Planar nanophotonic structures for intensity based readout refractive index sensing applied to dissolved methane detection

Abstract

As global temperatures rise, permafrost in the Arctic is thawing, stimulating increased release of methane, a key greenhouse gas. Accurate, low cost, and portable sensors are needed to measure the dissolved methane concentration in seawater and freshwater to quantify methane release in nature and to better understand how these sources are contributing to increasing global methane levels. This paper analyzes and compares three simple and low-cost planar nanophotonic and plasmonic structures as optical transducers for measuring the refractive index change of polydimethylsiloxane (PDMS) polymer films doped with cryptophane-A molecules, which selectively trap methane. To measure the concentration of dissolved methane, changes in the refractive index (RI) of PDMS functionalized to selectively trap methane molecules, can be measured via an optical readout mechanism. However, the range of the RI change is very narrow, from 1.41198 to 1.41358 for methane concentration levels ranging from 0 nM to 300 nM, which requires the use of highly sensitive optical sensors. Through numerical simulations, we evaluate the sensitivity of the proposed structures and demonstrate that they exhibit superior performance in the reflectance intensity readout mode to that of the standard surface-plasmon-polariton-mode Spreeta sensor. A practical implementation of this chip with a simple intensity-based measurement scheme is proposed. Integration of this planar structure into a small, portable, and low-cost dissolved methane sensor offers a way to make methane monitoring more widespread and accessible.