A Functionalized Microwave Biosensor for Rapid, Reagent-Free Detection of E. coli in Water Samples

Escherichia coli (E. coli) O157:H7 (O157), one of the most common Shiga toxin-producing E. coli, can contaminate water systems causing severe illnesses often accompanied with diarrhea and sometimes life threatening. Frequent monitoring of E. coli in water systems is critical to protect public health. Most traditional methods for E. coli detection are slow in responding to E. coli outbreaks due to the need for sample transportation from the site to the lab, expensive equipment, and highly trained personnel for the detection. This work presents a novel reagent-free detection method that employs a microwave biosensor functionalized with an antibody specific to E. coli to offer rapid and sensitive E. coli detection. By monitoring the resonance frequency shift caused by the binding between the E. coli in the water sample and the antibody coated on the sensor using a vector network analyzer (VNA), this microwave-based biosensor achieved a limit of detection (LOD) of 647 CFU/ml. This LOD can be further reduced to 6.47 CFU/ml with a simple preconcentration step prior to the sensing procedure. The sensor has also been tested to detect E. coli in natural water systems with a low-cost, palm-sized portable VNA, suggesting its excellent feasibility for real-time on-site E.coli detection.