Porous silica preconcentrator for selective ambient volatile organic compounds detection: Effects of surface functionalization and humidity

Abstract

With the increased demand for hand-held and ambient gas sensors, it is imperative to develop sensors that can offer both selective and sensitive detection. Gas preconcentration is a widely tried and tested method to increase the sensitivity of gas detectors. While it effectively lowers the limit of detection, it does not impact the selectivity of the detector. Therefore, preconcentrator materials have mostly been used in conjunction with selective detectors. In this work, we use the preconcentration method with a nonselective small and portable photoionization detector to introduce selectivity. For this purpose, we use a relatively slow heating rate, that allows for gradual desorption and analytes from the preconcentrator material–nanoporous silica. The characteristic desorption temperature of the volatile organic compounds (VOC) from the preconcentrator allows selective detection of the VOC. In this work, we study the effect of surface functionalization, to make it hydrophobic, and observe the adsorption–desorption behavior of polar (isopropyl alcohol) and non-polar (octane) gas molecules. The hydrophobic silica surface was found to improve the adsorption of non-polar octane, while it reduced the adsorption of polar isopropyl alcohol. The desorption temperature for isopropanol remained unchanged for both functionalized and non-functionalized preconcentrators; however, the desorption temperature for octane increased by 10 °C when the functionalized hydrophobic pSiO2 was used. We also observed the presence of humidity, a known interferent, did not heavily impact the sensing performance. These results are promising evidence that functionalized porous silica integrated with a photoionization detector can be used for selective gas detection in the ambient atmosphere.