Calorimetry at surfaces using high-resolution core-level photoemission

We have developed a method to measure simultaneously the internal energy of bulk and the first layer atoms of a crystal. The internal energy of bulk and the surface atoms of lithium (110) have been evaluated from 22 K up to above the melting transition, applying the Debye model to the thermal broadening of the respective 1s photoemission lines. Our measurements clearly reveal two phase changes: the known liquid to solid transition and the surface melting, occurring 50 K below the bulk melting point.     

Crystal-like nature of acoustic excitations in glassy ethanol

We report on inelastic x-ray scattering experiments on crystalline and glassy phases of ethanol in order to directly compare the influence of disorder on high frequency acoustic excitations. We find that both the dispersion and the line-width of the longitudinal acoustic excitations in the glass are the same as in the polycrystal in the reciprocal space portion covering the 1st and 2nd Brillouin zones. The structural disorder is found to play little role apart from an intrinsic angular averaging, and the nature of these excitations must essentially be the same in both glass and poly crystal.     

Gramicidin conducting dimers in lipid bilayers are stabilized by single-file ionic flux along them

Gramicidin D was incorporated in a biomimetic membrane consisting of a lipid bilayer tethered to a mercury electrode via a hydrophilic spacer, and its behavior was investigated in aqueous 0.1 M KCl by potential-step chronocoulometry and electrochemical impedance spectroscopy. The impedance spectra, recorded from 0.1 to 1 x 10(5) Hz over a potential range of 0.7 V, were fitted to a series of RC meshes, which were related to the different substructural elements of the biomimetic membrane. These impedance spectra were compared with those obtained by incorporating valinomycin, under otherwise identical conditions. The potential dependence of the stationary currents reported on bilayer lipid membranes by Bamberg and L?uger (Bamberg, E.; L?uger, P. J. Membrane Biol. 1973, 11, 177-194) as well as those extracted from potential-step chronocoulometric measurements was interpreted by relating the increase in gramicidin dimerization to a progressive increase in single-file K+ flux along the dimeric channels. An analogous approach was adopted in explaining the difference between the impedance spectra obtained with gramicidin D and those obtained with valinomycin. It is concluded that gramicidin has a low tendency to form dimers in the absence of ionic flux.     

Short-term changes in delta(13)C and delta(15)N signatures of water discharged from grazed grasslands

The composition of dissolved organic matter (DOM) in a soil is the product of a variety of soil processes. Changes in the composition of DOM in water discharged from soil should, therefore, give an important insight into modifications in these soil processes. We hypothesise that these processes in soils, under different grassland management regimes, would be affected to different extents by the short-term disturbance of a storm event and that evidence of this could be detected in delta(13)C and delta(15)N signatures in drainage and surface runoff waters. During a storm event we collected discharge waters from 1 ha grassland lysimeters, with or without artificial drainage, which received contrasting fertiliser inputs, and delta(13)C and delta(15)N signatures were determined. Changes in (13)C enrichment during the storm event were clearly identifiable, as were differences between plots for (13)C and (15)N, illustrating that this technique has potential to be a useful tool for identifying and investigating short- and long-term changes in soil organic matter dynamics. Copyright 1999 John Wiley & Sons, Ltd.     

Luminescence of CsPbBr3-like quantum dots in CsBr single crystals

Luminescence and decay kinetics of the Pb²⁺ aggregates in CsBr host crystals were measured in the 4–300 K temperature interval and in 10⁻¹⁰–10⁻³ time scale. Their emission properties are similar to those of CsPbBr₃ bulk crystal showing a subnanosecond free exciton emission in the 520–540 nm spectral region and slower trapped exciton emission in the 530–580 nm spectral region. An efficient energy exchange between the free and trapped exciton states is shown by the temperature dependencies of emission spectra. The quantum size effect is demonstrated in the high energy shift and broadening of the absorption and emission spectra and an estimate of the size of the CsPbBr₃-like aggregates is provided. Independent evidence of the presence of the CsPbBr₃ and Cs₄PbBr₆ aggregated phases in the CsBr host was obtained by X-ray structural studies.     

A DNA-based piezoelectric biosensor: strategies for coupling nucleic acids to piezoelectric devices

A DNA-based piezoelectric biosensor has been here studied in terms of probe immobilisation and DNA sample pre-treatment. The biosensor is specific for the detection of the mecA gene of methicillin-resistant Staphylococcus aureus (MRSA).Methicillin-resistant S. aureus is responsible of several infections in humans, like pneumonia, meningitis and endocarditic. MRSA is also a major cause of hospital-acquired infections worldwide. The antibiotics resistance is conferred by the gene mecA, codifying for an anomalous protein.Two different immobilisation procedures of the probe specific for mecA gene are reported: immobilisation via streptavidin-biotin interaction and direct immobilisation of thiolated probes.After the study with synthetic oligonucleotides, the system has been applied to the analysis of bacterial DNA from MRSA, amplified by polymerase chain reaction. These samples were pre-treated with two different denaturation procedures and the performances of the sensor in the two cases were compared.The two immobilisation methods and denaturation protocols were here used to study the influences of these parameters on the performances of the sensor, applied here to the detection of the mecA gene. Better results in terms of sensitivity and reproducibility were obtained when using the biotinylated probe and the PCR-amplified samples treated by a denaturation procedures involving the use of high temperature and blocking oligonucleotides.     

Electronic structure of crystalline copper phthalocyanine

The electronic structure of copper-phthalocyanine (CuPc) has been studied both experimentally and theoretically. Experiments have been performed on alpha and beta crystalline phases, using photoemission spectroscopy to probe core levels and valence band spectra. Different photon energies have been used, in order to probe different sample depths. Only minor differences have been observed in the experimental data on the two different phases, except for a small charge effect on the beta phase crystal. First-principles calculations have been performed using the density functional for molecular and three-dimensional periodic solids (Dmol(3)) code on both the single CuPc molecule and the beta phase, allowing the identification of the different atomic and angular contributions to the experimental density of states. In particular, the highest occupied molecular level is mainly due to Cu and N states. The comparison between theoretical data obtained for the CuPc in the beta phase and in the single molecule shows that the interchain interaction between the molecules is negligible, whereas slightly stronger intrachain interactions occur.     

Development of molecularly imprinted polymeric nanofibers by electrospinning and applications to pesticide adsorption

Novel polystyrene‐based molecularly imprinted polymer nanofibers were synthesized through the electrospinning technique. The molecularly imprinted polymers were prepared using a non‐covalent approach and atrazine as template. For comparison, nonimprinted polymer nanofibers were also synthesized. The morphology of the synthesized nanofibers was characterized using scanning electron microscopy. The adsorption of pesticides, atrazine, atrazine desisopropyl, atraton, carboxin, linuron, and chlorpyrifos was studied under equilibrium (batch) conditions. To describe the adsorption capability of the synthesized polymers, Langmuir and Freundlich models were used. The Freundlich model provided a better mathematical approximation of the sorption characteristic for polymers nanofibers. To evaluate the adsorption capacity in the presence of interferents experiments on river water samples spiked with a mixture of six pesticides were also performed. The results obtained for the highest concentration levels investigated, show a greater amount of pesticide adsorbed on molecularly imprinted polymers and non‐imprinted polymers compared to those obtained using commercial stationary phases used as reference.     

Au∕CuPc interface: a valence band photoemission investigation

The formation of the metal-organic interface depositing gold, by thermal evaporation in ultrahigh vacuum conditions, onto a copper phthalocyanine film has been studied by means of ultraviolet photoemission spectroscopy. The evolution of the valence band spectra and in particular of the highest occupied molecular orbitals (HOMO) and of the Au upper states has been followed as a function of gold film thickness. The HOMO peak position is strongly influenced by the Au deposition, suggesting an n-doping process of the organic film. The gold 6s state behavior indicates the formation of small metal clusters whose size increases as a function of the gold deposited. The doping of the organic film determines a strong reduction of the film ionization energy.