A SQUID measurement system for immunoassays

A SQUID measurement system is presented that was developed for analysis of antigen–antibody reactions in fluid samples. The detection method is based on the signal generated by the remanent or relaxing magnetization of nanoparticles which are attached as labels to the antibodies. The measurement system involves two 7.2-mm-diameter multiloop SQUID chip magnetometers (type W7A) with a typical field noise of √$\text{S}{}_{\mathrm{B}}$=1.5 fT/√$\text{Hz}$ arranged as an electronic gradiometer with a baseline of 160 mm. It is embedded in a liquid helium dewar with a tail bottom allowing 6 mm distance between SQUID and room temperature. A sophisticated direct-coupled read-out-electronics was realized with a bandwidth of 8 MHz. The read-out-electronics can be reset within τ_R ⩽0.5 μs. A 2 channel control unit was developed which enables environmental noise reduction. Data are acquired by a PC-controlled 8 channel system with 16-bit resolution and a maximum sampling frequency of f_s=196 kHz. First measurements are presented.