Synthesis and characterization of goethite (?-FeOOH) magnetic nanofluids

Magnetic nanofluids consisting of goethite nanoparticles in aqueous suspensions have been successfully synthesized using two sodium-free routes. Nanoparticle morphology, nanofluid stability and optical properties have been studied and compared with a goethite nanofluid whose nanoparticles are synthesized with a different protocol using NaOH. The synthesis procedure determines the nanoparticle size and morphology, which, in turn, is connected to optical properties and their sensitivity to external magnetic fields. The highest sensitivity is shown by the sample synthesized with NaOH, which is characterized by the largest nanoparticles (261±5 nm hydrodynamic size) and whose absolute transmittance values increase or decrease by about 11% and 20%, respectively, depending on to the mutual directions of magnetic field and electromagnetic wave polarization/wave vector, for an applied magnetic field of 5.60 mT. Nanoparticles in the fluid are therefore able to discriminate both the absolute value and the direction of the magnetic field and, moreover, being their response non-spectrally flat, specific spectral ranges exist where their response is maximized.