Rethinking cryptophane A for methane gas sensing: cross sensitivity to N2 and CO2 at ambient conditions

Abstract

Since the affinity of Cryptophane-A for methane was first reported in 1993, cryptophane-doped polymer films have been extensively studied as enrichment cladding layers in plasmonic, fiber-optic, and integrated waveguide-based optical sensors. While the use of cryptophane-doped layers has improved methane sensitivity compared to undoped claddings, controversy has grown over the years regarding their claimed selectivity and practical applicability. Key questions remain unresolved, including the extent of true methane enrichment at room temperature, cross-sensitivity to other gases, and the proportion of active cryptophane molecules within the polymer matrix. In this work, we employ Raman spectroscopy to provide direct and unambiguous evidence that Cryptophane-A exhibits measurable affinity for major ambient gases other than methane at room temperature. Notably, both nitrogen and carbon dioxide are shown to enter the cryptophane cavity and compete with methane for binding sites. This study underscores the value of Raman spectroscopy as a benchmark technique for investigating gas capture within host molecules at ambient conditions. It offers deeper insight into the binding behavior of Cryptophane-A and enables quantification of its relative affinities to atmospheric gases, thereby revealing both the limitations and the potential of cryptophane for future sensing applications.