Investigation of solid surfaces modified by Langmuir-Blodgett monolayers using sum-frequency vibrational spectroscopy and X-ray photoelectron spectroscopy

Langmuir-Blodgett (LB) monomolecular layers of alkylhydroxamic acids and alkylphosphonic acids on copper and iron substrates have been studied by X-ray photoelectron spectroscopy (XPS) and sum-frequency vibrational spectroscopy. According to the XPS results, the structures of the hydroxamic acid and phosphonic acid Langmuir-Blodgett films are very similar: the thickness of the layer of the hydrocarbon tails is typically 1.9-2.1 nm, while the layer of headgroups is about 0.3-0.35 nm thick. The tilt angle of the carbon chains is estimated to be 20-30 degrees with respect to the sample surface normal, and the molecules are connected to the substrate via their headgroups. Analysis of the P 2p and N 1s lines indicates the presence of deprotonated headgroups. The substrate Cu 2p line includes a component which can be assigned to Cu(2+) ions in a thin Cu(OH)(2) layer. The deposition of LB layers led to significant decrease of the hydroxide-related signal, which indicates that binding of the headgroups to the surface is accompanied by the elimination of water molecules. The sum-frequency spectra also clearly indicate that well-ordered monolayers can be formed by the Langmuir-Blodgett technique. Since the non-resonant background from the metal substrates renders the analysis of the spectra more difficult, model system samples on glass were prepared. It was found that the alkyl chains of the adsorbed acids predominantly adopt the all-trans conformation and form an ordered structure. Upper limits for the mean tilt angle of the terminal methyl groups are approximately 10-20 degrees.     

Subwords in Reverse-Complement Order

We examine finite words over an alphabet of pairs of letters, where each word w₁w₂ ... w_t is identified with its reverse complement where ( ). We seek the smallest k such that every word of length n, composed from Γ, is uniquely determined by the set of its subwords of length up to k. Our almost sharp result (k~ 2n = 3) is an analogue of a classical result for “normal” words. This problem has its roots in bioinformatics. Received October 19, 2005     

Synthesis and application of a fluorous Lawesson's reagent: convenient chromatography-free product purification

[reaction: see text] A fluorous analogue of Lawesson's reagent for thionation of carbonyl compounds has been developed and its use demonstrated on a series of amides, esters, and ketones. The separation of the Lawesson's reagent-derived byproducts can be achieved by a simple fluorous solid-phase extraction.     

A mass spectrometric line for tritium analysis of water and noble gas measurements from different water amounts in the range of microlitres and millilitres

This paper describes the procedure followed for noble gas measurements for litres, millilitres and microlitres of water samples in our laboratory, including sample preparation, mass spectrometric measurement procedure, and the complete calibrations. The preparation line extracts dissolved gases from water samples of volumes of 0.2 μ l to 3 l and it separates them as noble and other chemically active gases. Our compact system handles the following measurements: (i) determination of tritium concentration of environmental water samples by the ³He ingrowth method; (ii) noble gas measurements from surface water and groundwater; and (iii) noble gas measurements from fluid inclusions of solid geological archives (e.g. speleothems). As a result, the tritium measurements have a detection limit of 0.012 TU, and the expectation value (between 1 and 20 TU) is within 0.2 % of the real concentrations with a standard deviation of 2.4 %. The reproducibility of noble gas measurements for water samples of 20–40 ml allows us to determine solubility temperatures by an uncertainty better than 0.5 °C. Moreover, noble gas measurements for tiny water amounts (in the microlitre range) show that the results of the performed calibration measurements for most noble gas isotopes occur with a deviation of less than 2 %. Theoretically, these precisions for noble gas concentrations obtained from measurements of waters samples of a few microlitres allow us to determine noble gas temperatures by an uncertainty of less than 1 °C. Here, we present the first noble gas measurements of tiny amounts of artificial water samples prepared under laboratory conditions.     

Improving the visible-light photocatalytic activity of SnO<Subscript>x</Subscript>·SiO<Subscript>2</Subscript> glass systems by introducing SnO<Subscript>x</Subscript> nanoparticles

Tin silicate glass without SnO_x nanoparticles (SiO₂·SnO_x), a silica glass containing only SnO_x nanoparticles (SiO₂·SnO_xNP) and the improved product, which combines the tin silicate glass with SnO_x nanoparticles (SiO₂·SnO_x·SnO_xNP) was prepared. For the structural analysis ¹¹⁹Sn Mössbauer spectroscopy and X-ray diffraction were applied. The ¹¹⁹Sn Mössbauer spectra showed that the SiO₂·SnO_x·SnO_xNP sample had the largest Sn^II content (12.0%). It also had an outstanding methylene blue degradation with the first-order rate value with (18?±?2) × 10^?3 min^?1 with visible light irradiation.     

Modulation of water exchange in europium(III) DOTA-tetraamide complexes via electronic substituent effects

The chemical exchange saturation transfer (CEST) efficiency for a series Eu3+-based tetraamide complexes bearing p-substituents on a single coordinating pendant arm is highly sensitive to water exchange rates. The CEST effect increases in the order Me < MeO < F approximately CO2tBu < CN < H. These results show that CEST contrast can be modulated by changes in electron density at a single ligating atom, and this forms the basis of creating imaging agents that respond to chemical oxidation and reduction.     

Oxidative Conversion of a Europium(II)‐Based T1 Agent into a Europium(III)‐Based paraCEST Agent that can be Detected In Vivo by Magnetic Resonance Imaging

The Eu^II complex of 1,4,7,10‐tetraazacyclododecane‐1,4,7,10‐tetraacetic acid (DOTA) tetra(glycinate) has a higher reduction potential than most Eu^II chelates reported to date. The reduced Eu^II form acts as an efficient water proton T₁ relaxation reagent, while the Eu^III form acts as a water‐based chemical exchange saturation transfer (CEST) agent. The complex has extremely fast water exchange rate. Oxidation to the corresponding Eu^III complex yields a well‐defined signal from the paraCEST agent. The time course of oxidation was studied in vitro and in vivo by T₁‐weighted and CEST imaging.     

Molecular constants of aluminum monohalides: caveats for computations of simple inorganic molecules

Quantum chemical calculations have become an everyday tool in chemistry. There are commercial program packages and downloadable basis sets for most needs. However, many chemists rarely go beyond the routine use of these programs, rarely, if ever, checking the original references for basis sets. In this letter, we point out some of the pitfalls of such an approach. Structural parameters of the aluminum-monohalides, AlF, AlCl, AlBr, and AlI, have been calculated using the Gaussian 03 program package and different basis set combinations.     

DSC analysis of human fat tissue in idiopathic avascular necrosis of the femoral head

Avascular necrosis of human femoral head (ANFH) causes incongruity in the joint that leads to disability in patients requires total hip arthroplasty (THA). Several etiological factors of ANFH have been proposed in the literature but there are cases of idiopathic origin. We observed macroscopic variation in quality of the subcutaneous fat tissue in patients with ANFH compared to patients with osteoarthritis or hip fracture during THA procedures.The samples were analysed by histology, gas chromatography (GC) and differential scanning calorimetry (DSC).Conclusion: the alteration in the fatty acid profile did not cause histological changes, however we could detect biochemical changes using DSC and GC.     

Ellipsometry of silica nanoparticulate Langmuir-Blodgett films for the verification of the validity of effective medium approximations

The validity of various effective medium approximations (EMAs) (Bruggeman, Maxwell-Garnett) was studied for nanostructured systems, where the scale of inhomogeneities is comparable to the wavelength. Langmuir-Blodgett (LB) layers of St?ber silica nanospheres of diameters between 40 and 129 nm are excellent model structures for the experimental verification of the validity of the EMA methods in spectroscopic ellipsometry (SE) evaluation. Nanostructured mono- and multilayered silica films were investigated by SE and reflectance spectroscopy. The effective refractive index and film thickness were determined from the results of multiparameter fitting of SE spectra in the 300-759 nm wavelength region. The distribution of the effective refractive index in the particulate films was calculated assuming an ideal close-packed arrangement of particles. The average deviation from such a structure was deduced from the corrected model by introducing a "fill factor". In the EMA approximation, the spherical shape of the silica particle determines the optical behavior, rather than the "depth distribution" of silica or porosity. Therefore, the shape of particles has a dominant effect on the optical properties of nanoparticulate LB films.