Characterization of Magnetic Markers for Liquid-Phase Detection of Biological Targets

Abstract

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 <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</sub> of the aggregated particles included in the markers. We also determined the relationship between m and τ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</sub> . From the obtained result, we could classify the particles into three types: Type-1 particles with very small m and very short τ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</sub> , Type-2 particles with medium values of m and τN, and Type-3 particles with large m and very long τ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</sub> . 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 τN > τ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</sub> are primarily responsible for the observed signal in the liquid phase immunoassay using Brownian relaxation, where τ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">B</sub> is the Brownian relaxation time. The obtained results are useful for evaluating suitable markers for liquid phase immunoassays using Brownian relaxation.