Rethinking Cryptophane-A for Methane Gas Sensing: Cross-Sensitivity to N2 and CO2 at Ambient Conditions

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. In this work, we employ Raman spectroscopy to provide direct and unambiguous evidence that Cryptophane-A exhibits measurable affinity for three major atmospheric gases (carbon dioxide, methane, and nitrogen) at room temperature, debunking old beliefs of cryptophane-methane selectivity and reformulating the role of nitrogen. Notably, carbon dioxide shows a 1.5-times stronger affinity for Cryptophane-A than methane, while nitrogen relative affinity to methane was demonstrated to be 0.4. 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.