Exploration of thermal, electronic and dissipative interface effects for applications in bio- and chemosensors

Abstract

Self-assembled nanomolecular structures, in particular self-assembled monolayers (SAMs) consisting of either thiols or silanes, and biomimetic supported lipid bilayers/vesicles (SLBs/SLVs), have received great attention in various fields. In fact, they are useful structures for different applications such as biosensing platforms, corrosion prevention, micro- and nanofabrication, as well as biomaterials and drug delivery systems. Therefore, despite the known benefits of these interesting compounds, many interfacial aspects of these systems such as thermal, electronic and nanomechanical properties, remain a subject of debate. For instance, SAMs are known as versatile layers in biosensing platforms for tuning the surface properties and also as compact linker molecules for grafting of bioreceptors. However, their formation kinetics and their thermal transport properties are not fully understood yet. Similarly, SLBs and SLVs find a variety of applications in biosensing platforms. Moreover, they are candidates of interest for nanodrug delivery systems and for mimicking and studying the molecular pathways of eukaryotic and prokaryotic cellular systems. However, the knowledge of their nanomechanical and thermodynamic characteristics is limited. The main objectives of this PhD thesis are to explore the thermal, electronic, and nanomechanical interface effects of self-assembled nanomolecular structures using real-time, labelfree, and surface-sensitive biosensing platforms. Such platforms include the novel heat transfer method (HTM), quartz crystal microbalances (QCM) and electrochemical transducers. Firstly, the biophysical and thermodynamic properties of biomimetic supported lipid membranes with different geometries such as two dimensional SLBs, and three dimensional SLVs were studied. Specifically, the interactions of 2D-SLBs, and 3D-SLVs with the Ebola fusion peptide (EBO17) were evaluated in order to explore and understand the viral infection mechanism in this model system. In addition, the interaction of imidazolium-based ionic liquids, which was used as a toxicology model with 3D-SLVs was evaluated due to the limited knowledge of the cytotoxicity effects of these liquids. These liquids are often used as solvents in many applications such as bioprocessing. Furthermore, the effect of cholesterol on the phase behaviour of 3D-SLVs was monitored to provide new insights on how cholesterol stabilizes the lipid vesicle structures. Similarly, 2D-SLBs and 3D-SLVs were used to study the effect of antimicrobial and pore-forming melittin molecules (bee venom), serving as a pharmacological model on bacterial cell membranes. The biophysical and thermodynamic behaviour of these lipid geometries was monitored in real-time by QCM-D, while the morphological analysis was performed using dynamic light scattering (DLS). In these series of studies, we showed and proposed the application of QCM-D as a label-free surface sensitive technique to study the interactions of different molecules such as peptides, drugs and chemicals with lipid layers. This technique is a reliable and very informative methodology to understand the real-time action mechanisms of these molecules with model lipid membranes, which mimic real cell membranes without the hardly controllable complexity of truly biological membranes. Nevertheless, our approach paves the way to study the interactions of these types of molecules with complex, real cell membranes in the future. Furthermore, we proposed a mass sensitive methodology to study the nano-mechanical alterations in the phase transition of different lipids mixtures, which provides valuable information regarding the interaction mechanism of molecules with different lipid geometries. Secondly, the formation and electrical characterization of model cell membranes, in this case black lipid membranes (BLMs) were performed in porous filter supports, which is a useful model system for drug-permeability assays. In this regard, an electrophysiological setup consisting of a voltage amplifier and voltage waveform generator was employed to study the electrical properties of the BLMs. In this study, we showed successful application of cheap porous filters as supports to form stable and compact biomimetic lipid bilayers very similar to cell membranes. Furthermore, the thickness and compactness of the bilayers were evaluated using pore forming protein (ClyA) insertion, resulting in a stepwise increase of the leakage current associated with the opening of nanopores. The final important objective of this PhD was to explore the kinetics and thermal transport characteristics of alkanethiol SAMs in real-time. Furthermore, it was of interest to compare the heat blocking effect of SAMs with the same effect caused by much longer DNA fragments at the sold-liquid interface, which have been studied in previous research of our group. Therefore, the HTM was used to monitor heat-transport properties across the thiol nano-layers, while QCM-D was used as a complementary technique to document the time dependence of mass loading during SAM formation. In this study, we showed that the presence of 11- mercaptoundecanoic acid (11-MUA) SAMs on the gold surface with 1.5 nm length leads to a surprisingly strong increase of the interfacial heat-transfer resistance Rth which is comparable to the heat-flow blocking by 10 nm (30 base pairs) single-stranded DNA fragments. Furthermore, the real-time SAMs formation was observable in a two-step evolution, which can be attributed to a transition from a lying-down to a standing-up conformation of the thiols. By comparing thiols with different terminal groups (11-MUA and 1-dodecane thiol), we found evidence that the frequency matching of molecular vibrations of the terminal groups and the liquid (ethanol and water) is decisive for the efficiency of solid-to-liquid heat transfer. In this context we have also developed a simple algorithm to convert the data of heat-transfer resistance Rth to the thermal interface conductance G. This way, data obtained with the heattransfer method can be easily compared to other analytical methods and molecular dynamics simulations on interfacial heat transport.