Abstract
A high-Tc dc SQUID (superconducting quantum interference device) gradiometer wasdeveloped for magnetic immunoassays where magnetic nanoparticles are usedas markers to detect biological reactions. The gradiometer was fabricated on a5 × 10 mm2 SrTiO3 bicrystal substrate and has a gradiometer resolution of2.1 pT cm−1 Hz−1/2. A magnetic signal was detected from a sample of1 µl ofFe3O4 nanoparticlesin a 40 mg ml−1 solution kept in a microcavity fabricated on Si wafers withSi3N4 membranes using MEMS (micro-electro-mechanical-systems) technology. Itwas found that volumes as small as 0.3 nl in principle would be detectablewith our present device. This corresponds to a total number of particles of2.2 × 107. The estimated average dipole moment per particle is4.8 × 10−22 A m2. We are aiming at reading out immunoassays by detecting the Brownian relaxation ofmagnetic nanoparticles, and we also intend to integrate MEMS technology into our system.
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