Optical trapping of titania/silica core-shell colloidal particles

Manipulation of colloidal systems via optical trapping techniques requires a refractive index mismatch between particles and solvent which leads to st rong interparticle van der Waals interactions. Investigation of the behavio r of systems without such strong attractive interactions, however, requires the uncoupling of particle refractive index and particle-particle interact ions. To accomplish this, the synthesis of core-shell titania/silica partic les has been performed. By index matching a silica shell on a titania core using a mixture of toluene and propanol, the van der Waals interactions bet ween particles can be minimized. Due to the mismatch of the refractive inde x between the solvent and titania core, however, a strong trapping force ca n be generated, making optical manipulation feasible. In order to confirm t hat the silica shell was indeed matched, pure silica particles were synthes ized by the method of Stober (1968) and added to the core-shell system. In these mixed systems of core-shell and pure silica particles in silica-index -matching solvents, only the core-shell particles were trappable. (C) 2000 Academic Press.