Magnetic core-shell silica particles (2007)

This thesis deals with magnetic silica core-shell colloids and related functionalized silica structures. Synthesis routes have been developed and optimized. The physical properties of these colloids have been investigated, such as the magnetic dipole moment, dipolar structure formation and rotational dynamics of the magnetic silica spheres. Moreover attention has been paid to applications, such as functionalization of commercial silica sols and membranes as well as functionalization of magnetic silica with a catalyst. The synthesis of magnetic silica particles is presented in Chapter 2. Maghemite or cobalt ferrite nanoparticles irreversibly attached to the surface of monodisperse (fluorescent) silica spheres coated with a thiol-containing silane coupling agent. The resulting core-shell particles are subsequently covered with an outer silica shell. Electron- and confocal scanning laser microscopy show that these magnetizable silica colloids form dipolar chains when exposed to a homogeneous magnetic field if the outer silica shell is sufficiently thin. In Chapter 3, the method to synthesize silica composites via thiol-functionalization has been extended to other types of silica sols, such as commercially available silica nanoparticles, and porous silica grains to form composites with gold nanoparticles. Additionally, membranes of polyethersulfone are functionalized after first being encapsulated in a crust of silica nanoparticles. In Chapter 4, a proof-of-principle experiment is presented in which magnetic silica particles are coated with a catalyst. The large inducable dipole moment in these particles provides easy recycling of the catalyst. The magnetic properties of the core-shell particles from Chapter 2 are further investigated in Chapter 5 by measuring magnetization curves and the complex magnetic susceptibility as a function of both frequency and field amplitude. We demonstrate that silica microparticles with embedded cobalt ferrite nanoparticles have a permanent magnetic moment, even when the nanoparticles are oriented at random. This is revealed by susceptibility spectra of dispersed silica-cobalt ferrite particles, which show a characteristic frequency that corresponds to Brownian rotation of the microparticles, whereas the spectra of silica particles with maghemite only show contributions from Néel-relaxation inside the nanoparticles. An increase of the permanent magnetic moment is observed after treatment in a strong, saturating magnetic field. The dipole moment measured for these composite particles corresponds to a maximum interaction energy between the particles on the order of 0.5 to 1 kBT. The nearly diffusive rotation of these dipolar particles is studied in Chapter 6 by measuring the frequency-dependent complex magnetic susceptibility as a function of the particle concentration. In Chapter 7 static light scattering of the magnetic silica particles are performed at various optical contrasts. We investigate to what extent the theory for static light scattering of optically inhomogeneous particles in the Rayleigh-Gans-Debye approximation can be applied to spherical composite particles containing a core and two concentric shells, of which one containing magnetic metal oxide nanoparticles with a high refractive index. Although not able to describe the entire scattering profiles, we find iso-scattering points corresponding to the radius of single particles as obtained from electron microscopy.