Ethanol–water co-condensation into hydrophobic mesoporous thin films: example of a photonic ethanol vapor sensor in humid environment

Abstract

In this work, we report a new generation of solgel photonic vapor sensor based on water-repellent methylfunctionalized mesoporous silica thin films for the detection of alcohol vapors. Relative humidity, generally accountable for sensor performance deterioration, was demonstrated as a key parameter to detect and enhance sensor sensitivity concerning ethanol. The ethanol-assisted water vapor capillary condensation into hydrophobic mesopores was observed and investigated as an innovative sensing mechanism. We showed that water condensation was triggered by the surfactant-like behavior of alcohol molecules whose alkyl moieties preferentially interact with the hydrophobic walls and reversibly switch the surface energy from hydrophobic to hydrophilic. Influence of the ethanol and water pressure conditions for which the capillary condensation occurs were studied by in situ ellipsometry. This study revealed a synergic mechanism of coadsorption where the minimum ethanol concentration allowing water capillary adsorption decreases when humidity increases. Nanopatterned diffraction gratings films, fabricated via a nanoimprinting process, were demonstrated to be efficient transductor to transform the capillary-induced nanoscale variations of refractive index of the porous materials into optical signals easily measurable by conventional camera. Sensing results exhibited an ethanol vapor pressure threshold limit of detection of P/ P 0 EtOH = 0.07 at 100 % relative humidity and typical response and regeneration times are below one minute. Graphical Abstract

Dynamic Shaping of Femtoliter Dew Droplets

“…As proof-of-concept, we repeated the condensation experiment reported in the previous section on squared patterns but added ethanol (EtOH) in addition to water vapor. Relative vapor pressures of H 2 O and EtOH inside the chamber were controlled by bubbling dry air in a bubbler at 25 °C containing a solution of EtOH and H 2 O with 0, 1, 2.5, 5, and 10% wt% in alcohol as reported elsewhere. , The EtOH/H 2 O saturated air was “diluted” with dried air to reach a concentration of 25% in saturated vapors (as in the previous section). Condensation of EtOH/H 2 O vapors was induced by cooling down the surface at 2 °C.…”

“…After 1 min at room temperature, the thin film surrounded by the PDMS stamp was heated at 100 °C for 5 min on a hot plate. The PDMS stamp was then removed, and the titania thin film was finally calcinated at 450 °C for 5 min on a hot plate …”

“…The PDMS stamp was then removed, and the titania thin film was finally calcinated at 450 °C for 5 min on a hot plate. 35 Structural Characterization. Ellipsometry measurements were performed on a UV−visible (from 240 to 1000 nm) variable angle spectroscopic ellipsometer (VASE, 2000U Woollam), and the data analyses were performed with the Wvase32 software using Cauchy's models.…”

Method To Detect Ethanol Vapor in High Humidity by Direct Reflection on a Xerogel Coating

“…Systems exploiting only the change of the optical density are amongst the simplest ones, since transduction does not have to be performed spectroscopically. Simple reflected, transmitted, or guided signals can directly be exploited to detect the adsorption of EtOH, − or can also be exalted using photonic phenomena to enhance even more their sensitivity to volatile organic compounds (VOCs) down to 500 ppb for instance. − In these cases, highly porous and high surface area coatings such as silica xerogels were used as adsorbing matrix. However, the main issue related to detection based on adsorption-induced refractive index variation alone is its poor chemical selectivity.…”

“…Therefore, the selectivity between water and alcohol within such modified xerogel coatings is not high enough to permit an accurate quantification of EtOH in realistic operating situations of detection. Nonetheless, it was recently demonstrated that a quantitative analysis of EtOH vapor can be performed in saturated humid atmosphere by coupling xerogel sensitive materials with diffraction gratings . Even if the detection limit was found at 0.07 relative EtOH vapor pressure ( P / P 0 ), which is around 3 orders of magnitude above the sensitivity needed for medical and forensic applications, it remains a proof that it is possible to optically detect ethanol in presence of a high quantity of water.…”