Dependence of biomolecule detection on magnetic nanoparticle concentration

Abstract

This paper investigates the influence of magnetic nanoparticle (MNP) concentration on biomolecule detection with magnetic particle spectroscopy (MPS). Streptavidin-coated MNPs, dominated by Brownian relaxation, are used to detect biotinylated Immunoglobulin G (IgG). The conjugation of the biotinylated IgG onto the surface of the functionalized MNPs enlarges their hydrodynamic sizes, thus increasing the Brownian relaxation time and changing the MPS signal. Experimental samples are prepared with Brownian relaxation-dominated MNP samples with different mole concentrations from 6.67 to 0.67 nM and IgG samples with different mole concentrations from 0 to 8.33 nM. Experimental results of ac susceptibility spectra indicate that the Brownian relaxation time increases from 0.85 to 1.20 ms with decreasing the MNP concentration from 6.67 to 0.67 nM at a given IgG concentration of 5 nM. Experimental results of MPS show that the ratio of the 3rd to 1st harmonics decreases more significantly with increasing IgG concentration at a lower MNP concentration. The absolute variation in the ratio of 3rd to 1st harmonics of the MNPs is used to characterize the measurement sensitivity of IgG. It shows that the measurement sensitivity increases from 1.68 × 10−3 to 12.25 × 10−3 nM−1 at an excitation frequency of 180 Hz. This study demonstrates that a lower MNP concentration can improve the measurement sensitivity of biomolecules with MPS in the frequency range from 180 to 1418 Hz. We emphasize that our findings are of great importance to reach a highly sensitive approach for biomolecule detection.