Effective use of molecular recognition in gas sensing: Results from acoustic wave and in situ FT-IR measurements

To probe directly the analyte/film interactions that characterize molecular recognition in gas sensors, we recorded changes to the in situ surface vib rational spectra of specifically functionalized surface acoustic wave (SAW) devices concurrently with analyte exposure and SAW measurement of the exte nt of sorption. Fourier transform infrared external-reflectance spectra (FT -IR-ERS) were collected from operating 97-MHz SAW delay lines during exposu re to a range of analytes as they interacted with thin-film coatings previo usly shown to be selective: cyclodextrins for chiral recognition, nickel ca mphorates for Lewis bases such as pyridine or organophosphonates, and phtha locyanines far aromatic compounds. In most cases where specific chemical in teractions-metal coordination, "cage" compound inclusion, or pi-stacking-we re expected, analyte dosing caused distinctive changes in the IR spectra, t ogether with anomalously large SAW sensor responses. In contrast, control e xperiments involving the physisorption of the same analytes by conventional organic polymers did not cause similar changes in the IR spectra, and the SAW responses were smaller. For a given conventional polymer, the partition coefficients (or SAW sensor signals) roughly followed the analyte fraction of saturation vapor pressure. These SAW/FT-IR results support earlier conc lusions derived from thickness-shear mode resonator data.