Low T/sub c/ SQUID measurement system for magnetic relaxation immunoassays in unshielded environment

A novel technique for detecting immunochemical reactions based on a SQUID measurement system has recently been demonstrated. The reaction partners are labeled by magnetic nanoparticles which, after an immunochemical reaction, display changes of their magnetic relaxation behaviour due to a reduction of their mobility. We have developed a SQUID system dedicated to magnetic relaxation immunoassays (MARIAs) operating in unshielded environment. The system consists of 6 SQUID magnetometers electronically combined to a second-order gradiometer. The gradiometer is formed by electronic subtraction of FLL output signals or by the Three SQUID Gradiometer approach. By evaluating the amplitude of relaxation signals (average of 4 magnetization cycles) a minimum amount of (selected) Fe/sub 2/O/sub 3/-particles corresponding to 600 pmol Fe was detected demonstrating the potential of the measurement system for biochemical laboratory diagnostics.