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Redox Recycling Amplification using an Interdigitated Microelec-trode Array for Ionic Liquid-based Oxygen Sensors Richard Gondosiswanto, David Brynn Hibbert, Yu Fang, and Chuan Zhao Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b04945 • Publication Date (Web): 26 Feb 2018 Downloaded from http://pubs.acs.org on March 1, 2018

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Analytical Chemistry

Redox Recycling Amplification using an Interdigitated Microelectrode Array for Ionic Liquid-based Oxygen Sensors Richard Gondosiswanto,1 D. Brynn Hibbert,1 Yu Fang2,* and Chuan Zhao1,2,* 1

School of Chemistry, UNSW Sydney, NSW 2052, Australia 2 Key Laboratory of Applied Surface and Colloid Chemistry, Ministry of Education, School of Chemistry & Chemical Engineering, Shaanxi Normal University, Xi’an 710119, PR China ABSTRACT: A new design for a membrane-free gas sensor modified with a thin layer of ionic liquid is described. The new approach uses miniaturized interdigitated microelectrodes for detecting gases having reversible electrochemistry, for example dioxygen. Analyte molecules are reduced on the first working electrode creating an intermediate species (e.g. superoxide, O2•−, from dioxygen) that can be re-oxidized back to the original molecule at the second working electrode. The loop of redox reactions enhances the measured current leading to high sensitivity (3.29 ± 0.06 nA cm-2 ppm-1) and low detection limit (LOD = 174 ppm). The gas sensor design was demonstrated to monitor typical concentrations of oxygen with good accuracy and precision. The enhancement in the current is characteristic only of gas molecules with reversible electrochemistry, which indicates that the proposed gas sensor can analyze these molecules with greater sensitivity over those with irreversible electrochemistry.

In the past decades, there have been great developments in amperometric-based gas sensors to measure concentrations of gases such as oxygen, carbon dioxide, hydrogen sulfide, nitrogen-containing compounds and others.1-6 Sensor design varies in the dimensions of the sensor, choice of solvents, electrode composition, and membrane.1 A traditional design follows the Clark cell which consists of an electrode at which the gas is electrochemically reduced or oxidized, a solvent to capture and transport gas molecules from the atmosphere to the electrode, and a gas-permeable membrane (e.g. poly(tetrafluoroethylene)) to protect the electrodes and minimize solvent evaporation.1 The use of a membrane however creates several problems, one of the most important of which is the poor permeability to gas molecules which results in lesser sensitivity and longer response times. A membrane-less system is therefore desirable, if evaporation of solvent and stability issues can be overcome. Ionic liquids (ILs) have been regarded as ’green-solvents‘ and have been studied and used in many different applications, due to their advance properties such as low volatility, high conductivity, and electrochemical stability with a wide potential window (important in detecting species with high reduction/oxidation potentials).4 Furthermore, ILs are ‘tuneable’ by choosing different cations and anions to offer different physical and chemical properties (e.g. hydrophobicity, viscosity, and gas solubility).7 These superior properties make ILs suitable candidates for substituting conventional solvents in the gas sensor design. The negligible volatility of ILs circumvents the problem of solvent evaporation and thus, a membrane-free gas sensor design can be achieved. However, the high viscosity of ILs is still a hurdle for many applications, including gas sensors, where viscosity of the electrolyte is as much of a barrier to transport as the traditional membrane of an aqueous system. This results in slow response times and a decrease in the sensitivity (generated current) of the gas sensor.

Figure 1. 2D illustration of (a) single disk electrode, (b) microelectrodes array, and (c) interdigitated microelectrodes array. (d) 3D illustration of interdigitated microelectrodes array, in a thin layer of ionic liquid with the working principle of oxygen reduction in the first working electrode (yellow, WE1) to superoxide which diffuses to the second working electrode (orange, WE2) and is re-oxidized back to oxygen. Not to scale.

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In recent years several approaches to solve the problem of slow transport in ionic liquids have been developed.8-10 Sukeri et al. fabricated a nanoporous gold film on the electrode using an electrochemical method which in turn increased the active surface area of the electrode and current generated when compared to a bare macrodisk electrode.9 Zhou et al. fabricated a composite electrode made of graphene oxide, gold nanoparticles and IL to increase the active surface area for detecting traces of mercury in water.10 Gunawan et al. proposed another approach by miniaturizing the gas sensor device with only a microdroplet of IL on the electrode, thus providing a reduced diffusion pathway which improves both sensitivity and response time.11 Furthermore, reducing the size of the working electrode to micron dimensions can also increase the signal-tonoise ratio giving enhanced overall sensor performance.6,12 Figure 1a shows the typical IL-based gas sensor design using a macrodisk electrode. In this design, the analyte diffuses normally to the electrode. However, in a microelectrode design (Figure 1b), the diffusion of the analyte is hemispherical with increased flux.12-14 A single microelectrode, however, can only support a small current and thus microelectrode arrays have been proposed as a way to produce easily-measurable current densities, while maintaining the advantages discussed above.15,16 Herein, we introduce a new design for using arrays of interdigitated microelectrode lines integrated into a single chip for membrane-less gas sensing applications (Figure 1c). Interdigitated micro- to nano-electrodes arrays with two working electrodes have been reported previously, for example in a biochemical sensor17 or redox cycling amplification for an immunoassay study.18 Their applications in gas sensors are very rare. In this sensor design, the two microelectrodes arrays work in tandem by reducing gas analyte molecules (with reversible electrochemistry) on one, and then re-oxidizing the anionic products on the other. This creates an enhanced current and greater sensitivity for analytes with sufficiently reversible electrochemistry (Figure 1d).

High purity nitrogen, oxygen, and carbon dioxide were purchased from Air Liquide. Digital flowmeters (1 – 1500 mL min-1 for nitrogen carrier gas and 1 – 1000 mL min-1 for test gas oxygen or carbon dioxide) delivered reagent gases to the test chamber via a mixing chamber. The IL was dried under vacuum at 344 K overnight to ensure the removal of residual atmospheric gases and water vapor in the IL. After dropcasting the dried IL onto the electrodes, the cell was further purged with nitrogen for 1 h while the background current was monitored. The potential was held at E = –1.2 V vs Au for measurement of oxygen and E = –2.2 V vs Au for measurement of carbon dioxide. For the interference study using carbon dioxide mixed with oxygen, a 1:1 volume ratio was used at a total flow rate of 200 mL min-1.

EXPERIMENTAL SECTION Materials. The ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([BMP][Ntf2]) was purchased from Io-Li-Tech and used without further purification. 1-nitro-1-cyclohexene (99%) and ferrocene (98%) was purchased from Sigma-Aldrich and used as received. Two electrodes were obtained from Micrux Technologies; thin-film single disk electrodes (MDE) with gold as the working electrode (diameter, 1 mm) and platinum as both the counter and pseudo-reference electrode, and thin-film interdigitated microelectrodes array (IDA) with gold working (15 pairs of 10 µm electrodes), counter and reference electrodes. The two Micrux electrodes were found to be re-useable and had been used in all of the experiments. Electrochemical instrumentation. All voltammetric experiments were conducted with CHI 760d bi-potentiostat, (CHI Instrument Inc.) at room temperature. The Micrux electrode was connected to the bi-potentiostat using a drop-cell interface (ref. ED-DROP-CELL) obtained from Micrux Technologies. 1 µL of hydrophobic IL, [BMP][Ntf2] drop-casted on the electrodes was used for all of the experiments. All potentials were measured against a gold pseudo-reference electrode. Gas sensor instrumentation. The design of the gas sensor is adapted from our previous study11 and illustrated in Figure S1.

Figure 2. (a) Photograph of thin-film gold MDE as the working electrodes and platinum as the counter and reference electrode (left) and IDA with gold as the working, counter and reference electrode (right). (b) Reflection micrograph (magnification 10x) of IDA showing the 15 pairs of 10 µm gold electrodes array (scale bar is 100 µm). (c) Au mapping of IDA obtained from EDS (scale bar is 40 µm).

Physical characterization. Reflectance micrographs were recorded using a Nikon eclipse TS100-F inverted microscope (Nikon, Japan). Phase contrast micrographs were recorded using a Nikon eclipse LV150L microscope (Nikon). Scanning Electron Microscope (SEM) micrographs were taken using a FEI Nova NanoSEM 450 FE-SEM combined with Bruker


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Analytical Chemistry SDD-EDS detector (10 kV accelerating voltage) for the visualization of compositional differences across the electrode surface.

trodes). Figure 2b shows the reflection micrograph of the IDA with 15 pairs of 10 µm gold working electrodes and gap of 10 µm within each array. Figure 2c shows the energy dispersive X-ray spectroscopy (EDS) image of Au mapping of the IDA. The working principle of the IDA was first demonstrated using ferrocene (Fc) dissolved in a IL, [BMP][Ntf2] which is known to undergo a reversible one-electron oxidation to ferrocenium Fc+ (Fc – e ⇌ Fc+). Figure S2 shows the cyclic voltammogram of 5.0 mM Fc using IDA at different scan rates. The peak at +0.13 V corresponds to oxidation of Fc to Fc+, and the associated cathodic peak at –0.77 V corresponds to the reduction of Fc+ back to Fc. The peak-to-peak separation was observed to increase slightly with increasing scan rate, which is expected result of the electrochemistry in viscous ILs.19 As expected, the oxidation of Fc using IDA is diffusion controlled, as observed from the linear increase of the oxidation peak with the square root of the scan rates. Five concentrations of Fc in the IL, ranging from 0.313 mM to 5.00 mM, were analysed. Figure 3a shows the cyclic voltammetry of different concentrations of Fc at scan rates of 0.10 V s-1. The diffusion coefficient of Fc was calculated using the Randles-Sevcik equation, p 0.4463

.

(1)

Figure 4. Cyclic voltammograms at WE1 in [BMP][Ntf2] saturated with oxygen at different scan rates (0.50, 0.10, 0.15, 0.20, and 0.25 V s-1), T = 298 K.

Figure 3. (a) Cyclic voltammograms of 0.313, 0.625, 1.25, 2.50, and 5.0 mM Fc/Fc+ in [BMP][Ntf2] at a scan rate of 0.10 V s-1; inset shows the calibration curve of the current maxima at different concentrations. (b) Chronoamperograms of steady-state (t > 75 s, t is elapsed time from applying the shown potential at the electrode(s)) electrochemistry of 2.50 mM Fc (conditions are given on each curve). (c) Calibration plot of Fc oxidation with EWE2 on (red square) and disconnected (blue triangle); Black diamond indicates calibration plot of Fc+ reduction at the second electrode. All measurements were made using IDA.

RESULTS AND DISCUSSION The Micrux electrodes have dimensions of 10 × 6 × 0.75 mm on a glass substrate (Figure 2a). The MDE and IDA are electrodes fabricated by thin-film technologies (or thin-film deposition) protected by a layer of SU-8 resin with 2 mm electrochemical working cell (working, counter, and reference elec-

where ip is the peak current (A), n is the number of electrons in the charge transfer, F is the Faraday constant, A is the electrode surface area, C is the bulk concentration of Fc, R is the gas constant, T is the thermodynamic temperature and v is the scan rate (V s-1). The diffusion coefficient (D) of Fc in [BMP][Ntf2] was calculated to be (2.53 ± 0.12) × 10-7 cm2 s-1 which is consistent with the reported value of (2.69 ± 0.09) × 10-7 cm2 s-1.20 It is worth pointing out that since a pseudoreference electrode Pt or Au was used in the experiments, there was a small shift (up to 100 mV) in potential in the course of potential measurements, as also observed in previous studies.19 Therefore a potential that is suitably greater than the equilibrium potential is applied when detecting gases. For accurate measurements of potentials in ILs, an IUPACrecommended internal reference redox couple, such as Fc/Fc+ can be used to calibrate the potential.21,22 Figure 3b shows the chronoamperogram of Fc oxidation ([Fc] = 2.50 mM) using the IDA. When the potential of WE1 was



kept at EWE1 = -0.03 V vs Au, no reaction was observable, suggesting that no Fc+ is present in the IL droplet (brown line). When the potential at WE1 was stepped to EWE1 = +0.3 V vs Au (to oxidize Fc to Fc+), while the second working electrode (WE2) was disconnected (blue line), the current approaches steady-state of 0.25 µA after 150 s. When WE2 is switched on (EWE2 = -0.03 V vs Au), the Fc+ formed at, and diffused from, WE1 is reduced to Fc, which can diffuse back to WE1 leading to increased current. This can be observed from the increment in the steady state current in Figure 3b (WE1 red line) to 0.45 µA, which is nearly double that when WE2 was disconnected. The red line in figure 3b shows the current at WE2 generated from the reduction of Fc+ to Fc. For oxygen sensing applications, dioxygen is first reduced at WE1 to superoxide radicals, which diffuse to WE2 and are oxidized back to dioxygen. The regenerated oxygen diffuses back to WE1 leading to additional flux and increase current signal, thus improving the sensitivity of the oxygen sensor. Figure 3c shows the plot of the steadystate oxidation current (t = 150 s) against Fc concentrations with WE2 disconnected and at a potential of -0.03 V vs Au. In each case, a linear relationship can be obtained between the oxidation current and Fc concentration. However, upon application of a potential at WE2 for reduction of Fc+, the sensitivity (the slope of the calibration curves) (solid red) is significantly improved. Furthermore, current of the reduction of Fc+ at WE2 can be used to plot a second calibration curve (dash red), which can be used to improve the accuracy of the measurement of the concentration of the target analyte. Having verified the principle for improving sensitivity in an IL system by recycling a reversible couple (Fc/Fc+), we demonstrate its application for gas sensors. Dioxygen gas was chosen as a model system for gas sensing, since the electrochemistry of oxygen in ILs is well understood.23-25 Carter et al. pioneered the study of oxygen reduction in IL, 1-ethyl-3methylimidazolium chloride [EMIM]Cl mixed with aluminium chloride (AlCl3).26 Since then, numerous studies have shown that in aprotic hydrophobic ILs, dioxygen undergoes a reversible, one-electron reduction to superoxide radical anion, O2•– (Eq. 2).27 ⇌ • (2) In the presence of a source of protons, the reversible oneelectron reduction is replaced by an irreversible two-electron reaction as the intermediate product superoxide undergoes a series of protonation and disproportionation steps resulting in the final product hydrogen peroxide.27 The first re-oxidation of superoxide back to oxygen was observed in aprotic IL, 1butyl-3-methylimidazolium hexafluorophosphate, [Bmim][PF6] at a glassy carbon electrode.28 Similar voltammetry behavior was also observed using ILs with imidazolium-, tetraalkylammonium-, and tetraalkylphosphonium cations.23,2936 However, it was found that the intermediate superoxide is not stable in several IL cations such as imidazolium and phosphonium which create complexes, and is more stable in alicyclic and aliphatic ammonium cations such as quaternary pyrrolidinium and ammonium.25,37 To demonstrate our concept, an aprotic IL [BMP][Ntf2] was applied in this study due to its high hydrophobicity and known stability against reaction with superoxide anions.38

Figure 5. (a) Chronoamperograms of oxygen reduction in 1 µL [BMP][Ntf2] (E = −1.2 V vs Au) at oxygen concentration 2000 ppm using the gold MDE (blue) and MEA (red). (b) Calibration data for oxygen reduction fitted with a linear function at concentration of 1400 to 4800 ppm.

Figure 4 shows cyclic voltammograms obtained at different scan rates on WE1 in oxygenated [BMP][Ntf2], coated as a thin layer on the IDA surface. The reduction and oxidation peaks observed at –1.2 V and –0.8 V correspond respectively to the reduction of oxygen to superoxide and its re-oxidation to oxygen (Eq. 1). Such redox behavior is consistent with the reported oxygen redox processes in aprotic ILs.24,39 The peak current ratio of Iox/Ired is around 0.62 for all scan rates, suggesting that the oxygen redox processes are only quasi-reversible. This is as expected since there is a difference between diffusion coefficients of neutral and charged species. The superoxide radicals is known to diffuse slower through the viscous IL.23,25,32 Further electrochemical studies of oxygen reduction in this IL have been well reported previously.25 We first compared the ability to detect oxygen gas at a thin layer of IL on MDE and gold microelectrodes array (MEA, only WE1 in the IDA was used with WE2 disconnected). Figure 5a displays typical j-t curves of oxygen reduction in [BMP][Ntf2] (EWE1= –1.2 V vs Au) using MEA, compared to MDE (diameter, 1 mm), at an oxygen concentration of 2000 ppm (or µL L-1, as recommended by IUPAC). It can be observed that the MEA can produce significantly higher current densities compared to the MDE. The response times taken at 90% of the steady-state current density (τ90) using both electrodes was measured to be similar. The current densities with oxygen concentrations between 1400 to 4800 ppm were then measured. Linear calibration curves at MDE and MEA are shown in Figure 5b. The sensitivities (slopes of the calibration


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Analytical Chemistry lines) of MDE and MEA were b = (0.236 ± 0.008) nA cm-2 ppm-1 and b = (0.936 ± 0.04) nA cm-2 ppm-1, respectively. The limit of detection (LOD, 3sy/x/b) and limit of quantification (LOQ, 10sy/x/b) were calculated from the linear fit where sy/x is the standard error of the regression and b the sensitivity.40 The LOD and LOQ for MEA are 390 ppm and 1300 ppm, respectively. This increase in sensitivity at MEA is consistent with previous studies,12,41 and can be attributed to the enhanced mass transport illustrated in Figure 1a and b, where the diffusion profile of the electroactive species at a MDE is essentially linear but at MEA, it becomes hemispherical with increased flux.12-14 The working principle of the IDA for oxygen sensing is shown in Figure 1d. First, 1.0 µL of IL was drop-cast on the electrode to provide the thinnest practical layer of electrolyte. Before each measurement, the sensor was purged with nitrogen inside the gas chamber for one hour to remove residual oxygen and water. The current was monitored at EWE1 = –1.2 V vs Au, as different concentrations of oxygen were introduced into the gas chamber and reduced to its anionic product superoxide (O2•−). The generated superoxide then diffuses to WE2 which has a potential at which superoxide is oxidized back to dioxygen. The regenerated oxygen is then recycled between WE1 and WE2, thus enhancing the measured current. Figure 6a displays j-t curves of oxygen reduction in [BMP][Ntf2] using an IDA at oxygen concentrations of 1400 to 4800 ppm. The potential of the first working electrode (EWE1) was held at –1.2 V vs Au for the reduction of oxygen to the superoxide ion (as shown in Figure 4). The blue curve represents oxygen reduction current density at WE1 when WE2 was disconnected. The red curve represents current density at WE1 when WE2 was held at a potential (here –0.1 V vs Au) that can re-oxidize superoxide back to dioxygen. It can be observed that the current density increases significantly for all oxygen concentrations tested when the second electrode is turned on. The increase arises from the formation of a redox loop between WE1 and WE2 where oxygen is reduced to superoxide at WE1 and then is re-oxidized to oxygen at WE2. Figure 6b shows calibration data obtained from the current density measurements in Figure 6a fitted with a linear regression. In the linear range from 700 to 4800 ppm [O2], the sensitivity increased from 0.936 ± 0.04 nA cm-2 ppm-1 for IDA (WE2 disconnected) to 3.54 ± 0.07 nA cm-2 ppm-1 for IDA (EWE2 = –0.1 V). The LOD and LOQ of the IDA (EWE2 = –0.1 V) improved to 210 ppm and 700 ppm. The performance of the IDA gas sensor in term of sensitivity is far superior to previous reported IL-based oxygen sensor as shown in Table S1. Figure S3 demonstrates the repeatability of detecting oxygen at concentration of 4900 ppm with a relative sample standard deviation of 1.3% (n = 7). The gas sensing performance of the IDA electrode was also tested for greater oxygen volume fractions ranging from 3 to 20%. As shown in Figure 6c, when the second electrode is turned on, the calibration curve of the IDA is also steeper with a greater sensitivity of (20.8 ± 0.54) µA cm-2 ppm-1 (EWE2 = –0.1 V vs Au), compared to (13.4 ± 0.78) µA cm-2 ppm-1 (WE2 disconnected). Further enhancement of sensitivity can in principle be achieved by an IDA with greater number of microelectrodes and a smaller gap between each microelectrode. The electrochemical behavior of redox species (e.g. ferrocene and ferrocyanide) at IDA with different geometries (electrode width and gap) have been studied previously.42-45 It was found that

decreasing the electrode width and gap can increase the magnitude of the limiting currents. Moreover, the collection efficiency calculated from the ratio of the current from WE1 and WE2 was observed to approach unity (i.e. increasing the redox recycling amplification). Further, it was found that redox species with different diffusion coefficient have little effect on the collection efficiency. Therefore, decreasing the electrode spacing between the IDA can in principle give rise to higher redox recycling and collection efficiency, thus lead to higher sensitivity, wider dynamic range and higher signal-to-noise ratio to gas sensors. Further work is on the way to provide a quantitative model for gas sensor design. It is noted that when Fc is used as a redox probe, the enhancement in current using IDA is greater compared to that achieved in the oxygen system. This is due to the poorer reversibility of oxygen reduction compared to Fc0/+ in ILs.25 Therefore, for sensing applications, the IDA design is most suitable for detection of gas molecules with highly reversible electrochemistry. To demonstrate the effect we applied the IDA sensor for detection of carbon dioxide (CO2), which is irreversibly reduced.



Figure 6. (a) Chronoamperograms of oxygen reduction in [BMP][Ntf2] at different oxygen concentrations using IDA with EWE1 = -1.2 V vs Au and WE2 = disconnected (blue) and EWE2 = 0.1 V vs Au (red). (b) Calibration of current density vs concentration for the oxygen reduction from 1400 to 4800 ppm of [O2] fitted with linear trend line. (c) Calibration of current density vs concentration for the oxygen reduction from 3 to 20 % of [O2] fitted with linear trend line.

Figure 7a displays the cyclic voltammogram of CO2 reduction in [BMP][Ntf2] using the IDA at a scan rate of 0.10 V s-1. In aprotic medium, CO2 undergoes irreversible reduction at a peak potential of −2.2 V vs Au, which is attributed to CO2 reduction to oxalate and/or carbon monoxide and 46 bonateCO # . 2CO 2e → C O (3) & 2CO 2e → CO CO# (4) It is clearly demonstrated in Figure 7b that the current for different CO2 concentrations (b = (4.36 ± 0.08) µA cm-2 ppm-1) does not increase at all when the second electrode (EWE2) was held at +0.3 V vs Au, because only analytes with reversible electrochemistry can be recycled at WE2. Thus the IDA gas sensor design in principle offers improved performance against other gaseous molecules with irreversible electrochemistry.

Figure 7. (a) Cyclic voltammogram of carbon dioxide (CO2) reduction in [BMP][Ntf2] using IDA, v = 0.10 V s-1. (b) j-t curves of CO2 reduction in [BMP][Ntf2] (EWE1 = −2.2 V vs Au) at concentrations from 3000 to 30000 ppm with the second electrode on (red, EWE2 = 0.3 V vs Au) and disconnected (blue).

Interference studies using the sensor design were demonstrated for two cases. First, we studied the effect of an interferent

that can be electrochemically reduced at a potential where oxygen reduction occurs. Here, 1-nitro-1-cyclohexene was used as a model interferent since the reduction potential of this molecule occurs at the same potential as oxygen. Figure S4a shows the CV of 10 mM 1-nitro-1-cyclohexene dissolved in [BMP][Ntf2] under nitrogen atmosphere. One reduction peak at −1.3 V vs Au was observed without any immediate reoxidation peak on the anodic wave confirming that the reaction is irreversible. Such single peaks have also been seen in other nitro-compounds.47,48 Figure S4b shows the chronoamperograms of the reduction of 1-nitro-1-cyclohexene (EWE1 = 1.2 V vs Au) in [BMP][Ntf2] using the IDA. As observed, with or without the application of potential on WE2, the steady state current reached a similar value again confirming that the reduction is irreversible. This results also suggest that any irreversible interferent reduction will not alter the current at WE2 giving selectivity ability to the sensor design. Figure S4c shows the chronoamperograms of oxygen reduction in [BMP][Ntf2] at a concentration of 3000 ppm (green and black line) and with the addition of 10 mM 1-nitro-1-cyclohexene (red and blue line). Greater background currents of the chronoamperograms were observed with the addition of nitro compound into the IL droplet, which implies a change in current if this interferent were added to a working IDA oxygen sensor. However, the amplification of the redox cycling was calculated to be similar at 1.19 ± 0.02 (green and black line) and 1.21 ± 0.01 (red and blue). This result demonstrated that the redox recycling scheme can still work in the presence of this kind of electrochemical interference. The second case was to study the effect of an interferent that can chemically react with the intermediate product (superoxide) and so interrupt the redox recycling scheme. To demonstrate this, carbon dioxide was used as the model interferent since carbon dioxide can react chemically with superoxide radical to give peroxodicarbonate ion (Eq. 6 to 8).49,50 2O 2e ⇌ 2O• (5) • • CO O → CO& (6) • CO• CO → C O (7) ( & • C O• (8) ( O → C O( O Overall: O 2CO 2e → C O (9) ( Figure S5a shows cyclic voltammograms of oxygen reduction in [BMP][NTf2] with volume ratio 1:1 O2/N2 (red line) and 1:1 O2/CO2 (blue line). It can be observed that on the addition of CO2, the oxidation peak of superoxide disappeared, consistent with reactions in Eq. 6 to 8. Furthermore, for the O2/CO2 mixtures, the oxygen reduction current was also observed to increase. This behavior is due to the switch from one electron reduction to the overall two electron process (Eq. 9) for the reaction of carbon dioxide with superoxide.50,51 Figure S5b shows the chronoamperogram of oxygen reduction (volume concentration of 21%) using the IDA with EWE1 = −1.2 V vs Au and EWE2 = −0.1 V vs Au at different CO2 concentrations. At lesser concentrations of CO2 (400 to 800 ppm), the currents at WE1 and WE2 from oxygen reduction are not affected. This suggests that the reduction of oxygen to superoxide radical dominates the electrode reactions. However, when 1000 ppm CO2 was introduced into the system, a decrease in current was observed. This result suggests that at certain concentration of interferent (that can chemically react with the gas analyte), it will interfere with the redox recycling amplification. The effect of superoxide radicals chemically reacting with CO2 was


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Analytical Chemistry also studied for carbon dioxide reduction. Figure S6 shows the linear sweep voltammogram of carbon dioxide reduction in different concentrations of O2/CO2. As observed, on increasing the oxygen concentration, the reduction peak at −1.0 V vs Au (i) becomes more apparent whereas the carbon dioxide reduction peak (ii) at −2.3 V vs Au shifted to −1.9 V vs Au. Note that the reduction peak (ii), is a combination of carbon dioxide and oxygen reduction. Moreover, the reduction peak of carbon dioxide was also observed to decrease as the oxygen concentration increased, further demonstrating the chemical reaction of carbon dioxide with the superoxide radicals (Eq. 6 to 8).

CONCLUSION Ionic-liquid-based interdigitated microelectrode arrays (IDA) have been developed for detection of gases having reversible electrochemistry with improved sensitivity and selectivity over macro-disk electrodes. The enhanced diffusion of an electroactive species to a microelectrode array gives greater current densities than to a single disk electrode. The current densities are further improved by adding a second microelectrode array, which is interdigitated with the first, and held at a voltage to facilitate the reverse reaction. Thus, molecules having reversible electrochemistry can undergo a cycle of oxidation/reduction between the two arrays. The concept has been demonstrated for Fc/Fc+ and O2/O2•–. The dual currents can be used to improve the sensitivity, limit of detection and limit of quantification of the measurement of the analyte concentration. Furthermore, this sensor concept enhances sensitivity only for reversible molecules and thus could screen out any interfering molecules with irreversible electrochemistry. The application of an IDA can also be used to detect gas molecules that can react with a redox mediator. Furthermore, detection of dual or multiple gases in principle is possible on a single IDA chip.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website.

IDA sensor SI.pdf

AUTHOR INFORMATION Corresponding Author E-mail: [email protected]; [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT This work was financially supported by an Australian Research Council (ARC) Discovery Project (DP150101861), and the China’s “111” project (B14041).

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