Motion of redox molecules in solution monitored by the highly-sensitive EQCM technique

The behavior of redox molecules in solution that was not detected by electrochemical techniques was measured by a highly-sensitive electrochemical quartz crystal microbalance (EQCM) technique that has been improved in this study to obtain a high sensitivity of EQCM measurement in solution. The improved EQCM technique allowed to monitor the motion of a redox molecule, that is an access of the molecule to an electrode surface and repulsion from the surface during redox. An EQCM technique currently in use has measured adsorption of redox molecules on an electrode surface or polymerization on the surface caused by a chemical reaction following redox, which exhibits an enough large mass change response to detect with an EQCM measurement. However, access and repulsion of redox molecule, which is a slight motion of the molecule near on electrode surface, has not been detected and investigated by an EQCM technique, because the mass change response seems to be very small. In this study, the redox behavior of methyl viologen on a bare gold surface, pyridinethiol surface and methylpyridinethiol surface was investigated. Although the three electrodes give the same cyclic voltammogram of methyl viologen, the three are different in QCM response recorded at the same time as the voltammetry. Access/repulsion of methyl viologen within an electrical double layer was monitored by the highly-sensitive EQCM technique.     

Gravimetric measurement in redox polymer electrodes with the EQCM beyond the Sauerbrey limit

Gravimetric measurements during electrochemical redox switching of an Os-containing hydrogel film in contact with aqueous electrolyte have been obtained beyond the Sauerbrey rigid mass limit of the electrochemical quartz crystal microbalance (EQCM). For an acoustically non-rigid film the frequency-to-mass Sauerbrey relationship is not valid and the mass has been obtained from the experimental electroacoustic impedance data collected simultaneously with electrochemical experiments interpreted using Martin's viscoelastic model.     

Motion of large DNA molecules traveling from solution to gel under steady field

Direct observation of individual T4 DNA molecules at the interface between gel and pure solvent under steady electric field was carried out with fluorescence microscopy. Statistical analyses of the image data show that (1) when a DNA arrives at the gel surface, it remains there for a short time before the DNA stretches its arms to enter the gel, and (2) the time spent on the gel surface increases with an increase of the agarose percentage in the gel. © 1996 John Wiley & Sons, Inc.     

Studying the effects of bismuth on the electrochemical properties of lead dioxide layers by using the in situ EQCM technique

The charge-discharge characteristics and the aging mechanism of PbO₂ layers doped with bismuth in contact with sulfuric acid solutions were studied by using combined cyclic voltammetry and electrochemical quartz crystal microbalance (EQCM) techniques. For this purpose, thick lead dioxide layers (non-doped and doped with Bi) were electrodeposited on gold substrate from aqueous solutions of Pb(NO₃)₂ dissolved in nitric acid and they were investigated in sulfuric acid media. Based on the electrochemical and the mass change responses, it is concluded that during the electrodeposition, bismuth influences the structure of the PbO₂ formed. Bi(III) also inhibits the oxidation of lead sulfate and affects the reduction kinetics of lead dioxide. During successive cyclization (aging), the presence of bismuth accelerates the hydration of PbO₂.

     

Investigation of the electrochemical behaviour of lead dioxide in aqueous sulfuric acid solutions by using the in situ EQCM technique

The charge-discharge characteristics and the aging mechanism of PbO₂ layers in contact with sulfuric acid solutions of different concentrations (1.5–5.0 M) were studied by using combined cyclic voltammetry and electrochemical quartz crystal microbalance (EQCM) techniques. For this purpose, thick lead dioxide layers were electrodeposited on gold substrate from aqueous solutions of Pb(NO₃)₂ dissolved in nitric acid. Based on the electrochemical and the mass change responses, it was found that in more concentrated solutions of H₂SO₄, the main reduction reaction was the transformation of lead dioxide to lead sulfate. However, in less concentrated sulfuric acid media, the transformation of lead dioxide to lead(II) ion became the main reaction. These Pb²⁺ ions transformed into lead sulfate crystals later by a chemical reaction. Because the electrochemical oxidation of lead sulfate is less favourable in sulfuric acid medium of higher concentrations, thus, PbO₂ layers cannot be tested by continuous cyclization, which is necessary to study their aging parameters. Therefore, a delay step before each cyclic voltammogram was applied while the non-conductive lead sulfate dissolves or alternatively, by applying a pre-oxidation step prior to each cyclic voltammetry experiment to produce electrochemically significant amount of lead dioxide which can be reduced during the following negative potential sweep.

     

Permeability of micelles in surfactant-containing MCM-41 silica as monitored by embedded dye molecules

Mesoporosu (MCM-41 type) silica containing surfactant-embedded Congo Red has been prepared and tested against gas phase HCl and ammonia, as well as solutions of ionic species; it is shown that the hybrid (organic-inorganic) material is permeable to both gases and ionic species, and can act as a pH indicator and as a selective chelating agent.     

Determining the thermodynamic electrode potential in the presence of a mixed potential using EQCM technique

We give the theory explaining how to use Electrochemical Quartz Crystal Microbalance (EQCM) to determine the thermodynamic equilibrium electrode potential in a system where the open circuit potential is a mixed potential. The approach is applicable to electrodes of the first or the second kind in the presence of one or more soluble redox couples, at least one of which is irreversible. The key insight to be gained is that if the mass of an electrode of the first or the second kind does not change, its electrode potential is the thermodynamic equilibrium potential. This is true regardless of any other redox processes involving only soluble species that may be occurring at the electrode/solution interface. The model system used to test and confirm the theory was an electrode of the first kind, NiH_x,solid/Ni(II) undergoing active corrosion caused by hydrogen peroxide. The electrochemical quartz crystal microbalance method can be applied in fundamental and applied studies, e.g., of electroless deposition and corrosion systems containing multiple redox systems.     

Formation of 2D colloidal crystals by the Langmuir-Blodgett technique monitored in situ by Brewster angle microscopy

We report a method that combines Brewster angle microscopy and Langmuir-Blodgett films technique to obtain highly ordered 2D colloidal crystals of nanospheres. The deposition of Langmuir-Blodgett films of silica spheres monitored by Brewster angle microscopy allows to determine with accuracy the best physical conditions to transfer highly ordered monolayers of nanoparticles.     

Tensile tests of polypropylene monitored by SAXS. Comparing the stretch‐hold technique to the dynamic technique

Advanced instrumentation at synchrotron beamlines facilitates considerable reduction of the exposure required for the recording of low‐noise small‐angle X‐ray scattering (SAXS) patterns. Concerning the monitoring of tensile tests on polymer materials by SAXS means that change‐over from the stretch‐hold technique to more practical dynamic straining is possible, although the strain rate is still by a factor of 100 lower than that relevant in industry and service. As hard‐elastic polypropylene (PP) is tested, considerably different scattering patterns are recorded after switching to the dynamic technique. Even though during exposure the elongation is no longer constant when applying the dynamic technique, the images collected in stretch‐hold technique appear much more blurred. The observed immediate relaxation of nanostructure induced by stopping the extensometer is analyzed by means of the multidimensional chord distribution function (CDF): The extension of the crystalline lamellae is increasing. Their thicknesses are becoming non‐uniform. The range of order is shortening. Cross‐hatched lamellae are formed. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 721–726, 2008     

Thermally activated electron transport in single redox molecules

We have studied electron transport through single redox molecules, perylene tetracarboxylic diimides, covalently bound to two gold electrodes via different linker groups, as a function of electrochemical gate voltage and temperature in different solvents. The conductance of these molecules is sensitive to the linker groups because of different electronic coupling strengths between the molecules and electrodes. The current through each of the molecules can be controlled reversibly over 2-3 orders of magnitude with the gate and reaches a peak near the redox potential of the molecules. The similarity in the gate effect of these molecules indicates that they share the same transport mechanism. The temperature dependence measurement indicates that the electron transport is a thermally activated process. Both the gate effect and temperature dependence can be qualitatively described by a two-step sequential electron-transfer process.     

Adsorption properties of HKUST-1 toward hydrogen and other small molecules monitored by IR

Among microporous systems metal organic frameworks are considered promising materials for molecular adsorption. In this contribution infrared spectroscopy is successfully applied to highlight the positive role played by coordinatively unsaturated Cu2+ ions in HKUST-1, acting as specific interaction sites. A properly activated material, obtained after solvent removal, is characterized by a high fraction of coordinatively unsaturated Cu2+ ions acting as preferential adsorption sites that show specific activities towards some of the most common gaseous species (NO, CO2, CO, N2 and H2). From a temperature dependent IR study, it has been estimated that the H2 adsorption energy is as high as 10 kJ mol(-1). A very complex spectral evolution has been observed upon lowering the temperature. A further peculiarity of this material is the fact that it promotes ortho-para conversion of the adsorbed H2 species.     

Thermalization of photoexcited molecules in solution

Analysis of the steady-state absorption/fluorescence spectra of several laser dyes in room temperature solution suggests that the fluorescing molecules, if initially formed in a vibrationally excited state, lose their excess energy surprisingly slowly, remaining significantly warm on a nanosecond time scale. A similar analysis of the steady-state absorption/fluorescence spectra of GaAs confirms that in this case the carriers are fully thermalized, possessing no excess energy when they recombine on a microsecond time scale. A classical model which accounts qualitatively for slow molecular cooling is presented. We conclude that rapid molecular photoconversion processes are likely to involve incompletely cooled (thermalized) excited states.     

Magnetic focusing of redox molecules at ferromagnetic microelectrodes

The electrochemical behavior of Fe and Ni disk-shaped microelectrodes (25 to 250 μm radius) in a uniform magnetic field (1 T) is reported. Magnetization of the ferromagnetic microelectrode generates a magnetic field gradient across the depletion layer that can be used to focus paramagnetic molecules toward the electrode surface. The magnetic force acting on the depletion layer is given by F_∇B=(χ/μ)B·∇B, where χ and μ are the volume magnetic susceptibility and magnetic permeability of the depletion layer, respectively, and B is the magnetic field. Magnetic field focusing of the nitrobenzene radical anion at Fe and Ni microelectrodes is demonstrated.     

Single-molecule conductance of redox molecules in electrochemical scanning tunneling microscopy

Experimental data and theoretical notions are presented for 6-[1'-(6-mercapto-hexyl)-[4,4']bipyridinium]-hexane-1-thiol iodide (6V6) "wired" between a gold electrode surface and tip in an in situ scanning tunneling microscopy configuration. The viologen group can be used to "gate" charge transport across the molecular bridge through control of the electrochemical potential and consequently the redox state of the viologen moiety. This gating is theoretically considered within the framework of superexchange and coherent two-step notions for charge transport. It is shown here that the absence of a maximum in the Itunneling versus electrode potential relationship can be fitted by a "soft" gating concept. This arises from large configurational fluctuations of the molecular bridge linked to the gold contacts by flexible chains. This view is incorporated in a formalism that is well-suited for data analysis and reproduces in all important respects the 6V6 data for physically sound values of the appropriate parameters. This study demonstrates that fluctuations of isolated configurationally "soft" molecules can dominate charge transport patterns and that theoretical frameworks for compact monolayers may not be directly applied under such circumstances.     

Microvoltammetry of lithium nickel oxide single particle: deterioration of the redox behavior by water molecules

The microelectrode technique was applied to investigate the electrochemical behavior of LiNiO₂ which seriously influenced by water molecules. Cyclic voltammograms of humidified LiNiO₂ particles were compared to that of as-synthesized. As a result, a significant peak-potential shift was observed, which denotes that the reaction product caused by H₂O generates an electrochemical inactive material on the particle which prevents the Li⁺ insertion/extraction. The reaction product was considered to be NiO, and deterioration property has also been discussed.     

Dipoles-dipole association of mesogenic molecules in solution

The dielectric polarization has been used to study dipolar association of 4-n-pentyl-4'-cyanobiphenyl in benzene solution. The results have been interpreted with the assumption of a monomer-dimer equilibrium. To explain the relatively high effective dipole moment of the dimers, a new structure has been proposed for them.     

Modeling absorption spectra of molecules in solution

The presence of solvent tunes many properties of a molecule, such as its ground and excited state geometry, dipole moment, excitation energy, and absorption spectrum. Because the energy of the system will vary depending on the solvent configuration, explicit solute–solvent interactions are key to understanding solution-phase reactivity and spectroscopy, simulating accurate inhomogeneous broadening, and predicting absorption spectra. In this tutorial review, we give an overview of factors to consider when modeling excited states of molecules interacting with explicit solvent. We provide practical guidelines for sampling solute–solvent configurations, choosing a solvent model, performing the excited state electronic structure calculations, and computing spectral lineshapes. We also present our recent results combining the vertical excitation energies computed from an ensemble of solute–solvent configurations with the vibronic spectra obtained from a small number of frozen solvent configurations, resulting in improved simulation of absorption spectra for molecules in solution.     

Aggregation of rose bengal molecules in solution

Absorption, emission and excitation spectral data support the thesis that rose bengal forms H-type aggregates in water and polar, protic solvents.