Raman Spectra Of Solutions Research Articles

Raman spectra of oxidized sulfur species in hydrothermal fluids

Raman spectroscopic determination of sulfur species molalities in hydrothermal fluids requires correct assignment and knowledge of the scattering efficiencies of Raman bands. Therefore, we studied the Raman spectra of NaHSO4 and H2SO4 solutions experimentally to 700 °C, and of Na2SO4, NaHSO4, H2SO4, and H2SO3 solutions by ab initio molecular dynamics simulation at 727 °C. The results indicate that the scattering efficiencies of the νs(SO3), νas(SO3), and ν(S–OH) Raman bands of HSO4−(aq) depend on the H+ activity. The asymmetric shape of the νs(SO3) Raman band of HSO4−(aq) becomes more symmetric with increasing temperature, which correlates with decreasing hydrogen bonding in the molecular environment. Proton activity and ion pairing do not have a large effect on the change in the band asymmetry with temperature, and a resonance effect on the band shape is not observed. Therefore, we attribute the asymmetric shape of the νs(SO3) Raman band of HSO4−(aq) mostly to hydrogen bonding of the proton in the H–OSO3− molecule with water in its environment. The AIMD simulations clarify assignments of Raman bands of H2SO40, specifically to νs(SO2) and νas(SO2) at ∼1140 cm−1 and ∼1370 cm−1, to νs(SO4) and νas(SO4) at ∼970 cm−1 and ∼1220 cm−1, and to νs(S–(OH)2) and νas(S–(OH)2) at ∼750 cm−1 and ∼840 cm−1. In addition, the experiments showed that diamond is not inert to H2SO4 at high temperatures as reduction of S(VI) to S(IV) produces SO20 and oxidation of diamond generates CO20.

XGBoost algorithm assisted multi-component quantitative analysis with Raman spectroscopy

To improve prediction performance and reduce artifacts in Raman spectra, we developed an eXtreme Gradient Boosting (XGBoost) preprocessing method to preprocess the Raman spectra of glucose, glycerol and ethanol mixtures. To ensure the robustness and reliability of the XGBoost preprocessing method, an X-LR model (which combined XGBoost preprocessing and a linear regression (LR) model) and a X-MLP model (which combined XGBoost preprocessing and a multilayer perceptron (MLP) model) were developed. These two models were used to quantitatively analyze the concentrations of glucose, glycerol and ethanol in the Raman spectra of mixed solutions. The proportion map of hyperparameters was firstly used to narrow down the search range of hyperparameters in the X-LR and the X-MLP models. Then the correlation coefficients (R2), root mean square of calibration (RMSEC), and root mean square error of prediction (RMSEP) were used to evaluate the models’ performance. Experimental results indicated that the XGBoost preprocessing method achieved higher accuracy and generalization capability, with better performance than those of other preprocessing methods for both LR and MLP models.

Lifting Hofmeister's Curse: Impact of Cations on Diffusion, Hydrogen Bonding, and Clustering of Water.

Water plays a role in the stability, reactivity, and dynamics of the solutes that it contains. The presence of ions alters this capacity by changing the dynamics and structure of water. However, our understanding of how and to what extent this occurs is still incomplete. Here, a study of the low-frequency Raman spectra of aqueous solutions of various cations by using optical Kerr-effect spectroscopy is presented. This technique allows for the measurement of the changes that ions cause in both the diffusive dynamics and the vibrations of the hydrogen-bond structure of water. It is found that when salts are added, some of the water molecules become part of the ion solvation layers, while the rest retain the same diffusional properties as those of pure water. The slowing of the dynamics of the water molecules in the solvation shell of each ion was found to depend on its charge density at infinite dilution conditions and on its position in the Hofmeister series at higher concentrations. It is also observed that all cations weaken the hydrogen-bond structure of the solution and that this weakening depends only on the size of the cation. Finally, evidence is found that ions tend to form amorphous aggregates, even at very dilute concentrations. This work provides a novel approach to water dynamics that can be used to better study the mechanisms of solute nucleation and crystallization, the structural stability of biomolecules, and the dynamic properties of complex solutions, such as water-in-salt electrolytes.

Continuously wavelength tunable, continuous wave laser ideal for UV Raman spectroscopy

AbstractWe utilize a novel, high‐power, tunable, continuous wave (CW) deep UV laser to measure resonance Raman spectra of phenolate solutions with high signal‐to‐noise ratios (SNR). In UV resonance Raman (UVRR), increased coupling of the excitation light with a chromophore can transfer molecules into excited states that cause increased heating and photochemistry. Deep UV lasers have traditionally utilized high peak powers to enable efficient single‐pass nonlinear conversion from visible into near infrared light. Nonlinear phenomena such as the formation of transient radical species, Raman saturation, thermal heating, and dielectric breakdown can introduce extraneous light sources that can complicate the interpretation of the Raman spectrum. Dielectric breakdown can increase the baseline, increase noise, and sometimes saturate the detector, preventing Raman detection. Spontaneous Raman scattering intensities should scale linearly with the excitation light intensity. However, this linear behavior does not always occur with pulsed laser excitation. This occurs because stimulated Raman scattering can cause a superlinear intensity response, or transient absorption can cause sublinear intensity responses. CW laser excitation excites samples with electric fields that are much lower than typical pulsed laser excitation. This eliminates the nonlinear responses. The geometry of our new CW laser enables high gain in the harmonic generation cavities that achieve high harmonic generation efficiencies. Average power in the deep UV is >30 mW for wavelengths as short as 206 nm. In the work here, we demonstrate that CW excitation is ideal for resonance Raman measurements in general to reduce spectral complexity.

Centrohexaindane, a Unique Polyaromatic Hydrocarbon Bearing the Rare Cq (Cq )4 Core: Inelastic Neutron Scattering, Infrared and Raman Spectroscopy.

The structure and vibrational spectroscopy of centrohexaindane, 1, was investigated. This unusual molecule has a quaternary carbon atom that is coordinated to four further such quaternary carbon atoms as its core, each pair of which is bonded to an ortho-phenylene unit. Previous NMR studies have shown that the molecule has tetrahedral (Td ) symmetry in solution. The infrared and Raman spectra of chloroform and deuterochloroform solutions of 1 are completely in agreement with this conclusion, as the only modes that are visible are those allowed for Td symmetry. This is not the case in the solid state: X-ray powder diffraction indicates that the unit cell is triclinic or monoclinic with a volume in excess of 4000 Å3 . The vibrational spectroscopy is consistent with C1 site symmetry and the presence of at least two molecules in the primitive cell. It is likely that the space group is centrosymmetric.

Broadband, high-resolution spatial heterodyne Raman spectroscopy measurement based on a multi-Littrow-angle multi-grating.

This paper proposes a spatial heterodyne Raman spectrometer (SHRS) based on a multi-Littrow-angle multi-grating (MLAMG). Compared with a conventional multi-grating, the MLAMG not only provides higher spectral resolution and a broader spectral range, but is also easier to produce. A verification breadboard system is built using the MLAMG combined with four sub-gratings with a groove density of 300 gr/mm and Littrow angles of 4.6355°, 4.8536°, 5.0820°, and 5.3253°. This MLAMG-SHRS is used to obtain the Raman spectra of inorganic solids and organic solutions for different integration times, laser powers, suspension contents, and containers. The Raman spectra of mixed targets and minerals are also presented. The experiments demonstrate that the MLAMG-SHRS is suitable for broadband measurements at high spectral resolution in a wide range of potential applications.

Activated carbon@silver nanoparticles conjugates as SERS substrate for capturing malathion analyte molecules for SERS detection

AbstractMalathion is one of the commonly used organophosphate pesticides known to attack the central nervous system, posing a risk to humans and other animals upon exposure. The surface enhanced Raman spectroscopy (SERS) has been identified as an indispensable tool for chemical and biomolecular sensing. In this work, the facile fabrication of activated carbon (AC)‐based colloidal SERS active substrate dubbed AC@AgNPs was designed by trapping AgNPs on the surface of AC for detection of varying concentrations of malathion. Apart from the higher concentrations of malathion, the rest of the normal Raman spectra of malathion standard solutions exhibited weak Raman signals. The intensity of peaks for 0.47 mg L−1 were nearly non‐existent which is an indication that the malathion pesticide could only be detected up to 0.95 mg L−1 when using silica wafer. On the contrary, all the SERS spectra of malathion in wheat extracts adsorbed on AC@AgNPs substrate exhibited strong Raman signals. Quantitative analysis of malathion was performed by regression models developed using PLSR built with the raw spectra (no pretreatment), SNV‐PLSR, and SNV‐CARS‐PLSR. The model with the most remarkable performance was established by using SNV‐PLSR with r = 0.9869 and RPD = 4.61. This research shows that the proposed method can rapidly detect malathion residues in wheat, suggesting that it could be adopted for production process monitoring of other related food products to guarantee their safety for human and animal consumption.

Identification of Ion Pairs in Aqueous NaCl and KCl Solutions in Combination with Raman Spectroscopy, Molecular Dynamics, and Quantum Chemical Calculations

This work summarizes a theoretical analysis of the perturbation on Raman spectra in aqueous NaCl and KCl solutions with the aim to detect ion pairs. The experimental Raman spectra, both polarized and depolarized, are perturbed by these ions to a comparable extent or somewhat less by KCl than NaCl. This result appears to be contrary to the molecular dynamics (MD) simulation showing that the isolated and separated ions of KCl should have a larger perturbation than NaCl, as the solvation shell of K+ is larger than that of Na+. The apparent discrepancy signifies the ion pair formation which is more substantial for KCl than NaCl. The MD simulations and quantum chemical calculations revealed that KCl forms ion pairs more than NaCl and that the ion pair formation reduces the perturbation on the Raman spectra more for KCl. The present analysis shows that the perturbed Raman spectra provide a useful sign to evaluate the ion pair formation in aqueous solutions.

Multi-mode enhanced Raman scattering spectroscopy using aggregation-free hybrid metal/metal-oxide nanoparticles with intrinsic oxygen vacancies

Un-aggregated plasmonic nanoparticles with a metal oxide coating display persistent enhanced Raman spectra in solution. Enhancement can be further boosted with UV-irradiation (PIERS) to detect nanomolar concentrations of explosive dinitrotoluene.

A Raman spectroscopic and ab initio investigation of aqueous boron speciation under alkaline hydrothermal conditions: evidence for the structure and thermodynamic stability of the diborate ion.

Raman spectra of aqueous sodium borate solutions, with and without excess NaOH, NaCl, and LiCl, have been obtained from perpendicular and parallel polarization measurements acquired using a custom-built sapphire flow cell over the temperature range 25 to 300 °C at 20 MPa. The solvent-corrected reduced isotropic spectra include a large well-defined band at 865 cm-1 which overlaps with the boric acid B(OH)3 band at 879 cm-1, and becomes increasingly intense at elevated temperatures. This band does not correspond to the spectrum of any other previously reported aqueous polyborate ions, all of which have symmetric stretching bands at frequencies below that of borate, [B(OH)4]-, at 745 cm-1. Based on the classic high-temperature potentiometric titration study by R. E. Mesmer, C. F. Baes and F. H. Sweeton, Acidity measurements at elevated temperatures. VI. Boric acid equilibriums, Inorg. Chem., 1972, 11, 537-543, the new band was postulated to arise from a diborate ion, [B2(OH)7]- or [B2O(OH)5]-. Ab initio density functional theory (DFT), together with chemical modelling studies, suggest that it is most likely [B2(OH)7]-. Thermodynamic formation quotients derived from the peak areas showed variations with ionic strength as well as charge-balance discrepancies, which suggest one or more unidentified minor equilibrium species may also be present. The most likely candidate is the divalent diborate species [B2O2(OH)4]2- which is also predicted to have a band near 865 cm-1 and is postulated to be present as a sodium ion pair. These are the first quantitative Raman spectra ever reported for borate-rich solutions under such conditions and provide the first spectroscopic evidence of a diborate species at PWR reactor coolant temperatures.

Structural evolution of water-in-propylene carbonate mixtures revealed by polarized Raman spectroscopy and molecular dynamics.

The liquid structure of systems wherein water is limited in concentration or through geometry is of great interest in various fields such as biology, materials science, and electrochemistry. Here, we present a combined polarized Raman and molecular dynamics investigation of the structural changes that occur as water is added incrementally to propylene carbonate (PC), a polar, aprotic solvent that is important in lithium-ion batteries. Polarized Raman spectra of PC solutions were collected for water mole fractions 0.003 ≤ χwater ≤ 0.296, which encompasses the solubility range of water in PC. The novel approach taken herein provides additional hydrogen bond and solvation characterization of this system that has not been achievable in previous studies. Analysis of the polarized carbonyl Raman band in conjunction with simulations demonstrated that the bulk structure of the solvent remained unperturbed upon the addition of water. Experimental spectra in the O-H stretching region were decomposed through Gaussian fitting into sub-bands and comparison to studies of dilute HOD in D2O. With the aid of simulations, we identified these different bands as water arrangements having different degrees of hydrogen bonding. The observed water structure within PC indicates that water tends to self-aggregate, forming a hydrogen bond network that is distinctly different from the bulk and dependent on concentration. For example, at moderate concentrations, the most likely aggregate structures are chains of water molecules, each with two hydrogen bonds.

Backscattering and transmission Raman spectroscopy systems in the quantitative analysis concentration of acetone solutions

Raman spectroscopy is a set of techniques based on Raman scattering properties, widely applied to analyze the composition of various substances. The techniques consist of 1) backscattering, which collects Raman signals scattered from the surface of a sample; and 2) transmission, which collects Raman signals transmitted by the surface of a sample in the opposite direction of a light source (which triggers less fluorescence than backscattering). In this paper, we will measure the Raman spectrum of acetone solution by the transmission Raman spectroscopy and the backscattering Raman spectroscopy systems that we set up. Those accessories we use are a 532-nanometer diode laser with 100 milliwatt power as the light source, a focusing lens, an objective lens used for amplifying the Raman signals scattered from the sample, and a long-pass filter used to block light with a wavelength shorter than 532 nanometers. For the experimental samples, there are acetone solutions each prepared at 1M, 3M, 5M, 7M, 9M, 11M, and 13M. In the results of this experiment, we found that the intensity of Raman peaks for each Raman shift of acetone and the molar concentrations of acetone measured by both systems have a linear function in the backscattering Raman system and have a quadratic function in the transmissions Raman system because in backscattering system have noise by fluorescence effect more than in transmission system and it might be the effect of the Beer-Lambert law.

Electrical properties and Raman spectra in HoxMn1-xS solid solution

The conductivity of the HoxMn1-xS (0 < x ≤ 0.3) solid solution in the range of temperatures 80 K – 1000 K were measured. Step like temperature dependence and extremum conductivity at high temperature were found. The temperature dependence of Raman spectra in the frequency interval 100 cm−1 – 900 cm−1 in the 300K – 900K range temperature has been investigated in the of HoxMn1-xS solid solutions. Several oscillation modes have been detected. The intensity and frequency shift of temperature fluctuations are determined. A new Raman mode appearing at high temperature T > 660 K was established. The correlation between temperature dependencies of conductivity and frequencies and intensities of Raman modes were established. Raman spectra were explained in terms of impurities octahedral modes.

A spectroscopic approach to identifying the out-of-plane conformations of Ni(II) meso-tetraphenylporphyrin in solution

Ni(II) meso-tetraphenylporphyrin (NiTPP) is a model molecule whose substituent causes out-of-plane (oop) deformations of the porphyrin macrocycle. Although the conformers of NiTPP in the solution had been tentatively explored, the determination of its specific conformers was controversial. In this work, based on the conformational search through molecular dynamics (MD), three energy lowest-lying conformers of NiTPP were sorted out, namely, the S4 (staggered) conformer, the C2 (eclipsed) conformer, and the C1 (eclipsed-staggered) conformer. According to the high-precision free energy calculation based on density functional theory (DFT), the Raman and 1H-NMR spectra of the three conformers were calculated. The actual spectra of NiTPP in solution resulted from the sum spectra of the mixture conformers, so with the consideration of the corresponding conformation weights, the experimental spectra could be stimulated. Our results showed that only at the low-frequency ∼215 cm−1, did the Raman spectrum in solution differ significantly from that in solid powder. On the other hand, the 1H-NMR spectra of the NiTPP conformers were remarkably different, and none of the single conformer's 1H-NMR range could match the experimental 1H-NMR spectrum. However, with the consideration of the existence of three dominant conformations, the observed 1H-NMR spectrum could be well simulated, confirming the rational mixture of the three dominant conformers existing in the aqueous solution. Therefore, this work has not only resolved the different conformers of NiTPP in solution, but also provided an effective approach to combining DFT, Raman, and 1H-NMR analytical tools for identifying different oop conformations of metalloporphyrins in solution.

Second-Order Raman Scattering in Ferroelectric Ceramic Solid Solutions LiNbxTa1−xO3

In the second-order Raman spectra of ceramic solid solutions, LiNbxTa1−xO3 weak overtone bands of fully symmetric fundamental polar excitations were observed for the first time. The frequencies of the two bands exceeded the value of the overtone frequency corresponding to the fully symmetrical vibration 4A1(z). The possibility of the existence of phonon bound states of the antipolar type in the vibrational spectrum of LiNbxTa1−xO3 ceramics is predicted.

Detection of Glutamate Encapsulated in Liposomes by Optical Trapping Raman Spectroscopy.

The transmission of neuronal information is propagated through synapses by neurotransmitters released from presynapses to postsynapses. Neurotransmitters released from the presynaptic vesicles activate receptors on the postsynaptic membrane. Glutamate acts as a major excitatory neurotransmitter for synaptic vesicles in the central nervous system. Determining the concentration of glutamate in single synaptic vesicles is essential for understanding the mechanisms of neuronal activation by glutamate in normal brain functions as well as in neurological diseases. However, it is difficult to detect and quantitatively measure the concentration of glutamate in single synaptic vesicles owing to their small size, i.e., ∼40 nm. In this study, to quantitatively evaluate the concentrations of the contents in small membrane-bound vesicles, we developed an optical trapping Raman spectroscopic system that analyzes the Raman spectra of small objects captured using optical trapping. Using artificial liposomes encapsulating glutamate that mimic synaptic vesicles, we investigated whether spontaneous Raman scattered light of glutamate can be detected from vesicles trapped at the focus using optical forces. A 575 nm laser beam was used to simultaneously perform the optical trapping of liposomes and the detection of the spontaneous Raman scattered light. The intensity of Raman scattered light that corresponds to lipid bilayers increased with time. This observation suggested that the number of liposomes increased at the focal point. The number of glutamate molecules in the trapped liposomes was estimated from the calibration curve of the Raman spectra of glutamate solutions with known concentration. This method can be used to measure the number of glutamate molecules encapsulated in synaptic vesicles in situ.

Shell-Isolated Nanoparticle-Enhanced Raman Spectroscopy for Probing Riboflavin on Graphene.

Graphene research and technology development requires to reveal adsorption processes and understand how the defects change the physicochemical properties of the graphene-based systems. In this study, shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) and graphene-enhanced Raman spectroscopy (GERS) coupled with density functional theory (DFT) modeling were applied for probing the structure of riboflavin adsorbed on single-layer graphene substrate grown on copper. Intense and detailed vibrational signatures of the adsorbed riboflavin were revealed by SHINERS method. Based on DFT modeling and detected downshift of prominent riboflavin band at 1349 cm−1 comparing with the solution Raman spectrum, π-stacking interaction between the adsorbate and graphene was confirmed. Different spectral patterns from graphene-riboflavin surface were revealed by SHINERS and GERS techniques. Contrary to GERS method, SHINERS spectra revealed not only ring stretching bands but also vibrational features associated with ribityl group of riboflavin and D-band of graphene. Based on DFT modeling it was suggested that activation of D-band took place due to riboflavin induced tilt and distortion of graphene plane. The ability to explore local perturbations by the SHINERS method was highlighted. We demonstrated that SHINERS spectroscopy has a great potential to probe adsorbed molecules at graphene.

Investigation of a Raman scattering spectral model for seawater containing a composite salt solute.

To satisfy the demand for active remote sensing of ocean salinity, this paper proposes a Raman spectra, salinity, and temperature model for seawater. Seawater is a solution containing a composite salt solute, changes in the solute, temperature, and salinity of seawater can affect the intensity of Raman spectra. It is difficult to directly analyze the influence of various factors on the Raman spectra of seawater. Therefore, the Raman spectra of solutions containing a single solute and mixed solutions were detected, and the effect of solutions containing different solutes on the spectra was analyzed. The experimental results revealed the variation in the low- and high-frequency spectral intensities of the Raman spectra with salinity and temperature. The Raman spectra of seawater were modeled as a function of temperature and salinity using the low- and high-frequency area ratios, and the spectra of seawater at different salinities were obtained; the model calculation results are consistent with the experimental results within the entire range of seawater temperature and salinity. Because the Raman spectra were a function of temperature and salinity. To achieve high precision remote sensing of ocean salinity, it is necessary to use Brillouin scattering for remote sensing of ocean temperature.

PH determination of small sample volumes using Raman spectra of azo dyes

A method enabling pH-value determination in the 1.4–13.9 range of several nanoliters of aqueous solutions was developed using Raman spectra of four azo dyes: azorubine, ponceau 4R, allura red and tartrazine. Based on pre-resonance Raman spectra of dye solutions in phosphate buffer and HCl/KOH solutions, partial least squares models allowing pH measurement in the given range with an error of less than 3% were developed and validated. Spectra of several dozen samples with known pH values were recorded for each dye. As the concentration of hydrogen ions increased, changes in the intensity, position and half-width of bands were observed. In all analyzed systems, the most pronounced differences between the spectra were observed at two extreme pH values. The changes occurred mainly within vibrational bands of the azo group and the carbonyl group and were related to their protonation/deprotonation, along with the change in the pH of the solution.The results of cluster analysis and DFT calculations suggest that in a strongly acidic environment, the studied dyes appear in the form of 2- (azorubine, allura)/3- (ponceau 4R, tartrazine) anions with a negative charge located within the SO3− groups and a protonated azo group. The alkaline environment leads to further deprotonation and formation of 3- (azorubine, allura)/4- (ponceau 4R, tartrazine) anions. Principal component analysis confirmed the coexistence of two distinct forms of dye molecules in the studied solutions.

A Quantitative Study of Carmine Aqueous Solution Based on Drop-Coating Deposition Micro-Raman Spectroscopy (DCDR)

Carmine is a kind of colorant which is widely used in food, beverage, medicine, cosmetics and tobacco industry. However, excessive use of carmine may lead to the risks of carcinogenic, teratogenic and mutagenic, which seriously threaten the health and safety of consumers. In this paper, DCDR technology is utilized to develop a quantitative method for the detection of carmine, which requires only a small volume deposition of analyte solution (several μL) on a suitable hydrophobic substrate. The conventional Raman spectrum of carmine aqueous solution and corresponding Raman spectrum using DCDR method were compared, illustrating a much higher sensitivity for DCDR method. The Raman spectra of carmine aqueous solution with different concentrations of 100, 50, 10, 8, 4 and 2μg/mL are acquired from the spots on the “coffee-ring” with DCDR method. Using DCDR method, a good linear relationship has been observed between the intensities of the two characteristic peaks, 1364cm−1 as well as 1572cm−1, and the concentrations of the solution, with the linear correlation coefficient of R2>0.99. The results illustrate that DCDR method has a good potential in the quantitative analysis of colorant like carmine, providing a promising technique for a rapid detection for food additives.