Transient absorption studies and numerical modeling of iodine photoreduction by nanocrystalline TiO2 films

We have used transient absorption spectroscopy to study the reaction between photogenerated electrons in a dye-free nanocrystalline titanium dioxide film and an iodine/iodide redox couple. Recombination kinetics was measured by recording the transient optical signal following band gap excitation by a UV laser pulse. In the presence of a methanol hole scavenger in the electrolyte, a long-lived (0.1-1 s) red/infrared absorbance is observed and assigned to photogenerated electrons forming Ti(3+) species. In the presence of iodine and excess iodide in the electrolyte, the signal decays on a millisecond-microsecond time scale, assigned to reduction of the redox couple by photogenerated electrons in the TiO(2). The electron lifetime decreases inversely with increasing iodine concentration, indicating that the back reaction is first order in [I(2)]. No evidence for I(2)(-) is observed, indicating that the reaction mechanism does not involve the formation of I(2)(-) as an intermediate. The shape of the kinetics evolves from monoexponential at low [I(2)] to stretched-exponential as [I(2)] increases. A Monte Carlo continuous-time random walk model is implemented to simulate the kinetics and its [I(2)] dependence and used to address the order of the recombination reaction with respect to electron density, n. The model incorporates the diffusion of oxidized species from the electrolyte toward the TiO(2) surface as well as electron trapping and transport in the TiO(2). In the limit of low [I(2)], the monoexponential kinetics is explained by the recombination reaction being rate limited by the diffusion of the oxidized species in the electrolyte. The stretched-exponential behavior at high [I(2)] can be explained by the reaction being rate limited by the transport of electrons through a distribution of trap states toward reactive sites at the TiO(2)-electrolyte interface, similar to the mechanism proposed previously for the kinetics of electron-dye cation recombination. Such trap-limited recombination can also explain the superlinear dependence of electron recombination rate on electron density, which has been reported elsewhere, without the need for a reaction mechanism that is second order in n. In contrast, a second-order reaction mechanism in a trap-free medium cannot explain the observed kinetics, although a second-order mechanism incorporating electron trapping cannot be conclusively ruled out by the data. We propose that the most likely reaction scheme, that is first order in both [I(2)] and n, is the dissociative reduction of I(2) onto the metal oxide surface, followed by a second electron reduction of the resulting adsorbed iodine radical, and that empirical second-order behavior of the electron lifetime is most likely explained by electron trapping rather than by a second-order recombination mechanism.     

Effect of end‐tethered polymers on surface adhesion of glassy polymers

The adhesion between a glassy polymer melt and substrate is studied in the presence of end‐grafted chains chemically attached to the substrate surface. Extensive molecular dynamics simulations have been carried out to study the effect of the areal density ∑ of tethered chains and tensile pull velocity v on the adhesive failure mechanisms. The initial configurations are generated using a double‐bridging algorithm in which new bonds are formed across a pair of monomers equidistant from their respective free ends. This generates new chain configurations that are substantially different than the original two chains such that the systems can be equilibrated in a reasonable amount of cpu time. At the slowest tensile pull velocity studied, a crossover from chain scission to crazing is observed as the coverage increases, while for very large pull velocity, only chain scission is observed. As the coverage increases, the sections of the tethered chains pulled out from the interface form the fibrils of a craze that are strong enough to suppress chain scission, resulting in cohesive rather than adhesive failure. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 199–208, 2004     

Stereoselective synthesis of (2S,3R)- and (2R,3S)-iodoreboxetine; potential SPECT imaging agents for the noradrenaline transporter

With the aim of developing a new SPECT imaging agent for the noradrenaline transporter, a twelve-step stereoselective synthesis of iodinated analogues of (2S,3R)- and (2R,3S)-reboxetine has been achieved from 4-bromobenzaldehyde. The key steps involve a Sharpless asymmetric epoxidation to establish the stereogenic centres and a copper catalysed aromatic halogen exchange reaction to introduce the key iodine atom. In vitro testing of these compounds using a [(3)H]nisoxetine displacement assay with homogenised rat brain shows both compounds to have significant affinity, with K(i) values of 320.8 nM and 58.2 nM for (2S,3R)- and (2R,3S)-iodoreboxetine respectively.     

Synthesis and analysis of ZnO-CoMoO4 incorporated organic compounds for efficient degradation of azo dye pollutants under dark ambient conditions

In recent years, the degradation of organic dyes under dark conditions, at room temperature and atmosphere pressure, without additional lights or chemical stimulants, has been widely investigated. Here, a nanocomposite of ZnO-CoMoO₄ was synthesized using an organic template and investigated as a catalyst to degrade methyl orange in aqueous environment under dark, ambient conditions. The organic compounds of Abies Pindrow Royle were reacted with a precursor solution following sol–gel synthesis methodology to modify the chemistry and morphology of ZnO-CoMoO_4, so formed. The structure of the nanocomposite was confirmed by X-ray diffraction, Raman spectroscopy and energy dispersive X-ray spectroscopy while nanostructures were examined by field emission scanning electron microscopy. Organic functional groups were determined by Fourier transform infrared spectroscopy and Gas chromatography–mass spectrometry. The organic compound incorporated nanocomposite was revealed to be an excellent catalyst with 95% degradation of methyl orange in aqueous environment under dark ambient conditions within 10 min. The catalyst also revealed 99% degradation of azo dye in the presence of solar light. Furthermore, the catalysts illustrated good stability with pseudo first order kinetics (R² < 1) in the light as well as in the dark conditions with outstanding reusability till four cycles of experiments. Therefore, nanostructure and organic species of Abies Pindrow Royle were found to enhance the catalytic behavior of ZnO-CoMoO₄ towards methyl orange degradation even in dark conditions.     

Preparation of surfactant-stabilized gold nanoparticle–peptide nucleic acid conjugates

A simple, two-step method of producing stable and functional peptide nucleic acid (PNA)-conjugated gold nanoparticles using a surfactant stabilization step is presented. PNA are DNA analogs with superior chemical stability and target discrimination, but their use in metallic nanoparticle systems has been limited by the difficulty of producing stable colloids of nanoparticle–PNA conjugates. In this work, the nonionic surfactant Tween 20 (polyoxyethylene (20) sorbitan monolaurate) was used to sterically shield gold surfaces prior to the addition of thiolated PNA, producing conjugates which remain dispersed in solution and retain the ability to hybridize to complementary nucleic acid sequences. The conjugates were characterized using transmission electron microscopy, dynamic light scattering, and UV–visible absorbance spectroscopy. PNA attachment to gold nanoparticles was confirmed with an enzyme-linked immunoassay, while the ability of nanoparticle-bound PNA to hybridize to its complement was demonstrated using labeled DNA.     

Generation of dimethylsilylene and allylidene holmium complexes from trimethylsilylated allyl ligands

The reaction of K[1,3-(SiMe3)2C3H3] with partially hydrated holmium triflate leads to a dimeric complex (1) in which hydrogen abstraction from a trimethylsilyl group has occurred on two allyl ligands, forming dimethylsilylene units that bridge the holmium atoms. When the reaction time is prolonged, a different product (2) is isolated, in which in addition to two dimethylsilylene bridges, the metal centers are joined with a mu-eta1,eta3-allylidene ligand. Both crystallographic and computational studies provide evidence for delocalized bonding in the allylidene fragment.     

Least squares problems with inequality constraints as quadratic constraints

Linear least squares problems with box constraints are commonly solved to find model parameters within bounds based on physical considerations. Common algorithms include Bounded Variable Least Squares (BVLS) and the Matlab function lsqlin. Here, the goal is to find solutions to ill-posed inverse problems that lie within box constraints. To do this, we formulate the box constraints as quadratic constraints, and solve the corresponding unconstrained regularized least squares problem. Using box constraints as quadratic constraints is an efficient approach because the optimization problem has a closed form solution. The effectiveness of the proposed algorithm is investigated through solving three benchmark problems and one from a hydrological application. Results are compared with solutions found by lsqlin, and the quadratically constrained formulation is solved using the L-curve, maximum a posteriori estimation (MAP), and the χ² regularization method. The χ² regularization method with quadratic constraints is the most effective method for solving least squares problems with box constraints.     

Investigations of the antibacterial properties of ciprofloxacin@SiO2

The antibacterial activity of ciprofloxacin-encapsulated silica nanoshells synthesized from gold@silica core-shell nanoparticles has been investigated. The minimum inhibitory concentration of the material was found using the agar dilution method, and it showed better antibacterial activity compared to free ciprofloxacin in the case of Escherichia coli DH5, whereas the same activity was found for Lactococcus lactis MG 1363. Hydrophobicity measurements carried out in an octanol-water mixture suggested that ciprofloxacin@SiO2 is distributed almost equally in the aqueous and nonaqueous phases. The kinetics of the uptake of ciprofloxacin@SiO2 was compared with that of free ciprofloxacin. Fluorescence imaging studies carried out using fluorescein isothiocyanate@SiO2 showed that the nanoshells enter the bacterial cell. The uptake of silica shells has been probed by transmission electron microscopy also.     

Lidar detection using a dual-frequency source

A new technique of dual-frequency Doppler-lidar measurement is investigated. This technique is based on the use of a coherently locked, tunable, dual-frequency laser source and is shown to accurately measure velocities as small as 26 mum/s. It is generated by exploiting the nonlinear dynamics of a semiconductor laser through a proper combination of optical injection and operating conditions.