Magnetic colloidal suspensions have become very important in applications such as magnetic separation, biological research, and diagnostics. Under the influence of an external magnetic field paramagnetic particles aggregate to form chains aligned in the field direction. We have developed a technique in which the surface of the paramagnetic particles is functionalized and introduced to linker molecules which react to form permanent functional structures. Such linker molecules include streptavidin-biotin or gultaraldehyde-amine chemistry. In bulk solution these paramagnetic chains flex and curve due to thermal motion. Yet when an external magnetic field is applied, these paramagnetic chains collapse in the direction of the magnetic field and therefore can be mechanically manipulated according to the gradient of magnetic field.

Hybrid chains with hydrophilic and hydrophobic particles

We have created hybrid magnetic chains by using magnetic colloidal mixtures with particles either functionalized with fatty acid or biotin. The particles are first aligned to form chains in oil phase and then displaced with the gradient of magnetic field to aqueous phase filled with streptavidin molecules. The fatty acid modified particles flocculate rapidly in aqueous phase, and the biotin modified particles are linked with streptavidin. The streptavidin protein also diffuses between the two types of particles to adhere them into integrated chains. The biotin modified particles are marked with fluorescent compound to be easily distinguished under fluorescent filters. The folding behaviors of hydrophilic part of the chains in aqueous solution is studied. With such structures we can further pattern functional groups selectively onto the surface of either hydrophilic or hydrophobic particles and thus design intelligent materials.