Detecting hazardous substances, such as heavy metals in soil and water, has traditionally been done by field sampling followed by analysis in a laboratory equipped with very sophisticated instruments. Portable sensors that can identify contaminants in the field would be highly advantageous for both obtaining faster results and having the ability to monitor remediation efforts. Previous attempts to design such sensors have failed to produce devices with the same or better sensitivity and selectivity as laboratory instruments. This is especially true for sensing metals because many metal ions are very similar, making them difficult to detect at ultra-low concentrations with no interference by other metal ions.

One example is uranium, a naturally occurring radionuclide that almost always co-exists with thorium. Because of their similar physical and chemical properties, even some analytical equipment cannot distinguish between the two metals. It is important to know if uranium is present because of its adverse health effects to human exposure, including radiation poisoning and kidney damage. Over the past 60-plus years, uranium has been enriched for use in nuclear power plants, missiles, and nuclear weapons. Currently, with skyrocketing oil prices, nuclear power is being revisited as an important future energy source, so there could be a growing potential for environmental contamination. The uranyl ion is the stable form of uranium in water, and in this form poses greatest risk to environmental migration and human health. Detection by conventional means includes atomic absorption, emission, phosphorescence, or mass detection, all of which require expensive laboratory equipment.

To address this technology gap, the U.S. Army Engineer Research and Development Center (ERDC) teamed with the University of Illinois at Urbana-Champaign, Oregon State University at Corvallis, and Oak Ridge National Laboratory, TN, to conduct research on deoxyribonucleic acid (DNA) enzymes. To most people, DNA is associated with passing genetic traits from one generation to the next. In 1994, however, DNA was also shown to have catalytic activity, which pointed to its possible use as a metal-sensing platform. Catalytically active DNA (or DNAzymes) are isolated using a biological method called in vitro selection together with the heavy metal target of interest. DNAzymes show high metal-binding affinity and specificity.

The research team searched for DNAzymes specific for uranium by performing in vitro selection on commercially available libraries of DNA with the uranyl ion as the metal cofactor. After isolating the most active DNA strand toward the uranyl ion, they cut the DNA strand in two to create the DNAzyme strand and a complementary substrate DNA strand possessing a weak point, a ribonucleic acid (RNA) base. A “catalytic beacon” was constructed by tagging the DNAzyme with a quencher and the substrate strand with a fluorophore, so that when hybridized together no fluorescence is observed. Only in the presence of the uranyl ion does the DNAzyme cleave the substrate strand at the RNA base, separating the two strands and releasing the fluorescent signal. The sensor has a detection limit of 11 parts per trillion for the uranyl ion, a dynamic range up to 400 nanometers, and selectivity of greater than 1-million-fold over other metal ions, including thorium.

After testing in the laboratory, the sensor was demonstrated in the field for detecting uranium in soil samples. Solvent extracts from three different contaminated soils were analyzed using the catalytic beacon construct. Results from the catalytic beacon sensor compared favorably with results obtained from atomic emission performed in the laboratory.

The uranyl ion selective DNAzyme now takes its place next to lead selective DNAzymes to build multianalyte sensors. ERDC research is already exploiting these DNAzymes by introducing them onto portable labs on chip devices for field measurement of uranium and lead in ground water. Research to uncover DNAzymes for other heavy metal emerging contaminants, such as beryllium, tungsten, mercury, and copper, would represent a critical advance in creating simple, cost-effective, and portable metal sensors with similar sensitivity and selectivity as much more expensive and sophisticated analytical instruments. For DOD, the potential uses are many and could include monitoring remediation activities on formerly used defense sites, detecting the presence of contaminants at facilities targeted for base realignment and closure, and, because of the sensor’s portability, ensuring the health and safety of Soldiers deployed around the world. It will also allow real-time field testing and monitoring at Department of Energy sites.

For more information, please contact Dr. Donald Cropek at (217) 373-6737 or e-mail donald.m.cropek@usace.army.mil.