Multiplexed detection for biomolecules tagged to magnetic nanoparticles using a miniaturized AC magnetic susceptometer K. Park, S. Sonkusale, R. P. Guertin*, T. Harrah**, E. B. Goldberg** Department of Electrical and Computer Engineering, Tufts University *Department of Physics and Astronomy, Tufts University **Department of Molecular Biology and Microbiology, Tufts University Medford, MA 02155, USA kyoungchul.park@tufts.edu Abstract— A novel multiplexed sensing scheme based on Brownian relaxation for biomolecules tagged to magnetic nanoparticles (MNPs) in liquid environment is proposed. Feasibility of the technique has been verified by the experiments with the mixture of differently sized magnetic nanoparticles using a newly developed room temperature, miniaturized AC magnetic susceptometer. The AC magnetic susceptibility measurements of Brownian relaxation of MNPs verify the sensing modality that proves the resonant frequency of imaginary susceptibility is inversely proportional to effective hydrodynamic size of MNPs. The proposed Brownian sensing scheme has the potential for multiplexed analysis of multiple biological binding events on functionalized MNPs. The performances were verified for individual and mixtures of monodisperse iron oxide MNPs in solution with carboxylic acid group with core diameters of 15, 25, 35, 50nm using the proposed susceptometer. The approach is readily compatible with lab-on-chip applications in medical diagnostics, and can be used for affinity-based biosensing. I. INTRODUCTION Due to the relative simplicity of sample preparation, ease of processing and inherent biocompatibility, magnetic nanoparticles (MNPs) are increasingly of great interest in biotechnology and biomedicine as alternatives to conventional radioisotopes or fluorescent materials [1]. The development of bio-conjugated MNPs allows various opportunities for the application of MNPs for biomedical diagnostics [2]. Several sensing schemes using the magnetic stray field for MNPs bound to targets have been studied by means of magnetoresistance [3], Hall Effect [4], or superconducting quantum interference devices [5]. However, many established devices and techniques require extreme conditions such as high magnetic fields and/or low temperature and are also time consuming. An alternative method to detect the binding of biomolecules such as protein to the MNPs is via measurement of the AC magnetic susceptibility of the nanoparticles based on Brownian relaxation. AC magnetic susceptometry is a precise detection technique that capitalizes on the diffusive properties of MNPs in solution [6,7] and is appropriate for point of care diagnostics with potential for chip implementation[8]. The use of AC susceptometry for the detection of biomolecules using tagged MNPs was initially described theoretically by Connolly and St. Pierre [9]. Magneto-optical measurement [10] and fluxgate relaxometry [11] have recently been proposed to measure brownian relaxation of MNPs to acquire AC magnetic susceptibility as a function of frequency. Although previous studies have shown good performance with outstanding sensitivity, experiments have been done for only single sized MNPs with bulky conventional instruments. Our focus is on achieving multiplexed biosensing for the mixture of differently sized MNPs utilizing a newly developed, compact, room temperature, low cost, low power AC susceptometer. The susceptometer and the proposed approach are scalable for lab- on-chip application using planar microcoils and microfludics. II. THEORY OF BROWNIAN SENSING A. Brownian relaxation detection using AC susceptibility The principle of a Brownian relaxation detection scheme uses the random rotational motion of magnetically tagged sensors, determined via measurement of collective magnetic susceptibility as a function of the frequency of the applied magnetic field [12]. When the excitation frequency is close to the rotational motion frequency of the magnetically labeled sensor, a large increase in the loss component of the complex magnetic susceptibility occurs. This is observed as a peak frequency of the imaginary component of the complex magnetic susceptibility (90° out-of-phase: ' ' χ ). The application of this technique for biological diagnostics relies on a shift in the peak frequency of ' ' χ upon target binding to labeled MNPs. If a target molecule then binds to a specified receptor on the sensor, the hydrodynamic size of the sensor is effectively increased and there is a readily measurable shift of the frequency maximum to lower values with cubic dependence on hydrodynamic radius. The real ( ' χ ) and This research has been funded partially by the NIH, New England Regional Center or Excellence (NERCE) AI-057159. 978-1-4244-5335-1/09/$26.00 ©2009 IEEE 1204 IEEE SENSORS 2009 Conference