Rapid Detection of Pathogens by a 3D Biomolecular Filter and Automated Biosensor Measurement System

This paper investigates real-time detection of Salmonella on fresh apples 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 pathogen detection in the food system. The ME biosensor used in this work consists of a freestanding, strip-shaped ME resonator (1 mm x 0.2 mm x 30 um) as the signal transducer and E2-phage that specifically binds 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 and binds specifically with the target pathogen, the mass of the biosensor increases, 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, improved mass sensitivity can be obtained by measuring resonant frequency changes of higher harmonic modes. Additionally, measuring and analyzing multi-harmonic resonant frequencies can reduce possible false negative detection resulting from the Salmonella that are attached to the nodes of the vibrating sensors. As a proof-in-concept experiment, in-situ Salmonella detection on fresh apples was conducted. Both measurement (with phage) and control (without phage) sensors were prepared and placed directly on apples spiked with Salmonella. The sensors’ resonant frequency shifts were then measured in situ using a surface-scanning coil detector as a function of time. While the resonant frequency shifts of the control sensors remained negligible, those of the measurement sensors decreased largely as time progressed, indicating that specific binding of the Salmonella on the measurement sensors had occurred. In addition, the mass sensitivity was found to improve for higher harmonic modes (the 2^nd and 3^rd longitudinal modes) compared to the fundamental mode. False negative detection was also reduced, which was confirmed by inspecting the sensor surface using SEM and comparing the number of bound Salmonella to the measured frequency shift data. Hence, enhanced detection of Salmonella has been demonstrated using ME biosensors operating in multi-harmonic resonance modes.