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

Abstract

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(-).

Beyond Simple Cartoons: Challenges in Characterizing Electrochemical Biosensor Interfaces

“…For electrochemical sensors, the addition of redox active species in solution (acting as a “marker” or “indicator”), or using redox labeled adsorbates (probes), enables facile measurement of Faradaic electrochemical signals. − A number of redox species have been evaluated for use in the SAM based analytical determinations. − These redox markers can also report on the local molecular environment. − Determining redox characteristics is useful not only for characterizing the environment of the probes, but also for transduction if the target binding affects the microscopic environment to a measurable extent. This holds true when the capture probe is directly labeled with a redox active moiety.…”

Metal‐Supported Phospholipid Bilayers Formed by Redox Command

“…[8c,9,10] Vesicle-substrate interactions (van der Waals and electrostatic forces) must be sufficiently strong to surfactants to ferrocene-terminated SAMs. [22,23] Others reported the electrochemically-triggered surface deposition of polyelectrolytes and colloidal particles containing carboxylate and phosphate groups. [24] Motivated by these previous studies, the redoxmediated ion pairing reaction was exploited to trigger and drive the adsorption of phospholipids to form SLBs.…”

An integrated experimental-theoretical approach to understand the electron transfer mechanism of adsorbed ferrocene-terminated alkanethiol monolayers

“…Considering that the FcCnSH molecules attached to gold substrates are mostly used as an electron transfer mediator, an externally applied potential can as well modulate these interactions [24]. In addition, the electrogenerated oxidized ferrocene (Fc + ) moieties at the interface can form ion-pairs with the anion of the supporting electrolyte [24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40]. Consequently, the nature and the concentration of the anion in solution have also an effect on the kinetics and thermodynamics of the redox reaction of FcCnSH modified gold substrates.…”

“…This result is not M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPTsurprising recalling that not only the type of ions is changed but also that PB ions can be specifically adsorbed onto Au substrates[72][73][74][75].As already mentioned, the shape of the anodic current-potential profile depends on the type of interactions between adsorbed Fc molecules, generating sharp signals before cycling only at acid pH. From a thermodynamics point of view, this behaviour could be due to ion pairing between the oxidized Fc + on the Au(111) substrate and the anion of the supporting electrolyte[26][27][28][29][30][31][32]40]. The formation of these ion pairs, favours the oxidation process of the adsorbed FcC6SH molecules at acid pH and high4 ClO − concentration.…”