Super-resolution imaging of C-type lectin and influenza hemagglutinin nanodomains on plasma membranes using blink microscopy.

Dendritic cells express DC-SIGN, a C-type lectin (CTL) that binds a variety of pathogens and facilitates their uptake for subsequent antigen presentation. DC-SIGN forms remarkably stable microdomains on the plasma membrane. However, inner leaflet lipid markers are able to diffuse through these microdomains suggesting that, rather than being densely packed with DC-SIGN proteins, an elemental substructure exists. Therefore, a super-resolution imaging technique, Blink Microscopy (Blink), was applied to further investigate the lateral distribution of DC-SIGN. Blink indicates that DC-SIGN, another CTL (CD206), and influenza hemagglutinin (HA) are all localized in small (∼80 nm in diameter) nanodomains. DC-SIGN and CD206 nanodomains are randomly distributed on the plasma membrane, whereas HA nanodomains cluster on length scales up to several microns. We estimate, as a lower limit, that DC-SIGN and HA nanodomains contain on average two tetramers or two trimers, respectively, whereas CD206 is often nonoligomerized. Two-color Blink determined that different CTLs rarely occupy the same nanodomain, although they appear colocalized using wide-field microscopy. What to our knowledge is a novel domain structure emerges in which elemental nanodomains, potentially capable of binding viruses, are organized in a random fashion; evidently, these nanodomains can be clustered into larger microdomains that act as receptor platforms for larger pathogens like yeasts.

Controlling the fluorescence of ordinary oxazine dyes for single-molecule switching and superresolution microscopy.

Fluorescent molecular switches have widespread potential for use as sensors, material applications in electro-optical data storages and displays, and superresolution fluorescence microscopy. We demonstrate that adjustment of fluorophore properties and environmental conditions allows the use of ordinary fluorescent dyes as efficient single-molecule switches that report sensitively on their local redox condition. Adding or removing reductant or oxidant, switches the fluorescence of oxazine dyes between stable fluorescent and nonfluorescent states. At low oxygen concentrations, the off-state that we ascribe to a radical anion is thermally stable with a lifetime in the minutes range. The molecular switches show a remarkable reliability with intriguing fatigue resistance at the single-molecule level: Depending on the switching rate, between 400 and 3,000 switching cycles are observed before irreversible photodestruction occurs. A detailed picture of the underlying photoinduced and redox reactions is elaborated. In the presence of both reductant and oxidant, continuous switching is manifested by "blinking" with independently controllable on- and off-state lifetimes in both deoxygenated and oxygenated environments. This "continuous switching mode" is advantageously used for imaging actin filament and actin filament bundles in fixed cells with subdiffraction-limited resolution.

Single-molecule four-color FRET visualizes energy-transfer paths on DNA origami.

Fluorescence resonance energy transfer (FRET) represents a mechanism to transport light energy at the nanoscale, as exemplified by nature's light-harvesting complexes. Here we used DNA origami to arrange fluorophores that transport excited-state energy from an input dye to an output dye. We demonstrate that energy-transfer paths can be controlled on the single-molecule level by the presence of a "jumper" dye that directs the excited-state energy either to a red or to an IR output dye. We used single-molecule four-color FRET with alternating laser excitation to sort subpopulations and to visualize the control of energy transfer.

Mechanisms and advancement of antifading agents for fluorescence microscopy and single-molecule spectroscopy.

Modern fluorescence microscopy applications go along with increasing demands for the employed fluorescent dyes. In this work, we compared antifading formulae utilizing a recently developed reducing and oxidizing system (ROXS) with commercial antifading agents. To systematically test fluorophore performance in fluorescence imaging of biological samples, we carried out photobleaching experiments using fixed cells labeled with various commonly used organic dyes, such as Alexa 488, Alexa 594, Alexa 647, Cy3B, ATTO 550, and ATTO 647N. Quantitative evaluation of (i) photostability, (ii) brightness, and (iii) storage stability of fluorophores in samples mounted in different antifades (AFs) reveal optimal combinations of dyes and AFs. Based on these results we provide guidance on which AF should preferably be used with a specific dye. Finally, we studied the antifading mechanisms of the commercial AFs using single-molecule spectroscopy and reveal that these empirically selected AFs exhibit similar properties to ROXS AFs.

Intrinsically resolution enhancing probes for confocal microscopy.

In recent years different implementations of superresolution microscopy based on targeted switching (STED, GSD, and SSIM) have been demonstrated. The key elements to break the diffraction barrier are two distinct molecular states that generate a saturable nonlinear fluorescence response with respect to the excitation intensity. In this paper, we demonstrate that a nonlinearity can even be encoded in fluorescent probes, which then increase the resolution of a standard confocal microscope. This nonlinearity is achieved by an intensity dependent blocking of the resonance energy transfer between a donor and one or more acceptor fluorophores, utilizing radical anion states of the acceptor. In proof-of-principle experiments, we demonstrate a significant resolution increase using probes with different numbers of acceptor fluorophores. Quantitative description by a theoretical model paves the way for the development of fluorescent probes that can more than double the resolution of essentially any confocal microscope in all three dimensions.

Single-molecule kinetics and super-resolution microscopy by fluorescence imaging of transient binding on DNA origami.

DNA origami is a powerful method for the programmable assembly of nanoscale molecular structures. For applications of these structures as functional biomaterials, the study of reaction kinetics and dynamic processes in real time and with high spatial resolution becomes increasingly important. We present a single-molecule assay for the study of binding and unbinding kinetics on DNA origami. We find that the kinetics of hybridization to single-stranded extensions on DNA origami is similar to isolated substrate-immobilized DNA with a slight position dependence on the origami. On the basis of the knowledge of the kinetics, we exploit reversible specific binding of labeled oligonucleotides to DNA nanostructures for PAINT (points accumulation for imaging in nanoscale topography) imaging with <30 nm resolution. The method is demonstrated for flat monomeric DNA structures as well as multimeric, ribbon-like DNA structures.

Resolving single-molecule assembled patterns with superresolution blink-microscopy.

In this paper we experimentally combine a recently developed AFM-based molecule-by-molecule assembly (single-molecule cut-and-paste, SMCP) with subdiffraction resolution fluorescence imaging. Using "Blink-Microscopy", which exploits the fluctuating emission of single molecules for the reconstruction of superresolution images, we resolved SMCP assembled structures with features below the diffraction limit. Artificial line patterns then served as calibration structures to characterize parameters, such as the labeling density, that can influence resolution of Blink-Microscopy besides the localization precision of a single molecule. Finally, we experimentally utilized the adjustability of blink parameters to demonstrate the general connection of photophysical parameters with spatial resolution and acquisition time in superresolution microscopy.

Make them blink: probes for super-resolution microscopy.

In recent years, a number of approaches have emerged that enable far-field fluorescence imaging beyond the diffraction limit of light, namely super-resolution microscopy. These techniques are beginning to profoundly alter our abilities to look at biological structures and dynamics and are bound to spread into conventional biological laboratories. Nowadays these approaches can be divided into two categories, one based on targeted switching and readout, and the other based on stochastic switching and readout of the fluorescence information. The main prerequisite for a successful implementation of both categories is the ability to prepare the fluorescent emitters in two distinct states, a bright and a dark state. Herein, we provide an overview of recent developments in super-resolution microscopy techniques and outline the special requirements for the fluorescent probes used. In combination with the advances in understanding the photophysics and photochemistry of single fluorophores, we demonstrate how essentially any single-molecule compatible fluorophore can be used for super-resolution microscopy. We present examples for super-resolution microscopy with standard organic fluorophores, discuss factors that influence resolution and present approaches for calibration samples for super-resolution microscopes including AFM-based single-molecule assembly and DNA origami.

Correlated movement and bending of nucleic acid structures visualized by multicolor single-molecule spectroscopy.

At the end of the rainbow.... The structure and dynamics of three- and four- way DNA junctions are studied using triple alternating laser excitation. Steroid binding distorts the structure of three-way junctions with correlated bending of all branches towards a steeper pyramid (see picture). In four-way junctions, the dynamics of three distances are visualized simultaneously.

Superresolution microscopy on the basis of engineered dark states.

New concepts for superresolution fluorescence microscopy by subsequent localization of single molecules using photoswitchable or photoactivatable fluorophores are rapidly emerging and provide new ways to resolve structures beyond the diffraction limit. Here, we demonstrate that superresolution imaging can be carried out with practically every single-molecule compatible, synthetic fluorophore by controlling their emission properties. We prepare dark states by removing oxygen that extends the triplet state lifetime to several milliseconds. We further increase the duration of the off-states using electron transfer reactions to create radical ion states of severalfold longer lifetimes. Imaging single molecules, actin filaments, and microtubules in fixed cells as well as simulations demonstrate that the thus created dark states are sufficiently long for resolution of approximately 50 nm.

Super-resolution fluorescence imaging with blink microscopy.

Recently, a new approach for super-resolution microscopy has emerged which is based on the successive localization of single molecules. The majority of molecules are prepared to reside in a nonfluorescent dark state, leaving only a few single molecules fluorescing. The single molecules can subsequently be localized on the camera image. Successive localization of all molecules allows reconstruction of a super-resolved image of the labeled structure. A variety of ways for limiting the number of locatable molecules have been developed recently which expand this current field of imaging. Here we describe a super-resolution microscopy method that employs the use of reversible, generic dark states, for example radical ion states. This method requires only a single laser source and can be carried out with many fluorescent dyes, in some cases, even in living cells. We provide a step-by-step procedure for this method, which we have called Blink Microscopy.

Distance dependence of single-fluorophore quenching by gold nanoparticles studied on DNA origami.

We study the distance-dependent quenching of fluorescence due to a metallic nanoparticle in proximity of a fluorophore. In our single-molecule measurements, we achieve excellent control over structure and stoichiometry by using self-assembled DNA structures (DNA origami) as a breadboard where both the fluorophore and the 10 nm metallic nanoparticle are positioned with nanometer precision. The single-molecule spectroscopy method employed here reports on the co-localization of particle and dye, while fluorescence lifetime imaging is used to directly obtain the correlation of intensity and fluorescence lifetime for varying particle to dye distances. Our data can be well explained by exact calculations that include dipole-dipole orientation and distances. Fitting with a more practical model for nanosurface energy transfer yields 10.4 nm as the characteristic distance of 50% energy transfer. The use of DNA nanotechnology together with minimal sample usage by attaching the particles to the DNA origami directly on the microscope coverslip paves the way for more complex experiments exploiting dye-nanoparticle interactions.