Supported Lipid Membranes and Their Use for the Characterization of Biological Nanoparticles

Sammanfattning: Biological nanoparticles (BNPs) are nano-sized lipid vesicles of biological origin, which are involved in multiple biological processes. BNP characterization techniques are critical for improving the understanding of how these particles contribute to cellular communication, viral infections and drug-delivery applications. However, due to their small size (between 50 and 200 nm in diameter) and molecular heterogeneity, quantitative characterization of their physical, chemical and biological properties is demanding, especially since their large structural and compositional heterogeneity calls for methods with single nanoparticle resolution. To address this challenge, work in this thesis has been focused on investigating and using supported lipid bilayers (SLBs) and their two-dimensional fluidity as a platform for nanoparticle characterization. To investigate SLBs, we combined confocal microscopy with microfluidics to identify the mechanisms by which lipid vesicles are spontaneously converted into various types of planar membranes on a multitude of surfaces (Paper I) and found that most of the studied materials can support lipid film formation. In the context of SLB formation, specific focus was put on using total internal reflection fluorescence (TIRF) microscopy to monitor the kinetics of vesicle adsorption, rupture and spreading of individual SLB patches on glass (Paper II), revealing that the SLB formation process was driven by the autocatalytic growth and merger of multiple small SLB patches at appreciably high vesicle coverage. TIRF was also successfully employed to monitor lipid-enveloped drug permeation through an SLB formed on a mesoporous silica thin film (Paper III). The insights gained from investigating SLBs was also used for in depth characterization of BNPs using the surface-based flow-nanometry method, allowing for independent determination of size and fluorescence emission of individual BNPs tethered to a laterally fluid SLB formed on the floor of a microfluidic channel. This way we could demonstrate that the fluorescence emission from lipophilic dyes depends in a non-trivial way on nanoparticle size, and varies significantly between the different types of BNPs (Paper IV). The flow-nanometry concept was also used to elucidate the effect of vesicle size on their diffusivity on the SLB in the limit of few tethers (Paper V). The insights gained in this thesis work on lipid self-assembly at different surfaces and the possibility to use SLBs on silica for in-depth characterization of BNPs demonstrate this as a promising approach in the field of single nanoparticle analytics, which in future work will be possible to extend into a novel means to probe interactions between BNPs and cell-membrane mimics representing a near-native situation.