Browsing by Author "Coster, HGL"

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    Characterisation of alkyl-functionalised Si(111) using reflectometry and AC impedance spectroscopy
    (Elsevier, 2007-12-15) Wong, ELS; James, M; Chilcott, TC; Coster, HGL
    The past few years have seen a dramatic increase in the study of organic thin-film systems that are based on silicon–carbon covalent bonds for bio-passivation or bio-sensing applications. This approach to functionalizing Si wafers is in contrast to gold-thiol or siloxane chemistries and has been shown to lead to densely packed alkyl monolayers. In this study, a series of alkyl monolayers [CH³(CH²)nCH=CH²; n=7, 9, 11, 13, 15] were directly covalent-linked to Si(111) wafers. The structures of these monolayers were studied using X-ray reflectometry (XRR) and AC impedance spectroscopy. Both techniques are sensitive to the variation in thickness with each addition of a CH2 unit and thus provide a useful means for monitoring molecular-scale events. The combination of these techniques is able to probe not only the thickness, but also the interfacial roughness and capacitance of the layer at the immobilized surface with atomic resolution. Fundamental physical properties of these films such as chain canting angles were also determined. © 2007, Elsevier Ltd.
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    Comparative study of neutron reflectometry, x-ray reflectometry and electrical impedance spectroscopy of organic films on silicon
    (The Bragg Institute, Australian Nuclear Science and Technology Organisation, 2005-11-27) Wong, ELS; James, M; Coster, HGL; Chilcott, TC
    A comparative study has been made of the substructure and properties of functionalized organic layers on silicon substrates, using neutron reflectivity, X-Ray reflectivity and low frequency electrical impedance spectroscopy (EIS). All three techniques have similar spatial structural resolution. X-Ray reflectometry provides information on the electron density of substructural layers whilst EIS provides data on the electric polarizability and electrical conductance properties of individual substructural layers. Neutron reflectivity likewise provides data on the atomic mass density of the layers. With organic films the contrast in neutron scattering for the elements of interest such as C, O, N is not very large. However, replacement of H2O with D2O allows a much enhanced contrast to be obtained and provides information in particular of the interdigitation of water molecules into film. This impinges on the polarizability and electrical conductance as as measured by EIS. © 2005 The Authors
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    Immobilization of dendrimers on Si-C linked carboxylic acid-terminated monolayers on silicon(111)
    (Elsevier, 2006-12-05) Bocking, T; Wong, ELS; James, M; Watson, JA; Brown, CL; Chilcott, TC; Barrow, KD; Coster, HGL
    Poly(amidoamine) dendrimers were attached to activated undecanoic acid monolayers, covalently linked to smooth silicon surfaces via Si-C bonds. The resulting ultra-thin dendrimer films were characterized by X-ray photoelectron spectroscopy (XPS), X-ray reflectometry (XR) and atomic force microscopy (AFM). XPS results suggested amide bond formation between the dendrimer and the surface carboxylic acid groups. XR yielded thicknesses of 10 angstrom for the alkyl region of the undecanoic acid monolayer and 12 A for the dendrimer layer, considerably smaller than the diameter of these spherical macromolecules in solution. This was consistent with AFM images showing collapsed dendrimers on the surface. It was concluded that the deformation arose from a large number of amine groups on the surface of each dendrimer reacting efficiently with the activated surface, whereby the dendrimers can deform to fill voids while spreading over the activated surface to form a homogeneous macromolecular layer. © 2006, Elsevier Ltd.
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    The impact of pH on packing in tethered lipid bilayers
    (Australian Institute of Nuclear Science and Engineering, 2016-11-29) Cranfield, CG; Berry, T; Holt, SA; Le Brun, AP; Valenzuela, SM; Coster, HGL; Cornell, BA
    We report that increasing the H3O+ concentration when lowering the pH reduces the intrinsic ionic conduction through phospholipid bilayers (Fig 1A), which is counter to what might be expected from increasing the H3O+ concentration. We attribute the conduction decrease to a reduction of the molecular area per lipid (ao)[1]. These effects are seen at H3O+ concentrations in the range nM to µM despite these being very low concentrations compared to that of a typical bathing electrolyte solution of 135mM ionic concentration. We present a model, in which the pH dependent reduction in ao favours an increase in lipid packing. To support this model, we provide evidence of the effects of the hydronium ion on lipid geometry using neutron reflectometry (Fig 1C). Further examples will be given of the impact of the H3O ion concentration on the hydrogen bonding within the polar groups of lipid.
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    Ionic double layer of atomically flat gold formed on mica templates
    (Elsevier, 2009-05-30) Chilcott, TC; Wong, ELS; Coster, HGL; Coster, ACF; James, M
    Electrical impedance spectroscopy characterisations of gold surfaces formed on mica templates in contact with potassium chloride electrolytes were performed at the electric potential of zero charge over a frequency range of 6 × 10-3 to 100 × 103 Hz. They revealed constant-phase-angle (CPA) behaviour with a frequency exponent value of 0.96 for surfaces that were also characterised as atomically flat using atomic force microscopy (AFM). As the frequency exponent value was only marginally less than unity, the CPA behaviour yielded a realistic estimate for the capacitance of the ionic double layer. The retention of the CPA behaviour was attributed to specific adsorption of chloride ions which was detected as an adsorption conductance element in parallel with the CPA impedance element. Significant variations in the ionic double layer capacitance as well as the adsorption conductance were observed for electrolyte concentrations ranging from 33 [mu]M to 100 mM, but neither of these variations correlated with concentration. This is consistent with the electrical properties of the interface deriving principally from the inner or Stern region of the double layer. © 2009, Elsevier Ltd.