Silicon ring resonator with ZIF-8/PDMS cladding for sensing dissolved CO2 gas in perfluorocarbon solutions

Precise measurement of dissolved CO2 gas concentration in perfluorocarbon (PFC) solution is essential to the implementation and understanding of extra-pulmonary gas exchange with liquid ventilation. While electrochemical and optochemical sensing methods are widely used for dissolved CO2 sensing in aqueous solution, dissolved CO2 sensing in the PFC solution remains largely unexplored. We report a refractive index-based silicon photonic sensor for monitoring dissolved CO2 gas in the PFC solution. The sensor employs a high-Q silicon ring resonator with ZIF-8 cladding for selective and sensitive detection of dissolved CO2 gas. Further, a gas-permeable PDMS coating layer is applied to the ZIF-8 cladding for enabling gas diffusion between the PFC solution and the ZIF-8. The PDMS layer does not dissolve in PFC and its hydrophobic properties help protect the ZIF-8 from potential damage caused by aqueous body fluids in the PFC solution during the liquid ventilation process. The fabricated sensor exhibits stable and repeatable responses in the PFC solution with a fine resolution of ∼0.16 CO2 vol%, good selectivity for dissolved CO2 over dissolved O2, and a fast response time of ∼1.9 min, enabling reliable monitoring of dissolved CO2 gas level changes in the PFC solution.