Role of Phe‐D5 isotopically labeled analogues of bradykinin on elucidation of its adsorption mode on Ag,Au, and Cu electrodes. Surface‐enhanced Raman spectroscopy studies

Raman (RS), surface‐enhanced Raman scattering (SERS), electrochemistry, and isotopic effect methods were used to characterize selective adsorption of two isotopically labeled bradykinin analogues, [(Phe‐D₅)⁵]BK and [(Phe‐D₅)⁸]BK, a hormone which is known to be involved in small‐cell and non‐small‐cell lung carcinoma and prostate cancer. The investigated analogues contain Phe residue, at position 5 or 8 in the amino acid sequence, substituted by Phe‐D₅ (five protons of L‐phenylalanine ring substituted by deuterium). [(Phe‐D₅)⁵]BK and [(Phe‐D₅)⁸]BK were immobilized onto electrochemically roughened Ag, Au, and Cu electrode surfaces under different applied electrode potentials (−1.000 V to 0.200 V) in an aqueous solution containing 0.01 M phosphate buffer (pH = 7.0) and 0.1 M Na₂SO₄. Based on the analyses of the spectral profiles in the 920 – 1050 cm⁻¹ spectral range, specific conclusions were drawn with respect to the Phe⋅⋅⋅metal interactions and changes in the interaction that occurred when the adsorption conditions were varied. Copyright © 2013 John Wiley & Sons, Ltd.     

Characterization of adsorption mode of new B2 bradykinin receptor antagonists onto colloidal Ag substrate

In this paper, the surface‐enhanced Raman scattering (SERS) spectra of the potent B₂ bradykinin receptor antagonists, [D‐Arg⁰,Hyp³,Thi^5,8,L‐Pip⁷]BK, Aaa[D‐Arg⁰,Hyp³,Thi^5,8,L‐Pip⁷]BK, [D‐Arg⁰,Hyp³,Thi⁵,D‐Phe⁷,L‐Pip⁸]BK, and Aaa[D‐Arg⁰,Hyp³,Thi⁵,D‐Phe⁷,L‐Pip⁸]BK, were measured when immobilized onto a colloidal assembly of apparently randomly adhering Ag spheres with diameters of approximately 20 – 25 nm. The observed SERS bands corresponding to different vibrational modes of the molecule, attached to or near Ag, and the variations in these bands resulting from competitive interactions of the functional groups of the peptides with the SERS‐active Ag surfaces were analyzed in this study. Briefly, it was shown that Pip, in generally in vertical orientation, and Thi, in the edge‐on position, relative to the colloidal Ag surface interacted with this surface through their lone electron pairs on the nitrogen and sulfur atoms, respectively. The imide bond of the X‐Pro peptide linkage and the guanidine group of Arg were involved in the adsorption process. In addition, it was demonstrated that the specific differences in the amino acid sequences slightly influenced the mode of adsorption. For example, Aaa in Aaa[D‐Arg⁰,Hyp³,Thi^5,8,L‐Pip⁷]BK and Aaa[D‐Arg⁰,Hyp³,Thi⁵,D‐Phe⁷,L‐Pip⁸]BK and D‐Phe (vertical with respect to the colloidal Ag surface) in [D‐Arg⁰,Hyp³,Thi⁵,D‐Phe⁷,L‐Pip⁸]BK, and Aaa[D‐Arg⁰,Hyp³,Thi⁵,D‐Phe⁷,L‐Pip⁸]BK assisted in the adsorption of these peptides onto the colloidal Ag particles. To discuss these spectral alterations due to the different surface adsorption mechanisms of these peptides, the spectral changes were analyzed according to the adsorption process and Fourier‐transform‐Raman spectra. Copyright © 2013 John Wiley & Sons, Ltd.     

B2 bradykinin receptor antagonists: adsorption mechanism on electrochemically roughened Ag substrate

In this paper, the surface‐enhanced Raman scattering (SERS) spectra of the potent B₂ bradykinin receptor antagonists, [D‐Arg⁰,Hyp³,Thi^5,8,L‐Pip⁷]BK, Aaa[D‐Arg⁰,Hyp³,Thi^5,8,L‐Pip⁷]BK, [D‐Arg⁰,Hyp³,Thi⁵,D‐Phe⁷,L‐Pip⁸]BK, and Aaa[D‐Arg⁰,Hyp³,Thi⁵,D‐Phe⁷,L‐Pip⁸]BK, were measured when immobilized onto a highly specific electrochemically roughened SERS‐active Ag substrate characterized by the formation of a 50 – 150 nm Ag islands on its surface. The observed SERS bands corresponding to different vibrational modes of the molecule, attached to or near Ag, and the variations in these bands resulting from competitive interactions of the functional groups of the peptides with the SERS‐active Ag surfaces and reorientation occurring over time of adsorption were analyzed in this study. Copyright © 2012 John Wiley & Sons, Ltd.     

Energy barriers of single-adatoms diffusion on unreconstructed and reconstructed (110) surfaces

The present paper is aimed mainly to investigate theoretically the diffusion of Ag, Cu, Au and Pt adatoms on the (1 × 1) unreconstructed geometry for Ag, Cu and Pt (110), and reconstructed geometries ((1 × 2), (1 × 3) and (1 × 4)) for Pt and Au (110) surfaces. We consider the single adatom diffusion when additional atoms are deposited in adjacent row. For this study, we have used the molecular statics simulations combined with the embedded atom method. For several systems, we have calculated the activation barriers for hopping mechanism. For the diffusion on the unreconstructed surfaces, the trends for the activation barriers are the same for all considered systems except for Cu/Ag (110) system, where the activation barrier do not change. Further, our results indicate that additional atoms lead to a small decreasing of activation barriers for diffusion on reconstructed surfaces for some systems, while for other systems; the activation barrier remains practically unchanged.     

Growth, alloy formation and reaction with oxygen of Ag and Au submonolayers on W(1 1 0) studied by valence band and core level photoemission

We have studied epitaxial submonolayers of Ag and Au deposited one after another onto W(1 1 0) at room temperature and subsequently annealed at 600 K. Photoemission spectroscopy of valence bands and the Ag3d_5/2 core level has been used to monitor two-dimensional alloy formation. The extent of alloying depends on the order of deposition, composition and annealing. We have also studied the reaction of alloy surfaces to exposure of molecular oxygen at 300 and 600 K.     

Immobilization of galactose oxidase on self‐assembled monolayers of thiols on Au and Ag surfaces

Galactose oxidase (GalOD) was immobilized on self‐assembled monolayers of thiols on silver and gold surfaces using trans‐stilbene (4,4′‐diisothiocyanate)‐2,2′disulphonic acid (DIDS) as the bridging compound. DIDS is the symmetrical bifunctional reagent that reacted with the amine moiety of the thiol and with primary amino groups of enzyme. The Raman measurement revealed that onto cysteamine‐modified silver and gold electrodes, bands corresponding to the galactose oxidase (about 694, 1076, 1274 cm^—1 on Au and 762, 1058, 1274 cm^–1 on Ag ) appeared and clearly demonstrated its immobilization onto Au and Ag surfaces. Simultaneously, we have also observed changes in the ratio of trans–gauche conformers of adsorbed cysteamine molecules. Layers revealing high content of trans conformer are transformed into layers composed mainly of cysteamine molecule in gauche conformation after galactose oxidase adsorption. These observations deliver a strong support for enzyme immobilization on cysteamine‐modified gold and silver surfaces. The surface plasmon resonance experiment gave a surface coverage of ~8.4 × 10⁷ g/cm² for gold electrode modified cysteamine using DIDS chemistry and 1.1 × 10⁷ g/cm² for the cysteamine only modified gold substrate and demonstrated that galactose oxidase layers immobilized with DIDS coupling reagent are quite stable and cannot be easily removed from the surface by treatment with a buffer solution. The surface plasmon resonance results indicated that in this method, a multilayer of galactose oxidase have been immobilized. Our new method of covalent attachment of enzymes seems to be quite promising as a new way of manufacturing biosensors. Copyright © 2012 John Wiley & Sons, Ltd.     

Adsorption of 2‐amino‐5‐cyanopyridine on a gold surface as probed by surface‐enhanced Raman spectroscopy

The adsorption of 2‐amino‐5‐cyanopyridine (2‐ACP) was investigated in solution at different pH values by i n situ surface‐enhanced Raman scattering (SERS) spectroscopy combined with the electrochemical method. The assignments of the main bands were first performed on the basis of the spectral features of similar compounds and with the help of density functional theory calculations. The results revealed that the adsorption and the interfacial structure of 2‐ACP on the Au electrode depended on the applied potential and the pH values of the solution. In the natural solution, 2‐ACP was adsorbed on the surface with a vertical orientation by the CN group from − 0.4 to − 1.0 V, whereas in the − 0.4 to 0.8 V range, the N atom of the pyridine ring was bound to the surface. A transition region for the reorientation of the two adsorption modes was observed from − 0.8 to − 0.4 V. A flat configuration was preferred at an extremely negative potential. A similar surface adsorption behavior was observed in the alkali environment, while the Stark effect slope decreased because of the adsorption of OH⁻. Due to the protonation of N atom in the acidic solution, the potential region for the coexistence of two configurations ranged from − 0.4 to 0.2 V. Additionally, a similar adsorption configuration was proposed on the Au colloids at various pH values. The results revealed that the adsorption behavior became more complex on colloidal surfaces than that on a rigid electrode surface. Copyright © 2010 John Wiley & Sons, Ltd.     

Real‐time analysis of the microstructural evolution and optical properties of Cu(In,Ga)Se2 thin films as a function of Cu content

Thin films of Cu(In,Ga)Se₂ with various copper contents were deposited by co‐evaporation onto thermally oxidized silicon substrates. Characterization by real‐time spectroscopic ellipsometry reveals clear similarities among the samples, as well as key variations with Cu content. Although all films exhibit a Volmer–Weber nucleation and similar fundamental critical point energies in the analysis of optical properties, Cu‐rich films exhibit enhanced coalescence, smoother surfaces, larger grain sizes, as well as a sub‐bandgap absorption which is absent in Cu‐poor films. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Proline and hydroxyproline deposited on silver nanoparticles. A Raman,SERS and theoretical study

The Raman and surface‐enhanced Raman scattering (SERS) spectra of l ‐proline (Pro) and trans‐4‐hydroxy‐ l ‐proline (Hyp) were recorded. SERS spectra were obtained on colloidal Ag prepared by reduction with hydroxylamine. Allowing sufficient time for Pro and Hyp to adjust in the colloidal solution resulted fundamentally in obtaining unique and reproducible SERS spectra. Hyp stabilizes on the surface more rapidly than Pro. The spectral analysis indicates that Pro interacts with the Ag surface through the carboxylate group. The interaction of Hyp with the metal surface occurs through the amino, methylene and carboxylate moieties of the molecule. The spectroscopic results are supported by quantum chemical calculations, performed using extended Hückel theory (EHT) of the title compounds interacting with an Ag cluster model. The assignment of the Raman bands was supported by a normal coordinate analysis performed through Becke, three‐parameter, Lee–Yang–Parr/6‐311 G* + calculations. Copyright © 2011 John Wiley & Sons, Ltd.     

Adsorbed states of substituted α‐aminophosphinic acids on a silver electrode surface: comparison with a colloidal silver substrate

We investigated the interfacial structures of various aromatic (each compound contains one or two phenyls) di‐α‐amino ( L¹ – L³ ) and α‐amino‐α‐hydroxyphosphinic ( L⁴ – L⁶ ) acids immobilized onto an electrochemically roughened silver electrode surface in an aqueous solution using surface‐enhanced Raman scattering (SERS). These structures were compared to those on a colloidal silver surface to determine the relationship between adsorption strength and geometry. The presence of an enhanced ν_19a ring band in the SERS spectra of L⁶ , L² , and L³ on the electrode indicated that the benzene rings of those molecules interact with the electrode surface through localized CC bond(s). We observed significant band broadening of the benzene ring modes for all α‐hydroxyphosphinic acids on both substrates, except for L¹ deposited onto the electrode surface. This suggests the possibility of direct interaction between the ring and Ag, although the benzene ring–surface π overlap is weaker for the colloidal silver than for the Ag electrode. The downward shift in wavenumber and alternations in the enhancement of a ν₁₂ ring band indicate a general increase of tilt angle on both silver substrates in the order L³ < L⁴ < L⁵ < L⁶ . The altered enhancement of the bands due to the vibrations of the  NH₂ and O PO fragments, a finding observed on both silver substrates, strongly suggests that the groups interact with different strength and geometry with these substrates. Copyright © 2009 John Wiley & Sons, Ltd.     

Potential dependent thiocyanate adsorption on gold electrodes: a comparison study between SERS and SHINERS

Potential dependent adsorption of target molecules on electrode surface has long been analyzed by several analytical techniques at the electrochemical interfaces. Here, the adsorption of thiocyanate (SCN⁻) on gold electrodes [Au (111) and Au (poly)] is investigated by electrochemical shell isolated nanoparticle‐enhanced Raman spectroscopy (EC‐SHINERS) and surface‐enhanced Raman spectroscopy. Based on the experimental observation, C − N stretching mode of N‐bound SCN⁻ can be observed around 2080 cm⁻¹ throughout the whole potential range. The band corresponding to ν_C−N of S‐bound SCN⁻ appears as a shoulder at more negative potentials, and as a well‐defined band are more positive potentials. However, the overlapped bands provoke a negative shift in the frequency of S‐bound thiocyanate. Therefore, a change in the calculated Stark slope is observed. Interestingly, SHINERS has been employed to demonstrate the thiocyanate orientation and its effect on Raman spectra. Our results widen the opportunities of SHINERS to unravel the potential‐dependent adsorption behavior of target molecules on single‐crystal electrode surfaces. Copyright © 2016 John Wiley & Sons, Ltd.     

DFT calculations of adenine adsorption on coin metal (1 1 0) surfaces

Density functional theory is used to analyze in detail the adsorption of the adenine molecule on the (1 1 0) surfaces of Cu, Ag, and Au. While the adsorption configurations are similar in all three cases – the molecule bonds via two nitrogen atoms to the substrate – the details like charge transfer or local strain a rather different. The molecule–substrate interaction in case of Cu is stronger than for the more noble metals Ag and Au. Longe-range dispersion forces stabilize the adsorption configuration in dependence on the specific adsorption geometry. In case of Ag and Au, relativistic effects are found to be important.     

Scattering of surface states at step edges in nanostripe arrays

Surface states of noble metal surfaces split into Ag-like and Cu-like subbands in stepped Ag/Cu nanostripe arrays. The latter self-assemble by depositing Ag on vicinal Cu(111). Ag-like states scatter at nude step edges in Ag stripes, leading to umklapp bands, quantum size effects, and peak broadening. By contrast, Ag stripe boundaries become transparent to Cu-like states, which display band dispersion as in flat Cu(111). We find a linear relationship between the quantum size shift and peak broadening that applies in a variety of stepped systems, revealing the complex nature of step barrier potentials.     

Surface characteristics of Ag‐doped Au nanoparticles probed by Raman scattering spectroscopy

We have examined the surface characteristics of Ag‐doped Au nanoparticles (below 5 mol% of Ag) by means of the surface‐enhanced Raman scattering (SERS) of 2,6‐dimethylphenylisocyanide (2,6‐DMPI) and 4‐nitrobenzenethiol (4‐NBT). When Ag was added to Au to form ∼35‐nm‐sized alloy nanoparticles, the surface plasmon resonance band was blue‐shifted linearly from 523 to 517 nm in proportion to the content of Ag up to 5%. In the SERS spectra of 2,6‐DMPI, the N‐C stretching peak also shifted almost linearly from 2184 to 2174 cm⁻¹ when the Ag content was 5 mol% or less; the peak then remained the same as that of the pure Ag film. The potential variation of the SERS spectrum of 2,6‐DMPI in an electrochemical environment, as well as the effect of organic vapor, also showed a similar tendency. From the SERS of 4‐NBT, we confirmed the occurrence of a surface‐induced photoreaction converting 4‐NBT to 4‐aminobenzenethiol, when Ag was added to Au to form alloy nanoparticles. The photoreaction induction ability also increased linearly with the Ag content, reaching a plateau level at 5 mol% of Ag. All these observations suggest that the surface content of Ag should increase almost linearly as a function of the overall mole fraction of Ag and, once the Au/Ag nanoparticles reach 5 mol% of Ag, their surfaces are fully covered with Ag, showing the same surface characteristics of pure Ag nanoparticles. Copyright © 2011 John Wiley & Sons, Ltd.     

Contacting single bundles of carbon nanotubes with alternating electric fields

Single bundles of carbon nanotubes have been selectively deposited from suspensions onto sub-micron electrodes with alternating electric fields. We show that it is possible to control the trapping of a single bundle by the use of Ag as electrode material which, unlike Au, strongly interacts with the carboxyl functionalized carbon nanotubes. Excellent alignment of the bundles between Au or Ag electrodes occurs at frequencies above 1 kHz, with superior contacts being formed with Ag electrodes. Received: 22 May 2002 / Accepted: 21 June 2002 / Published online: 28 October 2002 RID="*" ID="*"Corresponding author. Fax: +49-7247/82-6368, E-mail: ralph.krupke@int.fzk.de     

Electroreflectance spectra of emersed metal electrodes

The reflectance spectra of Au and Ag electrodes were measured in the VIS-UV range after removal from aqueous electrolyte under potentiostatic control. It is shown, that the relative change, $\text{ΔR}\text{R}$, in the reflectance spectrum of the electrode, obtained by emersion at two different electrode potentials, is virtually the same as an in situ electroreflectance spectrum for the corresponding potential modulation. This demonstrates that the surface charge of the metal electrode induced by the electrode potential, is quantitatively retained during and after removal from the electrolyte, and it confirms the observation that the electric double layer of an electrode remains intact during and after emersion. The electroreflectance spectra of removed Au and Ag electrodes are shown and discussed. Emphasis is placed on the implications of these procedures for far UV electroreflectance measurements on metal surfaces utilizing synchrotron radiation.     

Surface-enhanced Raman scattering (SERS) activity of Ag, Au and Cu nanoclusters on TiO2-nanotubes/Ti substrate

Tubular arrays of TiO₂ nanotubes (ranging in diameter from 40 to 110 nm) on a Ti substrate were used as a support for Ag, Au or Cu deposits obtained by the sputter deposition technique, where the amount of metal varied from 0.01 to 0.2 mg/cm². Those composite supports were intended for surface-enhanced Raman scattering (SERS) investigations. Composite samples were studied with the aid of scanning electron microscopy (SEM) and Auger electron spectroscopy (AES) to reveal their characteristic morphological and chemical features. Raman spectra of pyridine (as a probe molecule) were measured at different cathodic potentials ranging from −0.2 down to −1.2 V after the pyridine had been adsorbed on the metal-covered TiO₂ nanotube/Ti substrates. In addition, SERS spectra on a bulk standard activated Ag, Au and Cu substrates were also measured. The SERS activity of the composite samples was strongly dependent on the amount of metal deposit, e.g. at and above 0.06 mg Ag/cm², the intensity of SERS signal was even higher than that for the Ag reference substrate. The high activity of these composites is mainly a result of their specific morphology. The high SERS sensitivity on the surface morphology of the substrate made it possible to monitor very small temporal changes in the Ag metal clusters. This rearrangement was not detectable with microscopic (SEM) or microanalytical (AES) methods. The SERS activity of Au or Cu clusters was distinctly lower than those of Ag. The spectral differences exhibited by the three kinds of composites as compared to the reference metal samples are discussed.     

Thermal desorption study of Oxygen adsorption on Pt, Ag & Au films deposited on YSZ

The Thermal Desorption or Temperature Programmed Desorption (TPD) technique has been used for the study of oxygen adsorption on Pt, Ag and Au catalyst films deposited on YSZ. The catalyst film was deposited on the one side of the YSZ specimen while on the other side gold counter and reference electrodes were deposited, constructing a three-electrode electrochemical cell similar to those used in Electrochemical Promotion studies. Oxygen adsorption has been carried out either by exposing the samples to gaseous oxygen (gas phase adsorption) or by the application of a constant current between the catalyst/working electrode and the counter electrode (electrochemical adsorption) or by mixed gas phase and electrochemical adsorption. Oxygen adsorption was carried out at temperatures between 200 and 480 °C. After exposure to gaseous oxygen, normal adsorbed atomic oxygen species have been observed on Pt and Ag surfaces while there was no detectable amount of adsorbed oxygen on Au. Electrochemical O²⁻ pumping to Pt, Ag and Au catalyst films creates strongly bonded “backspillover” anionic oxygen, along with the more weakly bonded atomic oxygen. Electrochemical O²⁻ pumping to Pt, Ag and Au catalyst films in presence of preadsorbed oxygen creates strongly bonded “backspillover” anionic oxygen, with a concomitant pronounced lowering of the T_p of the more weakly bound preadsorbed atomic oxygen. The two oxygen species co-exist on the surface. The activation energy for oxygen desorption or, equivalently, the binding strength of adsorbed oxygen was found to decrease linearly with increasing catalyst potential, for all three metal electrodes. Paper presented at the 4th Euroconference on Solid State Ionics, Renvyle, Galway, Ireland Sept. 13–19, 1997     

Enhancements in intensity and thermal stability of Raman spectra based on roughened gold substrates modified by underpotential deposition of silver

In this study, electrochemically roughened gold is modified with underpotential deposition (UPD) silver to investigate the effects on enhancements in the intensity and the thermal stability of surface‐enhanced Raman scattering (SERS). The SERS of Rhodamine 6G (R6G) adsorbed on the UPD Ag‐modified Au substrate exhibits a higher intensity by six‐fold of magnitude, as compared with that of R6G adsorbedon the unmodified Au substrate. Moreover, the SERS enhancement capabilities of UPD Ag‐modified Au and unmodified Au substrates are seriously depressed at temperatures higher than 200 and 150 °C, respectively. It indicates that the modification of UPD Ag can significantly depress the thermal destruction of SERS‐active substrates. Copyright © 2009 John Wiley & Sons, Ltd.