Abstract: Method for detecting the presence or absence of a biological or chemical substance in a particular sample mixed with a suspension with functionalized magnetic particles, comprising: providing a light source and detector, providing a constant magnetic force perpendicular to the light's propagation direction by applying a constant magnetic field gradient, and with an absolute value which is higher than 0.1 T and measuring the change of the magnetic particle's suspension transparency versus time and comparing it with the time-variation in absence of the targeted biological or chemical substance. The method of the invention allows monitoring the transparency irrespective of the emitted wavelength and particle's optical properties.