Magnetic nanoparticles biofunctionalized with antibodies are useful for specifically labeling target biomolecules. By measuring magnetic signals after the association between biofunctionalized magnetic nanoparticles and target biomolecules, the concentration of target biomolecules can be quantitatively detected. One of measuring methodologies is so-called immunomagnetic reduction (IMR), in which the reduction in ac magnetic susceptibility of magnetic reagent is a function of the concentration of target biomolecules. In this letter, the relationship between the magnetic reduction signal of reagent and the concentration of target biomolecules is explored. According to the experimental results on various kinds of target biomolecules, such as proteins and chemicals, the magnetic reduction signal as a function of concentration of various target biomolecules can be scaled to a universal curve. This universal curve is a logistic function. This implies that there exists a unique mechanism for the association between the target biomolecules and biofunctionalized magnetic nanoparticles in an IMR assay.
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