Per- and polyfluoroalkyl substances (PFAS) are persistent pollutants that have contaminated drinking water supplies around the world. Scientists and regulators would like to easily detect these substances as soon as they appear near the source of contamination and continuously track them in the field. Now, scientists have put together the basic components of an electrode sensor that could one day provide real-time remote sensing of PFAS in waterways (ACS Sens. 2020, DOI: 10.1021/acssensors.0c01894).
PFAS contamination “permeates many of the natural waterways here in North Carolina, says coauthor Jeffrey E. Dick, an analytical chemist at the University of North Carolina at Chapel Hill. His team is working with the US Army Corps of Engineers to improve detection of the substances, which have been used in firefighting foam, as stain repellents, and as nonstick coatings. One of the most widespread of these substances, perfluorooctanesulfonate (PFOS), has been linked to immune system and liver damage.
Instead of bringing water samples back to the lab for analysis, researchers would like to have a continuous field monitor. ”We knew that electrochemistry potentially would be sensitive and selective enough to detect PFAS, and it’s rather inexpensive,” Dick says.
In an earlier study, the team created a sensor to detect a PFAS known as GenX. The researchers coated electrodes with a polymer made using a method called molecular imprinting, which molds the polymer to the shape of the target molecule (Environ. Sci. Technol. Lett. 2020, DOI: 10.1021/acs.estlett.0c00341). Tested in the lab, the sensor detected current from the oxidation of ferrocene methanol, used as an electron donor. GenX molecules nestle into the electrode’s imprinted pores, blocking the oxidation and reducing the current in proportion to the concentration of GenX.
“But ferrocene derivatives are specialized synthetic compounds that we can’t use out in waterways,” Dick says. “That’s when we realized that we could use naturally available dissolved oxygen in the water for the redox reaction.” To test the idea, the team coated an electrode with a polymer imprinted to recognize PFOS. The scientists collected water samples from the Haw River in North Carolina and added PFOS to them at various concentrations. They then used electrical impedance spectroscopy to calibrate changes in current to PFOS concentrations. The electrode detected PFOS at levels as low as 3.4 pM, well below the US Environmental Protection Agency’s health advisory level of 140 pM.
“The next step is to optimize the selectivity of the polymer to PFOS,” Dick says. Although the sensor was selective toward PFOS, other substances in natural waterways could interfere with the signal. Future experiments will test different polymers for the best performance. The researchers hope to one day deploy a sensor not much larger than a coffee cup that could float in a river, record PFOS concentrations, and send the data to a researcher’s cellphone.
“Because PFAS compounds are hydrophobic and not electrochemically active, detecting them in waterways is difficult,” says Netz Arroyo, an electroanalytical chemist at Johns Hopkins University. He says the study authors creatively solved this problem by using the molecularly imprinted polymer. But before the sensor can be deployed, they must ensure that it is selective for PFOS alone, can respond to continuously changing concentrations, and doesn’t get fouled with bacteria, he adds.