Eric R. Dionne

Redox-Induced Ion Pairing of Anionic Surfactants with Ferrocene-Terminated Self-Assembled Monolayers: Faradaic Electrochemistry and Surfactant Aggregation at the Monolayer/Liquid Interface

Oxidoreduction of self-assembled monolayers (SAMs) of ferrocenyldodecanethiolate on gold in aqueous solutions of surface-active sodium n-alkyl sulfates (NaCnSO4) of 6, 8, 10, and 12 carbons is investigated by cyclic voltammetry and surface plasmon resonance. The effects of surfactant micellization and alkyl chain length on the redox response of the surface-tethered ferrocenes are examined. The SAM redox electrochemistry is sensitive to the surfactant aggregation state in solution. The nonideal behavior of the sodium alkyl sulfates at concentrations above the critical micelle concentration leads to a non-Nernstian variation of the SAM redox potential with concentration. The presence of micelles in solution results in decreased anodic-to-cathodic peak separations and anodic peak full widths at half-maximum. A longer alkyl chain length results in an increased ability of the alkyl sulfate anion to ion pair with the SAM-bound ferrocenium, resulting in oxidation of the ferrocene at lower potential. A comparison of the SAM redox potential at a fixed surfactant concentration of ideal behavior suggests a 4.5 × 10(4) difference in the ion-pairing abilities of the shorter-chain C6SO4(-) and longer-chain C12SO4(-). One-half of the available SAM-bound ferrocenes are oxidized in the NaCnSO4 electrolyte. Surfactant anions adsorb and assemble onto the SAM surface by specific ion-pairing interactions between the sulfate headgroups and oxidized ferrocenium species, forming an interdigitated monolayer in which the surfactant anions alternate between a heads-down and heads-up orientation with respect to the SAM. The work presented points to applications of ferrocenylalkanethiolate SAMs as anion-selective membranes, probes of micelle formation, and surfaces for the electrochemically switchable assembly of organosulfates.

Redox-Controlled Ion-Pairing Association of Anionic Surfactant to Ferrocene-Terminated Self-Assembled Monolayers

The redox-induced pairing from aqueous solution of a homologous series of sodium n-alkyl sulfate (NaCnSO4) surfactants of 6, 8, 10, and 12 carbons with gold-tethered self-assembled monolayers (SAMs) of ferrocenyldodecanethiolate (FcC12SAu) is investigated by cyclic voltammetry combined with surface plasmon resonance (SPR) spectroscopy. The adsorbed layer thicknesses and surface coverages are consistent with the formation of a monolayer of CnSO4(-) at the oxidized FcC12SAu SAM/aqueous solution interface. A comparison of the anodic charge density with the SPR data indicates that approximately 60% of the adsorbed surfactant anions are paired with SAM-bound ferroceniums, suggesting an interdigitated layer structure. The ion-pairing capabilities of the longer-chain NaC12SO4, NaC10SO4, and NaC8SO4 relative to the short-chain NaC6SO4 are compared using the relative ion-pair formation constants calculated from the apparent SAM redox potentials and IC50 values obtained from competitive association experiments. A longer alkyl chain increases the overall hydrophobicity of the CnSO4(-) anion, thereby increasing its ability to pair with and stabilize the ferrocenium in the nonpolar environment of the SAM. Binary mixtures of NaC12SO4 and NaC6SO4 of different compositions are used to demonstrate that the differences in ion-pairing abilities can be exploited to selectively pair and adsorb C12SO4(-).

Microcantilevers Bend to the Pressure of Clustered Redox Centers

The redox-activated deflection of microcantilevers has attracted interest for nanoactuation and chemical sensing. Microcantilever sensors are devices that transduce (bio)chemical reactions into a quantifiable nanomechanical motion via surface stress changes. Despite promising applications in analytical science, poor signal-to-noise ratios and a limited understanding of the molecular origins of the surface stress changes that cause the observed deflections remain obstacles to cantilever-based sensing becoming an established (bio)detection method, such as surface plasmon resonance and electrochemistry. We use phase-separated, binary self-assembled monolayers (SAMs) of ferrocenyldodecanethiolate and n-undecanethiolate as a model system to study the effect of the steric crowding of the redox centers on the surface stress change and cantilever deflection produced by the electrochemical oxidation of the surface-tethered ferrocene to ferrocenium. We correlate the measured surface stress change to the fraction of the clustered ferrocenyldodecanethiolate phase in the binary SAMs. The pairing of anions with the sterically crowded clustered ferroceniums induces a collective molecular reorientation which drives the cantilever deflection. The results provide fundamental insights into the response mechanism of microcantilever-based actuating and sensing technologies.