Two Metal-Molecule Binding Modes for Peptide Molecular Junctions Slawomir Sek † Department of Chemistry, UniVersity of Warsaw, Pasteura 1, 02093 Warsaw, Poland ReceiVed: May 22, 2007; In Final Form: June 25, 2007 The electrical behavior of gold-molecule-gold junctions incorporating 1,12-dodecanedithiol and 11-amino- 1-undecanethiol was investigated using scanning tunneling spectroscopy. The replacement of one terminal thiol group by the amine resulted in a change of the electronic conductance of the junction reflecting the difference in the nature of metal-molecule binding. The results were compared with the conductance of the molecular junctions incorporating short peptide consisting of a cystamine linker, L-alanine, and L-cysteine with an alternatively deprotected thiol or amine group. Using this approach, it is possible to form gold- molecule contacts using thiol or amine functional groups. The electron transmission through the peptide junctions formed using two different binding modes was relatively efficient. However, the junctions with gold-sulfur contacts showed a higher conductance as compared to those with a gold-amine contact. Introduction The electronic conduction properties of single organic molecules became an important issue in the studies of electron- transfer processes. 1 Understanding the electric behavior of the single molecules fixed between two metal electrodes is crucial for the future development of molecular electronics and nano- scale devices. 2-4 In a simple metal-molecule-metal junction, there are several important factors that determine the electrical behavior of the system. The first one is the chemical structure of the molecular bridge linking two metal electrodes. Using different bridging units, one can control the efficiency of electron transmission through the junction and symmetry of the current- voltage response. 5-10 Recently, it was shown that the conduc- tance of the molecular junctions could be strongly influenced by the geometry of the metal-molecule binding site 11,12 and the order within the molecular assembly trapped between two metallic electrodes. 12 In the latter case, it was demonstrated that for alkanedithiols, the structural transition of the alkyl chains from a gauche to an all-trans conformation leads to significant differences in the measured conductance. The variety of transitional conformers contributes also to the broad distribution of the measured currents. Thus, the determination of the single molecule conductance becomes difficult in such cases, and more complex analysis of the data is required. 12 Another important factor controlling the junction behavior is the nature of the contacts, which depends on the choice of the metal electrodes and the terminal functional groups, which connect the molecules to the electrodes. In this way, it is possible to control the strength of the interactions between the molecule and the electrode. The gold-sulfur contact is most commonly used for connecting the molecules to metal surfaces, 5-16 but other combinations are also known. A number of experimental studies has demonstrated that some functional groups such as the pyridyl, amine, selenide, and isocyanide groups can be considered as an alternative for thiol in Au-S binding. 14,17-21 Moreover, other metals, such as Pt, Pd, or Ag, could replace gold electrodes. 22-25 By choosing different metal-linker combinations, tuning the junction proper- ties becomes possible. It was shown that the nature of the metal-molecule contact influences the conductance and shape of the current-voltage characteristics observed for molecular junctions. For example, Kushmerick and co-workers observed that the extent of coupling between chemical linker and metal electrode determines the conductance of the junction and the extent of current rectification. 23,26 In this paper, the conductance measurements of the molecular junctions with symmetric and asymmetric gold-molecule contacts are reported. The experimental approach was based on the molecular junction method, which involves the entrap- ment of R,ω-functionalized molecules between a gold substrate and a gold tip of a scanning tunneling microscope. Several groups have demonstrated the use of this approach for the conductance measurements of alkanedithiols, 14,15 dithiolated DNA, 27-29 peptides, 7,8 viologens, 15,30,31 diamines, 17,18 and di- isonitryles. 17 In this study, the junctions were formed using the self-assembled monolayers of 1,12-dodecanedithiol (DDT), 11- amino-1-undecanethiol (AUT), and a short peptide consisting of a cystamine linker, L-alanine, and L-cysteine with protected amine and thiol moieties (CSA-Ala-(Boc)Cys(Acm)). In the latter case, the molecules were alternatively deprotected using a simple peptide chemistry to obtain free amine or thiol groups available for the adsorption on a gold scanning tunneling microscope tip. In this way, it was possible to introduce the peptide molecules into the junctions using two different binding modes: Au-S or Au-N. It should be noted that the terminal functional groups were chosen so as to provide a well-defined contact between the metal and the molecule. The amino group was chosen as an alternative for the thiol moiety because it can be considered to be a convenient functionality for coupling the molecules to metal leads in molecular junctions incorporating peptides. The effect of the replacement of the thiol group by an amine at one of the two gold-molecule contacts on the efficiency of electron transmission through the junction was considered. Experimental Procedures The starting materials for synthesis were purchased from Sigma, Aldrich, Fluka, and POCh (Gliwice, Poland). AUT was † Corresponding author. E-mail: slasek@chem.uw.edu.pl. 12860 J. Phys. Chem. C 2007, 111, 12860-12865 10.1021/jp073960h CCC: $37.00 © 2007 American Chemical Society Published on Web 08/03/2007