Analytical chemistry. How to detect weak pairs

High photon count rates improve the quality of super-resolution fluorescence fluctuation spectroscopy

Probing the diffusion of molecules has become a routine measurement across the life sciences, chemistry and physics. It provides valuable insights into reaction dynamics, oligomerisation, molecular (re-)organisation or cellular heterogeneities. Fluorescence correlation spectroscopy (FCS) is one of the widely applied techniques to determine diffusion dynamics in two and three dimensions. This technique relies on the temporal autocorrelation of intensity fluctuations but recording these fluctuations has thus far been limited by the detection electronics, which could not efficiently and accurately time-tag photons at high count rates. This has until now restricted the range of measurable dye concentrations, as well as the data quality of the FCS recordings, especially in combination with super-resolution stimulated emission depletion (STED) nanoscopy. Here, we investigate the applicability and reliability of (STED-)FCS at high photon count rates (average intensities of more than 1 MHz) using novel detection equipment, namely hybrid detectors and real-time gigahertz sampling of the photon streams implemented on a commercial microscope. By measuring the diffusion of fluorophores in solution and cytoplasm of live cells, as well as in model and cellular membranes, we show that accurate diffusion and concentration measurements are possible in these previously inaccessible high photon count regimes. Specifically, it offers much greater flexibility of experiments with biological samples with highly variable intensity, e.g. due to a wide range of expression levels of fluorescent proteins. In this context, we highlight the independence of diffusion properties of cytosolic GFP in a concentration range of approx. 0.01-1 µm. We further show that higher photon count rates also allow for much shorter acquisition times, and improved data quality. Finally, this approach also pronouncedly increases the robustness of challenging live cell STED-FCS measurements of nanoscale diffusion dynamics, which we testify by confirming a free diffusion pattern for a fluorescent lipid analogue on the apical membrane of adherent cells.

Single-Molecule Tracking and Its Application in Biomolecular Binding Detection

In the past two decades significant advances have been made in single-molecule detection, which enables the direct observation of single biomolecules at work in real time and under physiological conditions. In particular, the development of single-molecule tracking (SMT) microscopy allows us to monitor the motion paths of individual biomolecules in living systems, unveiling the localization dynamics and transport modalities of the biomolecules that support the development of life. Beyond the capabilities of traditional camera-based tracking techniques, state-of-the-art SMT microscopies developed in recent years can record fluorescence lifetime while tracking a single molecule in the 3D space. This multiparameter detection capability can open the door to a wide range of investigations at the cellular or tissue level, including identification of molecular interaction hotspots and characterization of association/dissociation kinetics between molecules. In this review, we discuss various SMT techniques developed to date, with an emphasis on our recent development of the next generation 3D tracking system that not only achieves ultrahigh spatiotemporal resolution but also provides sufficient working depth suitable for live animal imaging. We also discuss the challenges that current SMT techniques are facing and the potential strategies to tackle those challenges.

Femtoliter-scale optical nanofiber sensors

We report a robust and sensitive optical nanofiber sensor with a femtoliter-scale detection volume. The sensor is fabricated by embedding a 800-nm-diameter nanofiber into a microfluidic chip with probing light propagated perpendicular to a 5-μm-wide detection channel. To verify the effectiveness of the sensor, we present measurements of fluorescence intensity and refractive index (RI) with detection limits of 1 × 10(-7) M for fluorescein and 2.8 × 10(-4) RIU, respectively. The femtoliter-scale optical nanofiber sensor shown here may provide a compact and versatile sensing platform for sensitive and fast detection of ultra-low-volume samples, as well as studying the dynamics of single molecule.

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.

Fluorescence correlation spectroscopy at micromolar concentrations without optical nanoconfinement

Fluorescence correlation spectroscopy (FCS) is an important technique for studying biochemical interactions dynamically that may be used in vitro and in cell-based studies. It is generally claimed that FCS may only be used at nM concentrations. We show that this general consensus is incorrect and that the limitation to nM concentrations is not fundamental but due to detector limits as well as laser fluctuations. With a high count rate detector system and applying laser fluctuation corrections, we demonstrate FCS measurements up to 38 μM with the same signal-to-noise as at lower concentrations. Optical nanoconfinement approaches previously used to increase the concentration range of FCS are not necessary, and further increases above 38 μM may be expected using detectors and detector arrays with higher saturation rates and better laser fluctuation corrections. This approach greatly widens the possibilities of dynamic measurements of biochemical interactions using FCS at physiological concentrations.

Breaking the concentration limit of optical single-molecule detection

Over the last decade, single-molecule detection has been successfully utilized in the life sciences and materials science. Yet, single-molecule measurements only yield meaningful results when working in a suitable, narrow concentration range. On the one hand, diffraction limits the minimal size of the observation volume in optical single-molecule measurements and consequently a sample must be adequately diluted so that only one molecule resides within the observation volume. On the other hand, at ultra-low concentrations relevant for sensing, the detection volume has to be increased in order to detect molecules in a reasonable timespan. This in turn results in the loss of an optimal signal-to-noise ratio necessary for single-molecule detection. This review discusses the requirements for effective single-molecule fluorescence applications, reflects on the motivation for the extension of the dynamic concentration range of single-molecule measurements and reviews various approaches that have been introduced recently to solve these issues. For the high-concentration limit, we identify four promising strategies including molecular confinement, optical observation volume reduction, temporal separation of signals and well-conceived experimental designs that specifically circumvent the high concentration limit. The low concentration limit is addressed by increasing the measurement speed, parallelization, signal amplification and preconcentration. The further development of these ideas will expand our possibilities to interrogate research questions with the clarity and precision provided only by the single-molecule approach.

Improving zero-mode waveguide structure for enhancing signal-to-noise ratio of real-time single-molecule fluorescence imaging: a computational study

We investigated the signal-to-noise ratio (S/N) of real-time single-molecule fluorescence imaging (SMFI) using zero-mode waveguides (ZMWs). The excitation light and the fluorescence propagating from a molecule in the ZMW were analyzed by computational optics simulation. The dependence of the S/N on the ZMW structure was investigated with the diameter and etching depth as the simulation parameters. We found that the SMFI using a conventional ZMW was near the critical level for detecting binding and dissociation events. We show that etching the glass surface of the ZMW by 60 nm enhances the S/N six times the conventional nonetched ZMWs. The enhanced S/N improves the temporal resolution of the SMFI at physiological concentrations.

Regulation of DNA conformations and dynamics in flows with hybrid field microfluidics

Visualizing single DNA dynamics in flow provides a wealth of physical insights in biophysics and complex flow study. However, large signal fluctuations, generated from diversified conformations, deformation history dependent dynamics and flow induced stochastic tumbling, often frustrate its wide adoption in single molecule and polymer flow study. We use a hybrid field microfluidic (HFM) approach, in which an electric field is imposed at desired locations and appropriate moments to balance the flow stress on charged molecules, to effectively regulate the initial conformations and the deformation dynamics of macromolecules in flow. With λ-DNA and a steady laminar shear flow as the model system, we herein studied the performance of HFM on regulating DNA trapping, relaxation, coil-stretch transition, and accumulation. DNA molecules were found to get captured in the focused planes when motions caused by flow, and the electric field were balanced. The trapped macromolecules relaxed in two different routes while eventually became more uniform in size and globule conformations. When removing the electric field, the sudden stretching dynamics of DNA molecules exhibited a more pronounced extension overshoot in their transient response under a true step function of flow stress while similar behaviors to what other pioneering work in steady shear flow. Such regulation strategies could be useful to control the conformations of other important macromolecules (e.g., proteins) and help better reveal their molecular dynamics.

A zeptoliter volume meter for analysis of single protein molecules

A central goal in bioanalytics is to determine the concentration of and interactions between biomolecules. Nanotechnology allows performing such analyses in a highly parallel, low-cost, and miniaturized fashion. Here we report on label-free volume, concentration, and mobility analysis of single protein molecules and nanoparticles during their diffusion through a subattoliter detection volume, confined by a 100 nm aperture in a thin gold film. A high concentration of small fluorescent molecules renders the aqueous solution in the aperture brightly fluorescent. Nonfluorescent analytes diffusing into the aperture displace the fluorescent molecules in the solution, leading to a decrease of the detected fluorescence signal, while analytes diffusing out of the aperture return the fluorescence level. The resulting fluorescence fluctuations provide direct information on the volume, concentration, and mobility of the nonfluorescent analytes through fluctuation analysis in both time and amplitude.

Nanovesicle trapping for studying weak protein interactions by single-molecule FRET

Protein-protein interactions are fundamental biological processes. While strong protein interactions are amenable to many characterization techniques including crystallography, weak protein interactions are challenging to study because of their dynamic nature. Single-molecule fluorescence resonance energy transfer (smFRET) can monitor dynamic protein interactions in real time, but are generally limited to strong interacting pairs because of the low concentrations needed for single-molecule detection. Here, we describe a nanovesicle trapping approach to enable smFRET study of weak protein interactions at high effective concentrations. We describe the experimental procedures, summarize the application in studying the weak interactions between intracellular copper transporters, and detail the single-molecule kinetic analysis of bimolecular interactions involving three states. Both the experimental approach and the theoretical analysis are generally applicable to studying many other biological processes at the single-molecule level.

In vivo assembly and single-molecule characterization of the transcription machinery from Shewanella oneidensis MR-1

Harnessing the new bioremediation and biotechnology applications offered by the dissimilatory metal-reducing bacteria, Shewanella oneidensis MR-1, requires a clear understanding of its transcription machinery, a pivotal component in maintaining vitality and in responding to various conditions, including starvation and environmental stress. Here, we have reconstituted the S. oneidensis RNA polymerase (RNAP) core in vivo by generating a co-overexpression construct that produces a long polycistronic mRNA encoding all of the core subunits (alpha, beta, beta', and omega) and verified that this reconstituted core is capable of forming fully functional holoenzymes with the S. oneidensis sigma factors sigma(70), sigma(38), sigma(32), and sigma(24). Further, to demonstrate the applications for this reconstituted core, we report the application of single-molecule fluorescence resonance energy transfer (smFRET) assays to monitor the mechanisms of transcription by the S. oneidensis sigma(70)-RNAP holoenyzme. These results show that the reconstituted transcription machinery from S. oneidensis, like its Escherichia coli counterpart, "scrunches" the DNA into its active center during initial transcription, and that as the holoenzyme transitions into elongation, the release of sigma(70) is non-obligatory.

Single-molecule fluorescence resonance energy transfer in nanopipets: improving distance resolution and concentration range

In recent years fluorescence resonance energy transfer (FRET) has widely been used to measure distances, binding, and distance dynamics at the single-molecule (sm) level. Some basic constraints of smFRET are the limited distance resolution owing to low photon statistics and the restriction to high affinity interactions. We demonstrate that by confining molecules in nanopipets with an inner diameter of approximately 100 nm at the tip, FRET can be measured with improved photon statistics and at up to 50-fold higher concentrations. The flow of the donor/acceptor (Cy3B/ATTO647N) labeled double-stranded DNA conjugates was established by electrokinetic forces. Because of the small inner diameter of the nanopipet, every molecule passing the tip is detected applying alternating laser excitation (ALEX). Thus, the technique offers the advantage to study interactions with smaller association constants (<10(9) M-1) using minute sample amounts (<5 microL). The improved photon statistics reduces shot-noise contributions and results in sharper FRET distributions. Experimental results are supported by Monte Carlo simulations which also explain the occurrence of two populations in burst size distributions measured in nanopipet experiments. Because of the confinement of the molecules in nanopipets, the widths of FRET histograms are reduced to a degree where shot-noise is not the only limiting factor but also conformational dynamics of the linkers used to attach the chromophores have to be considered. In addition, our experiments emphasize the influence of photoinduced dark states on both the mean energy transfer efficiency and the width of FRET histograms.

Fueling protein DNA interactions inside porous nanocontainers

Vesicle encapsulation offers a biologically relevant environment for many soluble proteins and nucleic acids and an optimal immobilization medium for single-molecule fluorescence assays. Furthermore, the confinement of biomolecules within small volumes opens up new avenues to unique experimental configurations. Nevertheless, the vesicles' impermeability, even toward ions and other small molecules such as ATP, hinders more general applications. We therefore developed methods to induce pores into vesicles. Porous vesicles were then used to modulate the interaction between Escherichia coli RecA proteins and ssDNA by changing the extravesicular nucleotides. Repetitive binding and dissociation of the same RecA filament on the DNA was observed with a rebinding rate two orders of magnitude greater than in the absence of confinement, suggesting a previously unreported nucleation pathway for RecA filament. This method provides a biofriendly and simple alternative to surface tethering that is ideal for the study of transient and weakly interacting biological complexes.

Lateral mobility of membrane-binding proteins in living cells measured by total internal reflection fluorescence correlation spectroscopy

Total internal reflection fluorescence correlation spectroscopy (TIR-FCS) allows us to measure diffusion constants and the number of fluorescent molecules in a small area of an evanescent field generated on the objective of a microscope. The application of TIR-FCS makes possible the characterization of reversible association and dissociation rates between fluorescent ligands and their receptors in supported phospholipid bilayers. Here, for the first time, we extend TIR-FCS to a cellular application for measuring the lateral diffusion of a membrane-binding fluorescent protein, farnesylated EGFP, on the plasma membranes of cultured HeLa and COS7 cells. We detected two kinds of diffusional motion-fast three-dimensional diffusion (D(1)) and much slower two-dimensional diffusion (D(2)), simultaneously. Conventional FCS and single-molecule tracking confirmed that D(1) was free diffusion of farnesylated EGFP close to the plasma membrane in cytosol and D(2) was lateral diffusion in the plasma membrane. These results suggest that TIR-FCS is a powerful technique to monitor movement of membrane-localized molecules and membrane dynamics in living cells.

Alternating-laser excitation of single molecules

Single-molecule fluorescence spectroscopy addresses biological mechanisms and enables ultrasensitive diagnostics. We describe a new family of single-molecule fluorescence methods that uses alternating-laser excitation (ALEX) of diffusing or immobilized biomolecules to study their structure, interactions, and dynamics. This is accomplished using ratios that report on the distance between and the stoichiometry of fluorophores attached to the molecules of interest. The principle of alternation is compatible with several time scales, allowing monitoring of fast dynamics or simultaneous monitoring of a large number of individual molecules.

A fluorescence resonance energy transfer-based probe to monitor nucleosome structure

Nucleosomes are the basic units of eukaryotic chromatin structure. By restricting factor access to regulatory DNA sequences, nucleosomes significantly impact genomic processes such as transcription, and various mechanisms to alter nucleosome structure to relieve this repression have evolved. Both nucleosomes and processes that alter them are inherently dynamic in nature. Thus, studies of dynamics will be necessary to truly understand these relief mechanisms. We describe here the characteristics of a novel fluorescence resonance energy transfer-based reporter that can clearly signal the formation of a canonical nucleosome structure and follow conformational and compositional changes in that structure, both at the ensemble-average (bulk) and at the single molecule level. Labeled nucleosomes behave conformationally and thermodynamically like typical nucleosomes; thus they are relevant reporters of nucleosome behavior. Nucleosomes and free DNA are readily distinguishable at the single-molecule level. Thus, these labeled nucleosomes are well suited to studies of dynamic changes in nucleosome structure including single-molecule dynamics.

Fluorescence fluctuation spectroscopy in subdiffraction focal volumes

We establish fluorescence fluctuation spectroscopy (FFS) with nanoscale detection volumes generated by stimulated emission depletion. Our method applies fluorescence correlation spectroscopy and fluorescence intensity distribution analysis to extract molecular information about mobilities and fluorescence emission in solution. The combination of correlation analysis with that of photon intensity distributions reveals a fivefold squeezing of the detection volume over current diffraction-limited systems, which is in full agreement with the simultaneously demonstrated 25-fold reduction in (axial) focal transit time. Our method significantly extends the potential of far-field FFS, including for the noninvasive investigation of molecular reactions at higher concentrations.

Branching Out of Single‐Molecule Fluorescence Spectroscopy: Challenges for Chemistry and Influence on Biology

In the last decade emerging single-molecule fluorescence-spectroscopy tools have been developed and adapted to analyze individual molecules under various conditions. Single-molecule-sensitive optical techniques are now well established and help to increase our understanding of complex problems in different disciplines ranging from materials science to cell biology. Previous dreams, such as the monitoring of the motility and structural changes of single motor proteins in living cells or the detection of single-copy genes and the determination of their distance from polymerase molecules in transcription factories in the nucleus of a living cell, no longer constitute unsolvable problems. In this Review we demonstrate that single-molecule fluorescence spectroscopy has become an independent discipline capable of solving problems in molecular biology. We outline the challenges and future prospects for optical single-molecule techniques which can be used in combination with smart labeling strategies to yield quantitative three-dimensional information about the dynamic organization of living cells.

Rapid analysis of Forster resonance energy transfer by two-color global fluorescence correlation spectroscopy: trypsin proteinase reaction

In this study we introduce the combination of two-color global fluorescence correlation spectroscopy (2CG-FCS) and Förster resonance energy transfer (FRET) as a very powerful combination for monitoring biochemical reactions on the basis of single molecule events. 2CG-FCS, which is a new variation emerging from the family of fluorescence correlation spectroscopy, globally analyzes the simultaneously recorded auto- and cross-correlation data from two photon detectors monitoring the fluorescence emission of different colors. Overcoming the limitations inherent in mere auto- and cross-correlation analysis, 2CG-FCS is sensitive in resolving and quantifying fluorescent species that differ in their diffusion characteristics and/or their molecular brightness either in one or both detection channels. It is able to account for effects that have often been considered as sources of severe artifacts in two-color and FRET measurements, the most prominent artifacts comprising photobleaching, cross talk, or concentration variations in sample preparation. Because of its very high statistical accuracy, the combination of FRET and 2CG-FCS is suited for high-throughput applications such as drug screening. Employing beam scanning during data acquisition even further enhances this capability and allows measurement times of <2 s. The improved performance in monitoring a FRET sample was verified by following the protease cleavage reaction of a FRET-active peptide. The FRET-inactive subpopulation of uncleaved substrate could be correctly assigned, revealing a substantial portion of inactive or missing acceptor label. The results were compared to those obtained by two-dimensional fluorescence intensity distribution analysis.

Microscope Objective for Large-Angle Fluorescence Used for Rapid Detection of Single Nucleotide Polymorphisms in DNA Hybridization

A new type of microscope objective is used for the rapid detection of sequence-dependent affinity variations in DNA hybridization. We demonstrate that by performing probe/target hybridization on coverslips at room temperature terminal SNPs (single nucleotide polymorphisms) can be detected within seconds. The study of weak pair interaction, such as the association of very short DNA oligomers, requires the use of high analyte concentrations of both partners to generate a detectable amount of associated pairs. The background of high concentrations of unbound fluorescing analyte can easily hide the low signal of a weakly affine reaction and makes association extremely difficult to detect. Fluorescence detection is a powerful approach to analyze minute amounts of material, even single molecules, but it is usually limited to rather low concentrations. This limitation is now overcome due to the new type of microscope objective, which produces an extremely small detection volume at a water/glass interface.

Total internal reflection with fluorescence correlation spectroscopy: nonfluorescent competitors

Total internal reflection with fluorescence correlation spectroscopy is a method for measuring the surface association/dissociation rate constants and absolute densities of fluorescent molecules at the interface of a planar substrate and solution. This method can also report the apparent diffusion coefficient and absolute concentration of fluorescent molecules very close to the surface. Theoretical expressions for the fluorescence fluctuation autocorrelation function when both surface association/dissociation kinetics and diffusion through the evanescent wave, in solution, contribute to the fluorescence fluctuations have been published previously. In the work described here, the nature of the autocorrelation function when both surface association/dissociation kinetics and diffusion through the evanescent wave contribute to the fluorescence fluctuations, and when fluorescent and nonfluorescent molecules compete for surface binding sites, is described. The autocorrelation function depends in general on the kinetic association and dissociation rate constants of the fluorescent and nonfluorescent molecules, the surface site density, the concentrations of fluorescent and nonfluorescent molecules in solution, the solution diffusion coefficients of the two chemical species, the depth of the evanescent field, and the size of the observed area on the surface. Both general and approximate expressions are presented.

Near-field scanning optical microscopy in liquid for high resolution single molecule detection on dendritic cells

Clustering of cell surface receptors into micro-domains in the plasma membrane is an important mechanism for regulating cellular functions. Unfortunately, these domains are often too small to be resolved with conventional optical microscopy. Near-field scanning optical microscopy (NSOM) is a relatively new technique that combines ultra high optical resolution, down to 70 nm, with single molecule detection sensitivity. As such, the technique holds great potential for direct visualisation of domains at the cell surface. Yet, NSOM operation under liquid conditions is far from trivial. In this contribution, we show that the performance of NSOM can be extended to measurements in liquid environments using a diving bell concept. For the first time, individual fluorescent molecules on the membrane of cells in solution are imaged with a spatial resolution of 90 nm. Furthermore, using this technique we have been able to directly visualise nanometric sized domains of the C-type lectin DC-SIGN on the membrane of dendritic cells, both in air and in liquid.

Fluorescence-aided molecule sorting: analysis of structure and interactions by alternating-laser excitation of single molecules

We use alternating-laser excitation to achieve fluorescence-aided molecule sorting (FAMS) and enable simultaneous analysis of biomolecular structure and interactions at the level of single molecules. This was performed by labeling biomolecules with fluorophores that serve as donor-acceptor pairs for Förster resonance energy transfer, and by using alternating-laser excitation to excite directly both donors and acceptors present in single diffusing molecules. Emissions were reduced to the distance-dependent ratio E, and a distance-independent, stoichiometry-based ratio S. Histograms of E and S sorted species based on the conformation and association status of each species. S was sensitive to the stoichiometry and relative brightness of fluorophores in single molecules, observables that can monitor oligomerization and local-environment changes, respectively. FAMS permits equilibrium and kinetic analysis of macromolecule-ligand interactions; this was validated by measuring equilibrium and kinetic dissociation constants for the interaction of Escherichia coli catabolite activator protein with DNA. FAMS is a general platform for ratiometric measurements that report on structure, dynamics, stoichiometries, environment, and interactions of diffusing or immobilized molecules, thus enabling detailed mechanistic studies and ultrasensitive diagnostics.

Focal Volume Confinement by Submicrometer-Sized Fluidic Channels

Microfluidic channels with two lateral dimensions smaller than 1 microm were fabricated in fused silica for high-sensitivity single-molecule detection and fluorescence correlation spectroscopy. The effective observation volumes created by these channels are approximately 100 times smaller than observation volumes using conventional confocal optics and thus enable single-fluorophore detection at higher concentrations. Increased signal-to-noise ratios are also attained because the molecules are restricted to diffuse through the central regions of the excitation volume. Depending on the channel geometries, the effective dimensionality of diffusion is reduced, which is taken into account by simple solutions to diffusion models with boundaries. Driven by electrokinetic forces, analytes could be flowed rapidly through the observation volume, drastically increasing the rate of detection events and reducing data acquisition times. The statistical accuracy of single-molecule characterization is improved because all molecules are counted and contribute to the analysis. Velocities as high as 0.1 m/s were reached, corresponding to average molecular residence times in the observation volume as short as 10 micros. Applications of these nanofabricated devices for high-throughput, single-molecule detection in drug screening and genomic analysis are discussed.

Toward fluorescence nanoscopy

For more than a century, the resolution of focusing light microscopy has been limited by diffraction to 180 nm in the focal plane and to 500 nm along the optic axis. Recently, microscopes have been reported that provide three- to sevenfold improved axial resolution in live cells. Moreover, a family of concepts has emerged that overcomes the diffraction barrier altogether. Its first exponent, stimulated emission depletion microscopy, has so far displayed a resolution down to 28 nm. Relying on saturated optical transitions, these concepts are limited only by the attainable saturation level. As strong saturation should be feasible at low light intensities, nanoscale imaging with focused light may be closer than ever.