Hydrogel-Based Integrated Antenna-pH Sensor

In this paper, a miniaturized integrated antenna-sensor based on a pH-sensitive hydrogel is designed, fabricated and characterized. The proposed sensor geometry is based on a 5-mm-long slot antenna whose frequency is controlled by a back-metal coated on the pH-sensitive hydrogel that is patterned on a glass substrate. When the antenna is immersed in a solution whose pH changes from 5 to 12 it shows a variation of its resonant frequency by approximately 160 MHz around a center frequency of 21.7 GHz. Except for an abnormal behavior around the pH=l state caused by the back-metal-hydrogel bond, the pH-antenna frequency mapping exhibits a well-defined monotonic region from pH=5 to 12 thus covering many practical solutions.