An SPR-Based In Situ Methane Sensor for the Aqueous\nand Gas Phase

This paper presents\na fast, low-cost, precise methane sensor based\non refractive index changes in a cryptophane A (CryptA)-doped polystyrene\nmembrane. For the realization of this sensor, we built a surface plasmon\nresonance sensor with a refractive index resolution of 4.31 ×\n10<sup>–6</sup> and investigated the optimal membrane thickness,\ni.e., a polymer layer of sufficient sensitivity with the lowest response\ntime. For a membrane thickness of 760 nm, a limit of detection of\n135 ppm and a response time constant of 45 s were found. Despite a\ncomparable refractive index resolution and a higher CryptA content,\nthe limit of detection is 3 orders of magnitude larger than that of\na reported prototype. The sensor is capable of quantifying methane\nin the gas or aqueous phase. However, temperature, humidity, and ethanol\nvapor highly influence the signal. Although the principle of CryptA-based\nmethane measurement is a promising, fast, and low-cost method, it\nwill not be able to compete with state-of-the-art sensing in its current\nstate.