No abstract

Self-assembled organization of functional molecules on solid surfaces has developed into a powerful and sophisticated tool for surface chemistry and nanotechnology. A number of reviews on the topic have been available since the mid 1990s. This perspective article aims to focus on recent development in the investigations of electronic structures and assembling dynamics of electrochemically controlled self-assembled monolayers (SAMs) of thiol containing molecules on gold surfaces. A brief introduction is first given and particularly illustrated by a Table summarizing the molecules studied, the surface lattice structures and the experimental operating conditions. This is followed by discussion of two major high-resolution experimental methods, scanning tunnelling microscopy (STM) and single-crystal electrochemistry. In Section 3, we briefly address choice of supporting electrolytes and substrate surfaces, and their effects on the SAM structures. Section 4 constitutes the major body of the article by offering some details of recent studies for the selected cases, including in situ monitoring of assembling dynamics, molecular electronic structures, and the key external factors determining the SAM packing. In Section 5, we give examples of what can be offered by theoretical computations for the detailed understanding of the SAM electronic structures revealed by STM images. A brief summary of the current applications of SAMs in wiring metalloproteins, design and fabrication of sensors, and single-molecule electronics is described in Section 6. In the final two sections (7 and 8), we discuss the current status in understanding of electronic structures and properties of SAMs in electrochemical environments and what could be expected for future perspectives.

A long-standing issue in protein film voltammetry (PFV), particularly electrocatalytic voltammetry of redox enzyme monolayers, is the variability of protein adsorption modes, reflected in distributions of catalytic activity of the adsorbed protein/enzyme molecules. Use of well-defined, atomically planar electrode surfaces is a step towards the resolution of this central issue.We report here the voltammetry of copper nitrite reductase (CNiR, Achromobacter xylosoxidans) on Au(111)-electrode surfaces modified by monolayers of a broad variety of thiol-based linker molecules. These represent positively charged and electrostatically neutral, hydrophobic and hydrophilic, aliphatic and aromatic, and variable-length microenvironments, as well as their combinations. Optimal conditions for enzyme function seems to be a combination of hydrophobic and hydrophilic surface linker properties, which can lead to close to complete non-catalytic monolayer interfacial electron transfer function and electrocatalysis with activity approaching enzyme activity in homogeneous solution. Thiophenol (combined hydrophobic stacking and interdispersed water molecules), 4-methyl-thiophenol (hydrophobic and water molecules), and 3-and 4-aminothiophenol * Corresponding author. E-mail: ju@kemi.dtu.dk 1344 A. C. Welinder et al.(hydrophilic, hydrophobic) offer the overall most efficient micro-environments. Subtle differences with even small structural linker differences, however, lead to widely different electrocatalytic properties, strikingly illuminated by the ω-mercaptoamines. CuNiR thus shows highly efficient, close to ideal reversible electrocatalytic voltammetry on cysteaminecovered Au(111)-electrode surfaces, most likely due to two cysteamine orientations previously disclosed by in situ scanning tunnelling microscopy. Such a dual orientation exposes both a hydrophobic and a positively charged, hydrophilic surface feature. In contrast, the higher cysteamine homologues expose only the hydrophilic component with no electrocatalytic activity on these surfaces. These results offer a basis for rational surface design in forthcoming biological electrocatalysis useful both fundamentally and in novel biosensor technology.

We have explored the adsorption of zinc-free human insulin on the three low-index single-crystalline Au(111)-, Au(100)- and Au(110)-surfaces in aqueous buffer (KH(2)PO(4), pH 5) by a combination of electrochemical scanning tunnelling microscopy (in situ STM) at single-molecule resolution and linear sweep, LSV, cyclic, CV, and square wave (SQWV) voltammetry.Multifarious electrochemical patterns were observed. Most attention was given to reductive desorption caused by insulin binding to the Au-surfaces via up to three disulfide groups per insulin monomer, presumably converted to single Au-S links. SQWV suggested the Au-S bond strength order Au(111) > Au(110) > Au(100) based on the reductive desorption potentials. The voltammetric diversity was paralleled by different in situ STM insulin adsorption modes on the three surfaces. Single-molecule resolution was achieved in all cases. The coverage followed the order Au(110) > Au(100) > Au(111) and differs from the reductive desorption order that records the Au-S bonding element. Evenly distributed single molecules were scattered over large Au(111)-terraces, with intriguing molecular arrays disclosed near the terrace edges. In comparison, high-density molecular scale structures were observed both over the terraces and across terrace edges on Au(100). Larger rectangular structures also appeared (8-12% coverage). These are Au-islands from the lift of the reconstruction. Notably, 10 x 10 nm(2) patches of highly ordered much smaller structures, possibly from insulin decomposition emerged sporadically within the dense insulin adlayer. Insulin adsorbed in highest coverage on the Au(110) and followed the directional surface topology with insulin molecules aligned in the Au(110)-surface grooves, occasionally "spilling over" and merging into larger structures.Adsorption, Au-S binding, and insulin unfolding are all parts of insulin surface behaviour and reflected in both voltammetry and in situ STM. In spite of these complications, the data show that molecular scale resolution has been achieved and offer other perspectives of insulin surface science such as single-molecule mapping of the insulin monomer/dimer-hexamer interconversion.