carbonized polyaniline electrodes for p–nitrophenol sensing

    Composite Zeolite/Carbonized Polyaniline Electrodes for p-Nitrophenol Sensing ˇ Aleksandar Jovi´c, Aleksandar orevi´c, Maria Cebela, Ivana Stojkovi´c ˇ Simatovi´c, Radmila Hercigonja, Biljana Sljuki´ c PII: DOI: Reference:

S1572-6657(16)30430-1 doi: 10.1016/j.jelechem.2016.08.025 JEAC 2799

To appear in:

Journal of Electroanalytical Chemistry

Received date: Revised date: Accepted date:

7 May 2016 16 August 2016 18 August 2016

ˇ Please cite this article as: Aleksandar Jovi´c, Aleksandar orevi´c, Maria Cebela, Ivana ˇ Stojkovi´c Simatovi´c, Radmila Hercigonja, Biljana Sljuki´c, Composite Zeolite/Carbonized Polyaniline Electrodes for p-Nitrophenol Sensing, Journal of Electroanalytical Chemistry (2016), doi: 10.1016/j.jelechem.2016.08.025

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ACCEPTED MANUSCRIPT Composite Zeolite/Carbonized Polyaniline Electrodes for p–Nitrophenol Sensing

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Aleksandar Jović,a Aleksandar Đorđević,a Maria Čebela,b Ivana Stojković Simatović, a Radmila Hercigonja,a Biljana Šljukića,*

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Faculty of Physical Chemistry, University of Belgrade, Studentski trg 12–16, 11158 Belgrade, Serbia b Institute of Nuclear Sciences “Vinča”, University of Belgrade, Mike Petrovića Alasa 12–14, 11001 Belgrade, Serbia

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Abstract Electrodes based on composites of zeolites with carbonized polyaniline prepared in the presence of 5–sulfosalicylic acid are evaluated for both qualitative and quantitative determination of phenols in aqueous solutions. Zeolites used included NaX and NaY, as well as their transition metal (Mn and Cu) cation–exchanged forms, and they were all characterized using XRPD, FTIR and SEM. Cyclic voltammetry was used to study composites’ electrochemical response in the presence of p–nitrophenol, phenol and 5–aminophenol in acidic, neutral and alkaline media. Linear dependence of current on p–nitrophenol concentration in acidic media was obtained for 0.1 – 1 mM concentration range. The comparative evaluation of the electrochemical response of NaX/carbonized polyaniline composite and its individual components revealed significantly lower limit of detection obtained using composite electrode (1.27 μM) compared to that obtained using pure zeolite (135 μM) or pure carbonized polyaniline (94.5 μM) electrode. Composite electrode gave response to p–nitrophenol presence in neutral media as well, but it quickly disappeared with continuous scanning, while no clear response could be seen in highly alkaline media. Thus, this work demonstrates benefits of using novel composite based on zeolites and carbonized polyaniline for sensing of phenols in acidic aqueous solutions.

Keywords: zeolites; carbonized polyaniline; phenol; p–nitrophenol; 5–aminophenol

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ACCEPTED MANUSCRIPT Introduction

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Natural zeolites are rarely pure materials; they usually contain additives in the form of other zeolites, minerals, metals and quartz [1]. However, these zeolites are not broadly used for commercial purposes due to lack of purity and uniformity of structure. Synthetic zeolites are much more studied and more used for commercial purposes compared to natural mineral zeolite, as they show great uniformity in the composition and purity [2]. Zeolites are among the most important industrial catalysts because of their unique characteristics including ability of ion exchange, high adsorption capacity and high catalytic activity [3,4]. One of the most important classes of micropore zeolites are faujasite (FAU) zeolites and their cation exchanged forms that have broad application as a result of their micro framework structure [5–12]. Nowadays, with environmental hazards’ awareness increasing, utilization of zeolites for sensing of environmental pollutants is more frequent, with special attention given to their use in electrochemical sensors. Zeolites entrap electroactive species within their porous structure thus enabling direct determination of those species. Major features that make zeolites applicable as electrocatalytic material for detection of inorganic, organic and biological compounds [13–15] are their chemical stability and electrical conductivity [16,17]. The electrical conductivity of zeolites is of ionic nature, and exchangeable cations act as the main charge carriers [18]. Thus, conductivity of zeolites depends on the type of the charge balance cation (charge, size and electronic configuration) and the pore geometry. In recent years, modified carbon electrodes and composite electrodes received special attention for application in electroanalysis. They proved to be excellent electrochemical sensors suitable for detection of many organic and inorganic substances due to their high sensitivity [19–22]. They have further advantages of being inexpensive and simple to use, providing fast and reliable data. Composite zeolite/carbon electrodes demonstrated improved mechanical and chemical properties of the composite, compared to pure carbon material or to pure zeolite [14,23]. Different nanostructured carbon materials have been investigated for application in electroanalysis, including carbon nanotubes and their composites [24] and graphene and its composites [25–27]. These composites proved to have high sensitivity and selectivity, as well as excellent reproducibility, due to carbon materials high surface area and presence of edge–plane sites. The latest research is focused on the exploration of nitrogen–containing nanostructured carbons (NNCs) for sensor and biosensor [28,29]. Nanostructured N– containing conducting polymers, such as polyaniline (PANI), can be easily produced with high level of morphology control [30–32], with their subsequent carbonization yielding 2

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different types of NNCs (nanotubes, nanofibers, nanorods, nanosheets) [33–38]. These nanostructured carbonized PANIs (c–PANI) showed excellent properties as catalysts [36], electrocatalysts [35–39], field emitters [40], and supercapacitors [41,42]. Sensors and biosensors employing c–PANI–based electrode have been recently reported [43–45]. Within our previous work, c–PANIs were explored for electroanalytical application both as metal oxides supports [46,47], as well as electrode materials [48,49] for sensing of different analytes including heavy metals, nitrites and ascorbic acid.

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Volatile organic compounds (VOCs) are known to be toxic for both the environment and humans and therefore some of them are categorized as priority pollutants [50]. Phenol, one of typical VOCs, is commonly present in industrial wastewaters, and consequently represents one of the most common contaminants of water and soil, with high toxicity at very low concentrations. Currently, 6 million tons of phenol is produced in the world annually, with an increasing trend. With the increase of phenols production, pollution of wastewater also increases. Nitrophenols are essential components for production of various industrial chemicals, dyes, pharmaceuticals, pesticides, explosives, and therefore, they are regularly present in industrial and agricultural wastewaters. Nitrophenol is considered as one of the priority pollutants due to its high toxicity and hazard [51]. Aminophenols are important intermediates in pharmaceutical industry for analgesics and antipyretics, as well as for cosmetic products, and they are present in the azo colors and lubricants [52]. Aminophenol is relatively less toxic and it can be removed and mineralized easier than nitrophenol [53]. The phenol compounds have been detected by different analytical methods such as spectrophotometry [54], gas and liquid chromatography [55,56] and electrochemical methods [57–60]. The advantages of electrochemical methods compared to the others include short analysis time, simplicity, high reproducibility, high sensitivity and low cost [61]. In the present paper, NaX and NaY zeolites, as well as their cation–exchanged forms with Mn and Cu, were characterized using X–ray diffraction analysis, infrared spectroscopy and scanning electron microscopy. They were further used for preparation of composite electrodes with carbonized polyaniline prepared in an aqueous solution of 3,5– dinitrosalicylic acid. Application of these composite electrodes for determination of phenol, p–nitrophenol and 5–aminophenol in acidic, neutral and alkaline media was studied. Experimental Materials preparation and characterization 3

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The zeolite samples used in this work were synthetic NaY (SK–40) and NaX (Union Carbide), as well as their transition metal (Mn and Cu) cation–exchanged forms obtained by conventional ion–exchanged procedure [62]. Composition of zeolites determined by chemical analysis is presented in Table 1. The samples were characterized by X–ray powder diffraction (XRPD), Fourier–transformed infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) techniques. XRPD patterns were recorded using Bruker D5005 (Siemens) difractometer with a Ni filtered CuKα radiation in the 2θ range of 1 – 50◦. FTIR spectra of the powdered samples, dispersed in KBr and compressed into pellets, were recorded in 4000 – 400 cm–1 range at 64 scans per spectrum at 2 cm–1 resolution using Avatar 370FT–IR Spectrometer (Thermo Nicolet). The morphology of the zeolites and composite was investigated using a JEOL JSM–6610LV and Tescan MIRA3 field–emission gun scanning electron microscope, respectively.

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Table 1. Composition of zeolites expressed by Si/Al and M/Al ratios (M = Na, Cu, Mn). zeolite Si/Al M/Al Formula NaY 2.4 1.00 Na56(AlO2)56(SiO2)136 CuY 2.4 0.40 Na12Cu22(AlO2)56(SiO2)136 MnY 2.4 0.38 Na14Mn21(AlO2)56(SiO2)136 NaX 1.2 1.00 Na87(AlO2)87(SiO2)105 CuX 1.2 0.39 Na19Cu34(AlO2)87(SiO2)105 MnX 1.2 0.38 Na21Mn33(AlO2)87(SiO2)105 Carbonized polyaniline was prepared by the gram–scale template–free oxidative polymerisation of aniline with ammonium peroxydisulfate as an oxidant in an aqueous solution of 5–sulfosalicylic acid and subsequent carbonization of produced PANI (c–PANI– SSA) [39,63]. Electrochemical measurements All electrochemical measurements were carried out using Gamry PCI4/300 Potentiostat/Galvanostat with a three–electrode glass cell of 25 cm3 volume. Platinum and saturated calomel electrode (SCE) served as counter and reference electrode, respectively. All potentials in the paper are given relative to the SCE. Working electrodes were prepared by pipetting 10 µL of the corresponding catalytic ink onto glassy carbon (GC) substrate and leaving it to dry at 100°C overnight. Catalytic ink was prepared by ultrasonically mixing 5 mg of zeolite/c–PANI–SSA composite into 100 µL of poly(vinylidene fluoride) (Sigma–Aldrich) binder (5 %) in N–methyl 2–pyrrolidone (Sigma– Aldrich) solvent. Zeolite/c–PANI–SSA composite consisted of 25 wt.% of zeolite (NaX, 4

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NaY, CuX, CuY, MnX or MnY) and 75 wt.% of c–PANI–SSA as our study involving Ag– zeolite/c–PANI–SSA composites for nitrite and phenols sensing showed that this composition results in the best electrode’s analytical response. The same procedure was used to prepare working electrodes of zeolites (100%) and c–PANI–SSA (100%). Cyclic voltammograms (CVs) were recorded at a scan rate of 0.01 Vs–1 at room temperature. Electrolytes used were 0.1 M H2SО4 (96%, ACROS Organics™), pH 7 phosphate buffer and 0.1 M NaOH (> 98%, Centrohem). Phenolic compounds studied included phenol (PH, Aldrich), p–nitrophenol (p–NP, Aldrich) and 5–aminophenol (5–AP, Aldrich). Limit of detection (LOD) was calculated using 3 sigma method.

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Results and discussion

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Materials Characterisation XRPD patterns of NaY and NaX zeolites shown in Figure 1 reveal a typical Faujasite structure [Structure Commission of the International Zeolite Association, http://www.iza.structure]. Namely, observed peaks correspond to the Miller indices (hkl) of (111), (220), (311), (331), (440), (620), (533), (622), (642), (731), (733), (660), (555), (664), (931), (844) and (880) planes, that are in good agreement with those of the face–centered cubic crystal structure of Faujasite. The sharp and intense diffraction peaks are observed at both NaY and NaX patterns, confirming high crystallinity of these samples. Figure 1. XRPD patterns of (A) NaY, MnY and CuY and (B) NaX, MnX and CuX zeolites. No significant changes in the position of the reflections characteristic for the parent NaY were detected in the diffractograms of Mn2+ and Cu2+ cationic forms of Y zeolite, evidencing that structure of these zeolites is retained after ion–exchange processes. The relative intensity of few peaks in the XRPD patterns of MnY and CuY zeolites is slightly increased relative to the parent NaY. Peak positions at diffractograms of MnX and CuX were unchanged compared to those of NaX, but intensity of some CuX peaks was reduced, indicating a small loss of crystallinity of zeolite with Cu2+ charge–balance cations. This is most likely correlated with the locations of Cu2+ cations in the zeolite framework, pointing to the migration and redistribution of Cu2+ in the cationic positions following the ion–exchange, or distortions in the pores and intracrystalline voids due to strong interaction with lattice oxygen atoms [64–66]. Tosheva et al. [67] reported results of XRPD analysis which showed a certain

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loss of zeolite crystallinity as a result of the metal loading, with this loss being pronounced for the Cu–containing samples. The mid–infrared spectra of NaY and its cation modified forms are shown in Figures 2A with Table 2 summarizing the spectral frequencies of the bands assigned to the type of the vibration [68–72]. The presence of two main structural sensitive bands at 716 and 575 cm–1 in the spectrum of NaY confirms the high crystallinity of this zeolite. The appearance of these two bands in the spectra of MnY and CuY at the same frequencies as in the spectrum of NaY, without decrease in their intensity, confirmed the preserved crystallinity of the samples after introducing Mn2+ and Cu2+ ions in the zeolite lattice. Additionally, the structural sensitive band attributed to the symmetric stretching of Al–O bond appears in the spectra of NaY, MnY and CuY at the same frequency (789 cm–1), being of the same intensity.

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Table 2. FTIR peak assignment for NaY, MnY and CuY Zeolit –OH –OH Symmetr Asymmetr Symmetr Double e stretchi bendin ic Si–O– ic T–O ic Al–O ring ng g Si bond bond symmetr –1 –1 /cm /cm stretchin stretching stretchin ic g (T = Si, g stretchin –1 –1 /cm Al) /cm g –1 /cm /cm–1 NaY 3461 1641 1141 1105 789 716 MnY 3417 1641 1141 1105 789 716 CuY 3417 1642 1141 1105 789 716

D6R unit stretchi ng /cm–1

T–O bendin g vibrati on /cm–1

575 575 575

457 457 457

Figure 2. FTIR absorption spectra of (A) NaY, MnY and CuY and (B) NaX, MnX and CuX zeolites. The mid–infrared spectra of NaX and its cation–modified forms are shown in Figure 2B with Table 3 summarizing the spectral frequencies of the bands assigned to the type of the vibrations. The main structural sensitive bands attributed to double rings symmetric stretching (666 cm–1) and 6–membered (D6R) unit stretching (560 cm–1) appear in the spectrum of NaX, confirming its high crystallinity. One of two main structural sensitive bands (558 cm–1) in the spectrum of MnX has almost unchanged frequency and intensity of such band in the spectrum of NaX, while the other is shifted towards higher frequency (690 cm–1) and its intensity is decreased. Double rings symmetric stretching band in the spectrum of CuX is shifted to 600 cm–1 and both main structural sensitive bands are of low intensity. 6

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The structural sensitive band assigned to the framework vibrations (symmetric stretching of Al–O bond), appears in NaX and MnX spectra with the same frequency and intensity, but this band is missing in CuX spectrum. Thus, FTIR confirmed good crystallinity of MnX and lower crystallinity of CuX compared to NaX, but still with preserved characteristic crystal structure of faujasite.

D6R unit stretchi ng /cm–1

T–O bendin g vibratio n /cm–1

560 558 555

452 452 445

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Table 3. FTIR peak assignment for NaX, MnX and CuX samples zeolit –OH –OH Symmetr Asymmetr Symmetr Double e stretchi bendin ic Si–O– ic T–O ic Al–O rings ng g Si bond bond symmetr –1 –1 /cm /cm stretchin stretching stretchin ic g (T = Si, g stretchin –1 –1 /cm Al) /cm g –1 /cm /cm–1 NaX 3507 1650 1090 970 750 666 MnX 3416 1633 1090 981 749 690 CuX 3415 1635 1100 1013 – 600

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The influence of cations on the framework vibrations is evident, particularly in the case of CuX. The framework vibrations, especially the band near 750 cm–1, are sensitive to the cation type and charge. Abu–Zied [65] found that FTIR spectra of Cu–containing zeolites show continuous decrease of the intensity of the two main structural bands with increasing Cu content in the zeolite lattice, manifesting an evidence for structural damage. The unit cells of NaX and NaY contain 87 and 56 Na+ ions, respectively. In the process of ionic exchange one bivalent cation of transition metal replaces two monovalent Na+ cations. Having in mind the similar percentage of exchange of Na+ ions (about 80%) in both zeolites, the cation density and so the amount of the positive charge in MnX and CuX is much higher (ca. double) than that in MnY and CuY. The high density of positive charge in the frameworks of MnX and CuX affects the framework vibrations of these zeolites and so change the position and intensity of the band in IR spectra. Figure 3. SEM images of six studied zeolites

SEM was used to examine morphological features of studied zeolites, Figure 3. The particles of NaY and NaX appear to be spherical aggregates of distorted octahedral crystallites. 7

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Similar morphology has been reported in the literature [73]. The replacement of Na+ by Mn2+ and Cu2+ did not affect the morphological properties of the parent Y zeolite. However, replacement of Na+ by Mn2+ and Cu2+ in X zeolite seemed to result in particles of smaller size, with CuX particles forming some aggregates [65]. The SEM image of CuX illustrates further changes in its morphology due to the introduction of a high percent of Cu2+ into the zeolite lattice that also causes the formation of Cu, CuO, CuxOy or other Cu species on the surface of the zeolite particles [74,75]. SEM images of MnX/c–PANI–SSA composite are also included, with two types of particles being observed: large particles of the zeolite and the rod forms of c–PANI–SSA [48].

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Sensing of p–nitrophenol in acidic media Sensing of p–NP by zeolite/c–PANI–SSA composite electrodes was first studied in acidic media by cyclic voltammetry scanning the potential at a rate of 0.01 Vs–1. Unmodified GCE response in 1 mM p–NP solution in 0.1 M H2SO4 in 0 − 1.0 V potential range revealed a peak of ca. 8 μA at ca. +0.40 V, corresponding to electrochemical oxidation of p–NP, Figure 4A. However, noticeable decrease of peak current is seen after only 5 successive scans, due to the formation of a polymeric passivating layer.

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CV of NaX/c–PANI–SSА composite electrode in 1 mM p–NP solution in 0.1 M H2SO4 in 0 − 1.4 V potential range exhibited a clear peak at ca. 1.27 V and a broad peak at ca. 0.41 V corresponding to the p–NP oxidation, Figure 4B. The peak currents are observed to be several times higher than peak current value recorded at unmodified GCE. The peak corresponding to p–NP oxidation at NaY/c–PANI–SSA electrode appears at slightly higher potential (1.33 V), with peak current of ca. 0.093 mA being significantly lower than that recorded at NaX/c–PANI –SSA (ca. 0.3 mA), Figure 4B. Figure 4. CVs of GCE (A), NaX/c–PANI–SSA and NaY/c–PANI–SSA (B), MnX/c–PANI– SSA (C) and CuX/c–PANI–SSA (D) in 1 mM p–NP in 0.1 M H2SO4. CVs of NaX/c–PANI– SSA with increasing p–NP concentration (E) with standard addition plots (F) are also included.

The appearance of characteristic and well–defined oxidation peak at ca. 0.41 and 1.27/1.33 V is in agreement with previous studies of electrochemical oxidation of p–NP [76,77]. It has been reported that during p–NP electrooxidation products are formed giving rise to a broad anodic peak at ca. 0.5 V [77]. The proposed paths of p–NP oxidation assumes formation of p–nitrophenoxy radical, which is further oxidized to nitrophenoxy cation. Species formed 8

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during the first oxidation steps (p–nitrophenoxy radical and the corresponding p– nitrophenoxy cation) are highly reactive and they can be further chemically converted to phenolic or a hydroquinone intermediate by release of a nitro group or by its substitution by hydroxyl group. Indeed, hydroquinone, benzoquinone, 4–nitrocatechol, 1,2,4– trihydroxybenzene and 3,4,5–trihydroxy–nitrobenzene were detected by HPLC and GC–MS as main intermediates during p–NP degradation [78]. Peaks appearing in 0.4 – 0.65 V potential region are attributed to the oxidation of these species to p–benzoquinone and o– benzoquinone. Alternatively, formed reactive species (p–nitrophenoxy radical and the corresponding p–nitrophenoxy cation) can combine creating polymers. At the CV of MnX/c–PANI–SSA electrode in 1 mM p–NP in 0.1 M H2SO4 two characteristic broad anodic peaks corresponding to p–NP oxidation are observed at ca. 1.05 V and 1.27 V, as well as small broad cathodic wave at ca. 0.68 V, Figure 4C. This cathodic peak most likely corresponds to one of the products of p–NP oxidation, probably hydroquinone. Only one anodic peak could be seen at the CV of MnY/c–PANI–SSA electrode in 1 mM p–NP in 0.1 M H2SO4 at ca. 1.23 V (not shown), being notably smaller than that recorded at MnX/c–PANI–SSA (peak current of ca. 0.60 and ca. 0.32 mA for MnX/c–PANI–SSA and MnY/c–PANI–SSA, respectively). Similarly, a well–defined anodic peak at ca. 1.27 V appeared at the CV of CuX/c–PANI–SSA composite electrode in 1 mM p–NP in 0.1 mM H2SO4, Figure 4D. p–NP oxidation peak at CuY/c–PANI–SSA electrode appeared at the same potential (not shown), but it was of lower intensity (peak current of ca. 0. 34 and ca. 0.27 mA for CuX/c–PANI–SSA and CuY/c–PANI–SSA, respectively). The appearance of p–NP oxidation peaks suggests the possibility of employing proposed composite electrodes for electroanalytical determination of this compound in acidic media. Stability of electrodes for sensing of p–NP was also investigated as it has been reported that electrodes’ stability is one of the major problems for determination p–NP and phenolic compounds in general by electroanalytical methods [76]. Electrooxidation of p–NP at the composite electrodes seems to proceed with certain degree of electrodes passivation in the p– NP concentration range studied herein, as evidenced by some decrease of peak currents with continuous cycling (10 consecutive cycles run). Still, signals of the composite electrodes reached constant values that were easily quantifiable after few scans. Figure 4C, for instance, illustrates decrease of p–NP oxidation peak current decrease at MnX/c–PANI–SSA that stabilized after only three cycles. It is worth mentioning that stability of the activity of the prepared composite electrodes for p-NP is improved compared to that of electrodes previously studied by the authors or that of electrodes reported in the literature as evidenced by faster stabilisation of the oxidation peak currents with continuous cycling [76,77]. Subsequently, response of NaX/c–PANI–SSA was followed with increasing p–NP concentration in 0.1 M H2SO4 in order to investigate the possibilities of its application for the 9

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quantitative determination of p–NP in acidic media and to evaluate its detection limit. Therefore, CVs were recorded for a series of p–NP additions to 0.1 M H2SO4 in the concentration range from 100 µМ to 1 mM using 100 µМ steps, Figure 4E. Linear increase of peak current with increasing concentration of p–NP could be observed, Figure 4F. LOD of p–NP sensing with NaX/c–PANI–SSA composite electrode in acidic media was calculated by the 3σ method and found to be 1.75 µM. The experiments repetition gave reproducible result with standard deviation, SD, of 7.1%.

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Next, electrochemical response of electrodes based on pure NaX zeolite and based on c– PANI–SSA in 1 mM p–NP in 0.1 M H2SO4 was studied in order to examine the effect of individual components of the composite electrode on its electrochemical behavior. Both NaX– and c–PANI–SSA–based electrode gave CVs qualitatively similar to that of composite NaX/c–PANI–SSA electrode (not shown). Namely, electrodes based on pure NaX zeolite and on c–PANI–SSA gave peak corresponding to p–NP oxidation at ca. 0.36, i.e., at slightly lower potential than that at composite NaX/c–PANI–SSA electrode. Peak current of 96 μA at NaX is close, while peak current of ≈ 68 μA at c–PANI–SSA is lower than the value reported for the composite electrode. Subsequent 100 µМ addition of p–NP in the range from 100 µМ to 1 mM to 0.1 M H2SO4 resulted in the increase of peak current with increasing concentrations up to 0.6 mM at NaX and in the whole concentration range for c–PANI–SSA, Figure 4F. LODs of p–NP in acidic media obtained using electrodes based on pure NaX zeolite and on c–PANI–SSA were found to be 135 μM (SD = 6.7%) and 94.5 μM (SD = 4.8%), respectively. It should be pointed out that these values are significantly higher than LOD obtained using the composite electrode thus evidencing superiority of the composite electrode for p–NP sensing in acidic media. It is worth mentioning that LOD evaluated for p–NP in acidic media using NaX/c–PANI– SSA composite electrode is lower or comparable to LODs reported using different electrochemical sensors which indicates that composites electrodes studied herein could be successfully employed for p–NP determination. Namely, LODs for p–NP in acidic and neutral solutions reported in the literature range from 100 μM at hanging mercury drop electrode to 0.001 μM at ZnO nanoparticles/carbon nanotubes doped chitosan film [76,77,79–91]. Electrooxidation of phenolic compounds at zeolite–containing electrodes is believed to be controlled by adsorption of these compounds within zeolites large–area microporous structure (for instance, 904 m2 g–1 in case of NaY zeolites) [92,93]. Zeolites are reported to orient molecules that approach their surface, due to their electrical fields, thus prompting the increase in the rate of the redox reaction [94]. Large internal porous volume of zeolites allows solution to enter their microporous structure so that the available area is increased, 10

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leading to decrease of the oxidation potential and to increase of the rate of oxidation. Zeolites concentrate phenols within their porous structure thus increasing the amount of phenols available for reaction. c–PANI–SSA offers additional benefit in terms of high surface area. It is reported to be mainly microporous material, with a higher fraction of mesopores [42,95]. Its micropore and mesopore volume were reported to be 0.128 and 0.076 cm3g–1, respectively, while its micropore and mesopore surface area were reported to be 360 and 50 m2g–1, respectively [42]. However, the main benefit of c–PANI–SSA as electrode material is its high electrical conductivity (0.83 S cm–1) [42]. Thus, electrical conductivity of MnX/c–PANI–SSA composite was measured to be 0.075 S cm–1, which is orders of magnitude higher than that of zeolite. Nanostructured carbon materials have been also reported to enhance stability of the electrode response to phenolic compounds [96]. Different behaviour in terms of peak currents recorded was observed for composites with different zeolites cations, most likely due to the differences in their electrical conductivity and due to the differences in the strength of their interaction with phenols. Namely, phenols oxidation currents increased in the following order NaX/c–PANI–SSA < CuX/c–PANI–SSA < MnX/c–PANI–SSA. Furthermore, higher phenols oxidation currents were recorded using X zeolites than when using Y zeolites. As frameworks of all investigated zeolites are the same, this implies that some characteristics of charge–balance cation govern the electrochemical response. It has been shown that hydrogen from non–dissociated phenol molecule is attracted by oxygen from zeolites’ framework [97]. This hydrogen bonding is done through the shifting of electrons of the aromatic ring; the other possible interactions are those between aromatic ring of phenol with extra–framework cation, through a kind of charge transfer from phenolate anion to empty orbital of metal [98,99]. The transition metal cations Cu2+ and Mn2+ differ in electron configuration. Mn possesses 5 incomplete d–orbitals in comparison with 1 unpaired electron in 4s1 orbital in the case of Cu. Therefore the interaction of charge transfer is more evident in the case of MnX than in the case of CuX. In case of Na+ as the spin paired cation, hydrogen bonding is the only type of interaction between NaX and phenol and its compounds. Furthermore, electrical conductivity of studied zeolites differs since, as mentioned earlier, it depends of the nature of the change–balance cation. The electrical conductivity of hydrated faujasites has been attributed to the movement of ions inside supercage and sodalite cages [100], while it is not absolutely clear if electrical conductivity of zeolites is related to the movement of electrons or to the ion movement [101,102]. Ion radii of metal cations in investigated zeolites increase in the order Cu2+ (0.73 nm) < Mn2+ (0.82 nm) < Na+ (1.05 nm), implying that Cu2+ ions can move easier than the Mn2+ ions inside zeolite channels due to 11

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their smaller size. Nevertheless, CuX has lower electrical conductivity than MnX, which suggests that the mechanisms of electrical conductivity of CuX and MnX are not the same. As said above, electron configuration of Cu2+ and Mn2+ ions differ, so that Mn2+ ions are located within the zeolite can easily complete its uncompleted orbitals with the electrons of the oxygen from the zeolite lattice without moving. This represents the electron conductivity through the channels of the zeolite. It may be conclude that the electrical conductivity of CuX is better described by intrazeolite moving of ions, while in the case of MnX it is due to both the intrazeolite moving of ions and intrazeolite electron transport mechanism. Electrical conductivity of univalent Na+ ions is expected to be the lowest as they have half the charge of Mn2+ or Cu2+ and larger ionic radius compared to Mn2+ and Cu2+, so their movement inside the zeolite lattice is difficult. When it comes to comparison of X and Y zeolites with the same cation, X zeolite lattice negative charge is higher than of Y zeolite and hydrogen bonding between non–dissociated phenol molecule and oxygen from zeolites’ lattice is more pronounced in X than in Y zeolite. Furthermore, the charge balance cation can more easily take the electrons of the oxygen atom from the lattice of X zeolite than from the lattice of Y zeolite due to the higher density of negative charge linked to the AlO4 tetrahedra in X zeolites. Additionally, the higher number of bivalent cations in super and sodalite cages of, for instance, MnX (33 Mn2+ ions per unit cell) than in the MnY (21 Mn2+ ions per unit cell) contributes to higher electrical conductivity of the former. Hence, significantly lower peak currents for NaY/c–PANI – DNSA, MnY/c–PANI –DNSA and CuY/c–PANI –DNSA compared to NaX/c–PANI – DNSA, MnX/c–PANI –DNSA and CuX /c–PANI –DNSA, respectively, are expected. Interferences As phenolic compounds are likely to occur in a mixture, possibility of application of zeolite/c–PANI–SSA composite electrodes for determination of 5–AP and PH in acidic media was next investigated, Figure 5. CVs of NaX/c–PANI–SSA (Figure 5A) and MnX/c– PANI–SSA (Figure 5B) in 1 mM 5–AP in 0.1 M H2SO4 revealed a noticeable peak at ca. 0.64 and 0.55 V, respectively, due to the 5–AP oxidation at the composite electrodes. At CVs of NaX/c–PANI–SSA (Figure 5C) and MnX/c–PANI–SSA (Figure 5D) in 1 mM PH in 0.1 M H2SO4 two peaks arise due to electrochemical oxidation of PH: at ca. 0.51 and 1.14 V for NaX/c–PANI–SSA and at ca. 0.44 and 1.03 V for MnX/c–PANI–SSA [93,103]. Safavi et al. [104] reported formation of catechol (o–benzoquinone) and hydroquinone (p– benzoquinone) during the electrooxidaton phenol in pH 2 solution at a potential of ca. 0.6 V, in agreement with herein obtained results. It should be mentioned that during oxidation of PH at unmodified GCE in 0.1 M H2SO4, phenolic intermediate produced is giving rise to a

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broad peak of 24 μA at ca. 0.48 V (not shown), with this current being ca. 4 times lower than peak current value at the composite NaX/c–PANI–SSA electrode. It is worth noticing that appearance of peaks corresponding to electrooxidation of p–PH (ca. 0.41 and 1.27 V), 5–PH (0.64 V) and PH (0.51 and 1.14 V) at different potentials in 0.1 M H2SO4 suggests that NaX/c–PANI–SSA composite electrode could be used for simultaneous detection of phenols in acidic media.

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Figure 5. 1st CVs of NaX/c–PANI–SSA (A and C) and MnX/c–PANI–SSA (B and D) composite in 1 mM 5–AP and 1 mM PH in 0.1 M H2SO4, respectively.

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Sensing of p–nitrohenol in neutral media Next, possibility of using NaX/c–PANI–SSA composite electrode for detection of p–NP in neutral media was investigated. Control CV of NaX/c–PANI–SSA electrode recorded in pH 7 phosphate buffer as supporting electrolyte, revealed no peak (not shown). Conversely, CV of the composite electrode recorded in the presence of 1 mM p–NP in pH 7 phosphate buffer revealed appearance of a well–defined anodic peak at ca. 1.01 V (Figure 6A). Similarly, a peak at ca. 1.01 V corresponding to p–NP oxidation appeared at CV of composite MnX/c– PANI–SSA electrode in 1 mM p–NP in pH 7 phosphate buffer (Figure 6B), with somewhat higher peak currents than in case of NaX–based composite. However, with continuous scanning fast decrease of p–NP oxidation peak current and fast decrease in the composite electrodes activity could be observed (after only 3 and 2 cycles in case of NaX/c–PANI–SSA and MnX/c–PANI–SSA, respectively), thus excluding application of the proposed composite electrodes for sensors for p–NP in neutral media.

Figure 6. Successive CVs of (A) NaX/c–PANI–SSA and (B) MnX/c–PANI–SSA composite in 1 mM p–NP in pH 7 phosphate buffer solution.

Sensing of p–nitrohenol in alkaline media Last, possibility of using zeolite/c–PANI–SSA composite electrodes for determination of p– NP in alkaline environment was explored. Increase of current could be seen on CV of NaX/c–PANI–SSA composite electrode in 1 mM p–NP in 0.1 M NaOH (pH 13) compared to CV in 0.1 M NaOH supporting electrolyte, but with no clearly defined peak (not shown). Similarly, at CV of MnX/c–PANI–SSA composite electrode in 1 mM p–NP in 0.1 M NaOH solution no peak could be observed (not shown). Impossibility of electroanalytical detection of p–NP in highly alkaline media has been previously observed [76,105]. Previous study on 13

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effect of pH onto p–NP sensing with modified carbon paste electrodes reported a well– defined p–NP oxidation peak in the pH 2 – 6 range that complete disappeared at pH > 10 [105]. Namely, p–NP oxidation peak current increased in the pH 2 – 3.5 range, reaching a maximum at 3.5 and then decreased in the 3.5 – 10 pH range. The peak potential shifted with increase of pH so that two linear regions were observed with intersection point at pH 7.0. This change is explained by p–NP pKa value of 7.15 that indicates that in the given electrolyte p–PN deprotonation and sodium phenoxide formation occur [106].

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Conclusions Novel composites of zeolites with c–PANI–SSA were studied for electrochemical determination of p–NP, as well as PH and 5–AP in acidic media. Composite–based electrodes proved to have improved electrochemical response to phenols compared to their components. Namely, composite electrode gave significantly lower LOD (1.27 μM) compared to that obtained using pure zeolite (135 μM) or pure carbonized polyaniline (94.5 μM) electrode. The phenols oxidation currents increased in the order NaX/c–PANI–SSA < CuX/c–PANI–SSA < MnX/c–PANI–SSA composite. Likewise, higher phenols oxidation currents were obtained with composites containing X zeolites compared to those obtained with composites containing Y zeolites. Further improvement of the composite zeolite/c– PANI–SSA electrodes performance for sensing of p–NP, and phenols in general, could be achieved by using more sensitive electrochemical methods under the optimized conditions. Stability of the electrodes activity for p–NP oxidation was assessed by continuous cycling in p–NP solution in acidic media. Though the peak current was observed to decrease with cycling, it stabilised quickly at an easily quantifiable value. Thus, herein prepared electrodes could be used in phenols sensors for acidic media for application in environmental monitoring. Composite electrodes were also investigated for sensing of p–NP in neutral and alkaline media. The electrodes gave electrochemical response corresponding to p–NP oxidation in neutral media, but this response was found to disappear with further scanning due to electrodes passivation. No response to p–NP was observed in alkaline media.

Acknowledgements The authors would like to thank to the Ministry of Education, Science and Technological Development of Republic of Serbia for support within the project No. OI172043, OI172018 and III45012. The authors would also like to thank professor Gordana Ćirić–Marjanović and 14

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assistant professor Aleksandra Janošević Ležaić for synthesis of carbonized polyaniline used in this work, as well as to Dr Marko Spasenović, Institute of Physics, Serbia, for SEM analysis of composite.

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ACCEPTED MANUSCRIPT Research highlights Composites of X/Y zeolites and carbonized nanostructured polyaniline were prepared

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Stable determination of p-nitrophenol, phenol and 5-aminophenol was demonstrated

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Composites with X zeolites show higher phenols oxidation currents than Y zeolites

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Phenols oxidations currents increased in the order NaX < CuX < MnX

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