One of the main issues of light microscopy has always been the limited resolution caused by diffraction effects which were described by Ernst Abbe in 1873 . This leads to the problem that microscopy images with a high magnification look blurry and hide information about structural details in the respective sample. In recent years a couple of techniques spread which breaking Abbe’s diffraction limit gaining microscopy images with much higher resolution. These so called super-resolution techniques use either physical (i.e. STED  or SIM ) or chemical approaches (i.e. GSDIM  or DNA-PAINT ) to increase the optical resolution. This breakthrough was awarded with the Nobel Prize for chemistry in 2014. However, for a long time these methods had the problem that there was no standardized way for a quantitative determination of the achieved resolution of a certain super-resolution microscope. Additionally, it is often not clear which of the super-resolution methods is the most appropriate to answer a specific biological question. So far the most common way to demonstrate the performance of a super-resolution technique has been imaging the cytoskeleton. Hereby structures from eukaryotic cytoskeleton like microtubules or actin filaments are imaged diffraction limited as well as in super-resolution configuration. Then both images are shown together (Figure 1a, b) highlighting qualitatively the resolution improvement in an eye-catching, however just qualitative way. For a quantitative determination of the spatial resolution usually two filaments which lie parallel close to each other and are narrow resolvable are picked out (Figure 1c) and underwent a distance measurement. The shortest distance found that way then is called the achieved spatial resolution (Figure 1d).
This of course seems to be very arbitrary since there is no control on the distribution of the imaged filaments and especially the distances between them. In addition due to the fact that every distance is unique there is no statistical analysis possible. The quest to develop better resolution standards in every respect yielded to the development of DNA origami based super-resolution standards .
The DNA origami technique
The basic technology for constructing these defined structures on the nanoscale, invented in the year 2006 by Paul Rothemund, is the so called "DNA origami" technique . A DNA origami structure is constructed out of a long (about 7,000 to 8,000 base pairs) single stranded DNA molecule the so called "scaffold strand" (Figure 2a). By adding ~200 short single stranded DNA molecules (so called "staple strands") which have complementary sequences to different parts of the scaffold strand, the respective parts of the scaffold can be connected to each other (Figure 2b-d). By these defined connections the scaffold can be folded into any structure at will whereby the programming of the shape is carried out by the right choice of the staple strand sequences. By heating up the scaffold strands together with the short staple strands, buffer and salts and slowly cooling down to room-temperature the folding process occurs and the DNA origami structure folds self-assembled into the designed structure.
Typical DNA origami structures have dimensions of around 50 to 300 nm and can have the shape of flat rectangles, rods, cubes but essentially every shape, as seen in Figure 3.
The fact that the position of every short staple strand in the DNA origami structure is known offers the possibility to position organic dye-molecules with nanometer precision on the DNA origami structures. Therefore the respective staple strand needs to be labeled with an organic dye-molecule which is then incorporated into the DNA origami structure during the folding process (Figure 2e). Since every of the ~200 staple strands can be labeled individually at will, the DNA origami structures can be regarded as molecular breadboard which allow to position organic dye-molecules and essentially every object which can be attached to single-stranded DNA with nanometer precision (Figure 2f). This allows generating fluorescent marks on the DNA origami structures with defined distances and also defined numbers of dye-molecules which is then the ideal structure to test the achievable spatial resolution of fluorescence and especially super-resolution microscopes.
Because of the high parallelism of the DNA origami technique every image contains not only one single nanoruler but millions of identical nanorulers which ensures the possibility of measuring the achievable spatial resolution of super-resolution microscopes and to calculate statistical errors of this measurement . Also the imaging buffer as well as the density of dye molecules can be adjusted to the conditions which can be found in real samples of interest.
The GATTAquant GmbH is a young start-up company which is focused on the design and fabrication of DNA origami based nanorulers for any type of super-resolution microscopy. As a spin-off from the group of Prof. Philip Tinnefeld at TU Braunschweig the company holds immense expertise in the field of DNA nanotechnology, the field of photophysics and super-resolution imaging and especially the combination of both.