Characterization of magnetic markers for liquid-phase detection of biological targets

We characterized the properties of magnetic markers for liquid phase immunoassays using Brownian relaxation. Since magnetic markers are usually composed of aggregated magnetic nanoparticles, their properties are significantly affected by the degree of aggregation. Through the analysis of magnetization (M-H) and AC susceptibility measurements, we obtained the distribution of the magnetic moment m and the Neel relaxation time τ_N of the aggregated particles included in the markers. We also determined the relationship between m and τ_N. From the obtained result, we could classify the particles into three types: Type-1 particles with very small m and very short τ_N , Type-2 particles with medium values of m and τ_N, and Type-3 particles with large m and very long τ_N. The proportion of these three types of particles included in the marker was also evaluated. We show that Type-3 particles and some Type-2 particles with τ _N > τ_B are primarily responsible for the observed signal in the liquid phase immunoassay using Brownian relaxation, where τ_B is the Brownian relaxation time. The obtained results are useful for evaluating suitable markers for liquid phase immunoassays using Brownian relaxation.