Postnova Analytics GmbH 1 article published in JoVE Bioengineering Asymmetrical Flow Field-Flow Fractionation for Sizing of Gold Nanoparticles in Suspension Roland Drexel1, Vanessa Sogne1, Magdalena Dinkel1, Florian Meier1, Thorsten Klein1 1Research & Development, Postnova Analytics GmbH This protocol describes the use of Asymmetrical Flow Field-Flow Fractionation coupled with UV-vis detection for the determination of the size of an unknown gold nanoparticle sample.