The copper ion Cu²⁺ is essential in biochemistry but can be toxic in high concentrations. Now, thanks to work by two chemists in the US, it can be detected with greater sensitivity. 

Cu²⁺ has so far proved challenging to detect using conventional fluorescent sensors due to its quenching effect on fluorophores. To overcome this problem, Juewen Liu and Yi Lu at the University of Illinois at Urbana-Champaign used a colour change that takes place in a solution of gold nanoparticles as a selective and sensitive Cu²⁺ sensor. 

The process is started by a DNA enzyme (DNAzyme), which is made up of an enzyme strand and two DNA strands. The presence of Cu²⁺ triggers a reaction that links the two strands of DNA together, know as a ligation reaction. The linked strands are then released from the DNAzyme and chemically modify the nanoparticles, causing them to aggregate and change colour in a process that can be monitored by the naked eye.

Two DNA strands (red and green) link up in the presence of copper ions, causing gold nanoparticles to clump together

Liu and Lu had previously developed a Cu²⁺ sensor that relied on selective DNA cleavage reactions by metal ions. According to Lu, the new method is more selective because there is far less chance of other species interfering with the ligation reaction than the cleavage reaction.

Other researchers in the field have welcomed the results. Chengde Mao, associate professor of analytical chemistry at Purdue University, Indiana, US, said the most attractive feature of the new technique is its convenience, adding that it could be performed without the need for specialised instrumentation or technical training.

Yingfu Li from the Department of Biochemistry and Biomedical Sciences at McMaster University, Ontario, Canada, considers the work ground-breaking. '[It] will certainly get [scientists] even more interested in creating more and better DNAzymes for real-world applications,' he said.