Alexei Nabok

Study of adsorption of some organic molecules in calix[4]resorcinolarene LB films by surface plasmon resonance

Adsorption of some organic guest molecules in tetra-phosphorylated calix[4]-resorcinolarene LB films was studied by surface plasmon resonance (SPR) techniques. Two methods of registration have been used: measurement of the whole SPR curve at steady state of adsorption and in situ registration of SPR signal at fixed angle during adsorption. Fast and reversible adsorption of toluene and different hydrocarbon vapours was observed. Increasing both film thickness and refractive index implied the swelling behaviour of the LB film during adsorption and can be explained by the capturing and subsequent condensation of guest molecules in the calixarene matrix. Selectivity of SPR response to various hydrocarbons can be attributed to the difference of their condensation temperatures rather than to molecular dimensions.

Detection of β-amyloid peptide (1–16) and amyloid precursor protein (APP770) using spectroscopic ellipsometry and QCM techniques: A step forward towards Alzheimers disease diagnostics

A highly sensitive method of spectroscopic ellipsometry in total internal reflection mode (TIRE) was exploited for detecting β-amyloid peptide (Aβ(1-16)) in the direct immune reaction with monoclonal DE2 antibodies (raised against Aβ(1-16)) electrostatically immobilised on the surface of gold. A rapid detection of Aβ(1-16) in a wide range of concentrations from 5 μg/ml down to 0.05 ng/ml was achieved using a cost-effective and label-free direct immunoassay format. TIRE dynamic spectral measurements proved that the immune reaction between DE2 monoclonal antibodies and Aβ(1-16) is highly specific with the affinity constant K(D)=1.46×10(-8) mol/l. The same DE2 antibodies were utilised for detection of amyloid precursor protein APP(770), a larger protein containing Aβ(1-16) domain, using the quartz crystal microbalance (QCM) measurements in liquid. A combination of QCM and TIRE kinetics results allowed the evaluation of the originally unknown concentration of APP(770) in complete medium solution containing other proteins, salts, and amino acids.

Structural and electrical studies on sol-gel derived spun TiO2 thin films

Titanium dioxide thin films were prepared by spin coating of sol precursor onto microscopic glass slides, silicon and indium tin oxide (ITO) coated glass substrates. Spin speed was varied between 1000 and 6000 rpm. From the morphological analysis, it is found that thin films spun with speed ω≤4000 rpm assume higher ordered structure than those spun at a speed higher than 4000 rpm. Conduction at low voltages is believed to be variable range hopping at temperatures T<220 K while the ionization of donors situated at Ea = 0.46 eV below the conduction band edge becomes dominant at temperatures higher than 220 K. At high field charge transport is due to trap-controlled space charge limited mechanism. Traps with a density Nt≈1×1022 m−3 are thought to be situated at energy level Et = 0.3 eV below the conduction band and are associated with film nonstoichiometry and interface states.

Ellipsometric study of the adsorption of bovine serum albumin into porous silicon

The method of spectroscopic ellipsometry has been applied to study in situ the adsorption of bovine serum albumin (BSA). The porosity and amount of adsorbed BSA were determined by fitting the ellipsometric data to the Bruggeman effective medium approximation model. The presence of intermediate adsorbed layers of polyelectrolytes was found to increase protein adsorption.

Polyelectrolyte self-assembled thin films containing cyclo-tetrachromotropylene for chemical and bio-sensing

Abstract Thin composite films containing cyclo-tetrachromotropylene (Chromo1) were deposited onto glass slides by means of polyelectrolyte self-assembly technique. The films obtained were characterised by UV–visible spectral measurements, and a linear dependence of absorbance on the number of deposited layers has been found. It was also found that the shape of UV–visible spectra depends on the presence of ammonia in both gaseous and liquid phases. Other amines, such as n-butylamine, can also affect the spectra. An attempt of urease immobilisation has been done using polyelectrolyte self-assembly. Composite films containing both Chromo1 and urease demonstrated the possibility of optical registration of enzyme reactions, in particular, decomposition of urea which releases ammonia. After immersion into urea solution, the samples show spectral transformations similar to those observed in the presence of ammonia.

Size-quantization in extremely small CdS clusters formed in calixarene LB films

CdS nanoparticles have been formed within Y-type Langmuir-Blodgett (LB) films of cadmium salts of calix(8)- and calix(4)-arene by reaction with H2S. UV-vis absorption spectra of the LB films, measured at room temperature, show well-resolved transitions between size-quantization levels in CdS clusters. The size of the CdS particles, obtained by Gaussian fitting of the experimental spectra, is 1.5 +/- 0.3 nm, which is much less than those reported for fatty acid LB films. The particle size does net depend either on the type of calixarene or the number of LB layers. LB films were also characterised by X-ray diffraction and ellipsometry which show the film thickness do not change substantially after treatment with H2S. The mechanism of CdS nanoparticles formation is discussed

Complexation of metal ions with Langmuir-Blodgett films of novel calixarene azo-derivative

Abstract The complexation properties of Langmuir–Blodgett (LB) films of an amphiphilic calix(4)resorcinarene substituted by four arylazo groups with some heavy metal ions were studied using UV-vis spectral measurements. It was shown that soaking samples in metal salt solutions leads to irreversible decrease of absorbance at 460 nm. Efficiency of complexation is found to depend on both metal cations and anions. Effects of film thickness, solution concentration and time of exposure have also been studied. It was also shown that the addition of a small amount of aromatic compounds such as aniline, leads to a further decrease in the absorbance of the films. The presence of metal salts in the film was directly confirmed by XPS. The mechanism of ion complexation is discussed.

Transport mechanisms in porous silicon

The current transport mechanism through porous silicon (PS) films fabricated from 8 to 12 Ω cm p-type silicon (p-Si) substrates has been investigated using current–voltage I(V) measurements on metal/PS/p-Si/metal devices in the temperature range of 77–300 K. The characteristics for all devices show a rectifying behavior with ideality factor very close to unity. A value of 0.7 eV is obtained for the barrier height at the interface between PS and bulk p-Si at room temperature and the barrier height is found to increase with rising temperature. A band model is proposed in order to explain the observed characteristics.

Further optical studies on Langmuir-Blodgett films of octa-substituted metal-free phthalocyanines

Abstract Ellipsometry and optical spectroscopy in the ultraviolet-visible (UV-vis.) and infra-red ranges were used to study the correlation between structure and optical properties of Langmuir-Blodgett (LB) films of octa-substituted metal-free phthalocyanines containing different alkyl chains. The thickness and refractive index of LB films and the orientation of phthalocyanine moieties in the monolayer were obtained from ellipsometric measurements and then data were compared with those from Π A isotherms of the floating layer. UV-vis. absorption spectra of LB films of the molecules show Davydov splitting of the Q band. Small changes in alkyl length lead to significant changes in the Q-band shape. The optical characteristics of monolayers are found to be different from those of multilayer films.

Doping-Induced Conductivity Transitions in Molecular Layers of Polyaniline: Optical Studies of Electronic State Changes

The doping-induced conductivity transitions in molecular layers of polyaniline have been studied by monitoring the correlated optical and spectroscopic changes using spectroscopic and single wavelength extinction ellipsometry, also in total internal reflection mode (TIRE), together with reflection spectrometry. The measurements were performed on deposited multilayers as well as on a Langmuir monolayer at the air-water interface, as a function of acidic doping. We found that the characteristic spectroscopic features of conducting and insulating polyaniline persisted down to the single layer, both in the solid state and at the air-water interface. We also investigated in real time the modulation of conductivity induced by the intercalation of Li ions in the polyaniline film, by a combination of time-resolved ellipsometry and reflectivity spectra measurements. In this case, the enhanced sensitivity provided by the TIRE geometry, combined with the relatively fast time scale accessible by the single wavelength ellipsometry, allowed us to follow in detail in real time the doping/dedoping process.