V.I. Troitsky

Pattern recognition approach to the study of the interactions between metalloporphyrin Langmuir–Blodgett films and volatile organic compounds

Abstract In recent years metalloporphyrins and their derivatives have been proposed as sensitive layer for mass variation based transducers for the detection of volatile compounds. Their sensitivity/selectivity properties have suggested their application in electronic nose systems. Despite the positive applications appeared so far, a systematic study of the kind of interactions ruling the sensing mechanism has not yet been conducted. The selectivity towards a variety of compounds suggests the presence of weak Van der Waals or hydrogen bond interactions, but also certain evidences about coordination interactions with the central metal ion coordinated to the porphyrin were reported. In this paper, a study oriented towards a deeper comprehension of such mechanisms is introduced. From the experimental point of view, a set of quartz microbalances, coated with Langmuir–Blodgett films of various metalloporphyrins, were exposed to a number of volatile compounds at various concentrations. Linear sorption energy relationship theory provides, in many cases, a satisfactory model for sorption sensors in which Van der Waals and hydrogen bonding are responsible for the interaction. Nevertheless, the application of such a model results inadequate in the case here discussed. A more satisfactory treatment of the data was obtained considering a pattern recognition approach based, in particular, on a self-organizing neural network. This type of analysis has shown the role played by the central metal in determining the sensor selectivity; furthermore, the results have allowed to hypotize the presence of a coordination mechanism, superimposed to the weak interactions, between volatile compounds and metalloporphyrin films.

Deposition of uniform conductive polyaniline films and approach for their patterning

Abstract Conductive polyaniline Langmuir–Blodgett (LB) films are considered to be candidates for numerous applications. However, the quality of their deposition is usually non-satisfactory. In addition, the possibilities of many applications are restricted by the intrinsic low speed of deposition with the LB technique. We have found a solution to these problems and obtained uniform ultra thin polyaniline films using the Langmuir–Schaefer (LS) technique, which allows very fast monolayer transfer onto the substrate. A well known approach of deposition of mixed monolayers was used to obtain uniform films, but in our case the matrix compound was chosen in such a way as to provide a possibility of its selective dissolution from the deposited multilayers without the deterioration of uniformity of remaining polyaniline layer. Finally, we demonstrate an approach to polyaniline film patterning with an electron beam, which is based on the possibility to cross-link the polyaniline molecules without considerable loss of film conductivity and on further dissolution of polyaniline from non-irradiated areas.

Langmuir–Blodgett assemblies with patterned conductive polyaniline layers

Abstract Polyaniline (PANI) is used for the preparation of “molecular” electrodes in heterojunctions, which include either Langmuir–Blodgett (LB) films or LB assemblies composed of thin layers of different compounds. Langmuir–Schaefer (LS) technique is applied to provide high speed of film deposition. The uniformity of polyaniline films is achieved due to the deposition of mixed polyaniline/polycyanoacrylate films with further dissolution of polycyanoacrylate fraction. The required configuration of the heterojunction is produced by irradiating polyaniline films with an electron beam through the masks and pattern development in 1-methyl-2-pyrrolidone (NMP). Current–voltage characteristics of heterojunctions are measured to demonstrate the applicability of developed approach.

Conducting Langmuir-Blodgett films of hexadecyl-BEDT-TTF charger-transfer salts with inorganic compounds

Abstract A method for forming conducting Langmuir-Blodgett films of charge-transfer salts based on the interaction between surfactant donor molecules spread at the air/water interface and inorganic compounds dissolved in water is proposed. Films of hexadecylbis(ethylenedithio)tetrathiafulvalene with small additions of surfactant acceptor molecules were deposited from water subphases containing FeCl 3 or CuSo 4 . High quality films with conductivity of 2 Ω −1 cm −1 were deposited at pH of 3.8 when Fe 3+ ions at a concentration of 10 −4 M were used. In order to study the interaction of donor molecules with the compounds dissolved in water, electron probe analysis of the films was carried out. Variants of charge-transfer salt formation are discussed.

Langmuir-Blodgett films of bipolar lipids from thermophilic archaea

The Langmuir-Blodgett technique is applied for depositing thin films of two lipids extracted from the thermophilic archaeon Sulfolobus Solfataricus as well as of a derivative produced by hydrolysis of total lipid extract. These are monosubstituted glycerol-dialkyl-nonitol tetraether with a β-d-glucopyranosyl group (GL-GDNT), bisubstituted glycerol-dialkyl-nonitol tetraether with β-d-glucopyranosyl and phosphomyoinositol groups (GL-GDNT-PH) and asymmetric non-substituted glycerol-dialkyl-nonitol tetraether (GDNT). Films with satisfactory morphology are deposited by both horizontal and vertical lift techniques, but the results are not reproducible. To improve the quality of deposition and the uniformity of the film, mixtures of the lipid GL-GDNT and arachidic acid are used. After dissolution of arachidic acid from the multilayer by hexane highly uniform films are obtained. Studies of the monolayers at the air-water interface and of the deposited films show that probably the molecules in the monolayer are in a form of a U with close packing of isopranyl chains.

Thermal, Stability of Bipolar Lipid Langmuir Blodgett Films by X-Ray Diffraction

Abstract The thermal stability and structure of two uniform Langmuir - Blodgett films made of bipolar lipids from archaea have been studied. X-ray diffraction measurements at various temperatures from 20°C to 100°C were performed on these films, during heating and cooling cycles. The Bragg peak position, the integrated intensity and the peak width have been recorded as a function of temperature. Both intensity of diffraction peaks and layer thickness diminish at high temperatures while the correlation length, normal to the multilayer plane, displays some significant changes during the heating cycle. The data suggest the occurrence of some phase transitions. As a final result, the thermal stability of these two bipolar lipids LB-films is quite different and very high for the Phospho 2 LB film, which is stable till 100°C and is very promising for technological applications.

Kelvin probe investigation of the thickness effects in Langmuir–Blodgett films of pyrrolic macrocycles sensitive to volatile compounds in gas phase

Abstract Langmuir–Blodgett (LB) films are utilized as sensing materials for many different chemical sensors. This technique allows the fabrication of very thin films, down to one single monolayer. As is well known from classical material science, there is a minimum number of elementary blocks (atoms or molecules) that have to be aggregated before the material properties can be assessed. To this regard, molecular films tend to behave in a different way; since the basic block is a complex molecule, most of the properties are found in the single molecule. This opens the possibility of obtaining materials for which the surface to volume ratio can be maximized in order to increase, for instance, the sensor sensitivity. In this paper, some aspects related to the electronic structure of LB films of porphyrinoids are studied by the Kelvin probe technique as a function of the number of monolayers. Results show that the material properties are, in the case of corrole, confined to the single molecule, while in the case of sapphyrin, a build-up process seems to take place, so that many monolayers are necessary to establish the bulk material properties.

X-ray diffraction structural analysis of Langmuir-Blodgett films using a pattern recognition approach

Abstract A new approach for the analysis of X-ray diffraction patterns to obtain the structure of Langmuir-Blodgett (LB) films is reported. In particular, to solve the “phase problem” we apply a direct method, essentially a pattern recognition approach, which was recently proposed to determine without assumptions the structure of liquid-crystalline phases. This approach is used to analyse two different types of samples, namely LB films of fatty acid salts and of bipolar lipids extracted from thermoacidophilic archaeon Sulfolobus solfataricus . As a result, the electron density profiles allow us to fully characterize the molecular arrangement in both the investigated films.

An approach for fabrication of junctions with Langmuir–Blodgett films incorporated between molecular electrodes

Abstract Langmuir–Blodgett (LB) assemblies composed of monolayers of different compounds alternating in a predetermined sequence are potential candidates for development in the field of nanoelectronics. However, their poor mechanical stability restricts the possibility of fabrication of interconnected electronic elements because LB films can be easily destroyed during the deposition of metal electrodes. In this work, we present a technique for the fabrication of junctions for the electrical characterization of one-component LB films and LB assemblies enclosed between thin molecular electrodes. The molecular electrodes are produced using LB films of a charge-transfer salt. Such an application of these films became possible due to considerable improvements in their conductivity, quality of deposition and stability in comparison with the properties of films of similar compositions studied previously. Lateral patterning of electrodes is carried out by the irradiation of conductive films with an electron beam, which results in the loss of their conductivity. In this way, it is possible to prepare the samples for electrical investigations without any damage to incorporated LB films. The LB films of barium stearate, polycyanoacrylate, mixture of surfactant donor and acceptor, porphyrins, as well as the LB assemblies composed of thin layers of these compounds deposited in various sequences were enclosed between the molecular electrodes and studied. Probable conduction mechanisms are discussed on the basis of obtained data.

Surface potential study of selective interaction of potassium ions with valinomycin in Langmuir-Blodgett films

Abstract Langmuir-Blodgett films composed of valinomycin bilayers and protective bilayers of barium salt of disuccinilated glycerol-dialkylglycerol tetraether, which have been produced from lipid extract of thermophilic archaeon Sulfolobus solfataricus , are deposited and studied by the surface potential technique. The formation of K + -valinomycin complexes after immersion of the samples in solutions containing potassium ions is proved. Interaction of valinomycin molecules with sodium ions is absent. The lower limit of the surface density of complexes formed in the film via selective interaction is evaluated. The efficiency of the system is at least 6%. The development of sensing elements for potassium ions on the basis of such films seems to be possible.