Highly sensitive space-filling planar microwave biosensors for label-free E. coli bacteria sensing with enhanced detection capability

• A label-free microwave biosensor for bacteria detection with enhanced detection capability is proposed. • High selectivity and sensitivity is achieved by combining an appropriate biofunctionalization layer with a novel transducer geometry. • A space-filling curve inspired layout is proposed to fit a very long coplanar waveguide line for phase change sensing in a compact footprint. • A sensor demonstrator was developed for E. coli bacteria and fabricated using a single-layer metal-on-semiconductor microfabrication technology. • Effective sensitivity as high as 2.41 deg per order of CFU/ml was obtained where E. coli bacteria concentration as low as 100 CFU/ml is clearly distinguishable with a large margin. A novel microwave biosensor is proposed for label-free bacteria sensing. High selectivity and sensitivity and thus enhanced detection capability, is achieved by combining a biofunctionalization layer with a novel transducer geometry. The sensor operates on broadband transmission phase change sensing principle where the signal variation is proportional to medium‘s permittivity variation but also scales with transmission line length. The detection capability is further improved by maximizing the sensitive-to-total area ratio; to fit long line in compact footprint the use of space-filling curve for layout design is proposed. A coplanar waveguide hosted on an inorganic substrate is used as the sensing line to which the bound bacteria constitute a superstrate altering the propagation constant. For the best results, geometrical features with high electric field need to be in the bacteria scale and so microfabrication technology is used. Two demonstrators were developed and experimentally tested in the centimeter frequency range. Gold-on-silicon structure was used for implementation and volume production. A 20 mm long sensing line for which as much as 17 % of the total area is sensitive requires only ∼3 mm 2 of real estate including reference line and probe landings. Sensor coated with anti- E. coli bacteria antibody was tested for detecting gradually increasing concentrations of E. coli . The measurement results show that as low as 100 colony-forming-unit in 1 ml (100 CFU/ml) of bacteria suspension can be easily detected with sensitivity of 2.41° per order of CFU/ml, indicating potential to go even lower with detection threshold.