Detection of actuated clusters by scattering

A method for detecting clusters of superparamagnetic particles coated with a bioreactive agent is provided. A suspension of the superparamagnetic particles in a fluid to be analyzed is provided. The particles are allowed to form clusters due to an analyte present within the fluid and a magnetic field rotating at a given frequency is applied to the solution. Light is directed to the fluid and the amplitude of the intensity of scattered light at twice the frequency of the magnetic field is extracted. By determining the amplitude of the measured intensity of scattered light at twice the field depending on the frequency of the magnetic field a frequency-dependent measurement may be achieved. The frequency-dependent measurement may be used to determine the critical frequency of clusters, to distinguish clusters having different sizes or to measure the average value of the susceptibility and the spread of the susceptibility of the particles in the fluid. Furthermore, an apparatus for apparatus for detecting clusters of superparamagnetic particles is provided.