Ion-Transfer Voltammetry of Perfluoroalkanesulfonates and Perfluoroalkanecarboxylates: Picomolar Detection Limit and High Lipophilicity

Abstract

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(4) 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(7) 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.

Voltammetric Mechanism of Multiion Detection with Thin Ionophore-Based Polymeric Membrane

“…The bulk concentrations are maintained near the interface during the voltammetric transfer of an analyte ion, because the aqueous phase contains excess amounts of primary and secondary ions 16 and because the membrane is sufficiently thin. 5,7,18-20 Accordingly, the bulk concentrations of the ionophore in its free and complex forms, [L] and [L n x X z x ], respectively, can be used to define an overall formation constant, β n x , as $${\beta }_{n_{\mathrm{normalX}}}=\frac{\left[{\mathrm{normalL}}_{n_{\mathrm{normalX}}}{\mathrm{normalX}}^{z_{\mathrm{normalX}}}\right]}{{\left[\mathrm{normalL}\right]}^{n_{\mathrm{x}}}{\left[{\mathrm{X}}^{z_{\mathrm{X}}}\right]}_{\mathrm{m}}}$$ The mass balance of the ionophore is also given by using the bulk concentrations as $$L_{\mathrm{normalT}}=\left[\mathrm{normalL}\right]+n_{\mathrm{normalI}}\left[{\mathrm{L}}_{n_{\mathrm{I}}}{\mathrm{I}}^{z_{\mathrm{I}}}\right]+n_{\mathrm{normalJ}}\left[{\mathrm{L}}_{n_{\mathrm{J}}}{\mathrm{J}}^{z_{\mathrm{J}}}\right]$$ where L T is the total concentration of the ionophore. The membrane composition depends on the phase boundary potential across the membrane/water interface, which is externally controlled to drive the interfacial transfer of primary and secondary ions.…”

Ion transfer stripping voltammetry for the detection of nanomolar levels of fluoxetine, citalopram, and sertraline in tap and river water samples

“…Recently, it was demonstrated that a double-polymer-modified electrode in combination with ITSV is a powerful platform to reach a nanomolar limit of detection (LOD) for ions [32][33][34].…”

Voltammetry of the aqueous complexation–dissociation coupled to transfer (ACDT) mechanism with charged ligands

“…Advances in the investigation and modelling of transfer processes across soft interfaces are challenging and valuable given their interest in biological and environmental sciences 1 and their applications in chemical analysis, electrocatalysis and liquid|liquid extraction, among other fields. [2][3][4] Electrochemical methods have been proven to be very suitable for the study of the transfer of ionic species, [5][6][7] providing electrochemical strategies as alternatives to solid electrode electrochemistry for electroanalytical determinations. Ion transfer processes are usually coupled to chemical reactions in solution, which must be detected and characterized in order to fully elucidate the transfer mechanism.…”

“…Half-wave potential of the first wave obtained in NPV under chemical equilibrium conditions when z 1 a z 2 and two well-defined signals are obtained F Faraday constant I Current response in NPV I (1) First wave obtained in NPV when two well-defined signals are obtained I (2) Second wave obtained in NPV when two well-defined signals are obtained Total limiting current in NPV under chemical equilibrium conditions (I lim,eq = I (1) lim,eq + I (2) lim,eq ) when two well-defined signals are obtained I (1) lim,eq Limiting current of the first wave obtained in NPV under chemical equilibrium conditions when z 1 a z 2 and two well-defined signals are obtained I (2) lim,eq Limiting current of the second wave obtained in NPV under chemical equilibrium conditions when z 1 a z 2 and two well-defined signals are obtained I 1 peak Peak current of the first signal obtained in dNPV when two well-defined signals are obtained…”

“…Distance to the interface z 1 Electrical charge of species M z1+ z 2 Electrical charge of species ML z2+ g Square root of the ratio between the diffusion coefficients in the organic and in the aqueous solution…”