DNA Bases Assembled on the Au(110)/Electrolyte Interface: A Combined Experimental and Theoretical Study

Chemistry of cysteine assembly on Au(100): electrochemistry, in situ STM and molecular modeling

Cysteine (Cys) is an essential amino acid with a carboxylic acid, an amine and a thiol group. We have studied the surface structure and adsorption dynamics of l-cysteine adlayers on Au(100) from aqueous solution using electrochemistry, high-resolution electrochemical scanning tunnelling microscopy (in situ STM), and molecular modelling. Cys adsorption on this low-index Au-surface has been much less studied than Cys adsorption on Au(111)- and Au(110)-electrode surfaces. Chronopotentiometry was employed to monitor the adsorption dynamics at sub-second resolution and showed that adsorption is completed in 30 minutes at Cys concentrations above 100 μM. Two consecutive steps could be fitted to these data. Two separate reductive desorption peaks of Cys adlayers on Au(100) with a total coverage of 2.52 (±0.15) × 10-10 mol cm-2 were observed. In situ STM showed that the adsorbed Cys is organized in stripes with "fork-like" features which co-exist in (11 × 2)-2Cys and (7 × 2)-2Cys lattices, quite differently from Cys adsorption on Au(111)-electrode surfaces. Stripe structures with bright STM contrast in the center suggest that a second Cys adlayer on top of a first adlayer is formed, supporting the dual-peak reductive desorption of Cys adlayers. In addition, monolayers of both pure l-Cys and pure d-Cys and a 1 : 1 racemic mixture of l- and d-Cys on Au(100) were studied. Virtually identical macroscopic electrochemical features were found, but in situ STM discloses many more defects for the racemic mixture than for the pure enantiomers due to structural mismatch of l- and d-Cys. Density functional theory (DFT) calculations combined with a cluster model for the Au(100) surface were carried out to investigate the adsorption energy and geometry of the adsorbed monomer and dimer Cys species in different orientations, with detailed attention to the chirality effects. Optimized DFT geometries were used to construct model STM images, and kinetic Monte Carlo simulations undertaken to illuminate the growth of adsorbate rows and the mechanism of the adlayer formation as well as the Cys adsorption patterns specific to the Au(100)-electrode surface.

Redox Potentials and Electronic States of Iron Porphyrin IX Adsorbed on Single Crystal Gold Electrode Surfaces

Metalloporphyrins are active sites in metalloproteins and synthetic catalysts. They have also been studied extensively by electrochemistry as well as being prominent targets in electrochemical scanning tunneling microscopy (STM). Previous studies of FePPIX adsorbed on graphite and alkylthiol modified Au electrodes showed a pair of reversible Fe(III/II)PPIX peaks at about -0.41 V (vs NHE) at high solution pH. We recently used iron protoporphyrin IX (FePPIX) as an intercalating probe for long-range electrochemical electron transfer through a G-quadruplex oligonucleotide (DNAzyme); this study disclosed two, rather than a single pair of voltammetric peaks with a new and dominating peak, shifted 200 mV positive relative to the ≈-0.4 V peak. Prompted by this unexpected observation, we report here a study of the voltammetry of FePPIX itself on single-crystal Au(111), (100), and (110) and polycrystalline Au electrode surfaces. In all cases the dominating pair of new Fe(III/II)PPIX redox peaks, shifted positively by more than 200 mV compared to those of previous studies appeared. This observation is supported by density functional theory (DFT) which shows that strong dispersion forces in the FePPIX/Au electronic interaction drive the midpoint potential toward positive values. The FePPIX spin states depend on interaction with the Au(111) interface, converting all the Fe(II)/(III)PPIX species into low-spin states. These results support electrochemical evidence for the nature of the electronic coupling between FePPIX and Au-surfaces, and the electronic states of adsorbate molecules, with a bearing also on recent reports of magnetic FePPIX/Au(111) interactions in ultrahigh vacuum (UHV).

Shell-isolated nanoparticle-enhanced Raman spectroscopy study of the adsorption behaviour of DNA bases on Au(111) electrode surfaces

In situ EC-SHINERS is used to investigate the adsorption behaviour of DNA bases on atomically flat Au(111) electrode surfaces.