Immunoassays using high-Tc SQUIDs

Licentiate thesis, 2009

The development of a system for immunoassays using highly sensitive high-Tc superconducting quantum interference devices (SQUIDs) and magnetic nanoparticles (MNPs) is described in this thesis. High-Tc SQUIDs were fabricated using pulsed laser deposition and photolithography. The sensors are planar YBa2Cu3O(7-δ) (YBCO) dc SQUID gradiometers developed for operation inside a glass fibre liquid nitrogen cryostat operating at 68-80 K. The gradiometers, with baselines of 3 mm and 4 mm, were fabricated on SrTiO3 (STO) bicrystal substrates. The flux noise of the best performing SQUID was 4.6 μФ0/√Hz and the voltage modulation 40 μV. The extremely high magnetic field resolution of SQUIDs was used in combination with MNPs for sensitive detection of biomolecules. Two different measurement modalities were used: AC-susceptometry (ACS) and magnetorelaxometry (MRX) for frequency- and time-domain measurements, respectively. ACS provides high resolution information about the size distribution of the MNP-system and the fast MRX enables studies of the reaction kinetics as a function of time. The MNPs used were multi-core particles containing single domains of magnetic material. Two different types of MNPs were used: cobalt-ferrite (CoFe2O4) and magnetite (Fe3O4). The detection limit of the present setup at the noise floor of our SQUIDs was estimated to be 50 ng of MNPs, corresponding to roughly 10^8 MNPs (of CoFe2O4) with diameter of 100 nm. The feasibility of the present system for biomolecule detection using prostate-specific antibodies/antigens is shown in this thesis. Two different assay protocols were investigated: a cluster-type assay and a one-step assay. The cluster-type assay requires more complicated preparations but the sensitivity is higher. The one-step assay is simple and fast but less sensitive. The extrapolated biomolecule sensitivity is 18 ng/ml (or about 4x10^10 molecules) using the cluster-type assay and 10 μg/ml (roughly 10^12 molecules) using the faster one-step protocol. Future development includes incorporation of microfluidic chips for manipulation of droplets of MNPs based on electrowetting-on-dielectric (EWOD). This would reduce the sample volume and facilitate sample handling.