Voltammetric behaviour of ferrocene as probe molecule in olive oils using a phosphonium-based ionic liquid as electrolyte

Voltammetric measurements in non-polar solvents have been traditionally impeded by the lack of readily available, soluble and well-dissociated supporting electrolytes needed to enhance their low conductivity [1]. In the past, the use of mixed non-polar and polar solvents, containing conventional supporting electrolytes, has allowed undertaking some electrochemical measurements of compounds soluble only in non-polar media [2]. More recently, the extension of voltammetric methods into low-dielectric media has been made possible by the use of room temperature ionic liquids (RTILs) [3]. These can be employed in bulk as media to dissolve non polar compounds, or added in small amounts to the non polar solvents, as electrolytes, to enhance their conductivity. In this context, advantageous applications of RTILs have been recently reported for enhancing the conductivity also of natural liquid matrices such as edible vegetable oils, which are normally inaccessible to direct voltammetric measurements [4,5]. In particular, it has been shown that the addition of tetraalkylphosphonium-based RTILs to maize and olive oil samples has allowed to obtain meaningful cyclic voltammetric responses with a platinum microdisk electrode, which has been exploited as an “electronic tongue” for differentiating oil samples according to their quality and geographical origin [4], or for verifying the authenticity of Italian PDO extra-virgin olive oils [5]. These goals have been achieved with a “blind analysis” strategy based on the exploitation of the entire current-potential profiles in conjunction with chemometric pattern recognition and class-modelling techniques. The possibility of employing oil/RTIL mixtures to detect by voltammetry specific analytes in edible oils is also of interest. In this case, knowledge of the mass transport properties of an analyte in the media is an important step. With this in mind, in the present study we report on the voltammetric behaviour of ferrocene, taken as a probe molecule, in olive oil/RTIL mixtures. Because of its hydrophobic character, ferrocene can mime in fact possible analytes (i.e. natural components or contaminants) which could be present in edible oils. To this aim, the RTIL trihexyl(tetradecyl)phosphonium bis(trifluoromethylsulfonyl)imide ([P14,6,6,6]+[NTf2]-), which has proven to solubilise over a wide concentration range in the oil samples, has been here employed. Because of the high viscosity of the media investigated, and to avoid problems related to ohmic drop, measurements were taken with a Pt microdisk electrode.