A biosensor comprising: - at least one optical reservoir containing a 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, wherein the light source is an optical pickup head and wherein 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 an oscillating uniaxial magnetic field, the oscillating uniaxial 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 oscillating uniaxial 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, wherein light from the light source and the oscillating uniaxial magnetic field are propagating substantially parallel along an incoming direction; - a reflecting object positioned such that • the modulated light transmitted through the suspension of magnetic particles in the optical reservoir, and/or • light transmitted through the optical reservoir without being modulated by the suspension of magnetic particles is reflected back through the optical reservoir by the reflecting object along a direction of propagation being substantially parallel with the incoming direction, and - a detection unit measuring the intensity I trans of the light transmitted through the suspension of magnetic particles in the optical reservoir after the light has passed through the optical reservoir at least two times, wherein the intensity I trans of the transmitted light is detected at a frequency f y varying between a start frequency f y, start and an end frequency f y, end as the oscillating uniaxial 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. - at least one optical reservoir containing a 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, wherein the light source is an optical pickup head and wherein 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 an oscillating uniaxial magnetic field, the oscillating uniaxial 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 oscillating uniaxial 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, wherein light from the light source and the oscillating uniaxial magnetic field are propagating substantially parallel along an incoming direction; - a reflecting object positioned such that • the modulated light transmitted through the suspension of magnetic particles in the optical reservoir, and/or • light transmitted through the optical reservoir without being modulated by the suspension of magnetic particles is reflected back through the optical reservoir by the reflecting object along a direction of propagation being substantially parallel with the incoming direction, and • the modulated light transmitted through the suspension of magnetic particles in the optical reservoir, and/or • light transmitted through the optical reservoir without being modulated by the suspension of magnetic particles - a detection unit measuring the intensity I trans of the light transmitted through the suspension of magnetic particles in the optical reservoir after the light has passed through the optical reservoir at least two times,