Electrode Prepared with Combination of Different Conductive Polymers for Hydroquinone Determination in River Water

Modified electrode based on combination of different conductive polymers (polyaniline – poly(3-methylthiophene) – poly(3,3’-diaminobenzidine) (PANI – P3MT – PDAB)) was previously synthesized in dichloromethane containing HClO4 and tetrabutylammoniumperchlorate (TBAP) as supporting electrolyte. Herein, the determination of hydroquinone (HQ) with amperometric I-t method over this modified and its homopolymers coated electrode was successfully carried out in river water as a real sample and in solution consisting of NaHSO4/Na2SO4 (pH 2.0). For this method, potentials between 0.45 V and 0.65 V were applied and the best analytical response was obtained at 0.55 V using this modified electrode when compared to its homopolymers. The limit of detection (LOD), limit of quantification (LOQ) , the linear dynamic range and regression coefficient (R) were found as 1.31 × 10 mM, 4.37 × 10 mM, 4.37 × 10– 95.0 mM and 0.998, respectively. These results were compared to homopolymers coated and uncoated Pt electrodes. For checking on the accuracy of the developed method and the matrices interference, determination of HQ was performed in artificially contaminated river water samples with HQ concentration of 5.0 mM and 10.0 mM using this modified electrode and recovery values were calculated as 100.6% and 100.1%, respectively.


Introduction
Phenols are aromatic organic compounds having one or more hydroxyl groups.They are natural products resulting from the decomposition of some plant and animal organisms.Phenols also have a similar structure to some extensively used pesticides that are resistant to biodegradation.3][4] Products containing HQ are used to decolorize the skin.Overexposure to HQ causes irritation, skin sensitization and also discoloration of the nails. 5However, HQ is classified as an environmental pollutant because of the low degradability and high toxicity similar to other phenolic compounds by international organizations such as the European Union legislation and the US Environmental Protection Agency. 6Because of all these factors, determination of HQ is important.HQ is electroactive due to the presence of hydroxyl groups and can be analyzed electrochemically.HQ is usually oxidized to quinone by electrochemical oxidation and it can be determined based on the measurment of the resulting current by amperometric method.Nowadays, several analytical techniques, including high performance liquid chromatography, capillary electrochromatography, micellar chromatography and kinetic spectrophotometry are being replaced by inexpensive, rapid and reliable techniquies such as electrochemical methods 5 which are the most advantageous because of their simplicity, rapidness and sensitivity.
Conductive polymers are widely used to prepare modified electrodes.Among various conducting polymers, polyaniline (PANI) has attracted attention because Ülker and Kavanoz et al.: Electrode Prepared with Combination ... of its properties such as the presence of various oxidation forms, electrical and optical activity, low cost and environmental stability.These features of PANI film permit its use in a variety of areas such as rechargeable batteries, sensors, electrochemical capacitors and electrochromic devices.In addition, PANI film has the ability to catalyze redox reactions of some inorganic ions and organic compounds. 7,8PANI film can be synthesized by chemical and electrochemical polymerization of aniline monomer.Kavanoz et al. reported composite film of PANI with poly(vinylferrocenium) perchlorate on Pt electrode in dichloromethane medium. 9Another widely used conductive polymer is poly(3-methylthiophene) (P3MT) that can be directly synthesized on electrode surface by electrochemical polymerization of 3-methylthiophene.Synthesis of P3MT for modification of electrodes were reported in acetonitrile, 6,10 dichloromethane, 4 chloroform 11 and aqueous solution 12 and prepared P3MT modified electrodes have shown electrocatalytic effect on riboflavin, 13 some phenolics 14,15 and dopamine. 16lthough polymers of aromatic amines have been extensively studied, 17 studies related to polybenzidine and its derivatives have not been observed in the literature.Electrochemical polymerization of benzidine derivative of 3,3'diaminobenzidine (DAB) has been studied in ethanol 18 and acidic solution. 19,20Mulazımog ˘lu reported PDAB polymerization on glassy carbon electrode (GCE) for determination of phenol in tap water samples. 21Also, composite film of PDAB with poly(vinylferrocenium) perchlorate for HQ determination 22 and copolymer film with PANI and P3MT for determination of epinephrine were reported. 23n this study, we prepared a modified electrode by combination of conductive polymers such as PANI, P3MT and PDAB on Pt disc and used this modified electrode for amperometric determination of HQ in river water samples.Good analytical results (LOD, LOQ, the linear dynamic range and regression coefficient etc.), good repeatability and good recovery values without matrix effects were obtained.

1. Apparatus and Chemicals
Electrochemical studies were performed using an 1140B model CH Instruments system.The working cell has five inlets; three inlets were used for the electrodes: platinum disc as the working electrode; Pt spiral as the counter electrode; and Ag/AgCl (non-aqueous medium) and saturated calomel electrode (SCE) (aqueous medium) as the reference electrode.The other two inlets were used for nitrogen gas.Platinum discs (area = 7.85 × 10 -3 cm 2 ) were used as working electrodes for cyclic voltammetry.These electrodes were cleaned with a slurry of Cr 2 O 3 in water, washed with distilled water, rinsed with dichloromethane and dried.All chemicals were of analytical grade and used without further purification.Electropolymerization in non-aqueous solution was performed in dichloromethane under nitrogen (Linde).TBAP used as a supporting electrolyte in non-aqueous media was obtained by the reaction of tetrabutylammoniumhydroxide (TBAOH) (40% aqueous solution, Sigma-Aldrich) with perchloric acid (BDH) and recrystallized in 1:9 (v:v) water and ethyl alcohol solution, dried at 120 °C under vacuum for 12 h and stored under nitrogen. 24Buffer solutions (pH 1.0-6.5)were prepared by using NaHSO 4 (Merck) and Na 2 SO 4 (Merck) for amperometric experiments in aqueous media.HQ solutions were prepared in the sulfate buffer solution (SBS, pH 2.0).Before electrochemical experiments, dissolved oxygen in HQ solution was removed using nitrogen (Linde).

1. Preparation of PANI -P3MT-PDAB Film
As shown in Figure 1, in our previous work PANI -P3MT-PDAB film was synthesized on Pt disc electrode in dichloromethane solution layer by layer.Firstly, PANI films were potentiodynamically (between 1.80 V and -0.40 V, -0.20 V, 0.0 V and 0.20 V vs. Ag/AgCl (20 cycles)) deposited on Pt disc electrode in dichloromethane using polymerization solution containing 50.0 -400 mM aniline monomer, 100 mM HClO 4 and 100 mM TBAP as supporting electrolyte.Then, on the PANI film, P3MT-PDAB films were potentiodynamically (between 1.80 V and -0.40 V, -0.20 V, 0.0 V and 0.20 V vs. Ag/AgCl (20 cycles)) synthesized in dichloromethane using polymerization solution containing both monomers (20.0 -500 mM 3-methylthiophene and 1.0 mM 3,3'-diaminobenzidine) and 100 mM TBAP as supporting electrolyte.Cyclic voltammograms of these films deposited on the Pt disc electrode were obtained in methylene chloride containing only 0.10 M TBAP (blank solution) and peak currents and charge values of each film were compared.According to these results, the most electroactive PANI and P3MT films were obtained using 300 mM aniline and 500 mM 3-methylthiophene monomers, respectively (Figure 1a,b).Therefore, these monomer concentrations were determined as optimal in polymerization solutions.PANI and P3MT films have better electroactivity than PDAB film.The properties of PANI and P3MT films were enriched with regard to functional groups using PDAB film containing -NH 2 groups without affecting the electroactivitiy or conductivity of the PANI and P3MT films.Consequently, low DAB monomer concentration (1.00 mM) was preferred for the preparation of modified electrode.Therefore, synthesized PANI -P3MT-PDAB film has better electroactivitiy and more active functional groups. 25

2. pH behavior of PANI -P3MT-PDAB Film
pH behavior of the PANI -P3MT -PDAB film was investigated in SBS (pH 1.0 -6.5) in our previous study (Figure 2) 25 and the optimum pH value was determined as pH 2.0.

Cyclic Voltammetric Behavior of Hydroquinone Over PANI -P3MT-PDAB Film
The pH 2.0 solution was used for the characterization of HQ behavior in aqueous medium over Pt disc electrode, coated with the PANI -P3MT -PDAB film.The cyclic voltammograms over uncoated Pt electrode (Figure 3A-D (c)), PANI, P3MT, PDAB homopolymer films and PANI -P3MT -PDAB film (Figure 3A-D(a)) in SBS (pH 2.0) containing 5.0 mM HQ were measured.Also, these voltammograms were compared with HQ-free solution over coated Pt electrodes (Figure 3A-D(b)).The oxidation and reduction peaks of HQ over uncoated Pt electrode at pH 2.0 were observed at about 0.58 V and 0.05 V, respectively (Figure 3A-D(c)).The oxidation peak of HQ over PDAB film decreased and shifted to lower potential (from about 0.58 V to about 0.45 V) (Figure 3C(a,c)).Furthermore, because of the limited electroactivity or conductivity of PDAB, the oxidation peak current of HQ recorded on an uncoated Pt electrode was higher than on PDAB coated electrode. 25However, using PDAB in preparation of PANI -P3MT -PDAB modified electrode, the number of functional groups increased (the PDAB film contains many -NH and -NH 2 functional groups) and this enrichment provided greater hold of HQ by hydrogen bonding on this electrode surface.Thus, the current values obtained using PANI, P3MT and PDAB coated Pt electrodes containing HQ (Figure 3A-C(a)) were higher than on coated electrode surfaces without HQ (Figure 3A,C(b)).When we compared the interactions of HQ with uncoated and coated Pt electrodes, the current values obtained over PANI -P3MT -PDAB coated Pt electrode at pH 2.0 solution containing 5.0 mM HQ were higher than in all other cases (PANI, P3MT and PDAB coated Pt electrodes at pH 2.0 both containing HQ and without HQ) (Figure 3D(a)).As a result, because modified film structure contains many -NH, -N and -NH 2 sites, HQ molecules can form hydrogen bonds at these sites that originate from PANI and PDAB. 24,26In ad-

4. Amperometric Determination of Hydroquinone
The PANI-P3MT-PDAB film deposited on Pt disc electrode that was enriched in terms of functional groups was used for the determination of HQ in SBS (pH 2.0) and artificially contaminated river water samples with HQ by amperometric I-t method.The film was electrolyzed between 0.45 V and 0.65 V until steady-state current (about 1000 s) was reached in pH 2.0 solution vs SCE.After reaching the steady state of the film, first addition of HQ as 9.77 × 10 -4 mM was added from stock solution containing HQ dissolved in pH 2.0 solution.It was stirred for 30 s to ensure homogeneity and then the current values were recorded at the end of the 200 s period.Currents versus increasing concentrations of HQ were measured in a similar manner and amperometric I-t curve was recorded (Figure 4).When compared to calibration graphs, the best analytical parameters were obtained at 0.55 V (Table 1).Therefore, the amperometric I-t response of the PANI-P3MT-PDAB film was compared with its homopolymer coated and uncoated Pt electrodes at 0.55 V (Figure 5a).The steady-state currents measured on these amperometric I-t curves were plotted against HQ concentration (Figure 5b) and calibration graphs were constructed using these values (Figure 5c).According to the Figure 5c, we obtained the highest currents when we used the PANI-P3MT-PDAB film.LOD, LOQ, linear dynamic range and R 2 values were compared for PANI-P3MT-PDAB coated (between 0.45 and 0.65 V potentials), its homopolymer coated (0.55 V) and the uncoated platinum electrodes (0.55 V) in Table 1.
Table 1 shows the comparisons of analytical parameters for coated and uncoated Pt electrodes.3s/m and 10s/m (s: standard deviation, m: slope) equations were used to calculate the LOD and LOQ values. 4,25,27The parameter of "s" was calculated from the current values of the lowest concentration that generate a measurable current in three replicate measurements.As it is clear from the table, lower LOD value and wider linear range were obtained when the PANI-P3MT-PDAB coated electrode was used.It might be due to enrichment of PANI and P3MT film using PDAB film containing -NH 2 groups.Advantage of -N, -NH and -NH 2 group containing modified electrode is the ability to form hydrogen bonds with -OH groups in HQ structure.

5. Repeatability of the PANI-P3MT-PDAB Modified Electrode for Determination of Hydroquinone
Repeatability of current recorded from amperometric I-t curves (0.55 V) for HQ determination over PANI-P3MT-PDAB film deposited at optimum conditions on  Pt disc electrode 25 was tested by using at least three films for each measurement in SBS (pH 2.0) vs. SCE.
For each concentration of HQ (1.0, 5.0, 10.0 and 20.0 mM), the obtained amperometric I-t curves, currents, standard deviations (s) of currents, percent relative standard deviation (% RSD) and confidence interval are shown in Table 2. Confidence interval was calculated to evaluate the precision using combined standard deviation (s combined : 0.349 μA).

6. Determination of HQ in Artificially Contaminated River Water Samples
To check for the accuracy of the improved modified electrode and to demonstrate the matrix interference, electrochemical response of HQ was investigated in artificially contaminated river water from Turkey.Before the experiments, the pH of river water was adjusted to optimum working pH (2.0) with HCl solution and it was artificially contaminated by the addition of HQ.As previ- √ √Ν Ν ts ously described, amperometric I-t method was used for the determination of HQ in this solution.At optimum conditions (0.55 V), the PANI-P3MT-PDAB film was electrolyzed for about 1000 s (pH 2.0) to reach stability and then river water samples containing HQ (5.0 and 10.0 mM) were added and currents were measured.These results were compared to the standard HQ (5.0 and 10 mM) currents (Table 2), and then, the percentage of the recovery values was calculated.As seen in Table 3, there is no influence of the matrices for the determination of HQ in river water samples.
determination of HQ using amperometric I-t method and results of its homopolymers coated and uncoated Pt electrodes were compared.The validation parameters (LOD, LOQ and the linear dynamic range) of the PANI-P3MT-PDAB modified electrode were found as 1.31 × 10 -4 mM, 4.37 × 10 -4 mM and 4.37 × 10 -4 -95.0 mM, respectively.Assessment of reliability of the results obtained from repeatability experiments has shown satisfactory statistical results: standard deviations between 0.031 and 0.545; RSD between 1.316 and 3.80%.Matrix interference on the HQ determination was investigated in artificially contaminated river water from Turkey.Recovery values were found as 100.6% and 100.1%, respectively.Consequently, the developed method is found to be usable, simple and rapid.

Acknowledgments
This research was supported by The Scientific and Technological Research Council of Turkey (TÜBI • TAK) The obtained results were compared with other electrochemical methods [28][29][30][31][32] in Table 4.The PANI-P3MT-PDAB modified electrode displays better parameters such as lower LOD value and longer linear range in terms of analytical performance than the other modified electrodes.These might be due to enrichment of properties of the PANI-P3MT-PDAB film by combining the catalytic effects of each homopolymer and functional groups (-N, -NH, -NH 2 ) of PDAB and PANI.Advantage of this modified electrode is the ability to form hydrogen bonds with -OH groups in HQ structure.

Conclusion and Recommendation
The PANI-P3MT-PDAB modified electrode deposited with combination of different conductive polymers using aniline, 3-methylthiophene, 3,3'-diaminobenzidine monomers on Pt disc electrode was successfully used for the

Figure 1 .
Figure 1.Voltammogram of the deposition and schematic representation of PANI -P3MT-PDAB film a) PANI film on Pt disc electrode b) P3MT -PDAB film deposited on PANI film.

Figure 5 .
Figure 5. Determination of HQ (9.77 × 10 -4 -125.0 mM) over uncoated and PANI, P3MT, PDAB, PANI-P3MT-PDAB coated Pt electrodes at 0.55 V a) Amperometric I-t curves recorded vs. SCE b) A plot of the steady-state currents measured from amperometric I-t curve vs. increasing HQ concentrations c) Calibration curves for linearity range of HQ concentrations.

Table 1 .
Comparison of analytical parameters over uncoated and coated Pt electrodes for the determination of HQ.

Table 2 .
Assessment of reliability of the results obtained from repeatability experiments for the determination of HQ over PANI-P3MT-PDAB modified electrode * 95% confidence level was calculated.Confidence intervals were calculated using standard deviations ** group *** combined.

Table 3 .
Recoveries for determination of HQ in artificially contaminated river water from Turkey.

Table 4 .
Comparison of the improved method with the other modified electrodes for determination of HQ Project no: 111T445) and Recep Tayyip Erdogan University, Unit of Scientific Research (Project no: 2013.102.02.3).We thank to Dr. Selçuk Demir (Recep Tayyip Erdogan University, Faculty of Science & Arts, Department of Chemistry, Rize) for improving the language of this munucript.