Ion-transfer voltammetry of perfluoroalkanesulfonates and perfluoroalkanecarboxylates: Picomolar detection limit and high lipophilicity

Here we report on ion-transfer voltammetry of perfluoroalkanesulfonates and perfluoroalkanecarboxylates at the interface between a plasticized polymer membrane and water to enable the ultrasensitive detection of these persistent environmental contaminants with adverse health effects. The ion-transfer cyclic voltammograms of the perfluoroalkyl oxoanions are obtained by using a ∼1 μm thick poly(vinyl chloride) membrane plasticized with 2-nitrophenyl octyl ether. The cyclic voltammograms are numerically analyzed to determine formal ion-transfer potentials as a measure of ion lipophilicity. The fragmental analysis of the formal potentials reveals that the 10 times higher lipophilicity of a perfluoroalkanesulfonate in comparison to the alkanesulfonate with the same chain length is due to the inductive effect of perfluorination on lowering the electron density of the adjacent sulfonate group, thereby weakening its hydration. The fragmental analysis also demonstrates that the lipophilicities of perfluoroalkyl and alkyl groups with the same length are nearly identical and vary with the length. Advantageously, the high lipophilicity of perfluorooctanesulfonate allows for its stripping voltammetric detection at 50 pM in the presence of 1 mM aqueous supporting electrolytes, a ∼10 times higher concentration. Significantly, this detection limit for perfluorooctanesulfonate is unprecedentedly low for electrochemical sensors and is lower than its minimum reporting level in drinking water set by the U.S. Environmental Protection Agency. In comparison, the voltammetric detection of perfluoroalkanecarboxylates is compromised not only by the lower lipophilicity of the carboxylate group but also by its oxidative decarboxylation at the underlying poly(3-octylthiophene)-modified gold electrode during voltammetric ion-to-electron transduction.