Human plasma fibrinogen (HPF) was observed by atomic force microscopy (AFM) and Kelvin probe force microscopy (KPFM) conducted in non-contact mode. The HPF was adsorbed on a highly oriented pyrolytic graphite (HOPG) substrate as single molecules, as aggregated bundles, and as aggregated fibers. Topographic and phase images confirmed structural changes in the HPF after exposure to air, while topographic and KPFM images confirmed fibers with the width of a single HPF molecule. Additionally, KPFM confirmed the surface potential difference between the HPF and the HOPG, and periodical potential drop reflecting the E and D domains in the fiber.
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