Ultra-sensitive magnetoreduction measurement system and ultra-sensitive, wash-free assay using the same

An ultra-sensitive method for quantitatively determining a concentration of biomolecules in a sample solution, the method comprising:

• providing a reagent comprising magnetic nanoparticle, wherein the surfaces of the magnetic nanoparticles are coated with bio-receptors;
• measuring an ac magnetic susceptibility of the reagent (χac,o) at a mixture frequency (mf1+nf2), wherein m or n is independently an integer larger than zero;
• mixing the reagent with the sample solution containing the biomolecules that comprise multiple-active-epitope biomolecules or single-active-epitope biomolecules, wherein the bio-receptors coated on the surfaces of the magnetic nanoparticles become associated with the biomolecules;
• measuring the ac magnetic susceptibility of the reagent after the association (χac,φ) with the biomolecules at the mixture frequency (mf1+nf2);
• calculating a difference in the ac magnetic susceptibility of the reagent (Δχac,φ) at the mixture frequency (mf1+nf2) before and after the association with the biomolecules, wherein Δχac,φ≡(χac,o−χac,φ);
• and
• determining the concentration of the biomolecules in the sample solution.