Heavy metals are an ubiquitous and troublesome class of pollutants, and lead (Pb) occupies a prominent position as a contaminant requiring constant attention because of its numerous toxicological effects over a wide range of exposure. Lead is a Resource Conservation and Recovery Act (RCRA) metal, and its presence often defines a waste as hazardous. It is also an Environmental Protection Agency (EPA) Urban Air Toxic, meaning its emissions are regulated under the Clean Air Act Amendment of 1990. Anthropogenic sources of Pb from military operations require active monitoring and sensing to ensure environmental compliance and protection. Despite the recognized adverse effects of lead on aquatic and terrestrial biota, its presence is not actively monitored, in large part because of the lack of a field product that meets all requirements for remote, real-time, and in-situ measurement of lead in groundwater.
This SERDP Exploratory Development (SEED) project aimed to create a highly selective and sensitive miniaturized sensor for bioavailable lead (Pb2+) by combining the following two recent advances: (1) catalytic deoxyribonucleic acid (DNA) that is reactive only to Pb2+ and can be tagged to produce fluorescence only in the presence of the metal and (2) nanoscale fluidic molecular gates that can manipulate fluid flow and perform molecular separations on tiny volumes of material. Researchers developed both the chemistry needed to combine Pb2+-specific catalytic DNA with the molecular gates and the protocol for separating, sensing, and quantifying Pb2+ in a complex matrix.
Building on the capability of microfabricated, capillary electrophoresis columns in polydimethylsiloxane to precisely control fluidic movement, a three-dimensional arrangement of these channels was used to induce a sample to pass through a molecular gate consisting of a thin polymeric membrane. Specific recognition elements that cause a measurable response in the presence of a particular species were incorporated into these channels. During this project, the interior of the channels was chemically modified to bind a unique sequence of DNA. This sequence of DNA, obtained through in vitro selection, is highly selective toward Pb2+. It cleaves an associated strand of substrate DNA in the presence of Pb2+. Tagging the substrate DNA with a fluorophore enables detection of the substrate DNA fragments, providing a sensitive optical signal for the presence of Pb2+. This research combined these scientific advances into a miniaturized sensor that is capable of remote, selective, and sensitive detection of the presence of Pb2+.
Detection limit estimates indicate that the prototype Pb2+ sensor developed in this project is capable of lead detection below drinking water action limits. Furthermore, because identifying the Pb2+-specific catalytic DNA sequence was accomplished via a novel combinatorial search, success in the development of this prototype Pb2+ sensor lends itself to rapid development of other targeted chemical sensors based on catalytic DNA that are uniquely reactive to other metals or organic compounds. Thus, relevance of this research is magnified beyond Pb2+ to include field sensors for other chemicals of interest. (SEED Project Completed - 2005)