An ultrasensitive and ultraselective uranium sensor

Uranium poses a threat to human health and the environment, but the most sensitive methods of uranium detection require costly and bulky laboratory instruments. Therefore, keen analysis currently requires sending off potentially radioactively contaminated samples to well-funded, dedicated analytical laboratories. Now researchers have developed a sensor that will provide an inexpensive and portable way of detecting uranium in the field.

The uranium sensor is fluorescently labeled catalytic DNA, and it has sensitivity and selectivity that rival the most expensive laboratory equipment. The creation of the sensor occurred through a collaboration of researchers at the University of Illinois at Urbana-Champaign, the US Army Research and Development Center in Champaign, Oregon State University in Corvallis and Oak Ridge National Laboratory in Tennessee.

The sensor is a molecular beacon consisting of two annealed DNA strands, one of which has a fluorescent label on its 5' end (see figure). In the presence of the uranyl ion, the strand without the fluorophore enzymatically cleaves the strand that contains the fluorophore, releasing it. Once released and excited by a laser at 473 nm, it fluoresces 520-nm (bluish green) light. Quenchers on the 3' ends of both the substrate and enzymatic strands keep the fluorophore from fluorescing until the substrate strand is cleaved.

The researchers determined the appropriate DNA sequences by combinatorial selection, a method based on trial and error, using a large DNA pool in the presence of uranyl ions. They purchased the DNA samples, including the fluorophore and the quencher, from Integrated DNA Technologies of Coralville, Iowa, and used a fluorometer from Horiba Jobin Yvon of Edison, N.J.

As reported in the Feb. 6 online publication of PNAS, the researchers tested the sensor with various concentrations of the uranyl ion in aqueous solution and in soil samples suspected to contain uranium. The test took less than two minutes. The sensor detected uranyl ion concentrations as low as 11 parts per trillion, equivalent to 45 pM. They tested the sensor with solutions of 19 other metals and found it to be more than 1 million times more selective for uranium than for other metals.

Principal investigator Yi Lu said that the researchers plan to use a portable fluorometer for future experiments. He also said that additional tests on a large number of real-world samples are necessary to establish the sensor’s utility.