Raman Peak Intensity Research Articles

Structure and thermo-mechanical response of Na2O-TiO2-SiO2 glasses to temperature

In-situ high temperature Raman spectroscopy, Brillouin light scattering (BLS), and dilatometry have been used to study the structure and properties of xNa2O-yTiO2-(1-x-y)SiO2 glasses. Glasses with compositions x=10, 15, 20, and 25, and y=4, 7, and 10 were prepared through a traditional melt-quench method. Addition of Na2O decreases the temperature response of elastic moduli and increases the temperature response of the Poisson's ratio and the coefficient of thermal expansion (CTE). TiO2 increases the magnitude of elastic moduli, but does not change their responses to temperature appreciably. TiO2 has a much smaller effect in changing the CTE and the Poisson's ratio than Na2O. Changes in the position and intensity of Raman peaks with temperature have been discussed. Changes in the temperature responses of mechanical and material properties have been correlated to the underlying structural changes reflected in the temperature responses of Raman peak positions and intensities.

Influence of curvature strain and Van der Waals force on the inter-layer vibration mode of WS2 nanotubes: A confocal micro-Raman spectroscopic study

Transition-metal dichalcogenides (TMDs) nanostructures including nanotubes and monolayers have attracted great interests in materials science, chemistry to condensed matter physics. We present an interesting study of the vibration modes in multi-walled tungsten sulfide (WS2) nanotubes prepared via sulfurizing tungsten oxide (WO3) nanowires which are investigated by confocal micro-Raman spectroscopy. The inter-layer vibration mode of WS2 nanotubes, A1g, is found to be sensitive to the diameter and curvature strain, while the in-plane vibration mode, E12g, is not. A1g mode frequency shows a redshift by 2.5 cm−1 for the multi-layered nanotubes with small outer-diameters, which is an outcome of the competition between the Van der Waals force stiffening and the curvature strain softening. We also show that the Raman peak intensity ratio is significantly different between the 1–2 wall layered nanotubes and monolayer flat sheets.

Environmental Instability and Degradation of Single- and Few-Layer WTe2 Nanosheets in Ambient Conditions.

The ambient environmental instability and degradation mechanism of single- and few-layer WTe2 are investigated. Oxidation of W and Te atoms appears to be a main reason for degradation. Single-layer samples' Raman signals disappear within 20 min in air. Few-layer WTe2 exhibits saturating degradation behavior: only the top layer WTe2 is oxidized; the degraded layer can protect inner layers from further degradation.

Low-Temperature Phase Transformation Accompanied with Charge-Transfer Reaction of Polyiodide Ions Encapsulated in Single-Walled Carbon Nanotubes

In order to understand the reason why polyiodide ions encapsulated in single-walled carbon nanotubes (SWCNTs) drastically improve the electric conductivity and the water dispersibility of SWCNTs at low temperature, we performed in situ Raman measurements of polyiodide ions encapsulated in three kinds of SWCNTs having different mean tube diameters under low temperature down to −100 °C. It was found that, for all the three samples, the Raman peak intensities of polyiodide ions increase and the G-band peak position of SWCNTs shifts toward the higher-wavenumber side with decreasing temperature. It means that the charge-transfer from SWCNTs to encapsulated iodine molecules increases with decreasing temperature and that hole-doping level of SWCNTs increases at low temperature. Furthermore, the peak profiles changed with temperature drastically for polyiodide ions encapsulated in the SWCNT sample having the largest mean tube diameter of 2.5 nm. This change indicates the structural transformation of polyiodide ions in SWCNTs. These experimental results can be explained by the promotion of the chain-like polyiodide ion formation at low temperature. It was discussed with the control experiment using amylose that the promotion of the polyiodide ion formation at low temperature is characteristic for iodine molecules encapsulated in SWCNTs.

Quantitative analysis of thyroid-stimulating hormone (TSH) using SERS-based lateral flow immunoassay

We report the development of a surface-enhanced Raman scattering (SERS)-based lateral flow immunoassay (LFIA) sensor for the highly sensitive detection of thyroid-stimulating hormone (TSH) in clinical fluids. Here, Raman reporter-labeled gold nanoparticles (AuNPs) were used as SERS nano tags. Using this SERS-based LFIA test strip, the presence of TSH could be identified through a color change in the test line. In addition, quantitative evaluation of TSH was possible by monitoring the characteristic Raman peak intensity of SERS nano tags. The limit of detection (LOD) was estimated to be 0.025μIU/mL, which is approximately two orders of magnitude more sensitive than that achieved from the conventional colorimetric LFIA sensor based on the naked eye. Consequently, it is possible to diagnose hyperthyroidism as well as hypothyroidism since a TSH concentration lower than 0.5μIU/mL can be detected using this SERS-based LFIA detection technique. Therefore, this approach has strong potential feasibility for application in the early diagnosis of TSH hormone-related diseases.

Using Visible Laser-Based Raman Spectroscopy to Identify the Surface Polarity of Silicon Carbide

In this study we developed an approach to identify the surface polarity of silicon carbide (SiC) by using an excitation laser possessing a photon energy (2.33 eV) much lower than the band gap of 4H-SiC (3.30 eV). By gradually attenuating the intensity of the excitation laser, the effective depth that the laser could generate Raman signals could eventually be limited to within the ultrashallow region of the SiC wafer. Through three-dimensional finite-difference-time-domain (3D-FDTD) simulations, we found that the depth of the high electric field region could be limited from several micrometers below the surface to the near-surface region of 4H-SiC, merely by attenuating the power of the incident laser. Experimentally, we observed a clear trend in the Raman peak intensity ratio of the signals at 210 and 203 cm–1 in the FTA mode: the intensity ratio of the Si face was always higher than that of the C face regardless of the measurement position on the 4H-SiC wafer. Even through the carrier concentrations in the 4H-SiC wafer were nonuniform, the resulting variability in peak intensity did not influence the trend in the intensity ratio, which could, therefore, be used to identify the surface polarity. This approach might also allow characterization of different polytypes of SiC, for example, 6H-SiC and 3C-SiC, the optical band gaps of which are lower than that of 4H-SiC. Because this optical approach using low-photon-energy laser-based Raman spectroscopy is nondestructive, simple, and rapid and employs excitation light of low photon energy, it should be very applicable for characterizing the surface properties of various other crystalline materials.

Characterization of Aqueous Oleic Acid/Oleate Dispersions by Fluorescent Probes and Raman Spectroscopy.

Oleic acid (OA) and oleates form self-assembled structures dispersible in aqueous media. Herein, the physicochemical properties of OA/oleate assemblies were characterized using fluorescent probes and Raman spectroscopy, under relatively high dilution (<100 mM of total amphiphile) at 25 °C. Anisotropy analysis using 1,6-diphenyl-1,3,5-hexatriene showed that the microviscosity of the OA/oleate assembly was highest at pH 7.5 (the pH range of 6.9-10.6 was investigated). The fluorescence spectra of 6-lauroyl-2-dimethylaminonaphthalene revealed the dehydrated environments on membrane surfaces at pH < 7.7. The pH-dependent Raman peak intensity ratios, chain torsion (S = I1124/I1096) and chain packing (R = I2850/I2930), showed local maxima, indicating the occurrence of metastable phases, such as dispersed cubic phase (pH = 7.5), vesicle (pH = 8.5), and dispersed cylindrical micelle (pH = 9.7). These results suggest that large-scale OA/oleate assemblies could possess particular membrane properties in a narrow pH region, e.g., at pH 7.5, and 9.7.

Characterization of cell wall structure in dilute acid-pretreated biomass by confocal Raman microscopy and enzymatic hydrolysis

The chemical and ultrastructural properties of cell walls were investigated to determine the effect of dilute acid pretreatment on the hydrolysis of lignocellulosic biomass. Confocal Raman microscopy was used to gain a clear understanding of how dilute acid pretreatments destroy lignocellulosic cell walls. Total fermentable sugar (glucose and xylose) was high in oxalic acid hydrolysate (26.18 g/L) compared to that in sulfuric acid hydrolysate (24.34 g/L). Chemical composition of the pretreated biomass differed slightly according to the acid catalyst used. Oxalic acid pretreatment was effective for enzymatic hydrolysis, with 29.46 g/L of total fermentable sugar after 96 h. Optical microscopy showed that dilute acid pretreatment significantly changed cell wall structure, and broken and crushed cell walls could be clearly seen during pretreatment. Based on confocal Raman peak intensity, the ratio of lignin/cellulose [I(1600)/I(900)] was low for oxalic acid-pretreated biomass compared to sulfuric acid-pretreated biomass.

Laser Power Threshold of Chemical Transformation on Highly Uniform Plasmonic and Catalytic Nanosurface

Noble metallic nanosurface exhibits both plasmonic and catalytic functions. Surface-enhanced Raman scattering of para-aminothiophenol (p-ATP) was measured on a highly uniform two-dimensional silver nanoparticle array at different intensities of an excitation laser (532 nm) ranging from 4 to 4000 W/mm2. It was observed that p-ATP was chemically transformed to 4,4′-dimercaptoazobenzene (DMAB) with the laser intensities of ≥40 W/mm2 during the Raman measurement. At 4 W/mm2, the Raman peaks of DMAB disappeared, which indicates that the laser intensity was insufficient for the chemical transformation although it was sufficient for the Raman measurement. The highly uniform silver nanoparticle array allowed quantitative analysis on the Raman peak intensity. The threshold of the chemical transformation from p-ATP to DMAB was estimated to be ∼18.8 W/mm2 on the silver nanoparticle array whose enhancement factor is ∼104.

Characterization of chemical doping of graphene by in-situ Raman spectroscopy

We explored single-layer graphene and graphene field-effect transistors immersed in nitric acid using in-situ Raman spectroscopy. Two distinct stages were observed in the chemical doping process. The first stage involved blue shifts of the G and 2D peaks, whose saturation occurred rapidly with a time constant in the range of 10–25 s depending on the molar concentration of the acid. In the second stage, the intensity of the D peak, which was associated with structural defect formation, increased for a relatively long period of time. Since the major doping effects appeared during the first stage, the optimal doping conditions under which no noticeable structural defect formation occurred can be determined by monitoring the frequency shift. Transient doping concentrations along with structural defect densities were obtained from the Raman peak positions and intensities. We found that the doping-induced shift in the Dirac point in graphene field-effect transistors exhibited a fast response with respect to frequency shifts in the Raman spectra, which was attributed to the saturation of electrostatic gating effects.

Intensity Ratio of Resonant Raman Modes for (n,m) Enriched Semiconducting Carbon Nanotubes.

Relative intensities of resonant Raman spectral features, specifically the radial breathing mode (RBM) and G modes, of 11, chirality-enriched, single-wall carbon nanotube (SWCNT) species were established under second-order optical transition excitation. The results demonstrate an under-recognized complexity in the evaluation of Raman spectra for the assignment of (n,m) population distributions. Strong chiral angle and mod dependencies affect the intensity ratio of the RBM to G modes and can result in misleading interpretations. Furthermore, we report five additional (n,m) values for the chirality-dependent G(+) and G(-) Raman peak positions and intensity ratios; thereby extending the available data to cover more of the smaller diameter regime by including the (5,4) second-order, resonance Raman spectra. Together, the Raman spectral library is demonstrated to be sufficient for decoupling G peaks from multiple species via a spectral fitting process, and enables fundamental characterization even in mixed chiral population samples.

Intensity Ratio of Resonant Raman Modes for (n,m) Enriched Semiconducting Carbon Nanotubes

Relative intensities of resonant Raman spectral features, specifically the radial breathing mode (RBM) and G modes, of eleven, chirality-enriched, single-wall carbon nanotube (SWCNT) species were established under second-order optical transition excitation. The results demonstrate an under-recognized complexity in the evaluation of Raman spectra for the assignment of (n,m) population distributions in agreement with theory predicted by a symmetry-adapted nonorthogonal tight-binding model.[1] Strong chiral angle and mod dependencies affect the intensity ratio of the RBM to G modes. Furthermore, we report five additional values for chirality dependent G+ and G- Raman peak positions and intensity ratios extending the available data to cover more of the smaller diameter regime by including the first (5,4) second-order, resonance Raman spectra.[2] Together, the Raman spectral library is demonstrated to be sufficient for decoupling multiple species viaa spectral fitting process, and enables fundamental characterization even in mixed chiral population samples.

Pulsed cathodoluminescence and Raman spectra of MoS2 and WS2 nanocrystals and their combination MoS2/WS2 produced by self-propagating high-temperature synthesis

Molybdenum and tungsten disulfide nanoplates were produced by self-propagating high-temperature synthesis in argon atmosphere. This method provides an easy way to produce MoS2 and WS2 from nanoplates up to single- and several layers. The Raman peak intensities corresponding to in-plane E12g and out-of-plane A1g vibration modes and their shifts strongly depend on the thicknesses of the MoS2 and WS2 platelets indicating size-dependent scaling laws and properties. An electron beam irradiation of MoS2 and WS2 powders leads to an occurrence of pulsed cathodoluminescence (PCL) spectra at 575 nm (2.15 eV) and 550 nm (2.25 eV) characteristic to their intrinsic band gaps. For the combination of MoS2 and WS2 nanopowders, a PCL shoulder at 430 nm (2.88 eV) was observed, which is explained by the radiative electron-hole recombination at the MoS2/WS2 grain boundaries. The luminescence decay kinetics of the MoS2/WS2 nanoplates appears to be slower than for individual MoS2 and WS2 platelets due to a spatial separation of electrons and holes at MoS2/WS2 junction resulting in extension of recombination time.

Measuring mutual solubility in the H2O–CO2 system up to 200 bar and 100 °C by in situ Raman spectroscopy

The solubility control in the H2O–CO2 system under high pressure is of prime interest in numerous geochemical systems from hydrothermal fluids to CO2 geological storage. However, the number of experimental data is scarce in the range of interest of geological storage, especially in the CO2-rich phase. A new experimental device was built to measure mutual solubility in the CO2–H2O system without sampling by coupling a batch reactor with Raman immersion probes. The system was first calibrated by measuring the solubility of CO2 in water at 100°C. The results were provided with an accuracy of a few % between 40bar and 200bar and in agreement with other published experimental data sets and models. The linear correlation between Raman peak intensity and CO2 solubility in the aqueous phase was then used to provide new experimental data of CO2 solubility in water at 65°C from 3bar to 200bar. The Raman data of the CO2-rich phase or supercritical phase are compared to a thermodynamic models and the few experimental data available in literature to provide a new data set of H2O–CO2 mutual solubility at 100°C and up to 200bar.

Tip-Enhanced Raman Scattering Reveals Heterogeneity of Secondary Structures in Amyloid Fibrils Formed by Peptide CGNNQQNY

Tip-enhanced Raman Scattering (TERS) is an advanced technique allowing to measure Raman signals with high spatial resolution. By utilizing SERS effect in a probe coupled with Scanning Probe Microscopy the resolution reaches only a few nanometers breaking diffraction limit of optical microscopy. We have used TERS to study conformational features of amyloid aggregates formed by a peptide (CGNNQQNY) from the yeast prion protein Sup35. TERS results allowed us to identify a set of conformations that composes surfaces of morphologically different amyloid fibrils. Analysis of characteristic Raman bands in the amide III region showed that fibrils formed at pH 5.6 are composed of a mixture of peptide conformations (β-sheets, unordered and α-helix) while fibrils formed at pH 2 have primarily β-sheets. Additionally, by correlating the Raman peak positions corresponding to β-sheet structure in the amide III region with Ψ dihedral angle obtained from Molecular Dynamics simulations of 16-mer fibrils, we proposed that peptides have parallel arrangement of β-sheets for pH 2 fibrils and antiparallel arrangement for fibrils formed at pH 5.6. We developed a methodology for detailed analysis of the peptide secondary structure that enabled us to correlate intensity changes in TERS spectra. Extensive cross correlation analysis of Raman peak intensities corresponding to α-helix, β-sheet and unordered secondary structures in amide III region and amide I region indicated that both regions provide adequate representation of structural characteristics of amyloid surfaces. Furthermore, 2D correlation with the band corresponding to Cα-H in plane bending suggested high localization of structural properties and large contribution of unordered non-helical secondary structures. Here we also discuss various ways to prepare TERS active probes, their longevity and enhancement capabilities suitable for protein aggregation studies.Acknowledgements: NIH-GM096039, NSF-EPS-1004094 to YLL and 2P20GM103480-07 to AVK.

Detection of CEA in human serum using surface-enhanced Raman spectroscopy coupled with antibody-modified Au and γ-Fe2O3@Au nanoparticles

In this present work, a rapid and simple method to detect carcinoembryonic antigen (CEA) was developed by using surface-enhanced Raman spectroscopy (SERS) coupled with antibody-modified Au and γ-Fe2O3@Au nanoparticles. First, Au@Raman reporter and γ-Fe2O3@Au were prepared, and then modified with CEA antibody. When CEA was present, the immuno-Au@Raman reporter and immuno-γ-Fe2O3@Au formed a complex through antibody-antigen-antibody interaction. The selective and sensitive detection of CEA could be achieved by SERS after magnetic separation. Under the optimal conditions, a linear relationship was observed between the Raman peak intensity and the concentration of CEA in the range of 1–50ngmL−1 with an excellent correlation coefficient of 0.9942. The limit of detection based on two times ratio of signal to noise was 0.1ng/mL. The recoveries of CEA standard solution spiked with human serum samples were in the range of 88.5–105.9% with the relative standard deviations less than 17.4%. The method built was applied to the detection of CEA in human serum, and the relative deviations of the analysis results between the present method and electrochemiluminescence immunoassay were all less than 16.6%. The proposed method is practical and has a potential for clinic test of CEA.

Spectroscopic Studies of Chromium VI Formed in the Trivalent Chromium Conversion Coatings on Aluminum

In the present study, the evolution of hexavalent chromium evolution in a trivalent chromium conversion coating has been investigated by Raman spectroscopy and X-ray photoelectron spectroscopy. In terms of Raman spectroscopy, the presence of CrF3 in the coating (538–540 cm−1) generates an overlapped shift with the trivalent chromium hydroxide and oxide (536–550 cm−1). For the hexavalent chromium (845–866 cm−1) reactivity, post-treatment in sodium sulfite solution was revealed to be an effective process to eliminate Cr(VI) species on the coating surface. However, the diffusion kinetics of sulfite ions into the coating inner layer were found to become a limiting factor. The Raman peak intensity of hexavalent chromium in the air aged coating for 24 h revealed a significant decrease compared to that in the freshly-formed coating after the same conversion treatment process. This was attributed to Cr (VI) degradation at the base of coating cracks as evidenced by scanning electron and atomic force micrographs. Interestingly, 3.5 wt% NaCl droplets induced the trivalent chromium oxidation in the air-aged coating specimen, as evidenced by Raman peak at 845 cm−1. For this reason, the oxygen in the corrosive electrolyte caused the oxidation of trivalent chromium species; the electrolyte might play a role in coating degradation, which, in turn, permits an increase of oxygen ingress. The hexavalent chromium in the freshly-formed, air-aged and corroded trivalent chromium conversion coating on aluminum is an environmental concern for its application.

Spectroscopic study of energetic helium-ion irradiation effects on nuclear graphite tiles

Helium ion-irradiation effects on the nuclear graphite tiles were studied in order to understand the structural modifications and damages that can be produced by fusion reaction in tokamaks. The surface morphological changes due to increasing dose of the irradiation were examined by the field-effect scanning electron microscopy, and X-ray photoelectron spectroscopy elucidated the changes in the shallow surface bonding configurations caused by the energetic irradiation. Raman spectroscopy revealed the structural defects and diamond-like carbon sites that increased with increasing irradiation dose, and the average inter-defect distance was found from the Raman peak intensities as a function of the irradiation dose.

A SERS-based lateral flow assay biosensor for highly sensitive detection of HIV-1 DNA

User-friendly lateral flow (LF) strips have been extensively used for point-of-care (POC) self-diagnostics, but they have some limitations in their detection sensitivity and quantitative analysis because they only identify the high cut-off value of a biomarker by utilizing color changes that are detected with the naked eye. To resolve these problems associated with LF strips, we developed a novel surface-enhanced Raman scattering (SERS)-based LF assay for the quantitative analysis of a specific biomarker in the low concentration range. Herein, human immunodeficiency virus type 1 (HIV-1) DNA was chosen as the specific biomarker. Raman reporter-labeled gold nanoparticles (AuNPs) were employed as SERS nano tags for targeting and detecting the HIV-1 DNA marker, as opposed to using bare AuNPs in LF strips. It was possible to quantitatively analyze HIV-1 DNA with high sensitivity by monitoring the characteristic Raman peak intensity of the DNA-conjugated AuNPs. Under optimized conditions, the detection limit of our SERS-based lateral flow assay was 0.24pg/mL, which was at least 1000 times more sensitive compared to colorimetric or fluorescent detection methods. These results demonstrate the potential feasibility of the proposed SERS-based lateral flow assay to quantitatively detect a broad range of genetic diseases with high sensitivity.

Development of dental composites with reactive fillers that promote precipitation of antibacterial-hydroxyapatite layers

The study aim was to develop light-curable, high strength dental composites that would release calcium phosphate and chlorhexidine (CHX) but additionally promote surface hydroxyapatite/CHX co-precipitation in simulated body fluid (SBF). 80 wt.% urethane dimethacrylate based liquid was mixed with glass fillers containing 10 wt.% CHX and 0, 10, 20 or 40 wt.% reactive mono- and tricalcium phosphate (CaP). Surface hydroxyapatite layer thickness/coverage from SEM images, Ca/Si ratio from EDX and hydroxyapatite Raman peak intensities were all proportional to both time in SBF and CaP wt.% in the filler. Hydroxyapatite was, however, difficult to detect by XRD until 4 weeks. XRD peak width and SEM images suggested this was due to the very small size (~10 nm) of the hydroxyapatite crystallites. Precipitate mass at 12 weeks was 22 wt.% of the sample CaP total mass irrespective of CaP wt.% and up to 7 wt.% of the specimen. Early diffusion controlled CHX release, assessed by UV spectrometry, was proportional to CaP and twice as fast in water compared with SBF. After 1 week, CHX continued to diffuse into water but in SBF, became entrapped within the precipitating hydroxyapatite layer. At 12 weeks CHX formed 5 to 15% of the HA layer with 10 to 40 wt.% CaP respectively. Despite linear decline of strength and modulus in 4 weeks from 160 to 101 MPa and 4 to 2.4 GPa, respectively, upon raising CaP content, all values were still within the range expected for commercial composites. The high strength, hydroxyapatite precipitation and surface antibacterial accumulation should reduce tooth restoration failure due to fracture, aid demineralised dentine repair and prevent subsurface carious disease respectively.