Magnetic nanoparticle relaxation measured by a low-TcSQUID system

A low- T c SQUID system was developed for measuring magnetic relaxation of polymer-coated magnetic nanoparticles (MNPs) in a liquid carrier (e.g. water). The system consists of two low- T c SQUIDs which are electronically combined to form an axial gradiometer using high-bandwidth directly coupled FLL electronics. The system is operated in a magnetically shielded room. The magnetic relaxation of the investigated MNPs in a liquid carrier is dominated by Brownian motion. In a solid phase, when the MNPs are immobilized, the magnetization of the sample decays via the Néel mechanism. A similar situation occurs when the mobility of the MNPs is reduced by a biochemical binding reaction. This effect is used for identifying biological reactions for purposes of medical diagnostics, e.g. immunoassays. By investigating the magnetic relaxation of dried samples, quantities as small as 1 nmol Fe of -Fe 2 O 3 were detected. In the first agglomeration assay the binding reaction of the biochemical model biotin-avidin complexes can be clearly identified down to concentrations of <1 µg avidin in a volume of 150 µl of human blood.