Multiparameter Magnetic Relaxation Switch Assays

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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.