Refractive index modulation in magnetophoresis of bioreaction induced self-assembled magnetic fluid

We explore the refractive index (RI) modulation of bioreaction induced self-assembled magnetic fluid (SAMF) so as to facilitate the design of sensitive biosensors. Bovine serum albumin (BSA) is taken as a model protein to induce self-assembly of carboxylated magnetic nanoparticles (MNPs). A bidirectional magnetophoresis (BDMP) system is designed to generate uniform and large RI modulation to SAMF. A robust Mach-Zehnder interferometer compatible to the BDMP system is used for the RI detection. It is found that BDMP has greater ability in modulating the RI of magnetic fluid than the alignment of MNPs in a uniform magnetic field. The modulation depth of the RI increases with the increase of BSA concentration when applying a certain external magnetic field for a period of time. A larger magnetic field has greater ability to modulate the RI of SAMF. However, it may generate an over-modulation.