STM Study of Metal-Organic Coordination Networks Based on Amino-Acids
Final Report Abstract
The work done within this project expounds the characterization of the molecular self-assembly of biologically relevant species on noble metal surfaces. More specifically, we will consider the case of the amino acids methionine and tyrosine on the closed-packed noble metal surfaces Ag(lll) and Cu(lll). Like nucleic acids, amino acids are in general an interesting class of molecules as they represent the basic elements of living organisms. Through their inherent self-assembly and molecular-recognition capabilities, they automatically qualify as potential building units for the prospective non-covalent synthesis of functional nanostructures. Among the 20 natural amino acids, methionine and tyrosine have been selected due to the interesting chemical reactivity properties of their side-chains. Methionine, through its sulfur atom, and tyrosine through the phenyl and hydroxyl groups respectively, are assumed to bind in physiological conditions to metal ions, inducing conformational and thus functional changes in specific peptide chains. Hence, their study carries also some fundamental significance concerning their role in biological systems. The biomolecular systems were structurally resolved at the molecular level by means of real-space scanning tunneling microscopy (STM) measurements supported by diffraction data (HAS), whereas the spectroscopic x-ray absorption techniques (XPS and NEXAFS) provided information on the chemical and conformational states of the adsorbed species, respectively. The inherent properties of these biomolecules were used for the 'bottom-up' design of self-assembled nanostructures and to reach exquisite control of matter at the molecular level, in terms of rianoscale morphology, engineering of electronic quantum well states, and steering the alignment of single atoms. On Ag(lll), both homochiral amino acid self-assemblies are driven by zwitterionic hydrogen-bonded molecular dimerization involving the carboxylate and ammonium groups, where the high-symmetry axis of the underlying silver substrate influence the directionality of the biomolecular nanostructures. The long-range linear order of the biomolecular nanostructures mediated by the H-bonded dimerization process suggests a universal trend for the one-dimensional zwitterionic non-covalent synthesis of amino acid superstructures on Ag(lll). Exploiting this molecular self-assembly properties exhibits potentialities for the two-dimensional control and patterning of matter at the nanoscale. If the general amino acid self-assembly phenomenon can be exploited for the design of robust molecular nanoarchitectures on surfaces, the tuning of the amino acid side chain is a key control parameter for the structural determination of the final supramolecular assembly. The regularity, tunability and low-dimensionality of the methionine/Ag(lll) system were successfully exploited as one-dimensional electronic resonators for the engineering of electron wavefunctions, and as nanotemplates for the steering of the one-dimensional self-alignment process of transition metal adatoms. The characteristics of the stable molecule-substrate system are not only determined by the interactions between the adsorbed organic species, but the influence of the substrate plays a key role, as revealed in the methionine/Cu(lll) system. In the latter, the chemical properties of the supporting copper substrate have a catalytic function which mediates thermally induced chemical reactions of the supramolecufar ensemble, implying consequent structural alterations of the adsorbed nanosystem. The results of this work bring up new questions which motivate further studies.
Publications
- Interaction of cerium atoms with surface-anchored porphyrin molecules. J. Phys. Chem. C 112, 3453 - 3455 (2008)
Alexander Weber-Bargioni, Joachim Reichert, Willi Auwärter, Agustin Schiffrin, and Johannes V. Barth
