Gel turns clear for cocaine detection

Researchers from china have developed an aptamer cross-linked hydrogel that changes from bright red or blue to colourless when exposed to tiny amounts of cocaine. No sophisticated analysis is required, and it is hoped the gel will provide quick visual tests for small molecules such as drugs, explosives or water contaminants. 

The gel consists of polymer chains that are weakly held together by aptamers: short peptide chains that are engineered to bind to specific molecules. In this case, the Chinese researchers used an aptamer thatbinds strongly to cocaine. 

When the gel is exposed to small amounts of the drug, the aptamers release their hold on the polymer chains and instead bind to the cocaine. This causes the structure of the gel to collapse. 

In order to trigger a visible colour change when the gel collapses, gold nanoparticles are trapped inside the structure. As these escape, the gel turns clear. 

According to the researchers, as little as 20 nanograms of cocaine can be visually detected using this method in under ten minutes - which compares favourably with other systems that need complicated instrumentation for analysis. 

The low cost and simplicity of the method holds great promise for many applications - from airport security; to catch drug smugglers to rapid diagnostic systems in disaster areas. Also, the gel could be micro-patterned onto small "lab-on-a-chip" devices, which would be useful in forensic analysis or environmental monitoring.