According to the invention there is provided a method of detecting an analyte including the steps of: i) providing a sample which contains the analyte and magnetic nanoparticles, in which the magnetic nanoparticles include a magnetic body portion which acts as a signalling vector and at least one receptor moiety attached to the body portion for binding to the analyte; ii) applying a magnetic field across at least a portion of the sample to orient the magnetic nanoparticles with respect to the applied magnetic field; iii) introducing electromagnetic radiation into the sample; iv) detecting a physical property which varies in dependence on the orientation of the magnetic nanoparticles with respect to the applied magnetic field, wherein the physical property is associated with the interaction of the electromagnetic radiation with the magnetic body portion which thereby acts as a signalling vector; and v) correlating the detected physical property with the presence of the analyte.