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Pesticides in drinking water are never a good thing. To help combat this problem, Xinyan Bi and Kun-Lin Yang of the National University of Singapore have developed a new sensor that can continuously monitor a city’s water supply for two pesticides, imidacloprid and thiacloprid. Their work is described in a new AC paper (DOI 10.1021/ac801786a). The new device is based on a quartz crystal microbalance (QCM) transducer coupled to a 2D molecular imprinted monolayer (MIM). The sensor can discriminate between the two structurally related neonicotinoid molecules at levels below minimal European Union (EU) food-safety standards, even within a complex background matrix. (Because Singapore does not have its own standard for these two pesticides, Yang opted to follow the EU standard for the purposes of this study.) According to Yang, the impetus for the project stems from Singapore’s use of recycled water. “The government is very worried about [the] safety of the drinking water,” he explains. “They want to build a system which can continuously monitor the quality of the water and to detect any kind of pesticide residues or any dangerous chemicals in that recycled water.” Yang and Bi chose to focus on imidacloprid and thiacloprid, insecticides commonly used to treat fruiting vegetables, because discriminating between the two is particularly difficult. “Those two molecules present a very, very interesting challenge to the sensor system,” Yang says, “because they have a very, very similar molecular structure.” The ideal system should be capable of continuous on-line monitoring, have high specificity and sensitivity, and be reusable, Yang explains. LC, the typical analytical method for pesticide monitoring, does not meet those requirements, so instead the researchers decided to try a QCM-based system. The principle behind a QCM is simple. When an electric current is applied to a quartz crystal, it vibrates at a characteristic
KUN-LIN YANG
Pesticide monitoring with a quartz crystal microbalance
A QCM sensor device used in the on-line monitoring of thiacloprid and imidacloprid.
frequency (⬃9 MHz), and when molecules bind to the crystal surface, the frequency shifts. To create a system based on this principle, the researchers flowed a solution of analyte over the surface of the quartz crystal (a disc ⬃3 mm in diameter). “We just continuously monitor the frequency of the QCM, so we will be able to know whether there is binding to the surface or no binding,” Yang explains. But the crystal, Yang emphasizes, is not the sensor; it is merely a transducer. The sensor is a MIM on the surface of a gold film that is bonded to the crystal. To create the MIM, a molecular cast is effectively “poured” around a templateOin this case, pesticide molecules. The cast was made of long-chain alkanethiols, which self-assembled to form a monolayer on the gold film; once the cast set, the template molecules were removed, leaving a single layer of pesticide-shaped cavities. Yang and Bi are not the first to use molecular imprinting techniques to build a QCM sensor, but most molecular imprinted polymers are relatively thick, 3D structures that are ill suited for the current application. “If we take the
10.1021/AC8025306 2009 AMERICAN CHEMICAL SOCIETY
Published on Web 12/16/2008
traditional approach of molecular imprinting, then the response time will be very, very long, because time is required for the analyte molecules to diffuse through the polymer layers,” Yang explains. Previous attempts to cast MIMs in 2-mercaptoethanol yielded relatively poor specificity, Yang says. Therefore, the researchers built the new sensor from octanethiol or from hexadecanethiol, which produced tighter binding and better specificity than octanethiol. Yang says this difference is probably because the longer hydrocarbon chains pack together more efficiently to create a more rigid cast. The resulting sensors were both specific (a thiacloprid-imprinted QCM responded to thiacloprid but not imidacloprid, and vice versa) and sensitive. A thiacloprid-imprinted QCM could detect as little as 1 M of pesticide in water, and in a complex matrixOin this case, celery juiceOit detected the pesticides at a lower concentration than the EU standard of 0.5⫺1 mg pesticide/kg food mass. The sensors are also fast, binding to the pesticide molecules nearly instantaneously, and reset themselves after each binding event, which makes them reusable. A bispecific sensorOthat is, one sensitive to both pesticidesOdetected both thiacloprid and imidacloprid in spiked celery juice preparations, suggesting that the sensor could also monitor food safety. This strategy may also be applicable to other molecular classes, Yang says, so long as the templates are relatively planar. In particular, he points to two families of molecules: benzene-ring-containing polyaromatic hydrocarbons, which have been implicated as potential carcinogens, and explosives. First, however, the researchers must see how the sensor performs in the field. Yang and Bi plan to collaborate with the Public Utilities Board, the government authority in charge of Singapore’s water supply system, “and see whether we can make the system work in a real environment.” —Jeffrey M. Perkel
FEBRUARY 1, 2009 / ANALYTICAL CHEMISTRY
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