Quick cocaine detection

A new electronic, aptamerbased sensor that rapidly and cheaply detects cocaine illustrates the general power of such devices, say creators Kevin W. Plaxco, Alan J. Heeger, and colleagues at the University of California, Santa Barbara (J. Am. Chem. Soc. 2006, 128, 3138). The basic sensor consists of a nucleotide sequence designed by molecular evolution to bind to a specific target. The strand is tagged at one end by the redox indicator methylene blue (MB) and attached at the other end to a gold electrode. In the absence of a target-in this case, cocaine-the aptamer likely remains unfolded. If cocaine is present, the aptamer assembles into a three-armed structure with the cocaine molecule at the center. The MB tag either moves close to or binds with the electrode, thereby causing a change in voltage. The group ultimately found two nucleotide sequences that bind cocaine selectively in biofluid samples even in the presence of contaminants.