Paramagnetic particle detection for use with an immunoassay based biosensor

A biosensor and detection system that can provide measurement of analyte concentration within a test sample in four minutes is presented. The biosensor uses an immunoassay with micrometre-sized paramagnetic particle (PMP) labels for quantification. An immunoassay relies on the specific interaction between an antigen (analyte) and the corresponding antibody to immobilise the antigen to a substrate for measurement. In this system, the test samples and PMP labels are added to a reaction vessel on the biosensor and are attracted to a reaction surface at the base of the vessel using an electromagnet, where specific binding takes place. Unlike other immunoassay systems no extra washing or processing procedures are required. The bound labels, and associated antigen are detected, in situ, using a detector coil on the surface of the biosensor. The incorporation of a second coil under the biosensor allows for compensation for environmental changes such as thermal drift. The measurement of the resonant frequency of the detector coil in a parallel resonant circuit, prior to and after addition of the sample, provides a measure of the antigen in the sample. Good frequency stability is provided by the use of a phase locked loop arrangement. The resulting sensitivity was 1.17 mu g of PMP labels per 1 Hz of frequency change and the minimum mass of PMPs detected was 62.5 mu g. To demonstrate the utility of the technology and the rapidity of measurement, a model immunoassay for Troponin I (a cardiac marker) was used and discrimination at the clinically relevant value of 0.5 ng/ml Troponin I was achieved.