Mid-infrared chemical sensors utilizing plasma-deposited fluorocarbon membranes

In this study, plasma-polymerized films are evaluated as enrichment membranes deposited at the surface of mid-infrared transparent waveguides for liquid-phase chemical sensing utilizing evanescent field absorption spectroscopy. Fluorocarbon films were deposited onto zinc selenide (ZnSe) waveguides from plasma-polymerized pentafluoroethane (CF3CHF2) vapor. Excellent optical transmission of ZnSe waveguides after plasma deposition confirms compatibility of the infrared transparent substrate with this low-temperature, solvent-free film deposition process. The liquid-phase enrichment characteristics for plasma membranes were investigated via evanescent field absorption spectroscopy of a model analyte (tetrachloroethylene); the limits of detection were below 300 ppb (v/v) in water. Plasma-polymerized films are known for their excellent mechanical and chemical stability, while offering tunable chemical and physical characteristics during the deposition process. Future application of this coating strategy for depositing robust enrichment membranes with tunable batch production capability imparts an attractive route toward application-oriented development of next-generation mid-infrared chemical sensors applicable in harsh environments.