| A high transition temperature (Tc) Superconducting QUantum Interference Device (SQUID) is used to detect magnetically-labeled microorganisms. The targets are identified and quantified by means of magnetic relaxation measurements, with no need for unbound magnetic labels to be washed away. The binding rate between antibody-linked magnetic particles and targets can be measured with this technique.; Installed in a “SQUID microscope,” a YBa2Cu 3O7−δ SQUID is mounted on a sapphire rod thermally linked to a liquid nitrogen can; these components are enclosed in a fiberglass vacuum chamber. A thin window separates the vacuum chamber from the sample, which is at room temperature and atmospheric pressure. In one mode of the experiment, targets are immobilized on a substrate and immersed a suspension of ∼50 nm diameter superparamagnetic particles, coated with antibodies. A pulsed magnetic field aligns the magnetic dipole moments, and the SQUID measures the magnetic relaxation signal each time the field is turned off. Unbound particles relax within ∼50 μs by Brownian rotation, too fast for the SQUID system to measure. In contrast, particles bound to targets have their Brownian motion inhibited. These particles relax in ∼1 s by rotation of the internal dipole moment, and this Néel relaxation process is detected by the SQUID. This assay is demonstrated with a model system of liposomes carrying the FLAG epitope; the detection limit is (2.7 ± 0.2) × 105 particles. The replacement of the SQUID with a gradiometer improves the detection limit to (7.0 ± 0.7) × 103 particles.; In an alternate mode of the experiment, freely suspended targets (larger than ∼1 μm diameter) are detected. Since the Brownian relaxation time of the targets is longer than the measurement time, particles bound to targets are effectively immobilized and exhibit Néel relaxation. Listeria monocytogenes are detected using this method; the sensitivity is (1.1 ± 0.2) × 105 bacteria in 20 μL. For a 1 nL sample volume, the detection limit is expected to be 230 ± 40 bacteria. Time-resolved measurements, which yield the binding rate between particles and bacteria, are reported. Also, potential improvements to the system and possible applications are discussed. |
