Resonant Sensor Arrays for Wireless Characterization of Solvated Ions

A low-cost, passive resonant sensor was developed for wireless detection and measurement of ionic compounds. The sensor was fabricated as an open-circuit, Archimedean spiral composed of copper on a flexible, polyimide substrate. The sensor is interrogated by a two-loop antenna connected to a vector network analyzer (VNA) to monitor the scattering parameter response of the sensor when exposed to varying ionic concentrations. The sensor response was defined in terms of the resonant frequency and the peak-to-peak amplitude of the transmission scattering parameter profile (|S₂₁|). Potassium chloride (KCl) solutions with concentrations in the range of 100 nM – 4.58 M were tested on nine resonators having different length and pitch sizes to study the effect of sensor geometry on its response to ion concentration. The resonant sensors demonstrated an ion-specific response, caused by the variations in the relative permittivity of the solution, which was also a function of the resonator geometry. A lumped circuit model, which fit the experimental data well, confirms signal transduction via change in solution permittivity. Also, a ternary ionic mixture (composed of potassium nitrate (KNO₃), ammonium nitrate (NH₄NO₃), and ammonium phosphate (NH₄H₂PO₄)) response surface was constructed by testing 21 mixture variations on three different sensor geometries and the phase and magnitude of scattering parameters were monitored. It was determined that the orthogonal responses presented by resonant sensor arrays can be used for quantifying levels of target ions in ternary mixtures. Applications of these arrays include measuring the concentration of key ions in bioreactors, human sweat, and agricultural waters.