Applied Physics Letters, Volume 118, Issue 8, February 2021. Magnetic particles (MPs), a group of engineered particles in the nanometer and microscale, are valuable tools for separation of chemical or biological substance in environmental research, for target delivery of antibodies or proteins in biomedical applications, and for quantification of cells or biomolecules in biological systems. Despite the estimation of the amount of magnetic nanoparticles that were realized by magnetic particle spectroscopy, accurate quantification of MPs at single-particle resolution is still a challenge. Here, we used an elliptically optically pumped Mx atomic magnetometer combined with a peristaltic pump system to perform magnetic field measurements to quantify magnetic particles at the single-particle level under aqueous solution condition. Quantitative analysis of cobalt micro-nanoparticles revealed the volume-field linear character of magnetic particles. From the measured intensity of the magnetic field generated by MPs, we calculated the geometric parameters and numbers of MPs and estimated the detection limit of our developed atomic magnetometer. Our work paves a way for the practical use of MPs in the biomedical field as well as other applications in industry.

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