Effect of magnetoelastic biosensors' size on sensitivity for detecting Salmonella typhimurium

Abstract: Mass-sensitive, magnetoelastic resonance sensors have a characteristic resonant frequency that can be determined by monitoring the magnetic flux emitted by the sensor in response to an applied time-varying magnetic field. This magnetostrictive platform has a unique advantage over conventional sensor platforms in that measurement is wireless and remote. In this study, antibody-based magnetoelastic (ME) biosensor specific to Salmonella typhimurium has been developed by immobilizing bio-recognition element onto the physical transducer. Rabbit polyclonal antibody, which was a protein called immunoglobulin related to immune system, was used as the bio-recognition. A magnetoelastic platform was served as the transducer, since it offered wireless or remote detecting, which was a unique advantage over conventional sensor platforms. Amorphous magnetostrictive alloy was mechanically polished using fine grit paper to reduce the thickness to 15 μm for decreasing the initial mass, and magnetostrictive strips were made with different sizes using an auto controlled micro-dicing saw. Then the diced sensors were ultrasonically cleaned in methanol solution to remove grease and debris left by the dicing process. And antibody was immobilized on the magnetostrictive platform using Langmuir-Blodgett (LB) technique, in which seven mono layers containing antibody were transferred onto the magnetostrictive sensor surface using an LB film balance KSV 2200 LB. Due to the magnetoelastic nature of the amorphous magnetostrictive alloy, the sensor exhibited a physical resonance when it underwent a time-varying magnetic field, and a shift in resonance frequency of the magnetostrictive sensor depended only on the mass change when testing environmental parameters are invariable. In this study, the environment was kept constant, and the changes in the resonance frequency of biosensors attributing to the binding of Salmonella typhimurium were recorded by HP network analyzer 8751A over the testing period, and the binding was also confirmed by scanning electron microscopy (SEM) micrographs. According to the shift in resonance frequency due to the binding of Salmonella typhimurium, the binding density bound on the biosensor was calculated. In order to confirm the binding, up to ten different regions of each sensor surface were examined and photographed using SEM, the number of cells bound to the sensor surface was directly counted from the SEM images and statistically converted to an area density of bacteria attached to the sensor surface, and compared with that calculated from the shift in resonance frequency. The results showed that the developed magnetostrictive biosensors could be applied for detecting Salmonella typhimurium; there existed a detection limit, which was defined as the lowest concentration that could be sensitively detected, and it was found that the detection limit was closely related to the size of the biosensor, and the rule was that the smaller the size of the biosensor, the lower the detection limit. And the higher frequency shift could be obtained while detecting the same solution by using smaller size of biosensor. The detection limits of 5×103, 105 and 107 CFU/mL were obtained for the biosensors with the sizes of 2 mm×0.4 mm×15 μm, 5 mm×1 mm×15 μm and 25 mm×5 mm×15 μm, respectively. Good agreement was obtained between the physical densities of bacterial cells obtained from SEM images and the theoretical densities calculated from the measured frequency shift, with less than 10% difference in between.