Subnanomolar Ion Detection by Stripping Voltammetry with Solid-Supported Thin Polymeric Membrane Yushin Kim, Patrick J. Rodgers, Ryoichi Ishimatsu, and Shigeru Amemiya* Department of Chemistry, University of Pittsburgh, 219 Parkman Avenue, Pittsburgh, Pennsylvania 15260 Subnanomolar limits of detection (LODs) are obtained for stripping voltammetry based on ion transfer at the inter- face between the aqueous sample and the thin polymeric membrane supported with a solid electrode. It has been predicted theoretically that a lower LOD can be obtained for a more lipophilic analyte ion, which can be precon- centrated at a higher equilibrium concentration in the solid-supported thin polymeric membrane to enhance a stripping current response. This study is the first to experimentally confirm the general theoretical prediction for both cationic and anionic analytes. Proof-of-concept experiments demonstrate that a subnanomolar LOD of (8 ( 4) × 10 -11 M tetrapropylammonium is significantly lower than a LOD of less lipophilic tetraethylammo- nium. Importantly, stripping voltammetry of the cat- ionic analytes is enabled by newly introducing an oxidatively doped poly(3,4-ethylenedioxythiophene) film as the intermediate layer between a plasticized poly(vinyl chloride) membrane and a Au electrode. On the other hand, an undoped poly(3-octylthiophene) film is used as an intermediate layer for voltammetric detection of a lipophilic inorganic anion, hexafluoro- arsenate, an arsenical biocide found recently in waste- water. A LOD of (9 ( 2) × 10 -11 M hexafluoroarsenate thus obtained by ion-transfer stripping voltammetry is comparable to a LOD of 80 pM by inductively coupled plasma mass spectrometry with anion-exchange chro- matography. Great sensitivity for a lipophilic ion is potentially useful for environmental analysis because high lipophilicity of an ion is relevant to its bioaccu- mulation and toxicity. Dynamic ion transfer across the interface between two im- miscible electrolyte solutions, i.e., ITIES, enables highly sensitive stripping voltammetry. 1 In comparison to traditional stripping voltammetry, 2,3 ion-transfer stripping voltammetry at the liquid/ liquid interface is attractive for trace analysis of redox-inactive ions in environmental, biological, and biomedical samples. High sensitivity of this stripping method originates from preconcentra- tion of an aqueous analyte ion into a water-immiscible organic phase, which is driven by external control of the phase boundary potential at the interface. 1,4 The preconcentration step is followed voltammetrically by reverse extraction of the ion from the organic phase into the aqueous phase to yield a stripping ionic current with enhanced sensitivity. During the past decade, a limit of detection (LOD) of ion- transfer stripping voltammetry has been lowered to nanomolar levels while micromolar limits were originally reported for various ions including acetylcholine, 5 tetraethylammonium, 6 alkaline earth cations, 7 and protonated organic amines 8 by employing fluid organic phases. The improved sensitivity is mainly due to enhanced mass transfer of an analyte ion in the aqueous sample phase, which allows for more efficient preconcentration. Several to tens of nanomolar concentrations of Cd 2+ , 4 Zn 2+ , 4 Pb 2+ , 4,9 Hg 2+ , 9 and dodecylsulfonate 10 are detectable by rotating a plasticized poly(vinyl chloride) (PVC) membrane as a robust organic phase to hydrodynamically accelerate preconcentration of the analytes. 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