# Liquid-Phase detection of biological targets using magnetoresistive sensor and magnetic markers

Keiji Enpuku, Tatsuto Sakakibara, Takashi Yoshida

Research output: Contribution to journalArticlepeer-review

1 Citation (Scopus)

## Abstract

A magnetic immunoassay technique utilizing magnetoresistive sensor and Brownian relaxation of magnetic markers was developed for use in the liquid-phase detection of biological targets. The magnetic field noise spectrum of a detection system was 25 pT/Hz $$^{\mathrm {\mathbf {1/2}}}$$ at the measurement frequency, and the peak-to-peak field noise was about 35 pT when 72 measurements were averaged. We detected 10 ng of markers, and the minimum detectable number of the markers was estimated to be $$N-{\mathrm {\mathbf {m }}} =5.6 \times 10^{\mathrm {\mathbf {5}}}$$. We demonstrated the detection of biological targets called biotins. In the experiment, biotins were conjugated on the surface of the polystyrene beads with a diameter of 3.3 $$\mu$$ m. Streptavidin-coated magnetic markers were bound to the biotins. The bound and unbound (free) markers were magnetically distinguished by the Brownian relaxation of the free markers without using a washing process for maker separation. A clear correlation was obtained between the detected signal and the number ( $$N-{\mathrm {\mathbf {B}}}$$ ) of biotins. The minimum detectable number of biotins was as low as $$N-{\mathrm {\mathbf {B }}} =6.2\times 10^{\mathrm {\mathbf {5}}}$$. Since the sample volume was $$60~\mu$$ l, this value corresponds to a molecular number concentration of $$1.7\times 10~^{\mathrm {\mathbf {-17}}}$$ mol/ml. This result indicated the high sensitivity of the method used.

Original language English 6971373 IEEE Transactions on Magnetics 50 11 https://doi.org/10.1109/TMAG.2014.2323397 Published - Nov 1 2014

## All Science Journal Classification (ASJC) codes

• Electronic, Optical and Magnetic Materials
• Electrical and Electronic Engineering