Effect of Methanol Vapor on an MQ2-MQ7-MQ9-MQ135 Metal-Oxide Gas Sensor Array: Normalized Response Fingerprints, Dynamic Behavior, and Electronic-Nose Front-End Implications

Abstract

This work characterizes the methanol-vapor response of a four-channel MQ metal-oxide semiconductor gas-sensor array composed of MQ2, MQ7, MQ9, and MQ135 under controlled-flow conditions at approximately 1000 ppm. Sensor resistance was recorded once per second during methanol exposure and subsequent clean-air recovery. The extracted descriptors included baseline resistance, stabilized methanol resistance, normalized relative resistance response, response time, recovery time, response slope, exponential transient constants, and the array-level response fingerprint. All sensors showed resistance decreases under methanol, consistent with n-type SnO2-based sensors exposed to a reducing volatile compound. Under the present experimental conditions, MQ135 produced the largest normalized response (98.47%) and the strongest absolute slope (1.623 kΩ/s), MQ7 showed a similarly large normalized response (96.15%), MQ2 exhibited the shortest operational response time (25 s), and MQ9 exhibited the shortest operational recovery time (150 s). The combined amplitude-kinetic pattern provides a multidimensional feature space that may serve as a low-cost sensing front end for subsequent electronic-nose development. The present study is a sensor-array characterization at a single nominal methanol concentration; multi-concentration calibration, controlled humidity perturbation, independent-day reproducibility testing, and validated pattern-classification studies are required before broader analytical recognition claims can be made.