Tackling sample-related artifacts in membrane FCS using parallel SAF and UAF detection

Supercritical Angle Fluorescence Microscopy and Spectroscopy

Fluorescence detection, either involving propagating or near-field emission, is widely being used in spectroscopy, sensing, and microscopy. Total internal reflection fluorescence (TIRF) confines fluorescence excitation by an evanescent (near) field, and it is a popular contrast generator for surface-selective fluorescence assays. Its emission equivalent, supercritical angle fluorescence (SAF), is comparably less established, although it achieves a similar optical sectioning as TIRF does. SAF emerges when a fluorescing molecule is located very close to an interface and its near-field emission couples to the higher refractive index medium (n2 >n1) and becomes propagative. Then, most fluorescence is detectable on the side of the higher-index substrate, and a large fraction of this fluorescence is emitted into angles forbidden by Snell's law. SAF, as well as the undercritical angle fluorescence (UAF; far-field emission) components, can be collected with microscope objectives having a high-enough detection aperture (numerical aperture >n2) and be separated in the back focal plane by Fourier filtering. The back focal plane image encodes information about the fluorophore radiation pattern, and it can be analyzed to yield precise information about the refractive index in which the emitters are embedded, their nanometric distance from the interface, and their orientation. A SAF microscope can retrieve this near-field information through wide-field optics in a spatially resolved manner, and this functionality can be added to an existing inverted microscope. Here, we describe the potential underpinning of SAF microscopy and spectroscopy, particularly in comparison with TIRF. We review the challenges and opportunities that SAF presents from a biophysical perspective, and we discuss areas in which we see potential.

Measuring membrane association and protein diffusion within membranes with supercritical angle fluorescence microscopy

Supercritical angle fluorescence (SAF) detection combines the axial discrimination and exquisite signal-to-noise ratio of total internal reflection fluorescence (TIRF) with the lateral discrimination and convenience of confocal excitation. This combination makes SAF ideal for fluorescence correlation spectroscopy (FCS) on membranes and other structures in close proximity to the coverslip. Here we report a straightforward modification of a commercial microscope to implement SAF FCS and demonstrate in both model supported lipid bilayers and cellular systems that this approach shows an increase in signal from membrane-bound fluorophores relative to fluorophores in solution, benchmarked against line-scanning FCS. SAF FCS allowed us to demonstrate that activation of the T cell receptor resulted in the recruitment of the kinase Lck to the plasma membrane as well as a reduction in Lck mobility within the membrane.

Simultaneous Surface-Near and Solution Fluorescence Correlation Spectroscopy

We report the first simultaneous measurement of surface-confined and solution fluorescence correlation spectroscopy (FCS). We use an optical configuration for tightly focused excitation and separate detection of light emitted below (undercritical angle fluorescence, UAF) and above (supercritical angle fluorescence, SAF) the critical angle of total internal reflection of the coverslip/sample interface. This creates two laterally coincident detection volumes which differ in their axial extent. While detection of far-field UAF emission producesa standard confocal volume, near-field-mediated SAF produces a highly surface-confined detection volume at the coverslip/sample interface which extends only ~200 nm into the sample. A characterization of the two detection volumes by FCS of free diffusion is presented and compared with analytical models and simulations. The presented FCS technique allows to determine bulk solution concentrations and surface-near concentrations at the same time.

Eliminating unwanted far-field excitation in objective-type TIRF. Part II. combined evanescent-wave excitation and supercritical-angle fluorescence detection improves optical sectioning

Azimuthal beam scanning makes evanescent-wave (EW) excitation isotropic, thereby producing total internal reflection fluorescence (TIRF) images that are evenly lit. However, beam spinning does not fundamentally address the problem of propagating excitation light that is contaminating objective-type TIRF. Far-field excitation depends more on the specific objective than on cell scattering. As a consequence, the excitation impurities in objective-type TIRF are only weakly affected by changes of azimuthal or polar beam angle. These are the main results of the first part of this study (Eliminating unwanted far-field excitation in objective-type TIRF. Pt.1. Identifying sources of nonevanescent excitation light). This second part focuses on exactly where up beam in the illumination system stray light is generated that gives rise to nonevanescent components in TIRF. Using dark-field imaging of scattered excitation light we pinpoint the objective, intermediate lenses and, particularly, the beam scanner as the major sources of stray excitation. We study how adhesion-molecule coating and astrocytes or BON cells grown on the coverslip surface modify the dark-field signal. On flat and weakly scattering cells, most background comes from stray reflections produced far from the sample plane, in the beam scanner and the objective lens. On thick, optically dense cells roughly half of the scatter is generated by the sample itself. We finally show that combining objective-type EW excitation with supercritical-angle fluorescence (SAF) detection efficiently rejects the fluorescence originating from deeper sample regions. We demonstrate that SAF improves the surface selectivity of TIRF, even at shallow penetration depths. The coplanar microscopy scheme presented here merges the benefits of beam spinning EW excitation and SAF detection and provides the conditions for quantitative wide-field imaging of fluorophore dynamics at or near the plasma membrane.