The Feasibility of High Resolution, Multiple Labeling using Colloidal Particles of Similar Size but Different Shapes and Elemental Compositions

Abstract High resolution, quantitative cell labeling analysis for electron microscopy is typically accomplished using colloidal gold (cAu) particles which have a uniform diameter such that each particle conjugates no more than one ligand with monovalent specificity, such as an Fab antibody fragment. in this way, one particle labels exactly one target molecule. The corresponding particle size which satisfies this requirement, in the case of the Fab fragment, is 3-5nm. Under these constraints, multiple labeling cannot be performed using the standard cAu technology in which particles of various diameters are used such that each size corresponds to a unique label. Particles larger than 5nm may conjugate multiple Fab fragments, and thus a single particle may lable multiple target sites. Conversely, several particles smaller than 3nm may bind to a single Fab fragment so that several particles may label a single target molecule. Recent work in our lab has focused on the development of a labeling system which maintains the advantages of the standard cAu labeling technology, namely the use of electron-dense particles with excellent secondary and backscattered electron emission properties which make them readily detectable by both TEM and SEM, while at the same time offering greater versatility by permitting multiple labeling for high resolution, quantitative analysis. This novel labeling system makes use of colloidal particles which have unique and distinctive shapes, and composed of a variety of different metals including Au, Ag, Pt, Pd, Rh, and Ru. These elements all form stable aqueous colloidal suspensions, and the particles have surface charge properties similar to those of Au, permitting the adsorption of proteins and other ligands to them by the same conjugation techniques used with cAu (fig. 1).