Alvina V. Alexandrova’s research while affiliated with Frumkin Institute of Physical Chemistry and Electrochemistry and other places

Using X-ray techniques, crown-substituted chromoionophore Langmuir–Blodgett films were proved to preserve the fine structure and functional features of pre-organized monolayers.

In the present work, we investigate, by means of in situ UV-Vis reflection-absorption spectroscopy and X-ray reflectivity measurements, the effect of the mercury ion analyte on the supramolecular structure of the dithia-aza-crown-hemicyanine chromoionophore Langmuir monolayers upon their interaction at the air/water interface. It was revealed that while Hg2+ ions are not able to form complexes with ionophore crown ether groups and do not perturb the organization of the monolayer at low analyte concentrations, high enough concentrations lead to the change of its structure. Supramolecular architecture of the monolayer attains a type identical to the one observed in the case of analyte-binding by the barium-preorganized monolayer. Presented study brings further insight into this preorganization phenomenon.

Designing sensors for toxic compounds such as mercury salts in aqueous solutions still remains one of the most pressing tasks of modern chemical research, since many existing systems do not show enough sensitivity and/or response. In this regard, the opportunities offered by supramolecular approaches can be used to improve both these characteristics by creating a new self-organized smart system. Herein, we show that barium cations, that according to the data of X-ray standing waves do not bind directly to the ionophore molecules in the monolayers at the air/water interface, could be used to efficiently preorganize such molecules to achieve supramolecular architecture. We demonstrate that such preorganization ensures both low analyte detection threshold and high fluorescent response. We reveal the interrelation of the monolayer structure and receptor characteristics of a sensory system and show that such cation-induced preorganization in Langmuir monolayers of a hemicyanine dithia-aza-crown-substituted chromoionophore inhibits the formation of non-fluorescent aggregates with low receptor function, and allows the quantitative detection of mercury cations using a ratiometric fluorometric approach.

An ultrathin film, wherein a resonance energy transfer (FRET) from the energy donor monolayer to the analyte sensitive acceptor monolayer is controlled by definable cation binding, was produced, thus proving the feasible concept of non-covalently linked FRET couple based sensors.

This study takes a novel approach to the enhancement of receptor properties of thin film sensors based on hemicyanine dyes with dithia-aza-crown- ionophoric moiety. By means of in situ UV-Vis and XRR measurements it was revealed that the introduction of up to 0.25 mmol of Hg2+ under a preliminarily compressed monolayer, formed on pure water does not lead to cation binding. This is due to the formation of "head-to-tail" aggregates (H-type), in which ionophoric group is blocked by the neighboring molecule. However, the presence of barium cations in the subphase under the forming Langmuir monolayer of the mentioned compound, causes co-directional (head-to-head) orientation of chromoionophore fragments. This provides pre-organization of a monolayer structure that facilitates the binding of complementary mercury cations, even in a compressed state: asymmetric sandwich complexes containing two dye molecules coordinate a Hg2+ cation between them. This complex structure was confirmed by molecular modeling based on the electron density distribution calculated from XRR measurement data. Such preorganization of supramolecular ensembles induced by cations, that do not participate in the complex formation with macroheterocyclic receptors, may have applications in fields where strict control of molecular orientation at the interface is required, such as nanoelectronics, sensorics, catalysis, etc.

Planar supramolecular systems based on Langmuir monolayers of chromoionophores are widely used for sensor applications, and crown ether ionophoric groups are intensively studied. However, almost no attention is devoted to investigation of effect of alkyl moieties onto receptor properties of such systems. In the present work, we carry out a comparative study of Langmuir monolayers of three alkylated dithiaaza-crown substituted hemicyanine chromoionophores differing only in number of carbon atoms (12, 16, 21) in the hydrophobic alkyl substituent. A dependence of receptor properties of the studied monolayers on the length of alkyl chain is revealed. It is shown that the efficiency of mercury cations binding by both liquid-expanded and compressed monolayer of dithiaaza-crown substituted hemicyanine dyes formed on pure water subphase is identical for all three compounds with various length of alkyl chain substituent due to the key role of the interactions between chromoionophore fragments of neighboring molecules at the interface. Degree of system sensitivity enhancement due to barium cation-induced preorganization increases with increase of number of carbon atoms in the alkyl chain.