A novel strategy for the immobilization of cytochrome c on the surface of chemically modified electrodes is demonstrated and used to investigate the protein's electron-transfer kinetics. Mixed monolayer films of alkanethiols and omega-terminated alkanethiols (terminated with pyridine, imidazole, or nitrile groups that are able to ligate with the heme) are used to adsorb cytochrome c to the surface of gold electrodes. The use of mixed films, as opposed to pure films, allows the concentration of adsorbed cytochrome to remain dilute and ensures a higher degree of homogeneity in their environment. The adsorbed protein is studied using electrochemical methods and scanning tunneling microscopy.

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.

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Single-molecule electrical conduction studies are used to evaluate how the molecular linking unit influences the tunneling efficiency in metal-molecule-metal (m-M-m) junctions. This work uses conducting-probe atomic force microscopy (CP-AFM) to compare the molecular conduction of two π-bonded molecules: one with a single thiol linker, and another with a conjugated double thiol linker at both ends of the molecules. The results demonstrate that the molecule with conjugated double thiol linkers displays higher conduction in gold-molecule-gold junctions than nonconjugated single thiol-gold contacts. IntroductionIn recent years, improving the electrical conduction of molecular wires by using conjugated molecules has been a major focus in molecular electronics. However, the conduction of the molecule can become very large, so the metal-moleculemetal (m-M-m) contacts can dominate the response. Recent experimental 1-9 and theoretical [10][11][12][13][14][15] work has demonstrated the importance of metal-molecule contacts to the efficiency of charge transfer through m-M-m junctions. Particularly, for alkanethiol-based junctions, metal-molecule contact resistance decreased up to 2 orders of magnitude when chemically bonded contacts were used instead of nonbonded (mechanical) contacts. 1 Unlike electrical conduction studies on saturated molecules, fewer experimental studies address how the contacts effect conduction through conjugated molecular systems. This work demonstrates that the efficiency of charge transport through π-bonded molecules forming m-M-m junctions with thiol linking units between the metals and the molecule can be improved by using conjugated double thiol linkers at both ends of the molecule. This knowledge should be useful for the design and fabrication of more-efficient electrical and optical devices with potential use in molecular electronics.This work compares electrical conduction measurements for gold-molecule-gold tunnel junctions containing two different types of gold-molecule contacts: single gold-sulfur-Arsulfur-gold and double gold-S 2 C-Ar-CS 2 -gold contacts at both ends of the molecule, where Ar is a biphenyl moiety. The gold-S 2 C-Ar-CS 2 -gold contacts are postulated to provide two points of contact with each gold electrode and better conjugation to the molecule's π-system than the single goldsulfur-Ar-sulfur-gold contact. The junctions are formed using