An ATR-FTIR spectroscopic study of the competitive adsorption between oxalate and malonate at the water-goethite interface

The competitive adsorption between oxalate and malonate at the water-goethite interface was studied as a function of pH and total ligand concentrations by means of quantitative adsorption measurements and attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy. The results obtained show that ATR-FTIR spectroscopy resolves the individual spectroscopic features of oxalate and malonate when adsorbed simultaneously at the water-goethite interface. The characteristic peaks of all four types of predominating surface complexes existing in the single ligand systems were identified, namely one inner sphere and one outer sphere surface complex for each ligand. The quantitative adsorption data showed that oxalate partially out-competes malonate at the water-goethite interface. Evaluation of the peak area variations as a function of pH indicated that the stronger oxalate adsorption can be ascribed to the more stable inner sphere surface complex of oxalate, which in turn is related to the oxalate five-member chelate ring structure yielding a more stable complex compared to the six-member ring of malonate.     

Surface complexes of monomethyl phosphate stabilized by hydrogen bonding on goethite (α-FeOOH) nanoparticles

Typically, a significant fraction of phosphorus in soils is composed of organic phosphates, and this fraction thus plays an important role in the global phosphorus cycle. Here we have studied adsorption of monomethyl phosphate (MMP) to goethite (α-FeOOH) as a model system in order to better understand the mechanisms behind adsorption of organic phosphates to soil minerals, and how adsorption affects the stability of these molecules. The adsorption reactions and stability of MMP on goethite were studied at room temperature as a function of pH, time and total concentration of MMP by means of quantitative batch experiments, potentiometry and infrared spectroscopy. MMP was found to be stable at the water-goethite interface within the pH region 3-9 and over extended periods of time, as well as in solution. The infrared spectra indicated that MMP formed three predominating pH-dependent surface complexes on goethite, and that these interacted monodentately with surface Fe. The complexes differed in hydrogen bonding interactions via the auxiliary oxygens of the phosphate group. The presented surface complexation model was based on the collective spectroscopic and macroscopic results, using the Basic Stern approach to describe the interfacial region. The model consisted of three monodentate inner sphere surface complexes where the MMP complexes were stabilized by hydrogen bonding to a neighboring surface site. The three complexes, which had equal proton content and thus could be defined as surface isomers, were distinguished by the distribution of charge over the 0-plane and β-plane. In the high pH-range, MMP acted as a hydrogen bond acceptor whereas it was a hydrogen bond donor at low pH.     

Coadsorption of Cu(II) and glyphosate at the water-goethite (alpha-FeOOH) interface: molecular structures from FTIR and EXAFS measurements

The coadsorption of Cu(II) and glyphosate (N-(phosphonomethyl)glycine, abbreviated to PMG) at the water-goethite interface was studied by means of batch adsorption experiments, attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, and extended X-ray absorption fine structure (EXAFS) spectroscopy. The system was investigated over the pH range 3--9 and at total concentrations of 0.9 micromol and 2.2 micromol Cu(II) and PMG per m(2) of goethite. The collective quantitative and spectroscopic results show that Cu(II) and PMG directly interact at the water-goethite interface to form ternary surface complexes. Two predominating complexes have been identified. At pH 4 the IR and CuK-edge EXAFS data indicate a molecular structure where the phosphonate group of PMG bonds monodentately to the surface in an inner sphere mode, while carboxylate and amine groups coordinate to Cu(II) to form a 5-membered chelate ring. Hence, at pH 4, Cu(II) and PMG form a ternary surface complex on goethite with the general structure goethite-PMG-Cu(II). At the highest pH investigated (pH 9), the carboxylate group is still coordinated to Cu(II) but the phosphonate group is present in a relatively free, non-coordinated and/or disordered state. Although the spectroscopic data are not conclusive they indicate the formation of ternary surface complexes with the molecular architecture goethite-Cu(II)-PMG at high pH.     

Hydrogen-bonded and physisorbed CO in single-walled carbon nanotube bundles

Fourier transform infrared spectroscopy is used to study CO adsorption in single-walled carbon nanotubes. Evidence for adsorption in endohedral and groove/external surface sites is presented through displacement studies involving both CO and CO2. Blue-shifted CO stretching frequencies also indicate that CO hydrogen bonds to hydroxyl functionalities created on the nanotubes by acid purification steps. N2 surface area measurements are used to further understand the porosity of the nanotube samples and to help explain the spectroscopic results.     

Study of galactomannose interaction with solids using AFM, IR and allied techniques

Guar gum (GG) and locust bean gum (LBG) are two galactomannose polysaccharides with different mannose/galactose ratio which is widely used in many industrial sectors including food, textiles, paper, adhesive, paint, pharmaceuticals, cosmetics and mineral processing. They are natural nonionic polymers that are non-toxic and biodegradable. These properties make them ideal for industrial applications. However, a general lack of understanding of the interactions between the polysaccharides and solid surfaces has hindered wider application of these polymers. In this work, adsorption of locust bean gum and guar gum at the solid-liquid interface was investigated using adsorption tests, electrophoretic mobility measurements, FTIR, fluorescence spectroscopy, AFM and molecular modeling. Electrokinetic studies showed that the adsorption of GG and LBG on talc do not change its isoelectric point. In addition, GG and LBG adsorption on talc was found not to be affected by changes in solution conditions such as pH and ionic strength, which suggests a minor role of electrostatic force in adsorption. On the other hand, fluorescence spectroscopy studies conducted to investigate the role of hydrophobic bonding using pyrene probe showed no evidence of the formation of hydrophobic domains at talc-aqueous interface. Moreover, urea, a hydrogen bond breaker, markedly reduced the adsorption of LBG and GG on talc, supporting hydrogen bonding as an important role. In FTIR study, the changes in the infrared bands, associated with the CO stretch coupled to the CC stretch and OH deformation, were significant and therefore also supporting hydrogen bonding of GG and LBG to the solid surface. In addition, Langmuir modeling of adsorption isotherm further suggested that hydrogen bonding is the dominant force for polysaccharide adsorption since the adsorption free energy of these polymers is close to that for hydrogen bond formation. From molecular modeling, different helical structures are observed for LBG and GG because of their different galactose/mannose ratio and these polymers were found to adsorb flat on solid to let more of its OH groups in contact with the surface. All of the above results suggest that the main driving force for adsorption both of GG and LBG on talc is hydrogen bonding rather than hydrophobic force even though there is difference in G/M ratio between them.     

Adsorption and desorption of stearic acid self-assembled monolayers on aluminum oxide

Development of coatings to minimize unwanted surface adsorption is extremely important for their use in applications, such as sensors and medical implants. Self-assembled monolayers (SAMs) are an excellent choice for coatings that minimize nonspecific adsorption because they can be uniform and have a very high surface coverage. Another equally important characteristic of such coatings is their stability. In the present study, both the bonding mechanism and the stability of stearic acid SAMs on two aluminum oxides (single-crystal C-plane aluminum oxide (sapphire) and amorphous aluminum oxide (alumina)) are investigated. The adsorption mechanism is investigated by ex situ X-ray photoelectron spectroscopy and infrared (IR) spectroscopy. The results revealed that stearic acid binds to sapphire surfaces via a bidentate interaction of carboxylate with two oxygen atoms while it binds to alumina surfaces via both bidentate and monodentate interactions. Desorption kinetics of stearic acid self-organized on both aluminum oxide surfaces into water is explored by ex situ tapping mode atomic force microscopy, IR spectroscopy, and contact angle measurements. The results exhibit that the SAMs of stearic acid formed on sapphire are not stable in water and are continuously lost through desorption. Water contact angle measurements of SAMs that are immersed in water further indicate that the desorption rate of adsorbates from atomically smooth terrace sites is substantially faster than that of adsorbates from the sites of surface defects due to weaker molecular interaction with the smooth surface. A time-dependent desorption profile of SAMs grown on amorphous alumina reveals that contact angles decrease monotonically without any regional distinction, providing further evidence for the presence of adsorption sites with different types of affinity on the amorphous alumina surface.     

Adsorption and conformation of carboxymethyl cellulose at solid-liquid interfaces using spectroscopic, AFM and allied techniques

Carboxymethyl cellulose (CMC) is a polysaccharide which is widely used in many industrial sectors including food, textiles, paper, adhesives, paints, pharmaceutics, cosmetics and mineral processing. It is a natural organic polymer that is non-toxic and biodegradable. These properties make it ideal for industrial applications. However, a general lack of understanding of the interaction mechanism between the polysaccharides and solid surfaces has hindered the application of this polymer. In this work, adsorption of CMC at the solid-liquid interface is investigated using adsorption and electrophoretic mobility measurements, FTIR, fluorescence spectroscopy, AFM and molecular modeling. CMC adsorption on talc was found to be affected significantly by changes in solution conditions such as pH and ionic strength, which indicates the important role of electrostatic force in adsorption. The pH effect on adsorption was further proven by AFM imaging. Electrokinetic studies showed that the adsorption of CMC on talc changed its isoelectric point. Further, molecular modeling suggests a helical structure of CMC in solution while it is found to adsorb flat on the solid surface to allow its OH groups to be in contact with the surface. Fluorescence spectroscopy studies conducted to investigate the role of hydrophobic bonding using pyrene probe showed no evidence of the formation of hydrophobic domains at talc-aqueous interface. Urea, a hydrogen bond breaker, markedly reduced the adsorption of CMC on talc, supports hydrogen bonding as an important factor. In FTIR study, the changes to the infrared bands, associated with the CO stretch coupled to the CC stretch and OH deformation, were significant and this further supports the strong hydrogen bonding of CMC to the solid surface. In addition, Langmuir modeling of the adsorption isotherm suggests hydrogen bonding to be a dominant force for polysaccharide adsorption since the adsorption free energy of this polymer was close to that for hydrogen bond formation. All of the above results suggest that the main driving forces for CMC adsorption on talc are a combination of electrostatic interaction and hydrogen bonding rather than hydrophobic force.     

Vibrational spectroscopy on platinum single-crystal electrodes: Part I. In-situ infrared spectroscopic studies of the adsorption and oxidation of CO on Pt (111) in sulphuric acid

Platinum single-crystal electrodes of 5 mm diameter were prepared for in situ infrared spectroscopic measurements by melting platinum wires. The linear potential sweep voltammograms of hydrogen adsorption/desorption on Pt (111), (110) and (100) in 0.5 M sulphuric acid are in excellent agreement with those observed on smaller platinum single-crystal surfaces.The adsorption and oxidation of CO on Pt (111) in 0.5 M sulphuric acid was studied by in situ polarization modulated infrared reflection absorption spectroscopy. The effects of the initial adsorption potential and surface reconstruction on the nature and oxidation mechanism of the adsorbed CO layer are reported.     

Diamond Nanoelectrode Arrays for the Detection of Surface Sensitive Adsorption

Nanocrystalline diamond nanoelectrode arrays (NEAs) have been applied to investigate surface‐sensitive adsorption phenomena at the diamond–liquid interface. The adsorption of neutral methyl viologen (MV⁰) was used as a model system. The adsorption of MV⁰ was examined on hydrogen‐ and oxygen‐terminated surfaces. On the hydrogenated nanoelectrode surface, a sharp anodic stripping peak was observed upon oxidation of MV⁰, revealing strong adsorption of MV⁰. In contrast, a sigmoidal voltammogram was recorded with an oxygenated electrode surface, indicating there was no MV⁰ adsorption. The changes in the shapes of these voltammograms are due to the drastic changes that occur in the diffusion profiles during the transition. The diffusion profile changes from hemispherical diffusion on oxygen‐terminated surfaces to thin‐layer electrochemistry upon adsorption on hydrogen‐terminated surfaces. Different types and concentrations of buffer solutions were then used to vary the interaction of MV⁰ with diamond NEAs. The results suggest that the adsorption of MV⁰ on hydrogen‐terminated diamond NEAs is controlled by hydrophobic interactions. Therefore, diamond NEAs are ideal for the study of adsorption phenomena at the liquid–solid interface with voltammetry.     

Energetics of small molecule and water complexation in hydrophobic calixarene cavities

Calixarenes grafted on silica are energetically uniform hosts that bind aromatic guests with 1:1 stoichiometry, as shown by binding energies that depend upon the calixarene upper rim composition but not on their grafted surface density (0.02-0.23 nm(-2)). These materials are unique in maintaining a hydrophilic silica surface, as probed by H2O physisorption measurements, while possessing a high density of hydrophobic binding sites that are orthogonal to the silica surface below them. The covalently enforced cone-shaped cavities and complete accessibility of these rigidly grafted calixarenes allow the first unambiguous measurements of the thermodynamics of guest interaction with the same calixarene cavities in aqueous solution and vapor phase. Similar to adsorption into nonpolar protein cavities, adsorption into these hydrophobic cavities from aqueous solution is enthalpy-driven, which is in contrast to entropy-driven adsorption into water-soluble hydrophobic hosts such as beta cyclodextrin. The adsorption thermodynamics of several substituted aromatics from vapor and liquid are compared by (i) describing guest chemical potentials relative to pure guest, which removes differences among guests because of aqueous solvation and van der Waals contacts in the pure condensed phase, and (ii) passivating residual guest binding sites on exposed silica, titrated by water during adsorption from aqueous solution, using inorganic salts before vapor adsorption. Adsorption isotherms depend only upon the saturation vapor pressure of each guest, indicating that guest binding from aqueous or vapor media is controlled by van der Waals contacts with hydrophobic calixarene cavities acting as covalently assembled condensation nuclei, without apparent contributions from CH-pi or other directional interactions. These data also provide the first direct quantification of free energies for interactions of water with the calixarene cavity interior. The calixarene-water interface is stabilized by approximately 20 kJ/mol relative to the water-vapor interface, indicating that water significantly competes with the aromatic guests for adsorption at these ostensibly hydrophobic cavities. This result is useful for understanding models of water interactions with other concave hydrophobic surfaces, including those commonly observed within proteins.     

Adsorption of organic matter at mineral/water interfaces. 2. Outer-sphere adsorption of maleate and implications for dissolution processes

The effects of the adsorption of a simple dicarboxylate low molecular weight organic anion, maleate, on the dissolution of a model aluminum oxide, corundum (alpha-Al2O3), have been examined over a range of different maleate concentrations (0.125-5.0 mM) and pH conditions (2-10). In situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopic measurements indicate that maleate binds predominantly as an outer-sphere, fully deprotonated complex ([triple bond]AlOH2+ -Mal2-) at the corundum surface over the entire range of maleate concentrations and pH conditions investigated. In accordance with the ATR-FTIR findings, macroscopic adsorption data can be modeled as a function of maleate concentration and pH using an extended constant capacitance approach and a single [triple bond]AlOH2+ -Mal2- species. Outer-sphere adsorption of maleate is found to significantly reduce the protolytic dissolution rate of corundum under acidic conditions (pH < 5). A likely mechanism involves steric protection of dissolution-active surface sites, whereby strong outer-sphere interactions with maleate hinder attack on those surface sites by dissolution-promoting species.     

Tracking oxytetracyline mobility across environmental interfaces by second harmonic generation

This work examines the binding behavior of the antibiotic oxytetracycline (OTC) to mineral oxide/water interfaces in the presence and absence of organic functional groups using the interface-specific technique second harmonic generation (SHG). Studies show that OTC binding to fused quartz, methyl ester, carboxylic acid, and alkyl interfaces is fully reversible and highly dependent on solution pH, with appreciable adsorption occurring only at pH 8. Relative surface coverage at pH 8 is highest for the polar organic-functionalized surfaces, and surface saturation occurs for the methyl ester-functionalized fused quartz/water interface at 2 x 10(-5) M. Adsorption isotherm measurements indicate that the binding process is controlled by hydrogen bonding and hydrophobic interactions, with free energies of adsorption on the order of -40 kJ/mol for all interfaces studied. The results indicate that OTC transport in the environment will depend heavily on soil pH and composition and have implications for the development of bacterial antibiotic resistance.     

Interactions between nonpolar surfaces coated with the nonionic surfactant hexaoxyethylene dodecyl ether C12E6 and the origin of surface charges at the air/water interface

The interactions between nonpolar surfaces coated with the nonionic surfactant hexaoxyethylene dodecyl ether C12E6 were investigated using two techniques and three different types of surfaces. As nonpolar surfaces, the air/water interface, silanated negatively charged glass, and thiolated uncharged gold surfaces were chosen. The interactions between the air/water interfaces were measured with a thin film pressure balance in terms of disjoining pressure as a function of film thickness. The interactions between the solid/liquid interfaces were determined using a bimorph surface force apparatus. The influence of the nature of the surface on the interaction forces was investigated at surfactant concentrations below and above the cmc. The adsorption of the nonionic surfactant on the uncharged thiolated surface does not, as expected, lead to any buildup of a surface charge. On the other hand, adsorption of C12E6 on the charged silanated glass and the charged air/water interface results in a lowering of the surface charge density. The reduction of the surface charge density on the silanated glass surfaces is rationalized by changes in the dielectric permittivity around the charged silanol groups. The reason for the surface charge observed at the air/water interface as well as its decrease with increasing surfactant concentration is discussed and a new mechanism for generation of OH- ions at this particular interface is proposed.     

Dispersion stability of a ceramic glaze achieved through ionic surfactant adsorption

The adsorption of cetylpyridinium chloride (CPC) and sodium dodecylbenzenesulfonate (SDBS) onto a ceramic glaze mixture composed of limestone, feldspar, quartz, and kaolin has been investigated. Both adsorption isotherms and the average particle zeta potential have been studied in order to understand the suspension stability as a function of pH, ionic strength, and surfactant concentration. The adsorption of small amounts of cationic CPC onto the primarily negatively charged surfaces of the particles at pH 7 and 9 results in strong attraction and flocculation due to hydrophobic interactions. At higher surfactant concentrations a zeta potential of more than +60 mV results from the bilayered adsorbed surfactant, providing stability at salt concentrations < or = 0.01 M. At 0.1 M salt poor stability results despite substantial zeta potential values. Three mechanisms for SDBS adsorption have been identified. When anionic SDBS monomers either adsorb by electrostatic interactions with the few positive surface sites at high pH or adsorb onto like charged negative surface sites due to dispersion or hydrophobic interactions, the magnitude of the negative zeta potential increases slightly. At pH 9 this increase is enough to promote stability with an average zeta potential of more than -55 mV, whereas at pH 7 the zeta potential is lower at about -45 mV. The stability of suspensions at pH 7 is additionally due to steric repulsion caused by the adsorption of thick layers of neutrally charged Ca(DBS)2 complexes created when the surfactant interacts with dissolved calcium ions from the calcium carbonate component.     

Chemistry of aqueous silica nanoparticle surfaces and the mechanism of selective peptide adsorption

Control over selective recognition of biomolecules on inorganic nanoparticles is a major challenge for the synthesis of new catalysts, functional carriers for therapeutics, and assembly of renewable biobased materials. We found low sequence similarity among sequences of peptides strongly attracted to amorphous silica nanoparticles of various size (15-450 nm) using combinatorial phage display methods. Characterization of the surface by acid base titrations and zeta potential measurements revealed that the acidity of the silica particles increased with larger particle size, corresponding to between 5% and 20% ionization of silanol groups at pH 7. The wide range of surface ionization results in the attraction of increasingly basic peptides to increasingly acidic nanoparticles, along with major changes in the aqueous interfacial layer as seen in molecular dynamics simulation. We identified the mechanism of peptide adsorption using binding assays, zeta potential measurements, IR spectra, and molecular simulations of the purified peptides (without phage) in contact with uniformly sized silica particles. Positively charged peptides are strongly attracted to anionic silica surfaces by ion pairing of protonated N-termini, Lys side chains, and Arg side chains with negatively charged siloxide groups. Further, attraction of the peptides to the surface involves hydrogen bonds between polar groups in the peptide with silanol and siloxide groups on the silica surface, as well as ion-dipole, dipole-dipole, and van-der-Waals interactions. Electrostatic attraction between peptides and particle surfaces is supported by neutralization of zeta potentials, an inverse correlation between the required peptide concentration for measurable adsorption and the peptide pI, and proximity of cationic groups to the surface in the computation. The importance of hydrogen bonds and polar interactions is supported by adsorption of noncationic peptides containing Ser, His, and Asp residues, including the formation of multilayers. We also demonstrate tuning of interfacial interactions using mutant peptides with an excellent correlation between adsorption measurements, zeta potentials, computed adsorption energies, and the proposed binding mechanism. Follow-on questions about the relation between peptide adsorption on silica nanoparticles and mineralization of silica from peptide-stabilized precursors are raised.     

Chemisorption and diffusion of hydrogen on surface and subsurface sites of flat and stepped nickel surfaces

Plane-wave density functional theory calculations were performed to investigate the binding and diffusion of hydrogen on three flat Ni surfaces, Ni(100), Ni(110), and Ni(111), and two stepped Ni surfaces, Ni(210) and Ni(531). On each surface, the favored adsorption sites were identified by considering the energy and stability of various binding sites and zero-point energy corrections were computed. Binding energies are compared with experimental and theoretical results from the literature. Good agreement with experimental and previous theoretical data is found. At surface coverages where adsorbate-adsorbate interactions are relatively weak, the binding energy of H is similar on the five Ni surfaces studied. Favorable binding energies are observed for stable surface sites, while subsurface sites have unfavorable values relative to the gas phase molecular hydrogen. Minimum energy paths for hydrogen diffusion on Ni surfaces and into subsurface sites were constructed.     

DNA adsorption at functionalized Si/buffer interfaces studied by x-ray reflectivity

The adsorption of DNA on chemically homogeneous, functionalized, oxide-free single-crystal silicon surfaces is studied by x-ray reflectivity. The adsorption of monodisperse, 294 base-pair double-stranded DNA on a positively charged surface is detected through the deformation of the molecular monolayer of aminated alkyl-chain molecules covalently bonded to the surface. The adsorption of single-stranded DNA does not lead to the same deformation. A detailed quantitative characterization of the density profiles yield surface densities of the covalently grafted, molecular monolayers that are in excellent agreement with infrared spectroscopic measurements. The additional mass density that is measured following the adsorption of DNA corresponds either to the partial embedding of a densely-packed adsorbed layer or to a deeper penetration into the soft surface layer at a lower surface density of the adsorbed double-stranded DNA molecules. The adsorption is found to be irreversible under high added salt concentrations, suggesting a partial dehydration of the double-stranded DNA.     

Liquid hydrogen in protonic chabazite

Due to its fully reversible nature, H(2) storage by molecular adsorption could represent an advantage with respect to dissociative processes, where kinetic effects during the charging and discharging processes are present. A drawback of this strategy is represented by the extremely weak interactions that require low temperature and high pressure. High surface area materials hosting polarizing sites can represent a viable way toward more favorable working conditions. Of these, in this contribution, we have studied hydrogen adsorption in a series of zeolites using volumetric techniques and infrared spectroscopy at 15 K. We have found that in H-SSZ-13 zeolite the cooperative role played by high surface area, internal wall topology, and presence of high binding energy sites (protons) allows hydrogen to densify inside the nanopores at favorable temperature and pressure conditions.     

氢原子在Pt及Pt系双金属催化表面吸附的密度泛函理论研究

采用密度泛函理论(dFT)考察了Pt(100)、(110)、(111)三种表面氢原子的吸附行为, 计算了覆盖度为0.25 ML时氢原子在Pt 三种表面和M-Pt(111)双金属(M=Al, Fe, Co, Ni, Cu, Pd)上的最稳定吸附位、表面能以及吸附前后金属表面原子层间弛豫情况. 分析了氢原子在不同双金属表面吸附前后的局域态密度变化以及双金属表面d 带中心偏离费米能级的程度并与氢吸附能进行了关联. 计算结果表明, 在Pt(100), Pt(110)和Pt(111)表面, 氢原子的稳定吸附位分别为桥位、短桥位和fcc 穴位. 三种表面中以Pt(111)的表面能最低, 结构最稳定. 氢原子在不同M-Pt(111)双金属表面上的最稳定吸附位均为fcc 穴位, 其中在Ni-Pt 双金属表面的吸附能最低, Co-Pt 次之. 表明氢原子在Ni-Pt 和Co-Pt 双金属表面的吸附最稳定. 通过对氢原子在M-Pt(111)双金属表面吸附前后的局域态密度变化的分析, 验证了氢原子吸附能计算结果的准确性. 掺杂金属Ni、Co、Fe 的3d-Pt(111)双金属表面在吸附氢原子后发生弛豫, 第一层和第二层金属原子均不同程度地向外膨胀. 此外, 3d金属的掺入使得其对应的M-Pt(111)双金属表面d带中心与Pt 相比更靠近费米能级, 吸附氢原子能力增强, 表明3d-Pt系双金属表面有可能比Pt具有更好的脱氢活性.     

Surface-enhanced vibrational spectroscopy of B vitamins: what is the effect of SERS-active metals used?

Surface-enhanced Raman scattering (SERS) spectroscopy and surface-enhanced infrared absorption (SEIRA) spectroscopy are analytical tools suitable for the detection of small amounts of various analytes adsorbed on metal surfaces. During recent years, these two spectroscopic methods have become increasingly important in the investigation of adsorption of biomolecules and pharmaceuticals on nanostructured metal surfaces. In this work, the adsorption of B-group vitamins pyridoxine, nicotinic acid, folic acid and riboflavin at electrochemically prepared gold and silver substrates was investigated using Fourier transform SERS spectroscopy at an excitation wavelength of 1,064 nm. Gold and silver substrates were prepared by cathodic reduction on massive platinum targets. In the case of gold substrates, oxidation–reduction cycles were applied to increase the enhancement factor of the gold surface. The SERS spectra of riboflavin, nicotinic acid, folic acid and pyridoxine adsorbed on silver substrates differ significantly from SERS spectra of these B-group vitamins adsorbed on gold substrates. The analysis of near-infrared-excited SERS spectra reveals that each of B-group vitamin investigated interacts with the gold surface via a different mechanism of adsorption to that with the silver surface. In the case of riboflavin adsorbed on silver substrate, the interpretation of surface-enhanced infrared absorption (SEIRA) spectra was also helpful in investigation of the adsorption mechanism.