Spectral comparison of nanoporous silica-adsorbed organic molecules with gaseous and liquid states using a new waveguide technology

Abstract

Nanoporous materials play an important role in many applications including separations and sensing, therefore understanding surface interactions, molecular adsorption, and nanoconfinement phenomena are important. Here we use near-infrared spectroscopy to measure spectra of volatile organic compounds (VOCs) adsorbed inside a nanoporous silica waveguide and compare to the spectral features of the VOCs’ gas and liquid phase spectra. The 10 mm long nanoporous waveguide has a transparent optical window between 1200 and 2200 nm that allows sensitive near-infrared measurements of a range of polar and non-polar VOCs. For the non-polar compounds, the spectra of the molecules adsorbed in the nanopores have similar spectral features to those of the gas molecules in free space. However, for the polar compounds, there are more prominent spectral similarities between the adsorbed molecules and their corresponding liquid spectra. Interestingly, we also observed that the 10 mm path length porous waveguide was capable of sensing gas concentration at least one order of magnitude lower compared to a 400 mm path length conventional gas-cell. The novel optical waveguide allows for sensitive measurement in the near-infrared, demonstrating a strong potential to be used as a tool to study adsorption in nanoporous materials and as a gas sensor.