Molecular Design of Intercalation-Based Sensors. 2. Sensing of Carbon Dioxide in Functionalized Thin Films of Copper Octanediylbis(phosphonate)

Self-assembled thin films of layered copper alkanediylbis-(phosphonates) retain the amine-specific intercalation chemistry of the corresponding microcrystalline solids. Aliphatic and aromatic amines bind in a 1:1 ratio to coordinatively unsaturated copper ions in anhydrous Cu₂(O₃P(CH₂)₈PO₃); and by selecting an amine with an appropriate functional tail group, a chemically and sterically well-defined interlamellar binding site for CO₂ is created. Powder X-ray diffraction, Fourier transform infrared spectroscopy, and solid-state NMR experiments were used to study the intercalation of 3-aminopropanol, (3-aminopropyl)methyldihydroxysilane, and p-xylylenediamine, and their reversible reaction with CO₂ to form carbonates and carbamates, respectively. By growing these films on the electrodes of a quartz crystal microbalance device, a sensor can be fabricated for monitoring CO₂ in gas streams at concentrations of 0.5-19% (v/v). A Henrian response (frequency change directly proportional to CO₂ partial pressure) was observed, and the time required for equilibration of these devices with CO₂, using 5-layer films, was 3-4 min. Effective diffusion coefficients for CO₂ in the films were determined using a dual-transport model and were found to be in the range (6-9) × 10^-9 cm²/s.