Formation of spherical iron(III) oxyhydroxide nanoparticles sterically stabilized by chitosan in aqueous solutions

The interactions between the cationic polymer chitosan (Chit) and iron(III) were investigated. The solution properties were studied by pH-metry, viscometry and dynamic light scattering. Solid state iron(III)-Chit samples were also prepared and characterized by IR spectroscopy and electron microscopy. In aqueous solutions, the precipitation pH of the iron(III) oxyhydroxide (FeOOH) is significantly shifted towards the higher pH values in the presence of Chit indicating that some interaction takes place between the iron(III) and the polymer. However, the additivity of the pH-metric titration curves, the lack of variation both in the viscometric and IR spectra of Chit in the presence and absence of iron(III), indicate the lack of direct complexation between the Chit and ferric ions. Isolated FeOOH nanospheres of 5-10 nm diameter were observed on the transmission electron microscopic pictures of samples obtained from solutions containing iron(III) and Chit, while from DLS measurements hydrodynamic units with a few hundred nm in diameter were identified. Our data support that Chit acts as steric stabilizer and inhibits the macroscopic aggregation of the subcolloidal FeOOH particles. Thus the iron(III)-Chit interactions offer a simple and economic way to fabricate nanometric size FeOOH spheres, morphologically similar to the core of iron(III)-storage protein, ferritin.