Development of multisample biological immunoassay system using HTS SQUID and magnetic nanoparticles

We developed a prototype magnetic immunoassay system using a high temperature superconductor (HTS) superconducting quantum interference device (SQUID) to investigate the performance and usability of the magnetic immunoassay. The system is designed to measure multiple samples and liquid samples, and it can work in an unshielded environment at a medical facility. To reduce the disturbance from environmental noise, the SQUID and samples are covered with three-layers of permalloy magnetic shield. The SQUID and magnetic shield are set in an aluminum box which acts as an RF shield. A gradiometer with a 5 × 10 mm pickup coil, which is cooled by liquid nitrogen through a sapphire/Cu rod, is used as a sensor. We also developed a nonmagnetic sample disk with 12 reaction cells and examined 12 samples in one measurement sequence. The measurement process is controlled by a computer, which perform data averaging. Fe₃O₄ nanoparticles with a 25-nm diameter were used as test samples. After applying a magnetic field of about 0.1 T, we measured the remanent magnetic field from the Fe₃O₄ nanoparticles. The present system could detect 30 pg of Fe₃O ₄ nanoparticles. This result was obtained by averaging 100 trials under an unshielded laboratory environment. The measurement time for 100 trials was only 100 s.