ZnO2 nanohybrid thin film sensor for the detection of ethanol vapour at room temperature using reflectometric interference spectroscopy

• Nanohybrid thin film based ethanol sensor working at room temperature was developed.
• Reflectometric interference spectroscopy (RIfS) was used as measuring principle.
• The effect of noise level was reduced by real-time polynomial fitting of the spectra.
• The limit of detection and the sensitivity were improved by surface modification.
• The limit of detection of the sensor is 5 ppm in the 0–50 ppm concentration range.

Reflectometric interference spectroscopy (RIfS) was used to detect ethanol vapour in 10–100 ppm concentration on the surface of original and modified ZnO2 thin films in a gas flow platform. The interference pattern shift (Δλ) caused by the increase of the effective refractive index (in general terms the optical thickness) of the thin film due to vapour adsorption into the pores was investigated. The effect of noise level was reduced by polynomial fitting, hence the limit of detection (LOD) was improved significantly (by half an order of magnitude). Similarly, by surface functionalization (with butyltrichlorosilane, BTS) the sensitivity to ethanol vapour was increased by one order of magnitude (from 0.038 to 0.331 nm/ppm), and the limit of detection improved three-fold as well. The RIfS method was compared to a gravimetric technique (quartz crystal microbalance, QCM), and at the same time the adsorbed vapour amount (ns) that gives rise of Δλ was investigated: linear Δλ vs. ns curves were determined in the 0–60 ppm concentration range.

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