Nanoincorporation of iron oxides into carrageenan gels and magnetometric and morphological characterizations of the composite products

Carrageenan-based magnetic composites were prepared via in situ synthesis of iron oxides in a gelatinous network of the polysaccharide. The repeatable synthesis process involved three steps: immersion into ferrous salt (FeCl2) solution, alkali treatment in sodium hydroxide solution and oxidation with hydrogen peroxide, successively performed with τ- or κ-carrageenan hydrogels as the starting material. Field emission scanning electron microscope observations, X-ray diffractometry and superconducting quantum interference device (SQUID) magnetometry were carried out for the freeze-dried composites. Feroxyhite and magnetite and/or maghemite particles were produced after one cycle and multicycles of the in situ process, respectively, with a size less than several tens of nanometers, and were distributed throughout the inside as well as on the surface of numerous fibrillar entities constituting the carrageenan matrix. Nearly all the composite products explored displayed a superparamagnetic (SPM) property at 298 K; the estimated saturation magnetization (Ms) increased with increasing concentrations of FeCl2 in the gel immersion step and with repetition of the standardized process of iron oxide synthesis. By repeating the synthesis cycle 3–4 times, a composite of practically high Ms reaching ~25 emu (g sample)−1 was easily obtained without impairing the SPM character. Insight was provided into the evolution mechanism of the oxidation state and dimensional distribution of the cyclically loaded iron oxide nanoparticles through comparison with another composite series obtained by a co-precipitation method with a 1:2 mixture of ferrous/ferric salts