  In this paper, a new concept of magnetic nanoparticles as magneto-optical
markers is proposed, and a "magneto-optical gate" effect is explored.
High-sensitivity detection of magnetic nanoparticles (MNPs) was developed based
on this principle. Under irradiation from monochromatic light with a wavelength
much larger than the size of a given MNP, Rayleigh scattering occurs on the MNP
surface. The MNPs anisotropic and orientate themselves into chains, meaning
that the Rayleigh scattering now decreases under magnetic field excitation.
After applying an AC magnetic field of frequency f, the transmitted light
passing through the MNPs reagent generates an AC signal with a frequency of 2f.
Moreover, the 2f signal disappears after removal of the magnetic field. This
creates a "magneto-optical gate" effect. The instantaneous and highly sensitive
detection of magneto-optical markers with a concentration of 0.2 {\mu}g/mL and
an effective optical path length of 10 {\mu}m is realized based on this
"magneto-optical gate" effect.
