SO2 Gas Sensing by Nanometer-Thick Soap Films Using Conductometry and Fluorimetry

Conductometric and fluorometric sensing of SO₂ gas using common black films (CBF) of nanometer thickness (<20 nm) with large specific surface area (about 10⁸ m^–1) was shown. For conductometry, H₂O₂ molecules in the soap film oxidized SO₂ and increased the conductance by producing H₃O⁺, HSO₄^–, and SO₄^2– ions. Limit of detection (LOD) for this assay was 70 parts per billion (ppb), and a linear signal–concentration relation was seen up to 1 ppm (ppm) of SO₂ in the air. In the 1–25 ppm range, a monotonic yet nonlinear increasing correlation was seen. For fluorimetry, SO₂ reacted quantitatively with Rhodamine B hydrazide in CBF to form highly fluorescent Rhodamine B molecules. The LOD was 100 ppb, and a linear signal–concentration relation was seen for 0.1–25 ppm of SO₂ in the air. Sensing of SO₂ released by wine and beer samples was also demonstrated.