Peter M. Fredericks

Surface-enhanced Raman spectroscopy of amino acids adsorbed on an electrochemically prepared silver surface

Abstract An electrochemically roughened silver surface, in conjunction with a Raman microprobe spectrometer, was used to obtain SER spectra of a suite of 19 l -amino acids. The spectra obtained were very different from previously reported SER spectra obtained from silver hydrosols of amino acids. Analysis of the spectra showed that adsorption of all amino acids was via the ionised carboxylate group, and that the side chain of most of the molecules was also in close proximity to the surface. The spectra also indicated that, in contrast, the amine terminus was protonated and relatively far from the surface. Sulfur-containing amino acids also interacted with the silver surface through their sulfur atoms.

SURFACE-ENHANCED RAMAN SPECTROSCOPY OF PEPTIDES AND PROTEINS ADSORBED ON AN ELECTROCHEMICALLY PREPARED SILVER SURFACE

Abstract Surface-enhanced Raman (SER) spectra of a range of small peptides and proteins were obtained from an electrochemically roughened silver surface using a Raman microprobe spectrometer, and from analysis of these spectra the orientations of these molecules on the silver were elucidated. From the spectra it was proposed that all of the di- and tripeptides were adsorbed onto the silver through the carboxyl terminus, and that dominant peaks were due to the side chains of the amino acid residue closest to the carboxyl terminus, regardless of the SER scattering (SERS) activity of individual amino acid residues. SER spectra of the polypeptides and proteins were dominated by amide bands and aromatic side chain vibrations. For most samples, the SER spectrum could be related to the secondary structure of the protein on the silver surface.

Materials Characterization Using Factor Analysis of FT-IR Spectra. Part 1: Results

A method is presented which allows the FT-IR spectrum of a material to be related to many of the significant practical properties of that material. The method uses factor analysis of all or part of the spectra of a calibration set of well-characterized samples to determine the minimum set of factors and the associated factor loadings required to reproduce the spectra. Multiple linear regression of the factor loadings against measured properties gives close correlations which are used to predict the properties of unknown samples similar to those constituting the base set. The method is often fast compared with conventional techniques and, by suitable choice, of calibration sets, can give semiquantitative results for a widely varying set of samples, or quantitative results of accuracy comparable with conventional techniques for a calibration set and unknowns from a narrower range of samples. Examples of the use of the method are given for coal, bauxite, manganese dioxide ore, and diesel fuel.

Materials Characterization Using Factor Analysis of FT-IR Spectra. Part 2: Mathematical and Statistical Considerations

A method is described whereby the wealth of information present in the Fourier transform infrared spectrum of a material may be extracted and used to estimate useful properties of the material. Factor analysis is used to condense the data, after which the use of multiple linear regression allows correlations to be established for a calibration set, from which properties of unknown samples can be estimated.

SERS study of the interaction of thiourea with a copper electrode in sulphuric acid solution

The electrochemical interaction of thiourea with a copper electrode in sulphuric acid solution was investigated using Fourier transform Raman and in situ surface-enhanced Raman scattering (SERS) spectroscopy. SERS spectra of thiourea at a copper electrode were obtained in solutions containing greater than 5 ppm thiourea; the spectra obtained were consistent with adsorption of the molecule on the copper electrode via the sulphur atom. The SERS spectra provide evidence of complex formation involving thiourea and sulphate species at the electrode surface.

Chemical changes during natural oxidation of a high volatile bituminous coal

Abstract Core drilling of near surface seams near Wakefield NSW has provided high volatile bituminous coal samples that have been subject to varying degrees of natural in situ oxidation. Five levels of oxidation have been defined, mainly on visual evidence, ranging from unoxidised coal to highly oxidised material. Samples of each oxidation level have been investigated by chemical analysis, Fourier transform infrared spectroscopy (FTIR) and solid state carbon-13 nuclear magnetic resonance spectroscopy (NMR) with cross-polarization and magic angle spinning. The results show that mineral matter, moisture and oxygen content increase with oxidation, whereas carbon, hydrogen and specific energy decrease. Moisture determination is suggested as a quick, facile method for indicating the oxidation level for a coal sample from this seam. FTIR showed a marked loss of aliphatic C-H groups with increasing oxidation, and an increase in carbonyl and carboxylate groups. Aromatic C-H groups were much less affected by oxidation. Solid state carbon-13 NMR showed increasing carbon aromaticity with oxidation, but showed substantial carbonyl carbon only at the highest oxidation level. No evidence was found for an increase in aromatic substituents during oxidation; if anything, there is a small decrease in substituents.

Simple Technique for Measuring Surface-Enhanced Fourier Transform Raman Spectra of Organic Compounds

A silver surface, suitable for Fourier transform surface-enhanced Raman scattering (FT-SERS), has been prepared by subjecting a polished silver surface to an oxidation-reduction cycle in an electrochemical cell with KCl solution as the electrolyte. The surface produced by this method is shown, by scanning electron microscopy, to be very rough. The preparation of this surface is quite reproducible because the significant parameters of starting surface, electrolyte concentration, and potential can all be well controlled. After removal from the cell followed by washing and drying, organic compounds can be placed on the surface as solutions in aqueous or organic solvents, or by exposure of the surface to vapor. Typically, good FT-SERS spectra are obtained from 0.1–2 μg of sample, after evaporation of the solvent. These experiments were not optimized for minimum detection limits. The technique is very simple, and spectra can be obtained with ease for a wide variety of compounds. Most compounds gave FT-SERS spectra which were similar to their solid Raman spectra; however compounds containing groups expected to interact strongly with the silver surface gave FT-SERS spectra very different from their normal Raman spectra.

Spatially offset Raman spectroscopy (SORS) for the analysis and detection of packaged pharmaceuticals and concealed drugs

Spatially offset Raman spectroscopy (SORS) is a powerful new technique for the non-invasive detection and identification of concealed substances and drugs. Here, we demonstrate the SORS technique in several scenarios that are relevant to customs screening, postal screening, drug detection and forensics applications. The examples include analysis of a multi-layered postal package to identify a concealed substance; identification of an antibiotic capsule inside its plastic blister pack; analysis of an envelope containing a powder; and identification of a drug dissolved in a clear solvent, contained in a non-transparent plastic bottle. As well as providing practical examples of SORS, the results highlight several considerations regarding the use of SORS in the field, including the advantages of different analysis geometries and the ability to tailor instrument parameters and optics to suit different types of packages and samples. We also discuss the features and benefits of SORS in relation to existing Raman techniques, including confocal microscopy, wide area illumination and the conventional backscattered Raman spectroscopy. The results will contribute to the recognition of SORS as a promising method for the rapid, chemically specific analysis and detection of drugs and pharmaceuticals.

Fourier transform Raman spectroscopy of kandite clays

The Raman spectra of the kandite clay minerals, kaolinite, halloysite, dickite and nacrite, have been measured in the 180–3000 cm−1 region using Fourier transform near-IR Raman spectroscopy. These clays have a very small Raman cross-section and long data collection times were often required to obtain good spectra. Each clay has its own unique characteristic Raman spectrum which enables each kandite to be identified easily. In contrast, it is quite difficult to distinguish kandite clays by IR spectroscopy. Nacrite and dickite have relatively intense Raman peaks in the 1000–1100 cm−1 region, whereas kaolinite is characterized by an intense peak at 685 cm−1 and halloysite at 470 cm−1.

FT-Raman spectroscopy of wool—I. Preliminary studies

Abstract The FT-Raman spectrum of wool has been obtained using near-IR excitation. No significant fluorescence was observed and the spectra could be obtained routinely. No sample damage was observed for laser powers up to 400 mW. Several methods of sample presentation for both wet and dry wool were investigated and the optimum data collection conditions were determined for each. Dry dense plugs of wool fibers (∼0.16 g cm −3 ) were found to provide the best spectra. Due to improvements in band resolution and signal-to-noise ratio, several previously unobserved spectral features in the wool spectrum have become apparent. The assignment of the Raman-active vibrational modes of wool are reviewed and updated to include these features. Raman spectra obtained from chlorinated and untreated wool samples did not exhibit any significant differences. In contrast, the ATR spectra obtained from these samples exhibited significant differences in the SO stretching region.