| Five penta-substituted ferrocenes, each having one 11-SH-undecanoyl substituent, have been prepared from 1,1',3,3'-tetra-t-alkylferrocenes and attached as electrochemically-assembled monolayers to gold electrodes (t-alkyl = t-butyl (tetra-t-alkylferrocene = BUT-SH), 2-methyl-2-butyl (PENT-SH), 2-methyl-2-pentyl (HEX-SH), 2-methyl-2-hexyl (HEP-SH), and 2-methyl-2-nonyl (DEC-SH)). Cyclic voltammetry E 1/2 values determined for the Au/S-Fc+/0 redox couples in aqueous 0.1 M H2SO4 decreased significantly in the order shown when the electrolyte also contained 0.001 M of the anions NO 3- > BF4- > CF3SO 3- > ClO4- > ReO4 - > PF6- > perfluorooctanesulfonate (PFOS -). The largest DeltaE1/2(X- ) was greater than 0.5 V and was observed for the Au/S-DEC +/0 couple in 0.1 M H2SO4/0.001 M PFOS - relative to 0.1 M H2SO4 with no added anion. Additionally, for each of the 0.001 M added anions DeltaE 1/2(X-) became greater in the order Au/S-BUT+/0 < Au/S-PENT+/0 < Au/S-HEX+/0 < Au/S-HEP+/0 ≤ Au/S-DEC+/0. Equilibrium constants derived from DeltaE1/2(HSO4 -/X-) values suggest that differences in the electrochemical behavior for the five Au/S-Fc+/0 redox couples are manifestation of changes in the local dielectric constant surrounding Au/S-Fc+X - ion pairs. The novel ion-exchange selectivity of Au/S-HEP +/0 monolayers has been exploited to accomplish selective amperometric determination of part per billion levels of the emerging environmental contaminants PFOS- and perfluorooctanoic acid (PFOA) in aqueous mixtures. Finally, idealized electrode array outputs, derived from hydrodynamic voltammograms charted for each of the five Au/S-Fc+/0 monolayers from flow-injection data, demonstrate unique amperometric responses for structurally similar perfluorinated compounds: PFOS-, perfluorooctanoic acid (PFOA) and tetradecafluoroheptanoic acid. Unique current signatures are also demonstrated for structurally similar anions ClO4- and ReO4-. Collectively, these data demonstrate strong potential for a biologically-inspired sensing approach for interrogation of complex aqueous mixtures. |
