Metal-detecting biosensors spot water contaminants
Rebecca Lai, an associate professor of chemistry at the University of Nebraska-Lincoln is working on sensors at various stages of development that are capable of detecting mercury, silver or platinum. Similar technology could be used to find cadmium, lead, arsenic, or other metals and metalloids.
A primary purpose for the sensors will be to detect water contaminants. Instead of sending water samples away for time-consuming tests, people might someday use the biosensors to routinely monitor household water supplies for lead, mercury, arsenic or other dangerous contaminants.
Lai is among scientists searching for new and better ways to find gold. Not only aesthetically appealing and financially valuable, the precious metal is in growing demand for pharmaceutical and scientific purposes, including anti-cancer agents and drugs fighting tuberculosis and rheumatoid arthritis.
"Geochemical exploration for gold is becoming increasingly important to the mining industry," says Lai. "There is a need for developing sensitive, selective and cost-effective analytical methods capable of identifying and quantifying gold in complex biological and environmental samples."
Scientists have employed several strategies to find gold, such as fluorescence-based sensors, nanomaterials and even a whole cell biosensor that uses transgenic E.coli. Lai was a co-author of a 2013 study that explored the use of E. coli as a gold biosensor.
DNA, the carrier of genetic information in nearly all living organisms, might seem an unlikely method to detect gold and other metals. Lai’s research, however, exploits long-observed interactions between metal ions and the four basic building blocks of DNA: adenine, cytosine, guanine and thymine.
Different metal ions have affinities with the different DNA bases. The gold sensor, for example, is based on gold ions’ interactions with adenine. A mercury sensor is based upon mercury ions’ interaction with thymine. A silver sensor would be based upon silver ions’ interaction with cytosine.
NUtech Ventures, UNL’s affiliate for technology commercialization, is pursuing patent protection and seeking licensing partners for Lai’s metal ion sensors.
"Although these interactions have been well-studied, they have not been exploited for use in electrochemical metal ion sensing," says Lai and doctoral student Yao Wu in a recent Analytical Chemistry article describing the gold sensor entitled ‘Electrochemical Gold(III) Sensor with High Sensitivity and Tunable Dynamic Range‘.
The article examines how oligoadenines – short adenine chains – can be used in the design and fabrication of this class of electrochemical biosensors, which would be able to measure concentrations of a target metal in a water sample as well as its presence.
The DNA-based sensor detects Au(III), a gold ion that originates from the dissolution of metallic gold. The mercury and silver sensors also detect dissolved mercury and silver ions.
"The detected Au(III) has to come from metallic gold, so if gold is found in a water supply, a gold deposit is somewhere nearby," Lai says. The DNA-based biosensors will need more refinement before they can be made commercially available.
Lai’s sensor works by measuring electric current passing from an electrode to a tracer molecule, methylene blue in this case. In the absence of Au(III), the observed current is high because the oligoadenine probes are highly flexible and the electron transfer between the electrode and the tracer molecule is efficient.
But upon binding to Au(III) in the sample, the flexibility of the oligoadenine DNA probes is hindered, resulting in a large reduction in the current from the tracer molecule. The extent of the change in current is used to determine the concentration of AU(III) in the sample.
To allow the sensor to be reused multiple times, the Au(III) is later removed from the sensor with an application of another ligand.
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