Microwave Near-Field Detection of the Ion Concentration in Sealed Fluidic Systems

We have developed a near-field scanning microwave microscopy (NSMM) system that contains a high-quality factor resonator, a sharp metallic probe tip, a 6-degree scanning stage and a vector network analyzer. By applying incident GHz microwaves through the probe tip over samples under test and measuring the magnitude and phase shift of the reflection parameter S11, we have precisely detected the ion concentration of electrolytes in a variety of fluidic systems which are sealed under a 50 μm thick dielectric cover. As expected, the measured magnitude of S11 monotonically increases with the ion concentration, but not linearly, and is sensitive to the tip-sample spacing. This technique offers a useful way for on-site, real-time monitoring of the changes in electrolyte fluids of limited volume in a sealed device. Further work is needed to reveal the exact correlation between the deflection magnitude and ion concentration.