We present a method to identify and quantify methane using a hydrophobic ionic liquid (IL)-electrified metal electrode interface by electrochemical impedance spectroscopy. We investigated the mechanisms of the responses of the IL-electrified electrode interface to the exposure of methane and other interfering gases (H 2 , C 6 H 12 , SO 2 , NO, NO 2 , CO 2 , O 2 , H 2 O). Our results show that at low frequency the IL-electrified electrode interface shows a predominantly capacitive response. The IL-electrode double layer (EDL) was found to be the primary response layer while the transition zone and bulk region of the IL-electrode interface contribute little to the overall signal change. For recognition and quantification of methane using the Langmuir adsorption model and measurement of differential capacitance change, an optimum EDL interface structure was found to form at a specific DC bias potential. The cumulative results shown in this work suggest that an ideal IL-electrode interface can be formed by varying IL structure and applied DC bias electrode potential for a specific analyte and that the semi-ordered structure of IL-electrified interface can act as a recognition element for the sensitive and selective adsorption and detection of gaseous molecules.Due to their unique properties and increasing availability, room temperature ionic liquids (ILs) have received great interests in electrochemistry, catalysis, electronics, and energy conversion as well as interdisciplinary investigations on both fundamental and practical applications. 1-5 For example, ILs as solvent free and ion-coupled material exhibit strong benefits as non-aqueous electrolytes for enhancing the safety and robustness for sensor and transistor devices. 6-12 However, the low intrinsic conductivity of the ILs correlates with their high viscosity, which limits the response time and sensitivity of detection methods such as those based on amperometry and potentiometry. [13][14][15][16][17] Methane, which has been considered as a clean energy source and one of the most important greenhouse gases, has attracted significant interest to the characterization of its adsorption on surfaces and its quantification in atmosphere as well. Since methane is relative chemically and electrochemically inert, current methods for methane detection are either relatively high-cost (e.g. optical), which prevents widespread deployment, or lack the selectivity (e.g. catalytic bead) 18 demanded by various applications. Many methane sensors also need improvement regarding their size, power consumption and the ease of use. [19][20][21] Current results show that the potential-dependent interface of an IL and metal electrode is very sensitive to surface conditions on the electrode, such as the proton adsorption on an oxide electric interface 22 and the adsorption of CO on a metal electrode 23 . As shown in Figure 1, the molecular selectivity of an IL-electrode interface comes from the ordering of the electric double layer (EDL) as well as molecular interactions between the IL...