4060 IEEE TRANSACTIONS ON MAGNETICS, VOL. 49, NO. 7, JULY 2013 Magneto-Impedance Biosensor With Enhanced Sensitivity for Highly Sensitive Detection of Nanomag-D Beads Jagannath Devkota, Alejandro Ruiz, Pritish Mukherjee, Hariharan Srikanth, and Manh-Huong Phan Department of Physics, University of South Florida, Tampa, FL 33620 USA A magnetoimpedance (MI) biosensor based on Co-based amorphous ribbon was designed and tested to detect functionalized Nanomag-D magnetic beads. While previous studies were focused mainly on exploring the MI change for biosensing, we show that the sensitivity of the biosensor can be enhanced when the change in ac magnetoresistance (MR) or magnetoreactance (MX) is used. The frequency at which the sensitivity of the sensor is optimized can be tuned. This is of potential interest in developing functional biosensors with improved sensitivity and tunable frequency. Index Terms—Magnetic biosensors, magnetoimpedance (MI), magnetoreactance (MX), magnetoresistance (MR). I. INTRODUCTION M AGNETIC biosensors are of increasing research in- terest for their high potential to target biomolecules through the detection of functionalized magnetic beads [1], [2]. The important requirements of a biosensor regarding the detection of magnetic beads include high sensitivity, low power consumption, quick response, reliability, environment-friendly operation, and low cost. Magnetic sensors based on various principles, such as giant magnetoresistance (GMR) [3], [4] and superconducting quantum interference device (SQUID) [5], have been developed to detect magnetic beads. However, GMR sensors possess limited sensitivity and SQUID sensors require extremely low temperatures for operation. Recently, alterna- tive biosensors based on the giant magnetoimpedance (GMI) effect of soft ferromagnetic materials have been proposed for sensitive detection of magnetic biomarkers [6]–[9]. They are highly sensitive to the applied field, low power consuming, and operational at room temperature. The GMI effect is a large change in the complex impedance (where and are the resistance and reactance, respectively) of a soft ferromagnetic conductor subject to an external dc magnetic field [10]. GMI-based biosensors have been successfully employed to study the relative change in the impedance of amorphous rib- bons [6], [8] and microwires [7] from the presence of added magnetic particles, such as commercial dynabeads [6], estapor beads [7], and superparamagnetic iron oxide [8]. How- ever, targeting a very low concentration of a biological sample, such as cancer cells that have taken up magnetic nanoparticles [11], [12], requires more sensitivity for nanoparticle detection. Therefore, the increment of sensitivity and functional character of a sensor are of current focus in developing a new generation of diagnostics systems. In this paper, we report on a systematic study of GMI detection of functional magnetic beads, Nanomag-D, using Manuscript received October 30, 2012; accepted December 17, 2012. Date of current version July 15, 2013. Corresponding author: M. H. Phan (e-mail: phanm@usf.edu). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMAG.2012.2235414 a Co-based amorphous ribbon. Instead of limiting the focus exclusively to the impedance change for biosensing, as in previous studies [6]–[9], [11], [12], we show that the sensitivity of a GMI-based biosensor can be enhanced when a change in ac resistance and/or reactance is used. The frequency at which the highest sensitivity of the sensor is achieved can be tuned as well. Treating the surface of a ribbon with an appropriate concentration of acid is also shown to improve the sensitivity of detection of a ribbon-based GMI biosensor. II. EXPERIMENT Functionalized Nanomag-D beads (diameter, 250 nm) suspended in water with the original concentration of 10 mg/ml were purchased from Mircomod Partikeltechnologie GmbH, Germany. These beads are -coated composites of iron oxide nanoparticles and dextran and have potential medical applications [13], [14]. The sensor prototype has dimensions of and was designed using a Co-based amorphous ribbon prepared by a rapid quenching technique. The impedance was measured using an HP4192A impedance analyzer over a length of 1 cm by the four point measurement technique with a constant current of 5 mA flowing along the length of the ribbon in a frequency range of 0.1–13 MHz in the presence of axial dc magnetic field of up to 120 Oe. The impedance was first measured for a plain ribbon (P), and then with 30 of a Nanomag-D suspension (Nmag, 100 ) drop-cast on it for comparison. To improve the sensitivity of particle detection, the surface of the ribbon was also treated with 5 of dilute ( 4.5 vol-%) for 24 hrs and washed with distilled water. The impedance was measured again for the acid-treated ribbon (A), with 5 of water, and Nanomag-D suspension, separately. The magnetoresistance (MR), magnetoreactance (MX), and magnetoimpedance (MI) ratios were respectively defined as (1) (2) (3) 0018-9464/$31.00 © 2012 IEEE