A promising aspect of ESI is its application in the detection of elemental labels introduced into biomolecules for cell and molecular biological techniques. Even though colloidal gold labeling for electron microscopy (EM) is highly developed, availability of alternative labels, especially for double or triple labeling applications would be helpful because of difficulties with gold concerning i) detection (gold diameters ≤1nm), ii) discrimination due to gold particle size variations in one size class, and iii) different labeling efficiencies depending on gold granule size. An alternative labeling molecule should contain a high concentration of a specific chemical element which is not or in minor concentrations present in the system under surveillance, and has to have the potential to be discriminated from “biological” elements by ESI.

With respect to ESI, one candidate for elemental labeling is boron. It meets the criteria described above and substantial experience in the synthesis of labeling compounds exists. From the chemical point of view, the preferred labeling structure is a so called dendrimer, a highly branched regular three-dimensional monodisperse macromolecule. Dendritic structures offer a large variety of functionalities to incorporate an element detectable by energy filtering transmission electron microscopy (EFTEM).