Influence of Self-Assembling Redox Mediators on Charge Transfer at Hydrophobic Electrodes

We report an investigation of the influence of reversible self-assembly of amphiphilic redox-mediators on interfacial charge transfer at chemically functionalized electrodes. Specifically, we employed (11-ferrocenylundecyl)-trimethylammonium bromide (FTMA) as a model self-assembling redox mediator and alkanethiol-modified gold films as hydrophobic electrodes. By performing cyclic voltammetry (CV, 10 mV/s) in aqueous solutions containing FTMA above its critical micellar concentration (CMC), we measured anodic (<i>I</i>_a) and cathodic (<i>I</i>_c) peak current densities of 18 ± 3 and 1.1 ± 0.1 μA/cm², respectively, revealing substantial current rectification (<i>I</i>_a/<i>I</i>_c= 17) at the hydrophobic electrodes. In contrast, hydroxymethyl ferrocene (a non-self-assembling redox mediator) at hydrophobic electrodes and FTMA at bare gold electrodes, yielded relatively low levels of rectification (<i>I</i>_a/<i>I</i>_c= 1.7 and 2.3, respectively). Scan-rate-dependent measurements revealed <i>I</i>_a of FTMA to arise largely from the diffusion of FTMA from bulk solution to the hydrophobic electrode whereas <i>I</i>_c was dominated by adsorbed FTMA, leading to the proposal that current rectification observed with FTMA is mediated by interfacial assemblies of reduced FTMA that block access of oxidized FTMA to the hydrophobic electrode. Support for this proposal was obtained by using atomic force microscopy and quartz crystal microbalance measurements to confirm the existence of interfacial assemblies of reduced FTMA (1.56 ± 0.2 molecules/nm²). Additional characterization of a mixed surfactant system containing FTMA and dodecyltrimethylammonium bromide (DTAB) revealed that interfacial assemblies of DTAB also block access of oxidized FTMA to hydrophobic electrodes; this system exhibited <i>I</i>_a/<i>I</i>_c > 80. These results and others reported in this paper suggest that current rectification occurs in this system because oxidized FTMA does not mix with interfacial assemblies of reduced FTMA or DTAB formed at hydrophobic electrodes. More broadly, these results show that self-assembling redox mediators, when combined with chemically functionalized electrodes, offer the basis of new principles for controlling charge transfer at electrode/solution interfaces

Influence of Order within Nonpolar Monolayers on Hydrophobic Interactions

We report an experimental investigation of the influence of molecular-level order (crystallinity) within nonpolar monolayers on hydrophobic interactions. The measurements were performed using gold film-supported monolayers formed from alkanethiols (CH₃(CH₂)_<i>n</i>SH, with <i>n</i> ranging from 3 to 17), which we confirmed by using polarization–modulation infrared reflection–adsorption spectroscopy to exhibit chain-length-dependent order (methylene peak moves from 2926 to 2919 cm^–1, corresponding to a transition from liquid- to quasi-crystalline-like order) in the absence of substantial changes in chain density (constant methyl peak intensity). By using monolayer-covered surfaces immersed in either aqueous triethanolamine (TEA, 10 mM, pH 7.0) or pure methanol, we quantified hydrophobic and van der Waals contributions to adhesive interactions between identical pairs of surfaces (measured using an atomic force microscope) as a function of the length and order of the aliphatic chains within the monolayers. In particular, we measured pull-off forces arising from hydrophobic adhesion to increase in a nonlinear manner with chain length (abrupt increase between <i>n</i> = 5 and 6 from 2.1 ± 0.3 to 14.1 ± 0.7 nN) and to correlate closely with a transition from a liquid-like to crystalline-like monolayer phase. In contrast, adhesion in methanol increased gradually with chain length from 0.3 ± 0.1 to 3.2 ± 0.3 nN for <i>n</i> = 3 to 7 and then did not change further with an increase in chain length. These results lead to the hypothesis that order within nonpolar monolayers influences hydrophobic interactions. Additional support for this hypothesis was obtained from measurements reported in this paper using long-chain alkanethiols (ordered) and alkenethiols (disordered). The results are placed into the context of recent spectroscopic studies of hydrogen bonding of water at nonpolar monolayers. Overall, our study provides new insight into factors that influence hydrophobic interactions at nonpolar monolayers

Peptide-Assisted Directional Adsorption of Purple Membrane at the Liquid–Solid Interface

We report here an approach of directional adsorption of purple membrane (PM) on liquid–solid interfaces modified by peptide assemblies. Bacteriorhodopsin (bR) is the only protein in PM that can transport protons directionally from the cytoplasmic (CP) side to the extracellular (EC) side to achieve chemical energy for life and growth. Controlled adsorption of PM is critical to exploring novel properties in many areas, such as data storage, biosolar devices, and sensors. Here, we obtained oriented PM adsorption at the liquid–solid interface by modification with <i>de novo</i> peptides. EFM was utilized to distinguish the two sides of PM by measuring the surface potential of PM because of its high resolution in differentiating electrical characteristics. Furthermore, we confirmed the modulating effect by photoelectrical responses under laser irradiation

Interaction of the Hydrophobic Tip of an Atomic Force Microscope with Oligopeptides Immobilized Using Short and Long Tethers

We report an investigation of the adhesive force generated between the hydrophobic tip of an atomic force microscope (AFM) and surfaces presenting oligopeptides immobilized using either short (∼1 nm) or long (∼60 nm) tethers. Specifically, we used either sulfosuccinimidyl-4-(<i>N</i>-maleimidomethyl)­cyclohexane-1-carboxylate (SSMCC) or 10 kDa polyethylene glycol (PEG) end-functionalized with maleimide and <i>N</i>-hydroxysuccinimide groups to immobilize helical oligomers of β-amino acids (β-peptides) to mixed monolayers presenting tetraethylene glycol (EG4) and amine-terminated EG4 (EG4N) groups. When SSMCC was used to immobilize the β-peptides, we measured the adhesive interaction between the AFM tip and surface to rupture through a single event with magnitude consistent with the interaction of a single β-peptide with the AFM tip. Surprisingly, this occurred even when, on average, multiple β-peptides were located within the interaction area between the AFM tip and surface. In contrast, when using the long 10 kDa PEG tether, we observed the magnitude of the adhesive interaction as well as the dynamics of the rupture events to unmask the presence of the multiple β-peptides within the interaction area. To provide insight into these observations, we formulated a simple mechanical model of the interaction of the AFM tip with the immobilized β-peptides and used the model to demonstrate that adhesion measurements performed using short tethers (but not long tethers) are dominated by the interaction of single β-peptides because (i) the mechanical properties of the short tether are highly nonlinear, thus causing one β-peptide to dominate the adhesion force at the point of rupture, and (ii) the AFM cantilever is mechanically unstable following the rupture of the adhesive interaction with a single β-peptide. Overall, our study reveals that short tethers offer the basis of an approach that facilitates measurement of adhesive interactions with single molecules presented at surfaces

Nonadditive Interactions Mediated by Water at Chemically Heterogeneous Surfaces: Nonionic Polar Groups and Hydrophobic Interactions

We explore how two nonionic polar groups (primary amine and primary amide) influence hydrophobic interactions of neighboring nonpolar domains. We designed stable β-peptide sequences that generated globally amphiphilic (GA) helices, each with a nonpolar domain formed by six cyclohexyl side chains arranged along one side of the 14-helix. The other side of the helix was dominated by three polar side chains, from β³-homolysine (K) and/or β³-homoglutamine (Q) residues. Variations in this polar side chain array included exclusively β³-hLys (GA-KKK) and β³-hLys/β³-hGln mixtures (e.g., GA-QKK and GA-QQK). Chemical force measurements in aqueous solution versus methanol allowed quantification of the hydrophobic interactions of the β-peptide with the nonpolar tip of an atomic force microscope (AFM). At pH 10.5, where the K side chain is largely uncharged, we measured hydrophobic adhesive interactions mediated by GA-KKK to be 0.61 ± 0.04 nN, by GA-QKK to be 0.54 ± 0.01 nN, and by GA-QQK to be 0 ± 0.01 nN. This finding suggests that replacing an amine group (K side chain) with a primary amide group (Q side chain) weakens the hydrophobic interaction generated by the six cyclohexyl side chains. AFM studies with solid-supported mixed monolayers containing an alkyl component (60%) and a component bearing either a terminal amide or an amine group (40%) revealed analogous trends. These observations from two distinct experiment systems indicate that proximal nonionic polar groups have pronounced effects on hydrophobic interactions generated by a neighboring nonpolar domain, and that the magnitude of the effect depends strongly on polar group identity

High Transfection Efficiency of Homogeneous DNA Nanoparticles Induced by Imidazolium Gemini Surfactant as Nonviral Vector

Nonviral vectors are highly desirable for the development of efficient gene delivery systems. In this study, we report the monomolecular condensation of plasmid DNA and efficient cell transfection by imidazolium gemini surfactants ([C₁₂-4-C₁₂im]­Br₂), which could be a potential nonviral vector for efficient gene therapy. Homogeneous DNA/[C₁₂-4-C₁₂im]­Br₂ nanoparticles are formed with a diameter of approximately 100 nm and investigated by using atomic force microscopy. DNA condensates evolve from supercoiled DNA molecules, to individual toroids, to close-packed particles, and eventually to multimolecular aggregates with the increase of [C₁₂-4-C₁₂im]­Br₂ concentrations. Highly efficient gene transfection in vitro is demonstrated in human embryonic kidney 293 (HEK293) and HeLa cells, which could be attributed to the effective DNA condensation into uniform nanoparticles induced by [C₁₂-4-C₁₂im]­Br₂. In addition, the low cytotoxicity of [C₁₂-4-C₁₂im]­Br₂ at transfection concentration region verified by cell viability assay (3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyl tetrazolium bromide, MTT assay) also supports [C₁₂-4-C₁₂im]­Br₂ as an effective gene vector. The high gene transfection efficiency by [C₁₂-4-C₁₂im]­Br₂ as well as its low cytotoxicity could shed light on the rational molecular design of nonviral vectors for gene delivery systems