Lorentzian spatial intensity distribution in one-photon fluorescence correlation spectroscopy

Investigating axial diffusion in cylindrical pores using confocal single-particle fluorescence correlation spectroscopy

We explored the feasibility of using confocal fluorescence correlation spectroscopy to study small nanoparticle diffusion in hundred-nanometer-sized cylindrical pores. By modeling single particle diffusion in tube-like confined three-dimensional space aligned parallel to the confocal optical axis, we showed that two diffusion dynamics can be observed in both original intensity traces and the autocorrelation functions (ACFs): the confined two-dimensional lateral diffusion and the unconfined one-dimensional (1D) axial diffusion. The separation of the axial and confined lateral diffusion dynamics provides an opportunity to study diffusions in different dimensions separately. We further experimentally studied 45 nm carboxylated polystyrene particles diffusing in 300 nm alumina pores. The experimental data showed consistency with the simulation. To extract the accurate axial diffusion coefficient, we found that a 1D diffusion model with a Lorentzian axial collection profile needs to be used to analyze the experimental ACFs. The diffusion of the 45 nm nanoparticles in polyethyleneglycol-passivated 300 nm pores slowed down by a factor of ∼2, which can be satisfactorily explained by hydrodynamic frictions.

A straightforward approach for gated STED-FCS to investigate lipid membrane dynamics

Recent years have seen the development of multiple technologies to investigate, with great spatial and temporal resolution, the dynamics of lipids in cellular and model membranes. One of these approaches is the combination of far-field super-resolution stimulated-emission-depletion (STED) microscopy with fluorescence correlation spectroscopy (FCS). STED-FCS combines the diffraction-unlimited spatial resolution of STED microscopy with the statistical accuracy of FCS to determine sub-millisecond-fast molecular dynamics with single-molecule sensitivity. A unique advantage of STED-FCS is that the observation spot for the FCS data recordings can be tuned to sub-diffraction scales, i.e. <200 nm in diameter, in a gradual manner to investigate fast diffusion of membrane-incorporated labelled entities. Unfortunately, so far the STED-FCS technology has mostly been applied on a few custom-built setups optimised for far-red fluorescent emitters. Here, we summarise the basics of the STED-FCS technology and highlight how it can give novel details into molecular diffusion modes. Most importantly, we present a straightforward way for performing STED-FCS measurements on an unmodified turnkey commercial system using a time-gated detection scheme. Further, we have evaluated the STED-FCS performance of different commonly used green emitting fluorescent dyes applying freely available, custom-written analysis software.

STED-FLCS: An Advanced Tool to Reveal Spatiotemporal Heterogeneity of Molecular Membrane Dynamics

Heterogeneous diffusion dynamics of molecules play an important role in many cellular signaling events, such as of lipids in plasma membrane bioactivity. However, these dynamics can often only be visualized by single-molecule and super-resolution optical microscopy techniques. Using fluorescence lifetime correlation spectroscopy (FLCS, an extension of fluorescence correlation spectroscopy, FCS) on a super-resolution stimulated emission depletion (STED) microscope, we here extend previous observations of nanoscale lipid dynamics in the plasma membrane of living mammalian cells. STED-FLCS allows an improved determination of spatiotemporal heterogeneity in molecular diffusion and interaction dynamics via a novel gated detection scheme, as demonstrated by a comparison between STED-FLCS and previous conventional STED-FCS recordings on fluorescent phosphoglycerolipid and sphingolipid analogues in the plasma membrane of live mammalian cells. The STED-FLCS data indicate that biophysical and biochemical parameters such as the affinity for molecular complexes strongly change over space and time within a few seconds. Drug treatment for cholesterol depletion or actin cytoskeleton depolymerization not only results in the already previously observed decreased affinity for molecular interactions but also in a slight reduction of the spatiotemporal heterogeneity. STED-FLCS specifically demonstrates a significant improvement over previous gated STED-FCS experiments and with its improved spatial and temporal resolution is a novel tool for investigating how heterogeneities of the cellular plasma membrane may regulate biofunctionality.