This paper presents an investigation into the use of magnetoelastic biosensors for the rapid detection of Salmonella typhimurium on fresh spinach leaves. The biosensors used in this investigation were comprised of a strip-shaped, goldcoated
sensor platform (2 mm-long) diced from a ferromagnetic, amorphous alloy and a filamentous fd-tet phage which
specifically binds with S. typhimurium. After surface blocking with bovine serum albumin, these biosensors were,
without any preceding sample preparation, directly placed on wet spinach leaves inoculated with various concentrations
of S. typhimurium. Upon contact with cells, the phage binds S. typhimurium to the sensor thereby increasing the total
mass of the sensor. This change in mass causes a corresponding decrease in the sensor's resonant frequency. After 25
min, the sensors were collected from the leaf surface and measurements of the resonant frequency were performed
immediately. The total assay time was less than 30 min. The frequency changes for measurement sensors (i.e., phageimmobilized)
were found to be statistically different from those for control sensors (sensors without phage), down to 5 ×
106 cells/ml. The detection limit may be improved by using smaller, micron-sized sensors that will have a higher
probability of contacting Salmonella on the rough surfaces of spinach leaves.
This work demonstrated a direct detection of Salmonella on fresh food produce using groups of magnetoelastic
biosensors. The magnetoelastic biosensors were coated with E2 phage, which specifically binds with S. typhimurium.
The resonance frequency of the biosensor is measured using a pulse excitation system, which allows simultaneous
detection of multiple sensors. Multiple measurement and control biosensors were placed on fresh food surfaces that had
been spiked with a known amount of Salmonella. Binding with bacteria was allowed to occur for 30 minutes in a humid
air environment. The resonance frequencies of the groups of biosensors were then measured to determine the amount of
bound bacteria. By using a statistical experimental design and by taking the average of repeated measurements, possible
detection errors are decreased. By using multiple sensors at each site of interest, a higher portion of the contaminated
surface has contact with biosensors, allowing for more complete information on the food produce surface. Results from
SEM pictures of the sensor surface agree with the sensor frequency response results.
This paper presents the direct detection of Salmonella typhimurium on shell eggs using a phage-based magnetoelastic
(ME) biosensor. The ME biosensor consists of a ME resonator as the sensor platform and E2 phage as the biorecognition
element that is genetically engineered to specifically bind with Salmonella typhimurium. The ME biosensor,
which is a wireless sensor, vibrates with a characteristic resonant frequency under an externally applied magnetic field.
Multiple sensors can easily be remotely monitored. Multiple measurement and control sensors were placed on the shell
eggs contaminated by Salmonella typhimurium solutions with different known concentrations. The resonant frequency of
sensors before and after the exposure to the spiked shell eggs was measured. The frequency shift of the measurement
sensors was significantly different than the control sensors indicating Salmonella contamination. Scanning electron
microscopy was used to confirm binding of Salmonella to the sensor surface and the resulting frequency shift results.
Magnetoelastic sensors exhibit a characteristic resonance frequency upon the application of an alternating magnetic
field. In this research, magnetoelastic material was fabricated into micro-sized sensors coated with JRB7 phages to
specifically detect Bacillus anthracis spores. Research had shown that the sensor's resonant frequency decreases
linearly as its mass increases. As spores are captured, the mass increases. A high mass-sensitivity of up to 7.5 Hz/pg
allowed this sensor's use in applications requiring accurate sensing of a very low concentration of B. anthracis spores.
A B. anthracis spore weighs about 2 picograms. Two different sizes of sensors, 2000×400 μm and 1000×200 μm, were
used in this study. The resonant frequency and the sensitivity of the sensors were found to vary under different
magnitudes of DC biasing magnetic field. It was found that both the resonant frequency and the Q-value of the sensed
signal increase with an increase of the magnitude of the DC magnetic field until they approach magnetic saturation. As
the magnetic field was changed from low to high, it was observed that the signal amplitude increased to a maximum and
then decreased to undetectable. Finally, real-time detection of B. anthracis spores is performed under the optimum
magnetic field condition.
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