The inherent physical, optical and conductivity properties of ionic liquid - polymeric membranes; a self indicating, simultaneous response upon coordination to transition metal ions

Abstract

Ionic Liquids (IL’s) are organic salts that are liquid at room temperature. Typically, they contain a bulky, asymmetric organic cation and a small inorganic/organic anion held together via weak electrostatic interactions, which prevents them from forming a structured lattice. They exhibit many favourable physical and chemical characteristics, which has led to their use in a variety of analytical techniques 1. All IL’s display a measured ionic conductivity, plus liquid properties such as density and viscosity can be controlled by correct choice and/or chemical functionalisation of the ion pair 2. Ion Selective Electrodes (ISE’s) utilise polymeric membranes for the detection of a target ion in trace amounts. They typically require a polymer and plasticizer, plus also an ion-exchanging salt and an ionophore which will selectively bind to a target ion 3. The response of a target ion binding to an ionophore can be monitored electrochemically, or optically if the ion-ionophore complex produces a colour. There are several reports in which IL’s plasticize both polyvinylchloride (PVC) and polymethylmethacrylate (PMMA) based membranes for use in ISE’s 4. Furthermore IL’s also act as ion-exchangers, thereby eliminating the need for facilitated transport of target ions from aqueous to organic phase. We will describe the work done to date on the IL trihexyltetradecylphosphonium dicyanamide [P6,6,6,14][DCA] and its use in polymeric membranes typical of ISE’s. Once solidified into a PVC membrane, [P6,6,6,14][DCA] can not only act as the plasticizer and ion-exchanger, but also as the ionophore, producing a colorimetric response upon coordination to Cu2+ (yellow), Co2+ (blue), but also both ions simultaneously (green). The multifunctionality of [P6,6,6,14][DCA] leads to a dramatic simplification of membrane components, producing a system capable of a self-indicating, simultaneous, colorimetric response. As well as a selective optical response, we have also explored the possibility to see if the inherent conducting properties of these membranes can be exploited. Radio frequency (RF) detection provides a technique which can monitor the conductivity of a sample wirelessly, but also has the required sensitivity and is non-invasive on the sample. RF can not only discriminate between the coordinated and non-coordinated membranes, but also between the individual co-ordinated membranes. The resultant downward trend in conductivity from Blank > Cobalt > Mixture > Copper has been validated by Electrochemical Impedance Spectroscopy (EIS) and by portable X-Ray Fluorescence (XRF). XRF shows that the results obtained from RF and EIS are directly related to the binding selectivity of the ligand [DCA]-. We have observed the highest binding levels for membranes exposed to Cu2+ ions, thereby producing the lowest RF conductivity signal and the highest impedance values. The opposite applies for Co2+, we have observed lower binding values, conversely producing the highest co-ordinated RF signal and the lowest coordinated impedance values. IL’s have been shown to bind to a variety of heavy metal ions 5, lanthanide ions6 plus important target analytes such as CO2 7 . If a change in conductivity can be presumed upon binding to the analyte, then the inherent conductivity of IL’s could be exploited in future electrochemical sensing.