Highly selective and sensitive simple sensor based

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Analytica Chimica Acta 899 (2015) 66e74

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Analytica Chimica Acta journal homepage: www.elsevier.com/locate/aca

Highly selective and sensitive simple sensor based on electrochemically treated nano polypyrrole-sodium dodecyl sulphate film for the detection of para-nitrophenol Abraham Daniel Arulraj a, Muthunanthevar Vijayan b, Vairathevar Sivasamy Vasantha c, * a b c

Alagappa University, Karaikudi, Tamil Nadu, 630 003, India Central Electrochemical Research Institute, Karaikudi, 630 006, India Department of Natural Products Chemistry, Madurai Kamaraj University, Palkalai Nagar, Madurai, 625 021, India

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 An electrochemically treated nano polypyrrole/sodium dodecyl sulphate film modified glassy carbon electrode was prepared and applied for determination of p-NP.  A very good linear detection range (from 0.1 nM to 100 mM) was obtained.  The best LOD (0.1 nM) of p-NP was obtained, without any interference.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 23 May 2015 Received in revised form 10 September 2015 Accepted 30 September 2015 Available online 13 October 2015

An ultrasensitive and highly selective electrochemical sensor for the determination of p-nitrophenol (pNP) was developed based on electrochemically treated nano polypyrrole/sodium dodecyl sulphate film (ENPPy/SDS film) modified glassy carbon electrode. The nano polypyrrole/sodium dodecyl sulphate film (NPPy/SDS film) was prepared and treated electrochemically in phosphate buffer solution. The surface morphology and elemental analysis of treated and untreated NPPy/SDS film were characterized by FESEM and EDX analysis, respectively. Wettability of polymer films were analysed by contact angle test. The hydrophilic nature of the polymer film decreased after electrochemical treatment. Effect of the pH of electrolyte and thickness of the ENPPy/SDS film on determination of p-NP was optimised by cyclic voltammetry. Under the optimised conditions, the p-NP was determined from the oxidation peak of phydroxyaminophenol which was formed from the reduction of p-NP in the reduction segment of cyclic voltammetry. A very good linear detection range (from 0.1 nM to 100 mM) and the best LOD (0.1 nM) were obtained for p-NP with very good selectivity. This detection limit is below to the allowed limit in drinking water, 0.43 mM, proposed by the U.S. Environmental Protection Agency (EPA) and earlier reports. Moreover, ENPPy/SDS film based sensor exhibits high sensitivity (4.4546 mA mM1) to p-NP. Experimental results show that it is a fast and simple sensor for p-NP. © 2015 Elsevier B.V. All rights reserved.

Keywords: p-Nitrophenol Electrochemical sensor Contact angle test Square wave voltammetry

1. Introduction * Corresponding author. E-mail address: [email protected] (V.S. Vasantha). http://dx.doi.org/10.1016/j.aca.2015.09.055 0003-2670/© 2015 Elsevier B.V. All rights reserved.

Nitroaromatic compounds are used in the manufacturing of

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dyes, plasticizers, pesticides, fungicides and explosives [1]. In particular, p-nitrophenol (p-NP) is one of the nitrophenols cited in the list of priority pollutants of the U.S.A. Environmental Protection Agency (EPA) due to its toxicity (carcinogen, teratogenic and mutagenic) and persistence [2,3]. The harmful effects of p-NP on humans include headache, fever, breathing trouble and even death at high levels of exposure [4]. The p-NP is one of the examples that can be found in wastewater and agricultural run-off due to biodegradation of parathion and methyl parathion [5]. This chemical compound is an inhibitor of acetylcholinesterase [6]. Furthermore, the p-NP is a usable model chemical pollutant in the elaboration of biodegradability tests due to its erratic biodegradability. The p-NP exists not only in industrial wastewater but also in freshwater and marine environments [7,8]. The permissible limit of p-NP in the environment by different agents like United States EPA and the European Commission are 0.43 mM and 0.72 nM, respectively [9]. Thus, there is a need to develop simple and reliable sensing devices for the determination of trace amounts of p-NP in the environment. Hitherto, various methods have been employed for sensitive detection of p-NP including spectrophotometry [10,11], high performance liquid chromatography [12e15], fluorescence [16e18] and electrophoresis [19e22]. However, these approaches involve complex sample preparation procedures and generally require expensive instrumentation. On the other hand, electrochemical methods have received considerable attention in the determination of p-NP, because of their great advantages, such as simple operation, inexpensive, fast response and in situ detection [23,24]. Different modified electrodes have been used for sensitive detection of p-NP such as carbon nanotubes, metal NPs, and conducting polymers [25e28]. Zhang et al., reported a novel method for simultaneous determination of NP isomers at nano-gold modified glassy carbon electrode [29]. However, No report is available for the detection of p-NP using PPy film in the literature. In this work, a new and simple sensing material was developed for designing a highly selective and sensitive electrochemical sensor for p-NP. This new modified electrode paves a way to the potential application for electrochemical detection of picomolar level of p-NP. To the best of our knowledge, this modified electrochemical sensor represents the best low detection limit, higher sensitivity and wide linear detection range than the other sensors reported earlier. Well-separated square wave voltammetric peaks were observed for p-NP and o-NP during simultaneous determination. 2. Experimental 2.1. Materials and equipment All chemicals were of analytical grade and used without further purification. All aqueous solutions were prepared with doubly distilled water. p-NP, o-NP, Pyrrole monomer, Sodium dodecyl sulphate (SDS), Lithium perchlorate (LiClO4), Sodium hydroxide, disodium hydrogen phosphate (Na2HPO4), monosodium dihydrogen phosphate (NaH2PO4), potassium chloride and sodium acetate were purchased from SigmaeAldrich. Potassium ferricyanide and ferrocyanide were purchased from Merck. Phosphate buffer solution (PBS, 0.1 M, pH ¼ 7) was prepared from NaH2PO4 and Na2HPO4 salts. The 10 mM stock solutions of p-NP and o-NP were prepared by using doubly distilled water. Both the Nitrophenol solutions in low concentration were prepared just before using. Cyclic voltammetry (CV) and Square wave voltammetry (SWV) measurements were performed using Bio-logic Science Instruments (Model: SP-150 s/n 0555 electrochemical workstation). The cell setup contained a glassy carbon electrode (GCE) (3 mm) as

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working electrode, Ag/AgCl (saturated KCl) electrode as reference electrode and Pt wire as counter electrode. The pH measurements were carried out with Cyberscan 510 pH meter EUTECH instrument, Singapore. The surface morphology of the polymer films were studied using Zeiss FESEM. EDAX analysis with Bruker Quantx EDS detector. Wettability was studied using the contact angle Data physics instrument. 2.2. Electro chemical preparation of ENPPy/SDS film The GC electrode was polished using 0.05 mm alumina powder followed by rinsing thoroughly with doubly distilled water. After successive sonication in 1:1 acetone and double distilled water, and dried at room temperature. The electrochemical polymerization was carried out in solution containing 0.1 M pyrrole, 0.01 M SDS and 0.05 M LiClO4 in the potential range from 0.40 V to 0.75 V at scan rate of 50 mV s1. Then the above modified electrode was electrochemically treated in 0.1 M phosphate buffer solution (pH 7.0) by scanning between 0.80 V and 1.30 V for several cycles until to get very stable background current. Then, the electrode surface was washed with distilled water. 3. Results and discussion 3.1. FESEM images of NPPy/SDS film and ENPPy/SDS film The surface morphologies of the NPPy/SDS film (before cycling) and ENPPy/SDS film (after cycling in phosphate buffer solution) were characterized by FESEM (Fig. 1). As shown in Fig. 1AeD, the NPPy/SDS film and ENPPy/SDS film exhibit a similar morphology such as globular structure, but the size of the polymer particle decreases after electrochemical treatment and more over nano size cracks are also formed in the polymer matrix. This morphological change must be due to that the replacement of macro size SDS ions by smaller phosphate ions during dedoping may create nano cracks [30]. 3.2. Elemental analysis of NPPy/SDS film and ENPPy/SDS films The elemental analysis of the NPPy/SDS film and ENPPy/SDS films were analysed by EDX technique (Fig. 2A and B). In the spectrum of NPPy/SDS film, the peaks for sulfur and sodium are appeared which indicate that SDS ions have been successfully incorporated in the PPy-matrix during electropolymerization. However, the amount of sulfur atoms decreased from 0.81 atomic % to 0.29 atomic % after electrochemical treatment. Similarly, oxygen atom increases from 13.21 atomic % to 24.03 atomic % and carbon atoms decreases from 59.68 atomic % to 45.33 atomic % which indicates that the SDS ions must be expelled by phosphate ions during cycling in the phosphate buffer solution. Moreover, interestingly the EDX spectra shows peak for phosphorous in the ENPPy/ SDS film. Thus, the results support the replacement of the SDS ions by phosphate ions. 3.3. Ion exchange properties of the NPPy/SDS film Oxidation of “conducting polymers” generates fixed positive charges on the polymer back bones. In the reduced state, the PPy matrix is electrochemically neutral. The neutral PPy films are therefore relatively poor ionic conductors in addition to poor electronic conductors. To alter this situation, a number of polymer structures have recently been synthesised which contain electroinactive molecules with fixed negative charges. This has been achieved mainly by electropolymerizing pyrrole in the presence of macro anions like poly (styrene-sulfonate), poly-(vinylsulphate),

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Fig. 1. FESEM images of NPPy/SDS film (A & B) and ENPPy/SDS film (C & D).

sodium dodecyl sulphate, Fe (CN)3 6 etc., which are thereby incorporated into the PPy matrix irreversibly [31]. Hence, these compounds show high capacitance current due to incorporation of macro polyanions and cations. In this study, SDS ions were incorporated into PPy matrix during synthesis and then replaced by small anions such as phosphate ions which induces some changes in morphological and physical properties of polymer chain. Qingli Hao et al. explained in details about  2 the exchange of anions such as ClO 4 , HSO4 or SO4 (in H2SO4),  3 H2PO4 or PO4 in polyaniline films [32]. In order to study the effect of the phosphate anions on the surface morphology and physical properties of NPPy/SDS film, cyclic voltammograms were reordered for the NPPy/SDS film in phosphate buffer solution (Fig. 3). During cycling, there is a drastic change in the shape and size of the cyclic voltammogram of the NPPy/SDS film. In order to explain the changes clearly, the cyclic voltammogram can be divided into four regions as “a”, “b”, “c” and “d”. Here the changes in the regions “a” and “b” are removal of cations which were incorporated during polymerization in order to compensate the negative charges created by irreversible incorporation of SDS. The changes in the regions “c” and “d” are corresponding to exchange SDS- anions by phosphate ions in the polymer matrix. This is due to that when the NPPy/SDS film is cycled in phosphate buffer solution; the SDS anions are expelled by the doping of phosphate ions from the electrolyte. But unlike doping of SDS anions, the phosphate doping/ dedoping is reversible. Hence, the amount of fixed negative charges by the SDS anions decreases. Hence the sharp cation peaks in the

regions “a” and “b” are disappeared during cycling. So the net charge inside the polymer matrix is decreased. Hence, after 50 cycles, the ENPPy/SDS film exhibits very low capacitance than the NPPy/SDS film. A similar approach has been used for the incorporation of macro anions and cations into polymers by several research groups [33e37]. 3.4. Wettability study of NPPy/SDS film and ENPPy/SDS films The water wettability on NPPy/SDS film and ENPPy/SDS films were measured (shown in Fig. 4) by contact angle method. The contact angle of water on NPPy/SDS film is less than 5 , but the contact angle on ENPPy/SDS film is 29.4 , it indicates that the surface property of NPPy/SDS film has been “turned” towards hydrophobic after electrochemical treatment. This is due to replacement of SDS and sodium ions from the polymer backbone [38] (Fig. 4). The EDX results are also support the above observation. 3.5. Electrochemical studies of p-NP at NPPy/SDS film and ENPPy/ SDS films The electrochemical behavior of p-NP was studied at the bare GCE, SDS film and PPy film, NPPy/SDS film and ENPPy/SDS film modified electrodes in the potential range from 0.80 V to 1.30 V in phosphate buffer solution (pH 7) (Fig. S1). There are no obvious oxidation peaks for p-NP at the bare GCE. But, the SDS film, PPy film and NPPy/SDS film modified electrodes show small humps

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Fig. 2. A. EDAX results of NPPy/SDS film, B. EDAX results of ENPPy/SDS film.

at þ0.11 V, þ0.16 V and þ0.34 V for p-NP. However, the ENPPy/SDS film electrode depicts a sharp reduction and oxidation peaks at 0.79 V and þ0.15 V, respectively. According to earlier reports, pNP was reduced into p-hydroxyaminophenol with four electrons and proton transfer at 0.79 V in the reduction side and on the reverse scan, p-hydroxyaminophenol was oxidised to nitrosophenol with transfer of two electrons and protons þ0.15 V [39e46] as in Eqs. (1) & (2) (Fig. A). The electrochemical reactions involved are as follows:

Fig. 3. Cyclic voltammogram of NPPy/SDS film modified electrode during electrochemical scanning in phosphate buffer solution. Scan rate: 50 mV s1.

Fig. A. Redox mechanism of p-NP at ENPPy/SDS film modified electrode.

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Fig. 4. Wettability test of (A) NPPy/SDS film, (B) ENPPy/SDS film.

Further, ENPPy/SDS film modified electrode shows several fold enhanced current response for p-NP than other modified GCEs. The results clearly validate that ENPPy/SDS film modified electrode possesses very good electrocatalytic ability towards p-NP. The catalytic effect of ENPPy/SDS film can be explained as follows: firstly as seen in the FESEM images, the electrochemical treatment leaves nano cracks and also decreasing particle size of the polymer by changing the big globular structure into smaller one. These cracks can act as micro electrochemical cells for the catalysis of p-NP; hence they allow p-NP to diffuse into the polymer matrix through capillary action [30]. Secondly it may also be due to hydrophobic interaction between hydrophobic parts of the polymer film and pNP. Similar interactions were observed between dopamine e PDMA film [47] and dopamine e PEDOT film [48]. 3.6. Optimisation 3.6.1. Effect of pH on electrochemical redox behavior of p-NP at ENPPy/SDS film The effect of pH on the electrochemical behavior of p-NP (56 mM) at ENPPy/SDS film electrode was studied over the pH ranging from 3 to 10 (Fig. S2). The change in anodic peak current vs potential of p-hydroxyaminophenol with pH was plotted (Fig. S2B and Fig. S2C, repectively). As shown in Fig. S2, the anodic peak current increases gradually with pH from 3 to 7 and then decreases upto 10.0. Therefore, pH ¼ 7.0 is selected as the optimum pH for further studies. Since the pKa value of p-NP is 7.16 [49], with increasing the pH of the electrolyte, the deprotonation of p-NP is occurring upto pH 7. Hence at pH ¼ 7, all p-NP can exist in neutral form. But, above the pH ¼ 7, p-NP is existing as anion (p-nitrophenoxide) [50,51]. Since, the neutral form of p-NP is hydrophobic in nature compare to ionic form, hence the interaction between p-NP and ENPPy/SDS film must be stronger through hydrophobic force. As shown in Fig. S2C, with increasing pH values, the anodic peak potential of p-hydroxyaminophenol shifts towards less positive side. The slope of the straight line plotted between pH vs potential is 72 mV pH1. This value is very close to 59 mV pH1, which indicates that the electrochemical redox reaction of p-NP involves an equal number of electrons and protons [52]. 3.6.2. Effect of thickness of the polymer on determination of p-NP The thickness of the ENPPy/SDS film was controlled by the number of cycles during polymerisation. The number of cycles of

electropolymerization was varied from 2 to 20. As shown in Fig. S3, the reduction or oxidation peak current increases predominantly from 2 to 14 cycles and then oxidation peak current remains almost constant after 14 cycles. Hence, 14 cycles is optimised to prepare the ENPPy/SDS film modified GC electrode [53]. The phenomenon must be due to increase of thickness of the polymer film resulted in increase of total electro active of the polymer film. The ENPPy/SDS film obtained at lower cycles must covers the electrode surface partially with more pinholes, whereas the polymer obtained at 14 cycles may be having optimum active surface area with more pinholes. But, the polymer obtained beyond 14 cycles must be having enough active surface area with very poor porous nature due to high thickness of polymer film. Hence, the polymer obtained at 14 cycles can interact effectively with p-NP through hydrophobic interaction and thin layer effect. The pores of the thin layer can act as micro cells which can act also as capillary tubes for the diffusion of p-NP into the electrode surface [54]. It indicates that the 14 cycles of polymer thickness is enough to cover the electrode surface uniformly. 3.7. Determination of p-NP at ENPPy/SDS film electrode The cyclic voltammetric response of p-NP (pH 7.0) at ENPPy/SDS film is shown in Fig. 5A & B. Fig. 5B depicts the relationship between peak currents (anodic and cathodic) verses different concentration of p-NP. Under the optimum conditions, the ENPPy/SDS film reveals a linear relationship in the examined concentration range of 0.1 mMe56 mM. From the cyclic voltammograms, it is observed that the oxidation peak current of p-hydroxyaminophenol is higher than the reduction peak current of p-NP (Fig. 5). This is due to that at lower concentration of p-NP (