On-chip preconcentration and infrared detection of toxic gases using a nanoporous waveguide

Selective detection of toxic volatile organics in the gas-phase is required for many applications. Here we report a novel optical waveguide for near infrared (NIR) spectroscopy for quantitative detection of volatile organic compounds (VOCs), enabling sensitive and selective measurements of multiple VOCs simultaneously. The adsorption of the VOC molecules inside the nanopores allows detection at a much lower concentration compared to traditional gas-cell detection, where infrared light propagates through the gaseous molecules in the free space. Results show that a 10 mm long porous core waveguide offers a significant enhancement in signal of 1800x per millimeter pathlength compared to a 400 mm long conventional gas-cell. Moreover, lowering the temperature of the waveguide facilitates increased adsorption of molecules inside the pores and maximizes the signal as demonstrated for gaseous toluene at four different concentrations. An increase in signal by a factor of 10 is observed as the temperature decreases from 70 to 10°C. At 10°C waveguide temperature, the limit of detection for toluene is 2.5 ppm. Additionally, we measured a binary mixture of toluene and 1-octene at different temperatures, showing the selectivity of the method. The combined effect of nanopore adsorption and temperature-assisted condensation allows for selective and sensitive on-chip detection of volatile organics. The proposed waveguide sensor technology has promising potential for industrial gas leak detection, air quality monitoring, and medical applications.