A clever combination of functional DNA and gold nanoparticles has produced a point-of-care diagnostic system that could allow medical workers in developing countries to achieve faster and cheaper detection of many common diseases.

Modern DNA tests can easily pick up sequences that belong to bacteria and viruses, but many of these rely on specialized equipment and training. According to Warren Chan, a biomedical engineering professor at the University of Toronto, gold nanoparticles (GNPs) provide a signal that’s simple enough to read with the naked eye. “When GNPs are monodispersed in solution, they appear red,” he says. “When they aggregate together, oscillating electrons between adjacent particles combine, and they appear blue.”

GNPs alone can’t detect DNA, but Kyryl Zagorovsky, a PhD candidate in Chan’s lab, solved this problem using functional DNA sequences called DNAzymes. Just as protein-based enzymes can change their shape on binding and thereby cleave certain molecules, so DNAzymes can break apart other sequences when bound to the right target. Zagorovsky coated GNPs with one of two sequences, A or B. He then made linker sequences complimentary to A on one end and B on the other, which aggregated the gold nanoparticles together. Finally, he created DNAzymes that can split the linker sequence only when they are bound to a target. A single bound DNAzyme can split many linkers, amplifying the signal and turning the solution from blue to red.

In a paper published in Angewandte Chemie, Chan and Zagorovsky successfully detected DNA sequences from gonorrhea, syphilis, hepatitis B and malaria. Moreover, the ratio of blue to red emission can be measured to get an idea of the target’s concentration; the system is sensitive enough to detect levels as low as 50 picomoles per litre. “It might seem complicated, but you actually have just three components, which could all be put together as a powder,” says Zagorovsky. “This makes it optimal for use in the field.” He and Chan plan to complete some clinical tests over the summer, and to eventually produce a spin-off company or license the technology to others in order to commercialize the new tool.