Assembly of Amyloid beta (A beta) peptides, in particular A beta-42 is central to the formation of the amyloid plaques associated with neuro-pathologies such as Alzheimer's disease (AD). Molecular assembly of individual A beta-42 species was observed using a simple fluorescence microscope. From the molecular movements (aka Brownian motion) of the individual peptide assemblies, we calculated a temporal evolution of the hydrodynamic radius (R-H) of the peptide at physiological temperature and pH. The results clearly show a direct relationship between R-H of A beta-42 and incubation period, corresponding to the previously reported peptide's aggregation kinetics. The data correlates highly with in solution-based label-free electrochemical detection of the peptide's aggregation, and A beta-42 deposited on a solid surface and analysed using atomic force microscopy (AFM). To the best of our knowledge, this is the first analysis and characterisation of A beta aggregation based on capturing molecular trails of individual assemblies. The technique enables both real-time observation and a semi-quantitative distribution profile of the various stages of A beta assembly, at microM peptide concentration. Our method is a Promising candidate for real-time observation and analysis of the effect of other pathologically-relevant molecules such as metal ions on pathways to A beta oligomerisation and aggregation. The method is also a promising screening tool for AD therapeutics that target A beta assembly. (C) 2008 Elsevier Inc. All rights reserved.