ORIGINAL PAPER Ultrasensitive detection of Dynabeads protein A using the giant magnetoimpedance effect Tao Wang & Yong Zhou & Chong Lei & Jian Lei & Zhen Yang Received: 22 February 2013 / Accepted: 21 May 2013 # Springer-Verlag Wien 2013 Abstract We report on a biosensing system for ultrasensitive detection of Dynabeads protein A (DPA) that employs the magnetoimpedance (GMI) effect. The system is capable of detecting DPA via magnetic signals in the form of a magnetoimpedance change. The GMI ratio shows distinctive changes because of the induced fringe field produced by the superparamagnetic Dynabeads. The GMI ratio undergoes an overall downturn at high frequencies, but the drop becomes smaller with increasing DPA concentration. This phenomenon has not been observed so far. At a concentration of 0.1 μg mL -1 , the GMI ratio drops by 8.53 % at a frequency of 1.4 MHz. In other word: almost 90 Dynabeads can be detected. We believe that this novel scheme has a large poten- tial in high-sensitivity and miniaturized immunoassays. Keywords Dynabeads protein A . Ultrasensitive detection . Giant magnetoimpedance ratio . Drop . Flexible biosensing system . Superparamagnetic Introduction Magnetic beads with good dispersion are very important in sensing applications because they are easier to be detected with an applied magnetic field due to their outstanding superparamagnetism. Functionalized magnetic beads are widely applied in biomedical field [1–6], especially in magnetic-labeled targeted detection [7, 8]. The magnetic beads are pre-coupled with a bioprobe, it can be an antibody, protein or antigen, DNA/RNA probe or any other molecule with an affinity for the desired target. The Dynabeads pro- tein A are uniform superparamagnetic Dynabeads [9] cova- lently coupled with recombinant protein A (PA), PA has a high specificity for immunoglobulins and is therefore suit- able for the one-step capture of antibody [10, 11]. Further- more, the PA is readily adsorbed on the surface of gold nanoparticles (GNPs) [12, 13]. The GNPs are easily func- tionalized with recognition molecules (antibodies, antigens, oligonucleotides and so on.) by methods that lead to highly stable conjugates, and provided these are properly blocked non-specific interactions with other surfaces can be reduced almost to zero [14]. Micro-magnetic sensors having small size and quick response are in great demand for establishing ad- vanced intelligent measurement and control systems for mod- ern technology [15]. They are widely used in nearly all engi- neering and industrial sectors, such as high-density magnetic recording, target detection and tracking, magnetic marking and labelling, geomagnetic measurements, and biomagnetic measurements [16]. The giant magnetoimpedance (GMI) ef- fect was firstly observed in FeCoSiB amorphous wires by Mohri and his co-workers [17]. The GMI effect is a phenom- enon that the impedance of soft magnetic amorphous metals undergoes a large change when subjected to an external mag- netic field. The strong dependence of permeability in soft magnetic materials on applied magnetic field is the origin of the GMI effect. This effect has been extensively studied in soft amorphous ferromagnetic wires [18–20], thin films [21–23], and ribbon [24–26] in last decade. Magnetic sensors that are based upon the GMI effect have attracted much attention because they offer several advantages (higher sensitivity, higher stability, and higher linearity) over conventional mag- netic sensors such as fluxgate sensors, hall sensors and giant magnetoresistive sensors. The GMI sensors have been intro- duced into the biochemical analytical field to detect magnetic ferrofluid [27], micro-beads [28–30] and nanoparticles [31, 32] as a biosensor prototype in order to develop a new T. Wang (*) : Y. Zhou (*) : C. Lei : J. Lei : Z. Yang Key Laboratory for Thin Film and Microfabrication Technology, Ministry of Education, Research Institute of Micro/Nanometer Science and Technology, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China e-mail: skins@sjtu.edu.cn e-mail: yzhou@sjtu.edu.cn Microchim Acta DOI 10.1007/s00604-013-1016-5