Stopped-Flow FT-IR Spectroscopy of Aqueous Solutions Using Attenuated Total Reflectance

By combining attenuated total reflection (ATR) Fourier transform infrared (FTIR) difference spectroscopy with perfusion of chemical compounds on membrane proteins deposited on the surface of an internal reflection prism, we demonstrate that redox transitions in various bioenergetic systems can be probed. After partial drying of reaction centers (RCs) from Rb. sphaeroides on the ATR prism, the film was rehydrated with buffer. The IR beam penetrates the ATR prism and the evanescent wave propagates through the sample film which is enclosed in a flow cell. The rRedox state changess in the RCs film were manipulated by changing the chemical composition (ferricyanide, ferrocyanide or dithionite) of the flowing buffer. Illumination of the same film was also performed to generate the P+QA- charge-separated state. Chemically-induced ATR/FTIR difference spectra were recorded ofcorresponding to (a) the oxidation of the primary donor (P+/P); and to (b) the reduction of the primary quinone acceptor (QA-/QA); were obtained, as well as ; as well as (c) the light-induced P+QA-/PQA difference spectrum. The calculated QA-/QA spectrum (c-a) agrees with the chemically-induced spectrum (b). ATR/FTIR spectroscopic studies on RCs from Rps. viridis wand PS I will be also presented. More generally, the perfusion-induced ATR/FTIR technique can be widely applied to investigate molecular vibrational changes triggered by redox- and pH-changes, or ligand binding.

Inverse modeling of thin layer flow cells for detection of solubility, transport and reaction coefficients from experimental data

Honey adulteration, especially in sugar and jaggery syrup, has serious health, economic, and quality consequences. Thus, the study investigated the use of ATR-FTIR and chemometric methods to identify honey adulterants. To distinguish pure honey from adulterated samples, we used ATR-FTIR spectral characteristics. ATR FTIR analysis of pure honey reveals C–H bending in 1,2,3-trisubstituted molecules at 775 cm−1, 1,4-disubstituted molecules at 816 cm−1, C–N stretching of an amine at 1026 cm−1, C=C stretching of alkenes at 1642 cm−1, and C–H stretching in alkenes at 2931 cm−1. Sugar syrup and jaggery syrup were also analysed using ATR FTIR. The adulterants’ ATR FTIR spectra were identical due to alene’s C=C at 986 cm−1 and 989 cm−1 and anhydride’s CO-O-CO stretching at 1042 and 1046 cm−1 in sugar and jaggery syrup, respectively. Both adulterants show aromatic esters, as shown by C–O esters at 1262 cm−1 and O-H bends at 1416 cm−1. We used PCA to improve spectral data discrimination and prediction, and it was able to distinguish samples above 95% variance. The study demonstrated that ATR-FTIR spectroscopy and chemometrics can quickly and non-destructively assess sugar or jaggery syrup in honey. This method could protect consumers and ensure honey authenticity.

Tilt angles of liquid crystal molecules, 5CB, have been investigated in the liquid crystal cells with the polyimide (PI) LB alignment films on the aluminum electrodes using the attenuated total reflection (ATR) measurement. The ATR properties were evaluated from the surface plasmon polariton (SPP) and the guided wave excitation modes (GWEM) when various DC voltages were applied to the cell. The tilt angles of the liquid crystal molecules were calculated by fitting the theoretical ATR curves to the experimental ones using a three layers model. Furthermore, the electric field intensities in the cell were calculated at several incident angles.

A dialkylsubstituted imidazolium iodide ionic liquid (1-methyl-3-[3-(trimethoxy-lambda4-silyl)propyl]imida-zolium iodide, MTMSPIm+I-) was prepared with the intention of using it as a quasi-solid-state electrolyte for dye-sensitized photoelectrochemical (DSPEC) cells of Grätzel design, while the analogous electrolyte in a gel state was made by the addition of tetramethoxysilane (TMOS) in the molar ratio MTMSPIm+I-:TMOS = 1:1. The structure of the MTMSPIm+I- in its non-hydrolyzed and hydrolyzed states and in its fully condensed form, obtained after ageing the sols for various times (from a few hours to a few weeks) and heating them at 200 degrees C (fully condensed form), was studied employing time-dependent infrared attenuated total reflection (ATR) and 29Si NMR spectroscopic measurements. The structure of the condensed species was correlated with the viscosity and the specific conductivity measurements of MTMSPIm+I- sols and TMOS/MTMSPIm+I- gels during their ageing. The final product of the condensation of MTMSPIm+I- sols was described as a positively-charged ladder-like polysilsesquioxane with Tn end groups exhibiting a single T3 signal in 29Si NMR spectra and characteristic doublet bands at 1138 and 1049 cm(-1) in IR. This structure was retained to a large extent in TMOS/MTMSPIm+I- gels, confirming their nanocomposite structure. The results of the ATR infrared time-dependent spectroscopic studies showed that in the course of condensation of sols, the refractive index of the modes attributed to the polysilsesquioxane species exhibited strong dispersion, which led to shifts in the vibrational band positions in the experimental ATR spectra. This effect accompanies the sol-to-gel transformations and has not yet been considered as a possible error in analysis of the ATR spectra of sols and gels. The calculation procedure for obtaining the corresponding transmission spectra is briefly outlined and the results applied in this work.

Metasurface-enhanced ATR sensor for aqueous solution in the terahertz range

We present a novel methodology to simultaneously monitor of the degree of cure and curing shrinkage of thermosetting formulations. This methodology is based on the observation of changes in the infrared absorption of reactive functional groups and the groups used as a standard reference for normalization. While the optical path length is exact and controlled in transmission infrared spectroscopy, in attenuated total reflection Fourier transform infrared (ATR FT-IR), the exact determination of volume changes requires the measurement of the refractive indices of the studied system throughout the curing process or at least an indirect parallel measurement of this property. The methodology presented here allows one to achieve quantitative measurements of the degree of cure and shrinkage for thermosets using in situ ATR FT-IR spectroscopy.

This thesis covers several spectroscopic techniques and reports on the development of instrumentation for spectroscopic applications. A discrete frequency mid-infrared spectrometer is developed for the analysis of transient processes at a single wavelength at a time. The spectrometer is based on a commercially available quantum cascade laser and is tuneable over a broad wavenumber range. Different data acquisition approaches are compared and the spectrometer is used to study the cis-trans switching dynamics of stilbene in ethanol.<br/>Subsequently, the spectrometer is reconfigured to study the swelling dynamics of a polymer brush in switching humidities. By sequentially analysing the transient swelling process at different wavelengths, an absorbance spectrum in time is built. The swelling dynamics are compared to a measurement of the same brush using spectroscopic ellipsometry.<br/>A thin-layer electrochemical flow cell capable of working at high pressures is designed and built. The cell has a titanium counter electrode and a boron-doped diamond working electrode. The manufactured cell is characterized to see if it matches design criteria. The cell is used in a EC-HPLC-MS study of a red pigment, identifying its redox products. Chromatograms of several products are presented.<br/>Finally, a flow cell with an integrated visible light waveguide sensor is developed. The waveguide sensor is supplied in chip form and several approaches to bonding the sensor to an electrode are presented. The final assembly of the flow cell from an electrode and a sensor chip works using patterned tape bonding, but has a relatively low success rate of assembly.<br/>

Evaluation of liquid crystal molecules on polyimide LB films using attenuated total reflection measurement

Aggregation of silver colloidal particles in the hydrosol via chemisorption of octadecanethiol (ODT) molecules was investigated based on in situ UV–visible transmission and infrared attenuated total reflection (ATR) measurements. As‐prepared silver hydrosol exhibits a single absorption peak at 395 nm because of the plasma resonance of the metal particles. Upon addition of ODT‐dissolved ethanol to the hydrosol, the resonance band red‐shifts and broadens, the detailed features of which depend on the ODT concentration and elapsed time. Both the red‐shift and the broadening are attributed to particle aggregation in the solution via ODT chemisorption. Aggregation on a germanium prism surface by the addition of ODT‐dissolved ethanol to the hydrosol was investigated using infrared ATR spectroscopy. At moderate ODT concentrations, intense aggregation via ODT chemisorption occurs strongly on the prism surface, allowing observation of ODT absorption bands. Angle‐dependent infrared ATR measurements for the ethanol and ODT molecules at the prism/solution interface clearly show that aggregation on the prism surface proceeds via the exclusion of ethanol. Copyright © 2004 John Wiley &amp; Sons, Ltd.

Development of a new quantitative method for determining low concentrations of aqueous polyatomic anions using attenuated total reflectance (ATR) FTIR spectroscopy is described. Evaporated thin-film coatings of anion-selective tetraalkylated ferrocenium salts were applied to the surface of ATR crystals, which enabled anion detection limits to be lowered up to 23 000-fold below those achieved using the commercially available spectrometer with identical uncoated ATR crystals. Linear calibration curves based on d(absorbance)/dt, which is related to the rate of anion exchange in the thin film, were established in the 0.04-30 microM range. Limits of detection (10-min analyses) for perchlorate, chlorate, trifluoromethanesulfonate, perfluoro-n-butanesulfonate, perfluoro-n-octanesulfonate, tetrafluoroborate, hexafluorophosphate, and pinacolylmethylphosphonate in aqueous solution were 0.03, 0.2, 0.05, 0.07, 0.06, 0.06, 0.6, and 0.7 microM, respectively, using the thin-film coatings. This simple detection/quantification method afforded good reproducibility with relatively fast detection times.

Boosting Electrocatalytic CO ₂ Reduction with Conjugated Bimetallic Co/Zn Polyphthalocyanine Frameworks

A combination of solid phase micro‐extraction (SPME) with attenuated total reflection (ATR) infrared spectrometry provides a fast and sensitive way to detect organic compounds in aqueous solutions. It is especially useful for detection of chlorinated organic compounds in environmental samples. Currently, analyses of organic compounds in aqueous solutions are limited to low‐polarity compounds by the SPME/ATR‐IR sensing method. This limitation was mainly caused by the low polarity nature of the SPME phase. To increase the capability of this method to detect more polar compounds and also to increase the sensitivity in detection of organic compounds, the principle of “like‐dissolve‐like” was used to design a specific SPME phase for a certain class of chlorinated compounds. To demonstrate this concept, chlorinated phenols were used as probe molecules and polyvinyl chloride was chemically modified with phenol, ‐naphthol and ‐naphthol to provide SPME phases with a similar chemical structure to chlorinated phenols. These polymers were used as SPME phases and their performance were compared with the commonly used SPME phases (i.e., polystyrene and polyisobutylene). Results indicated that naphthols attached to PVCs provided much lower compactness, which allows fast speed in absorption of phenols. Meanwhile, due to the structural similarity between naphthols attached to PVCs and phenols, much higher partition coefficients were found for these chemically modified PVCs than conventionally used polymers. To further increase the sensitivity for analysis of chlorinated phenols, the common influencing factors, such as pH values and salt effect were also investigated. Apparently, pH values of the solutions did not influence the structure of the modified PVCs significantly. In absorption of chlorinated phenols in aqueous solutions with different pH values, the observed IR signals were decreased greatly in pH higher than 6 due to the charged form of chlorinated phenols that were presented. Results of the salt effect indicated that three times stronger of IR signals can be obtained if 20% (w/vol) of NaCl was added.

A fully automated titration system with infrared detection was developed for investigating interfacial chemistry at high pressures. The apparatus consists of a high-pressure fluid generation and delivery system coupled to a high-pressure cell with infrared optics. A manifold of electronically actuated valves is used to direct pressurized fluids into the cell. Precise reagent additions to the pressurized cell are made with calibrated tubing loops that are filled with reagent and placed in-line with the cell and a syringe pump. The cell's infrared optics facilitate both transmission and attenuated total reflection (ATR) measurements to monitor bulk-fluid composition and solid-surface phenomena such as adsorption, desorption, complexation, dissolution, and precipitation. Switching between the two measurement modes is accomplished with moveable mirrors that direct the light path of a Fourier transform infrared spectrometer into the cell along transmission or ATR light paths. The versatility of the high-pressure IR titration system was demonstrated with three case studies. First, we titrated water into supercritical CO2 (scCO2) to generate an infrared calibration curve and determine the solubility of water in CO2 at 50 °C and 90 bar. Next, we characterized the partitioning of water between a montmorillonite clay and scCO2 at 50 °C and 90 bar. Transmission-mode spectra were used to quantify changes in the clay's sorbed water concentration as a function of scCO2 hydration, and ATR measurements provided insights into competitive residency of water and CO2 on the clay surface and in the interlayer. Finally, we demonstrated how time-dependent studies can be conducted with the system by monitoring the carbonation reaction of forsterite (Mg2SiO4) in water-bearing scCO2 at 50 °C and 90 bar. Immediately after water dissolved in the scCO2, a thin film of adsorbed water formed on the mineral surface, and the film thickness increased with time as the forsterite began to dissolve. However, after approximately 2.5 h, the trend reversed, and a carbonate precipitate began to form on the forsterite surface, exposing dramatic chemical changes in the thin-water film. Collectively, these applications illustrate how the high-pressure IR titration system can provide molecular-level information about the interactions between variably wet scCO2 and minerals relevant to underground storage of CO2 (geologic carbon sequestration). The apparatus could also be utilized to study high-pressure interfacial chemistry in other areas such as catalysis, polymerization, food processing, and oil and gas recovery.

The surface exciton polaritons associated with the A, B and C excitons in ZnO are investigated by the attenuated total reflection (ATR) measurements. The concept of the surface admittance is introduced to understand the characteristic nature of the surface polaritons in anisotropic ZnO crystal. The experimental ATR spectra and the dispersion curves for the respective excitons are analyzed taking account of the anisotropic and non-local exciton dielectric functions and of the surface layer effect. The real and virtual surface polariton modes are identified on the B exciton from the experimental dispersion curves for particular geometries. Besides, a pseudo-surface mode is observed as additional ATR dips and is interpreted as the guided-wave mode propagating along an optical wave-guide-like structure, ZnO-surface layer-MgF 2 gap, under specific conditions.