Author: Ahlam Alharbi
Alharbi, Ahlam, 2024 Exploring Structural Characteristics of Tethered Bilayer Lipid Membranes (tBLMs) and Carboxylic-Terminated Self-Assembled Monolayers (SAMs), Flinders University, College of Science and Engineering
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Tethered bilayer lipid membranes (tBLMs) are widely recognized as excellent models for biological membranes, and they have been utilized to examine many membrane-related processes and features. They have technological applications and are a useful tool for biophysical studies of membrane proteins. While structural details of tBLMs have been explored, for further applications, an in-depth characterization of their molecular structures would be very useful. However, most high-resolution techniques require vacuum environments, while membranes typically only exist in aqueous media. This thesis made first steps towards developing and characterizing procedures to employ vacuum-based techniques for the analysis of membrane samples.
Angle-resolved X-ray photoelectron spectroscopy (AR-XPS) and neutral impact collision ion scattering spectroscopy (NICISS) emerged as ideal choices for this study due to their ability to provide concentration depth profiles with high resolution. To our knowledge, artificial phospholipid bilayers have not yet been structurally investigated under vacuum. Knowing the concentration depth profiles of the functional groups within the bilayers will provide valuable information in this field.
In addition to AR-XPS and NICISS, other techniques are also employed in this thesis for structural analysis. These include electrochemical impedance spectroscopy (EIS) as a complementary technique, along with ultraviolet photoelectron spectroscopy (UPS) and metastable induced electron spectroscopy (MIES).
In order to characterize a tBLM, we first investigated different membrane components, such as non-wetted bilayers and organic self-assembled monolayers (SAMs). The latter represents a simplified and stable model of tBLMs with a monolayer structure.
This thesis is divided into eight chapters, structured as follows:
Chapter 1 provides an overview of natural cell membranes and introduces various model membrane systems, particularly tBLMs, which have been developed in a controlled manner using only the fundamental components of natural membranes to mimic their structure and functionality. Within this chapter, we also discuss previous common techniques employed for probing the structure of tBLMs and explain why vacuum-based techniques are necessary.
In Chapter 2, three main subjects are addressed: the description of the techniques employed, the materials and methods used for sample preparation, and the explanation of the developed measurement method.
In Chapter 3, we have successfully determined the overall molecular structure of non-wetted tBLMs. The concentration depth profiles of tBLMs were determined using AR-XPS, enabling, for the first time, identification of the position of functional groups within the bilayer aligned parallel to the surface normal. tBLMs are typically produced and function in aqueous solution. In this chapter, we present evidence using EIS that tBLMs maintain functionality even after being dried out. However, these tBLMs were to some degree unable to maintain their functional integrity when transferred from their original formation site and exposed to air followed by vacuum conditions. This observation could serve as a valuable case study for evaluating the feasibility of utilizing such model systems in potential biosensor applications that may be exposed to dehydration conditions. This finding led us to explore alternative systems with surface characteristics similar to those of membranes, while demonstrating enhanced stability, in order to undertake wetting investigations.
The following Chapters 4, 5, 6, and 7 explore distinct aspects of the properties of carboxylic-terminated SAMs, specifically 16-mercaptohexadecanoic acid (16-MHDA), 11-mercaptoundecanoic acid (11-MUDA), and α-lipoic acid (α-LA) SAMs. These SAMs are used in this thesis as simplified forms of the double layers due to their comparable surface features (hydrophilic interfaces) while offering improved stability.
In Chapter 4, our focus lies in understanding the structure and orientation of the selected SAMs. Given that these SAMs are used in this thesis as model monolayer systems for wetting studies, exploring their orientation becomes essential. This study is motivated by a lack of consensus in the literature regarding the structure of these SAMs, particularly their orientation. The results of our study revealed that the molecules are not straight and that the carboxyl groups are below the surface.
In Chapter 5, through the use of the SAMs for their well-defined composition and thickness, the study focuses on determining the stopping power of Ne+-ions and understanding their trajectory through matter at low primary energies using NICISS technique. The term "stopping power" refers to the energy lost by a particle beam as it passes through a material. This research was motivated by the necessity of stopping power information when conducting structural analysis with Ne+-ions. In contrast to He-NICISS, commonly used ions in NICISS, Ne-NICISS facilitates distinguishing heavier elements and is therefore more suitable for analysing biological membranes containing elements with similar masses, for example, P and N found in tBLM films. Our findings revealed that although we still might be able to distinguish between the elements, using Ne-NICISS is not suitable for determining concentration depth profiles at energies of 5 keV and below.
In Chapter 6, we have used COOH-terminated SAMs as markers for tBLMs to explore the wetting behavior and develop effective handling methods. This chapter focuses into exploring the potential of NICISS to investigate the structural characteristics of SAMs under closely natural conditions “an aqueous environment”. Our study primarily yielded layer thickness information, rather than detailed structural insights. The wetting behavior and stability of SAMs in different selected solvents were also examined.
In Chapter 7, the focus is on examining the X-ray-induced damage to the SAMs and exploring its potential implications for the orientation and electronic structure of these thin films. This study was undertaken due to indications during the investigation of SAMs structures that X-ray exposure caused damage. Our goal was to further investigate this phenomenon. The existing literature lacks sufficient details to understand how X-rays influence the crucial properties of SAMs, such as orientation and the presence of COOH groups at the surface, as well as the electronic structure of the COOH-terminated SAMs. This study observed that prolonged exposure times to X-rays caused significant damage to the SAMs, particularly affecting the COOH groups. The analysis revealed that such damage could lead to changes in work function, indicating alterations in the electronic distribution within the outermost layer and/or potential modifications in electronic structure at the SAM/Au layer interface.
Chapter 8 provides a final summary of the work.
Keywords: Self-Assembled Monolayers,tBLM,stopping power,Ne+ projecties,X-ray damage,Carboxylic-terminated SAMs
Subject: Physics thesis
Thesis type: Doctor of Philosophy
Completed: 2024
School: College of Science and Engineering
Supervisor: Gunther Andersson