A miniaturized 2.4 GHz re-entrant cavity has been designed, manufactured and tested as a sensor for microfluidic compositional analysis. It has been fully evaluated experimentally with water and common solvents, namely methanol, ethanol, and chloroform, with excellent agreement with the expected behaviour predicted by the Debye model. The sensor’s performance has also been assessed for analysis of segmented flow using water and oil. The samples’ interaction with the electric field in the gap region has been maximized by aligning the sample tube parallel to the electric field in this region, and the small width of the gap (typically 1 mm) result in a highly localised complex permittivity measurement. The re-entrant cavity has simple mechanical geometry, small size, high quality factor, and due to the high concentration of electric field in the gap region, a very small mode volume. These factors combine to result in a highly sensitive, compact sensor for both pure liquids and liquid mixtures in capillary or microfluidic environments.

In this article, a highly sensitive and compact microwave sensor with quality factor (Q ≈ 3000) is proposed for identifying and sensing liquid samples using a central gap ring resonator (CGRR). The high Q is a result of surface current redistribution. Our sensor is designed to operate at 2.5 GHz with an active gap region, where the sample flows, in which the electric field is at its maximum amplitude. The sensor was simulated, fabricated, and tested for a new application to identify the condition of oil used as a coolant and an insulating material in high-voltage transformers. The efficiency of oil in this application depends on its insulation capability and water content. Since both of these factors correlated with the oil's dielectric constant, the CGRR sensor showed very good sensitivity for detecting the ratio of water in oil from the measurements of Q change (and its link to the oil's breakdown voltage) involving fresh and used oil samples.

A new type of microwave microfluidic sensor is presented which allows the correction of the systematic errors in complex permittivity measurement when the ambient temperature changes. The sensor has been designed and fabricated using two half-wave microstrip resonators, where the two resonators have been assembled with a common junction at the input and output ports which allows the transmission coefficients of the two resonators to be measured using the same two input and output ports. The sensor has been tested using water with a temperature sweep from 21 °C to 39 °C. The results show an excellent agreement between the measured and expected results after the correction procedure has been applied.

Clostridium difficile is a major cause of hospital acquired infection that poses significant diagnostic challenges. In this paper, a new type of resonant microwave applicator is proposed for the liberation of DNA from C. difficile spores via microwave disruption, followed by rapid detection using a sandwich hybridization assay. A split-ring resonator is designed to operate at 2.4 GHz with a 3 mm active gap region that contains an isolate volume of approximately 10 mm ³ , exposed to pulsed microwaves of 12 W rms power. The parallel electric field configuration maximizes the interaction between the microwaves and the sample. In a proof of principle study, in combination with pathogen specific DNA probes, we have used the system to correctly identify virulent strains of C. difficile using magnetic bead extraction of DNA suitable for point-of-care application.