ZnSe Internal Reflection Element Research Articles

Insight into Heterogeneous Distribution of Protein Aggregates at the Surface Layer Using Attenuated Total Reflection-Fourier Transform Infrared Spectroscopic Imaging.

Monoclonal antibodies (mAbs) have been used as therapeutics for the last few decades. It is necessary to investigate the stability of these mAbs under stress conditions and to elucidate aggregation mechanisms as a means of developing approaches which minimize the problem. Attenuated total reflection (ATR)-FTIR spectroscopic imaging allows probing of a sample at a depth of penetration of around 0.5-5 μm, which makes it suitable for the study of aggregated proteins when accumulated as a layer close to the surface of the ZnSe internal reflection element (IRE). Here, macro ATR-FTIR spectroscopic imaging, along with a variable angle of incidence accessory, have been used to differentiate between the secondary structure of proteins in bulk solution and those that have precipitated onto or near the ZnSe IRE surface. IgG spectra obtained from protein samples in individual wells have been averaged, extracted, and preprocessed, and the Amide I bands of the protein samples were compared and further analyzed to reveal protein distribution at the ZnSe IRE surface. These findings show depth profiling of IgG aggregates at the ZnSe IRE surface (0.5-5 μm) and do not follow a trend of decreasing protein presence with an increasing angle of incidence or increasing depth of penetration, suggesting an irregular distribution of aggregates in the z-direction.

ATR-FTIR study of Bacillus sp. and Escherichia coli settlements on the bare and Al2O3 coated ZnSe internal reflection element

Marine microorganism accumulated on the surface of ships or pipelines would accelerate fouling organisms, such as mussels and barnacles, adhered on the surface. It was significant to understand the bio-interaction between the microorganisms and the surface. Attenuated total reflection (ATR) Fourier transform infrared (FTIR) spectroscopy was used to study the initial stages of marine microorganism adhering to surfaces, because it could probe the microorganism interaction to the surface regardless of the water interference. Bacillus sp. and Escherichia coli were selected to study the initial attachment on different surfaces, because they were typical fouling microorganisms and showed opposite Gram stain results. The assays were conducted respectively in dried and settled bacteria on two different surfaces (ZnSe, Al2O3 coated on ZnSe). IR spectra of settled bacteria showed amide I band red shift and amide II band blue shift in aqueous environment on both surfaces compared with the dry bacteria. The reasons of amide bands shift were investigated and it was discovered that the hydrogen bond between the water and the protein of the bacteria led to the protein secondary structure change. ATR-FTIR provided an approach to study the attachment process and showed dynamic changing process on the surface, and it could be an appropriate approach to study the interaction between proteins and chemicals.

ATR-FTIR spectrokinetic analysis of the CO adsorption and oxidation at water/platinum interface

Attenuated total reflection infrared (ATR-IR) spectroscopy is a powerful tool to investigate reaction pathways in liquid(reactive)/solid(catalyst) systems. Catalysts are commonly deposited on an internal reflection elements (IRE) as layers of powders or as films (e.g. metal film), and they are exposed to the liquid phase reactants. To obtain quantitative information of intrinsic reaction rates, the chemical engineering aspects of an ATR flow-through cell must be evaluated. Particularly, mass transport in the ATR cell has to be characterized. We present here an analysis of the mass transfer from the flowing solution to the surface of the ATR crystal, where the catalyst is deposited. Criteria to determine kinetic parameters under chemical control were developed on the base of non-dimensional Péclet (Pe) and Sherwood (Sh) numbers, and Thiele modulus (φcl). A Pt thin film deposited on a ZnSe IRE by vapor deposition and a layer of Pt/Al2O3 porous catalyst were used to study the adsorption of carbon monoxide and oxidation of preadsorbed carbon monoxide on aqueous phase. Experimental data of the evolution of the linearly adsorbed CO (2048cm−1) were fitted using a microkinetic model to obtain reaction constants. Results reported here serve as a practical guide to quickly determine the operational limits of an ATR cell with a porous layer of catalyst deposited onto the IRE.

In situ ATR-IR study of prochiral 2-methyl-2-pentenoic acid adsorption on Al2O3 and Pd/Al2O3

Adsorption and desorption of trans-2-methyl-2-pentenoic acid (MPeA) in dichloromethane (CH(2)Cl(2)) were investigated by using in situ attenuated total reflection infrared (ATR-IR) spectroscopy. A liquid flow-through spectroscopic cell allowed for high quality spectra to be obtained from deposited thin films of Al(2)O(3) and 1 wt% Pd/γ-Al(2)O(3) on a ZnSe internal reflection element. The MPeA molecules adsorb on both Al(2)O(3) and Pd surfaces molecularly and dissociatively under the concentration range examined (2-16 mM). In the case of molecular adsorption, both monomer (ν(C=O) ~ 1720 cm(-1)) and dimer (ν(C=O) ~ 1685 cm(-1)) species are observed to adsorb, with the relative amount of monomer to dimer dependent on the surface and the liquid phase acid concentration. In the case of dissociative adsorption, the acid adsorbs predominantly in a bridged bidentate configuration, as adjudged by the ca. 150-220 cm(-1) separation between asymmetric and symmetric vibrational bands. All of these species are found to be strongly adsorbed on both Al(2)O(3) and 1 wt% Pd/γ-Al(2)O(3) surfaces, even under pure solvent flow after adsorption.

Casein Aggregates Built Step-by-Step on Charged Polyelectrolyte Film Surfaces Are Calcium Phosphate-cemented

The possible mechanism of casein aggregation and micelle buildup was studied in a new approach by letting α-casein adsorb from low concentration (0.1 mg·ml(-1)) solutions onto the charged surfaces of polyelectrolyte films. It was found that α-casein could adsorb onto both positively and negatively charged surfaces. However, only when its negative phosphoseryl clusters remained free, i.e. when it adsorbed onto a negative surface, could calcium phosphate (CaP) nanoclusters bind to the casein molecules. Once the CaP clusters were in place, step-by-step building of multilayered casein architectures became possible. The presence of CaP was essential; neither Ca(2+) nor phosphate could alone facilitate casein aggregation. Thus, it seems that CaP is the organizing motive in the casein micelle formation. Atomic force microscopy revealed that even a single adsorbed casein layer was composed of very small (in the range of tens of nanometers) spherical forms. The stiffness of the adsorbed casein layer largely increased in the presence of CaP. On this basis, we can imagine that casein micelles emerge according to the following scheme. The amphipathic casein monomers aggregate into oligomers via hydrophobic interactions even in the absence of CaP. Full scale, CaP-carrying micelles could materialize by interlocking these casein oligomers with CaP nanoclusters. Such a mechanism would not contradict former experimental results and could offer a synthesis between the submicelle and the block copolymer models of casein micelles.

Detection of Spores Using Electric Field-Assisted FTIR-ATR

An approach that integrates an electric field with an attenuated total reflection Fourier transform infrared spectroscopy (FTIR-ATR) flow through cell was used to detect spores in aqueous environments. A "proof of concept" in terms of the principle features of the method is described. It is shown that under an electric field, the negatively charged spores migrate and are concentrated on the surface of a ZnSe internal reflection element (IRE). No coating on the IRE is required, and a maximum amount adsorbed was obtained within the time needed to record the first spectrum. The amount adsorbed depends on both the pH and the ionic strength. Lowering the pH decreases the charge density and reduces the lateral-lateral repulsion force, leading to a higher packing density on the IRE. Reversal of the field does not overcome the strong attraction between the spores and the IRE. However, repeated measurements can be performed as the spores are completely and rapidly removed from the IRE by simply adding the next sample. The intensity of the infrared bands is due to mass loading of the spores on the IRE and setting a minimum value of 1 x 10(-3) absorbance; this requires a total of approximately 4 x 10(6) spores/cm(2). The theoretical detection limit in terms of spore concentration for our cell cavity height of 1.5 mm is approximately 10 ppm or 2.5 x 10(7) spores/cm(3).

Adsorption of sodium dodecyl sulfate (SDS) at ZnSe and α-Fe 2O 3 surfaces: Combining infrared spectroscopy and batch uptake studies

Adsorption of sodium dodecyl sulfate (SDS) at the solid/aqueous interface was examined as a function of pH and SDS concentration ([SDS]) using attenuated total reflectance–Fourier transform infrared (ATR–FTIR) spectroscopy and batch uptake experiments. Two types of sorbent surfaces were compared: (i) a hydrophobic zinc selenide (ZnSe) ATR internal reflection element (IRE) and (ii) the same surface coated with hydrophilic nanoparticulate α-Fe 2O 3 (hematite). The results indicate that adsorption to the ZnSe IRE is affected by both electrostatic attraction and hydrophobic interaction. Batch adsorption and ATR–FTIR spectral results are consistent with SDS forming outer-sphere complexes at the α-Fe 2O 3 surface. There is also no evidence for ligand (SDS)-promoted dissolution of hematite. Adsorption to hematite is dominated by anion exchange and surfactant self-assembly. ATR–FTIR data indicate that adsorption to both surfaces shows a strong pH dependence at low [SDS] and negligible pH dependence when [SDS] exceeds the critical micelle concentration (cmc). Adsorption to ZnSe IRE shows small variation with [SDS], apparently due to a lack of surfactant self-assembly at the interface. Adsorption to α-Fe 2O 3 is a rapid process; equilibrium is reached within a few minutes. Conversely, adsorption to the ZnSe IRE shows strong longer time dependence; evidently, hydrophobic interfacial reactions constitute a much slower process.

A Comparison of Pseudomonas Aeruginosa Biofilm Development on ZnSe and TiO2 Using Attenuated Total Reflection Fourier Transform Infrared Spectroscopy

The growth of Pseudomonas aeruginosa PAO1 biofilms on ZnSe internal reflection elements (IREs) was compared with their growth on TiO(2)-coated ZnSe over several days using attenuated total reflection Fourier transform infrared (ATR-FT-IR) spectroscopy. The effect of the TiO(2) coating on the IR spectra of reference compounds and cell suspensions was determined to aid in the interpretation of the data. The presence of TiO(2) on the surface of a ZnSe IRE tripled the size of the amide II peak and facilitated the detection of pyoverdin production due to its increased adsorption on the coated surface. A 50% increase in the length of the lag phase was observed for PAO1 growth on TiO(2)-coated surfaces as compared to growth on ZnSe. Biofilms on both surfaces exhibited a growth maximum for all components, followed by restructuring at the surface characterized by a decrease in the signal. The composition of biofilms grown on TiO(2) was relatively constant after the restructuring phase, while the extracellular polymeric substance (EPS) component of the biofilms grown on ZnSe gradually increased. The peak due to the carbohydrate component of EPS was much larger in the spectra of biofilms than in those of planktonic cells. The increase of the pyoverdin signal over time in the spectra of the biofilms on TiO(2) closely followed the overall increase in biomass. However, no signal from pyoverdin was detected in the presence of ferric ions.

In-situ monitoring of Cryptosporidium parvum oocyst surface adhesion using ATR-FTIR spectroscopy

Surface chemistry and molecular interaction mechanisms of Cryptosporidium parvum oocysts with a ZnSe internal reflection element (IRE) surface were investigated as a function of pH and ionic strength in NaCl and CaCl 2 background electrolyte using in-situ ATR-FTIR spectroscopy. Since the surface properties of oocysts play an important role in adhesion behavior, the effects of surface modifications that are commonly employed to inactivate the pathogen for laboratory studies, including viable (control), formalin-, and heat-inactivation, were also examined. The ATR-FTIR spectra of oocyst surfaces exhibit amide, carboxylate, phosphate, and polysaccharide functional groups. Results indicate that changes in solution chemistry strongly impact oocyst adhesion behavior in aqueous systems. Increasing ionic strength from 1 to 100 mM or decreasing pH from 9.0 to 3.0 resulted in an increase in oocyst adhesion to the IRE surface as measured by IR absorbance. For equivalent ionic strength, the adhesion rate was found to be independent of CaCl 2 versus NaCl electrolyte solution, but was increased following formalin and heat treatments. This latter effect correlated with molecular changes reflected in spectral data. The ratio of amide I:amide II band intensities increased, and sugar ring vibrations at 1023 cm −1 became sharper and more intense following formalin treatment. Similar changes in the polysaccharide region were observed following heat treatment, and protein secondary structure was also altered from mainly parallel β-sheet to anti-parallel β-sheet conformation.

Mechanistic Investigation of the Heterogeneous Hydrogenation of Nitrite over Pt/Al2O3 by Attenuated Total Reflection Infrared Spectroscopy

The mechanism of the heterogeneous hydrogenation of nitrite over a Pt/Al2O3 catalyst layer deposited on a ZnSe internal reflection element was investigated in water using attenuated total reflection infrared spectroscopy. In addition to adsorbed nitrite, hydrogenation intermediates NO(ads), “HNO”(ads), and HNO2−(ads) are formed on the platinum surface. Hydrogenation of the surface intermediates mainly results in NH4+, but also traces of N2O are observed as well, which is believed to be an intermediate in the formation of nitrogen. “HNO”(ads) is the most prominent surface species during steady state operation and is therefore involved in the rate-determining step. Some NO(ads) accumulates at steps in transient experiments, showing a very low reactivity toward N2O. The results show that although the reaction pathways of nitrite hydrogenation on platinum and palladium are rather similar; the rate-determining steps on the metals are clearly different.

An ATR-FTIR study of sulphate sorption on magnetite; rate of adsorption, surface speciation, and effect of calcium ions

The adsorption of sulphate on magnetite was studied in-situ using ATR-FTIR spectroscopy. Synthetic magnetite particles were deposited on a ZnSe internal reflection element and the spectra of sulphate adsorbed at pH 4–8.5 were recorded. Two different ionic strengths were used viz. 0.01 M and 0.1 M NaCl. The spectra of adsorbed sulphate on magnetite coated ZnSe were compared with the spectra of sulphate solutions at the same pH values and in contact with uncoated ZnSe. The spectrum of adsorbed sulphate at pH 4 showed three maxima at 979, 1044, and 1115 cm −1 indicating a monodentate adsorption in which the T d symmetry of SO 4 2 − is lowered to C 3v. At pH 6.5, sulphate adsorbed as an outer-sphere complex with two weak bands appearing at 1102 and 980 cm −1. Moreover, spectra of the adsorbed sulphate at pH 4 were recorded as a function of time and sulphate concentration. The equilibrium absorbance at different concentrations fitted a Langmuir type adsorption isotherm. The Langmuir affinity constant K at pH 4 was determined from the slope and intercept of the Langmuir plot to be K = 1.2344 × 10 4 M − 1 and the Gibbs free energy of adsorption Δ G ads 0 was estimated from this value to be −33.3 kJ/mol. Kinetic analysis indicated that adsorption at pH 4 is fast, whilst the desorption kinetic at the same pH is very slow. In addition, the effect of Ca ions on sulphate adsorption was also studied. It was shown that Ca ions increased the sulphate adsorption on magnetite at pH 8.5.

Detection of Bacillus Globigii Spores Using a Fourier Transform Infrared–Attenuated Total Reflection Method

The use of an alumina-coated ZnSe internal reflection element (IRE) to detect spores by attenuated total reflection infrared spectroscopy (FTIR-ATR) was investigated. Two methods for coating the IRE with alumina are described. It is shown that the adsorption proceeds through an interaction of the carboxylate groups on Bacillus globigii (BG) and positively charged sites on the alumina. The amount adsorbed is highly dependent on solution pH and passes through a maximum value near pH 5, which is dictated by the charge density on the spores and the charge density on the alumina surface. Furthermore, it is shown that lateral-lateral repulsion between the spores limits the maximum adsorbed amount, giving rise to a detection limit of 10(7) spores per cm2 of the IRE.

Attenuated Total Reflection Fourier Transform Infrared Spectroscopy Analysis of Human Hair Fiber Structure

The structure of human hair was studied by attenuated total reflection Fourier transform infrared (ATR-FT-IR) spectroscopy. The use of Ge, ZnSe, and Si internal reflection elements, various incident light angles, and difference spectra allowed detailed analysis of the cuticle, cortex, and cuticle-cortex intercellular regions without physically or mechanically removing the cuticle of the hair fiber. The ATR-FT-IR data showed the cuticle to be composed of protein having predominantly beta-sheet and disorder and beta-turn configurations. In contrast, the cortex spectra showed alpha-helical structures due to the presence of intermediate filaments of alpha keratin plus beta-sheet, beta-turn and disorder structures. In the cuticle-cortex interface region the protein structures were primarily disorder and beta-turn with small amounts of beta-sheet configurations. The spectral analyses are consistent with the general information on hair fiber structure proposed in the literature.

In situ ATR-IR study of nitrite hydrogenation over Pd/Al 2O 3

The mechanism of nitrite hydrogenation over a Pd/Al 2O 3 catalyst layer deposited on a ZnSe internal reflection element was investigated in water using attenuated total reflection infrared spectroscopy. Nitrite hydrogenates to NO (ads), NH 2(ads), and NH + 4 on the palladium surface. Hydrogenation of adsorbed NO on palladium results in the formation of a reaction product that is not infrared-active (most likely nitrogen), whereas no NH + 4 is formed from NO (ads). NH + 4 is formed solely from hydrogenation of the NH 2(ads) intermediate. The present study clearly shows that formation of nitrogen and NH + 4 proceeds via two separate pathways, based on which a revised reaction scheme is proposed.

ATR-FTIR study of lipopolysaccharides at mineral surfaces

Lipopolysaccharides (LPS) are ubiquitous in natural aqueous systems because of bacterial cell turnover and lysis. LPS sorption and conformation at the mineral/water interface are strongly influenced by both solution and surface chemistry. In this study, the interaction of LPS with various surfaces (ZnSe, GeO 2, α-Fe 2O 3, α-Al 2O 3) that vary in surface charge and hydrophobicity was investigated using attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy. The presence of Ca 2+ (versus Na +) in LPS solutions resulted in aggregate reorientation and increased sorptive retention. ATR-FTIR spectra of Na-LPS systems are consistent with reduced surface affinity and are similar to those of solution phase LPS. Ca-LPS spectra reveal hydrophobic interactions of the lipid A region at the ZnSe internal reflection element (IRE). However, pH-dependent charge controls Ca-LPS sorption to hydrophilic surfaces (GeO 2, α-Fe 2O 3, and α-Al 2O 3), where bonding occurs principally via O-antigen functional groups. As a result of accumulation at the solid–liquid interface, spectra of Ca-LPS represent primarily surface-bound LPS. Variable-angle ATR-FTIR spectra of Ca-LPS systems show depth-dependent trends that occur at the spatial scale of LPS aggregates, consistent with the formation of vesicular structures.

Relation between s-Polarized and p-Polarized Internal Reflection Spectra: Application for the Spectral Resolution of Perpendicular Vibrational Modes

The orientation of chemical bonds in thin films is commonly probed with polarized internal reflection Fourier transform infrared spectroscopy. Here we demonstrate how the internal reflection spectra obtained using s-polarized light are related with the corresponding p-polarized spectra. The relation between s- and p-polarized internal reflection spectra is applied to resolve the absorption bands of vibrational modes with components oriented perpendicular to the substrate. This is successfully demonstrated using Langmuir-Blodgett films containing smectite clay minerals and a surfactant deposited on ZnSe and Ge internal reflection elements.

In situ ATR-IR study of CO adsorption and oxidation over Pt/Al 2O 3 in gas and aqueous phase: Promotion effects by water and pH

The adsorption and oxidation of carbon monoxide over a Pt/Al 2O 3 catalyst layer deposited on a ZnSe internal reflection element was investigated both in gas phase and water using attenuated total reflection infrared spectroscopy. A preparation method is described that results in a strongly attached layer that is stable for many days in a water flow. Both adsorption and oxidation of CO are largely affected by the presence of liquid water. It influences the metal particle potential as well as the CO molecule directly, which is reflected in large red shifts (45 cm −1) and a fourfold higher intensity when the experiments are carried out in water. Furthermore, the rate of CO oxidation changes significantly when carried out in water compared with gas phase. Finally, with increasing pH, CO stretching frequencies shift to lower wavenumbers, accompanied by a large increase in CO oxidation rate.

ATR-FTIR Spectroscopy Reveals Bond Formation During Bacterial Adhesion to Iron Oxide

The contribution of various bacterial surface functional groups to adhesion at hematite and ZnSe surfaces was examined using attenuated total reflectance (ATR) Fourier transform infrared (FTIR) spectroscopy. When live Shewanella oneidensis, Pseudomonas aeruginosa, and Bacillus subtilis cells were introduced to a horizontal hematite (alpha-Fe(2)O(3))-coated internal reflection element (IRE), FTIR peaks emerged corresponding to bacterial phosphate group binding. These IR peaks were not observed when bacteria were introduced to the uncoated ZnSe IRE. When cells were added to colloidal suspensions of alpha-Fe(2)O(3) at pH 7, spectra included peaks corresponding to P-OFe and nu(COOH), the latter being attributed to bridging of carboxylate at mineral surface OH groups. Selected model organic compounds with P-containing functionalities (phenylphosphonic acid [PPA], adenosine 5'-monophosphate [AMP], 2'-deoxyadenyl(3'-->5')-2'-deoxyadenosine [DADA], and deoxyribonucleic acid [DNA]) produce spectra with similar peaks corresponding to P-OFe when adsorbed to alpha-Fe(2)O(3). The data indicate that both terminal phosphate/phosphonate and phosphodiester groups, either exuded from the cell or present as surface biomolecules, are involved in bacterial adhesion to Fe-oxides through formation of innersphere Fe-phosphate/phosphonate complexes.

CO Adsorption and Oxidation at the Catalyst−Water Interface: An Investigation by Attenuated Total Reflection Infrared Spectroscopy

Adsorption of carbon monoxide and oxidation of preadsorbed carbon monoxide from gas and aqueous phases were studied on a platinum catalyst deposited on a ZnSe internal reflection element (IRE) using attenuated total reflection infrared (ATR-IR) spectroscopy. The results of this study convincingly show that it is possible to prepare platinum metal layers strongly attached to an IRE, which are stable for over 3 days in aqueous-phase experiments. It is shown that ATR-IR spectroscopy is a suitable technique to study adsorption and catalytic reactions occurring at the interface of a solid catalyst in an aqueous reaction mixture, even with an extreme low-surface-area catalyst. Clearly, ATR-IR spectroscopy allows for a direct comparison of reactions on a catalytic surface in gas and liquid phases on the same sample. CO was found to adsorb both linearly and bridged on the platinum metal layer when adsorbed from the gas phase, but only linear CO was detected in aqueous solution, although with 5 times higher intensity. Oxidation of preadsorbed CO on platinum occurs in both gas phase, wetted gas, and aqueous media and was found to be 2 times faster in the aqueous phase compared to gas-phase oxidation because of a promoting effect of water. Moreover, during oxidation at room temperature, CO2 adsorbed on Pt/ZnSe was detected in both gas and aqueous phases.

Attenuated total reflectance infrared studies of liposome adsorption at the solid–liquid interface

Interfacial interactions between liposomes and the solid–liquid interface (i.e. a ZnSe internal reflection element, modified to mimic a biological surface) were studied by Fourier transform infrared (FTIR) spectroscopy in attenuated total reflectance (ATR) mode. Both conventional liposomes, containing lecithin and cholesterol and Stealth ® liposomes containing poly(ethylene)glycol (PEG)5000- or PEG2000-lipids were investigated. IR bands due to the liposome components were observed to increase with time and enabled the liposome adsorption kinetics and thermodynamics to be quantified. The liposome solution conditions, surface properties and compositions have all been shown to influence liposome adsorption. Free energies of adsorption were determined to be in the range from −10.0 to −11.0 kJ mol −1 and slightly reduced by PEG incorporation. The adsorption rate constant is decreased with increased solution pH and decreased ionic strength; this reflects the importance of electrostatics in controlling liposome adsorption. Increasing the level and molecular weight of PEG incorporation in the liposomes significantly reduced both the rate and extent of liposome adsorption; steric hindrance is considered to play a key role. Findings from this research will improve the understanding of liposome interaction during drug delivery, give insight into the actions of liposomes in the body and may form the basis for improved liposome formulations.