Oleg A. Raitman

Electrical contacting of glucose dehydrogenase by the reconstitution of a pyrroloquinoline quinone-functionalized polyaniline film associated with an Au-electrode: an in situ electrochemical SPR study

A novel method to generate an integrated electrically contacted glucose dehydrogenase electrode by the surface reconstitution of the apo-enzyme on a pyrroloquinoline quinone (PQQ)-modified polyaniline is described. In situ electrochemical surface plasmon resonance (SPR) is used to characterize the bioelectrocatalytic functions of the system.

Supramolecular control of photochemical and electrochemical properties of two oligothiophene derivatives at the air/water interface.

Two geometric isomers of oligothiophene derivatives containing two crowned styryl fragments in 2- or 3-positions of thiophene rings are able to form stable monolayers on the water subphase. The organizing of crown-containing oligothiophenes in monolayers is guided by the π-stacking interaction of hydrophobic styrylthiophene fragments and interaction of hydrophilic macrocycles with the water subphase. The difference in structure of oligothiophene molecules leads to the formation of distinct monolayer architectures with various electrochemical and optical characteristics.

Orientation-induced redox transformations in Langmuir monolayers of double-decker cerium bis[tetra-(15-crown-5)-phthalocyaninate] and multistability of its Langmuir-Blodgett films

The spectral, electrochemical, and optical properties of Langmuir-Blodgett films (LBFs) and cast films from a solution of new double-decker cerium bis[tetra-(15-crown-5)-phthalocyaninate] (Ce(R4Pc)2) are studied. Based on analysis of compression isotherms and quantum-chemical calculations, schemes of the organization of Ce(R4Pc)2 molecules at different states of its monolayers are proposed. Correlation dependences are determined in order to relate the optical and electrochemical characteristics of monolayers and LBFs of sandwich-type lanthanide phthalocyaninates to the ionic radii of their metal centers. The valent state of Ce ions in a monolayer-forming complex is determined, and a sequence of redox transformations occurring in LBF uppon appliance of a potential is proposed, one of the transformations being associated with the Ce3+/Ce4+ redox transition. Orientation-induced intramolecular electron transfer is revealed in the planar supramolecular system. It is shown that, during the formation of a monolayer from a Ce(R4Pc)2 solution, a tetravalent metal center passes to a trivalent state. Monolayer compression to a high surface pressure reverts the complex to the electronic state typical of the solution. The reversible transformations observed upon the monolayer compression result from intramolecular electron transfer from the 4f-orbital of Ce to the phthalocyanine ring and backwards. The high operation rate and the reversibility of switching between the stable states, which are determined by means of the surface plasmon resonance technique, upon a stepwise change in the electrode potential within the range of 200–850 mV may underlie the development of optoelectronic systems. With a large number of molecules in a stacking aggregate, changes in the distance between the decks of the complex that occur with changes in the oxidation level of the metal center can substantially modulate the sizes of molecular ensembles. A supramolecular device capable of performing mechanical work can be developed based on this effect.

Control of photochemical properties of monolayers and Langmuir-Blodgett films of amphiphilic chromoionophores

Control of the structure of ultrathin films that determine the steric conditions of the passing of intermolecular interactions is one of the most promising methods of implementing the advantages of planar supramolecular systems as basic elements of nanosize information devices. This work studies the behavior of Langmuir monolayers of a new amphiphilic crown-substituted chromoionophore I on a water subphase. It is found that dithiaazacrown ether in molecule I selectively binds Hg2+ cations both in organic solvents and from aqueous subphase. The electronic absorption spectra of the monolayer showed that H-aggregation occurs actively in the course of two-dimensional compression on deionized water, which hinders the complexation process, while the presence of Na+ and Ba2+ cations in the subphase results in the effective inhibition of this aggregation. This conclusion is confirmed by spectral fluorimetric studies of monolayers of a dye with a similar structure that contains a chromophoric group with a much higher fluorescence quantum yield. Monolayer aggregation on deionized water at the surface pressure values of just 4–6 mN/m leads to the three- to fourfold fluorescence quenching, while in the case of subphases containing inert (noncomplementary to the dye ionophoric fragment) cations, the compression of the monolayer to pressures of 25–30 mN/m reduces the fluorescence intensity by no more than 25–35%. It was thus found that variations in the subphase composition allows one to monitor the degree of aggregation of the monolayer and the efficiency of cation recognition.

Layer-by-layer assembly of porphyrin-based metal–organic frameworks on solids decorated with graphene oxide

Fabrication of metal–organic frameworks (MOFs) as films on surfaces is a challenging and actively developed field in the context of potential applications of surface-integrated MOFs (SURMOFs) in sensing, catalysis and molecular electronics. Herein, we report a method for the preparation of composites of graphene oxide (GO) and zinc tetracarboxyphenyl porphyrin SURMOFs, which are potentially useful as synergetic catalysts on planar solid supports. The SURMOF films were assembled via layer-by-layer coordination of zinc porphyrins with ZnAc2 clusters on the ultrathin surface coatings of GO. We found that non-covalent adsorption of zinc acetate on the GO films is essential for anchoring porphyrin molecules in the growth-initiating template layer. UV-vis spectroscopy provided evidence for a stepwise assembly of the deposited SURMOF films. By using a combination of X-ray diffraction, scanning electron microscopy and atomic force microscopy methods, we show that the resulting GO-associated SURMOFs exhibit a leaflet-like planar morphology with out-of-plane periodicity. Because of the influence of the well-defined surface geometry, the molecular organization in thin SURMOFs is different from that of synthetic bulk materials with a similar composition. The results of BET method measurements suggest that the fabricated SURMOF films have exceptionally large surface areas and adsorption capacities. Our results provide a basis for new types of composite SURMOF structures using GO surface coatings as growth-guiding scaffolds for metal–organic coordination networks integrated with various solid supports.

Electrochemically controlled multistability of ultrathin films of double-decker cerium phthalocyaninates

The spectral and electrochemical properties of Langmuir-Blodgett (LB) films of homoleptic double-decker cerium bis-[tetra-15-crown-5]-phthalocyaninate Ce[R4Pc]2 and its heteroleptic analog [Pc]Ce[R4Pc] were studied. It was shown that during formation of Langmuir monolayers upon contact of both complexes solutions with water subphase the metal center with valent state IV in the solution transforms into one with the valent state III in the monolayer. Upon cyclic compression-expansion of these monolayers, orientation-induced reversible intramolecular transfers of electron from 4f orbital of cerium ion to the phthalocyanine macrocycle (compression) and in reverse direction (expansion) occur in a planar supramolecular system. Scheme of reversible electrochemical transformations occurring in ultrathin films of double-decker cerium crown-phthalocyaninates, one of which is associated with the Ce+3/Ce+4 redox-transition of metal center of the complex, was proposed and substantiated using the results of spectro-electrochemical investigations. It was shown that one of these transformations is associated with the Ce+3/Ce+4 redox transition of metal center of the complex and upon change of metal center oxidation state its size changes, and, consequently, the distance between the decks of the complex also changes. This change in the distance can lead to a substantial (13–15%) modulation of the linear size of molecular assemblies with a large number of molecules in stack. On the basis of this effect the supramolecular device that can perform mechanical work can be developed. Using surface plasmon resonance technique we have demonstrated that a step change in electrode potential in region of 200–1000 mV induces a corresponding optical response reflected in a step change of the resonance angle. High operation speed and reversibility of switching between stable states can serve as a basis for development of optoelectronic switching systems.

Physicochemical properties of solutions and ultrathin films of triple-decker gadolinium tetra-15-crown-5-phthalocyaninate

The ability of the triple-decker gadolinium complex with tetra-15-crown-5-phthalocyanine Gd2(R4Pc)3, (R = 15-crown-5) (1) to form monolayers and Langmuir-Blodgett films (LBFs) was studied for the first time. The charge characteristics of molecules of a triple-decker phthalocyaninate in monolayer, as well as their orientation and adhesion to the water subphase, were controlled by changing the surface pressure, pH, and subphase composition (aqueous solutions containing triethylenetetramine (TETA) and metal cations). It was shown that the presence of Na+ and protonated TETA aminogroups in resulted in an increasing limiting monolayer area and significant decreasing of the monolayer liquid state region. It is proven that the observed effects are caused by the conformational and charge transitions of peripheral crown ethers induced by their interaction with cations. A comparison of the differential reflectance spectra of the complex monolayer on the deionized water surface with UV-Vis absorption spectra of three-layer Gd2(R4Pc)3 LBF and complex solution in chloroform shows that partial complex oxidation and intensive stacking formation occurs already at the stage of monolayer formation on the subphase surface. Electrochemical studies of three-layer LBFs performed at the indium-tin-oxide electrodes (ITO-electrodes) using cyclic voltammetry (CV) showed three reversible redox waves in the potential range −200 to +1100 mV (vs. Ag+/AgCl). All registered peaks remain the same position and intensity upon multiple cycling. plasmon resonance (SPR) measurements allow to register three stable redox states of studied LBF upon applying the external potential. Such behavior shows the possibility to use multistep redox transformations of studied complex LBF for developing of stable and reproducible switchable optoelectronic systems.

Photonic transduction of electrochemically-triggered redox-functions of polyaniline films using surface plasmon resonance spectroscopy

Surface plasmon resonance (SPR) spectroscopy is used to follow the swelling and shrinking processes of a polyaniline redox-active polymer; SPR provides a reading signal for the electrochemical stimuli that activate the polymer.

Conformational tuning of sensing Langmuir-Blodgett membranes for selective determination of metal ions, anions, and molecular fragments

In this work, we describe a novel approach to prepare ionophore-based Langmuir-Blodgett (LB) sensors with desirable analytical characteristics via fine tuning of macrocycle conformation during the assembly of the membrane. In this context, two related types of LB sensors of dicetyl cyclen and its complex with Zn/sup 2+/ were constructed. The multi-use mass-sensitive and electrochemical LB-multilayer sensors of DCC were used for selective and accurate detection of divalent copper traces (down to 10/sup -9/M) in aqueous solution, containing other similar metal ions (Zn/sup 2+/ and Ni/sup 2+/). The SPR-based imide recognition modeled with uracil binding and the phosphate recognition were investigated with SAM-supported LB monolayers of zinc(II)-dycetyl cyclen complexes. Controlled steric constraints imposed on Zn(II)-DCC structure upon membrane formation resulted in exceptional selectivity toward inorganic HPO/sub 4//sup 2-/ (at least down to 10/sup -8/M of anions).

Langmuir-Blodgett composite films for the selective determination of calcium in aqueous solutions

A sensitive membrane of a new type was suggested for the determination of calcium ions in aqueous solutions. Polylayer membranes were formed by the Langmuir-Blodgett method from octadecylamine and 1,2-bis-(2-aminophenoxy)ethane-N,N,N,N-tetraacetic acid. When brought in contact with an aqueous solution of a calcium salt, the ultrathin film sorbs metal ions. The chelator then transports metal ions over the whole membrane volume, and the ions are accumulated in the form of calcium hydroxide crystallites in regions enriched in the diphilic base. The main analytic parameters were determined by piezoquartz weighing, electrochemical impedance spectroscopy, and surface plasmon resonance spectroscopy. The response of sensor elements of this type was shown to be a linear function of the logarithm of the concentration of calcium ions over the concentration range 10−8–10−1 mol/l at the contact time between the film and the solution under study no more than 10 s. The conclusion was drawn that the membranes obtained remained highly sensitive with respect to calcium ions in 0.1 M solutions of NaCl and were selective with respect to other biologically relevant cations.