Porphyrin self-assembly at electrochemical interfaces: role of potential modulated surface mobility

The self-assembly of 5,10,15,20-tetra(4-pyridyl)-21H,23H-porphine (TPyP) on Au(111) electrodes was investigated. The adlayer structure was found to depend on the electrode potential. At positive potentials (>0.5V(SCE)), a disordered layer of TPyP is formed on the Au(111) electrode. STM images showed that the disordered molecules are immobile. At negative potentials (-0.2V(SCE)), however, the molecules are highly mobile and can no longer be imaged by STM, though they remain on the surface. At intermediate potentials (-0.2 to +0.2V(SCE)), the TPyP formed a highly ordered adlayer. Once the ordered adlayer is formed, it persists even after the potential is stepped to higher values (0.5-0.8 V(SCE)). These results can be explained by the role of potential modulated adsorbate-substrate interaction and surface mobility. This suggests the intriguing prospect of using electrode potential to tune surface interactions and to drive surface processes, e.g., molecular self-assembly, in electrochemical systems.     

In situ infrared spectroscopy at electrochemical interfaces

An insight into the in situ FTIR spectroscopy method as applied in Electrochemistry is given. The particular aspects inherent to the electrochemical method are described in a concise form. Selected examples cover the results of about the last 8 years, on a variety of systems including carbon monoxide, small organic molecules and double-layer components (hydrogen, anions and water). The experimental data refer mostly to adsorption on well-defined single-crystal surfaces. Analogies and differences with data from the metal/gas interface are discussed.     

In-situ voltage tunneling spectroscopy at electrochemical interfaces

Nanophysics at electrochemical interfaces, probing the physical properties of nanostructures, requires laterally resolved in-situ spectroscopy, in particular voltage tunneling spectroscopy (VTS), which is at present not yet established. In-situ spectroscopy is required to achieve reliable and reproducible measurements of the intrinsic properties of nanostructures in an electrochemical environment, which are mainly determined in small nanostructures by surface atoms rather than bulk atoms. In contrast to tunneling spectroscopy in ultrahigh vacuum, tip and substrate double-layer capacitances as well as Faradaic currents play an important role in voltage tunneling spectroscopy at electrochemical interfaces. Deoxygenation of the electrolyte, fast measurements using appropriate instrumentation, and minimization of the unisolated tip apex and substrate surface areas exposed to the electrolyte are the key parameters to achieve reliable in-situ voltage tunneling spectroscopy data at electrochemical interfaces. The presented data show that bias voltage intervals of more than 1000 mV can be utilized for spectroscopic investigations in aqueous electrolytes, which allow the in-situ study of discrete electronic levels in nanostructures.     

Alignment of electronic energy levels at electrochemical interfaces

The position of electronic energy levels in a phase depends on the surface potentials at its boundaries. Bringing two phases in contact at an interface will alter the surface potentials shifting the energy levels relative to each other. Calculating such shifts for electrochemical interfaces requires a combination of methods from computational surface science and physical chemistry. The problem is closely related to the computation of potentials of electrochemically inactive electrodes. These so-called ideally polarizable interfaces are impossible to cross for electrons. In this perspective we review two density functional theory based methods that have been developed for this purpose, the workfunction method and the hydrogen insertion method. The key expressions of the two methods are derived from the formal theory of absolute electrode potentials. As an illustration of the workfunction method we review the computation of the potential of zero charge of the Pt(111)-water interface as recently published by a number of groups. The example of the hydrogen insertion method is from our own work on the rutile TiO(2)(110)-water interface at the point of zero proton charge. The calculations are summarized in level diagrams aligning the electronic energy levels of the solid electrode (Fermi level of the metal, valence band maximum and conduction band minimum of the semiconductor) to the band edges of liquid water and the standard potential for the reduction of the hydroxyl radical. All potentials are calculated at the same level of density functional theory using the standard hydrogen electrode as common energy reference. Comparison to experiment identifies the treatment of the valence band of water as a potentially dangerous source of error for application to electrocatalysis and photocatalysis.     

In situ video-STM studies of dynamic processes at electrochemical interfaces

Atomic-scale dynamic processes during Cu(100) dissolution/deposition in pure and Cu-containing 0.01 M HCl solution were studied in situ by high-speed electrochemical STM (video-STM). Direct observations of the equilibrium fluctuations at atomic kinks in the steps on the crystal surface due to the local removal/addition of atoms reveal the same anisotropic behavior found previously in Cu-free electrolytes, caused by the influence of the ordered (2 x 2) Cl adlayer on the kink structure. A first quantitative analysis of these fluctuations and interpretation in terms of a local current exchange density was attempted. In addition, observations on the nucleation of vacancy- or ad-rows at terrace corners and within the Cu steps are presented and the relevance of these processes for the macroscopic current density is discussed.     

Redox and redox-coupled processes of heme proteins and enzymes at electrochemical interfaces

Modern bioelectrochemical methods rely upon the immobilisation of redox proteins and enzymes on electrodes coated with biocompatible materials to prevent denaturation. However, even when protein denaturation is effectively avoided, heterogeneous protein electron transfer is often coupled to non-Faradaic processes like reorientation, conformational transitions or acid-base equilibria. Disentangling these processes requires methods capable of probing simultaneously the structure and reaction dynamics of the adsorbed species. Here we provide an overview of the recent developments in Raman and infrared surface-enhanced spectroelectrochemical techniques applied to the study of soluble and membrane bound redox heme proteins and enzymes. Possible biological implications of the findings are critically discussed.     

Polymer-based bilayer interfaces for electrochemical rectification

In this paper, the electrochemical current rectification phenomenon exhibited at an electrochemical interface constituted by a glassy carbon electrode covered with a bilayer of polymer films is discussed. The authors have shown that Methylene Blue (MB) redox species can be confined to a very thin insulating polymer film formed from orthophenylene diamine. The poly(opd) film exhibited excellent blocking properties to redox molecules in solution. On the other hand, the insulating poly(opd) film trapped with MB could mediate electron transfer between the redox molecules in solution and the electrode. Further, a second polymeric layer (Nafion film) trapped with ferrocene redox species was formed as the outer layer over the inner poly (opd) film containing MB. This bilayer-modified electrode, due to the significant difference in the redox potentials of the MB and ferrocene species immobilized in the inner and outer layers, respectively, exhibits unidirectional current flow and the results of the voltammetric investigations on the modified electrodes are described in this communication.     

Surface mobility and diffusion at interfaces of polystyrene in the vicinity of the glass transition

Symmetric polydisperse (M_w = 23 × 10⁴, M_w/M_n = 2.84) and monodisperse (M_w = 21 × 10⁴, M_w/M_n < 1.05) polystyrene (PS), and asymmetric polydisperse PS/poly(2,6-dimethyl 1,4-phenylene oxide) (PPO) interfaces have been bonded in the vicinity of the glass transition temperature (T_g) of PS. In a lap-shear joint geometry, strength develops in all cases with time to the fourth power, which indicates that it is diffusion controlled. Strength developing at short times at the polydisperse PS/PS interface, at 90°C, is higher than that at the monodisperse interface, at 92°C (at T_g − 13°C in both cases), presumably due to the contribution of the low molecular weight species. The decrease of strength at the PS/PPO interface when the bonding temperature decreases from 113 to 70°C, i.e., from T_g + 10°C to T_g − 33°C of the bulk PS, indicates a high molecular mobility at the surface as compared to that in the bulk, and can be expressed by a classical diffusion equation, which is valid above T_g (of the surface layer). © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 567–572, 1998     

Grand-canonical approaches to understand structures and processes at electrochemical interfaces from an atomistic perspective

Electrochemical interfaces between an electrode and an electrolyte are often covered by ions from the solution. These adsorbed ions can strongly modify the properties of the interfaces. Furthermore, in electrocatalysis, the reacting species typically have to get into contact with the surface of the electrocatalyst, where the reaction then proceeds. Hence the understanding of the interaction of solvated species with electrode surfaces and the determination of the resulting adsorbate structures as a function of electrochemical control parameters such as electrode potential and electrolyte concentrations are crucial in electrochemistry. Here, grand-canonical theoretical approaches to derive adsorbate structures at electrochemical interfaces from an atomistic perspective will be reviewed. Special emphasis will be put on the distinction between the validity of the approach and the approximations that are typically made when using this approach.     

Molecular-level investigation on electrochemical interfaces by Raman spectroscopy

The structure and dynamics of electrode/liquid interfaces play an increasingly important role in electrochemistry. Raman spectroscopy is capable of providing detailed structural information at molecular level and new insight into the interfacial structure, adsorption, reaction, electrocatalysis and corro-sion. In this account we will summarize some progresses of surface Raman spectroscopy in the study of electrochemical interfaces, mainly based on our group's work, laying emphasis on the detection sensitivity, spectral resolution, time resolution and spatial resolution as well as the hyphenated technique.     

Reversible electrochemical monitoring of surface confined reactions at liquid-liquid interfaces by modulation of ion transfer fluxes

The adsorption of the neutral surfactant Brij35 at a liquid-liquid interface is reversibly monitored via its disturbance of an electrochemically imposed ion flux across the interface, forming a promising experimental tool for the detection of surface confined reactions at such liquids and polymers.     

Probing nanoscale interfaces with electrochemical surface-enhanced Raman scattering

Surface-enhanced Raman scattering (SERS) is a powerful tool for studying nanoscale molecule-metal interfaces across a range of electrochemical applications. SERS combines molecular-level information with a high degree of surface specificity, making it an ideal tool for understanding interfacial processes, from understanding how analytes and electrolytes organize near metal surfaces to following surface-mediated reactions in real time. However, because SERS relies on the excitation of localized surface plasmons, additional effects such as the production of hot charge carriers and photothermal heating can impact electrochemical SERS signals. These effects must also be considered when using SERS for quantitative electrochemical studies.     

Glycan-modified interfaces in biosensing: an electrochemical approach

Glycans (complex carbohydrates) attached to various interfacial layers represent a simple but functional model of a cellular surface, which is covered densely by glycans. In this review, we discuss interactions of surface-confined glycans with various glycan-binding proteins, viruses, or pathogenic bacteria. Such glycan receptive interfaces can be applied for biosensing of these analytes. In this review, we focus on the application of glycan interfaces for label-free electrochemical biosensing.     

Reconstitution of supramolecular organization involved in energy metabolism at electrochemical interfaces for biosensing and bioenergy production

How the redox proteins and enzymes involved in bioenergetic pathways are organized is a relevant fundamental question, but our understanding of this is still incomplete. This review provides a critical examination of the electrochemical tools developed in recent years to obtain knowledge of the intramolecular and intermolecular electron transfer processes involved in metabolic pathways. Furthermore, better understanding of the electron transfer processes associated with energy metabolism will provide the basis for the rational design of biotechnological devices such as electrochemical biosensors, enzymatic and microbial fuel cells, and hydrogen production factories. Starting from the redox complexes involved in two relevant bacterial chains, i.e., from the hyperthermophile Aquifex aeolicus and the acidophile Acidithiobacillus ferrooxidans, examination of protein–protein interactions using electrochemistry is first reviewed, with a focus on the orientation of a protein on an electrochemical interface mimic of a physiological interaction between two partners. Special attention is paid to current research in the electrochemistry of essential membrane proteins, which is one mandatory step toward the understanding of energy metabolic pathways. The complex and challenging architectures built to reconstitute a membrane-like environment at an electrode are especially considered. The role played by electrochemistry in the attempt to consider full bacterial metabolism is finally emphasized through the study of whole cells immobilized at electrodes as suspensions or biofilms. Before the performances of biotechnological devices can be further improved to make them really attractive, questions remain to be addressed in this particular field of research. We discuss the bottlenecks that need to be overcome in the future.