Bubbles and tiny electrodes may hold the key to faster, more cost-effective detection of perfluorinated surfactants that can contaminate drinking water. Researchers have developed an electrochemistry-based method to detect surfactants, specifically perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA), with high sensitivity and specificity (Anal. Chem. 2019, DOI: 10.1021/acs.analchem.9b01060).
Perfluorinated surfactants are highly stable due to perfluoroalkyl moieties, and are common in products like nonstick coatings and fire-fighting foam. Chronic exposure to two such perfluoroalkyl substances, PFOS and PFOA, has been linked to health issues in humans. Though these two chemicals are no longer used in industry, they persist in the environment and can contaminate drinking water.
Long Luo, an analytical chemist at Wayne State University, began his search for a novel way to detect these harmful chemicals after one such PFOS/PFOA contamination event in a Michigan town during the summer of 2018. The most commonly used detection method uses high-performance liquid chromatography with tandem mass spectrometry (HPLC-MS/MS), which requires complex instrumentation and can cost up to $300 per sample, Luo says. Hoping to develop a simpler, less expensive method, the team turned to electrochemistry.
Their method is based on a phenomenon known as electrochemical bubble nucleation. Applying electric potential to an electrode in an aqueous solution splits water into hydrogen gas and oxygen. Ramping up the current increases gas concentration near the electrode until a bubble forms, blocking the electrode surface and causing the current to drop. Surfactants reduce surface tension and make it easier for such bubbles to form, meaning the amount of current required to form those bubbles is inversely related to surfactant concentration.
To test their method, Luo and his collaborators fabricated tiny platinum electrodes less than 100 nm in diameter (smaller electrodes are more sensitive). The team could detect PFOS and PFOA concentrations as low as 80 µg/L and 30 µg/L, respectively. Preconcentrating samples using solid-phase extraction moved the limit of detection below 70 ng/L—the health advisory level for drinking water set by the U.S. Environmental Protection Agency. The method also remained sensitive and selective for surfactant detection even in the presence of a 1,000-fold greater concentration of poly(ethylene glycol), a nonsurfactant molecule with a molecular weight similar to that of PFOS.
“Electrochemical methods, in general, have great promise for measuring very low concentrations of contaminants in complex matrices,” says Michelle Crimi, an environmental engineer at Clarkson University. “I look forward to hearing more about the future of this technology, including its validation in field-contaminated water samples.”
Creating a handheld device for testing water in streams and other field sites—not just drinking water—is the ultimate goal, Luo says. An important step in that process will be developing a pretreatment phase to eliminate other surfactants that also promote bubble formation at electrodes, like sodium dodecyl sulfate. Such interference would be unlikely in drinking water samples, Luo says, because most compounds are not as stable as perfluoroalkyl substances and are destroyed during water treatment processing.