This paper investigates a rapid method of detecting bacterial pathogens on 2D food, such as leafy greens. The method is based on our previous work that uses freestanding phage-coated magnetoelastic (ME) biosensors and a surface-scanning coil detector for direct bacterial detection on the surface of food without commonly used sample preparation steps (washing, concentration, enrichment, etc). The ME biosensors used in this work were ferromagnetic alloy strips coated with E2 phage that specifically binds with Salmonella Typhimurium. First, the biosensors (1 mm × 0.2 mm × 30 µm) were placed in rectangular, indexed cavities (1.1 mm × 0.22 mm × 20 µm) that are etched in a glass wafer. A sheet magnet was added to the back of the wafer to hold the sensors in place. The glass wafer with the biosensors and sheet magnet was then flipped and lowered onto Salmonella-tainted spinach leaves. At this time, a weight was added such that the leaves got flattened to improve the chance for the biosensors to physically contact with Salmonella cells. Various weights ranging from 0 g to 120 g were tested to investigate their effects on the Salmonella binding.

Measurement sensors loaded with E2 phage and control sensors devoid of the phage were prepared and used. The control sensors served as the reference to compensate for the environmental effects such as temperature and humidity. The biosensors were placed on the surface of the spinach leaves for 2 mins for the binding to occur. The degree of the binding of Salmonella to the measurement sensors was determined by measuring the resonant frequency shift of the sensors using a surface-scanning detection system and confirmed by optical microscopy. The binding of Salmonella was found to increase as the spiked Salmonella concentration increased (10³ cfu/ml to 10⁸ cfu/ml). In addition, significant resonant frequency shifts of the measurement sensors (4,800 ± 160 Hz) were observed when 60 g of weight was added. However, further increases in weight did not improve the Salmonella binding with E2 phage. Rather, heavier weights increased the probability of non-specific attachment of Salmonella cells and other residuals from the leaf surface.