Raman spectroscopic detection for perchlorate at low concentrations

Perchlorate (ClO4-) has recently emerged as a widespread environmental contaminant found in groundwater and surface water, and there is a great need for rapid detection and monitoring of this contaminant. In this study, we explore the use of surface-enhanced (SERS) and normal Raman spectroscopy for detecting ClO4- at low concentrations. We found that ClO4- is SERS active and, for the first time, were able to detect ClO4- at concentrations as low as 10(-6)-10(-7) M (or 10-100 microg/L) through the application of silver SERS substrates or selective sorbents such as bifunctional anion-exchange resins. The use of selective sorbents greatly enhanced the reproducibility and sensitivity of ClO4- detection by normal Raman spectroscopy. Further exploration and research may allow application of these techniques for in situ, real-time detection and monitoring of ClO4- in environmental samples at even lower concentrations.     

Extended-range spectroscopic pH measurement using optimized mixtures of dyes

The spectroscopic technique for pH measurement is a well-established laboratory technique that can give high-accuracy pH values. Recent studies have shown the advantage of this technique over standard potentiometric methods for pH measurements in fresh water and seawater and also at high temperatures and pressures. However, a limitation of the spectroscopic technique is that a single pH dye is sensitive only over a narrow pH range. We have developed optimized dye mixtures that are both sensitive and accurate over a broad pH range. The measurement is robust and simple, requires a minimum of two wavelengths, and is independent of the volume of the dye mixture added. Optimization of the dye mixture formulation to maximize accuracy in a broad range of pH involves varying both the dye type and its mole fraction and also accounting for spectral noise. This technique has been successfully applied for in situ pH measurements of oilfield formation waters.     

Independent components in spectroscopic analysis of complex mixtures

We applied two methods of “blind” spectral decomposition (MILCA and SNICA) to quantitative and qualitative analyses of UV absorption spectra of several non-trivial mixture types. Both methods use the concept of statistical independence and aim at the reconstruction of minimally dependent components from a linear mixture. We examined mixtures of major ecotoxicants (aromatic and polyaromatic hydrocarbons), amino acids and complex mixtures of vitamins in a veterinary drug. Both MICLA and SNICA were able to recover concentrations and individual spectra with minimal errors comparable with instrumental noise. In most cases their performance was similar to or better than that of other chemometric methods such as MCR-ALS, SIMPLISMA, RADICAL, JADE and FastICA. These results suggest that the ICA methods used in this study are suitable for real life applications.     

Development of rapid oral bacteria detection apparatus based on dielectrophoretic impedance measurement method

In this study, a bacteria detection apparatus based on dielectrophoretic impedance measurement (DEPIM) method was demonstrated for rapid evaluation of oral hygiene. The authors integrated a micro electrode chip on which bacteria were captured by dielectrophoresis (DEP), an AC voltage source to induce DEP force, and an impedance measurement circuit to a portable instrument that enables rapid and automated oral bacterial inspection in hospitals and clinics. Special considerations have been made on effects of high electrical conductivity of oral samples on DEP force and DEPIM results. It was shown experimentally and theoretically that using a higher electric field frequency for the DEP bacteria trap and the impedance measurement could realise DEPIM application to bacteria inspection from oral samples with higher conductivity. Based on these investigations, the authors optimised the frequency condition of the DEPIM suitable for inspecting an oral sample along with the design and development of a portable DEPIM apparatus for on-site inspection of oral bacteria. Under the optimised frequency condition, DEPIM results were in good agreement with the conventional culture method showing significant applicability of the DEPIM apparatus for practical rapid oral bacteria inspection.     

Raman spectroscopic investigation of sodium borosilicate glass structure

Raman spectra of sodium borosilicate glasses with a wide range of Na₂O/B₂O₃ ratios were systematically measured. Variations of the spectra with glass composition were studied to interpret the implied distribution of Na⁺ ions between silicate and borate units. When Na₂O/B₂O₃ is less than 1, all Na⁺ ions are associated with borate units as indicated by the absence of the 1100 cm⁻¹ band of Si-O⁻ non-bridging bond stretching. For the (1−x)Na₂O · SiO₂ ·xB₂O₃ glass withx≦0.4 the peak-height ratio of the 950 cm⁻¹ band to the 1080 cm⁻¹ band was used to analyse semiquantitatively the distribution of the Na⁺ ions between silicate and borate units. Sodium ions are divided between silicate and borate units approximately in proportion to the amount of SiO₂ and B₂O₃ present in these glasses. Some of the high sodium content glasses were crystallized and their spectra were compared with the bulk glass spectra. The distribution of Na⁺ ions in the glass was quite different from their distribution after crystallization. Spectra of high silica glasses that had been heat-treated for phase separation indicated exclusion of borate units from the silica network and the formation of borate groups. For high boron content glasses, no change was observed on heat treatment. Raman bands due to borate groups seem to be little affected by their environments. Also affiliated with the Department of Geosciences.     

An FTIR spectroscopic study of hydrogen-bonding competition in entrainer-cosolvent mixtures

In chemical separation processes such as supercritical extraction the use of an entrainer cosolvent can dramatically improve selectivity and yield. Ideally, in an extraction process, an entrainer cosolvent should complex with only the desired solute, pulling it from the feed. But not all cosolvents are entrainers, and a cosolvent that is effective in one application may not be effective in others. Often, competing hydrogen bonding interactions limit the effectiveness of an entrainer cosolvent. In this paper FTIR spectroscopy is used to study hydrogen bonding competition in solute/solvent/entrainer cosolvent mixtures. The extent of hydrogen bonding is determined from analysis of hydrogen-bonded and non-hydrogen-bonded infrared absorption peaks. Since these peaks overlap, curvefitting and Fourier self-deconvolution techniques are used to resolve them. Concentrations of monomeric and hydrogen-bonded species are modeled using the associated perturbed anisotropic chain theory (APACT). Using APACT it is shown that the equilibrium constant, derived from activities, can be written as the product of a temperature-dependent term and the ratio of concentrations: K=(RT) ^vIIC _i ^vi . This gives a statistical mechanical basis for the empirical observation that for hydrogen-bonding equilibria, the ratio of concentrations is approximately equal to the ratio of activities.Paper presented at the Tenth Symposium on Thermophysical Properties, June 20–23, 1988, Gaithersburg, Maryland, U.S.A.     

Diffusion measurements in binary liquid mixtures by Raman spectroscopy

It is shown that Raman spectroscopy allows determination of the molar fractions in mixtures subjected to molecular diffusion. Spectra of three binary systems, benzene/n-hexane, benzene/cyclohexane, and benzene/acetone, were obtained during vertical (exchange) diffusion at several different heights (z) as a function of time. A procedure to determine time-dependent concentration profiles and diffusion coefficients is described in detail for one system, and results are given for the two other cases. For the system benzene/cyclohexane, much lower diffusion coefficients than reported in the literature were found, even in a thermostatically controlled diffusion cell, recording spectra through circulating water. For the system benzene/acetone, the determined diffusion coefficients were in good agreement with the literature data. The limitations of the Raman method are discussed, and it is concluded that many more systems ought to be studied. It is pointed out that diffusion profiles can be obtained in ternary and higher systems, where proper measurements are almost nonexistent.     

Raman scattering spectroscopic study of n-pentane under high pressure

The Raman spectroscopy of n-pentane was investigated in a Moissanite anvil cell from 0.07 GPa to 4.77 GPa at ambient temperature. The result shows that the CH3 symmetric stretching vibration (2877 cm(-1)) and asymmetric stretching vibration (2964 cm(-1)), the CH2 symmetric stretching vibration (2856 cm(-1)) and asymmetric stretching vibration (2935 cm(-1)), and -(CH2)n- in-phase twist mode (1303 cm(-1)) shifted to higher wavenumbers almost linearly with increasing pressure. Around 2.4 GPa an abrupt visible change took place, indicating a sort of phenomenon of freezing due to over-pressurization. In the pressure range of 2.84 to 4.77 GPa a high-pressure phase transition may occur in the crystallized n-pentane. By determining pressure with the method of solid-liquid coexistence, we concluded that the equilibrium freezing pressure of n-pentane is 1.90 +/- 0.05 GPa at 27 degrees C.     

Raman spectroscopic imaging markers for fatigue-related microdamage in bovine bone

Raman spectroscopic markers have been determined for fatigue-related microdamage in bovine bone. Microdamage was induced using a cyclic fatigue loading regime. After loading, the specimens were stained en-bloc with basic fuchsin to facilitate damage visualization and differentiate fatigue-induced damage from cracks generated during subsequent histological sectioning. Bone tissue specimens were examined by light microscopy and hyperspectral near-infrared Raman imaging microscopy. Three regions were defined-tissue with no visible damage, tissue with microcracks, and tissue with diffuse damage. Raman transects, lines of 150-200 Raman spectra, were used for initial tissue surveys. Exploratory factor analysis of the transect Raman spectra has identified spectroscopically distinct chemical microstructures of the bone specimens that correlate with damage. In selected regions of damage, full hyperspectral Raman images were obtained with 1.4-microm spatial resolution. In regions of undamaged tissue, the phosphate nu1 band is found at 957 cm(-1), as expected for the carbonated hydroxyapatic bone mineral. However, in regions of visible microdamage, an additional phosphate nu1 band is observed at 963 cm(-1) and interpreted as a more stoichiometric, less carbonated mineral species. Raman imaging confirms the qualitative relationship between the Raman spectral signature of bone mineral and the type of microdamage in bovine bone. Two tentative explanations for the presence of less carbonated phosphate in damaged regions are proposed.     

Lamellar structure in heat-treated polyethylene. A Raman spectroscopic study

Measurement of the low frequency Raman spectrum of polyethylene affords a method of following the effects of sample deformation on lamellar structure and of the process of annealing on lamellar thickness.     

Surface-enhanced Raman spectroscopy of bacteria and pollen

A technique for distinguishing biological material based on surface-enhanced Raman scattering (SERS) is reported in this work. Of particular interest is biological material that can be airborne. Silver colloidal particles with diameters in the range 10 to 20 nm and with a characteristic ultraviolet-visible (UV-VIS) absorption band at 400 nm were used to obtain SERS spectra of Escherichia coli, Pseudomonas aeruginosa, and Salmonella typhimurium bacteria and a number of tree and grass pollens (Cupressus arizonica (cypress), Sequoia sempervirens (redwood), Populus deltoides (cottonwood), Poa pratensis (Kentucky bluegrass), and Anthoxanthum odoratum (sweet vernal grass)). While differences in the SERS spectra among the bacteria were small, we found that the pollen spectra we analyzed could readily be distinguished from the bacteria spectra, and there were significant differences between pollen from different families. In order to obtain reproducible results, we studied the parameters controlling the interaction between the analyte and the nanoscale metallic surface. Our results show that the volume ratio of analyte to colloidal particles must be within a narrow range of values to optimize the signal-to-noise ratio of the SERS spectra and minimize the fluorescence from the analyte. Also, we found that the time-dependent behavior of colloidal/bacterial suspensions (or adsorption rate of the silver colloid particles on the bacteria) is strongly dependent on pH, density of bacteria in solution, and even, to some extent, the type of bacteria.     

Discrimination of bacteria using surface-enhanced Raman spectroscopy

Raman spectroscopy has recently been shown to be a potentially powerful whole-organism fingerprinting technique and is attracting interest within microbial systematics for the rapid identification of bacteria and fungi. However, while the Raman effect is so weak that only approximately 1 in 10(8) incident photons are Raman scattered (so that collection times are in the order of minutes), it can be greatly enhanced (by some 10(3)-10(6)-fold) if the molecules are attached to, or microscopically close to, a suitably roughened surface, a technique known as surface-enhanced Raman scattering (SERS). In this study, SERS, employing an aggregated silver colloid substrate, was used to analyze a collection of clinical bacterial isolates associated with urinary tract infections. While each spectrum took 10 s to collect, to acquire reproducible data, 50 spectra were collected making the spectral acquisition times per bacterium approximately 8 min. The multivariate statistical techniques of discriminant function analysis (DFA) and hierarchical cluster analysis (HCA) were applied in order to group these organisms based on their spectral fingerprints. The resultant ordination plots and dendrograms showed correct groupings for these organisms, including discrimination to strain level for a sample group of Escherichia coli, which was validated by projection of test spectra into DFA and HCA space. We believe this to be the first report showing bacterial discrimination using SERS.     

Discrimination of bacteria and bacteriophages by Raman spectroscopy and surface-enhanced Raman spectroscopy

Detection of pathogenic organisms in the environment presents several challenges due to the high cost and long times typically required for identification and quantification. Polymerase chain reaction (PCR) based methods are often hindered by the presence of polymerase inhibiting compounds and so direct methods of quantification that do not require enrichment or amplification are being sought. This work presents an analysis of pathogen detection using Raman spectroscopy to identify and quantify microorganisms without drying. Confocal Raman measurements of the bacterium Escherichia coli and of two bacteriophages, MS2 and PRD1, were analyzed for characteristic peaks and to estimate detection limits using traditional Raman and surface-enhanced Raman spectroscopy (SERS). MS2, PRD1, and E. coli produced differentiable Raman spectra with approximate detection limits for PRD1 and E. coli of 10(9) pfu/mL and 10(6) cells/mL, respectively. These high detection concentration limits are partly due to the small sampling volume of the confocal system but translate to quantification of as little as 100 bacteriophages to generate a reliable spectral signal. SERS increased signal intensity 10(3) fold and presented peaks that were visible using 2-second acquisitions; however, peak locations and intensities were variable, as typical with SERS. These results demonstrate that Raman spectroscopy and SERS have potential as a pathogen monitoring platform.     

Defects in individual semiconducting single wall carbon nanotubes: Raman spectroscopic and in situ Raman spectroelectrochemical study

Raman spectroscopy and in situ Raman spectroelectrochemistry have been used to study the influence of defects on the Raman spectra of semiconducting individual single-walled carbon nanotubes (SWCNTs). The defects were created intentionally on part of an originally defect-free individual semiconducting nanotube, which allowed us to analyze how defects influence this particular nanotube. The formation of defects was followed by Raman spectroscopy that showed D band intensity coming from the defective part and no D band intensity coming from the original part of the same nanotube. It is shown that the presence of defects also reduces the intensity of the symmetry-allowed Raman features. Furthermore, the changes to the Raman resonance window upon the introduction of defects are analyzed. It is demonstrated that defects lead to both a broadening of the Raman resonance profile and a decrease in the maximum intensity of the resonance profile. The in situ Raman spectroelectrochemical data show a doping dependence of the Raman features taken from the defective part of the tested SWCNT.     

High pressure Raman spectroscopic study of structural phase transition in samarium oxide

High pressure Raman spectroscopic study of Sm₂O₃ poly crystal was performed up to 21.0 GPa and room temperature using a diamond anvil cell. Pressure induced phase transition was observed at 2.6 GPa in the pressure increasing process. This phase transition corresponds to the monoclinic B type phase → the hexagonal A type transformation. The A type phase was stable up to 21.0 GPa. In the pressure release process, the A type phase was stable above 1.8 GPa, and was completely reverted to the B type phase at 1.1 GPa. The phase transition was confirmed to be reversible with a hysteresis of approximately 1.0 GPa.     

Strain measurement in diacetylene-containing copolymers using Raman spectroscopy

Rigid diacetylene-containing block copolymers are shown to have Raman spectra similar to those of polydiacetylene single crystals. The vibrational frequencies of certain main-chain Raman-active modes of the copolymers are sensitive to deformation which enables strain measurement to be made by following the shift in the Raman band positions. Measurements of the stress concentrations around defects in copolymer specimens during deformation have been carried out using Raman spectroscopy and they have been compared with theoretical analyses of stress concentrations. There is good agreement between the theoretical and experimental measurements and it has been demonstrated that the use of Raman spectroscopy allows the measurement of stress or strain in complex situations for which no theoretical solutions exist.     

Raman spectroscopic studies of the phase transitions in hexane at high pressure

Raman spectroscopic study of n-hexane was carried out in a cubic zirconia anvil cell up to approximately 2.0 GPa. Under high pressure, the C-H stretching region of the spectrum at 2850-3000 cm(-1) shows measurable changes in frequency, bandwidth, and intensity. These Raman bands shift towards higher frequencies with increasing pressure. At about 1.4 GPa, phase transition from liquid to solid was induced by compression, as was simultaneously observed with the built-in microscope.