Behavior of multi-component magnetic colloidal systems in tunable magnetic fields and applications in biosensing

A system consisting of multiple-component beads, such as superparamagnetic beads, nonmagnetic beads and magnetorheological (MR) fluid, can display some very amazing and special properties when subjected to an external magnetic field because the MR fluid can act on both types of beads synchronously as a magnetic medium. Some novel structures and phenomena were discovered and are discussed in our work, including 'ring-structures', 'small-ring' and 'ring-chains' in static or rotational magnetic fields. If both probe and target molecules are attached consisting of functionalized superparamagnetic beads and non-magnetic beads, respectively, the ring-structure could be maintained due to biomolecular bonding, even after removing the external magnetic field. Using these remnant rings, we raised two protocols for biosensing: a two-dimensional biosensor using a magnetic self-assembled colloidal ring-structure, and an improved magneto-optical transmittance (MT) method. In the former protocol, we define the small nonmagnetic particles as petals because the whole structure looks like a flower. It was proved that the number of remnant ring petals was a function of the concentration of the target molecules', with a concentration range from 0.0768 ng/mL similar to 3.8419 ng/mL, making it a promising technology for applications involving biosensing. In the latter protocol, the use of larger individual units made the magnetic particle chain longer, which was considered to be a promising way of improving the sensitivity of the MT method.