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Quantitative determination of anionic surfactants using optical sensors with microtiter plate reader
Sensors and Actuators B 203 (2014) 181–186
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Quantitative determination of anionic surfactants using optical sensors with microtiter plate reader Nedime Dürüst ∗ , Sibel Nac¸, Nazangül Ünal Department of Chemistry, Abant Izzet Baysal University, Gölköy, Bolu 14280, Turkey
a r t i c l e
i n f o
Article history: Received 10 May 2014 Received in revised form 23 June 2014 Accepted 24 June 2014 Available online 1 July 2014 Keywords: Anionic surfactants Membrane sensors Microtiter plate reader Scatchard plot
a b s t r a c t In this study, it is shown that anionic surfactants can be readily determined by using polymer film-based optodes containing an appropriate chromoionophore. The optodes are prepared with poly(vinyl chloride) (PVC), polyurethane (PU), bis(2-ethylhexyl) sebacate (DOS), and 2 ,7 -dichlorofluorescein octadecylester (DCFOE). These optodes exhibit reproducible and sensitive absorbance changes in response to the varying titrant concentrations. Three different cationic surfactants, cetyltrimethylammonium bromide (CTAB), benzyldimethyltetradecylammonium chloride (zephiramine, Zeph), and benzyldimethylhexadecylammonium chloride (CDB), are employed as titrants to determine anionic surfactant concentrations. In addition, membrane electrodes were also employed for estimating the interactions between the cationic and anionic species. The reaction stoichiometries are compared based on the electrode measurements and Scatchard plots are prepared to estimate the strength of binding interaction of titrants with the target surfactants. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Surfactants are substances with molecular structures consisting of a hydrophilic and a hydrophobic part and are widely used in various industries, in agriculture, in the plastics industry and in clinical laboratories. Different types of surfactants are added to laundry and cleaning detergents, cosmetics, food, paints, pesticides and petroleum products. As the everyday production and use of surfactants increases, it has become necessary to monitor their levels and impact on different parts of the environment [1]. Most of these compounds are directly discharged into the environment, causing troublesome environmental problems. Therefore, various methods, such as voltammetry [2], spectrophotometry [3,4], and flow-injection spectrophotometry [5,6] flow Injection fluorometry [7] have been reported for the determination of anionic surfactants. Potentiometric sensors were also used previously [8] for determination of these kind of compounds. In recent years, the chemistry employed in the sensing membranes of these type of electrodes have also been adapted to develop new optical polyion-sensitive sensors and numerous polyanions were determined [9,10]. These optical sensors were developed using plasticized poly(vinyl chloride) films by casting on glass plates. It has been reported that
∗ Corresponding author. Tel.: +90 3742541000. E-mail address: durust [email protected] (N. Dürüst). http://dx.doi.org/10.1016/j.snb.2014.06.099 0925-4005/© 2014 Elsevier B.V. All rights reserved.
an anion exchanger and a pH-sensitive dye can be used in the plasticized poly(vinyl chloride) films to detect polyanions [9–13]. Chan et al. prepared an optode containing 1,2-benzo-7(diethylamino)-3-(octadecanoylimino) phenoxazine (ETH 5294) as a chromoionophore and they determined some anionic surfactants with such films [14]. In this paper, we report the quantitative determination of several anionic surfactants using 2 ,7 -dichlorofluorescein octadecylester (DCFOE)-based polymeric film optodes. We use sensing film-coated wells of microtiter plates and an absorbance reader in this work, since plate has 96 wells, and thus in comparison with a routine spectrophotometric analysis much higher sample throughput can be achieved using only 300 L of sample. We also report the mass ratios between the target anionic surfactants and cationic titrants. In addition, membrane electrodes based on an ion-exchanger are also used to estimate the binding interactions between target anionic surfactants and titrant surfactants.
2. Experimental 2.1. Chemicals and apparatus Bis(2-ethylhexyl) sebacate(DOS), high-molecular weight poly(vinyl chloride) (PVC, 81387), polyurethane (PU), 2 ,7 dichlorofluorescein (DCF), 1-iodooctadecane, tetrahydrofuran (THF), TRIS [tris(hydroxymethyl) aminomethane], sodium
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dodecylbenzenesulfonate (NaDBS), potassium polyvinylsulfate (PVSK), benzyldimethyltetradecyl ammonium chloride (zephiramine, Zeph), cetyltrimethylammonium bromide (CTAB), chloride (CDB), and 2cetyldimethyllbenzylammonium nitrophenyloctyl ether (NPOE), tetrahydrofuran (THF), were purchased from Sigma-Aldrich Chemical Co. Sodium dodecylsulfate (NaDS) was obtained from Calbio Chem. Calcium dinonylnaphthalene sulfonate (DNNS) was a kind gift from King Industries (Norwalk, CT). 2 ,7 -Dichlorofluorescein octadecylester (DCFOE) was synthesized as described by Wang et al. [12]. The buffer solution used in all experiments was 50 mM Tris–HCl, pH 7.4, containing 120 mM NaCl. Other reagents were analytical grade. All solutions were prepared using 0.055 S cm−1 deionized water produced by a Tka Smart 2 Pure water purification system. A UV–vis double-beam spectrophotometer (Hitachi U-2900) was used for preliminary spectrophotometric studies. All titrations were performed by using a microtiter plate absorbance reader (Molecular Devices Versamax Model). A Thermo Scientific Orion 4 Star pH/ISE meter was used to adjust the pH of buffer solutions. 2.2. Preparation of polymeric film sensors and titration Film optodes were prepared as previously described [12,13]. The film composition was 1 wt% chromoionophore (DCFOE), 30 wt% polymer (15 wt% PVC + 15 wt% PU), and 69 wt% DOS (as plasticizer). The cocktail solution was prepared by dissolving a total amount of 200 mg of film components in 2 mL of freshly distilled THF. This solution was then uniformly dispensed (10 L/well) into each Ubottomed microwell of microtiter plates by means of a multipette dispenser with combitips. The thickness of the membranes in the wells was ca. 20 m. Microtiter plates used were polypropylenebased (Sigma M4404). The microtiter plate films were dried in a dust-free environment for 1 day. Lifetimes of the plates are at least one week. All the wells of microtiter plates were filled in with 300 L buffer (Tris–HCl, pH 7.4) and equilibrated in buffer until the polymer film-coated wells had a stable absorbance value. This waiting time was at least 20 min. Absorbance measurements of optode films in buffer were made at 540 and 620 nm. Titrant solutions were prepared from their stock solution by appropriate dilution. Titration of each surfactant anion was carried out by adding given aliquots of the cationic standard solutions to a series of separate eppendorf plastic tubes containing a fixed volume of a given anionic surfactant solution. Each analyte–titrant mixture in plastic tubes were prepared as 1000 L total volume, so that the final mixture can be assayed using three different wells of the microtiter plate. Before these solutions were transferred to the wells, they were mixed thoroughly (for 3–5 min). Then, 300 L aliquots of the cationic titrant-target anionic surfactant solutions were transferred into each well of a microtiter plate coated with the polycationsensitive film optode to determine the unbound cationic titrant level. After all the wells were filled in with these solution mixtures, they were incubated for at least 3 min. Then wells were emptied and filled in with plain buffer solution again and the absorbances were recorded in buffer solution at same wavelengths. All measurements and pre-equilibrations were carried out at ambient temperature (∼25 ◦ C). Using the method outlined above, each of the target anionic surfactants (Sodium dodecylsulfate, NaDS; sodium dodecylbenzenesulfonate, NaDBS; and potassium poly(vinyl-sulfate) (PVSK), were titrated by using microtiter plate format optodes. The spectrophotometric titration curves were prepared by plotting the total absorbance change from the baseline value (A) vs. the concentration of the cationic titrant solutions (i.e., zephiramine, Zeph; cetyltrimethylammonium bromide, CTAB; and cetyldimethyllbenzyl-ammonium chloride, CDB). The proportional titration curves were obtained for various concentrations of all
target surfactants using these three different cationic titrants. The mass ratios between the three target surfactants and three cationic titrants were estimated from the end-points of the titration curves. Film electrodes were also prepared [15]. Concisely, the membrane composition for electrodes was 1 wt% DNNS(ion-exchanger dinonylnaphthalene sulfonate), 49.5 wt% NPOE (2-nitrophenyloctyl ether), 49.5 wt% M48 (polyurethane). All membranes were cast from a cocktail solution containing a total of 1 g of the components dissolved in 8 mL THF. Electrodes were prepared by dip coating these solutions onto the end of rounded glass rods protruding out of a narrow-bore Tygon tube and dried overnight. The electrodes were used for estimating the binding interaction between cationic and anionic surfactants. They were also used to make a comparison with the reaction stoichiometries obtained using the microtiter plate optical sensing films. 2.3. Sample analyses Several real samples containing target surfactants were analyzed. Known weights of the samples (100–200 mg) were dissolved completely in 10 mL of ultrapure deionized water and titrated with the polycationic titrants using the DCFOE-based microtiter plate-format optodes as the endpoint detection system. Anionic surfactant concentrations present in samples were found by using the endpoints of the titration curves and stoichiometries calculated from titration curves of standard surfactant solutions. Recovery studies were also carried out in the samples containing the target anionic surfactants. The ability of this method to detect the anions in the samples was assessed by spiking the sample with varying levels of the target compounds. For recovery studies, two different samples were tested by using Zeph, and CTAB as titrants. 3. Results and discussion 3.1. Response of film to cationic titrants The DCFOE-based PU/PVC membrane film itself has a maximal absorbance at ∼465 nm at pH 7.40 but this value shifts to longer wavelengths, in the range of 530–540 nm, in the presence of the cationic species used here as titrants. All preliminary experiments were carried out using the UV–vis spectrophotometer and then all absorbance changes for analytical titrations were monitored at over this wavelength range using the microtiter plate reader employing an optical filter covering this range of wavelengths. Fig. 1 shows the maximum absorption shift of a polymeric sensing film in the presence and absence of the titrant zephiramine. The colorless cationic titrant extracts into the film converts the initial yellow color of the dye of film to pink, as it displaces the proton. The continuous extraction of the lipophilic titrant continues and results in an increase in the absorbance values at 540 nm. The color change that is observed here within microplate wells coated with the polymer sensing film is shown visually in Fig. 2. 3.2. Titration of anionic surfactants in the buffer solutions The anionic surfactants were determined by using the DCFOEbased film optodes with the titrants Zeph, CTAB, and CDB. Three of the typical spectrophotometric titration curves obtained for varying levels of NaDS, NaDBS, and PVSK in the buffered saline are shown in Figs. 3–5, respectively. End-points which are proportional to the concentration of each analyte were obtained from the titration curves. The reaction stoichiometries between the analytes and the titrants were obtained from the end-points and are summarized in Table 1.
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0.14 0.12 0.1
∆A
0.08 0.06 0.04 0.02 0
Fig. 1. Absorption spectra of the film optode obtained from UV–vis spectrophotometer before (left) and after (right) exposed to 500 g mL−1 of Zeph. at pH 7.4.
20
0
40
60
80
100
120
140
CTAB (µg/mL) Fig. 4. Proportional titration curves of 0 ( ), 5 ( ), 10 ( ), 20 ( ), and 40 ( ) g mL−1 of NaDBS with titrant CTAB. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
3.3. Analysis of some samples containing anionic surfactants
Fig. 2. View of the microplate wells after the addition of cationic titrant that converts the initial yellow color of film to pink. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
After having been obtained reproducible results for each target anionic surfactants in the buffered solutions, efforts were made to assess the ability to determine certain target anion surfactants in real samples using the titrimetric approach with optical detection of the cation/anion end-point using the microtiter plate arrangement. Solutions of samples containing surfactants were prepared at given concentrations (100–200 mg sample/10 mL) by mixing well and filtering under vacuum. These sample solutions were then titrated with the different cationic titrants. Varying anionic surfactant concentrations were calculated from the endpoint break of the titration curves. Ten different product samples
0,2
0.09
0.07
0,15
∆A
∆A
0.05
0,1
0.03 0 µg/mL PVSK
0,05
10 µg/mL PVSK
0.01
20 µg/mL PVSK 40 µg/mL PVSK
0
-0.01 0
20
40
60
80
100
120
CDB (µg/mL) Fig. 3. Proportional titration curves of 0 ( ), 5 ( ), 10 ( ), 20 ( ), and 40 ( ) g mL−1 of NaDS with titrant CDB. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
0
10
20
30
40
50
60
70
Zephiramine (µg/mL) Fig. 5. Proportional titration curves of 0 ( ), 10 ( ), 20 ( ), and 40 ( ) g mL−1 of PVSK with titrant zephiramine. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Table 1 Spectrophotometric titration data for different levels of anionic surfactants using cationic surfactant as titrant with lipophilic cation-sensitive film optodes. Titrant (Zeph) NaDBS (g)
Neutralization end point* (g)
Mass ratio (Zeph/NaDBS)
5 10 20 40
4 8 18 37
0.80 0.80 0.90 0.93 Average: 0.86 (±0.07)
NaDS (g)
Neutralization end point* (g)
Mass ratio (Zeph/NaDS)
5 10 20 40
6 13 26 53
1.20 1.30 1.30 1.33 Average: 1.28 (±0.06)
PVSK (g)
Neutralization end point* (g)
Mass ratio (Zeph/PVSK)
10 20 40
6 10 23
0.60 0.50 0.58 Average: 0.56 (±0.05)
NaDBS (g)
Neutralization end point* (g)
Mass ratio (CTAB/NaDBS)
5 10 20 40
4 10 21 41
0.80 1.00 1.05 1.03 Average: 0.97 (±0.11)
NaDS (g)
Neutralization end point* (g)
Mass ratio (CTAB/NaDS)
5 10 20 40
6 12 24 47
1.20 1.20 1.20 1.18 Average: 1.20 (±0.01)
NaDBS (g)
Neutralization end point* (g)
Mass ratio (CDB/NaDBS)
5 10 20 40
4 9 18 40
0.80 0.90 0.90 1.00 Average: 0.90 (±0.08)
NaDS (g)
Neutralization end point* (g)
Mass ratio (CDB/NaDS)
5 10 20 40
6.5 12 25 50
1.30 1.20 1.25 1.25 Average: 1.25 (±0.04)
Titrant (Zeph)
Titrant (Zeph)
Titrant (CTAB)
Titrant (CTAB)
Table 2 Total surfactant concentration levels measured in some cleaning products using film optodes. Sample analyzed
Total surfactant, as NaDS (% m m−1 )*
Hair shampoo 1 Liquid soap for hand Washing liquid 1(for dishes) Hair shampoo 2 (different brand) Shower gel 1 Washing liquid 2(for dishes) (different brand) Face cleaning gel Surface cleaner Toothpaste Shower gel 2 (different brand)
5.86 (±0.39) 6.84 (±0.59) 5.47 (±0.30) 11.10 (±0.47) 8.98 (±1.03) 17.19 (±0.78) 3.75 (±0.42) 5.27 (±0.23) 5.00 (±0.58) 6.67 (±0.87)
* The amount of analyte (g) in 100 g sample. Average of at least three measurements. All experiments were carried out in 50 mM Tris–HCl, pH 7.4, containing 120 mM NaCl.
cation-sensitive DCFOE-based microtiter plate-format optodes. The results are expressed as % m m−1 (g anionic surfactant/100 g sample) and are shown in Table 2. The representative graphs related to the titration of the hair shampoo-1, the face cleaning gel, and the washing liquid-1 (for dishes) are presented in Figs. 6–8, respectively. In order to investigate whether the sample matrix would have an effect on the determination, we spiked some real samples as well. The accuracy of the method was confirmed by these recovery experiments. For this purpose, the proportional amounts of NaDS were added to the solutions of these samples. Two of the studied samples, shower gel 1 and shower gel 2, containing anionic surfactant were examined. For example, titration curves of recovery studies with Zeph for the sample mentioned above are shown in Fig. 9. The data obtained for the sample tested are in good agreement with the proportional amounts added of surfactant. The results obtained show average polyanion recoveries of 96.3 (±1.8) %, and 101.0 (±3.6) %, for shower gel 1 (titrated with zeph), and for shower gel 2 (titrated with CTAB) samples, respectively. The recovery of anionic surfactants added to some real samples is quantitative. In this regard, we can conclude that the effect of the sample matrix is negligible based on the high recovery percentages.
Titrant (CDB)
0.25 0.2
Titrant (CDB)
* Each neutralization end-point is the average values of six measurements with standard deviations given in parentheses. The proposed method provides good detection limits (around 1 ppm) for all the anionic surfactants.
were tested; hair shampoo (1), hair shampoo (2), shower gel (1), shower gel (2), liquid soap for hands, washing liquid (1) (for dishes), washing liquid (2) (for dishes), face cleaning gel, surface cleaner, and toothpaste were purchased from the local markets and were tested. Total surfactant amounts in these samples were measured by the spectrophotometric titration method using the
∆A
0.15 0.1 0.05 0 0
20
40
60
Zephiramine (µg/mL) Fig. 6. A typical spectrophotometric titration curve for hair shampoo 1 containing surfactant: blank ( ), and 200 g mL−1 sample ( ). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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185
0.3 0.25
∆A
0.2 0.15 0.1
0
µg/mL
200 µg/mL Sample + 10 µg/mL NaDS
0.05
+ 20 µg/mL NaDS + 40 µg/mL NaDS
0 0
30
60
90
120
150
Zephiramine (µg/mL)
3.4. Binding interactions between cationic and anionic species Polymeric membrane electrodes based on the ion-exchanger were also used for examining the interaction between cationic and anionic surfactants by using Scatchard Plot. To conduct such measurements, polymeric membrane electrodes were used [16]. The electrode was first calibrated by adding increasing concentrations of cationic surfactant to buffer solution. Then the potentiometric titrations of anionic surfactants were performed by adding increasing concentrations of cationic surfactant (1 mg mL−1 ) to buffer solution containing a fixed amount of anionic species. The free cationic surfactant concentration (F) for each data point on the titration curve was determined graphically from the previously prepared calibration curve. The bound cationic surfactant
Fig. 9. Spectrophotometric titration curves obtained from recovery studies of shower gel 1 containing surfactant spiked with known amounts of NaDS.
A
0.35 0.3 0.25 0.2
∆A
Fig. 7. A typical spectrophotometric titration curve for face cleaning gel containing surfactant: blank ( ), and 200 g mL−1 sample ( ). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
0.15
0 µg/mL NaDBS 5 µg/mL NaDBS
0.1
10 µg/mL NaDBS
0.05
20 µg/mL NaDBS 40 µg/mL NaDBS
0 0
0.4
20
40
60
80 100 120 140 160 180
Zephiramine (µg/mL)
B 400
300
0.2
ΔmV
∆A
0.3
200
0 µg/mL NaDBS 10 µg/mL NaDBS
0.1
20 µg/mL NaDBS
100
2,5 µg/mL NaDBS 5 µg/mL NaDBS
0 0
20
40
60
80
100
120
Zephiramine (µg/mL) Fig. 8. A typical spectrophotometric titration curve for washing liquid 1(for dishes) containing surfactant: blank ( ), and 400 g mL−1 sample ( ). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
0 0
4
8
12 16
20 24
28 32
36 40
Zephiramine (µg/mL) Fig. 10. Proportional titration curves of NaDBS with titrant zephiramine by using optical sensors (A) and potentiometrical sensors (B).
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200000
dodecylsulfate (NaDS) and sodium dodecylbenzenesulfonate (NaDBS) via a simple titrimetric approach. Microtiter plates and readers are essential equipment in all life sciences and analytical laboratories, and since the microtiter plate-format optodes can provide high sample throughput (96 samples in less than 5 min), this new method should be attractive to many researchers and quality control chemists who need to measure levels of anionic surfactant species. In addition, relatively rapid titrations can be done by means of the MPOs, and hence, they are very appropriate for the routine analysis. The new method also provides good detection limits (around 1 ppm) for all the anionic surfactant species tested.
y = -1E+06x + 724043
160000
B'/F
120000
80000
Acknowledgements
40000
0,74
0,7
0,66
0,62
0,58
0,54
0,5
0
We are extremely grateful to the TÜBI˙ TAK (The Scientific and Technological Research Council of Turkey, Grant 112T708) and Abant Izzet Baysal University, Directorate of Research Projects Commission (BAP Grant 2010.03.03.343) for financial support.
B' Fig. 11. Typical Scatchard plot for the binding of NaDBS to zephiramine. B and B /F values were calculated from the potentiometric titration data of blank and 10 g mL−1 NaDBS.
concentration (B) was calculated by subtracting F from the total cationic surfactant concentration added. The cation-anion binding constant (Keq ) was estimated by recasting the data in the form of a Scatchard plot of B’ versus B’/F (B’, the moles of bound titrant per mole of anionic surfactant). The reaction stoichiometries were also compared by using the electrode method. As shown in Fig. 10, it can be clearly seen from the curves that the reaction stoichiometries obtained from the titrations conducted with the both type of sensors (optical and electrochemical) are in good accord with each other. For example, as can be seen for Zeph/NaDBS in Fig. 10, the mass ratio values were found to be 0.86 (±0.07) and 0.70 (±0.02) for the spectrophotometric and potentiometric methods, respectively. Fig. 10A and B shows the titration curves obtained by using both types of sensors. Fig. 11 shows the Scatchard plot for the binding of NaDBS to zephiramine using the potentiometric data. The slope of the line is the negative value of the equilibrium constant (Keq ) betweeen analyte and titrant. Keq is depicted as y = mx + b on the plot. Since Keq is large (106 ) the reaction between titrant and analyte is in favor of products and this is quite available for titration. 4. Conclusion In summary, in this study, we clearly demonstrate the potential analytical applications of microtiter plate-formate optodes (MPOs) for the measurement of anionic surfactants such as sodium
References [1] E. Olkowska, Z. Polkowska, J. Namiesnik, Talanta 88 (2012) 1–13. [2] M.J. Spencer, G.G. Wallace, P.T. Crisp, T.W. Lewis, Anal. Chim. Acta 244 (1991) 197–200. [3] D. Gorenc, B. Adam, B. Gorenc, Microchim. Acta 103 (1991) 311–315. [4] I. Kasahara, K. Hashimoto, T. Kawabe, A. Kunita, K. Magawa, N. Hata, S. Taguchi, K. Goto, Analyst 120 (1) (1995) 1803–1807. [5] Q. He, H. Chen, Fresenius J. Anal. Chem. 367 (2000) 270–274. [6] T. Masadome, Anal. Lett. 34 (2001) 2711–2719. [7] T. Masadome, Anal. Lett. 31 (1998) 1071–1079. [8] J.M. Esson, N. Ramamurthy, M.E. Meyerhoff, Anal. Chim. Acta 404 (2000) 83–94. [9] S. Dai, Q. Ye, E. Wang, M.E. Meyerhoff, Anal. Chem. 72 (2000) 3142–3149. [10] N. Dürüst, M.E. Meyerhoff, N. Ünal, S. Nac, Anal. Chim. Acta 699 (2011) 107–112. [11] E. Wang, M.E. Meyerhoff, V.C. Yang, Anal. Chem. 67 (1995) 522–527. [12] E. Wang, G. Wang, L. Ma, C.M. Stivanello, S. Lam, H. Patel, Anal. Chim. Acta 334 (1996) 139–147. [13] S.B. Kim, H.C. Cho, G.S. Cha, H. Nam, Anal. Chem. 70 (1998) 4860–4863. [14] W.H. Chan, A.W.M. Lee, J.Z. Lu, Anal. Chim. Acta 361 (1998) 55–61. [15] N. Dürüst, M.E. Meyerhoff, J. Electroanal. Chem. 602 (2007) 138–141. [16] N. Dürüst, M.E. Meyerhoff, Anal. Chim. Acta 432 (2001) 253–260.
Biographies Nedime Dürüst is currently working as a Professor of Analytical Chemistry in Abant Izzet Baysal University, Bolu, Turkey. She is the Head of Chemistry Department. Her research activities include the electrochemical and optical chemical sensors, the investigation of acidity constants of new synthesized heterocyclic compounds, phytochemical analyses of polyphenolic compounds. Sibel Nac¸ received her B.Sc. degree from the University of Abant Izzet Baysal in Chemistry in 2008 and completed her M.Sc. degree on the optical sensors in Chemistry in 2010 at the same University. She currently works as a research chemist at a pharmaceutical company in Istanbul, Turkey. Nazangül Ünal received the B.Sc. degree in 2005 and M.Sc. degree in 2010 both in Chemistry from the University of Abant Izzet Baysal, Bolu, Turkey.
Contents lists available at ScienceDirect
Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Quantitative determination of anionic surfactants using optical sensors with microtiter plate reader Nedime Dürüst ∗ , Sibel Nac¸, Nazangül Ünal Department of Chemistry, Abant Izzet Baysal University, Gölköy, Bolu 14280, Turkey
a r t i c l e
i n f o
Article history: Received 10 May 2014 Received in revised form 23 June 2014 Accepted 24 June 2014 Available online 1 July 2014 Keywords: Anionic surfactants Membrane sensors Microtiter plate reader Scatchard plot
a b s t r a c t In this study, it is shown that anionic surfactants can be readily determined by using polymer film-based optodes containing an appropriate chromoionophore. The optodes are prepared with poly(vinyl chloride) (PVC), polyurethane (PU), bis(2-ethylhexyl) sebacate (DOS), and 2 ,7 -dichlorofluorescein octadecylester (DCFOE). These optodes exhibit reproducible and sensitive absorbance changes in response to the varying titrant concentrations. Three different cationic surfactants, cetyltrimethylammonium bromide (CTAB), benzyldimethyltetradecylammonium chloride (zephiramine, Zeph), and benzyldimethylhexadecylammonium chloride (CDB), are employed as titrants to determine anionic surfactant concentrations. In addition, membrane electrodes were also employed for estimating the interactions between the cationic and anionic species. The reaction stoichiometries are compared based on the electrode measurements and Scatchard plots are prepared to estimate the strength of binding interaction of titrants with the target surfactants. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Surfactants are substances with molecular structures consisting of a hydrophilic and a hydrophobic part and are widely used in various industries, in agriculture, in the plastics industry and in clinical laboratories. Different types of surfactants are added to laundry and cleaning detergents, cosmetics, food, paints, pesticides and petroleum products. As the everyday production and use of surfactants increases, it has become necessary to monitor their levels and impact on different parts of the environment [1]. Most of these compounds are directly discharged into the environment, causing troublesome environmental problems. Therefore, various methods, such as voltammetry [2], spectrophotometry [3,4], and flow-injection spectrophotometry [5,6] flow Injection fluorometry [7] have been reported for the determination of anionic surfactants. Potentiometric sensors were also used previously [8] for determination of these kind of compounds. In recent years, the chemistry employed in the sensing membranes of these type of electrodes have also been adapted to develop new optical polyion-sensitive sensors and numerous polyanions were determined [9,10]. These optical sensors were developed using plasticized poly(vinyl chloride) films by casting on glass plates. It has been reported that
∗ Corresponding author. Tel.: +90 3742541000. E-mail address: durust [email protected] (N. Dürüst). http://dx.doi.org/10.1016/j.snb.2014.06.099 0925-4005/© 2014 Elsevier B.V. All rights reserved.
an anion exchanger and a pH-sensitive dye can be used in the plasticized poly(vinyl chloride) films to detect polyanions [9–13]. Chan et al. prepared an optode containing 1,2-benzo-7(diethylamino)-3-(octadecanoylimino) phenoxazine (ETH 5294) as a chromoionophore and they determined some anionic surfactants with such films [14]. In this paper, we report the quantitative determination of several anionic surfactants using 2 ,7 -dichlorofluorescein octadecylester (DCFOE)-based polymeric film optodes. We use sensing film-coated wells of microtiter plates and an absorbance reader in this work, since plate has 96 wells, and thus in comparison with a routine spectrophotometric analysis much higher sample throughput can be achieved using only 300 L of sample. We also report the mass ratios between the target anionic surfactants and cationic titrants. In addition, membrane electrodes based on an ion-exchanger are also used to estimate the binding interactions between target anionic surfactants and titrant surfactants.
2. Experimental 2.1. Chemicals and apparatus Bis(2-ethylhexyl) sebacate(DOS), high-molecular weight poly(vinyl chloride) (PVC, 81387), polyurethane (PU), 2 ,7 dichlorofluorescein (DCF), 1-iodooctadecane, tetrahydrofuran (THF), TRIS [tris(hydroxymethyl) aminomethane], sodium
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dodecylbenzenesulfonate (NaDBS), potassium polyvinylsulfate (PVSK), benzyldimethyltetradecyl ammonium chloride (zephiramine, Zeph), cetyltrimethylammonium bromide (CTAB), chloride (CDB), and 2cetyldimethyllbenzylammonium nitrophenyloctyl ether (NPOE), tetrahydrofuran (THF), were purchased from Sigma-Aldrich Chemical Co. Sodium dodecylsulfate (NaDS) was obtained from Calbio Chem. Calcium dinonylnaphthalene sulfonate (DNNS) was a kind gift from King Industries (Norwalk, CT). 2 ,7 -Dichlorofluorescein octadecylester (DCFOE) was synthesized as described by Wang et al. [12]. The buffer solution used in all experiments was 50 mM Tris–HCl, pH 7.4, containing 120 mM NaCl. Other reagents were analytical grade. All solutions were prepared using 0.055 S cm−1 deionized water produced by a Tka Smart 2 Pure water purification system. A UV–vis double-beam spectrophotometer (Hitachi U-2900) was used for preliminary spectrophotometric studies. All titrations were performed by using a microtiter plate absorbance reader (Molecular Devices Versamax Model). A Thermo Scientific Orion 4 Star pH/ISE meter was used to adjust the pH of buffer solutions. 2.2. Preparation of polymeric film sensors and titration Film optodes were prepared as previously described [12,13]. The film composition was 1 wt% chromoionophore (DCFOE), 30 wt% polymer (15 wt% PVC + 15 wt% PU), and 69 wt% DOS (as plasticizer). The cocktail solution was prepared by dissolving a total amount of 200 mg of film components in 2 mL of freshly distilled THF. This solution was then uniformly dispensed (10 L/well) into each Ubottomed microwell of microtiter plates by means of a multipette dispenser with combitips. The thickness of the membranes in the wells was ca. 20 m. Microtiter plates used were polypropylenebased (Sigma M4404). The microtiter plate films were dried in a dust-free environment for 1 day. Lifetimes of the plates are at least one week. All the wells of microtiter plates were filled in with 300 L buffer (Tris–HCl, pH 7.4) and equilibrated in buffer until the polymer film-coated wells had a stable absorbance value. This waiting time was at least 20 min. Absorbance measurements of optode films in buffer were made at 540 and 620 nm. Titrant solutions were prepared from their stock solution by appropriate dilution. Titration of each surfactant anion was carried out by adding given aliquots of the cationic standard solutions to a series of separate eppendorf plastic tubes containing a fixed volume of a given anionic surfactant solution. Each analyte–titrant mixture in plastic tubes were prepared as 1000 L total volume, so that the final mixture can be assayed using three different wells of the microtiter plate. Before these solutions were transferred to the wells, they were mixed thoroughly (for 3–5 min). Then, 300 L aliquots of the cationic titrant-target anionic surfactant solutions were transferred into each well of a microtiter plate coated with the polycationsensitive film optode to determine the unbound cationic titrant level. After all the wells were filled in with these solution mixtures, they were incubated for at least 3 min. Then wells were emptied and filled in with plain buffer solution again and the absorbances were recorded in buffer solution at same wavelengths. All measurements and pre-equilibrations were carried out at ambient temperature (∼25 ◦ C). Using the method outlined above, each of the target anionic surfactants (Sodium dodecylsulfate, NaDS; sodium dodecylbenzenesulfonate, NaDBS; and potassium poly(vinyl-sulfate) (PVSK), were titrated by using microtiter plate format optodes. The spectrophotometric titration curves were prepared by plotting the total absorbance change from the baseline value (A) vs. the concentration of the cationic titrant solutions (i.e., zephiramine, Zeph; cetyltrimethylammonium bromide, CTAB; and cetyldimethyllbenzyl-ammonium chloride, CDB). The proportional titration curves were obtained for various concentrations of all
target surfactants using these three different cationic titrants. The mass ratios between the three target surfactants and three cationic titrants were estimated from the end-points of the titration curves. Film electrodes were also prepared [15]. Concisely, the membrane composition for electrodes was 1 wt% DNNS(ion-exchanger dinonylnaphthalene sulfonate), 49.5 wt% NPOE (2-nitrophenyloctyl ether), 49.5 wt% M48 (polyurethane). All membranes were cast from a cocktail solution containing a total of 1 g of the components dissolved in 8 mL THF. Electrodes were prepared by dip coating these solutions onto the end of rounded glass rods protruding out of a narrow-bore Tygon tube and dried overnight. The electrodes were used for estimating the binding interaction between cationic and anionic surfactants. They were also used to make a comparison with the reaction stoichiometries obtained using the microtiter plate optical sensing films. 2.3. Sample analyses Several real samples containing target surfactants were analyzed. Known weights of the samples (100–200 mg) were dissolved completely in 10 mL of ultrapure deionized water and titrated with the polycationic titrants using the DCFOE-based microtiter plate-format optodes as the endpoint detection system. Anionic surfactant concentrations present in samples were found by using the endpoints of the titration curves and stoichiometries calculated from titration curves of standard surfactant solutions. Recovery studies were also carried out in the samples containing the target anionic surfactants. The ability of this method to detect the anions in the samples was assessed by spiking the sample with varying levels of the target compounds. For recovery studies, two different samples were tested by using Zeph, and CTAB as titrants. 3. Results and discussion 3.1. Response of film to cationic titrants The DCFOE-based PU/PVC membrane film itself has a maximal absorbance at ∼465 nm at pH 7.40 but this value shifts to longer wavelengths, in the range of 530–540 nm, in the presence of the cationic species used here as titrants. All preliminary experiments were carried out using the UV–vis spectrophotometer and then all absorbance changes for analytical titrations were monitored at over this wavelength range using the microtiter plate reader employing an optical filter covering this range of wavelengths. Fig. 1 shows the maximum absorption shift of a polymeric sensing film in the presence and absence of the titrant zephiramine. The colorless cationic titrant extracts into the film converts the initial yellow color of the dye of film to pink, as it displaces the proton. The continuous extraction of the lipophilic titrant continues and results in an increase in the absorbance values at 540 nm. The color change that is observed here within microplate wells coated with the polymer sensing film is shown visually in Fig. 2. 3.2. Titration of anionic surfactants in the buffer solutions The anionic surfactants were determined by using the DCFOEbased film optodes with the titrants Zeph, CTAB, and CDB. Three of the typical spectrophotometric titration curves obtained for varying levels of NaDS, NaDBS, and PVSK in the buffered saline are shown in Figs. 3–5, respectively. End-points which are proportional to the concentration of each analyte were obtained from the titration curves. The reaction stoichiometries between the analytes and the titrants were obtained from the end-points and are summarized in Table 1.
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0.14 0.12 0.1
∆A
0.08 0.06 0.04 0.02 0
Fig. 1. Absorption spectra of the film optode obtained from UV–vis spectrophotometer before (left) and after (right) exposed to 500 g mL−1 of Zeph. at pH 7.4.
20
0
40
60
80
100
120
140
CTAB (µg/mL) Fig. 4. Proportional titration curves of 0 ( ), 5 ( ), 10 ( ), 20 ( ), and 40 ( ) g mL−1 of NaDBS with titrant CTAB. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
3.3. Analysis of some samples containing anionic surfactants
Fig. 2. View of the microplate wells after the addition of cationic titrant that converts the initial yellow color of film to pink. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
After having been obtained reproducible results for each target anionic surfactants in the buffered solutions, efforts were made to assess the ability to determine certain target anion surfactants in real samples using the titrimetric approach with optical detection of the cation/anion end-point using the microtiter plate arrangement. Solutions of samples containing surfactants were prepared at given concentrations (100–200 mg sample/10 mL) by mixing well and filtering under vacuum. These sample solutions were then titrated with the different cationic titrants. Varying anionic surfactant concentrations were calculated from the endpoint break of the titration curves. Ten different product samples
0,2
0.09
0.07
0,15
∆A
∆A
0.05
0,1
0.03 0 µg/mL PVSK
0,05
10 µg/mL PVSK
0.01
20 µg/mL PVSK 40 µg/mL PVSK
0
-0.01 0
20
40
60
80
100
120
CDB (µg/mL) Fig. 3. Proportional titration curves of 0 ( ), 5 ( ), 10 ( ), 20 ( ), and 40 ( ) g mL−1 of NaDS with titrant CDB. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
0
10
20
30
40
50
60
70
Zephiramine (µg/mL) Fig. 5. Proportional titration curves of 0 ( ), 10 ( ), 20 ( ), and 40 ( ) g mL−1 of PVSK with titrant zephiramine. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Table 1 Spectrophotometric titration data for different levels of anionic surfactants using cationic surfactant as titrant with lipophilic cation-sensitive film optodes. Titrant (Zeph) NaDBS (g)
Neutralization end point* (g)
Mass ratio (Zeph/NaDBS)
5 10 20 40
4 8 18 37
0.80 0.80 0.90 0.93 Average: 0.86 (±0.07)
NaDS (g)
Neutralization end point* (g)
Mass ratio (Zeph/NaDS)
5 10 20 40
6 13 26 53
1.20 1.30 1.30 1.33 Average: 1.28 (±0.06)
PVSK (g)
Neutralization end point* (g)
Mass ratio (Zeph/PVSK)
10 20 40
6 10 23
0.60 0.50 0.58 Average: 0.56 (±0.05)
NaDBS (g)
Neutralization end point* (g)
Mass ratio (CTAB/NaDBS)
5 10 20 40
4 10 21 41
0.80 1.00 1.05 1.03 Average: 0.97 (±0.11)
NaDS (g)
Neutralization end point* (g)
Mass ratio (CTAB/NaDS)
5 10 20 40
6 12 24 47
1.20 1.20 1.20 1.18 Average: 1.20 (±0.01)
NaDBS (g)
Neutralization end point* (g)
Mass ratio (CDB/NaDBS)
5 10 20 40
4 9 18 40
0.80 0.90 0.90 1.00 Average: 0.90 (±0.08)
NaDS (g)
Neutralization end point* (g)
Mass ratio (CDB/NaDS)
5 10 20 40
6.5 12 25 50
1.30 1.20 1.25 1.25 Average: 1.25 (±0.04)
Titrant (Zeph)
Titrant (Zeph)
Titrant (CTAB)
Titrant (CTAB)
Table 2 Total surfactant concentration levels measured in some cleaning products using film optodes. Sample analyzed
Total surfactant, as NaDS (% m m−1 )*
Hair shampoo 1 Liquid soap for hand Washing liquid 1(for dishes) Hair shampoo 2 (different brand) Shower gel 1 Washing liquid 2(for dishes) (different brand) Face cleaning gel Surface cleaner Toothpaste Shower gel 2 (different brand)
5.86 (±0.39) 6.84 (±0.59) 5.47 (±0.30) 11.10 (±0.47) 8.98 (±1.03) 17.19 (±0.78) 3.75 (±0.42) 5.27 (±0.23) 5.00 (±0.58) 6.67 (±0.87)
* The amount of analyte (g) in 100 g sample. Average of at least three measurements. All experiments were carried out in 50 mM Tris–HCl, pH 7.4, containing 120 mM NaCl.
cation-sensitive DCFOE-based microtiter plate-format optodes. The results are expressed as % m m−1 (g anionic surfactant/100 g sample) and are shown in Table 2. The representative graphs related to the titration of the hair shampoo-1, the face cleaning gel, and the washing liquid-1 (for dishes) are presented in Figs. 6–8, respectively. In order to investigate whether the sample matrix would have an effect on the determination, we spiked some real samples as well. The accuracy of the method was confirmed by these recovery experiments. For this purpose, the proportional amounts of NaDS were added to the solutions of these samples. Two of the studied samples, shower gel 1 and shower gel 2, containing anionic surfactant were examined. For example, titration curves of recovery studies with Zeph for the sample mentioned above are shown in Fig. 9. The data obtained for the sample tested are in good agreement with the proportional amounts added of surfactant. The results obtained show average polyanion recoveries of 96.3 (±1.8) %, and 101.0 (±3.6) %, for shower gel 1 (titrated with zeph), and for shower gel 2 (titrated with CTAB) samples, respectively. The recovery of anionic surfactants added to some real samples is quantitative. In this regard, we can conclude that the effect of the sample matrix is negligible based on the high recovery percentages.
Titrant (CDB)
0.25 0.2
Titrant (CDB)
* Each neutralization end-point is the average values of six measurements with standard deviations given in parentheses. The proposed method provides good detection limits (around 1 ppm) for all the anionic surfactants.
were tested; hair shampoo (1), hair shampoo (2), shower gel (1), shower gel (2), liquid soap for hands, washing liquid (1) (for dishes), washing liquid (2) (for dishes), face cleaning gel, surface cleaner, and toothpaste were purchased from the local markets and were tested. Total surfactant amounts in these samples were measured by the spectrophotometric titration method using the
∆A
0.15 0.1 0.05 0 0
20
40
60
Zephiramine (µg/mL) Fig. 6. A typical spectrophotometric titration curve for hair shampoo 1 containing surfactant: blank ( ), and 200 g mL−1 sample ( ). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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0.3 0.25
∆A
0.2 0.15 0.1
0
µg/mL
200 µg/mL Sample + 10 µg/mL NaDS
0.05
+ 20 µg/mL NaDS + 40 µg/mL NaDS
0 0
30
60
90
120
150
Zephiramine (µg/mL)
3.4. Binding interactions between cationic and anionic species Polymeric membrane electrodes based on the ion-exchanger were also used for examining the interaction between cationic and anionic surfactants by using Scatchard Plot. To conduct such measurements, polymeric membrane electrodes were used [16]. The electrode was first calibrated by adding increasing concentrations of cationic surfactant to buffer solution. Then the potentiometric titrations of anionic surfactants were performed by adding increasing concentrations of cationic surfactant (1 mg mL−1 ) to buffer solution containing a fixed amount of anionic species. The free cationic surfactant concentration (F) for each data point on the titration curve was determined graphically from the previously prepared calibration curve. The bound cationic surfactant
Fig. 9. Spectrophotometric titration curves obtained from recovery studies of shower gel 1 containing surfactant spiked with known amounts of NaDS.
A
0.35 0.3 0.25 0.2
∆A
Fig. 7. A typical spectrophotometric titration curve for face cleaning gel containing surfactant: blank ( ), and 200 g mL−1 sample ( ). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
0.15
0 µg/mL NaDBS 5 µg/mL NaDBS
0.1
10 µg/mL NaDBS
0.05
20 µg/mL NaDBS 40 µg/mL NaDBS
0 0
0.4
20
40
60
80 100 120 140 160 180
Zephiramine (µg/mL)
B 400
300
0.2
ΔmV
∆A
0.3
200
0 µg/mL NaDBS 10 µg/mL NaDBS
0.1
20 µg/mL NaDBS
100
2,5 µg/mL NaDBS 5 µg/mL NaDBS
0 0
20
40
60
80
100
120
Zephiramine (µg/mL) Fig. 8. A typical spectrophotometric titration curve for washing liquid 1(for dishes) containing surfactant: blank ( ), and 400 g mL−1 sample ( ). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
0 0
4
8
12 16
20 24
28 32
36 40
Zephiramine (µg/mL) Fig. 10. Proportional titration curves of NaDBS with titrant zephiramine by using optical sensors (A) and potentiometrical sensors (B).
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200000
dodecylsulfate (NaDS) and sodium dodecylbenzenesulfonate (NaDBS) via a simple titrimetric approach. Microtiter plates and readers are essential equipment in all life sciences and analytical laboratories, and since the microtiter plate-format optodes can provide high sample throughput (96 samples in less than 5 min), this new method should be attractive to many researchers and quality control chemists who need to measure levels of anionic surfactant species. In addition, relatively rapid titrations can be done by means of the MPOs, and hence, they are very appropriate for the routine analysis. The new method also provides good detection limits (around 1 ppm) for all the anionic surfactant species tested.
y = -1E+06x + 724043
160000
B'/F
120000
80000
Acknowledgements
40000
0,74
0,7
0,66
0,62
0,58
0,54
0,5
0
We are extremely grateful to the TÜBI˙ TAK (The Scientific and Technological Research Council of Turkey, Grant 112T708) and Abant Izzet Baysal University, Directorate of Research Projects Commission (BAP Grant 2010.03.03.343) for financial support.
B' Fig. 11. Typical Scatchard plot for the binding of NaDBS to zephiramine. B and B /F values were calculated from the potentiometric titration data of blank and 10 g mL−1 NaDBS.
concentration (B) was calculated by subtracting F from the total cationic surfactant concentration added. The cation-anion binding constant (Keq ) was estimated by recasting the data in the form of a Scatchard plot of B’ versus B’/F (B’, the moles of bound titrant per mole of anionic surfactant). The reaction stoichiometries were also compared by using the electrode method. As shown in Fig. 10, it can be clearly seen from the curves that the reaction stoichiometries obtained from the titrations conducted with the both type of sensors (optical and electrochemical) are in good accord with each other. For example, as can be seen for Zeph/NaDBS in Fig. 10, the mass ratio values were found to be 0.86 (±0.07) and 0.70 (±0.02) for the spectrophotometric and potentiometric methods, respectively. Fig. 10A and B shows the titration curves obtained by using both types of sensors. Fig. 11 shows the Scatchard plot for the binding of NaDBS to zephiramine using the potentiometric data. The slope of the line is the negative value of the equilibrium constant (Keq ) betweeen analyte and titrant. Keq is depicted as y = mx + b on the plot. Since Keq is large (106 ) the reaction between titrant and analyte is in favor of products and this is quite available for titration. 4. Conclusion In summary, in this study, we clearly demonstrate the potential analytical applications of microtiter plate-formate optodes (MPOs) for the measurement of anionic surfactants such as sodium
References [1] E. Olkowska, Z. Polkowska, J. Namiesnik, Talanta 88 (2012) 1–13. [2] M.J. Spencer, G.G. Wallace, P.T. Crisp, T.W. Lewis, Anal. Chim. Acta 244 (1991) 197–200. [3] D. Gorenc, B. Adam, B. Gorenc, Microchim. Acta 103 (1991) 311–315. [4] I. Kasahara, K. Hashimoto, T. Kawabe, A. Kunita, K. Magawa, N. Hata, S. Taguchi, K. Goto, Analyst 120 (1) (1995) 1803–1807. [5] Q. He, H. Chen, Fresenius J. Anal. Chem. 367 (2000) 270–274. [6] T. Masadome, Anal. Lett. 34 (2001) 2711–2719. [7] T. Masadome, Anal. Lett. 31 (1998) 1071–1079. [8] J.M. Esson, N. Ramamurthy, M.E. Meyerhoff, Anal. Chim. Acta 404 (2000) 83–94. [9] S. Dai, Q. Ye, E. Wang, M.E. Meyerhoff, Anal. Chem. 72 (2000) 3142–3149. [10] N. Dürüst, M.E. Meyerhoff, N. Ünal, S. Nac, Anal. Chim. Acta 699 (2011) 107–112. [11] E. Wang, M.E. Meyerhoff, V.C. Yang, Anal. Chem. 67 (1995) 522–527. [12] E. Wang, G. Wang, L. Ma, C.M. Stivanello, S. Lam, H. Patel, Anal. Chim. Acta 334 (1996) 139–147. [13] S.B. Kim, H.C. Cho, G.S. Cha, H. Nam, Anal. Chem. 70 (1998) 4860–4863. [14] W.H. Chan, A.W.M. Lee, J.Z. Lu, Anal. Chim. Acta 361 (1998) 55–61. [15] N. Dürüst, M.E. Meyerhoff, J. Electroanal. Chem. 602 (2007) 138–141. [16] N. Dürüst, M.E. Meyerhoff, Anal. Chim. Acta 432 (2001) 253–260.
Biographies Nedime Dürüst is currently working as a Professor of Analytical Chemistry in Abant Izzet Baysal University, Bolu, Turkey. She is the Head of Chemistry Department. Her research activities include the electrochemical and optical chemical sensors, the investigation of acidity constants of new synthesized heterocyclic compounds, phytochemical analyses of polyphenolic compounds. Sibel Nac¸ received her B.Sc. degree from the University of Abant Izzet Baysal in Chemistry in 2008 and completed her M.Sc. degree on the optical sensors in Chemistry in 2010 at the same University. She currently works as a research chemist at a pharmaceutical company in Istanbul, Turkey. Nazangül Ünal received the B.Sc. degree in 2005 and M.Sc. degree in 2010 both in Chemistry from the University of Abant Izzet Baysal, Bolu, Turkey.