Sensors and Actuators B 156 (2011) 651–656 Contents lists available at ScienceDirect Sensors and Actuators B: Chemical journa l h o mepage: www.elsevier.com/locate/snb A microfluidic sensor based on ferromagnetic resonance induced in magnetic bead labels Esha Chatterjee a , Tim Marr b , Pallavi Dhagat b , Vincent T. Remcho a,∗ a Department of Chemistry, Oregon State University, 153 Gilbert Hall, Corvallis, OR 97331, United States b School of Electrical Engineering and Computer Science, Oregon State University, 1148 Kelley Engineering Center, Corvallis, OR 97331, United States a r t i c l e i n f o Article history: Received 21 September 2010 Received in revised form 26 January 2011 Accepted 8 February 2011 Available online 3 March 2011 Keywords: Microfluidics Sensors Ferromagnetic resonance Magnetic beads a b s t r a c t This report details preliminary studies towards the development of a microfluidic sensor that exploits ferromagnetic resonance, excited in magnetic bead labels, for signal transduction. The device consists of a microwave circuit in which a slotline and a coplanar waveguide are integrated with a biochemically activated sensor area. The magnetic beads are immobilized in the sensor area by bio-specific reactions. A microwave signal applied to the slotline is coupled to the coplanar waveguide only in the presence of magnetic beads at the functionalized sensor area. Ferromagnetic resonance in the beads further enhances the coupling. This inductive detection technique lends itself to miniaturization, is inexpensive to fabricate and can be adapted for the detection of a wide range of molecules for which bio-specific ligands are available. Experimentally, the variation of the output signal as a function of the location of magnetic beads was studied for the proposed technique. Subsequently, a prototype device was constructed by biotiny- lation of the sensor area and integration with a microfluidic chip fabricated in polydimethyl siloxane (PDMS). Preliminary experiments were conducted on this prototype using streptavidin-functionalized magnetic beads as labels. It was shown that the magnetic beads, immobilized at the sensor area by streptavidin–biotin linkage, produced a distinct ferromagnetic resonance response easily discernable from the background signal. © 2011 Elsevier B.V. All rights reserved. 1. Introduction An important step towards the development of a portable, stan- dalone, point-of-use diagnostic device involves integration of a highly sensitive transduction system with the sensing module. Typ- ically, immunoassay-based techniques employ a label attached to a biochemical probe for detection. Labels may be molecules such as radioisotopes, fluorescent dyes or enzymes, or may be particu- late in nature, such as quantum dots and nanoparticles. Label-free methods of detection, such as surface plasmon resonance (SPR) and quartz crystal microgravimetry (QCM) are gaining popularity as they bypass the traditional labeling steps to provide a rapid and highly specific means of identifying and quantifying analytes. While these label-free techniques offer advantages in terms of reduced assay time, which is essential in the field of rapid-onset-illness diagnostics, they cannot at present match the detection limits of traditional labeled assays such as ELISA or radioimmunoassay (RIA) ∗ Corresponding author. Tel.: +1 541 737 8181; fax: +1 541 737 2062. E-mail addresses: chattere@onid.orst.edu (E. Chatterjee), marrt@eecs.oregonstate.edu (T. Marr), dhagat@eecs.oregonstate.edu (P. Dhagat), Vincent.Remcho@oregonstate.edu (V.T. Remcho). [1]. SPR can only detect molecules in close proximity of the sens- ing surface, which is not suitable for typical microfluidic devices since they have channel heights much greater than 200 nm [2]. Also, instrumentation based on QCM and SPR are not as amenable to miniaturization and batch fabrication [3]. Conventional techniques for detection of labeled molecules are highly sensitive but suffer from certain drawbacks. Radioactive labels have a limited shelf life and stringent waste disposal requirements. Optical detection requires the use of instrumentation that is bulky and difficult to miniaturize [4]. Thus, there is a need for investigation into alterna- tive methods of transduction that are highly sensitive, conducive to miniaturization and suitable for mass production. This report presents a novel high-sensitivity inductive detection technique for potential immunosensing applications using mag- netic beads as labels: a microwave circuit, designed for operation at frequencies from 2 GHz to 4 GHz, was used to excite ferromag- netic resonance (FMR) in beads immobilized at the sensor area and detect the resulting signal. The biotin–avidin pair was employed to demonstrate the potential of this technique for bio-specific detection. High sensitivity was achieved both by operating at high frequency and matching the microwave excitation to the ferro- magnetic resonance frequency in the beads. To our knowledge, this is the first report that demonstrates inductive detection of 0925-4005/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2011.02.012