I

Ma

b

a

ARRAA

KASING

1

itpcbtassrvresTis

fpti

0d

Electrochimica Acta 56 (2011) 5650–5655

Contents lists available at ScienceDirect

Electrochimica Acta

journa l homepage: www.e lsev ier .com/ locate /e lec tac ta

onic liquid–polymer electrolyte for amperometric solid-state NO2 sensor

artina Nádhernáa,b, Frantisek Opekarb, Jakub Reitera,∗,1

Institute of Inorganic Chemistry of the ASCR, v.v.i., 250 68 Husinec-Rez, Czech RepublicDepartment of Analytical Chemistry, Faculty of Science, Charles University in Prague, Albertov 2030, 128 40 Prague 2, Czech Republic

r t i c l e i n f o

rticle history:eceived 20 February 2011eceived in revised form 4 April 2011ccepted 5 April 2011vailable online 13 April 2011

a b s t r a c t

A new ionic liquid–polymer electrolyte was successfully tested in the planar amperometric solid-statesensor sensitive towards nitrogen dioxide. The electrolyte consists of 1-butyl-3-methylimidazolium hex-afluorophosphate (BMIPF6) and poly(ethylene glycol) methyl ether methacrylate (PEGMEMA) in the ratio57:43 mol.% and exhibits ionic conductivity 1.6 × 10−4 S cm−1 at 20 ◦C, high electrochemical stability (over4 V on gold or glassy carbon) and thermal stability (over 230 ◦C). The analyte, gaseous nitrogen dioxide in

eywords:mperometric gas sensorolid-state sensoronic liquiditrogen dioxide

air, was determined using the electrochemical reduction at −900 mV vs. Pt/air on gold minigrid indicat-ing electrode with Pt/air as a reference electrode. The sensor response is linear in the NO2 concentrationrange 0.3–1.1 ppm and is reproducible and long-term stable.

© 2011 Elsevier Ltd. All rights reserved.

old minigrid

. Introduction

A significant group of chemical sensors is based on electrochem-cal principles [1,2]. Besides numerous applications in the solution,he detection of environmentally important compounds in gaseoushase is being intensively studied. The parameters of the chemi-al sensor are primarily determined by the type of material of theasic sensor parts and their configuration. One important part ofhe sensor is the electrolyte. Some sensors require the presence ofliquid phase, e.g. the Clark [3] or Severinghaus [4] type of sen-

or with a gas permeable membrane and a liquid electrolyte, orensors using an ion selective electrode [5]. These sensors give aeliable response at room temperature, but the disadvantage is theaporisation of the electrolyte with time and complicated prepa-ation and maintenance. The present development in the field oflectrochemical gas sensors is strongly oriented towards solid-stateensors, i.e., those that do not contain a macroscopic liquid phase.he solid-state electrolytes should exhibit good ionic conductiv-ty along with chemical, thermal and electrochemical long-termtability.

As a solid electrolyte, many inorganic materials were used [6],or example, silver iodide [7], zirconium phosphate and phos-

honate [8], indium-doped tin phosphate [9], but in some casesheir low conductivity has to be increased by elevating operat-ng temperature. Due to these obstacles, recent development of

∗ Corresponding author. Tel.: +420 266172198; fax: +420 220941502.E-mail address: reiter@iic.cas.cz (J. Reiter).

1 ISE member.

013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2011.04.022

solid-state electrolytes with good ionic conductivity is concen-trated on the solid polymer electrolytes [10–14]. The absenceof the macroscopic liquid phase is important also due to thedemand for device miniaturisation. Inherent ion-conducting poly-mers (e.g. Nafion®) or various binary (polymer-salt) or ternarysystems (polymer-solvent-salt) have been used as ion-conductingelectrolytes in different electrochemical devices [15–20]. Unfor-tunately, hydrophilic polymers such as Nafion exhibit strongdependence of the overall conductivity on humidity. Moreover, dueto the permanent threat of long-term evaporation of the presentsolvent, electrolytes without volatile solvents are highly appreci-ated.

In our previously developed NO2 solid-state sensor, a ternarysystem composed of polyvinylchloride (PVC), 2-nitrophenyloctylether (NPOE), tetrabutylammonium hexafluorophosphate(TBAPF6) served as the electrolyte [21,22]. To maintain theadvantages of the gel polymer electrolytes or solid polymerelectrolytes, we focused on a combination of an ionic liquid andpolymer forming together a self-standing stable electrolyte withsufficiently high ionic conductivity and electrochemical stability.The function of an embedded solution of salt in a solvent is thereforejoined in the hydrophobic ionic liquid. Generally, in comparisonto water or non-aqueous solvents [23], the ionic liquids employtheir advantageous properties. Especially the non-volatility, highthermal and electrochemical stability is appreciated in variouselectrochemical sensors [24–27]. To the best of our knowledge,

this is the first application of a binary polymer-ionic liquid systemserving a solid-state electrolyte is such a device.

The present work describes the development and charac-terisation of binary polymer-ionic liquid electrolytes and their

himica

adso

2

2

m(eap((clgMm

dc2fcspft

2e

tTs

taiePoavaM

si1HastAmv

hN

M. Nádherná et al. / Electroc

pplication in the amperometric gas sensor for nitrogen dioxideetection. The principle of NO2 detection is based on the mea-urement of current generated by the electrochemical reductionf NO2.

. Experimental

.1. Synthesis of ionic liquids and polymer electrolyte preparation

Several monomers or macromonomers, 2-hydroxyethylethacrylate (HEMA), poly(ethylene glycol) methacrylate

PEGMA; Mn 360 and Mn 526) and poly(ethylene glycol) methylther methacrylate (PEGMEMA; Mn 300; all from Sigma–Aldrich)nd ionic liquids, 1-butyl-3-methylimidazolium hexafluorophos-hate (BMIPF6), 1-butyl-3-methylimidazolium tetrafluoroborateBMIBF4) and 1-ethyl-3-methylimidazolium tetrafluoroborateEMIBF4; all prepared according to literature [28–30]) wereombined in various molar ratios. As cross-linking agents, ethy-ene glycol dimethacrylate (EDMA) for HEMA and poly(ethylenelycol) dimethacrylate (PEGDMA; Mn 330) for PEGMA and PEG-EMA were used in the concentration 0.3 mol.% of monomer oracromonomer.Polymer electrolytes were prepared by a method similar to that

escribed in our previous papers [17,31]. The initial liquid mixtureontains the monomer, ionic liquid, and polymerisation initiator,,2′-azo-bis(isobutyronitrile) (AIBN; Sigma–Aldrich; recrystallisedrom acetone; 1 mol.% of monomers). The thermally initiated radi-al polymerisation proceeded for a period of 150 min at 80 ◦C. Theamples for the basic electrochemical characterisation were pre-ared in a cell formed of polypropylene plate, packing distancerame (silicone rubber) and glass plate. The preparation of the elec-rolytes for the sensor is described in Section 2.3.

.2. Electrochemical and thermogravimetric investigation oflectrolyte

The initial electrochemical characterisation of prepared elec-rolytes was performed on the potentiostat PGSTAT 30 (Eco Chemie,he Netherlands) including the FRA-2 module for impedance mea-urements.

The voltammetric measurements of prepared polymer gel elec-rolytes were performed in a glove box (MBraun, USA; argontmosphere, O2 and H2O below 2 ppm) with a gold (BASi, 1.6 mmn diameter) or glassy carbon (BASi, 3 mm in diameter) workinglectrode and a glassy carbon counter electrode. The solid-stateMMA–Cd–Cd2+ system was used as a reference electrode devel-ped in our laboratory for electrochemical investigation of liquidnd polymer aprotic systems with E(PMMA–Cd–Cd2+) = −0.44 Vs. SCE in propylene carbonate [32]. The surface of the workingnd the counter electrode was polished by abrasives (0.3 alumina,etroohm) and soft cloth after each measurement.Conductivity measurements were performed using impedance

pectroscopy, whereby the influence of the temperature was stud-ed in the range from 0 to 90 ◦C using a circulating bath Ministat25-cc (precision of the temperature ±0.1 ◦C, Huber, Germany).ere, a slice of gel (2 cm × 2 cm) was sandwiched between two par-llel stainless steel electrodes and a single potential impedancepectrum was measured in the frequency range from 200 kHzo 1 Hz. The obtained spectrum was analysed by the EcoChemieutolab software producing the values of the equivalent circuit ele-ents. The resulting ohmic resistance value was converted in the

alue of specific resistivity or conductivity.The thermogravimetric analysis (TGA) was done in air at the

eating rate of 5 ◦C min−1 with the Simultaneous Thermal Analysisetzsch STA 409 (Germany).

Acta 56 (2011) 5650–5655 5651

2.3. Sensor construction and apparatus

The sensors tested have been designed and fabricated asdescribed previously [22,33]. In the three-electrode arrange-ment, the gold minigrid (33.21 mm2 grid total geometric surface,0.005 mm wire diameter, 1500 wires/inch; Goodfellow, UnitedKingdom) serves as an indicating electrode, platinum as an aux-iliary and pseudoreference Pt/air electrode. The testing apparatuswas described in Ref. [33]. The air was pumped by a membranepump (Cole-Palmer, USA) and divided into two streams – mainand reference. A known amount of nitrogen dioxide was addedto the main flow using a calibrated permeation tube (Vici AGInternational, USA) with NO2 production of 148 ng min−1 (±2%).The NO2 concentration range (0.6–2.0 ng ml−1, thus 0.3–1.1 ppm)was obtained by changing the air flow-rate along the permeationsource. To humidify both air streams, two flasks were included inthe apparatus with Mg(NO3)2 saturated solution providing stableand reproducible relative humidity (RH) of 54%. The sensor wasplaced inside a glass chamber with volume 8 cm3. The whole mea-surement was performed at room temperature and gas flow rate of1 ml s−1. The potentiostat and current meter were laboratory madeof the operational amplifiers.

3. Results and discussion

3.1. Compatibility of ionic liquids with methacrylates

The first task was to find a compatible combination of an ionicliquid with a polymer. Various monomers and macromonomerswere combined with imidazolium-based ionic liquids as can beseen in Table 1. Contrary to the previously reported gel electrolytescontaining polymer, ionic liquid and aprotic solvent [29], here onlya binary system polymer-ionic liquid is prepared.

Preparation of a homogeneous electrolyte requires a chemicalcompatibility of both monomer and polymer with a particularionic liquid. Generally, the aprotic monomers and polymers arenon-miscible with polar ILs. For example, methyl methacrylateand styrene were found to be non-compatible with 1-ethyl-3-methylimidazolium tetrafluoroborate or 1-butylpyridiniumtetrafluoroborate due to the phase-to-phase separation [34]. Ourprevious experiments showed that 2-ethoxyethyl methacrylate(a representative of more polar aprotic monomers) is misciblewith BMIPF6, but the phase-to-phase separation occurs during thepolymerisation and results in the ionic liquid exclusion from thepolymer [29].

The pairs PEGMEMA–BMIPF6 and PHEMA–BMIBF4 form sam-ples with suitable mechanical properties allowing both manipula-tion and electrochemical tests such as impedance and voltammetricmeasurements. The other samples were chalky, easily breakableand they disintegrate when strained. For the further experiments,PEGMEMA–BMIPF6 electrolyte was used due to its higher conduc-tivity and suitable mechanical properties allowing both fabricationof a sensor and electrochemical tests. Their storage in air did notshow any visible changes with time.

3.2. Electrochemical and thermogravimetric characterisation ofthe electrolyte

Neat ionic liquids reach conductivity values close to 10−2 S cm−1

at 20 ◦C, e.g. conductivity of BMIPF6, BMIBF4 and EMIBF4 is 1.40,2.90 and 13.8 mS cm−1, respectively. The ionic conductivity of theionic liquid is reduced due to the presence of polymer. Impedance

measurements showed a reasonable ionic conductivity in thecase of PEGMEMA–BMIPF6 electrolyte (1.6 × 10−4 S cm−1 at 20 ◦C),while the value for the PHEMA–BMIBF4 sample is much lower(8.2 × 10−6 S cm−1 at 20 ◦C).

5652 M. Nádherná et al. / Electrochimica Acta 56 (2011) 5650–5655

Table 1Properties of prepared combinations of ionic liquids with polymers (conductivity value is valid for 20 ◦C).

Ionic liquid/polymer BMIPF6 BMIBF4 EMIBF4

HEMA Bad mechanical properties (chalky) 75:25 (mol.%) good mechanicalproperties (� = 8.2 × 10−6 S cm−1)

Phase to phase separation

ptd9i

�

nttedtfiant

e9rp

atioaai

FP

bis(oxalato)borate) PEOEMA-PC-LiBOB shows that the ternary sys-

PEGMA Bad mechanical properties (chalky)PEGMEMA 43:57 (mol.%) good mechanical

properties (� = 1.6 × 10−4 S cm−1)

Fig. 1 presents the relationship between the ionic conductivity ofrepared polymer electrolytes and temperature. The data are plot-ed in Arrhenius coordinates (specific conductivity is plotted as aecadic logarithm). The values for the temperature region from 0 to0 ◦C can be fitted with the Vogel–Tamman–Fulcher (VTF) equation

n the logarithmic form:

T1/2 = A exp[ −EA

R(T − T0)

](1)

In this particular relationship A is the parameter related to theumber of charge carriers, EA is the activation energy for conduc-ion, R is the universal gas constant and T0 the ideal glass transitionemperature indicating the temperature at which the free volumextrapolates to zero. The analysis of the experimental conductivityata in terms of the VTF relationship leads to the determination ofhree empirical parameters: A, EA and T0, when T0 is determined bytting the experimental data with relationship (1). The conductivityctivation energy corresponds to a slope in the Arrhenius coordi-ates (see Fig. 1) and explains how conductivity is influenced byemperature.

Comparison of the neat BMIPF6 and the PEGMEMA–BMIPF6lectrolyte shows that the conductivity is reduced by a factor of. The conductivity activation energies are 7.1 and 7.5 kJ mol−1,espectively, which indicates only a small influence of the presentolymer on the EA.

The imidazolium-based ionic liquids with weakly coordinatingnions such as hexafluorophosphate or tetrafluoroborate exhibithe electrochemical stability window over 4 V. The cathodic limits restricted by the presence of an acidic H(2) on carbon C(2)

f the imidazolium ring that undergoes an irreversible reductiont ca. −1.8 V vs. Cd/Cd2+. The anodic oxidation limit at 2.5 V isssigned to the decomposition of the anion [35]. The electrochem-cal stability of used aprotic polymer is generally high, when the

ig. 1. Arrhenius plot of conductivity for neat ionic liquid BMIPF6 andEGMEMA–BMIPF6 (43:57 mol.%) electrolyte (temperature range 0–90 ◦C).

Phase to phase separation Phase to phase separationBad mechanical properties (chalky) Phase to phase separation

methyl ether terminal group in PEGMEMA improves the stabilitycompared to HEMA or PEGMA. Methacrylate and acrylate-basedpolymer electrolytes have been widely tested in lithium-ion batter-ies, where the highest demands for electrochemical stability occur[18,36–38].

The electrochemical stability window of PEGMEMA–BMIPF6electrolyte was measured by linear sweep voltammetry on goldand by cyclic voltammetry on glassy carbon as shown in Fig. 2.The first electrode material was chosen because we expected ahigher catalytic activity of gold compared to glassy carbon. Bothmeasurements showed cathodic stability values down to −1.8 Vin agreement with our previous experiments [29] and resultspublished elsewhere [28,39]. In the case of LSV on gold, a small irre-versible wave attributed to reduction of water traces was observedat −1 V vs. Cd/Cd2+. Due to its low current density (wave height lessthan 2.5 �A cm−2) can be omitted as negligible. The anodic stabil-ity was found to be over 2.2 V vs. Cd/Cd2+, where the limiting factorbecomes the stability of the anion.

The absence of an organic solvent in the structure of the mem-brane substantially improves the thermal stability of the material.The tested ionic liquids are thermally stable up to 300–400 ◦C (Refs.[40,41]), when a single-step decomposition reaction occurs. Asshown in Fig. 3, the PEGMEMA–BMIPF6 electrolyte starts to decom-pose at Tdec of 235 ◦C followed by three, hardly distinguishableconsecutive decomposition steps. The overall decomposition of theelectrolyte is finished at 470 ◦C.

The comparison to a TGA curve of a gel polymer electrolyte(poly(2-ethoxyethyl methacrylate)-propylene carbonate-lithium

tem polymer-solvent-salt shows a distinctively lower thermalstability. The presence of an organic solvent limits the thermal

Fig. 2. Linear sweep voltammograms of PEGMEMA–BMIPF6 (43:57 mol.%) elec-trolyte on gold electrode, at 5 mV s−1. Inserted: cyclic voltammogram of the sameelectrolyte on glassy carbon (reference PMMA–Cd–Cd2+, counter glassy carbon).

M. Nádherná et al. / Electrochimica Acta 56 (2011) 5650–5655 5653

FPh

ss

stipm

3d

nowstgiP

[

N

otir

A

A

N

N

t

2

Fig. 4. Steady-state polarisation curves of the sensor with gold minigrid (NO2 con-centration 0.54 ppm, flow rate 1 ml s−1, RH 54%) for pure air (background) – empty

on a step back to zero concentration, the value of 0.1 I(max) wasreached within 51 s under the experimental conditions specified inFig. 5.

ig. 3. TGA curves for PEGMEMA–BMIPF6 (43:57 mol.%; solid line) andEOEMA–PC–LiBOB gel electrolyte (34:62:4 mol.%, dash line) – 5 ◦C min−1

eating rate, temperature range 30–530 ◦C, air atmosphere.

tability of the sample to 110–120 ◦C, however the weight loss isuppressed by the present polymer.

In the case of PEGMEMA–BMIPF6 binary system, the thermaltability is determined rather by the stability of polymer than byhe ionic liquid. It is well known that methacrylates decomposen two exothermic reaction processes [42,43]: degradation of theolymer end groups (240–280 ◦C) and total decomposition of theonomer units (above 310 ◦C).

.3. Steady-state voltammetric curve – optimisation of theetection potential

Initially, a steady-state polarisation curve of the reduction ofitrogen dioxide at the indication electrode at the relative humidityf 54% and the analyte concentration of 0.6 ppm. The measurementas done in the potential range from +300 to −1000 mV vs. Pt/air as

hown in Fig. 4. The suitable potential for the NO2 detection lies inhe limiting current region, where the difference between the back-round current and the current of NO2 reduction with backgrounds the highest. According to this, the potential value of −900 mV vs.t/air was used in further measurements.

The generally accepted overall reaction of the NO2 reduction19,44–47] is

O2 + 2H+ + 2e− → NO + H2O. (2)

However, the reaction mechanism may be complex and dependn many factors, such as the electrode material and properties ofhe electrolyte. It is assumed that the reduction on gold surface firstnvolves chemical oxidation of gold, followed by electrochemicaleduction of the gold oxides formed [19,46,47]

u + NO2 → AuO + NO (3)

uO + 2H+ + 2e− → Au + H2O (4)

A mechanism involving the nitrite anion is also possible [48],

O2 + e− → NO2− (5)

O2− + 2H+ + e− → NO + H2O. (6)

In both these cases, water is oxidised at the auxiliary anode andhe overall reaction in the sensor is completed to

NO2 → 2NO + O2. (7)

circle, air + NO2 reduction current – half full circle, NO2 reduction current afterbackground subtraction – full circle.

The presence of the mechanism described by Eqs. (5) and (6)would explain the decrease in current at very negative potentialsby the disproportionation reaction (see Fig. 4):

3NO2− + H2O → 2NO + NO3

− + 2OH−. (8)

3.4. Dynamic behaviour and concentration dependence

A typical sensor response to a step increase and decrease inthe NO2 concentration is depicted in Fig. 5. For description of thedynamic behaviour of the sensor we can define I(t) as the cur-rent at time t and I(max) as the stationary response current. Ona step increase in the NO2 concentration from zero to 1.08 ppm,the response equal to 0.9 I(max) was attained within 13 s, whereas

Fig. 5. Sensor response at −900 mV to a step increase in NO2 concentration from 0to 1.08 ppm and back (flow rate 1 ml s−1, RH 54%).

5654 M. Nádherná et al. / Electrochimica

Fig. 6. Dependence of the gold minigrid sensor signal (reduction current withoutbackground) on the NO2 concentration (flow rate 1 ml s−1, RH 54%, measured atpv

c(

I

w(t1l

T07

ntbircwea

tdPctt

sasIcb

[[[

[[[

[[[[[[[[

[[[[[

[[[31] J. Reiter, J. Michálek, J. Vondrák, D. Chmelíková, M. Prádny, Z. Micka, J. Power

otential of −900 mV, NO2 concentration range 0.3–1.1 ppm, each point is averagealue from 6 measurements and standard deviations are marked as error bars).

The dependence of the sensor response, current I, on the NO2oncentration, c(NO2), was linear in the tested concentration rangesee Fig. 6), 0.3–1.1 ppm, and can be described by the equation

= 0.4390 c(NO2), (9)

here I is current in �A and c(NO2) concentration in ppm1 ppm = 1.848 ng ml−1); the zero value of the intercept lies withinhe confidence interval. Average noise of the baseline was about.3 × 10−3 �A. From the triple of this value and using Eq. (9), the

imit of detection, 0.01 ppm, was obtained.The sensor response was monitored over a period of 10 months.

he relative standard deviation of the signal for NO2 concentration.32 ppm and 1.08 ppm (at RH 54%) was determined to be 9.1 and.2%, respectively.

The water molecules participate in the NO2 reduction mecha-ism. Provided that the water content in the test air is much higherhan the NO2 concentration, the electrode reaction will primarilye controlled by the NO2 concentration and should be practically

ndependent of the changes in the water content. The air satu-ated with water contains 19.42 g m−3 of water. At RH = 54%, theontent of water is 19.42 g m−3 × 0.54 = 10.5 g m−3 at 22 ◦C. Theater/NO2 concentration ratio is ≈5700 for the NO2 concentration

qual to 1 ppm = 1.85 mg m−3; this ratio is higher than 1000, event RH = 12%.

Water can also influence the electrolyte properties. In con-rast to, e.g., Nafion, whose conductivity and dimensions changeramatically with changing water content, the properties of ourEGMEMA–BMIPF6 electrolyte are influenced less by the waterontent, mainly due to hydrophobicity of the ionic liquid used ando generally different structure of the electrolyte; this is an advan-age of the electrolyte tested.

Nevertheless, it has been experimentally observed that the sen-or sensitivity is not fully resistant to changes in the RH; it increaseslmost linearly by ca. 3 nA ppm−1 per 1% RH. This means that theensitivity changes by 10% when the RH value is altered by ca. 13%.

f the test air RH value changes even more, then it is necessary to re-alibrate sensor at the actual RH, or an electronic correction muste introduced.

[[[

Acta 56 (2011) 5650–5655

4. Conclusions

The binary polymer-ionic liquid electrolyte was prepared asa desirable electrolyte for the solid-state sensor for nitrogendioxide. The material properties of the electrolyte consisting of1-butyl-3-methylimidazolium hexafluorophosphate BMIPF6 andpoly(ethylene glycol) methyl ether methacrylate PEGMEMA wereinvestigated using electrochemical and thermoanalytical methods.As a suitable composition from application point of view, the ratio57:43 mol.% was used.

The functionality of the electrolyte was successfully tested in aminiaturised solid-state sensor with gold minigrid indicating elec-trode. The initial tests include the optimisation of the detectionpotential, description of the sensor dynamic behaviour and mon-itoring of the long-term stability. The sensor response is linear inthe NO2 concentration range 0.3–1.1 ppm and is reproducible andlong-term stable.

Further research will include detailed study of the sensor perfor-mance under various experimental conditions including the effectof the three-phase boundary length (determined by the minigridelectrode geometry), the influence of interfering gases, and relativehumidity.

Acknowledgements

This work was supported by the Grant Agency of theAcademy of Sciences of the Czech Republic (KJB200320801 andKJB200320901), the Ministry of Education, Youth and Sports(LC523 and MSM0021620857) and by the Academy of Sciences(AV0Z40320502). We thank Glazura Ltd. (Czech Republic) for sup-plying special platinum ink.

References

[1] W. Göpel, J. Hesse, J.N. Zemel (Eds.), Sensors. A Comprehensive Survey, vols. 2and 3, VCH, Weinheim, 1989.

[2] B.G. Eggins, Chemical Sensors and Biosensors, John Wiley & Sons, 2002.[3] R. Ramamoorty, P.K. Dutta, S.A. Akbar, J. Mater. Sci. 38 (2003) 35.[4] K. Wiegran, T. Trapp, K. Cammann, Sens. Actuators B 57 (1999) 120.[5] M. Shortreed, E. Bakker, R. Kopelman, Anal. Chem. 68 (1996) 2656.[6] J.W. Fergus, Sens. Actuators B 134 (2008) 1034.[7] S. Suzuki, K. Nagashima, Anal. Chim. Acta 144 (1982) 261.[8] G. Alberti, F. Cherubini, R. Palombari, Sens. Actuators B 24/25 (1995) 270.[9] A. Tomita, Y. Namekata, M. Nagao, T. Hibino, J. Electrochem. Soc. 154 (2007)

172.10] J.R. Stetter, J. Li, Chem. Rev. 108 (2008) 352.11] F. Opekar, K. Stulík, Anal. Chim. Acta 385 (1999) 151.12] D.W. Stergiou, T. Stergiopoulos, P. Falaras, M.I. Prodromidis, Electrochem. Com-

mun. 11 (2009) 2113.13] K. Wallgren, S. Sotiropoulos, Electrochim. Acta 46 (2001) 1523.14] C.Y. Chiou, T.C. Chou, Sens. Actuators B 87 (2002) 1.15] A. Fort, C. Lotti, M. Mugnaini, R. Palombari, S. Rocchi, V. Vignoli, Microelectron.

J. 40 (2009) 1308.16] K.C. Ho, W.T. Hung, J.C. Yang, Sensors 3 (2003) 290.17] J. Reiter, O. Krejza, M. Sedlaríková, Sol. Energy Mater. Sol. Cells 93 (2009) 249.18] J. Reiter, R. Dominko, M. Nádherná, I. Jakubec, J. Power Sources 189 (2009) 133.19] F. Opekar, Electroanalysis 4 (1992) 133.20] F. Opekar, K. Stulík, Crit. Rev. Anal. Chem. 32 (2002) 253.21] J. Langmaier, F. Opekar, Z. Samec, Sens. Actuators B 41 (1997) 1.22] Z. Hohercáková, F. Opekar, Sens. Actuators B 97 (2004) 379.23] H. Ohno (Ed.), Electrochemical Aspects of Ionic Liquids, John Wiley-VCH, New

York, 2005.24] M.C. Buzzeo, C. Hardacre, R.G. Compton, Anal. Chem. 76 (2004) 4583.25] R. Wang, T. Okajima, F. Kitamura, T. Ohsaka, Electroanalysis 16 (2004) 66.26] L. Yu, Y. Huang, X.X. Jin, A.J. Mason, X.Q. Zeng, Sens. Actuators B 140 (2009) 363.27] Z.L. Wei, Z.J. Li, X.L. Sun, Y.J. Fang, J.K. Liu, Biosens. Bioelectron. 25 (2010) 1434.28] P. Bonhôte, A.P. Dias, N. Papageorgiou, K. Kalyanasundaram, M. Grätzel, Inorg.

Chem. 35 (1996) 1168.29] J. Reiter, J. Vondrák, J. Michálek, Z. Micka, Electrochim. Acta 52 (2006) 1398.30] A. Lewandowski, A. Swiderska, Solid State Ionics 169 (2004) 21.

Sources 158 (2006) 509.32] J. Reiter, J. Vondrák, Z. Micka, Solid State Ionics 177 (2007) 3501.33] P. Hrncírová, F. Opekar, K. Stulík, Sens. Actuators B 69 (2000) 199.34] A. Noda, M. Watanabe, Electrochim. Acta 45 (2000) 1265.

himica

[

[

[[

[[

[

[

[

[44] J.M. Sedlak, K.F. Blurton, J. Electrochem. Soc. 123 (1976) 1476.[45] J.M. Sedlak, K.F. Blurton, Talanta 23 (1976) 811.[46] S.C. Chang, J.R. Stetter, Electroanalysis 2 (1990) 359.

M. Nádherná et al. / Electroc

35] J.N. Barisci, G.G. Wallace, D.R. MacFarlane, R.H. Baughman, Electrochem. Com-mun. 6 (2004) 22.

36] J.T. Lee, M.S. Wu, F.M. Wang, H.W. Liao, C.C. Li, S.M. Chang, C.R. Yang, Elec-trochem. Solid State Lett. 10 (2007) 97.

37] H.S. Kim, J.H. Shin, S.I. Moon, M.S. Yun, J. Power Sources 119 (2003) 482.38] C. Gerbaldi, J.R. Nair, G. Meligrana, R. Bongiovanni, S. Bodoardo, N. Penazzi,

Electrochim. Acta 55 (2010) 1460.

39] M. Galinski, A. Lewandowski, I. Stepniak, Electrochim. Acta 51 (2006) 5567.40] H.L. Ngo, K. LeCompte, L. Hargens, A.B. McEwen, Thermochim. Acta 357 (2000)

97.41] J.G. Huddleston, A.E. Visser, W.M. Reichert, H.D. Willauer, G.A. Broker, R.D.

Rogers, Green Chem. 3 (2001) 156.

[[

Acta 56 (2011) 5650–5655 5655

42] Y.H. Liao, M.M. Rao, W.S. Li, C.L. Tan, J. Yi, L. Chen, Electrochim. Acta 54 (2009)6396.

43] A.M. Grillone, S. Panero, B.A. Retamal, B. Scrosati, J. Electrochem. Soc. 146 (1999)27.

47] J.S. Do, R.Y. Shieh, Sens. Actuators B 37 (1996) 19.48] I. Bergman, J. Electroanal. Chem. 157 (1983) 59.