Application of an RF-SQUID to Detect Magnetic Particles Used in Immunoassays

We developed a detection system for magnetic particles based on a two-hole bulk RF-SQUID. In this system, the particles were detected by measuring the magnetic flux change caused by the displacement of magnetized samples across the SQUID hole. Commercial silica coated magnetic particles of iron oxide were used in the experiments. Dilutions from a 50 micrograms/milliliter dispersion of particles were performed in a viscous media consisting of PEG in milliQ water to avoid precipitation and aggregation. The signal intensities from the magnetic flux variations were consistent with the dilution factors. We were able to measure samples containing down to 250 picograms of particles, which generated about one flux quantum inside the SQUID.