Biosensors and Bioelectronics 20 (2004) 585–591 A staphylococcal enterotoxin B magnetoelastic immunosensor Chuanmin Ruan a,b , Kefeng Zeng a,b , Oomman K. Varghese a,b , Craig A. Grimes a,b,∗ a Department of Electrical Engineering, 217 Materials Research Laboratory, The Pennsylvania State University, University Park, PA 16802, USA b Department of Materials Science and Engineering, 217 Materials Research Laboratory, The Pennsylvania State University, University Park, PA 16802, USA Received 19 January 2004; received in revised form 27 February 2004; accepted 4 March 2004 Available online 17 April 2004 Abstract A magnetoelastic immunosensor for detection of staphylococcal enterotoxin B (SEB) is described. The magnetoelastic sensor is a newly developed mass/elasticity-based transducer of high sensitivity having a material cost of approximately $0.001/sensor. Affinity-purified rabbit anti-SEB antibody was covalently immobilized on magnetoelastic sensors, of dimensions 6 mm × 2 mm × 28 m. The affinity reaction of biotin–avidin and biocatalytic precipitation are used to amplify antigen–antibody binding events on the sensor surface. Horseradish peroxidase (HRP) and alkaline phosphatase were examined as the labeled enzymes to induce biocatalytic precipitation. The alkaline phosphatase substrate, 5-bromo-4-chloro-3-indolyl phosphate (BCIP) produces a dimer, which binds tightly to the sensor surface, inducing a change in sensor resonance frequency. The biosensor demonstrates a linear shift in resonance frequency with staphylococcal enterotoxin B concentration between 0.5 and 5 ng/ml, with a detection limit of 0.5 ng/ml. © 2004 Elsevier B.V. All rights reserved. Keywords: Staphylococcal enterotoxin B; Magnetoelastic sensor; Immunosensor; Biocatalytic precipitation; Biotin–avidin 1. Introduction Immunosensors, in which antibodies are immobilized onto electrodes, optical fibers, piezoelectric crystals, or semiconductor chips, have attracted great interest in re- cent years for monitoring biological and chemical agents in a variety of applications. First and foremost a useful immunosensor requires a means for molecular recognition, that is sensing the specific antigen–antibody binding reac- tion at the surface of transducer. A useful immunosensor also requires a means of signal-transfer, where the specific binding event corresponds to changes in either an opti- cal, electrical, magnetic or acoustic parameter. A number of different signal-transduction platforms have been de- signed for monitoring antigen–antibody binding reactions at a sensor/solution interface, including surface plasmon resonance (Naimushin et al., 2002; Slavik et al., 2002; Medina, 2003), optical coupling (Akkoyun and Bilitewski, 2002), capacitance affinity sensors (Berggren and Johansson, 1997), amperometric immunosensors (Wang et al., 1998), impedance-based sensors (Ruan et al., 2002), and quartz ∗ Corresponding author. Tel.: +1-814-8659142; fax: +1-814-8656780. E-mail address: cgrimes@engr.psu.edu (C.A. Grimes). crystal microbalance-based sensors (Janshoff et al., 2000; Makower et al., 2003). A thick-film mass-sensitive magnetoelastic sensor plat- form has recently been designed for chemical sensing (Zorn et al., 2003) and biological sensing (Ruan et al., 2003a,b). Critical features of the sensor platform are low cost, making practical use of the sensors on a disposable basis, ease of use, excellent target sensitivity as well as specificity. The fundamental operating principle of the magnetoelastic sen- sors involves a change in sensor resonance frequency due to mass loading of the sensor, which herein is associated with binding of an analyte to a receptor immobilized on the surface of the ribbon-like magnetoelastic sensor. Theoreti- cally, the fundamental resonance frequency of a ribbon-like sensor of length L, with width and thickness much smaller than length, Young’s modulus E and density ρ is given by (Stoyanov and Grimes, 2000; Schmidt and Grimes, 2001): f r =  E ρ 1 2L (1) A small mass load m evenly deposited on a sensor of mass m 0 shifts the measured resonant frequency by an amount (Stoyanov and Grimes, 2000; Schmidt and Grimes, 0956-5663/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.bios.2004.03.003