Metal‐Supported Phospholipid Bilayers Formed by Redox Command

Abstract

induce the rupture of adsorbed vesicles and formation of a continuous bilayer via vesicle-substrate and/or vesicle-vesicle interactions. Vesicle fusion typically works with a narrow range of material surfaces, notably smooth hydrophilic surfaces (water contact angle θ W < 20°), [11] such as glass, silica, and mica. [8c,12,13a] Adsorbed vesicles remain intact on gold surfaces employed for electrochemical and surface plasmon resonance sensing; [9c,13] these are typically polycrystalline [14] and present θ W ≳ 60° in ambient atmosphere. [11,12,13a,15] Although methods have been developed to form bilayers on gold (e.g., surface functionalization with hydrophilic organic films, [16] solvent-assisted lipid bilayer formation [13b] or the addition of a vesicledestabilizing agent [17] ), there remains a need for active strategies that are fast, versatile, and scalable.

Gold-Supported Lipid Membranes Formed by Redox-Triggered Vesicle Fusion on Binary Self-Assembled Monolayers: Ion-Pairing Association and Surface Hydrophilicity

“…The CVs shown herein for the pure FcC 11 SAu SAM (χ Fc surf = 1.0) in the presence of SUVs of DMPS (Figure A) or POPS (Figure S6) are similar to those reported in earlier work using single-component FcC 12 SAu SAMs . An oxidation peak appears at around 610 mV (vs Ag/AgCl) for the gel-phase DMPS and 540 mV for the fluid-phase POPS during the anodic potential sweep.…”

“…NaF was added as a supporting electrolyte to reduce the resistance of the aqueous SUV suspensions. The poor ion-pairing ability of F – compared to ClO 4 – facilitates the ion-pairing association of the anionic or zwitterionic phospholipid headgroup with the oxidized ferrocenium. , No redox peaks were observed on potential cycling of the Fc-terminated SAM in the NaF (aq) electrolyte in a previous study . Moreover, we found that the small size of the F – counterion does not hinder the fusion of SUVs of zwitterionic phospholipid to the SAM surface …”

“…While cyclic voltammetry informs on the electrochemical behavior and ion-pair association of the phospholipids with the electrogenerated ferroceniums, we previously demonstrated that holding the applied potential at oxidizing values for a period of minutes is more effective than potential cycling at producing adsorbate layers of high phospholipid surface coverage (i.e., ≳90% on pure ferrocene-terminated SAMs) and uniform thickness . The applied potential is stepped from open circuit to 640 mV (vs Ag/AgCl) to generate the maximum concentration of ferrocenium and held at 640 mV for a period of up to 5 min to allow the adsorbed vesicle−bilayer transformation to go to completion before stepping back to open circuit.…”

“…These values yield effective adsorbate film thicknesses of 5.1 ± 0.4 nm for DMPS and 4.3 ± 0.3 nm for POPS that are close to the anhydrous bilayer thicknesses determined by X-ray and neutron scattering (4.4 nm for DMPS and 4.2 nm for POPS). , Similarly, the effective thicknesses calculated for DPPC, DOPC, and POPG (Table ) are in agreement with previously published data. , Ion-pair formation between the phosphate and/or carboxylate of the anionic phospholipid head and positively-charged ferrocenium induces sufficient tension or deformation of the adsorbed vesicles to cause them to rupture and form a bilayer. In the case of the zwitterionic phospholipids, ion pairing of the headgroup phosphate with the ferrocenium leaves a positive charge (choline group) in the region of the vesicle–SAM surface interaction that is compensated by the electrolyte anion …”

“…We recently reported a proof-of-concept demonstration that the redox-triggered fusion of SUVs to the surface of ferrocene-terminated SAMs on gold produces single bilayer membranes of high surface coverage (≳90%) within minutes at room temperature . The electrochemical oxidation of the SAM-bound ferrocene (Fc) involves both interfacial electron transfer and the formation of an ion pair between the cationic ferrocenium (Fc + ) generated and an electrolyte anion (X – ) in solution to neutralize the excess positive charge at the SAM/aqueous interface , normalF normalc normalS normalA normalM + normalX false( normala normalq false) − ⇌ ( F c + X − ) S A M + normale − …”