Ion-exchange and electrochemical properties of tetra-t-alkylferrocenium monolayers on gold electrodes.

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-talkylferrocene = 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 E1/2 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 NO3– > BF4– > CF3SO3– > ClO4– > ReO4 – > PF6– > perfluorooctanesulfonate(PFOS–). The largest ΔE1/2(X−) was greater than 0.5 V and was observed for the Au/SDEC+/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 ΔE1/2(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 ΔE1/2(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 ionexchange 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.