Toward Membrane-Free Amperometric Gas Sensors: An Ionic Liquid

ARTICLE pubs.acs.org/JPCC

Toward Membrane-Free Amperometric Gas Sensors: An Ionic Liquid
Nanoparticle Composite Approach Shi-Quan Xiong,†,‡ Yan Wei,†,§ Zheng Guo,† Xing Chen,† Jin Wang,*,† Jin-Huai Liu,† and Xing-Jiu Huang*,†,|| †

)

Research Center for Biomimetic Functional Materials and Sensing Devices, Institute of Intelligent Machines, Chinese Academy of Sciences, Hefei 230031, People's Republic of China ‡ Department of Chemistry, University of Science and Technology of China, Hefei 230026, People's Republic of China § Department of Chemistry, Wannan Medical College, Wuhu 241002, People's Republic of China School of Mechanical and Electronic Engineering, Wuhan University of Technology, Wuhan 430070, People's Republic of China ABSTRACT: Room temperature ionic liquid (RTIL)
gold nanoparticle composites have been demonstrated to overcome a major obstacle for creating an extremely thin layer of RTIL on the surface of an electrode for membrane-free amperometric gas sensors. A combination of the advantages of RTILs and the superior catalytic effect of gold nanoparticles provides some new brillant abilities of RTIL
gold nanomaterial composites, which includes not only larger current and ultrafast response but also real diversity, morphology-dependent activity, and lower reduction potentials in contrast with conventional screen-printed carbon electrode (SPCE). Several ionic liquids with different viscosities and surface tensions, such as 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C2mim][NTf2]), 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C4mim][NTf2]), 1-butyl-2,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide ([C4dmim][NTf2]), and 1-hexyl-3-methylimidazolium trifluorotris(pentafluoroethyl) phosphate ([C6mim][FAP]), and gold nanorods (GNRs) and and quasi-platonic gold nanoparticles (QPGNPs) have been chosen for the measurements. The response of the sensor was quantified by both cyclic voltammetry and chronoamperometry. Such sensors indeed possess the characteristics required for a really versatile, long-lived membraneless sensor.

1. INTRODUCTION Present amperometric sensors are confronted with specific problems, such as relatively long response times and short sensor lifetime according to recent investigations on various gas sensors manufacturers and suppliers (Crowcom,1 Alphasense,2 City Technology,3 Draeger,4 Honeywell,5 etc.). Since (water) vapor pressure is approximately identical with ambient in temperate climates, strong aqueous sulfuric acid is often preferred as an electrolyte so as to prolong the lifetime of sensors. It is necessary to provide restrictions on the nature of the electrode processes, which can be used for the detection of various gases. For example ammonia is protonated under these conditions and becomes electroinactive toward oxidation. These restrictions can encourage the use of nonaqueous solvents although acetonitrile while often an attractive voltammetric medium is generally too volatile (boiling point 81.6 °C), DMSO freezes at 18.6 °C, and DMF is hard to maintain in a sufficiently pure state to allow sensitive amperometric measurements. Propylene carbonate is suitable under some circumstances (boiling point 241.7 °C), notably oxidations, but it too suffers from impurity problems.6
9 To overcome the problems arising from solvent volatility, the use of low vapor pressure solvents is recommended. Owing to their high thermal stability,10
12 low volatility,13 wide potential windows,14 good intrinsic conductivity,10,11,15
18 and tunableness,10,15 A green solvent, room temperature ionic liquid r 2011 American Chemical Society

(RTIL)19
21 allows, in principle, the opportunity for overcoming the problems identified above with respect to high temperature measurements and short sensor lifetimes. A membranefree microelectrode/array modified with a thin layer of a RTIL has been considered.22,23 The use of ionic liquid as electrolyte eliminates the need for a membrane and added supporting electrolyte. We found a thickness-dependent electrochemical behavior in these works, i.e., the thinner the layer of RTIL, the more rapid the response of the sensor.22 However, it was very difficult to reduce the thickness of the RTIL due to the surface properties of the microarray, even though numerous attempts, such as spin coating, evaporating dilute solutions of RTIL in nonaqueous solvents, and physical blotting of the RTIL, were made. While an extremely thin layer of RTIL on the surface of the array was successfully achieved, particularly via blotting, the results have so far been irreproducible. It was also found that the film was unstable over a period of hours if the RTIL layer is too thin, gradually retreating to form small discontinuous droplets on the chip surface, which is attributed to uptake of atmospheric moisture altering the RTIL surface tension.

Received: May 9, 2011 Revised: August 2, 2011 Published: August 03, 2011 17471

dx.doi.org/10.1021/jp204309b | J. Phys. Chem. C 2011, 115, 17471–17478

The Journal of Physical Chemistry C In this paper, we show an ionic liquid
gold nanomaterial composite approach for a membrane-free amperometric O2 sensor. The motivation of the present work was to use gold nanorods (GNRs) and quasi-platonic gold nanoparticles (QPGNPs) to overcome the above limitations and study the GNR- and QPGNP-catalyzed oxygen reduction. Both larger current responses and more rapid response times observed in a fixed RTIL film thickness (196 μm), as well as morphology-dependent activity and lower reduction potentials, were investigated. The described system corresponds to a significantly enhanced response to O2 compared to pure membrane-free sensors.

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Scheme 1. Schematic Showing the Construction of ILs/Gold Nanomaterial Composites Deposited as a Film onto the Surface of a Working Electrode Area in a SPCE, Where the Film Is Surrounded by the Same Blank ILs

2. EXPERIMENTAL SECTION 2.1. Chemical Reagents. 1-Ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C2mim][NTf2]), 1-butyl-3methylimidazolium bis-(trifluoromethylsulfonyl)imide ([C4mim][NTf2]), and their bromide salt precursors were prepared by standard literature procedures.24,25 1-Butyl-2,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide ([C4dmim][NTf2]) and 1-hexyl-3-methylimidazolium trifluorotris(pentafluoroethyl) phosphate ([C6mim][FAP]) were supplied by Merck KGaA (Darmstadt, Germany). The success of the above procedures in purifying the ionic liquids was sensitively assessed by means of voltammetric measurements on blank solvents. Impurity-free nitrogen and oxygen were purchased from Nanjing Special Gases Factory Co., Ltd., for electrochemical experiments as described below. All experiments were carried out at a temperature of 295 ( 3 K. HAuCl4 3 4H2O (99.9%), NaBH4 (99%), vitamin C (99.9%), cetyltimethylammonium bromide CTAB (99%), (3-aminopropyl)trimethoxysliane (APTMS), trisodium citrate, and AgNO3 (99.9%) were obtained from Aldrich. All the reagents and solvents, which were used in the synthesis of gold nanomaterials, were analytical grade and used as received without further purification. 2.2. Preparation of Gold Nanorods (GNRs). Synthesis of gold nanorods was adopted by seed-mediated methods with some modifications.26 A 1.5 mL portion of a 0.02 M NaBH4 solution was added to 100 mL of aqueous solution containing 0.25 mM HAuCl4 and 0.25 mM trisodium citrate under constant stirring. The solution turned from pale-yellow to wine red color, indicating that gold nanoparticles could be formed.27 The solution was used as gold seed solution 2
4 h after its preparation so as to degrade the excess borohydride. CTAB (1.5 g) was added to 50 mL of 0.25 mM HAuCl4 aqueous solution under gentle stirring at 40 °C in order to completely dissolve the CTAB. Subsequently, a mixture of 1 mL of acetone and 750 μL of cyclohexane was added to the aforementioned HAuCl4/CTAB solution. After cooling to room temperature, the solution could be used as a growth solution. A 0.3 mL gold seed solution and 25 μL of 0.02 M AgNO3 were added to 50 mL of growth solution. Then 500 μL of 0.05 M freshly prepared vitamin C was added to the solution under thorough stirring for 30 s. The color of the solution should be gradually turned to violet color within 20 min. The stock gold solution had to be allowed to settle for 24 h so that the gold nanorods could be formed. Purification of gold nanorod solution should be performed due to the partially interfering spherical gold nanoparticles. Au(III)/CTAB complex was prepared by dissolving 3.64 g of CTAB and 0.02 g of HAuCl4 3 3H2O in 100 mL of deionized (DI) water. Subsequently, 1 mL of this solution was added to the suspension of gold nanorods and spheres in CTAB with gentle

stirring and the mixture was allowed to settle overnight. After the supernatant had been removed, the remanents were redispersed in 10 mL of 0.1 M CTAB solution followed by addition of another 1 mL of Au (III)/CTAB solution. The process should be repeated 3
4 times so as to obtain the purified gold nanorods. 2.3. Preparation of Quasi-Platonic Gold Nanoparticles (QPGNPs). A 50 μL portion of 0.1 M sodium citrate and HAuCl4 were diluted with 10 mL of fresh redistilled water, respectively. Next, 50 μL of 1 M NaBH4 was quickly added to the mixture under stirring so as to yield gold nanoseeds. The resulting mixture was aged for 4 h in order to allow the hydrolysis of unreacted NaBH4. After the aging period, 100 mL of 0.1 M CTAB solution was mixed with 0.25 mL of 0.1 M HAuCl4, 50 μL of 1 M NaOH, and 50 μL of 1 M vitamin C. The growth solution was preheated to 60 °C under reflux condition so as to prevent the loss of water. Finally, 0.2 mL of as-prepared gold nanoseed solution was added to the growth solution and the color of the growth solution was changed to purple, indicating that quasi-platonic gold nanoparticles (QPGNPs) should be formed. Purification of QPGNPs has to be carried out in order to separate from spherical gold nanoparticles. A glass slide was pretreated with piranha solution (7:3 v/v 98% H2SO4/30% H2O2) for 1 h. (Caution! Piranha solution is a very strong oxidizing agent and particularly dangerous. Hence, it should be handled with great care.) Then the glass slide was immersed in 2-propanol solution of 2% APTMS in the presence of 0.5% acetic acid for 3 h, and the modified substrate was rinsed with DI water. Finally, the APTMS-modified glass substrate was soaked in the aforementioned growth solution for several hours, which induced the QPGNPs to be isolated from the growth solution and more QPGNPs could be removed from the glass substrate. 2.4. Ionic Liquids/Gold Nanomaterials Composite. The ILs/gold nanomaterials composites were prepared by adding 5 mL of suspensions of GNRs and QPGNPs to 500 μL of blank ionic liquids, respectively. The mixtures were purged under vacuum (Edwards high-vacuum pump, model ES 50) for at least 48 h until the solvents and impurities were thoroughly removed. A required amount of the ILs/gold nanomaterials was placed into the carbon working electrode on screen-printed carbon electrodes (SPCEs); the blank RTILs were used to cover counter and reference electrodes (Scheme 1). Note: the IL/gold nanomaterial composites must be prepared as a paste, otherwise the gold 17472

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Figure 1. TEM images of GNRs (a) and QPGNPs (b) used in this paper.

nanomaterials flow from the working electrode into the area of counter and reference electrodes). 2.5. Apparatus. Electrochemical experiments were performed with SPCEs (ref 110, DropSens, Edificio Severo Ochoa, Spain). The electrochemical cell consists of a three-electrode arrangement with a carbon (4 mm diameter) serving as the working electrode, with a carbon used as the counter electrode. A silver pseudoreference electrode completed the circuit. Cyclic voltammetry (CV) and chronoamperometry were performed using a type II μ-Autolab (Eco Chemie, Utrecht, Netherlands) potentiostat, which was interfaced with a PC using GPES (version 4.9) software for Windows. The electrodes were housed in a poly(tetrafluoroethylene) (PTFE) cell specifically designed to allow samples to be studied under a controlled environment. Before the oxygen was added to the cell, the ionic liquid was purged under vacuum (0.5 Torr) for 2 h to remove impurities. Oxygen and nitrogen were introduced to the electrochemical cell in desired ratios using a W€ osthoff triple gas-mixing pump (Bochum), accurate to (1%. After being introduced into the cell, the gas was allowed to diffuse through the ionic liquid until full equilibration between gaseous and dissolved oxygen had occurred. This occurred typically after 30 min for 30 μL of RTIL prior to starting a set of experiments (unless otherwise stated in the text) and was determined by periodically recording cyclic voltammograms until a maximum in the reduction wave had been reached. Transmission electron microscopy (TEM) was performed using a JEM-2010 microscope equipped with an Oxford INCA EDS operated at 200 kV accelerating voltage (Quantitative method: Cliff Lorimer thin ratio section). UV
vis spectra were recorded with a Shimadzu UV-2550 spectrophotometer.

3. RESULTS AND DISCUSSION 3.1. Characterization of GNRs and QPGNPs. The gold nanoparticles were first examined using TEM. Figure 1 displays typical images of gold nanorods (GNRs) and quasi-platonic gold nanoparticles (QPGNPs) used in this paper. As compared to the QPGNPs with average widths of about 45 nm (Figure 1b), the averaged dimension of GNRs can be fixed at about 50 nm in length and 15 nm in diameter (Figure 1a). Also, no agglomerates of smaller nanoparticles were observed. Importantly, as seen from TEM images, it seems that QPGNPs can provide more active sites for catalytic reaction than GNRs, as will be discussed later.

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Figure 2. UV
vis absorption spectroscopy of GNRs and QPGNPs used in this paper.

Figure 3. Overlaid cyclic voltammograms of GNRs and QPGNPs in nitrogen-saturated 0.5 M H2SO4 (scan rate 0.1 V s
1).

The feature of GNRs and QPGNPs was further confirmed using UV
vis absorption spectroscopy. Figure 2 shows the optical absorption of the samples used in this experiment. Two plasmon resonance absoption of the gold nanorods, i.e., transverse and longitudinal LSPR bands, can be observed. The absorption peak at about 510 nm should be attributed to transverse oscillation of electrons regardless of the aspect ratio. However, the other sensitive absorption can be due to the longitudinal oscillation of the electrons about 760 nm, which are related to the aspect ratio of the nanorods.28 As far as the QPGNPs are concerned, the strong and broad band that ranged between 660 and 1000 nm can be assigned as in-plane dipole plasmonic modes. To confirm that the nanomaterials in the composites were indeed formed from gold, we used a voltammetric procedure developed previously to “fingerprint” the characteristic signal of gold nanomaterials.29 The voltammetric responses were recorded for the electrodes using either blank SPCE or the GNRs and QPGNPs modified SPCE in 0.5 M H2SO4 over the potential range 0 to +1.3 V (vs Ag). In the absence of Au nanoparticles, no peak was observed in either anodic or cathodic scan. However, the GNRs and QPGNPs modified SPCE exhibited a sharp reduction peak about +0.45 V, characteristic of the reduction of surface oxides of gold (Figure 3). In contrast, oxidation peaks at about +0.8 and +1.0 V (vs Ag) are registered for the electrode constructed from Au nanomaterials along with the reduction 17473

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Figure 4. Typical cyclic voltammograms (scan rate 0.01 V s
1) at different ionic liquids for the reduction of oxygen in a 100% O2 on the GNR and QPGNP composites. Data for bare SPCE are included for comparison purposes.

peak at +0.45 V (vs Ag) analogous to others. The presence of the oxidation peak suggests a gold “macro”electrode behavior, which corresponds to the presence of the partial gold film in the SPCE surface. Thus, we can conclude that the GNRs and QPGNPs observed in Figure 1 are indeed formed from metallic gold and that they are in electrical contact with the macroelectrode surface. By measuring the area under the reductive peak outlined in Figure 3, it is possible to estimate the electroactive surface area of the GNRs and QPGNP-modified electrodes. This was conducted based on the amount of charge consumed during the reduction of the Au surface oxide monolayer, and a reported value of 400 μC cm
2 was used for the calculation.29 It is clear that the area of the SPCE is correlated to modification of the Au nanoparticles. Surprisingly, modification by QPGNPs results in the increase of electroactive surface area from 24.5  10
3 to 89.3  10
3 cm2 in comparison with the modification by GNRs, a nearly 4-fold increase. Looking at the values, it is expected that the gold nanomaterial does affect the response of the constructed electrode toward the reduction of oxygen, which will be discussed next. 3.2. Distinct Activities toward Oxygen Reduction. Four room temperature ionic liquids with different cations and anions, namely, [C2mim][NTf2], [C4mim][NTf2], [C4dmim][NTf2], and [C6mim][FAP], were used as suitable solvents for electrochemical experiments. [C2mim][NTf2] and [C4mim][NTf2] were chosen since they all showed no noticeable voltammetric features in the absence of oxygen when fully purged under vacuum.30 [C6mim][FAP] was chosen as it contains the [FAP]
anion, which is considered highly hydrophobic, and this ionic liquid shows little water uptake under atmospheric conditions.31,32 [C4dmim][NTf2] was chosen because they have the widest anodic windows under atmospheric conditions out of 12 RTILs studied.32

Figure 4 displays cyclic voltammograms recorded for the reduction of oxygen (100% v/v) in four different ionic liquids at the GNRs and QPGNPs composites with typical data for bare SPCE presented for comparison. As can be observed in Figure 4a, a very broad reductive feature is obtained at about
1.45 V in [C2mim][NTf2] on a bare SPCE at a scan rate of 0.01 V s
1. Expectedly, two important catalytic activities of the SPCE loaded with the GNRs and QPGNPs were obtained, i.e., (1) a significant positive shift of the O2 reduction peak from
1.45 to
0.83 and
0.76 V upon loading of the GNRs and QPGNPs, respectively (at the bulk Au SPE, the reduction peak was observed at about
1.14 V vs Ag). (2) The substantial lowering of the reduction potential observed for oxygen at IL/gold nanoparticle composite is coupled to significantly larger current signals. The reduction current on the GNRs and QPGNPs composites is about 6 and 12 times that of the bare SPCE, respectively. The different behaviors for each of the three electrodes could be mainly attributed to the loading of Au nanoparticles (rods or quasiplatonic particles) onto the SPCE. The IL/gold nanoparticle composite is believed to increase the electron transfer rates for reduction of O2 and hence lowers reduction potentials compared to conventional SPCE. In the meantime, the surface-specific activity of QPGNPs toward the reduction of oxygen was found to be almost 2-fold higher than that of GNRs. It has been widely accepted that the facet of crystals may have a dramatic impact on their catalytic feature. According to the literature,33 the activity of Pd catalysts for the reduction of oxygen strongly depends on its morphology, i.e., nanorods and conventional nanoparticles. In this case, the superior oxygen reduction activity of Pd nanorods, as revealed by density functional theory calculations, could be ascribed to the exceptionally weak interaction between an O adatom and a Pd(110) facet. Therefore, it is reasonable that the active crystal 17474

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Figure 5. Current
time recording obtained on the GNR composites in (a) [C2mim][NTf2] (at
0.83 V), (b) [C4mim][NTf2] (at
0.92 V), (c) [C4dmim][NTf2] (at
0.96 V), and (d) [C6mim][FAP] (at
1.45 V). Data were collected from continuous CVs (scan rate 0.01 V s
1) for the reduction of oxygen in varying % volume of O2.

Figure 6. Current
time recording obtained on the QPGNP composites in (a) [C2mim][NTf2] (at
0.77 V), (b) [C4mim][NTf2] (at
0.84 V), (c) [C4dmim][NTf2] (at
0.72), and (d) [C6mim][FAP] (at
0.90 V). Data were collected from continuous CVs (scan rate 0.01 V s
1) for the reduction of oxygen in varying % volume of O2.

facet of QPGNPs might be an important factor contributing to its excellent catalytic behavior. It is also important to consider that QPGNPs could provide more active sites (as observed by TEM) for the reduction of oxygen. Further research on this contribution would be of value and is in hand.

Experimentally, this contribution was further evidenced by the CVs in [C4mim][NTf2] (Figure 4b), [C4dmim][NTf2] (Figure 4c), and [C6mim][FAP] (Figure 4d). The results were found to be similar to that observed in the above [C2mim][NTf2] experiments. In addition, it can clearly be observed that 17475

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the reduction potential and current in four ionic liquids are different on GNR and QPGNP composites. For instance, for [C2mim][NTf2], [C4mim][NTf2], and [C4dmim][NTf2], it is about
0.83,
0.93, and
0.98 V, respectively. Whereas it is about
1.45 V for [C6mim][FAP], which is more negative than those shown in panels a, b, and c of Figure 4. This suggests that the nature of the cation or anion might have a significant effect on the voltammetric behavior. Similar voltammetry for the reduction of oxygen could be seen on a 10 μm diameter gold electrode in different RTILs.34 3.3. Voltammetric Quantification of Oxygen. Continuous voltammetric quantification was obtained by continuously Table 1. Comparison of Percentage of Total Responsea reduction current/μA

electrode C2mimNTf2 bare SPCE GNRs QPGNPs C4mimNTf2 bare SPCE

at low

at high

first

second

conc

conc

scan

scan


3.09
21.5


3.64
30.6

21.2 50.9

56.3 88.8


41.3


51.1

89.5

95.7

18.5

40.4


2.39


2.95

GNRs


23.3


26.1

50.3

82.1

QPGNPs


28.9


31.5

58.2

89.2

12.5

21.3

C4dmimNTf2 bare SPCE

C6mimFAP

total response (%) at


1.82


2.22

GNRs


10.9


12.2

30.7

61.5

QPGNPs bare SPCE


21.2
2.36


23.5
2.63

36.2 9.76

78.4 13.5

GNRs


23.2


40.7

65.7

87.6

QPGNPs


54.8


62.6

75.7

92.8

Percentage of total response is given by (I1st
-ILowConc)/(IHighConc
ILowConc) and (I2nd
ILowConc)/(IHighConc
ILowConc). a

cycling at 0.01 V s
1 while altering the O2 content (data not shown). As the v/v % O2 increases, there was a corresponding increase in the current for O2 reduction. Figure 5 displays plots for the amperometric responses of the four different ionic liquids with respect to the different O2 contents, by plotting the reduction current for every CV recorded vs the time corresponding to that specific data point. Changes in the O2 content resulted in clear steps that can be observed in Figure 5. To determine whether the current
time behavior on the QPGNPs composites is similar to that observed in the above experiments, current
time characteristics were measured in different RTILs and the results are presented in Figure 6. As expected, the current increases stepwise with each successive change of the volume of O2. The plot of current as a function of volume % of O2 will be addressed in following section. A comparison of the current change at the first and second responses in the four different ionic liquids (Note: the data were extracted after some complete cyclings after a change in O2 content considering the different reduction potentials in four different ionic liquids) is shown in Table 1. Examining the table, the percentage of total response of the sensors is clearly improved by the modification of GNRs and QPGNPs. This is consistent with that reported previously on the thin layer behavior of RTIL, i.e., the total response could be significantly improved as the thickness of the RTIL layer is decreased. In addition, an evaluation of relative response time of different electrodes could be estimated by careful analysis of the reduction currents obtained in the two CVs recorded after a change in O2 content (highlighted in pink in Figures 5 and 6 and labeled first response and second response in (a)). However, more precise data should obtain according to chronoamperometric transients, as will be discussed below. Together with the better response of QPGNP composites, the results suggest that a morphology tailored nanoparticle composite approach toward membrane-free amperometric gas sensors is reliable.

Figure 7. Plots of current as a function of volume % O2 in (a) [C2mim][NTf2], (b) [C4mim][NTf2], (c) [C4dmim][NTf2], and (d) [C6mim][FAP] on the GNR and QPGNP composites with linear trend lines (R > 0.99 for all). Data for bare SPCE were included for comparison. 17476

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Figure 8. Potential step chronoamperometric transient for the reduction of O2 in (a) [C2mim][NTf2], (b) [C4mim][NTf2], (c) [C4dmim][NTf2], and (d) [C6mim][FAP] on the GNR and QPGNP composites. N2 gas was maintained for t = 0 to 150 s, then 100% O2 was introduced into the chamber from t = 150 s.

Figure 7 shows the resulting plots of peak current vs O2 content in these studies. For comparison, the data for bare SPCE were also included in Figure 7. As can be seen, in all cases, a high degree of linearity, with R > 0.99, is observed, suggesting that the IL/nanoparticle composite approach for membrane-free amperometric gas sensing system sensitively responds to changes in the oxygen level, producing a linear response over the range of investigated concentrations. Importantly, it can be noted that the sensitivity of IL/GNR and IL/QPGNP was found to be about 4
18 times higher than that of bare SPCE. From Figure 7, it is also seen that QPGNPs catalyst shows much higher activity than GNRs toward the reduction of oxygen. The results confirm that the GNRs and QPGNPs have significant catalytic effect on the reduction of oxygen and verify the data obtained in Figure 4. Figure 8 displays chronoamperometric transients recorded in four different ionic liquids on the GNR and QPGNP composites at 100% O2. The response time can be theoretically evaluated as22 τ ¼ d2 =D where D is the diffusion coefficient of gaseous analyte through the ionic liquid, τ is the response time, and d is the thickness of the ionic liquid layer. The film thickness was estimated as 196 μm based upon the RTIL sitting on the electrode as a rectangle, whereas in reality a dome of liquid can be expected above the electrode. Using the previously determined diffusion coefficient of O2 in [C2mim][NTf2] (2.5  10
10 m2 s
1),23 [C4mim][NTf2] (8.76  10
10 m2 s
1),34 [C4dmim][NTf2] (5.05  10
10 m2 s
1),34 and [C6mim][FAP] (3.5  10
10 m2 s
1),23 the estimated response times on bare SPCE are 153, 43, 75, and 109 s, respectively. As seen in Figure 8, the experimental response time is close to the theoretical value, and the discrepancy is tolerable by considering the calculation errors in the RTIL layer.

Significantly, the experimental response time of IL/GNR and IL/QPGNP is less than 10 s for each case. This could be explained by the GNRs and QPGNPs in the composite greatly decreasing the thickness of the RTIL layer (The reduction of oxygen actually occurs on the surface of gold nanoparticles) and also provides an accelerated electron transfer pathway for the reduction of oxygen. As suggested previously,22 the slower diffusion of analyte within the more viscous medium resulted in slower time responses. However, the results shown in Figure 8 demonstrate that the viscosity has no effect on the response and could be ignored. 3.4. Morphology-Tailored Activity toward Oxygen. It is anticipated that there is a strong morphology dependence of the catalytic activity of gold nanoparticles toward the reduction of oxygen. It has been reported that the surface-specific activity increases by 10-fold upon changing the morphology of Pd catalysts from nanoparticles to nanorods. In the present work, the surface-specific activity increases markedly by 2-fold upon changing from nanorods to quasi-platonic nanoparticles. To make sure of the underlying mechanism of such a morphologydepended activity of gold nanoparticles, work is ongoing in an attempt to further study through a combination of electrochemical experiments and density functional theory calculations.

5. CONCLUSIONS In summary, we have experimentally demonstrated a new design comprising ionic liquid
nanoparticle composites toward membrane-free amperometric gas sensors. The composite overcomes a major obstacle for creating an extremely thin layer of RTIL on the surface of electrode due to its ease of fabrication and handling (that is, thin-layer behavior could be easily relized herein via utilization of the composite). The coverage of the gold 17477

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The Journal of Physical Chemistry C nanocatalysts surface by the ionic liquid protects the metal against oxidation and keeps the gold catalysts active and stable.35 The as-prepared composite significantly enhances both the reduction current above that observed for a bare SPCE and sensitivity and simultaneously lowers response time (