The present invention can provide a method of determining the presence, location, quantity, or a combination thereof, of a biological substance, comprising: (a) exposing a sample to a plurality of targeted nanoparticles, where each targeted nanoparticle comprises a paramagnetic nanoparticle conjugated with one or more targeting agents that preferentially bind with the biological substance, under conditions that facilitate binding of the targeting agent to at least one of the one or more biological substances； (b) subjecting the sample to a magnetic field of sufficient strength to induce magnetization of the nanoparticles； (c) measuring a magnetic field of the sample after decreasing the magnetic field applied in step b below a threshold； (d) determining the presence, location, quantity, or a combination thereof, of the one or more biologic substances from the magnetic field measured in step (c).
