Molecular Recognition at Model Organic Interfaces: Electrochemical Discrimination Using Self-Assembled Monolayers (SAMs) Modified via the Fusion of Phospholipid Vesicles

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Abstract

Supported lipid layers were formed via the fusion of large unilamellar vesicles of 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) to mixed self-assembled monolayers (SAMs) on gold comprised of ferrocene-functionalized hexadecanethiol chains (FcCO2C16SH) diluted in either hexadecanethiol (C16SH) or 1-hydroxylhexadecanethiol (HOC16SH). For the former case, the DMPC adsorbs predominantly as a single layer to form a hybrid bilayer membrane (HBM). The structures obtained in this way were characterized by methods that include electrochemical measurements, ellipsometry, and surface plasmon resonance (SPR). Cyclic voltammetry (CV) reveals that the electrochemistry of the ferrocene groups present in the SAM is strongly perturbed by the adlayer structure. The electrochemical behaviors of the ferrocene groups incorporated into a mixed SAM prepared using the more polar hydroxyl terminated thiol are quite different. The adsorption of DMPC via vesicle fusion in this case leads to the adsorption of bilayer assemblies of the lipid on top of the SAM. The coverages of the DMPC suggested by the SPR data lie between the values expected for fusion processes depositing either one or two bilayers of the lipid on top of the SAM. The electrochemical properties of the ferrocene moieties present in this structure were found to be largely unperturbed following the DMPC adsorption. Subsequent studies revealed that the adsorbed DMPC strongly influences the interactions of the tethered ferrocene groups with secondary aqueous molecular redox probes present in the electrolyte solution; permselective properties are seen in this adlayer structure. The varying degrees of electrochemical rectification seen in CV surveys demonstrated that probes such as K4Fe(CN)6, C5H5Fe(C5H4CH2N(CH3)3)PF6, and Ru(NH3)6Cl3 appear to penetrate the DMPC layer while species such as C5H5Fe[C5H4CH2N+H(CH3)2], C5H5Fe(C5H4CH2OH), and C5H5Fe(C5H4COO-) do not. We believe that molecular scale defect structures present in the adsorbed DMPC layers confer the molecular discrimination properties seen. A qualitative structural model is proposed.

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