The basic principle of GSDIM (also referred to as dSTORM) is similar to PALM and STORM with using a stochastic readout of individual molecules prepared in the on-state, while the majority of molecules are kept in the off-state. The major advantage over PALM and STORM is that both states can be realized as electronic states, i.e. much faster switching times, thus faster recording, can be realized.
After nanoscale precise read-out of the localization of individual molecules they are switched back to the dark-state.
The following depicts a Jablonski diagram of a GSDIM process of a fluorescent molecule:
After excitation (1) from the ground state S0 into the first excited state S1, the molecule has two ways of returning into the ground state: The regular fluorescence process (2) from S1 (on state), or the transition via a a dark state D (off state), e.g. the Triplet state. In the latter case the molecule returns radiation free back into the ground state (3).
The GSDIM principle suggest, to keep only a small fraction of molecules in the "on" state (i.e. the ground state is depleted) at a given time to be able to record positions of individual molecules separately.
The recording scheme is similar to the PALM / STORM principle:
GSDIM implements a stochastic read out of the molecule positions. As such the image is taken iteratively by operlapping all identified positions. Inherently, the method must rely on algorithms to calculate the location of each identified molecule.
On the implementation side a GSDIM microscope is very simple: a widefield-setup with a single-molecule sensitive area detector and only one light source responsible for both switching and excitation.
The GSDIM method provides a theoretically unlimited localization accuracy of individual dyes (clusters). The localization accuracy is depending on the number of photons (m) being emitted by one dye (cluster):
where Δx denotes the localization accuracy of each individual molecule, and λ denotes the excitation wavelength.
To deliver a high-resolution image the GSDIM concept must leverage numerical methods to overlay all calculaterd positions of individual molecules.