Stephen H. Donaldson Jr.

arXiv2019

A Multi-Modal Miniature Surface Forces Apparatus (uSFA) for Interfacial Science Measurements

Advances in the research of intermolecular and surface interactions result from the development of new and improved measurement techniques and combinations of existing techniques. Here, we present a new miniature version of the Surface Force Apparatus (the uSFA) that has been designed for ease of use and multi-modal capabilities with retention of the capabilities of other SFA models including accurate measurement of surface separation distance and physical characterization of dynamic and static physical forces (i.e., normal, shear, and friction) and interactions (e.g., van der Waals, electrostatic, hydrophobic, steric, biospecific). The small physical size of the uSFA, compared to previous SFA models, makes it portable and suitable for integration into commercially available optical and fluorescence light microscopes, as demonstrated here. The large optical path entry and exit ports make it ideal for concurrent force measurements and spectroscopy studies. Examples of the use of the uSFA in combination with surface plasmon resonance (SPR) and Raman spectroscopy measurements are presented. Due to the short working distance constraints associated with Raman spectroscopy, an interferometric technique was developed and applied for calculating the inter-surface separation distance based on Newtons rings. The introduction of the \muSFA will mark a transition in SFA usage from primarily physical characterization to concurrent physical characterization with in situ chemical and biological characterization to study interfacial phenomena, including (but not limited to) molecular adsorption, fluid flow dynamics, determination of surface species and morphology, and (bio) molecular binding kinetics.

physics.ins-det↗

arXiv2018

Label-free imaging of cholesterol and lipid distributions in model membranes

Over recent decades, lipid membranes have become standard models for examining the biophysics and biochemistry of cell membranes. Interrogation of lipid domains within biomembranes is generally done with fluorescence microscopy via exogenous chemical probes. However, most fluorophores have limited partitioning tunability, with the majority segregating in the least biologically relevant domains (i.e., low-density liquid domains). Therefore, a molecular-level picture of the majority of non-labeled lipids forming the membrane is still elusive. Here, we present simple, label-free imaging of domain formation in lipid monolayers, with chemical selectivity in unraveling lipid and cholesterol composition in all domain types. Exploiting conventional vibrational contrast in spontaneous Raman imaging, combined with chemometrics analysis, allows for examination of ternary systems containing saturated lipids, unsaturated lipids, and cholesterol. We confirm features commonly observed by fluorescence microscopy, and provide an unprecedented analysis of cholesterol distribution at the single-membrane level.

physics.bio-ph↗