Sensitivity analysis of multilayer microporous polymer structures for terahertz volatile gas sensing.

The sensitivity of multilayer microporous polymer structures (MPSs) for in situ and label-free organic vapor sensing is investigated in terahertz frequency. The porous structure provides a large hydrophilic surface area and numerous micropores to adsorb or fill polar vapors, thereby leading to greatly enhanced wave-analyte interaction with an apparent terahertz signal change. Different configurations of MPS with distinct geometric parameters are fabricated to study the structure-dependent sensitivity. The signal responses from the acetone-vapor-filled MPS, represented as effective absorption coefficient and refractive index variation, are proportional to the amounts of vaporized molecules. The responsivity is independent of MPS stacking configuration but can be significantly improved by decreasing the micropore volume. The linear responsivity range for the acetone vapor concentration is as wide as 200 ppm, and the detection limit can be as low as 1 ppm corresponding to the molecular density of 31 pmol/mm³. Different concentrations of toxic methanol adulterated in alcoholic aqueous solutions are successfully identified in their vapor phase by using the MPS-based terahertz sensor with an optimal sensitivity.