Brownian magnetic relaxation of water-based cobalt nanoparticle ferrofluids

Following the synthesis of monodispersed 20 nm cobalt nanoparticles via a thermal decomposition method we have successfully transferred these hydrophobic nanoparticles into the water phase using tetramethylamonium hydroxide pentahydrate as phase transfer agent and 12-aminododecanoic acid as a stabilizing agent. Frequency dependent ac susceptibility of water-based cobalt nanoparticle ferrofluids was measured at room temperature in the frequency range of 0.01–1000 Hz. In addition to the “high” frequency magnetic relaxation peak at 200 Hz, which is determined by the solvent viscosity and hydrodynamic volume of the nanoparticles, a lower frequency relaxation peak attributed to the interaction of surface coatings is observed. The position of the latter varies with the solution pH in the range of 0.02–0.05 Hz. This variation is explained as due to the change of chemical and charge state of the surfactant molecules. The peak at low frequency may be potentially used to study protein configuration changes in solution or other dynamic properties of biomolecules. © 2006 American Institute of Physics. DOI: 10.1063/1.2172203