Biosensing utilizing magnetic markers and superconducting quantum interference devices

Magnetic biosensing techniques that are based on the use of bio-functionalized magnetic nanoparticles (magnetic markers) and superconducting quantum interference devices (SQUIDs) are expected to have various advantages when compared with conventional biosensing methods. In this paper, we review the recent progress made in magnetic biosensing techniques. First, we describe the most important parameters of magnetic markers that are intended for use in biosensing, i.e., the magnetic signal and the relaxation time that are determined by the Brownian and/or Neel relaxation mechanisms. We note that these parameters are significantly dependent on the marker size, and as a result, commercial markers exhibit a wide variety of values for these key parameters. Next, we describe three measurement methods that have been developed based on the magnetic properties of these markers, i.e., AC susceptibility, relaxation and remanence-based measurement methods. The weak (picotesla-range) signals emitted by the markers can be measured precisely with a SQUID system using these methods. Finally, we give examples of biosensing for in vitro and in vivo medical diagnosis applications. For in vitro diagnosis, high-sensitivity detection of various biological targets has been demonstrated without use of any washing process to separate the bound and free markers. For in vivo applications, detection of the quantities and the three-dimensional positions of the markers that have been injected into the test subject are demonstrated. These results confirm the effectiveness of magnetic biosensing techniques.