Room temperature ionic liquids as optical sensor matrix materials for gaseous and dissolved CO2

Sensors and Actuators B 117 (2006) 295–301

Room temperature ionic liquids as optical sensor matrix materials for gaseous and dissolved CO2 Ozlem Oter, Kadriye Ertekin ∗ , Derya Topkaya, Serap Alp University of Dokuz Eylul, Faculty of Arts and Science, Department of Chemistry, 35160 Tinaztepe, Izmir, Turkey Received 14 July 2005; received in revised form 18 November 2005; accepted 21 November 2005 Available online 28 December 2005

Abstract A new optical CO2 sensor based on the spectrophotometric signal changes of the bromothymol blue/tetraoctylammonium (BTB− /TOA+ ) ion pair in room temperature ionic liquids (RTILs) has been proposed. Ionic liquids, also known as green chemistry reagents, have been used for the first time as a matrix material in the optical CO2 sensor design. In the first stage of the study, determination of the acidity constant (pKa ) of the modified BTB has been performed in the employed ionic liquids. In the second stage, response of the sensor composition to gaseous and dissolved CO2 has been evaluated. It should be noted that the solubility of CO2 in water miscible ionic liquids is about 10–20 times as that observed in the conventional solvents, polymer matrices or in water and enhance the response of the sensing agent. The detection limits were 1.4% for gaseous, and 10−6 M [HCO3 − ] for dissolved CO2 . CO2 can be completely extruded from the ionic liquid by heating, under vacuum or sonification. The regenerated ionic liquid has been used for CO2 sensing without any loss of efficiency. © 2005 Elsevier B.V. All rights reserved. Keywords: Room temperature ionic liquids (RTILs); Carbon dioxide; Optical sensor; Bromothymol blue

1. Introduction The concentration of CO2 , the most potent greenhouse gas after water vapour, in the atmosphere has increased by more than 30% from the pre-industrial era, which increases the average temperature of the earth and results in dramatic changes in climate and the ecosystem. The continuous and accurate monitoring of CO2 levels in atmosphere has a vital importance and probably will be an exigency for governments in near future. The present CO2 sensing techniques are based on infrared (IR) absorptiometry, electrochemical Severinghouse electrode and optical sensors. However, in spite of the sensitiveness of the IR absorptiometry sensor, it is subject to strong interference from water vapour and is an expensive system. On the other hand, Severinghouse electrode detects CO2 due to the changes in the pH of the solution and is markedly affected from electromagnetic disturbances, from interfering acidic and basic gases ∗

Corresponding author. Present address: Dokuz Eylul Universitesi FenEdebiyat Fakultesi Kimya Bolumu 35160, Tinaztepe Buca, Izmir, Turkey. Tel.: +90 232 453 5072/2274; fax: +90 232 453 4188/2153. E-mail addresses: [email protected], [email protected] (K. Ertekin). 0925-4005/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.snb.2005.11.040

and from osmotic pressure in the sample. Recently, the optical CO2 sensors based on the absorbance or fluorescence changes of pH indicators have been developed. They offer several attractive features, which include electrical isolation, reduced noise interference, ability of miniaturization and remote sensing. Amao and Nakamura [1] designed an optical CO2 sensor based on the overlay of the CO2 induced absorbance change of pH indicator dye ␣-naphtolphytalein with the fluorescence of tetraphenylporphyrin using ethyl cellulose and polystyrene membranes, and obtained 53.9% signal change from 100% N2 to 100% CO2 . M¨uller and Hauser [2] performed an optode for measurements of low concentrations of dissolved CO2 . The useful measuring range was found between 10−3 and 10−1 M NaHCO3 . They recorded response and recovery times of 6 and 20 min, respectively. Neurauter et al. [3a] measured gaseous CO2 levels in an ethyl cellulose matrix in the concentration range of 0–30 hPa. In fact, in most of the optical sensors, indicators are adsorbed or immobilized in a solid matrix and when they are in contact with the sample, leaching can occur. Besides these, there are some problems about the lifetimes of the sensors due to evaporation of water or poisoning by ambient air and interfering gases such as NO2 , SO2 or O2 . To overcome these difficulties, Ertekin et al. designed a fiber optic sensing device in a capillary reservoir

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filled with the florescent indicator HPTS in an ethyl cellulose solution [4]. He and Rechnitz proposed a fiber optic sensor in which a pipet tip was used that was filled with an aqueous indicator solution [5]. Lee and Chou designed an electrochemical sensor and used ionic liquids for the determination of ethanol [6], but to our knowledge, ionic liquids are used for the first time as a matrix material in the optical CO2 sensor design. A number of investigations have shown that CO2 is reversibly soluble in imidazolium based ionic liquids [7–12]. Solubility and selectivity of the ionic liquid can be tuned by choice of cations, anions and substituents on ionic liquids. Bates et al. developed a task specific ionic liquid (TSIL) for CO2 capture by attaching an amine group to the ionic liquid for CO2 deposition purposes [10]. Due to the strong Coulombic attraction between the ions of these liquids they exhibit no measurable vapour pressure up to their thermal decomposition point; >300 ◦ C. This lack of vapour pressure make these materials highly attractive for gas processing and they may be considered as “liquid solids” incorporating some of the most useful physical properties of both phases. Although there have been numerous details concerning their properties, probably the most important one is that room-temperature ionic liquids have been utilized as “clean solvents” and “catalysts” for green chemistry. Additionally, their gas sorption–desorption dynamic is very fast and desorption by vacuuming is completely reversible. These characters make the RTILs exceptionally promising. Here, we offer a new optical sensor prepared by the simple immobilization of the modified bromothymol blue into water miscible room temperature ionic liquids; 1-methyl-3-butylimidazolium tetrafluoroborate (RTILI) and 1-methyl-3-butylimidazolium bromide (RTIL-II).

solutions were prepared using deionised and distilled water. NMethylimidazole, n-butyl bromide and sodium tetrafluoroborate were purchased from Merck and Fluka. Water miscible and room temperature ionic liquids were chosen as matrix materials and synthesized in our laboratory according to the conventional literature method [13a–f]. The typical chemical structures of the ionic liquids and modified indicator dye are shown in Fig. 1. 2.2. Cocktail preparation protocols Sensing cocktails were prepared by dissolving either the sodium salt or the ion pair form of the indicator dye in two different ionic liquids. Cocktail 1 contains 0.014 mg of ion pair form in 1 mL of ionic liquid (1-methyl-3-butylimidazolium tetrafluoroborate) and 80 ␮L of tetraoctylammonium hydroxide (TOAOH). Cocktail 2 was the same as the cocktail 1 but does not contain TOAOH. Other cocktail compositions were given in Table 1. Cocktails 3 and 4 were the same as the cocktails 1 and 2 except the ionic liquid. Here, we used a different ionic liquid; 1-methyl3-butylimidazolium bromide. Cocktails 5 and 6 exactly match cocktails 1 and 3 except that of the form of the dye. Instead of the sodium salt, the ion pair of the dye was used (see Table 1). 2.3. Carbon dioxide sensing studies CO2 and N2 gases were mixed in the concentration range of 0–100% in a home made gas mixing chamber by controlling the gas flow rates with sensitive flow-meters. The output flow rate of the gas mixture was maintained at 500 mL min−1 . Gas mixtures were introduced into the sensor agent containing cuvette via a diffuser needle under atmospheric conditions.

2. Experimental

2.4. Dissolved CO2 sensing studies

2.1. Materials and equipment

Standard solutions were prepared freshly from a 1 M NaHCO3 stock solution prior to the measurements. Dilute solutions of sodium hydrogen carbonate were used to form dissolved CO2 calibration graphs. CO2 -free standard solutions were prepared with doubly distilled water after boiling, bubbling with nitrogen, and kept in closed containers. Table 2 gives calculated H2 CO3 concentrations [the sum of hydrated CO2 (aq) and real H2 CO3 ] and corresponding PCO2 values in solutions made up from NaHCO3 . Concentrations were calculated by using the following equations, where K1 and K2 are referred to the dissolution equilibrium constant of gaseous CO2 in water and the

All solvents and chemicals were of analytical grade and purchased from Merck, Fluka and Riedel. Solvents for the spectroscopic studies were used without further purification. Absorption spectra were recorded by using a Shimadzu UV-1601 UV–Vis spectrophotometer. pH measurements were recorded with a WTW pH meter under magnetic stirring after a conditioning process in ILs. Carbon dioxide and nitrogen gas cylinders were of 99.9% purity and obtained from Gunes Company, Izmir, Turkey. Sodium bicarbonate was from Riedel. All of the Table 1 Compositions of cocktails employed as CO2 sensing agent Coctail no.

Ionic liquid

Dye

Additive

C-1 C-2 C-3 C-4 C-5 C-6

RTIL-I (1 mL) RTIL-I (1 mL) RTIL-I (1 mL) RTIL-II(1 mL) RTIL-I (1 mL) RTIL-II (1 mL)

BTB− /TOA+ (9.74 × 10−5 M) BTB− /TOA+ (9.74 × 10−5 M) BTB− /TOA+ (1.95 × 10−5 M) BTB− /TOA+ (1.95 × 10−5 M) Na-BTB (9.74 × 10−5 M) Na-BTB (1.95 × 10−5 M)

TOAOH (80 ␮L) – TOAOH (80 ␮L) – TOAOH (80 ␮L) TOAOH (160 ␮L)

RTIL-I and RTIL-II are referred to the room temperature ionic liquids; 1-methyl-3-butylimidazolium tetrafluoroborate and 1-methyl-3-butylimidazolium bromide. BTB− /TOA+ and Na-BTB are bromothymol blue/tetraoctylammonium and sodium bromothymol blue, respectively.

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Fig. 1. Schematic structure of the employed water miscible room temperature ionic liquids and BTB− /TOA+ ion pair.

equilibrium constant of carbonic acid formation, respectively. K3 and K4 are the first and second acid dissociation constants of the carbonic acid [14,15]. CO2 (g) ↔ CO2 (aq),

log K1 = −0.47

CO2 (aq) + H2 O ↔ H2 CO3 , H2 CO3 ↔ H+ + HCO3 − , −

+

HCO3 ↔ H + CO3

−2

log K2 = −1.41

log K3 = −6.38 3. Results and discussion

,

log K4 = −10.38

−

In a HCO3 solution, the relationship between the partial pressure of dissolved CO2 (g) (PCO2 ) and proton concentration is as follows: ([H+ ] + [H+ ] [Na+ ] − Kw [H+ ]) K3 ([H+ ] + 2K4 ) 3

αPCO2 = [H2 CO3 ] =

2

where α = K1 K2 [H2 O], Kw is the water dissociation constant and [Na+ ] is the concentration of sodium ions present. Table 2 Calculated concentration of [H2 CO3 ] and corresponding PCO2 in solutions prepared with NaHCO3 Total NaHCO3 (mol L−1 )

[H2 CO3 ] (mol L−1 )

PCO2 in RTILs (atm)

2 × 10−6

1.95 × 10−7

5.02 × 10−6 1.79 × 10−5 7.17 × 10−5 5.98 × 10−4 4.78 × 10−3

2 × 10−5 2 × 10−4 2 × 10−3 2 × 10−2

Twenty microlitres portions of standard solutions of NaHCO3 were added into the sensing agent containing cuvette and sonicated for 10 s. The change in absorbance due to addition of different concentrations of dissolved CO2 was measured. The time when 90% equilibrium was reached (τ 90 ) was recorded as the response time. All the experiments were carried out at room temperature of 25 ± 1 ◦ C.

6.97 × 10−7 2.79 × 10−6 2.33 × 10−5 1.86 × 10−4

3.1. Dissociation constant (pKa ) of indicator dye in RTILs The knowledge of dissociation constant (pKa ) of BTB in RTILs is of fundamental importance in order to provide information on chemical reactivity range of the indicator dye. Indicators with pKa values between 6.8 and 10.0 are essential for sensitive PCO2 optodes. In order to evaluate the availability of BTB− /TOA+ ion pair in the RTILs for CO2 sensing, determination of the acidity constant has been performed. pH induced absorption spectra of the cocktail composition were recorded in the pH range of 8.70–10.49 in both of the ionic liquids. Dye dopped-RTIL-II was titrated with 0.5 M HCl, and after each addition, pH values of the media were recorded (see Fig. 2a). Here a relative signal change of approximately 65% has been reached. Fig. 3 shows the plot of absorbance intensity of BTB in RTIL-II versus measured pH values. In RTIL-I, BTB exhibited very similar results. pKa values were found as 9.68 and 9.74 for RTIL-I and II, respectively. Given pKa values were calculated by using non-linear fitting algorithm of Gauss–Newton–Marquardt

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Fig. 2. pH induced absorption spectra of the BTB in RTIL-II after titration with 0.5 M HCl in the pH range of 8.7–10.49.

method, pKa = pH + log




Ax − A b Aa − A x



where Aa and Ab are the absorbance intensities of acidic and basic forms and Ax is the intensity at a pH near to the midpoint [16]. The calculated pKa values reveal that the BTB− /TOA+ ion pair can be used as an absorption based CO2 indicator when doped into RTILs. 3.2. Dissolved CO2 sensing properties How the RTIL based sensing cocktail may work as a colorimetric sensor for CO2 can be explained by the following equilibrium reaction. BTB− (color A) + xH2 O + CO2(g) ↔ HCO3 − + (x − 1)H2 O + BTBH (color B)

Fig. 3. Plot of absorbance intensities vs. measured pH in the pH range of 8.72–10.49 in RTIL-II.

Fig. 4. Absorption spectra of BTB− /TOA+ ion pair in (I) EtOH and (II) in RTILI after addition of HCO3 − solutions in the concentration range of 2 × 10−6 to 2 × 10−2 mol L−1 . (a) 0; (b) 2 × 10−6 ; (c) 2 × 10−5 ; (d) 2 × 10−4 ; (e) 2 × 10−3 ; (f) 2 × 10−2 mol L−1 .

Here, color A is the color of the unprotonated form of the bromothymol blue in RTILs which is blue (turquoise) and color B is the color of the protonated form which is greenish yellow. For the preparation of the modified sensing agent (BTB− /TOA+ ) stoichiometric amounts of tetraoctylammonium bromide (in dichloromethane) and bromothymol blue solution (in water solution of 1% Na2 CO3 ) were vigorously shaken in a separatory funnel for 5 min, extracted, and the organic phase was washed three times with 3 mL portions of a 0.1 M solution of NaOH. Finally, the ion pair was treated in rotary evaporator as reported by Weigl and Wolfbeis [3b]. In order to evaluate the effect of the RTIL matrix material to the dye performance, we recorded the absorption spectra of the BTB− /TOA+ ion pair both in ethanol and RTILs. From EtOH to RTIL, the blue shift at 420 nm has disappeared in both of the RTILs. The absorption maximum around 420 nm shifted to 414 nm, exhibiting a 6 nm blue shift. The absorption maximum around 625 nm exhibited a slight red shift and appeared at 628 nm. Fig. 4 shows the absorption spectra of the BTB− /TOA+ ion pair in EtOH in comparison with in RTIL-I after addition

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Fig. 5. Sensor calibration curve in the concentration range of 2 × 10−6 to 2 × 10−2 mol L−1 [HCO3 − ], λabs max = 628 nm.

of HCO3 − solutions in the concentration range of 2 × 10−6 to 2 × 10−2 mol L−1 . The relative signal changes of absorption spectra of the cocktails 1–6 were monitored after addition of certain concentrations of HCO3 − solutions. The cocktail 1 (C-1), which contained 1methyl-3-butylimidazolium tetrafluoroborate, exhibited the best response to different concentrations of HCO3 − solutions in the direction of an increase in signal intensity at 628 nm. Due to the presence of the isobestic point at 512 nm, BTB− /TOA+ ion pair allows ratiometric measurements in RTIL-I. However, the isobestic point becomes unclear in RTIL -II. The linearized calibration curve of the sensor composition C-1 is shown in Fig. 5. The calibration plot of the sensor can be described by the equation of y = −0.1818x + 1.2534 and the correlation coefficient of 0.9984. The dynamic working range is logarithmic and covers the concentration range of 1 × 10−6 to 2 × 10−2 mol L−1 [HCO3 − ]. In most of the sensor designs, TOAOH was added into the sensing cocktail as a counter ion in order to increase the sensitivity and lifetime, and an enhancement has been reported due to the presence of TOAOH [2]. However, we could not observe any significant enhancement in sensor response after addition of HCO3 − solutions in the TOAOH containing cocktail compositions. In our case, the matrix material was basic enough, the initial pH of the matrix was around pH = 10.45, and the dye was completely in the deprotonated form. Besides the proper initial pH value, the water miscible RTILs also allow the uptake of water molecules and the direct addition of HCO3 − solutions into the sensing medium.

299

Fig. 6. Absorption spectrum in RTIL-I after exposure to certain CO2 partial pressures: (a) 0.0; (b) 20%; (c) 40%; (d) 60%; (e) 80%; (f) 100% PCO2 .

ality of the sensor, we blew CO2 gas through the water during the gas-phase studies. The sensor compositions exhibited relative signal changes around 60% (61–54%) in the direction of a decrease in absorption intensities at λabs max = 628 nm (RTIL-I) and λabs max = 636 nm (RTIL-II) upon exposure to CO2 . The long wavelength absorption band of the BTB− /TOA+ ion pair exhibited a slight red shift (8 nm) from RTIL-I to II. Except the small red shift, both of the ionic liquids exhibited similar response to gaseous CO2 . The best results have been obtained with cocktail compositions of C-1 and C-5. 3.4. Response, recovery and stability characteristics The response and recovery related data of sensing cocktails were acquired by recording the absorbance intensity signals while exposing to 100% CO2 and 100% N2 , respectively. The τ 90 response times in-RTIL-I and II were 60 and 68 s, respectively. In the concentration range of 0.0–100% PCO2 , in the RTIL-I containing cocktails the regeneration could not be succeeded more than 67%. However, the initial signal intensities of the cocktails could be completely recovered after switching in the

3.3. Gas phase studies The representative absorption spectrum of Cocktail 1 as a function of PCO2 and related calibration graph are illustrated in Figs. 6 and 7, respectively. (A0 − A)/A0 is the intensity ratio of protonated (A) and deprotonated (A0 ) forms of the indicator. The linearized calibration plot of the sensor composition (C-1) can be described by the equation of y = 0.0034x + 0.2855 and the correlation coefficient of R2 = 0.9645. Since hydration of CO2 and subsequent protolysis is essential for the function-

Fig. 7. Linearized calibration curve of the sensor composition C-1 after exposure to CO2 in the partial pressure range of 0.0–100% PCO2 .

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ILs can be used repeatedly instead of solvents. It should be remembered that our municipal water treatment facilities are not designed for removing of synthetic chemicals or solvents and typically only consist of sand bed filtration and disinfection. As discussed earlier we indicated that the RTIL-I and RTIL-II could be effectively used as alternative matrix materials to conventional solvents and solid matrices in absorption based studies. ILs combine the attractive physical properties of solid and liquid phases and can be useful in construction of the inexpensive and field available reservoir type sensor design. The compatibility of the sensor composition(s) with the solid-state optical components (in particular, LEDs and fiber optics) also makes them promising optical sensor matrix materials. Fig. 8. The reproducibility of sensor (C-4) after six times exposure to carbon dioxide in the partial pressure range of 0–100% PCO2 .

concentration range of 0.0–80% PCO2 . In the RTIL-II containing cocktails, regeneration performance was better than that of the RTIL-I containing ones. It was fully reversible on going from 100% CO2 to 100% N2 , and no hysterisis was observed during the experiments. The τ 90 response times of the sensor compositions were 60 and 126 s for C-4, C-6 and C-1, C-2, C-3, C-5, respectively. The complete recovery time was 240 s. When the cocktail solution was stored in sealed brown bottles away from sunlight over a period of 1 month, no drift in the sensitivity was observed. Fig. 8 exhibits the response behaviour of the C-4 in the partial pressure range of 0.0–100% PCO2 . At each time the reagent phase was regenerated with pure gaseous nitrogen (99.99%). Between 1st and 6th cycles, the level of reproducibility of upper signal levels was 0.4352 ± 0.014 (n = 6). The average regeneration time under batch conditions was about 2.5 min for all compositions. The detection limits (the concentration of the PCO2 or [HCO3 − ] giving a signal equal to the blank signal, plus three standard deviation of the blank) were 1.4% for gaseous and 10−6 M [HCO3 − ] for dissolved CO2. The independence of the cocktail compositions from pH or other acidogenic species like H2 S was not tested. These are preliminary results and our efforts are focused on production of a Teflon membrane separated sensor design. The compatibility of the sensor composition with the solidstate optical components (in particular, LEDs) can be useful in construction of inexpensive and field available instrumentation. Our work along these lines but on different dyes is in progress. 4. Conclusion Ionic liquids (ILs) are discussed as green solvents. Several unique properties of the ILs, mainly their negligible vapour pressure, reversible CO2 solubility and selectivity, capability of dye dissolution, water-miscible characteristic and optical transparency, make them promising matrix materials for CO2 sensor design. The lack of vapour pressure makes the ILs highly attractive for laboratory workers during gas processing. Additionally, due to their reversible CO2 adsorption–desorption property,

Acknowledgements Funding this research was provided by the TUBITAK— (Kariyer Project—104M268) and Scientific Research Funds of Dokuz Eylul University (04 kb Fen 104). References [1] Y. Amao, N. Nakamura, Optical CO2 sensor with the combination of colorimetric change of ␣-naphtholphthalein and internal reference fluorescent porphyrin dye, Sens. Actuators B: Chem. 100 (2004) 347–351. [2] B. M¨uller, P.C. Hauser, A fluorescence optical sensor for low concentrations of dissolved carbon dioxide, Analyst 121 (1996) 339–343. [3] (a) G. Neurauter, I. Klimant, O.S. Wolfbeis, Microsecond lifetime-based optical carbon dioxide sensor using luminescence resonance energy transfer, Anal. Chim. Acta 382 (1–2) (1999) 67–75; (b) B.H. Weigl, O.S. Wolfbeis, New hydrophobic materials for optical carbon dioxide sensors based on ion pairing, Anal. Chim. Acta 302 (1995) 249–254. [4] K. Ertekin, I. Klimant, G. Neurauter, O.S. Wolfbeis, Chracterization of a rezervoir-type capillary optical microsensor for pCO2 measurements, Talanta 59 (2003) 261–267. [5] X. He, G.A. Rechnitz, Linear response function for fluorescence-based fiber-optic CO2 sensors, Anal. Chem. 67 (1995) 2264–2268. [6] Y.G. Lee, T.C. Chou, Ionic liquid ethanol sensor, Biosens. Bioelectron. 20 (2004) 33–40. [7] C. Cadena, J.L. Anthony, J.K. Shah, T.I. Morrow, J.F. Brennecke, E.J. Maginn, Why is CO2 so soluble in imidazolium based ionic liquids? Am. Chem. Soc. 126 (2004) 5300–5308. [8] L.A. Blanchard, Z. Gu, J.F. Brennecke, High-pressure phase behaviour of ionic liquid/CO2 systems, J. Phys. Chem. B 105 (2001) 2437–2444. [9] J.L. Anthony, E.J. Maginn, J.F. Brennecke, Solubilities and thermodynamic properties of gases in the ionic liquid 1-n-butyl-3methylimidazolium hexafluorophosphate, J. Phys. Chem. B 106 (2002) 7315–7320. [10] E.D. Bates, R.D. Mayton, I. Ntai, J.H. Davis, CO2 capture by a taskspecific ionic liquid, J. Am. Chem. Soc. 124 (2002) 926–927. [11] A.P.S. Kamps, D. Tuma, J. Xia, G. Maurer, Solubility of CO2 in the ionic liquid [bmim][PF6 ], Chem. Eng. Data 48 (2003) 746–749. [12] D. Camper, P. Scovazzo, C. Koval, R. Noble, Gas solubilities in room temperature ionic liquids, Ind. Eng. Chem. Res. 43 (2004) 3049–3054. [13] (a) J. Fuller, R.T. Carlin, H.C. Delong, D. Haworth, The properties of ionic liquids, Chem. Commun. (1994) 299–300; (b) P. Bonhote, A.P. Dias, M. Armand, N. Papageorgiou, M. Gratzel, Hydrophobic, highly conductive ambient-temperature molten salts, Inorg. Chem. 35 (1996) 1168–1178; (c) T. Welton, Room temperature ionic liquids, solvents for synthesis and catalysis, Chem. Rev. 99 (1998) 2071–2083;

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Biographies Ozlem Oter has a BSc in chemistry and a MSc in analytical chemistry from Dokuz Eylul University, Izmir, Turkey. Her current research interests

301

include development of new optical sensors for pH, CO2 , O2 and some cations. Kadriye Ertekin has a BSc in chemistry, a MSc and PhD in analytical chemistry from Ege University, Izmir, Turkey. She works as an assistant professor in Dokuz Eylul University. Her current research interests include photo-characterization of newly synthesized dyes, optical sensors for pH, CO2 , O2 and cations. Derya Topkaya has a BSc in chemistry from Ondokuz Mayis University and a MSc in organic chemistry from Dokuz Eylul University. Her research interests include synthesis and photo-characterization of pH sensitive fluorescent dyes and ionic liquids for CO2 sensing. Serap Alp has a BSc in chemistry, a MSc and PhD in organic chemistry from Ege University, Izmir, Turkey. She works as an associated professor in Dokuz Eylul University. Her current research interests include synthesis and photo-characterization of fluorescent dyes, ionic liquids and development of optical sensors for pH, CO2 , O2 and cations.