Scanning electrochemical microscopy (SECM) studies of catalytic EC' processes: theory and experiment for feedback, generation/collection and imaging measurements

This paper describes the use of scanning electrochemical microscopy (SECM) in the tip generation/substrate collection (TG/SC), or feedback, mode and substrate generation/tip collection (SG/TC) mode to measure homogeneous kinetics in the catalytic EC' process. Theoretical analyses of both configurations have been developed numerically to allow the optimal conditions for sensitive kinetic measurements to be determined. This is shown to involve collection efficiency measurements as a function of tip-substrate electrode distance in the case of TG/SC measurements and tip (collector current) images in a plane normal to the substrate electrode for the SG/TC mode. An important consideration for the SECM configuration (particularly for TG/SC and feedback measurements) is that the electroinactive co-reactant may be depleted more significantly than with other electrode geometries, because of cycling of the redox couple in the tip/substrate electrode gap, while the co-reactant can only enter this gap by hindered diffusion. The approaches described are examined through studies of the oxidation of amidopyrine by electrogenerated Fe(CN) in 0.5 mol dm(-3) aqueous KOH solution. A second-order rate constant of 390 ± 80 dm(3) mol(-1) s(-1) is obtained from TG/SC measurements, consistent with SG/TC quantitative imaging measurements. The consistency of the kinetic measurements confirms the validity of the approaches described. The kinetic constant is lower than expected based on previous ultramicroelectrode (UME) studies, and this is attributed to the fact that background currents for the direct heterogeneous oxidation of amidopyrine are more significant with conventional UME measurements, which will tend to enhance the current measured and may therefore lead to an overestimation of kinetic constants. The TG/SC approach, on the other hand, provides a means of making dual-electrode collection efficiency measurements with diffusional feedback of the redox couple, leading to superior voltammetric responses and enabling more accurate kinetic determination.     

Interaction of Zn(II) with quinolone drugs: structure and biological evaluation

Zinc complexes with the third-generation quinolone antibacterial drugs levofloxacin and sparfloxacin have been synthesized and characterized. The deprotonated quinolones act as bidentate ligands coordinated to zinc ion through the pyridone and a carboxylato oxygen atom. The crystal structures of [bis(aqua)bis(levofloxacinato)zinc(II)], 1, and [bis(sparfloxacinato)(1,10-phenanthroline)zinc(II)], 3, have been determined by X-ray crystallography. The biological activity of the complexes has been evaluated by examining their ability to bind to calf-thymus DNA (CT DNA) by UV spectroscopy and viscosity measurements. UV studies of the interaction of the complexes with DNA have revealed that they can bind to CT DNA probably by the intercalative binding mode which has also been verified by DNA solution viscosity measurements. The DNA binding constants have been also calculated. A competitive study with ethidium bromide (EB) showed that the complexes exhibit the ability to displace the DNA-bound EB indicating that they bind to DNA in strong competition with EB for the intercalative binding site. The interaction of the complexes with human and bovine serum albumin proteins has been studied by fluorescence spectroscopy showing that the complexes exhibit good binding propensity to these proteins having relatively high binding constant values. The biological properties of the complexes have been evaluated in comparison to the previously reported Zn(II) complexes with the first- and second-generation quinolones oxolinic acid and enrofloxacin.     

A comparative study on the use of the extended-Cauchy dispersion equation for fitting refractive index data in crystals

This study attempts a comparative evaluation of chromatic dispersion equations in 61 crystalline solids. Based on previously published data, it is demonstrated that the extended-Cauchy equation (which in the past has been mainly used with glasses) outperforms Sellmeier models and other alternative approaches in the cases of 43 crystals, including BBO, BiBO, KTP, etc. Accumulated Cauchy coefficients for these 43 materials are presented. Performance characteristics of the extended-Cauchy model are related to specific experimental conditions, such as the number of available refractive index data, as well as the spectral location and bandwidth of measurement. The number of Cauchy coefficients required for reaching practically maximum fitting accuracy is determined. It is shown that typically, extended-Cauchy equations constructed by use of only five experimental data may be used for precise modeling of two-octave spanning spectral bandwidths.     

Di(n-butyl) itaconate end-functionalized polymers: Synthesis by group transfer polymerization and solution characterization

In this study we present our efforts to homo- and co-polymerize, using group transfer polymerization (GTP), a dialkyl diester of itaconic acid, a naturally-derived unsaturated dicarboxylic acid. The monomer employed, di(n-butyl) itaconate (DBI), did not seem to polymerize by GTP to high conversion, but it displayed the remarkable ability to contribute to the polymer 1–2 units, suggesting that it would make a good end-functionalizing reagent for GTP. This was demonstrated by preparing by GTP hydrophilic homopolymers, and in situ adding to them DBI which is hydrophobic. The thus-obtained amphiphilic end-functionalized copolymers formed in water small micelles, whose aggregation numbers and radii of gyration were characterized using small-angle neutron scattering.     

Supported Au Nanoparticles-Catalyzed Regioselective Dehydrogenative Disilylation of Allenes by Dihydrosilane

Supported Au nanoparticles on TiO₂ catalyze the unprecedented dehydrogenative disilylation of monosubstituted and 1,1-disubstituted allenes by Et₂SiH₂ exclusively on the terminal double bond in a stereoselective manner. Treatment of the disilylation products with H₂O, in a one-pot operation also catalyzed by Au/TiO₂, leads to 3-alkylidene-1,2,5-oxadisilolanes, an unknown class of heterocyclic compounds, which are excellent scaffolds for the stereoselective synthesis of alkenes under Hiyama-type cross-coupling conditions.     

A new family of octanuclear Mn complexes with a rod-like topology

Three new octanuclear compounds were prepared from reactions of [Mn(O₂CR)₂]·2H₂O (R = Et or Ph) with the diols 1,3-propanediol (pdH₂) or 2-methyl-1,3-propanediol (mpdH₂) in the presence of NaN₃. All three compounds [Mn₈(N₃)₄(O₂CR)₆(L)₄(py)₆] (L = pd²⁻, R = Et 1; L = mpd²⁻, R = Et 2; L = pd²⁻, R = Ph 3) (py = pyridine) possess a novel near-planar, rod-like topology. Dc and ac magnetic susceptibility studies in the 2–300 K range for complexes 1 and 2 revealed the presence of dominant antiferromagnetic exchange interactions, leading to diamagnetic ground spin states.     

Uncertainty of cooperation in random scale-free networks

We study the spatial prisoner’s dilemma game where the players are located on the nodes of a random scale-free network. The prisoner’s dilemma game is a powerful tool and has been used for the study of mutual trust and cooperation among individuals in structured populations. We vary the structure of the network and the payoff values for the game, and show that the specific conditions can greatly influence the outcome of the game. A variety of behaviors are reproduced and the percentage of cooperating agents fluctuates significantly, even in the absence of irrational behavior. For example, the steady state of the game may be a configuration where either cooperators or defectors dominate, while in many cases the solution fluctuates between these two limiting behaviors.     

The effect of ionic and electronic properties of titanium oxide on the electrochemical growth and redox behavior of polyaniline on titanium surfaces

Polyaniline (PANI) was electrodeposited directly from aniline (ANI)-containing sulfuric acid media on Ti|TiO₂ electrodes, which were prepared in ANI-free 0.5 M H₂SO₄ solution by applying suitable anodization conditions in order to control the donor density (N _D) in the oxide layer of a fixed thickness. By using the suggested procedure, reproducible conditions for the cyclic voltammetric growth of PANI on Ti can be achieved, and most importantly, the properties of Ti|TiO₂|PANI electrodes can be modulated to meet the requirements for specific applications. The polymerization rate and the deposited amount of PANI were found to depend strongly on the N _D estimated by Mott-Schottky analysis. The redox behavior of the resulted Ti|TiO₂|PANI electrodes indicated differences in acid solutions as compared with the corresponding behavior of other PANI-modified metal surfaces. Analysis of the anodic and cathodic electrochemical response of the prepared Ti|TiO₂|PANI electrodes discloses the significant role of ionic and electronic properties of the intervening oxide between the metal support and PANI.     

Optical Gain in Semiconducting Polymer Nano and Mesoparticles

The presence of excited-states and charge-separated species was identified through UV and visible laser pump and visible/near-infrared probe femtosecond transient absorption spectroscopy in spin coated films of poly[N-9″-heptadecanyl-2,7-carbazole-alt-5,5-(4,7-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) nanoparticles and mesoparticles. Optical gain in the mesoparticle films is observed after excitation at both 400 and 610 nm. In the mesoparticle film, charge generation after UV excitation appears after around 50 ps, but little is observed after visible pump excitation. In the nanoparticle film, as for a uniform film of the pure polymer, charge formation was efficiently induced by UV excitation pump, while excitation of the low energetic absorption states (at 610 nm) induces in the nanoparticle film a large optical gain region reducing the charge formation efficiency. It is proposed that the different intermolecular interactions and molecular order within the nanoparticles and mesoparticles are responsible for their markedly different photophysical behavior. These results therefore demonstrate the possibility of a hitherto unexplored route to stimulated emission in a conjugated polymer that has relatively undemanding film preparation requirements.