Array of water|room temperature ionic liquid micro-interfaces

Cyclic Voltammetry and AC Voltammetry were used to characterise the micro-interface array between water and a commercially available room temperature ionic liquid (RTIL) trihexyltetradecylphosphonium tris (pentafluoroethyl)trifluorophosphate ([P14,6,6,6][FAP]) for the first time. The interface array was formed within the micropores of a silicon chip membrane (30 pores and 23 m diameter). The polarisable potential window and capacitance curves were recorded, and the transfers of three cations (tetraalkylammoniums) and three anions (tetraphenylborate, hexafluorophosphate and tetrafluoroborate) across the interface were studied. The shapes of the voltammograms revealed that the RTIL filled the pores and that the interface was located at/near the pore mouths. This is the first report of voltammetry at an array of water|RTIL microinterfaces, rather than at a single interface or porous polymer supported-interface. This work is particularly relevant to the sensing/extraction of redox-inactive ions

Low-cost microarray thin-film electrodes with ionic liquid gel-polymer electrolytes for miniaturised oxygen sensing

A robust, miniaturised electrochemical gas sensor for oxygen (O2) has been constructed using a commercially available Pt microarray thin-film electrode (MATFE) with a gellified electrolyte containing the room temperature ionic liquid (RTIL) 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C2mim][NTf2]) and poly(methyl methacrylate) (PMMA) in a 50:50 mass ratio. Diffusion coefficients and solubilities for oxygen in mixtures of PMMA/RTIL at different PMMA doping concentrations (0-50% mass) were derived from potential step chronoamperometry (PSCA) on a Pt microdisk electrode. The MATFE was then used with both the neat RTIL and 50% (by mass) PMMA/RTIL gel, to study the analytical behavior over a wide concentration range (0.1 to 100 vol% O2). Cyclic voltammetry (CV) and long-term chronoamperometry (LTCA) techniques were employed and it was determined that the gentler CV technique is better at higher O2 concentrations (above 60 vol%), but LTCA is more reliable and accurate at lower concentrations (especially below 0.5% O2). In particular, there was much less potential shifting (from the unstable Pt quasi-reference electrode) evident in the 50% PMMA/RTIL gel than in the neat RTIL, making this a much more suitable electrolyte for long-term continuous oxygen monitoring. The mass production and low-cost of the electrode array, along with the minimal amounts of RTIL/PMMA required, make this a viable sensing device for oxygen detection on a bulk scale in a wide range of environmental conditions

Detection of sub-ppm Concentrations of Ammonia in an Ionic Liquid: Enhanced Current Density Using "Filled" Recessed Microarrays

The voltammetric detection of less than 1 ppm of ammonia gas in the room temperature ionic liquid (RTIL) 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C2mim][NTf2]) is demonstrated using low-cost planar electrode devices. Three commercially available planar devices were employed, all with platinum (Pt) working electrodes: a thin-film electrode (TFE), a screen-printed electrode (SPE), and a microarray thin film electrode (MATFE), along with an "ideal" conventional Pt microdisk electrode for comparison. The microholes on the recessed MATFE were also "filled" with electrodeposited platinum, to improve radial diffusion characteristics to the microhole and generate higher current densities. Current density was lowest for the TFE and SPE surfaces (linear diffusion), higher for the MATFE (mixed radial and linear diffusion), and even higher for the filled MATFE (predominantly radial diffusion). Linear sweep voltammetry (LSV) and potential-step chronoamperometry (PSCA) at 10-100 ppm of NH3 gave linear behavior for current vs concentration. Limits of detection (LODs) were in the range of ca. 1-9 ppm, lower than the minimum exposure limit (25 ppm) for NH3. The best stability, reproducibility, and the lowest LODs were observed on the recessed and filled MATFEs. These were employed to detect lower concentrations of ammonia (0.1-2 ppm), where linear behavior was also observed, and LODs of 0.11 (recessed) and 0.02 (filled) were obtained. These are believed to be the lowest LODs (to date) reported for ammonia gas in neat ionic liquids. This is highly encouraging and suggests that RTILs and low-cost miniaturized MATFEs can be combined in amperometric sensor devices to easily and cheaply detect ammonia gas at ppb concentrations. (Graph Presented)

Electrochemical behaviour of dissolved proton species in room temperature ionic liquids.

Understanding the nature of dissolved species in ionic liquids is important, particularlywhen using them as reaction media to replace volatile organic solvents. Electrochemicaltechniques such as cyclic voltammetry and potential-step chronoamperometry are verypowerful tools used to study electrochemical reactions, elucidate mechanisms andquantify diffusion parameters. These techniques have been used in various studies on thebehaviour of dissolved species in ionic liquids. Particularly, the behaviour of hydrogen or?protic? species in ionic liquids can give some insight into the hydrogen bondingcharacteristics of the ionic liquids (individual cation/anion combination) and eventuallythe pH properties of the solvents. We have looked 1,2 at the direct oxidation of hydrogengas in ten ionic liquids with various cation/anion combinations. The mechanism involvesthe two-electron oxidation of hydrogen to the electrogenerated proton, which is thoughtto then combine with the anion (A-) of the ionic liquid. The appearance and position ofthe reverse (reduction) peak on the voltammogram is thought to depend on three factors:(1) the stability of the solvated proton, HA, (2) the position of equilibrium of theprotonation reaction HA = H+ + A-, and (3) any follow-up chemistry e.g. dissociation orreaction of the solvated proton, HA. This is discussed for all ten ionic liquids studied.Solubilities of hydrogen gas are found to be in the range ca. 3-10 mM and diffusioncoefficients are calculated to be of the order 10-10 m2 s-1, with no evidence that the Stokes-Einstein law applies for the diffusion of hydrogen gas in ionic liquids

Electrochemical studies of hydrogen chloride gas in several room temperature ionic liquids: Mechanism and sensing

The electrochemical behaviour of highly toxic hydrogen chloride (HCl) gas has been investigated in six room temperature ionic liquids (RTILs) containing imidazolium/pyrrolidinium cations and range of anions on a Pt microelectrode using cyclic voltammetry (CV). HCl gas exists in a dissociated form of H+ and [HCl2]- in RTILs. A peak corresponding to the oxidation of [HCl2]- was observed, resulting in the formation of Cl2 and H+. These species were reversibly reduced to H2 and Cl-, respectively, on the cathodic CV scan. The H+ reduction peak is also present initially when scanned only in the cathodic direction. In the RTILs with a tetrafluoroborate or hexafluorophosphate anion, CVs indicated a reaction of the RTIL with the analyte/electrogenerated products, suggesting that these RTILs might not be suitable solvents for the detection of HCl gas. This was supported by NMR spectroscopy experiments, which showed that the hexafluorophosphate ionic liquid underwent structural changes after HCl gas electrochemical experiments. The analytical utility was then studied in 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C2mim][NTf2]) by utilising both peaks (oxidation of [HCl2]- and reduction of protons) and linear calibration graphs for current vs. concentration for the two processes were obtained. The reactive behaviour of some ionic liquids clearly shows that the choice of the ionic liquid is very important if employing RTILs as solvents for HCl gas detection

Comparative Study of Screen Printed Electrodes for Ammonia Gas Sensing in Ionic Liquids

Commercially available screen printed electrodes (SPEs) have been used for electrochemical ammonia (NH3) gas sensing in the room temperature ionic liquid 1-ethyl-3-methylimidazolium bit(trifluoromethylsulfonyl)imide ([C2mim][NTf2]). The SPEs consist of a 4mm diameter working electrode surface (carbon, platinum or gold) with a silver reference and C/Pt/Au counter electrode. No obvious voltammetric response was observed for NH3 oxidation on the carbon SPE, however, clear oxidation peaks were observed on Pt and Au. Linear calibration graphs were obtained for oxidation peak current vs. concentration in the range 240-1360ppm NH3 on both Pt and Au SPEs, giving limits of detection of 50ppm and 90ppm, respectively. The voltammetry on Au was complicated by additional peaks (most likely due to water impurities in the RTIL), which leads us to suggest that Pt is the preferred electrode surface material. The conditions of the experiment were chosen to be as close to real conditions as possible (no pre-vacuuming of the RTIL and no polishing/electrochemical cleaning of the SPE surface before experiments) suggesting that Pt SPEs in conjunction with non-volatile RTILs may provide cheaper alternative sensing materials compared to those currently used in commercial amperometric gas sensing devices

Electroreduction of 2,4,6-Trinitrotoluene in Room Temperature Ionic Liquids: Evidence of an EC2 Mechanism

The reduction of 2,4,6-trinitrotoluene (TNT) has been studied in eight room temperature ionic liquids (RTILs) on a gold (Au) microdisk electrode and a Au thin film electrode (TFE). Three reduction peaks were observed in all RTILs, corresponding to the reductions of each of the three nitro groups in the TNT structure. TNT was the easiest to reduce in imidazolium RTILs, followed by pyrrolidinium and then tetraalkylphosphonium. Diffusion coefficients (D) and electron counts (n) were calculated from potential-step chronoamperometry on the first reduction peak. D's ranged from 0.7 × 10-11 to 4.1 × 10-11 m2 s-1, and a plot of D against the inverse of viscosity was linear, indicating that the Stokes-Einstein relation holds well for TNT in RTILs. The electron count was one in most RTILs-in stark contrast to the widely accepted six-electron reduction in protic solvents. An electrogenerated red solid was formed after the first reduction peak, believed to be an azo (or azoxy) compound formed by dimerization of two TNT radicals, although characterization of the product(s) proved difficult. The behavior at different concentrations revealed different degrees of chemical reversibility of reduction peak. This evidence points toward the possibility of an EC2 mechanism, which was supported by digital simulation of the experimental voltammograms. Understanding the reduction mechanism of TNT is essential if RTILs are to be used for TNT sensing applications, particularly at high concentrations

Effect of Ionic Liquid Structure on the Oxygen Reduction Reaction under Humidified Conditions

The oxygen reduction reaction (ORR) is widely studied in room-temperature ionic liquids (RTILs) but typically in dry environments. Because water is known to affect diffusion coefficients and reaction outcomes, the influence of water on the ORR is expected to be significant. We have therefore studied the effect of RTIL structure on the ORR at different relative humidity (RH) levels using cyclic voltammetry. A broad range of cations including imidazolium, ammonium, pyrrolidinium, pyridinium, sulfonium, and phosphonium, and anions such as [BF4]-, [PF6]-, [NTf2]-, and [FAP]- were employed. The cation was found to have a large effect on the reduction current of oxygen, even at low humidity levels (65 RH%). Consequently, the choice of cation needs to be carefully considered when selecting a suitable RTIL solvent for oxygen reduction in humidified environments. The size, structure, and hydrophobicity of the ions were found to dictate the degree at which the RTIL is susceptible to changes in humidity. The physical characteristics of the RTIL electric double layer on platinum electrode surfaces were further investigated by atomic force microscopy force-curve studies in three selected RTILs. The results suggest that there is a significant amount of water incorporated at the electrode-RTIL interface in [C2mim][NTf2] and [N4,1,1,1][NTf2] but not in the more hydrophobic [P14,6,6,6][NTf2]. The presence of moisture has a significant impact on ORR currents in [C2mim][NTf2], even at extremely low humidity levels, which was verified by the higher level of water incorporation in [C2mim][NTf2] compared with [N4,1,1,1][NTf2] and [P14,6,6,6][NTf2]. Hydrophobic and large RTIL cations and anions (e.g., [P14,6,6,6]+ and [FAP]-) are recommended for applications where a stable ORR current response is required under humidified conditions

Formation of 3-dimensional gold, copper and palladium microelectrode arrays for enhanced electrochemical sensing applications

Microelectrodes offer higher current density and lower ohmic drop due to increased radial diffusion. They are beneficial for electroanalytical applications, particularly for the detection of analytes at trace concentrations. Microelectrodes can be fabricated as arrays to improve the current response, but are presently only commercially available with gold or platinum electrode surfaces, thus limiting the sensing of analytes that are more electroactive on other surfaces. In this work, gold (Au), copper (Cu), and palladium (Pd) are electrodeposited at two different potentials into the recessed holes of commercial microelectrode arrays to produce 3-dimensional (3D) spiky, dendritic or coral-like structures. The rough fractal structures that are produced afford enhanced electroactive surface area and increased radial diffusion due to the 3D nature, which drastically improves the sensitivity. 2,4,6-trinitrotoluene (TNT), carbon dioxide gas (CO2), and hydrogen gas (H2) were chosen as model analytes in room temperature ionic liquid solvents, to demonstrate improvements in the sensitivity of the modified microelectrode arrays, and, in some cases (e.g., for CO2 and H2), enhancements in the electrocatalytic ability. With the deposition of different materials, we have demonstrated enhanced sensitivity and electrocatalytic behaviour towards the chosen analytes

Sensors for Highly Toxic Gases: Methylamine and Hydrogen Chloride Detection at Low Concentrations in an Ionic Liquid on Pt Screen Printed Electrodes

Commercially available Pt screen printed electrodes (SPEs) have been employed as possible electrode materials for methylamine (MA) and hydrogen chloride (HCl) gas detection. The room temperature ionic liquid (RTIL) 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C2mim][NTf2]) was used as a solvent and the electrochemical behaviour of both gases was first examined using cyclic voltammetry. The reaction mechanism appears to be the same on Pt SPEs as on Pt microelectrodes. Furthermore, the analytical utility was studied to understand the behaviour of these highly toxic gases at low concentrations on SPEs, with calibration graphs obtained from 10 to 80 ppm. Three different electrochemical techniques were employed: linear sweep voltammetry (LSV), differential pulse voltammetry (DPV) and square wave voltammetry (SWV), with no significant differences in the limits of detection (LODs) between the techniques (LODs were between 1.4 to 3.6 ppm for all three techniques for both gases). The LODs achieved on Pt SPEs were lower than the current Occupational Safety and Health Administration Permissible Exposure Limit (OSHA PEL) limits of the two gases (5 ppm for HCl and 10 ppm for MA), suggesting that Pt SPEs can successfully be combined with RTILs to be used as cheap alternatives for amperometric gas sensing in applications where these toxic gases may be released