Multiparameter Magnetic Relaxation Switch Assays

Abstract

Magnetic nanoparticles (NPs) can serve as magnetic relaxation switches (MRSw's), switching from a dispersed to a clustered state, or the reverse, due to the presence of molecular targets, with changes in the spin−spin relaxation time of water (T₂). Biotinylated NP probes reacted with an avidin molecular target to form stable NP clusters, which permitted several NMR parameters to be measured as a function of cluster size. Associated with avidin-induced NP cluster formation was an increase in the spin−spin relation rate (1/T₂), while the spin−lattice relaxation rate (1/T₁)was unaffected. On the basis of the selective effects of NP cluster formation on T₂, we developed a T₁/T₂ interrogation method where NP probe concentration and avidin analyte were unknown and both were determined. A third NMR parameter examined was the replication of T₂ measurements, which were used to rapidly determine whether the ratio of avidin to biotinylated NP was optimal or whether additional biotinylated NP was needed. The T₁/T₂ and T₂ replication interrogation methods illustrate how MRSw assays can employ multiple parameters, instead of relying only on T₂, to obtain information about the reaction of NPs with molecular targets.