A miniaturized AC magnetic susceptometer for detecting biomolecules tagged to magnetic nanoparticles

We demonstrate a miniaturized AC magnetic susceptometer to detect biomolecules tagged to magnetic nanoparticles (MNPs). The proposed compact susceptometer measures the magnetic relaxation spectrum of biomolecules tagged with MNPs in an aqueous suspension. The relaxation time corresponds to the Brownian relaxation time and depends on the hydrodynamic size of the MNPs. Conventional susceptometers are bulky, expensive and are designed for extreme conditions (e.g. low temperature or high DC magnetic field). Our handheld prototype is composed of AC current source, precise millimeter sized differential sensing coils, and a ultra-sensitive lock-in amplifier. The performances were verified for 15, 25 nm Fe
₃
O
₄
-COOH MNPs suspended in solution. The result for each sample show the peak frequency of imaginary susceptibility is inversely proportional to effective hydrodynamic size of MNPs.