Liquid-phase immunoassays using brownian relaxation of magnetic markers

Anwarul Kabir Bhuiya, Tetsu Mitake, Masaki Asai, Tomoya Ito, Schunichi Chosakabe, Takashi Yoshida, Keiji Enpuku, Akihiko Kandori

Research output: Contribution to journalArticlepeer-review

7 Citations (Scopus)


We have developed a liquid-phase immunoassay technique using Brownian relaxation of magnetic markers. In this method, biological targets are fixed on the surface of large polymer beads whose size is typically a few μm. When the markers are bound to the targets, their Brownian relaxation time is dominated by that of the polymer bead, becoming much longer than that of unbound (free) markers. The resulting difference between the magnetic properties of the bound and free markers was detected by relaxation measurements. Therefore, we can magnetically distinguish between the bound and free markers, i.e., we can omit a time consuming washing process called bound/free separation. We developed a detection system using a magneto-resistive (MR) sensor and showed that we can detect 1.4× 107 bound markers in 60 μl of solution. If we assume that a single marker is bound to a single target, this sensitivity can be expressed as 3.8× 10-16mol/ml (or 0.38 fmol/ml) in terms of the molecular-number concentration. We also demonstrated the detection of biological targets called biotins, which were conjugated on the surface of the polystyrene beads with a diameter of 3.3 μm. A strong relationship was obtained between the number of bound markers and the number of biotin-conjugated polymer beads, which confirmed the validity of the present method.

Original languageEnglish
Article number6027621
Pages (from-to)2867-2870
Number of pages4
JournalIEEE Transactions on Magnetics
Issue number10
Publication statusPublished - Oct 2011

All Science Journal Classification (ASJC) codes

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


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