Detection of sulfur dioxide at low parts-per-million concentrations using low-cost planar electrodes with ionic liquid electrolytes

Simon Doblinger; Junqiao Lee; Zoe Gurnah; Debbie S Silvester

doi:10.1016/j.aca.2020.05.037

Detection of sulfur dioxide at low parts-per-million concentrations using low-cost planar electrodes with ionic liquid electrolytes

Anal Chim Acta. 2020 Aug 8:1124:156-165. doi: 10.1016/j.aca.2020.05.037. Epub 2020 May 20.

Authors

Simon Doblinger¹, Junqiao Lee¹, Zoe Gurnah², Debbie S Silvester³

Affiliations

¹ Curtin Institute for Functional Molecules and Interfaces, And School of Molecular and Life Sciences, Curtin University, GPO Box U1987, Perth, WA, 6845, Australia.
² Curtin Institute for Functional Molecules and Interfaces, And School of Molecular and Life Sciences, Curtin University, GPO Box U1987, Perth, WA, 6845, Australia; Department of Chemistry, University of Southampton, University Road, Southampton, SO171BJ, UK.
³ Curtin Institute for Functional Molecules and Interfaces, And School of Molecular and Life Sciences, Curtin University, GPO Box U1987, Perth, WA, 6845, Australia. Electronic address: d.silvester-dean@curtin.edu.au.

PMID: 32534668
DOI: 10.1016/j.aca.2020.05.037

Abstract

Sulfur dioxide (SO₂) is a toxic gas at low parts-per-million (ppm) concentrations, with a permissible exposure limit (PEL) of 2 ppm. Its detection is mandatory, particularly in the fields of occupational health and safety, and environmental pollution. In this work, ppm concentration detection of sulfur dioxide was performed in six room temperature ionic liquids (RTILs), as well as on two different electrode materials - platinum and gold - and with two different electrode geometries, i.e. macro thin-film electrodes (TFEs) and microarray thin-film electrodes (MATFEs). Calibration curves were established for 10-200 ppm SO₂ using cyclic voltammetry to determine the optimum combination of RTIL, electrode surface and geometry for the sensing. The RTIL 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonium)imide ([C₄mpyrr][NTf₂]) with a platinum thin-film electrode was found to give the best response due to the relatively low viscosity of the RTIL combined with the high sensitivity and a clean blank response. On MATFEs, deposited sulfur particles - confirmed using scanning electron microscopy (SEM) coupled to an energy dispersive spectrometer - were found to passivate and block some of the microholes, leading to unstable long-term chronoamperometric responses. To the best of our knowledge, this is the first observation of sulfur deposition from SO₂ reduction in aprotic ionic liquids. Consecutive additions of 2 ppm SO₂ were studied in [C₄mpyrr][NTf₂] on a TFE using long-term chronoamperometry, showing excellent reproducibility upon successive additions. This demonstrates that small volumes of RTILs can be combined with miniaturized, low-cost TFEs and applied for the reliable and continuous monitoring of sulfur dioxide gas at concentrations lower than the permissible exposure limit of 2 ppm.

Keywords: Cyclic voltammetry; Gas sensing; Long-term chronoamperometry; Room temperature ionic liquids; Sulfur dioxide; Thin-film electrodes.