ATR FTIR Study of the Interaction of TiO2 Nanoparticle Films with ß-Lactoglobulin and Bile Salts.

The technique of in situ particle film attenuated total reflection Fourier transform infrared spectroscopy (ATR FTIR) has been used to probe the adsorption and coadsorption (sequential) of a common food protein (ß-lactoglobulin, BLG) and two representative bile salts (taurocholic acid and glycocholic acid, abbreviated as TCA and GCA) onto the surface of titanium dioxide (TiO2) nanoparticles. Evaluating of binding interactions between commonly used (historically now, in some countries) food additives and food components, as well as the body's own digestion chemicals, is a critical step in understanding the role of colloidal phenomena in digestion and bioavailability. TCA is found to adsorb onto TiO2 but without any significant ability to be retained when it is not present in the aqueous phase. GCA is also found to adsorb via two distinct binding mechanisms, with one type of adsorbed species being resistant to removal. BLG adsorbs, is irreversibly bound, and has altered conformation when adsorbed at pH 2 (stomach conditions) to the conformation when adsorbed at pH 6.5 (small intestine conditions). This altered conformation is not interface-dependent and is mirrored in the solution spectra of BLG. Sequential coadsorption studies indicate that TCA and GCA adsorb onto TiO2 nanoparticle surfaces and display similar degrees of reversibility and binding in the presence or absence of preadsorbed BLG.

Adsorption of Carboxymethyl Cellulose onto Titania Particle Films Studied with in Situ IR Spectroscopic Analysis.

Adsorption of carboxymethyl cellulose (CMC) in aqueous solution onto a titania nanoparticle film has been studied using in situ attenuated total reflectance infrared spectroscopy (ATR-IR). CMC was adsorbed onto the positively charged titania surface in neutral, partially charged, and fully charged state. The response of the adsorbed polyelectrolyte layer was monitored upon changing the electrolyte pH and ionic strength. The degree of dissociation of the CMC increased upon adsorption onto the titania surface and changed with the surface coverage. Ionic strength change was observed to influence the degree of dissociation of the adsorbed CMC similar as when in solution. No significant peak shifts were observed in the spectrum of the adsorbed CMC during adsorption or in response to changing solution conditions; therefore, inner-sphere complexation between the carboxyl groups and the titania could not be confirmed. The effect of ion identity on the adsorption process was studied using soft and hard cations and mono- and divalent cations. The presence of a divalent counterion was observed to cause changes in the carboxymethyl vibrations, which can be related to formation of intra- or interchain linkages.

Adsorption of a Polyethoxylated Surfactant from Aqueous Solution to Silica Nanoparticle Films Studied with In Situ Attenuated Total Reflection Infrared Spectroscopy and Colloid Probe Atomic Force Microscopy.

Polyethoxylated (PEO) surfactant adsorption to silica under aqueous conditions is an important physical process in a multitude of industries. Consequently, a considerable number of spectroscopic and other studies have been carried out to ascertain the molecular/structural details of the adsorbed surfactant and the kinetics of PEO surfactant adsorption. However, the use of infrared spectroscopy to probe surfactant adsorption at the silica/aqueous solution interface has been limited because of the instability of silica particle films under aqueous conditions and the opacity of silicon prisms below 1300 cm-1 typically employed for these studies. The work presented here provides infrared spectroscopic measurements of silica particle films formed from differing suspension pH on a diamond internal reflection prism to probe silica particle film stability as a function of pH. The films formed from a suspension pH of 2.5 were found to be the most stable owing to a sol-gel transition of the colloidal suspension upon drying and the reduction in electrostatic repulsion between silica nanoparticles, creating a tightly packed nanoparticle film. Colloid probe atomic force microscopy (CP-AFM) was used to confirm the alteration of surface forces between silica nanoparticles as a function of pH. Particle films from silica suspensions of pH 2.5 were formed in situ on an attenuated total reflection infrared diamond prism and used to probe Triton X-100 adsorption from an aqueous solution. The obtained infrared spectra revealed a critical surface aggregation concentration at a solution concentration of 0.14 mmol L-1, Triton X-100 forms discrete micelles at the silica surface, and the PEO head group preferentially adopts a helical conformation. Most intriguingly, a breakup of the silica particle film was observed at the critical micelle concentration of the surfactant. This is due to the repulsive steric forces arising from the interactions between the PEO corona of the surfactant micelles formed at the silica surface, as confirmed by the CP-AFM measurements.

Microscopic and infrared spectroscopic comparison of the underwater adhesives produced by germlings of the brown seaweed species Durvillaea antarctica and Hormosira banksii.

Adhesives from marine organisms are often the source of inspiration for the development of glues able to create durable bonds in wet environments. In this work, we investigated the adhesive secretions produced by germlings of two large seaweed species from the South Pacific, Durvillaea antarctica, also named 'the strongest kelp in the word', and its close relative Hormosira banksii The comparative analysis was based on optical and scanning electron microscopy imaging as well as Fourier transform infrared (FTIR) spectroscopy and principal component analysis (PCA). For both species, the egg surface presents peripheral vesicles which are released soon after fertilization to discharge a primary adhesive. This is characterized by peaks representative of carbohydrate molecules. A secondary protein-based adhesive is then secreted in the early developmental stages of the germlings. Energy dispersive X-ray, FTIR and PCA indicate that D. antarctica secretions also contain sulfated moieties, and become cross-linked with time, both conferring strong adhesive and cohesive properties. On the other hand, H. banksii secretions are complemented by the putative adhesive phlorotannins, and are characterized by a simple mechanism in which all constituents are released with the same rate and with no apparent cross-linking. It is also noted that the release of adhesive materials appears to be faster and more copious in D. antarctica than in H. banksii Overall, this study highlights that both quantity and quality of the adhesives matter in explaining the superior attachment ability of D. antarctica.

Histomorphometric and histologic evaluation of titanium-zirconium (aTiZr) implants with anodized surfaces.

The choice of implant surface has a significant influence on osseointegration. Modification of TiZr surface by anodization is reported to have the potential to modulate the osteoblast cell behaviour favouring more rapid bone formation. The aim of this study is to investigate the effect of anodizing the surface of TiZr discs with respect to osseointegration after four weeks implantation in sheep femurs. Titanium (Ti) and TiZr discs were anodized in an electrolyte containing DL-α-glycerophosphate and calcium acetate at 300 V. The surface characteristics were analyzed by scanning electron microscopy, electron dispersive spectroscopy, atomic force microscopy and goniometry. Forty implant discs with thickness of 1.5 and 10 mm diameter (10 of each-titanium, titanium-zirconium, anodized titanium and anodized titanium-zirconium) were placed in the femoral condyles of 10 sheep. Histomorphometric and histologic analysis were performed 4 weeks after implantation. The anodized implants displayed hydrophilic, porous, nano-to-micrometer scale roughened surfaces. Energy dispersive spectroscopy analysis revealed calcium and phosphorous incorporation into the surface of both titanium and titanium-zirconium after anodization. Histologically there was new bone apposition on all implanted discs, slightly more pronounced on anodised discs. The percentage bone-to-implant contact measurements of anodized implants were higher than machined/unmodified implants but there was no significant difference between the two groups with anodized surfaces (P > 0.05, n = 10). The present histomorphometric and histological findings confirm that surface modification of titanium-zirconium by anodization is similar to anodised titanium enhances early osseointegration compared to machined implant surfaces.

Surficial Siloxane-to-Silanol Interconversion during Room-Temperature Hydration/Dehydration of Amorphous Silica Films Observed by ATR-IR and TIR-Raman Spectroscopy.

Silica has been frequently studied using infrared and Raman spectroscopy due to its importance in many practical contexts where its surface chemistry plays a vital role. The majority of these studies have utilized chemical-vapor-deposited films in vacuo after high-temperature calcination. However, room-temperature hydration and dehydration of thin silica particle films has not been well characterized in spite of the importance of such films as substrates for polymer and surfactant adsorption. The present study has utilized ATR-IR spectroscopy and thin silica particle films exposed to varying humidity to clearly show reversible conversion between surface siloxanes and hydrogen-bonded silanols without the need for semiempirical peak deconvolution. The IR spectra from corresponding hydration experiments on deuterated silica films has confirmed the vibrational mode assignments. The variation of humidity over silica films formed from silica suspensions of differing pH gave IR spectra consistent with the change in the relative populations of siloxide to silanol surface groups. In addition, total internal reflection Raman spectroscopy has been used to provide further evidence of room-temperature dehydroxylation, with spectral evidence for the presence of three-membered siloxane rings when films are dehydrated under argon. The confirmation of room-temperature siloxane-to-silanol interconversion is expected to benefit understanding in many silica surface chemical contexts.

Spark anodization of titanium-zirconium alloy: surface characterization and bioactivity assessment.

Titanium (Ti) and its alloys have been popularly used as implant biomaterial for decades. Recently, titanium-zirconium (TiZr) alloy has been developed as an alternative implant material with improved strength in load bearing areas. Surface modification is one of the key factors to alter the surface properties to hasten osseointegration. Spark anodic oxidation (anodization) is one such method that is reported to enhance the bone formation around implants. This study aims to anodize TiZr and study its surface characteristics and cytocompatibility by cell culture experiments using osteoblast-like cells. Titanium (Ti) and TiZr discs were anodized in an electrolyte containing DL-α-glycerophosphate and calcium acetate (CA) at 300 V. The surface characteristics were analyzed by scanning electron microscopy, electron dispersive spectroscopy, X-ray diffraction (XRD), atomic force microscopy and goniometry. Using osteoblast-like cells viability, proliferation, differentiation and mineralization was assessed. The anodized surfaces demonstrated increased oxygen, entrapped calcium and phosphorous from the electrolyte used. XRD analysis confirmed the presence of anatase in the oxide layer. Average roughness increased and there was a significant decrease in contact angle (P < 0.01) following anodization. The anodized TiZr (aTiZr) surfaces were more nano-porous compared to anodized Ti (aTi). No significant difference was found in the viability of cells, but after 24 h the total number of cells was significantly higher (P < 0.01). Proliferation, alkaline phosphatase activity and calcium deposits were significantly higher on anodized surfaces compared to machined surfaces (P < 0.05, ANOVA). Anodization of TiZr resulted in a more nanoporous and hydrophilic surface than aTi, and osteoblast biocompatibility appeared comparable to aTi.

Oxidative IR spectroelectrochemistry of copper in methanol containing carbon monoxide.

IR spectroelectrochemistry was used to examine the electro-oxidation behavior of carbon monoxide in methanol at a polycrystalline copper electrode. Under such neutral conditions copper electrodes are coated with ill-defined copper oxides and hydroxides and at the oxidative potentials can be expected to generate soluble copper species. The electrochemistry displayed complex behavior suggesting that methanol oxidation was one prominent reaction. However, the spectroscopy revealed that very little methanol oxidation had occurred and that carbon monoxide was not adsorbed to the copper electrode. Instead, the electro-oxidation generated an intense IR band at 2107 cm(-1) that was attributed to a soluble [Cu(I)CO](+) species.

In situ ATR FTIR study of dextrin adsorption on anatase TiO2.

The adsorption of two dextrin-based polymers, a regular wheat dextrin (TY) and a carboxymethyl-substituted (CM) dextrin, onto an anatase TiO(2) particle film has been studied using in situ attenuated total reflection (ATR) FTIR spectroscopy. Infrared spectra of the polymer solutions and the polymer adsorbed at the anatase surface were acquired for two solution conditions: pH 3 and pH 9; below and above the isoelectric point (IEP) of anatase, respectively. Comparison of the polymer solution spectra and the adsorbed layer spectra highlighted a number of spectral differences that were attributed to involvement of the carboxyl group of CM Dextrin interacting with the anatase surface directly and the adsorption of oxidized dextrin chains in the case of regular dextrin (TY) at high pH. The adsorption/desorption kinetics were determined by monitoring spectral peaks of the pyranose ring of both polymers. Adsorption equilibrium was not established for Dextrin TY for many hours, whereas CM Dextrin reached equilibrium in its adsorption within 60 min. The extent of desorption of Dextrin TY (observed by flowing a background electrolyte dextrin-free solution) was extensive at both pH values, which reflects the poor affinity and binding of the polymer on anatase. In contrast, CM Dextrin underwent almost no desorption, indicating a high affinity between the carboxyl groups of the polymer and the anatase surface.

Redox transformations at nanodiamond surfaces revealed by in situ infrared spectroscopy.

Attenuated total reflectance infrared spectroscopy is used to monitor nanodiamond surface group transformations in the presence of aqueous IrCl(6)(2-). Electron transfer between the nanoparticle surface and the solution redox species results in oxidation of ∼8.5% of surface alcohol groups, with concomitant formation of unsaturated ketone or quinone-like moieties.

Alginate ion adsorption on a TiO2 particle film and interactions of adsorbed alginate with calcium ions investigated by attenuated total reflection infrared (ATR-IR) spectroscopy.

The adsorption of alginic acid on a TiO(2) particle film from aqueous solution was investigated by attenuated total reflection infrared (ATR-IR) spectroscopy. ATR-IR spectra recorded at different pHs confirmed that alginate adsorption to TiO(2) is favored at pH 3.0 and no significant adsorption occurs above pH 5.0. Upon adsorption the carboxylic acid groups of alginic acid are converted to the carboxylate form and bind to surface Ti(IV) ions via bridging bidentate structures. Spectral analyses of the carboxylic acid and carboxylate stretching vibrations indicated that about three in four -COOH groups are converted to -COO(-) groups as they bind coordinately to TiO(2). Additionally, the spectral data at pH 8.0 showed specific interactions of Ca(2+) ions with the free COO(-) groups of the polysaccharide backbone of adsorbed alginic acid.

Adsorption of enterobactin to metal oxides and the role of siderophores in bacterial adhesion to metals.

The potential contribution of chemical bonds formed between bacterial cells and metal surfaces during biofilm initiation has received little attention. Previous work has suggested that bacterial siderophores may play a role in bacterial adhesion to metals. It has now been shown using in situ ATR-IR spectroscopy that enterobactin, a catecholate siderophore secreted by Escherichia coli, forms covalent bonds with particle films of titanium dioxide, boehmite (AlOOH), and chromium oxide-hydroxide which model the surfaces of metals of significance in medical and industrial settings. Adsorption of enterobactin to the metal oxides occurred through the 2,3-dihydroxybenzoyl moieties, with the trilactone macrocycle having little involvement. Vibrational modes of the 2,3-dihydroxybenzoyl moiety of enterobactin, adsorbed to TiO(2), were assigned by comparing the observed IR spectra with those calculated by the density functional method. Comparison of the observed adsorbate IR spectrum with the calculated spectra of catecholate-type [H(2)NCOC(6)H(3)O(2)Ti(OH)(4)](2-) and salicylate-type [H(2)NCOC(6)H(3)O(2)HTi(OH)(4)](2-) surface complexes indicated that the catecholate type is dominant. Analysis of the spectra for enterobactin in solution and that adsorbed to TiO(2) revealed that the amide of the 2,3-dihydroxybenzoylserine group reorientates during coordination to surface Ti(IV) ions. Investigation into the pH dependence of enterobactin adsorption to TiO(2) surfaces showed that all 2,3-dihydroxybenzoyl groups are involved. Infrared absorption bands attributed to adsorbed enterobactin were also strongly evident for E. coli cells attached to TiO(2) particle films. These studies give evidence of enterobactin-metal bond formation and further suggest the generality of siderophore involvement in bacterial biofilm initiation on metal surfaces.

The physico-chemical characterization of casein-modified surfaces and their influence on the adhesion of spores from a Geobacillus species.

To gain a better understanding of the factors influencing spore adhesion in dairy manufacturing plants, casein-modified glass surfaces were prepared and characterized and their effect on the adhesion kinetics of spores from a Geobacillus sp., isolated from a dairy manufacturing plant (DMP) was assessed using a flow chamber. Surfaces were produced by initially silanizing glass using (3-glycidyloxypropyl) trimethoxysilane (GPS) or (3-aminopropyl) triethoxysilane to form epoxy-functionalized (G-GPS) or amino-functionalized glass (G-NH(2)) substrata. Casein was grafted to the G-GPS directly by its primary amino groups (G-GPS-casein) or to G-NH(2) by employing glutaraldehyde as a linking agent (G-NH(2)-glutar-casein). The surfaces were characterised using streaming potential measurements, contact angle goniometry, infrared spectroscopy and scanning electron microscopy. The attachment rate of spores suspended in 0.1 M KCl at pH 6.8, was highest on the positively charged (+14 mV) G-NH(2) surface (333 spores cm(-2) s(-1)) compared to the negatively charged glass (-22 mV), G-GPS (-20 mV) or G-GPS-casein (-21 mV) surfaces (162, 17 or 6 spores cm(-2) s(-1) respectively). Whilst there was a clear decrease in attachment rate to negatively charged casein-modified surfaces compared to the positively charged amine surface, there was no clear relationship between surface hydrophobicity and spore attachment rate.

In situ infrared spectroscopic analysis of the water modes of [Zr4(OH)8(H2O)16]8+ during the thermal dehydration of ZrOCl2.8H2O.

An in situ infrared spectroscopic analysis of the thermal dehydration of zirconyl chloride octahydrate was carried out to identify bending mode vibrations of distinctive water molecules in this well-defined zirconium(IV) cluster cation. TG-MS analysis revealed the particular temperatures where one water molecule at a time was removed from the solid hydrate. In situ IR data unveiled remarkable spectral changes featuring isosbestic behavior. We were able to experimentally distinguish between delta(H(2)O) modes from coordinatively bound water molecules (Zr-(OH(2))(3), 1595 cm(-1)), tetrahedrally coordinated lattice water (H(2)O dimer, 1620 cm(-1)), as well as strongly H-bonded lattice waters accommodating hydrated protons (1705 and 1666 cm(-1)). Spectral changes in the range from 1050 and 900 cm(-1) during dehydration are discussed.

Salt modulates bacterial hydrophobicity and charge properties influencing adhesion of Pseudomonas aeruginosa (PA01) in aqueous suspensions.

The influence on cell hydrophobicity of differential extension with ionic strength of lipopolysaccharide molecules (LPS), which exist as charged and uncharged polymers at the surface of the gram-negative bacterium Pseudomonas aeruginosa (PA01), has been investigated. Attenuated total reflection infrared (ATR-IR) spectral absorptions from a single layer of cells adsorbed to ZnSe increased in intensity with increasing NaCl concentration up to 0.1 mol L(-1). Dynamic contact angle measurements (Wilhelmy plate tensiometry) made with a ZnSe plate having an adsorbed cell layer and the adherence of the cells to hexadecane suggest that PA01 cells were most hydrophobic in contact with 0.1 mol L(-1) NaCl solutions. These data indicate a charge screening induced compression of the charged LPS polymers decreasing the cell-surface approach distance and increasing the cell hydrophobicity due to the greater surface predominance of the uncharged LPS polymers. Interestingly, adsorbed cell layers in 0.3 mol L(-1) NaCl had a lower IR absorption intensity, and PA01 cells suspended in 0.3 mol L(-1) were found to be more hydrophilic, indicating that other factors influence the cell-surface approach distance and hydrophobicity. The examination of cell electrophoretic mobility variation with NaCl concentration suggests that the compression of charged polysaccharides increases the polysaccharide charge density and may also reduce the flow of liquid through the polysaccharide layer affecting the effective potential at the interface, the cell hydrophobicity, and the cell-surface approach distance.

Silicic acid adsorption and oligomerization at the ferrihydrite-water interface: interpretation of ATR-IR spectra based on a model surface structure.

Oxide surfaces can promote specific lateral interactions between adsorbed species that become concentrated in specific orientations at an interface. In this article, in situ attenuated total reflectance (ATR) IR spectra were collected over time (from 0 to approximately 100 h) as the iron oxide ferrihydrite reacted with H(4)SiO(4) (between 0.007 and 1.65 mM) and at a pH of 4, 7, or 10. Under all conditions, the first product formed was a monomeric surface species with distinct bands at 945 and 880 cm(-1), and a bidentate (2)C complex with SiO(4) sharing corners with two edge-linked Fe octahedra was proposed. Once a certain surface concentration (Gamma(Si)) of monomers was reached, a condensed oligomeric surface species with Si-O-Si linkages was observed on the surface with bands at 1005, 917, and 827 cm(-1) and one or more bands at >1050 cm(-1). This species was observed as a minor surface component at Gamma(Si) that was up to 10 times lower than the calculated density of (2)C adsorption sites on ferrihydrite and became the dominant surface species at higher Gamma(Si). This formation of a specific oligomer is rationalized on the basis of a recent model for the ferrihydrite surface, with the arrangement of (2)C adsorption sites on the (021) ferrihydrite face causing adjacent Si monomers to be held in an orientation that is conducive to the formation of a condensed Si species upon insertion of a solution H(4)SiO(4). Therefore, this model predicts that the ferrihydrite surface may act as a template for oligomerization in one dimension forming segments of pyroxene-like structures. The ATR-IR spectra and changes in the surface species' composition with time are consistent with such a model.

Structure and conformation of methyl-terminated poly(ethylene oxide)-bis[methylenephosphonate] ligands adsorbed to boehmite (AlOOH) from aqueous solutions. Attenuated total reflection infrared (ATR-IR) spectra and dynamic contact angles.

A ligand with a bisphosphonate headgroup and methyl-terminated poly(ethylene oxide) 550 tail has been adsorbed to a boehmite (AlOOH) particle film from aqueous solutions at 9, 18, 26, and 40 degrees C with a range of coverages. In situ attenuated total reflectance infrared (ATR-IR) spectra of the PEO-modified boehmite particle films has been used to monitor surface coverage. PEO-related IR absorptions showed variation in band shape with coverage and temperature which reflect changes in the configuration and crowding of PEO ligand tails. The integrated absorbance of PEO-related absorptions arising from CH(2) wagging and twisting vibrations indicated the average conformation about the C-C and C-O bonds. At all values of coverage and temperature, the PEO tails of the PEO-phosphonate ligands were found to have a predominantly gauche conformation about C-C bonds and a predominantly trans conformation about C-O bonds. The proportion of PEO with this predominant configuration, termed TGT, was found to vary with coverage and temperature and was most prevalent for PEO-modified boehmite surfaces with a ligand coverage of approximately 0.95 at 18 degrees C. Dynamic water contact angle measurements showed that PEO-modified surfaces with the greatest proportion of PEO in the TGT configuration were also the most hydrophilic, thus indicating that PEO in the TGT configuration was more hydrated or polar than other configurations. Variation in the proportion of PEO in the TGT configuration with temperature and coverage have been used to explain the variable resistance to protein adhesion reported for PEO-modified surfaces. Factors influencing the configuration of surface-bound PEO with changes in ligand coverage and temperature have been discussed.

Structure and conformation in mixtures of methyl-terminated poly(ethylene oxide) and water. Principal component analysis and band fitting of infrared absorptions.

Infrared absorption spectra of aqueous mixtures of poly(ethylene oxide) dimethyl ether (PEO), with a number average molecular weight of 500 and a water volume fraction between 0 and 0.96 have been recorded at 18, 26, and 40 degrees C. Composition and temperature were found to influence the intensity and wavenumber of PEO-related absorptions. Spectral band fitting of the CH(2) wagging/twisting region of the spectra showed that the proportion of gauche and trans conformers of the C-C and C-O bonds varied with composition. Principal component analysis (PCA) of the PEO-related absorptions showed that the configuration with a gauche conformation about the C-C bond and trans conformation about the C-O bond was favored at all the compositions and temperatures tested but was decreasingly favored as the temperature was increased. PCA also enabled the assignment of spectral features associated with a specific conformation of either the C-C or C-O bond. Water-related O-H stretching absorptions indicated that hydrogen bonds were formed between water and PEO ether oxygen atoms. Variation in the wavenumber and integrated absorbance of band-fitted contributions to the water O-H stretching region suggest that the degree of water-water hydrogen bonding changes with composition. At water volume fractions between 0.2 and 0.6, almost all of the water appeared to be strongly hydrogen bonded. The smallest infrared absorption intensity from weakly hydrogen bonded water was recorded for mixtures with a water volume fraction of around 0.4. Such PEO-water mixtures had the greatest proportion of trans conformers about the C-O bond, and there were unexpectedly large PEO and water absorption intensities. Correlations between the average PEO configuration and the water O-H stretching absorptions suggest that, while the conformation about the C-C bond was mainly determined by the polarity of the mixture, the conformation about the C-O bond was influenced by the formation of a strongly hydrogen bonded PEO-water network.

In situ IR spectroscopic studies of the avidin-biotin bioconjugation reaction on CdS particle films.

Avidin-biotin bioconjugation reactions have been carried out on CdS nanoparticle films in H2O and D2O and investigated using in situ ATR-IR spectroscopic techniques. The experimental procedure involved the sequential adsorption of mercaptoacetic acid, the protein avidin, and the subsequent binding of the ligand biotin. The IR spectra of the solution-phase species mercaptoacetic acid, avidin, and biotin, at pH=7.2 were generally found to be similar in both H2O and D2O, with some minor peak shifts due to solvation changes. The IR spectra of the adsorbed species suggested that avidin may have undergone a conformational change upon adsorption to the CdS surface. In general, adsorption-induced conformational changes for avidin are likely, but to our knowledge have not been previously reported. The conformation of adsorbed avidin appeared to change again upon the binding of biotin, with the spectral data suggesting partial reversion to its native solution conformation.

Adsorption/desorption kinetics from ATR-IR spectroscopy. Aqueous oxalic acid on anatase TiO2.

Adsorption and desorption kinetics at the solid/solution interface have been monitored using attenuated total reflection infrared (ATR-IR) spectroscopy to evaluate this approach as an alternative to equilibrium (adsorption isotherm) measurements of adsorption affinity. The adsorption and desorption kinetics of oxalate ion to anatase TiO2 have been measured by using aqueous 1 x 10(-4) mol L(-1) oxalic acid solutions at pH 4 and thin films of TiO2 particles deposited on an internal reflection prism. The adsorption kinetics were obtained from the absorbance versus time behavior of major adsorbed oxalate infrared absorptions with flow of oxalic acid solution followed by flow of solution not containing oxalic acid to measure the desorption kinetics. Regression analysis of the desorption data based on Langmuir kinetics yielded three distinct pseudo-first-order rate constants with desorption half-lives of 300, 14, and 2 min, indicating the presence of three adsorbed oxalate species of different adsorption affinities. The most slowly desorbing and most strongly bound adsorbate species is likely to be a bidentate chelating oxalate ion from comparisons with the IR spectra of coordination compounds involving oxalate ligands. Regression analysis of the adsorption data was unable to yield the corresponding pseudo-first-order adsorption constants and prevented the calculation from kinetics data of Langmuir adsorption affinity constants. Measurement of adsorption and desorption kinetics by ATR-IR spectroscopy is expected to provide a relatively rapid means of assessing the presence of species of different adsorption affinities in systems in which their spectra are not well differentiated.