Immobilisation of redox active molecules as precursors for nanowire self-assembly
Campagnoli, Elena (2007) Immobilisation of redox active molecules as precursors for nanowire self-assembly. PhD thesis, Dublin City University.
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Spontaneously adsorbed monolayers of [Co(ttp-CH2-SH)2](PF6)2 have been formed on platinum microelectrodes by exposure to micromolar solutions of the complex in 0.1 M TBABF4 in acetonitrile, ttp-CH2-SH is 4'-(p-(thiolmethyl)-phenyl)-2,2':6',2''- terpyridine. Resonance Raman spectroscopy on roughened polycrystalline platinum macro electrodes show that the molecule undergoes adsorption by sulphur atom onto the platinum surface. The monolayers show reversible and well defined cyclic voltammetry when switched between Co2+ and Co3+ forms, with a peak to peak splitting of 0.040±0.005 V up to 200Vs'' and an FWHM of 0.138±0.010 V. Adsorption is irreversible leadi* ng to the maximum surface coverage, 6.3±0.3xl0‘ 1 m1 olc2m' for 2.5<[Co(ttp-Ctf2-SH)2]<10 (o,M. The rate of monolayer formation appears to be controlled not by mass transport or interfacial binding but by surface diffusion of the complex. The surface diffusion coefficient is 5 .5± l.lx l0 '7 cm V indicating that prior to formation of an equilibrated monolayer, the adsorbates have significant mobility on the surface. The electron transfer process across the monolayer-electrode interface has been probed by high speed chronoamperometry and the standard heterogeneous electron transfer rate constant, k°, is approximately 3.06±0.03xl04 s '1. The reorganization energy is. 18.5 kJmol'1.
Self-assembled monolayers of pentanethiol-beta-cyclodextrins have been immobilised on gold polycrystalline macroelectrodes and electrochemically and spectroscopically characterised. The adsorption dynamics was followed by monitoring the change in capacitance during time and a surface diffusion process was found to be the rate determining step. The layers appear to have a mixed structure most likely between a hexagonal close-packed monolayer and a brick configuration. Blocking behaviour depends on the hydrophobicity of the solution phase redox active probe. Scan rate dependent cyclic voltammetry has been used to characterise the defect density within the assemblies. In general, the layers are stable enough in time and potential window to probe the host-guest properties of the immobilized cyclodextrin cavities. The modified gold electrodes were exposed to micromolar solutions of cobalt biphenylterpyridine, in order to electrochemically and spectroscopically characterise the inclusion process. The host-guest reaction occurred for the in-situ experiments and it was characterised in terms of equilibrium constant, 2.78±0.7><104 M'1, and free Gibbs energy of inclusion, 25 kJ m ol1. Finally, control experiments of the adsorption of the redox active probe onto the bare gold electrodes further confirmed the occurring of the inclusion.
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