Patent ID: 8034634

Claim:
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 (χ ac,o ) at a mixture frequency (mf 1 +nf 2 ), 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 (χ ac,φ ) with the biomolecules at the mixture frequency (mf 1 +nf 2 ); calculating a difference in the ac magnetic susceptibility of the reagent (Δχ ac,φ ) at the mixture frequency (mf 1 +nf 2 ) before and after the association with the biomolecules, wherein Δχ ac,φ ≡(χ ac,o −χ ac,φ ); and determining the concentration of the biomolecules in the sample solution.