LAUSR

Abstract This article reports a smart water-based ferrofluid that can transform from stable state to unstable state for separating the contained magnetic nanoparticles after use. The ferrofluid is synthesized by preparing polyethylenimine modified Fe3O4 nanoparticles via a one-pot method, and then improved by acidification treatment and ultracentrifugation-based washing. The resultant ferrofluid and intermediate products have been systematically characterized, verifying that the ferrofluid possesses superparamagnetism, high saturation magnetization as well as strong colloidal stability, and the magnetic nanoparticles contained in the ferrofluid have small aggregation size, strong electropositivity and high saturation magnetization. It has been confirmed that the acidification treatment and ultracentrifugation-based washing greatly activate the positive charge and reduce the aggregation size of polyethylenimine modified Fe3O4 nanoparticles, which is the key to our successful synthesis of this new type of ferrofluid. Furthermore, the adsorption behavior of the ferrofluid on the anionic Ponceau S dye has been systematically investigated, demonstrating that the ferrofluid can adsorb anionic Ponceau S dye in a short time (