An Electrochemical Impedance Spectroscopy Study on the Use of Fabry-Pérot Etalon Devices as Metal–Insulator–Metal Capacitive-Based Sensors

Fabry-Pérot optical interferometers (etalons) were constructed by sandwiching a stimuli-responsive poly(N-isopropylacrylamide) (pNIPAm)-based microgel layer between two thin gold layers. These etalons have previously been employed as low-cost, robust, qualitative colorimetric-based point-of-need (PON) sensors for various analytes. In this work, the ability of etalon devices to interface with electronics and function as quantitative, capacitive-based sensors was explored. Electrochemical impedance studies suggested that dry state etalons behaved as traditional metal-insulator-metal (MIM) capacitors, while hydrated state etalons possessed more supercapacitor-like qualities. As pNIPAm is well-known for its thermoresponsivity, the potential of the devices to act as capacitive-based sensors was examined using temperature as a stimulus to demonstrate proof-of-concept. The change in capacitance of the devices was observed to be directly related to the ability of the microgels to deswell as the temperature was varied. This study demonstrated that pNIPAm-based etalon devices can be used as quantitative, capacitive-based sensors, and have promise as versatile, cheap, easy-to-use, and robust PON sensors.