Surface-enhanced Raman and electron spectroscopic investigations of lead-modified silver surfaces.
Name:
azu_td_9010481_sip1_m.pdf
Size:
6.711Mb
Format:
PDF
Description:
azu_td_9010481_sip1_m.pdf
Publisher
The University of Arizona.Rights
Copyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author.Abstract
Surface enhanced Raman scattering (SERS) is a powerful means for obtaining vibrational data from the metal/electrolyte or metal/gas interfacial environment. However, SERS is only observed for a limited number of metal surfaces under certain experimental conditions. Before this method can become a universal tool, the enhancement mechanism(s) must be understood. The results reported in this dissertation assess both electronic and chemical contributions to the SERS mechanism. The electronic properties of the metal are altered by systematic deposition of Pb or Cu onto a substrate that supports intense SERS, Ag. The chemical nature of the interface is altered with different probe molecules. The effect of Pb deposition on the SERS enhancing ability of Ag electrodes has previously been investigated with strongly adsorbed probe molecules. The behavior of cyanide species in the presence of Pb⁺² is complicated by the necessity of maintaining low solution pH to prevent Pb(OH)₂ precipitation; thus, the predominant solution species is HCN. Although previous reports state that no SERS can be detected from cyanide-containing solutions below pH 6, intense SERS signals can be obtained at pH 2 if sufficiently positive electrode potentials are maintained. The two unresolved SERS bands observed in acidic solutions are attributed to HCN which interacts with the Ag surface in end-on and side-on configurations. The predominant effect of Pb deposition on HCN SERS is HCN displacement. Enhancement due to charge transfer processes is not significant, while electromagnetic effects dictate the residual SERS intensity remaining after the initial HCN displacement. The supporting electrolyte anion affects the rate of change of the potential dependent C≡N stretch in basic CN⁻ media. A correlation between the rate of frequency change and anion charge/radius ratio was observed at potentials near and slightly negative of the Ag potential of zero charge in basic CN⁻ media. These results demonstrate the extraordinary sensitivity of SERS to interfacial conditions. The contributions from chemical and electromagnetic enhancement are further assessed by following excitation wavelength dependence of the SERS intensity of pyridine and Cl⁻ as a function of Cu coverage. Contributions from both are observed, but chemical enhancement is less evident for Cu than for Pb deposition. This is related to the smaller change in work function that occurs as a consequence of Cu versus Pb deposition on Ag surfaces.Type
textDissertation-Reproduction (electronic)
Degree Name
Ph.D.Degree Level
doctoralDegree Program
ChemistryGraduate College
Degree Grantor
University of ArizonaCollections
Related items
Showing items related by title, author, creator and subject.
-
COMPUTER SIMULATION OF SURFACE GROUND MOTIONS INDUCED BY NEAR SURFACE BLASTS.(The University of Arizona., 1982)
-
Reactive ion-surface collisions with terminally labeled Langmuir-Blodgett films and mechanistic investigation of peptide fragmentation by surface-induced dissociation(The University of Arizona., 1999)Two major lines of investigation, both involving collisions of ions with organic thin films on metal, are described in this dissertation. The first topic involves studies of low-energy (e.g., 10-250 eV) ion-surface interactions with Langmuir-Blodgett (L-B) films labeled at the outermost surface carbon atom, either by isotopes (deuterium or 13C) or by fluorine. The L-B films are prepared from the labeled fatty acids. The ion-surface collision results suggest that the outermost surface atoms/groups are the main determinant of energy transfer, electron transfer, and ion-surface reactions for polyatomic projectile ions (e.g., benzene, pyrazine). The results presented in this highly interdisciplinary area could be of interest to ion chemists, surface scientists, molecular physicists, physical chemists and others. In addition, the results promise a novel surface characterization technique using ion-surface interactions in the future. The second research topic involves studies that utilize surface-induced dissociation (SID) for mechanistic investigations of peptide fragmentation. Easily prepared self-assembled monolayers (SAMs) of alkanethiols or fluoroalkanethiols on gold are used in SID of peptides. A long-term goal associated with the work on peptide fragmentation is to provide improved and additional predictive rules of peptide dissociation for the computer-aided interpretation of MS/MS spectra in MS-based high throughput peptide/protein sequencing. For example, MS/MS results of fixed-charge derivatized peptides unequivocally demonstrate that in the absence of an available mobile proton, selective cleavages at the peptide bond immediately C-terminal to an Asp residue (Asp-Xxx), initiated by the Asp side chain acidic hydrogen, dominate the MS/MS spectra. SID on a series of dendrimers was also performed to investigate the effect of different charge states on the ion fragmentation. The dendrimers serves as model compounds that have a number of protonation sites with similar gas-phase basicities compared to peptides that have a more heterogeneous population of basic sites. In contrast to previously reported results for multiply protonated peptides of comparable size and charge states, no dependence of SID characteristic collision energy on the charge state of the dendrimers is observed. This supports the idea that it is a mobile proton available to the amide group moiety, instead of simply an additional positive charge, that promotes the lower energy cleavage of peptide bonds.
