Improved Pathogen Detection Using Magnetoelastic Biosensors Operating Under Multi-Harmonic Resonance Modes
Authors: Du Songtao , Horikawa Shin , Chai Yating , Wikle Howard Clyde , Chin Bryan A 
Published: February 2020

This paper presents an investigation into improved pathogen detection using phage-coated magnetoelastic (ME) biosensors that operate under multi-harmonic resonance modes. The ME biosensor is a class of mass-sensitive biosensors that is being developed for rapid, on-site detection of pathogenic bacteria in the food system. The ME biosensor used in this work consists of a freestanding, strip-shaped ME resonator as the signal transducer and an engineered phage probe specifically binding with the pathogen of interest. The ME resonator is made of an amorphous, ferromagnetic alloy with magnetostrictive properties and hence can be placed into mechanical resonance by an externally applied time-varying magnetic field. When this phage-coated ME biosensor comes into contact with the target pathogen, the pathogen is specifically captured and bound to the biosensor by the phage. The bound pathogen then causes the mass of the biosensor to increase, resulting in a decrease in the biosensor's resonant frequency. The conventional theory describes that the mass sensitivity of the ME biosensor (i.e., the ratio of the resonant frequency change to the mass change) is proportional to the harmonic, n, defined as an integer multiple of the fundamental resonant frequency (n = 1, 2, 3, ···). Hence, the mass sensitivity can be improved by measuring resonant frequency changes of higher harmonic modes. Additionally, there is an advantage of measuring multi-harmonic resonant frequency changes over solely measuring the fundamental resonant frequency change. For each harmonic mode, there exist nodes in a vibrating sensor where no displacement occurs. Hence, no resonant frequency changes can be detected when the target mass is loaded at these nodes, leading to false-negative detection. To improve the sensor’s detection capabilities as well as to avoid the false detection, this research takes the approach of measuring multi-harmonic resonant frequency changes in response to the attachment of the target pathogen. As a model study, detection experiments of Bacillus anthracis Stern spores using 4 mm × 0.8 mm × 30 µm ME biosensors were conducted. The JRB7 phage, which possesses high binding affinity for B. anthracis spores, was used as the detection probe and coated on the sensor surfaces. Up to the 15th harmonic resonant frequency was successfully measured, and the resonant frequency change of each harmonic mode was compared to one another. As anticipated, the mass sensitivity was found to improve as the harmonic, n, increases. The measured results also showed close agreement with theoretical calculation results. Furthermore, localized masses (spores) were attached to a biosensor, and measurement of multi-harmonic resonant frequency changes was performed. Depending on the location of the attached spores, the resonant frequency change of each harmonic mode varied. When spores were loaded at certain locations of a vibrating sensor, no detectable frequency changes were observed for some harmonic modes, indicating that the spores were loaded at the nodes of these harmonic modes. However, the other harmonic modes were found to show resonant frequency changes successfully, eliminating false-negative detection. Hence, poof-in-principle has been demonstrated.