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Stimulated Emission Depletion Microscopy (STED)

Imaging below the optical diffraction limit

Stimulated emission depletion microscopy (STED) is a fluorescence microscopy technique that overcomes the diffraction limited resolution of confocal microscopes. The resolution enhancement is essentially based on switching off the fluorescence of dye molecules by stimulated emission using intense laser light in the outer regions of the diffraction limited excitation focus. This intense radiation causes almost all of the excited molecules to return to the ground state. Fluorescence from the remaining excited dye molecules in the center of the excitation focus is then detected and used to form the high resolution images. An even further resolution enhancement  is possible by applying time gates to the collected data (gated STED or gSTED). As STED creates an effectively smaller observation volume, it can also be applied to other methods such as FCS. In that case, collecting data at different observation volume diameters can help to disentangle complex 2D diffusion scenarios in heterogeneous samples such as biological membranes.

Sketch of a typical STED set-upThe STED method uses pairs of synchronized laser pulses. The first laser pulse generated by, e.g. a picosecond pulsed diode laser, is used to excite the fluorescence dye and produces an ordinary diffraction limited focus. The excitation pulse is immediately followed by a depletion pulse, which is red-shifted in frequency to the emission spectrum of the dye. By spatially arranging the STED pulse in a doughnut shape using specially designed phase plates, only the fluorescence from molecules at the periphery of the excitation focus is quenched via stimulated emission. In the center of the doughnut, where the STED laser intensity is zero, fluorescence remains unaffected and is detected by single-photon sensitive detectors (e.g., Single Photon Avalanche Diodes, SPADs).

Consequently the essential components of a STED microscope set-up are:

  • pulsed laser source for excitation and depletion
  • single photon sensitive detector
  • dichroic mirror (to overlay excitation and depletion lasers and to separate fluorescence signal from excitation light)
  • phase plate for transforming the STED pulse into a doughnut shape
  • objective
  • data acquisition unit, e.g., a TCSPC module
  • suited scanning system

Interview with Stefan W. Hell about STED

PicoQuant offers a complete solution for STED microscopy:

MicroTime 200 STEDMicroTime 200 STED

Time-resolved confocal fluorescence microscope with super-resolution capability

The MicroTime 200 STED is a powerful instrument capable of STED microscopy, Fluorecence Corelation Spectroscopy and its daughter techniques as well as Fluorescence Lifetime Imaging (FLIM) with single molecule detection sensitivity. It contains the complete optics and electronics for recording virtually all aspects of the fluorescence dynamics of microscopic samples or femtoliter volumes and with a spatial resolution below 50 nm. The instrument gains its exceptional sensitivity and flexibility in combination with unprecedented ease-of-use from a unique fusion of miniaturized and highly sophisticated state-of-the-art technologies. Although, these technologies enable to run an instrument of comparable complexity and power without having to spend more time on instrument maintenance than on original scientific content, the MicroTime 200 STED remains an open platform that allows the advanced scientist to easily built upon the open character of the instrument in order to realize highly customized applications.


Sketch of a typical STED set-upThe following core components are needed to build a system capable of STED measurements, which are (partly) available from PicoQuant:

Crimson beads

Confocal image of crimson beads with 20 nm diameterSTED image of crimson beads with 20 nm diameter

The example shows a comparison between confocal and STED images of Crimson beads (20 nm diameter). Fluorescent beads are often used to evaluate the resolution of a microscope. The FWHM in the confocal image is 260 nm, and in the STED image it is 38 nm. The resolution enhancement is clearly visible.

Set-up:


Dual Species Separation

Confocal image - U2OS cell labeled for tubulin with Abberior STAR 635p (green) and giantin with Atto647N STED image - U2OS cell labeled for tubulin with Abberior STAR 635p (green) and giantin with Atto647N

Two fluorescent species can be distinguished by pulsed interleaved excititation (PIE) with two slightly shifted wavelengths and detection of the fluorescence in two spectral bands. Fluorescence pattern matching was used to identify the contributions of each fluorophore. The example shows part of a U2OS cell labeled for tubulin with Abberior STAR 635p (green) and giantin with Atto647N (red) utilizing a single STED laser wavelength and excitation with two different diode lasers at 640 nm and 660 nm.

Set-up:

Sample courtesy of Markus Sauer, University of Würzburg, Germany


Cytoskeleton – microtubules

Confocal image of tubulin structures in a HeLa cell labeled with Abberior STAR635pSTED image of tubulin structures in a HeLa cell labeled with Abberior STAR635p

The example shows tubulin structures in a HeLa cell labeled with Abberior STAR635p. Super-resolution permits the clearer distinction of the filamentous structures in the STED image.

Set-up:


DNA origamiDNA origami

Specifically labeled DNA origami is now commonly used to evaluate the resolution of diffraction-limit-breaking microscopes. Here, origami with two rows of 11 dye molecules with a distance of 71 nm was used. Image size: 4 µm x 4µm

Set-up:

The samples were provided by GATTAquant GmbH and the Tinnefeld lab, TU Braunschweig, Germany
 


gSTED improves FCS

gSTED FCS of Atto655 in waterThe example shows FCS of Atto655 in water using the standard confocal (blue) and STED (red) configuration. Due to the smaller detection volume in the STED experiment, the FCS curve is shifted to shorter lag times. By applying a time-gate that rejects early photons, the observation volume is decreased even more, leading to a further shift of the FCS curve towards shorter lag times (gSTED, black)

Set-up:


STED images of Crimson beads

STED images of Crimson beads

The example shows a comparison between confocal and STED images of Crimson beads (20 nm diameter) recorded at different repetition rates of the two lasers. The beads were excited using the LDH-P-C-640B and the LDH-P-FA-765 was used as STED laser. The upper parts of the images shows the beads imaged with a standard confocal setup, whereas the lower parts show the STED images of the same sample region. The resolution enhancement is clearly visible in the plotted cross-sections through one bead, with the STED images reaching a spatial resolution of approx. 50 nm.

Images courtesy of A. Honigmann, C. Eggeling and S.W. Hell, MPI Göttingen


STED images of vimentin fibers

STED image of Vimentin fibersThe example shows vimentin fibers stained with ATTO 565 via primary and secondary antibodies. The Solea laser has been used to excite the sample at 560 nm. A lateral resolution of approx. 50 nm is obtained, which can be expected for a fiber with a diameter of 10 nm, covered by antibodies with a diameter of approx. 8 nm each.

Data courtesy of J. Engelhardt and S.W. Hell, DKFZ Heidelberg
 

Latest 10 publications related to STED

The following list is an extract of 10 recent publications from our bibliography that either bear reference or are releated to this application and our products in some way. Do you miss your publication? If yes, we will be happy to include it in our bibliography. Please send an e-mail to info@picoquant.com containing the appropriate citation. Thank you very much in advance for your kind co-operation.