning it blue.