An oxygen amperometric gas sensor based on its electrocatalytic reduction in room temperature ionic liquids

A conveniently assembled membrane-free amperometric sensor is proposed for the detection of oxygen in gaseous atmospheres which exploits some profitable properties of room temperature ionic liquids (RTILs), such as their high electrical conductivity, negligible vapour pressure and good thermal stability. The advantages offered by this type of medium were increased by adding small amounts of a further low melting salt bearing a quinone moiety, which allowed the reduction of O2 to occur through an electrocatalytic pathway taking place at quite lower potentials than those required by its direct reduction. The rate constant (14,160 ± 370 M−1 s−1) of this electrocatalytic process was determined by resorting to linear sweep voltammetric measurements. The performance of this device was assayed under both flowing and static stop-flow conditions on synthetic O2 + N2 atmospheres with a controlled oxygen content, which was changed in a wide range (200–106 ppm v/v). At room temperature, repeatable (±2.7%) and linearly dependent current signals were recorded, allowing a detection limit of 140 ppm v/v (equivalent to 6.2 × 10−6 mol of O2 per L of gaseous atmosphere) to be inferred. At higher temperatures (100 °C) a lower detection limit (10 ppm v/v, equivalent to 4.5 × 10−7 mol of O2 per L of gaseous atmosphere) could be instead estimated. The possibility of profiting from this sensor for monitoring oxygen under reduced pressure was also assayed, in view of the growing importance attached to the evaluation of residual O2 in food packaging under vacuum or controlled atmospheres.