Detection of per- and polyfluoroalkyl substances (PFAS) by interrupted energy transfer
PFAS, a family of highly fluorinated substances, represent
a danger for humans and the environment. Particularly problematic
members of this family, such as perfluorooctane sulfonate (PFOS) and
perfluorooctanoic acid (PFOA) appear to cause organ damage and cancer,
as well as disrupting the endocrine system. In the journal Angewandte
Chemie, researchers have now introduced a new method for an
economical, easy-to-use fluorescence sensor for sensitive on-site
testing for PFAS in water samples.
© Wiley-VCH, re-use with credit to 'Angewandte Chemie' and a link to the original article.
The term per- and polyfluoroalkyl substances (PFAS) refers
to a group of organic compounds in which most or all the hydrogen atoms
bound to the carbon atoms have been replaced with fluorine atoms. They
are used to provide water-, oil-, and dirt-resistance to a variety of
products, such as nonstick pans, outdoor clothing, and packaging. They
may also be found in fire-suppressing foam, paint, and car polish. These
compounds are highly useful—and highly dangerous when they find their
way into the environment: they do not break down and thus become
concentrated in plants, animals, and people.
Limits of 100 ng/l for individual specific PFAS substances
and 500 ng/l for the total of all PFAS were set for drinking water in
the EU. In Germany, water providers must begin testing drinking water
for PFAS in 2026. The US Environmental Protection Agency has set
stricter limits: for the most widespread PFAS (PFOS and PFOA), the upper
limit is set at 4nm/l for each substance.
The usual method used to detect such trace amounts involves
chromatography and mass spectrometry, is time-consuming and expensive,
and requires complex equipment and experienced personnel. Timothy M.
Swager and Alberto Concellón at the Massachusetts Institute of
Technology (MIT) in Cambridge, USA, have now introduced a technique for
making a portable, inexpensive test that uses fluorescence measurements
to easily and selectively detect PFAS in water samples.
The test is based on a polymer—in the form of a thin film
or nanoparticles—with fluorinated sidechains that have fluorinated dye
molecules (squaraine derivatives) embedded in them. The special polymer
backbone (poly-phenylene ethynylene) absorbs violet light and transfers
the light energy to the dye by an electron exchange (Dexter mechanism).
The dye then fluoresces red. If PFAS are present in the sample, they
enter the polymer and displace the dye molecules by a fraction of a
nanometer. This is enough to stop the electron exchange and thus the
energy transfer. The dye’s red fluorescence is “switched off”, while the
blue fluorescence of the polymer is “switched on”. The degree of
fluorescence change is proportional to the concentration of PFAS.
This new technique, which has a detection limit in the µg/l
range for PFOA and PFOS is suitable for on-site detection in highly
contaminated regions. Detection of trace amounts of these contaminants
in drinking water can be achieved with sufficient precision after
pre-concentration of the samples by solid-phase extraction.
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About the Author
Dr. Timothy M. Swager is a Professor of Chemistry at the
Massachusetts Institute of Technology. His research is at the
chemistry/materials interface and he has pioneered the use of novel
materials in the creation of chemical sensors with ultra-trace detection
capabilities. Dr. Alberto Concellón was a postdoctoral researcher at
MIT, and is presently at Ramón y Cajal Researcher at the University of
Zaragoza, Spain working on functional self-assembled materials.