A biosensor comprising: - a suspension of magnetic particles; - an optical reservoir containing the suspension of magnetic particles; - a light source emitting light at a wavelength λ with an intensity I, the light being directed at the optical reservoir and being adapted for interacting with the suspension of magnetic particles, where the light entering the optical reservoir has an intensity I in and light transmitted through the optical reservoir has an intensity I trans; - a magnetic field generation unit generating a uniaxial oscillating magnetic field oscillating at a frequency f x being variable between a start frequency f x, start and an end frequency f x, end, the uniaxial oscillating magnetic field being applied to the optical reservoir containing the suspension of magnetic particles whereby the suspension of magnetic particles is modulated such that the intensity I trans of light transmitted through the suspension of magnetic particles is modulated compared to the intensity I in of the light entering the optical reservoir; and - a detection unit measuring: • the intensity I trans of the light transmitted through the suspension of magnetic particles in the optical reservoir, and • an in-phase (V') component and an out-of-phase (V") component of the transmitted light, wherein the modulation of the intensity I trans of the transmitted light and the in-phase (V') component and the out-of-phase (V") component of the transmitted light are detected at a frequency f y varying between a start frequency f y, start and an end frequency f y, end as the uniaxial oscillating magnetic field is swept from the start frequency f x, start to the end frequency f x, end, wherein the detected frequency f y is the second harmonic ( f y = 2 f x) or higher harmonic components of the intensity I trans of the transmitted light. - a suspension of magnetic particles; - an optical reservoir containing the suspension of magnetic particles; - a light source emitting light at a wavelength λ with an intensity I, the light being directed at the optical reservoir and being adapted for interacting with the suspension of magnetic particles, where the light entering the optical reservoir has an intensity I in and light transmitted through the optical reservoir has an intensity I trans; - a magnetic field generation unit generating a uniaxial oscillating magnetic field oscillating at a frequency f x being variable between a start frequency f x, start and an end frequency f x, end, the uniaxial oscillating magnetic field being applied to the optical reservoir containing the suspension of magnetic particles whereby the suspension of magnetic particles is modulated such that the intensity I trans of light transmitted through the suspension of magnetic particles is modulated compared to the intensity I in of the light entering the optical reservoir; and - a detection unit measuring: • the intensity I trans of the light transmitted through the suspension of magnetic particles in the optical reservoir, and • an in-phase (V') component and an out-of-phase (V") component of the transmitted light, wherein the modulation of the intensity I trans of the transmitted light and the in-phase (V') component and the out-of-phase (V") component of the transmitted light are detected at a frequency f y varying between a start frequency f y, start and an end frequency f y, end as the uniaxial oscillating magnetic field is swept from the start frequency f x, start to the end frequency f x, end, wherein the detected frequency f y is the second harmonic ( f y = 2 f x) or higher harmonic components of the intensity I trans of the transmitted light. • the intensity I trans of the light transmitted through the suspension of magnetic particles in the optical reservoir, and • an in-phase (V') component and an out-of-phase (V") component of the transmitted light,