Fig. 1.

Principles of single aggregate measurements. A) Fluorophore labelled monomer can undergo intermolecular FRET between a donor and acceptor fluorophore when aggregates are formed. This allows aggregates to be distinguished from monomer. Detection of coincident red and blue fluorescence allows the number of aggregates to be counted and their relative size to be estimated by comparing the fluorescence intensity to that of the monomer. The extent of FRET can also be measured which provides information about the compaction of the aggregate and if one or multiple species are present. B)Total internal reflection fluorescence microscopy can be used to detect aggregates on a glass coverslipusing the transient binding nature of dyes such as Thioflavin T or the pentameric form of formyl thiophene acetic acid (pFTAA), whose quantum yield increases significantly when bound to beta sheet structures in the aggregates. C) The transient binding of a fluorophore labelled DNA imaging strand to the docking strand on an aptamer that binds aggregates allows single molecule localisation to be performed. This enables aggregates of Aβ to be imaged at super-resolution. The scale bar is 500 nm.