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Interfacial electroactive assemblies: from molecular electronics to biological applications

Mallon, Colm (2009) Interfacial electroactive assemblies: from molecular electronics to biological applications. PhD thesis, Dublin City University.

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Abstract

Spontaneously adsorbed monolayers of di-6A, 6B-deoxy-6-(4-pyrid-ylmethyl)amino-ƴ-cyclodextrin (ƴ-CD-(py)2) were formed on platinum electrodes. AC voltammetry showed significantly lower capacitance values for electrodes exposed to ƴ-CD-(py)2 solutions overnight compared to bare electrode values. Co-adsorption of 1-nonanethiol in the presence of a 10-fold excess of cavity guest 1-adamantylamine created layers which exhibited greater blocking ability to the solution phase probe [Fe(CN)6]4−. Complete blocking was achieved by insertion of a high-affinity guest 1-adamantylamine into the cavity. Raman spectra of the ƴ-CD-(py)2/1-nonanethiol layer exhibited features associated with both pyridine-functionalised CD and alkane moieties. Significantly, co-adsorption of 1-nonanethiol dramatically effected the ability of the ƴ-CD-(py)2 layer to complex the electroactive, high affinity guest, [Co(biptpy)2]2+. A redox response for the Co2/3+ couple was not observed at the pure ƴ-CD-(py)2 layer, but the molecular recognition properties were turned on by co-adsorbing the alkanethiol molecules with the CD layer. The binding of [Co(biptpy)2]2+ to co-adsorbed monolayers depends on the bulk concentration of guest and was modelled by the Langmuir isotherm, yielding a free energy of adsorption, △Gads, of -29 kJ.mol−1 for the Co2+ state and a limiting surface coverage 1.49 ± 0.25 x 10−11 mol.cm−2. The rate of electron transfer from the cobalt metal center to the electrode surface was found to be of the order of 1 x 105 s−1 by high speed chronoamperometry. Molecular junctions incorporating monolayers of ƴ-CD-(py)2, co-adsorbed with 1-nonanethiol have been formed by bringing macroscopic platinum and mercury electrodes together. The mercury electrode was either modified with an alkanethiol layer for bi-layer junction formation, or remained unmodified for monolayer junction formation. The junctions were characterised by determining the effect of junction thickness on the magnitude of the tunnelling current through alkanethiol layers. A tunnelling co-efficient,β, of 0.88 ± 0.01 per carbon atom was determined for these alkanethiol bilayer junctions. Significantly, for bilayer junc tions incorporating CD layers, the tunnelling current depends markedly on the nature of the CD guest. Junctions where nonconjugated guests, such as 1-adamantylamine, were included in the CD showed an order of magnitude lower current than junctions incorporating the conjugated guest C60. Moreover, monolayer junctions of CD backfilled with 1-nonanethiol exhibited potential-dependent currents in the presence of CD guest molecule [Co(biptpy)2]2+ but not for [Co(tpy)2]2+, which is structurally analogous but cannot associate with CD. The effect of electrode displacement on these potential dependent currents indicated a redox cycling or electron hopping mechanism of electron transport. Fibrinogen has been adsorbed at planar and 820 nm nano-cavity gold surfaces. AC voltammetry showed that the capacitance values of electrodes exposed to fibrinogen solutions overnight were lower, by 30 ±5 μF.cm−2, than those seen at bare gold electrodes. AFM of the protein at the planar surfaces showed a fibrous network of adsorbed protein. Oregon Green labelled fibrinogen layers were imaged using fluorescence microscopy at both the planar and nano-cavity surfaces. The effect of electrode potential on the fibrinogen layer was investigated. It was found that the protein desorbed at a potential of -1.2 V. The rate of this desorption process was investigated by capacitance studies, which showed a two stage desorption process from the planar surface, where k1 = 0.400 ± 0.065 s−1 and k2 = 0.011 ± 0.001 s−1. The desorbed protein was collected in solution and UV-visible spectroscopy studies showed that 6.3 x 10−13 mol.cm−2 of protein is desorbed. SDS-PAGE gel electrophoresis studies showed that the desorbed protein was fragmented by the adsorption-desorption process. Selective modification of the nano-cavity arrays resulted in localisation of the protein predominately inside the nano-cavities. The desorption of the dye labelled protein was investigated using fluorescence microscopy at both the planar and nano-cavity surfaces. The diffusion of the protein out of the confocal laser volume was seen to be slower at the nano-cavity surfaces, compared to the planar surface, indicating that the exit of the protein from the cavities is a rate limiting step. An increase in the fluorescence signal was observed at the nano-cavity surface and an enhancement factor of 500 is estimated.

Item Type:Thesis (PhD)
Date of Award:March 2009
Refereed:No
Supervisor(s):Keyes, Tia E. and Forster, Robert J.
Uncontrolled Keywords:physical chemistry; interfacial supramolecular chemistry; protein deposition; tunnelling junctions; electrochemistry; microscopy; microcavity;
Subjects:Physical Sciences > Chemistry
DCU Faculties and Centres:DCU Faculties and Schools > Faculty of Science and Health > School of Chemical Sciences
Use License:This item is licensed under a Creative Commons Attribution-NonCommercial-No Derivative Works 3.0 License. View License
Funders:Irish Research Council for Science Engineering and Technology, Science Foundation Ireland
ID Code:2336
Deposited On:02 Apr 2009 13:49 by Tia Keyes. Last Modified 16 Nov 2009 17:31

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