Double-stranded DNA(poly(dA) 30 ‚poly(dT) 30 ) -modified gold electrodes, prepared by air-drying/adsorption method, have been investigated by various techniques, including cyclic voltammetry (CV), quartz crystal microbalance (QCM), electrochemical scanning tunneling microscopy (EC-STM), and surface-enhanced Raman scattering spectroscopy (SERS). CV and QCM results show that an average surface coverage of (7.5 ( 0.2) × 10 -12 mol cm -2 was obtained for poly(dA) 30 ‚poly(dT) 30 -modified gold electrodes, close to the value for a saturated monolayer of ds-DNA lying flat on surfaces. EC-STM was used to evidence directly that ds-DNA forms a highly ordered and compact monolayer film on the gold substrate, whereas singlestranded DNA(poly(dT) 30 ) adopts a coiled configuration and, therefore, cannot form an ordered structure on the gold substrate. Moreover, it was demonstrated, for the first time, by SERS experiments that partial denaturation of duplexes occurs arising from the different interfacial orientations of A and T bases on the gold electrode surface. The adsorptive nature of the surface-bound ds-DNA was also elucidated, which results in the obtained DNA-modified gold surfaces stable in a wide range of potentials.
Abstract:In situ electromodulated reflectance Fourier transform infrared (FTIR) spectroscopy has been employed to study the adsorption of 4-cyanopyridine (4-CNPy) at an Au(1 I I) electrode surface. The vibrational spectra have been used to study (i) the dependence of the band intensity on the surface coverage, (ii) the character of surface coordination, and (iii) the stability of adsorbed 4-CNPy molecules. It has been observed that the vibrational bands in the spectra acquired in the electroreflection experiment are significantly broader than the corresponding spectra acquired in a transmission cell. Some weaker bands seen in the spectra recorded in transmission were not observed in the electroreflectance experiment. The electroreflectance spectra were dominated by the two ring deformation bands observed at 1416 cm-' and 1554 cm-I. The intensities of these bands correlated well with the surface concentrations of 4-CNPy molecules determined from independent electrochemical studies. The integrated absorption intensities of the bands recorded in the electroreflection experiments were larger by a factor of five than the absorption intensities measured in the transmission cell. This indicates that the electric field of the photon acting on a molecule, present in front of the gold electrode, is significantly enhanced by reflection from the electrode. The infrared experiments suggest that at positive potentials the 4-CNPy molecules are coordinated to the metal surface through the nitrogen atom of the aromatic ring. The 4-CNPy molecules are oxidized at the Au electrode at potentials higher than 0.6 V (SCE) and are reduced to form (4-CNPy)-' ion at potentials lower than -1 .I V (SCE).
The dissociative adsorption of amino acids on Pt and Au electrodes in 0.1 M NaOH solutions was studied by cyclic voltammetry and surface-enhanced Raman spectroscopy (SERS). The intermediate species has been determined as adsorbed cyanide, which is designated by a potential-dependent vibration band around 2110 cm -1 on both Pt and Au surfaces. The dissociation of glycine can be observed on Pt surface in a wide potential region to form cyanide, while the dissociation of serine and threonine occurs at relatively high potentials along with the oxidation of their functional groups. The onset potential of dissociation of amino acids on the Pt surface increases in the order glycine < threonine < serine. It has been revealed that the self-inhibition of amino acid oxidation is originated from the strongly adsorbed cyanide, which is oxidized at potentials above 0.2 V vs SCE. On gold surfaces, cyanide species can be formed only from anodic oxidation of amino acids. The present study reveals characteristic interactions between amino acid molecules and metallic electrode surfaces, as well as the role of amine group in the adsorption configuration.
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