A magnetic nanoparticles relaxation sensor for protein–protein interaction detection at ultra-low magnetic field

Functionalized magnetic nanoparticles (MNPs) can serve as magnetic relaxation sensors (MRSs) to detect different biological targets, because the clustering of magnetic particle may cause the spin–spin relaxation time (T2) decrease of the surrounding water protons. However, the application of MNPs in clinical NMR systems faces the challenge of poor stability at magnetic field strengths in the order of tesla. The recently developed ultra-low field (ULF) NMR technique working at microtesla (μT) range then becomes a candidate. Herein, we incorporated superconducting quantum interference device (SQUID) as the detector in the ultra-low field system to enhance the sensitivity. We functionalized the Fe3O4 nanoparticles with the gama-aminobutyrate type A receptor-associated proteins (GABARAP), which specifically interact with calreticulin (CRT). As a result of the interaction between GABARAP and CRT, the clustering of the functionalized MNPs generates local magnetic fields, which accelerate the dephasing of the water protons in the vicinity. We analyzed the relation between T2 values and the CRT concentrations at 211μT and the low detection limit for CRT is 10pg/ml, which is superior to the immunoblot system. The high sensitivity of the ULF NMR system for protein–protein interaction detection demonstrates the potential to use this inexpensive, portable system for quick biochemical and clinical assays.