Sensors for Highly Toxic Gases: Methylamine and Hydrogen Chloride Detection at Low Concentrations in an Ionic Liquid on Pt Screen Printed Electrodes (original) (raw)
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Electrochemical Oxidation and Sensing of Methylamine Gas in Room Temperature Ionic Liquids
The electrochemical behavior of methylamine gas in several room temperature ionic liquids (RTILs), [C 2 mim][NTf 2 ], [C 4 mim][NTf 2 ], [C 6 mim][FAP], [C 4 mpyrr][NTf 2 ], [C 4 mim][BF 4 ], and [C 4 mim][PF 6 ], has been investigated on a Pt microelectrode using cyclic voltammetry. A broad oxidation wave at approximately 3 V, two reduction peaks, and another oxidation peak were observed. A complicated mechanism is predicted based on the voltammetry obtained, with ammonia gas as a likely byproduct. The currents obtained suggest that methylamine has a high solubility in RTILs, which is important for gas-sensing applications. The analytical utility of methylamine was then studied in [C 4 mpyrr][NTf 2 ] and [C 2 mim]-[NTf 2 ]. A linear calibration graph with an R 2 value of 0.99 and limits of detection of 33 and 34 ppm were obtained, respectively, suggesting that RTILs are favorable nonvolatile solvents for the electrochemical detection of highly toxic methylamine gas.
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 (NH 3 ) gas sensing in the room temperature ionic liquid 1-ethyl-3-methylimidazolium bit(trifluoromethylsulfonyl)imide ([C 2 mim][NTf 2 ]). The SPEs consist of a 4 mm 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 NH 3 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 240-1360 ppm NH 3 on both Pt and Au SPEs, giving limits of detection of 50 ppm and 185 ppm, 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.
As a result of the toxic and corrosive nature of chlorine gas, simple methods for its detection are required for monitoring and control purposes. In this paper, the electro-chemical behavior of chlorine on platinum working electrodes in the room temperature ionic liquid (RTIL) 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C 2 mim][NTf 2 ]) is reported, as a basis for simple sensor devices. Cyclic voltammetry (CV) and chronoamperometry (CA) on a Pt microelectrode revealed the two-electron reduction of Cl 2 to chloride ions. On the CV reverse sweep, an oxidation peak due to the oxidation of chloride was observed. The reduction process was diffusion controlled at the concentrations studied (≤4.5% in the gas phase), in contrast to a previous report (J. Phys. Chem. C 2008, 112, 19477), which examined only 100% chlorine. The diffusion-controlled currents were linear with gas-phase concentration. Fitting of the CA transients to the Shoup and Szabo expression gave a diffusion coefficient for chlorine in the RTIL of ca. 2.6 × 10 −10 m 2 s −1. Furthermore, determination of the equilibrium concentration of Cl 2 in the RTIL phase as a function of gas-phase concentration enabled a value of 35 to be determined for the Henry's law dimensionless volatility constant. The electrochemical behavior of chlorine on a Pt screen-printed electrode was also investigated, suggesting that these devices may be useful for chlorine detection in conjunction with suitable RTILs.
The demonstration of prolonged amperometric detection of oxygen in room-temperature ionic liquids (RTILs) was achieved by the use of mechanical polishing to activate platinum screen-printed electrodes (Pt-SPEs). The RTILs studied were 1-ethyl-3-methylimidazolium bis-(trifluoromethylsulfonyl)imide ([C 2 mim][NTf 2 ]) and N-butyl-N-methyl-pyrrolidinium bis(trifluoromethylsulfonyl)-imide ([C 4 mpyrr][NTf 2 ]). It was found that voltammetry on polished Pt-SPEs exhibited less deterioration (in terms of voltammogram shapes, stability of peak currents, and appearance of contaminant peaks) from long-term consecutive cycling under 100% vol oxygen flow in both RTILs. The detection capability of these RTIL/Pt-SPE systems, initially subjected to long-term consecutive voltammetric cycling, was also investigated by cyclic voltammetry (CV) and long-term chronoamperometry (LTCA). Current versus concentration plots were linear on both unpolished and polished electrodes for 10−100% vol O 2 (using CV) and 0.1−5% vol O 2 (using LTCA). However, sensitivities and limits of detection (LODs) from CV were found to improve significantly on polished electrodes compared to unpolished electrodes, particularly in [C 2 mim][NTf 2 ], but also moderately in [C 4 mpyrr][NTf 2 ]. The lowest LODs (of ca. 0.1% vol O 2) were found on polished SPEs using LTCA, with the most stable responses observed in [C 4 mpyrr][NTf 2 ]. Calibration graphs could not be obtained on unpolished electrodes in both RTILs using LTCA. The results show that polishing markedly improves the analytical performances of Pt-SPEs for oxygen sensing in RTILs. The reusability of such disposable Pt-SPEs, after the surfaces had been experimentally fouled, was also demonstrated through the use of polishing. Mechanical polishing of Pt-SPE devices offers a viable approach to performance improvement for amperometric gas sensing.
Sensors and Actuators B: Chemical, 2017
The growing impact of airborne pollutants and explosive gases on human health and occupational safety has escalated the demand of sensors to monitor hazardous gases. This paper presents a new miniaturized planar electrochemical gas sensor for rapid measurement of multiple gaseous hazards. The gas sensor features a porous polytetrafluoroethylene substrate that enables fast gas diffusion and room temperature ionic liquid as the electrolyte. Metal sputtering was utilized for platinum electrodes fabrication to enhance adhesion between the electrodes and the substrate. Together with carefully selected electrochemical methods, the miniaturized gas sensor is capable of measuring multiple gases including oxygen, methane, ozone and sulfur dioxide that are important to human health and safety. Compared to its manually-assembled Clark-cell predecessor, this sensor provides better sensitivity, linearity and repeatability, as validated for oxygen monitoring. With solid performance, fast response and miniaturized size, this sensor is promising for deployment in wearable devices for real-time point-of-exposure gas pollutant monitoring.
Sensors and actuators. B, Chemical, 2017
Intense study on gas sensors has been conducted to implement fast gas sensing with high sensitivity, reliability and long lifetime. This paper presents a rapid amperometric method for gas sensing based on a room temperature ionic liquid electrochemical gas sensor. To implement a miniaturized sensor with a fast response time, a three electrode system with gold interdigitated electrodes was fabricated by photolithography on a porous polytetrafluoroethylene substrate that greatly enhances gas diffusion. Furthermore, based on the reversible reaction of oxygen, a new transient double potential amperometry (DPA) was explored for electrochemical analysis to decrease the measurement time and reverse reaction by-products that could cause current drift. Parameters in transient DPA including oxidation potential, oxidation period, reduction period and sample point were investigated to study their influence on the performance of the sensor. Oxygen measurement could be accomplished in 4 s, and th...
Electrochemical Sensing of Oxygen Gas in Ionic Liquids
The work presented in this thesis aimed to investigate the potentiality of screen printed electrodes (SPEs), when used in conjunction with non-volatile room temperature ionic liquids (RTILs), for the amperometric sensing of gases. O 2 was selected as the model gas for these studies. Cyclic voltammetry (CV) and amperometry techniques were employed for these investigations. Experiments were conducted with an inert background atmosphere of N 2 gas.
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 [HCl 2 ] À in RTILs. A peak corresponding to the oxidation of [HCl 2 ] À was observed, resulting in the formation of Cl 2 and H +. These species were reversibly reduced to H 2 and Cl À , 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 ([C 2 mim][NTf 2 ]) by utilising both peaks (oxidation of [HCl 2 ] À 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.
The electrochemical reduction of oxygen (O 2) has been studied on commercially-available integrated Pt thin-film electrodes (TFEs). Chemically reversible (but electrochemically quasi-reversible) cyclic voltammetry was observed in the room temperature ionic liquid (RTIL) 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C 2 mim][NTf 2 ]), showing superior behaviour of TFEs compared to screen-printed electrodes for oxygen sensing. As a step towards the preparation of robust gas sensors, the RTIL was mechanically stabilised on the TFE surface by the addition of poly(methyl methacrylate) (PMMA). At a PMMA concentration in the RTIL of ca. 50% mass, electrolyte flow was not evident. O 2 reduction peak currents were found to decrease systematically with increasing PMMA content, reflecting the higher viscosity of the electrolyte medium. Linear calibration graphs were obtained for 0–100% vol. oxygen at all PMMA–RTIL mixtures studied. The sensitivities decreased as [PMMA] increased, but the limits of detection were relatively unchanged. Mechanical stability of the sensors was tested in different orientations (flat, upside down, sideways) with both the neat RTIL and 50% mass electrolyte. Whilst the electrochemical responses were dramatically changed for the neat RTIL, the responses in the PMMA– RTIL mixture were independent of electrode orientation. Additionally, the oxygen response in the PMMA–RTIL mixture was less affected by atmospheric impurities and moisture, compared to the neat RTIL. This suggests that these low-cost miniaturised devices can successfully be used for oxygen sensing applications in field situations, especially where portability is essential.
Applications of ionic liquids in electrochemical sensors
Analytica Chimica Acta, 2008
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