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Browsing by Author "Le Brun, AP"

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90° magnetic coupling in a NiFe/FeMn/biased NiFe multilayer spin valve component investigated by polarized neutron reflectometry
(American Institute of Physics, 2014-07-17) Callori, SJ; Bertinshaw, J; Cortie, DL; Cai, JW; Le Brun, AP; Zhu, T; Klose, F
We have observed 90° magnetic coupling in a NiFe/FeMn/biased NiFe multilayer system using polarized neutron reflectometry. Magnetometry results show magnetic switching for both the biased and free NiFe layers, the latter of which reverses at low applied fields. As these measurements are only capable of providing information about the total magnetization within a sample, polarized neutron reflectometry was used to investigate the reversal behavior of the NiFe layers individually. Both the non-spin-flip and spin-flip neutron reflectometry signals were tracked around the free NiFe layer hysteresis loop and were used to detail the evolution of the magnetization during reversal. At low magnetic fields near the free NiFe coercive field, a large spin-flip signal was observed, indicating magnetization aligned perpendicular to both the applied field and pinned layer. © 2020 AIP Publishing LLC.
Advances in sample environments for neutron scattering for colloid and interface science
(Elsevier, 2024-05) Le Brun, AP; Gilbert, EP
This review describes recent advances in sample environments across the full complement of applicable neutron scattering techniques to colloid and interface science. Temperature, pressure, flow, tensile testing, ultrasound, chemical reactions, IR/visible/UV light, confinement, humidity and electric and magnetic field application, as well as tandem X-ray methods, are all addressed. Consideration for material choices in sample environments and data acquisition methods are also covered as well as discussion of current and potential future use of machine learning and artificial intelligence. Crown Copyright © 2024 Published by Elsevier B.V
Annexin V-containing cubosomes for targeted early detection of apoptosis in degenerative retinal tissue
(Royal Society of Chemistry, 2018-10-26) Ding, Y; Chow, SH; Liu, GS; Wang, B; Lin, TW; Hsu, HY; Duff, AP; Le Brun, AP; Shen, HH
New drug delivery materials targeting damaged ocular tissues are of particular interest. In this work, we have formulated annexin/phosphatidylserine/phytantriol and annexin/phosphatidylserine/monoolein cubosomes based on incorporation of 1,2-dipalmitoyl-sn-glycero-3-phospho-L-serine (PS) lipid and annexin V (ANX) protein with phytantriol (Phy) and monoolein (MO) respectively. The incorporation of ANX is important because it can be used as a diagnostic tool for in vivo apoptosis detection due to its high affinity to phosphatidylserine in the presence of Ca2+. We have also prepared PS–Phy and PS–MO cubosomes without ANX as a comparison, and characterized them using dynamic light scattering, cryo-TEM images and small-angle X-ray scattering, showing that PS–Phy cubosomes have greater chemical stability, and that ANX–PS–Phy cubosomes have the potential for in vivo drug delivery. In addition, we have reconstituted an apoptotic biomimetic membrane on a surface to gain insights into cubosome–bilayer interactions using a quartz-crystal microbalance and neutron reflectometry. The neutron reflectivity data reveal that there is exchange of materials between the biomimetic apoptotic bilayer and ANX–PS–Phy cubosomes, with an accumulation of ANX between the membrane and cubosomes possibly being the reason for the reduced cytotoxicity of ANX–PS–Phy cubosomes. A rat model of laser-induced choroidal neovascularization showed that ANX–PS–Phy cubosomes specifically targeted apoptotic cells in vivo. We propose that ANX–PS–Phy cubosomes are a potential candidate for ocular drug delivery for eye diseases. © The Royal Society of Chemistry 2018
The antimicrobial peptide aurein 1.2 disrupts model membranes via the carpet mechanism
(Royal Society of Chemistry, 2012-01-01) Fernandez, DI; Le Brun, AP; Whitwell, TC; Sani, MA; James, M; Separovic, F
The membrane interactions of the antimicrobial peptide aurein 1.2 were studied using a range of biophysical techniques to determine the location and the mechanism of action in DMPC (dimyristoylphosphatidylcholine) and DMPC/DMPG (dimyristoylphosphatidylglycerol) model membranes that mimic characteristics of eukaryotic and prokaryotic membranes, respectively. Neutron reflectometry and solid-state NMR revealed subtle changes in membrane structure caused by the peptide. Quartz crystal microbalance with dissipation, vesicle dye leakage and atomic force microscopy measurements were used to investigate the global mode of peptide interaction. Aurein 1.2 displayed an enhanced interaction with the anionic DMPC/DMPG membrane while exhibiting primarily a surface interaction with both types of model membranes, which led to bilayer disruption and membrane lysis. The antimicrobial peptide interaction is consistent with the carpet mechanism for aurein 1.2 with discrete structural changes depending on the type of phospholipid membrane. © 2012, Royal Society of Chemistry
Arginine catabolism is essential to polymyxin dependence in Acinetobacter baumannii
(Elsevier, 2024-07) Han, ML; Alsaadi, Y; Zhao, JX; Zhu, Y; Lu, J; Jiang, X; Ma, W; Patil, NA; Dunstan, RA; Le Brun, AP; Wickremasinghe, H; Hu, X; Wu, Y; Yu, HH; Wang, J; Barlow, CK; Bergen, PJ; Shen, HH; Lithgow, T; Creek, DJ; Velkov, T; Li, J
Polymyxins are often the only effective antibiotics against the "Critical" pathogen Acinetobacter baumannii. Worryingly, highly polymyxin-resistant A. baumannii displaying dependence on polymyxins has emerged in the clinic, leading to diagnosis and treatment failures. Here, we report that arginine metabolism is essential for polymyxin-dependent A. baumannii. Specifically, the arginine degradation pathway was significantly altered in polymyxin-dependent strains compared to wild-type strains, with critical metabolites (e.g., L-arginine and L-glutamate) severely depleted and expression of the astABCDE operon significantly increased. Supplementation of arginine increased bacterial metabolic activity and suppressed polymyxin dependence. Deletion of astA, the first gene in the arginine degradation pathway, decreased phosphatidylglycerol and increased phosphatidylethanolamine levels in the outer membrane, thereby reducing the interaction with polymyxins. This study elucidates the molecular mechanism by which arginine metabolism impacts polymyxin dependence in A. baumannii, underscoring its critical role in improving diagnosis and treatment of life-threatening infections caused by "undetectable" polymyxin-dependent A. baumannii. ª 2024 The Author(s). Published by Elsevier Inc. This is an open access article under the CC BY-NC licence
The assembly mechanism and mesoscale architecture of protein–polysaccharide complexes formed at the solid–liquid Interface
(American Chemical Society, 2022-10-04) Biswas, S; Melton, LD; Nelson, ARJ; Le Brun, AP; Heinrich, F; McGillivray, DJ; Xu, AY
Protein-polysaccharide composite materials have generated much interest due to their potential use in medical science and biotechnology. A comprehensive understanding of the assembly mechanism and the mesoscale architecture is needed for fabricating protein-polysaccharide composite materials with desired properties. In this study, complex assemblies were built on silica surfaces through a layer-by-layer (LbL) approach using bovine beta-lactoglobulin variant A (βLgA) and pectin as model protein and polysaccharide, respectively. We demonstrated the combined use of quartz crystal microbalance with dissipation monitoring (QCM-D) and neutron reflectometry (NR) for elucidating the assembly mechanism as well as the internal architecture of the protein-polysaccharide complexes formed at the solid-liquid interface. Our results show that βLgA and pectin interacted with each other and formed a cohesive matrix structure at the interface consisting of intertwined pectin chains that were cross-linked by βLgA-rich domains. Although the complexes were fabricated in an LbL fashion, the complexes appeared to be relatively homogeneous with βLgA and pectin molecules spatially distributed within the matrix structure. Our results also demonstrate that the density of βLgA-pectin complex assemblies increased with both the overall and local charge density of pectin molecules. Therefore, the physical properties of the protein-polysaccharide matrix structure, including density and level of hydration, can be tuned by using polysaccharides with varying charge patterns, thus promoting the development of composite materials with desired properties. © 2024 American Chemical Society
Asymmetric phospholipid: lipopolysaccharide bilayers; a Gram-negative bacterial outer membrane mimic
(The Royal Society, 2013-10-16) Clifton, LA; Skoda, MWA; Daulton, E; Hughes, AV; Le Brun, AP; Lakey, JH; Holt, SA
The Gram-negative bacterial outer membrane (OM) is a complex and highly asymmetric biological barrier but the small size of bacteria has hindered advances in in vivo examination of membrane dynamics. Thus, model OMs, amenable to physical study, are important sources of data. Here, we present data from asymmetric bilayers which emulate the OM and are formed by a simple two-step approach. The bilayers were deposited on an SiO2 surface by Langmuir–Blodgett deposition of phosphatidylcholine as the inner leaflet and, via Langmuir–Schaefer deposition, an outer leaflet of either Lipid A or Escherichia coli rough lipopolysaccharides (LPS). The membranes were examined using neutron reflectometry (NR) to examine the coverage and mixing of lipids between the bilayer leaflets. NR data showed that in all cases, the initial deposition asymmetry was mostly maintained for more than 16 h. This stability enabled the sizes of the headgroups and bilayer roughness of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine and Lipid A, Rc-LPS and Ra-LPS to be clearly resolved. The results show that rough LPS can be manipulated like phospholipids and used to fabricate advanced asymmetric bacterial membrane models using well-known bilayer deposition techniques. Such models will enable OM dynamics and interactions to be studied under in vivo-like conditions. © 2013, The Royal Society.
Correction to structural characterization of a model gram-negative bacterial surface using lipopolysaccharides from rough strains of Escherichia coli
(American Chemical Society, 2014-07-24) Le Brun, AP; Clifton, LA; Halbert, CE; Lin, B; Meron, M; Holden, PJ; Lakey, JH; Holt, SA
In the original publication, the schematic of lipopolysaccharide (LPS) from Escherichia coli in Figure 1 is incorrect. A corrected version of the figure and accompanying legend is below. Figure 1. Schematic of the organization of Escherichia coli LPS. LPS was from the rough mutant J5 strain of E. coli O111:B4, which produces an Rc chemotype with a core oligosaccharide as described by Müller-Loennies et al. (1) The original R mutants, which defined the different chemotypes were from Salmonella minnesota, so in this paper we use the terms Ra/Rc to denote the chemotype of E. coli LPS used according to this convention. Kdo, 2-keto-3-deoxyoctonic acid; Hep, l-glycero-D-manno heptose; Glc, glucose; Gal, galactose; GlcN, glucosamine. The Lipid A tails consists of four (R)-3-hydroxy-mystic acids, one myristic acid, and one lauric acid. Additional phosphates and ethanolamines on Kdo and Hep have been omitted for clarity. © 2014 American Chemical Society. CC-BY - Open Access
Developments on the platypus neutron reflectometer
(Australian Institute of Nuclear Science and Engineering (AINSE), 2020-11-11) Nelson, A; Le Brun, AP; Huang, TY; Paul, O; Holt, SA
PLATYPUS is the initial neutron reflectometer at the Australian Centre for Neutron Scattering with the capability to study surface and interface systems ranging from biomolecules, soft matter through to magnetic thin films [1-3]. There have been a number of significant improvements to both the instrument and data reduction and treatment software [4] over the last two years. On the hardware front the original detector has been replaced yielding higher count-rate capabilities, greater detection efficiency at shorter wavelengths and significantly lower background. The slits which define the neutron beam have been replaced with upgraded positioning mechanisms enabling greater flexibility in experimental setup. These changes have significantly enhanced the instrument performance with improved reproducibility. This presentation will highlight the enhancements and recent publications.
Effect of divalent cation removal on the structure of gram-negative bacterial outer membrane models
(American Chemical Society, 2015-01-13) Clifton, LA; Skoda, MWA; Le Brun, AP; Ciesielski, F; Kuzmenko, I; Holt, SA; Lakey, JH
The Gram-negative bacterial outer membrane (GNB-OM) is asymmetric in its lipid composition with a phospholipid-rich inner leaflet and an outer leaflet predominantly composed of lipopolysaccharides (LPS). LPS are polyanionic molecules, with numerous phosphate groups present in the lipid A and core oligosaccharide regions. The repulsive forces due to accumulation of the negative charges are screened and bridged by the divalent cations (Mg2+ and Ca2+) that are known to be crucial for the integrity of the bacterial OM. Indeed, chelation of divalent cations is a well-established method to permeabilize Gram-negative bacteria such as Escherichia coli. Here, we use X-ray and neutron reflectivity (XRR and NR, respectively) techniques to examine the role of calcium ions in the stability of a model GNB-OM. Using XRR we show that Ca2+ binds to the core region of the rough mutant LPS (RaLPS) films, producing more ordered structures in comparison to divalent cation free monolayers. Using recently developed solid-supported models of the GNB-OM, we study the effect of calcium removal on the asymmetry of DPPC:RaLPS bilayers. We show that without the charge screening effect of divalent cations, the LPS is forced to overcome the thermodynamically unfavorable energy barrier and flip across the hydrophobic bilayer to minimize the repulsive electrostatic forces, resulting in about 20% mixing of LPS and DPPC between the inner and outer bilayer leaflets. These results reveal for the first time the molecular details behind the well-known mechanism of outer membrane stabilization by divalent cations. This confirms the relevance of the asymmetric models for future studies of outer membrane stability and antibiotic penetration. © 2014 American Chemical Society. This is an open access article published under a Creative Commons Attribution (CC-BY) License, which permits unrestricted use, distribution and reproduction in any medium, provided the author and source are cited.
Electro-polymerization rates of diazonium salts are dependent on the crystal orientation of the surface
(Elsevier, 2022-11-15) Rahpeima, S; Le Brun, AP; Raston, CL; Darwish, N
Electro-polymerization of diazonium salts is widely used for modifying surfaces with thin organic films. Initially this method was primarily applied to carbon, then to metals, and more recently to semiconducting Si. Unlike on other surfaces, electrochemical reduction of diazonium salts on Si, which is one of the most industrially dominant material, is not well understood. Here, we report the electrochemical reduction of diazonium salts on a range of silicon electrodes of different crystal orientations (111, 211, 311, 411, and 100). We show that the kinetics of surface reaction and the reduction potential is Si crystal-facet dependent and is more favorable in the hierarchical order (1 1 1) > (2 1 1) > (3 1 1) > (4 1 1) > (1 0 0), a finding that offers control over the surface chemistry of diazonium salts on Si. The dependence of the surface reaction kinetics on the crystal orientation was found to be directly related to differences in the potential of zero charge (PZC) of each crystal orientation, which in turn controls the adsorption of the diazonium cations prior to reduction. Another consequence of the effect of PZC on the adsorption of diazonium cations, is that molecules terminated by distal diazonium moieties form a compact film in less time and requires less reduction potentials compared to that formed from diazonium molecules terminated by only one diazo moiety. In addition, at higher concentrations of diazonium cations, the mechanism of electrochemical polymerization on the surface becomes PZC-controlled adsorption-dominated inner-sphere electron transfer while at lower concentrations, diffusion-based outer-sphere electron transfer dominates. These findings help understanding the electro-polymerization reaction of diazonium salts on Si en route towards an integrated molecular and Si electronics technology. © 2022 Elsevier Inc
Electrochemical grafting of organic films on silica
(Royal Society of Chemistry (RSC), 2022-10-11) Datson, Z; Dief, E; Li, T; Le Brun, AP; Darwish, N
Traditionally, self-assembled monolayers formed on silicon require the removal of the insulating and chemically inert silica layer that naturally forms on the surface of crystalline silicon. The removal of silica is thought to be necessary in order to expose the conducting Si–H surface, which is reactive towards molecules. Here we report the unexpected result of electrochemical formation of thin organic films on silica-terminated silicon with silica thickness up to 20 nm. The process is facilitated by the electrochemical generation of aryl radicals that react with silanol groups at the distal end of silica. © Royal Society of Chemistry 2024.
Engineered self-assembling monolayers for label free detection of influenza nucleoprotein
(Springer Nature, 2015-06) Le Brun, AP; Soliakov, A; Shah, DSH; Holt, SA; McGill, A; Alison, JH
Integrating nanotechnology into useable devices requires a combination of bottom up and top down methodology. Often the techniques to measure and control these different components are entirely different, so methods that can analyse the nanoscale component in situ are of increasing importance. Here we describe a strategy that employs a self-assembling monolayer of engineered protein chimeras to display an array of oriented antibodies (IgG) on a microelectronic device for the label free detection of influenza nucleoprotein. The structural and functional properties of the bio-interface were characterised by a range of physical techniques including surface plasmon resonance, quartz-crystal microbalance and neutron reflectometry. This combination of methods reveals a 13.5 nm thick engineered-monolayer that (i) self-assembles on gold surfaces, (ii) captures IgG with high affinity in a defined orientation and (iii) specifically recognises the influenza A nucleoprotein. Furthermore we also show that this non-covalent self-assembled structure can render the dissociation of bound IgG irreversible by chemical crosslinking in situ without affecting the IgG function. The methods can thus describe in detail the transition from soluble engineered molecules with nanometre dimensions to an array that demonstrates the principles of a working influenza sensor. © The Author(s) 2015. Open Access. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
Epitaxial graphene growth on cubic silicon carbide on silicon with high temperature neutron reflectometry: an operando study
(Royal Society of Chemistry, 2024-01-19) Pradeepkumar, A; Cortie, DL; Smyth, E; Le Brun, AP; Iacopi, F
The growth of graphene on silicon carbide on silicon offers a very attractive route towards novel wafer-scale photonic and electronic devices that are easy to fabricate and can be integrated in silicon manufacturing. Using a Ni/Cu catalyst for the epitaxial growth of graphene has been successful in the mitigation of the very defective nature of the underlying silicon carbide on silicon, leading to a consistent graphene coverage over large scales. A more detailed understanding of this growth mechanism is warranted in order to further optimise the catalyst composition, preferably via the use of operando characterization measurements. Here, we report in situ neutron reflectometry measurements of (Ni, Cu)/SiC films on silicon wafers, annealed from room temperature to 1100 °C, which initiates graphene formation at the buried (Ni, Cu)/SiC interface. Detailed modelling of the high temperature neutron reflectometry and corresponding scattering length density profiles yield insights into the distinct physical mechanisms within the different temperature regimes. The initially smooth solid metallic layers undergo intermixing and roughening transitions at relatively low temperatures below 500 °C, and then metal silicides begin to form above 600 °C from interfacial reactions with the SiC, releasing atomic carbon. At the highest temperature range of 600–1100 °C, the low neutron scattering length density at high temperature is consistent with a silicon-rich, liquid surface phase corresponding to molten nickel silicides and copper. This liquid catalyst layer promotes the liquid-phase epitaxial growth of a graphene layer by precipitating the excess carbon available at the SiC/metal interface. © The Authors - Open Access CC BY-NC
A high temperature operando study of epitaxial graphene growth on cubic silicon carbide using neutron reflectometry
(Taylor & Francis, 2024-05-30) Pradeepkumar, A; Cortie, DL; Smyth, E; Le Brun, AP; Iacopi, F
The synthesis of epitaxial graphene (EG) on cubic silicon carbide on silicon substrates holds vast promise for scalable graphene-based electronics and photonics applications integrated with silicon technology. The 3C-SiC/silicon platform is particularly challenging due, among other factors, to the highly defective nature of the 3C-SiC [Citation1]. We have pioneered the use of a Ni/Cu catalyst to obtain a continuous coverage of graphene despite the defective cubic silicon carbide surface template, inferring that this considerable improvement was also to the liquid-phase epitaxial growth condition at 1100°C [Citation2]. A detailed understanding of the graphene growth mechanism through operando analysis is critical in order to optimize further the Ni/Cu catalyst composition and further refine the graphene synthesis process. © 2024Informa UK Limited
High-density lipoprotein function is modulated by the SARS-CoV-2 spike protein in a lipid-type dependent manner
(Elsevier B. V., 2023-09) Correa, YB; Del Giuduce, R; Waldie, S; Thépaut, M; Gerelli, Y; Moulin, M; Delauney, C; Fieschi, F; Haertlein, M; Le Brun, AP; Forsyth, VT; Moir, M; Russell, RA; Darwish, TA; Brinck, J; Wodaje, T; Jansen, M; Martín, C; Roosen-Runge, F; Cárdenas, M; Micciulla, S; Pichler, H
There is a close relationship between the SARS-CoV-2 virus and lipoproteins, in particular high-density lipoprotein (HDL). The severity of the coronavirus disease 2019 (COVID-19) is inversely correlated with HDL plasma levels. It is known that the SARS-CoV-2 spike (S) protein binds the HDL particle, probably depleting it of lipids and altering HDL function. Based on neutron reflectometry (NR) and the ability of HDL to efflux cholesterol from macrophages, we confirm these observations and further identify the preference of the S protein for specific lipids and the consequent effects on HDL function on lipid exchange ability. Moreover, the effect of the S protein on HDL function differs depending on the individuals lipid serum profile. Contrasting trends were observed for individuals presenting low triglycerides/high cholesterol serum levels (LTHC) compared to high triglycerides/high cholesterol (HTHC) or low triglycerides/low cholesterol serum levels (LTLC). Collectively, these results suggest that the S protein interacts with the HDL particle and, depending on the lipid profile of the infected individual, it impairs its function during COVID-19 infection, causing an imbalance in lipid metabolism. © Crown Copyright 2023. Published by Elsevier Inc. Open Access - CC BY licence 4.0.
HIV and influenza fusion peptide interactions with (dis)ordered lipid bilayers: understanding mechanisms and implications for antimicrobial and antiviral approaches
(Elsevier, 2024-09) Miłogrodzka, I; Le Brun, AP; Banaszak Holl, MM; van 't Hag, L
The interactions of viral fusion peptides from influenza (E4K and Ac-E4K) and human immunodeficiency virus (gp41 and Ac-gp41) with planar lipid bilayers and monolayers was investigated herein. A combination of surface-sensitive techniques, including quartz crystal microbalance with dissipation (QCM-D), Langmuir-Blodgett area-pressure isotherms with Micro-Brewster angle microscopy, and neutron reflectometry, was employed. Differences in the interactions of the viral fusion peptides with lipid bilayers featuring ordered and disordered phases, as well as lipid rafts, were revealed. The HIV fusion peptide (gp41) exhibited strong binding to the DOPC/DOPS bilayer, comprising a liquid disordered phase, with neutron reflectometry (NR) showing interaction with the bilayer's headgroup area. Conversely, negligible binding was observed with lipid bilayers in a liquid ordered phase. Notably, the influenza peptide (E4K) demonstrated slower binding kinetics with DOPC/DOPS bilayers and distinct interactions compared to gp41, as observed through QCM-D. This suggests different mechanisms of interaction with the lipid bilayers: one peptide interacts more within the headgroup region, while the other is more involved in transmembrane interactions. These findings hold implications for understanding viral fusion mechanisms and developing antimicrobials and antivirals targeting membrane interactions. The differential binding behaviours of the viral fusion peptides underscore the importance of considering membrane composition and properties in therapeutic strategy design. © 2024 The Authors. Published by Elsevier Inc. Open Access article under the CC BY-NC licence.
Hot commissioning and first user experiments on the Spatz neutron reflectometer
(Australian Nuclear Science and Technology Organisation, 2021-11-26) Le Brun, AP; Huang, TY; Pullen, SA; Nelson, A; Holt, SA
The Spatz neutron beam instrument is the latest to be installed and commissioned in the Neutron Guide Hall at the 20 MW OPAL Research Reactor. Spatz is a time-of-flight neutron reflectometer used for studying nanoscale structures at surfaces and interfaces and utilises a vertical sample geometry / horizontal scattering geometry. The instrument is situated at the end position of the CG2B neutron guide and views the cold neutron source (CNS). The disc chopper cascade that pulses the neutron beam to produce the time-of-flight is very configurable to provide a wavelength resolution between 1 to 12 %. The detector is a helium-3 two dimensional detector that is capable of measuring both specular and off-specular reflectivity. The sample stage can support a range of different sample environments including multiple solid-liquid cells, an atmospheric chamber with temperature control, the ATR-FT-IR spectrometer for simultaneous infra-red spectroscopy and neutron reflectometry measurements, electrochemical cells, etc. The geometry of the instrument and the sample environment available means that Spatz is well suited to studying phenomena at the gas-solid interface and solid-liquid interface. The Spatz instrument has been fully commissioned with neutrons and the results of the commissioning are presented. This includes measurements using the ‘Bragg mirror’ consisting of 25 bilayers of nickel and titanium, different solid substrates of silicon, quartz and sapphire, spin-coated polymer samples, and films under liquid. Reflectivity down to 10-7 can be achieved within 1 hour measuring time with good counting statistics in most cases. Early user experiments cover a range of science including investigating the thermal stability of organic solar cell materials and proteins interacting with biomimetic phospholipid cell membranes. © 2021 The Authors
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.
The impact of surface coverage on the kinetics of electron transfer through redox monolayers on a silicon electrode surface
(Elsevier, 2015-12-20) Ciampi, S; Choudhury, MH; Ahmad, SABA; Darwish, N; Le Brun, AP; Gooding, JJ
The impact of the coverage of ferrocene moieties, attached to a silicon electrode modified via hydrosilylation of a dialkyne, on the kinetics of electron transfer between the redox species and the electrode is explored. The coverage of ferrocene is controlled by varying the coupling time between azidomethylferrocene and the distal alkyne of the monolayer via the copper assisted azide-alkyne cycloaddition reaction. All other variables in the surface preparation are maintained identical. What is observed is that the higher the surface coverage of the ferrocene moieties the faster the apparent rates of electron transfer. This surface coverage-dependent kinetic effect is attributed to electrons hopping between ferrocene moieties across the redox film toward hotspots for the electron transfer event. The origin of these hotspots is tentatively suggested to result from minor amounts of oxide on the underlying silicon surface that reduce the barrier for the electron transfer. © 2015 Elsevier Ltd.

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