Simultaneous electrochemical determination of

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Simultaneous electrochemical determination of hydrazine and hydroxylamine by CuO doped in ZSM-5 nanoparticles as a new amperometric sensor† Sedigheh Rostami,a Seyed Naser Azizi

*a and Shahram Ghasemib

In this work, a novel electrochemical sensor using a carbon paste modified electrode with CuO doped in ZSM-5 nanoparticles (CuO/ZSM-5 NPs/CPE) is successfully fabricated for simultaneous determination of hydrazine (HY) and hydroxylamine (HA). The prepared materials are characterized with scanning electron microscopy (SEM), transmission electron microscopy (TEM), powder X-ray diffraction (XRD), energy dispersive X-ray (EDX) and Brunauer–Emmett–Teller (BET) techniques. For sensitive determination of HY and HA, differential pulse voltammetry (DPV) and amperometry techniques are used. Furthermore, Received 23rd July 2017, Accepted 4th October 2017 DOI: 10.1039/c7nj02685d

in amperometric measurements, the current response and the HY concentration have linear relationships in the ranges of 25 mM to 0.9 mM and 0.9–4.5 mM. Also, there are linear relationships in the ranges of 20 mM to 0.9 mM and 0.9–7.0 mM for HA. Also, low detection limits of 3.6 mM and 3.2 mM (S/N = 3) are achieved for HY and HA, respectively. The sensor has good stability, reproducibility and anti-interference

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capability as well as simplicity.

1. Introduction Fuel cells are one of the most hopeful sources of alternative energy by providing high-energy output and also without emitting any harmful by-products. Fuel cells are efficient and environmentally acceptable energy is generated in them by direct electrochemical oxidation either by hydrogen (proton exchange membrane fuel cell, PEM) or methanol (direct methanol fuel cell, DMFC).1,2 Recently, there has been increasing research interest in the development of HA and HY fuel cells.3–5 HY (N2H4) is a reactive and water-soluble vaporizable inorganic compound with strong reducing ability widely used in corrosion inhibitors and antioxidants, in the production of several insecticides, herbicides and pesticides, and also in the synthesis of some pharmaceutical materials.4,6 HA (NH2OH) is recognized as a kind of reducing agent considerably used in industrial and pharmaceutical science. It is known as an important intermediate in the cycles related to nitrogen and plays a key role in life science. HA is extensively used as a crude and primary material for the synthesis and production of pharmaceutical intermediates and final drugs. However, HA is a commendable mutagen, mildly toxic and detrimental to creatures.7,8 Hence, quantitative determination a

Analytical division, Faculty of Chemistry, University of Mazandaran, Postal code, 47416-95447, Babolsar, Iran. E-mail: [email protected] b Faculty of Chemistry, University of Mazandaran, Babolsar, Iran † Electronic supplementary information (ESI) available. See DOI: 10.1039/c7nj02685d

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of HA is also very necessary for investigations related to biology and industry methods.9 There are different methods such as potentiometry, spectrophotometry, gas chromatography, and mass spectrometry for measuring of HY and HA.10–16 But, these methods have disadvantages like having a low detection limit and precision. Hence, electroanalytical techniques such as amperometry and voltammetry are used because they are more sensitivite and more reliable.17–23 Unfortunately, HY and HA need large over-potentials at usual carbon electrodes which makes them inappropriate for determination using common electrochemical methods.24–29 As far as we know, there are many reports on the determination and detection of HY and HA using voltammetric and amperometric methods30–36 but only in some cases are both HY and HA determined simultaneously.37–39 Benvidi et al. in 2015 used a magnetic bar carbon paste electrode (CPE) modified with reduced graphene oxide/Fe3O4 nanozeolites and a heterogeneous mediator for simultaneous determination of HY and HA.39 Also, in other work, Zare et al. applied nano-scale islands of ruthenium oxide as a bifunctional electrocatalyst for simultaneous catalytic oxidation of HY and HA.37 Zeolites are one of the nanoporous substances that are possible supports in many modified electrodes.40 Zeolites, as microporous aluminosilicate crystallites, have been widely used for their properties such as their large surface area, high size selectivity, and appropriate pore-size distribution.41 In addition, nanozeolites are ones with dimensions below 100 nm that have

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larger surface area and higher catalytic activity than conventional zeolites. Zeolite-supported electrodes combine the significant properties of zeolites with the high sensitivity of electrochemical method which leads to significant betterment in their catalytic activity compared to the other supported electrodes. ZSM-5 zeolite has medium pores with sizes of 5.1–5.6 Å and three dimensional channels of 10-membered rings.42,43 The incorporation of materials in both the form of metals and metal oxides into the pores and channels of zeolites has opened a new window of applications such as sensors and electronics applications.44 Recent studies showed that doping of semiconductors such as CuO into a suitable support increases the activity of them. Among the different supports, zeolites due to their unique properties including ion exchange, size, charge and shape selectivity, are good candidates to support some materials and semiconductors.45 Hao et al. investigated the incorporation of Cu and CuO into MCM-41.46 Some researchers have introduced CuO nanoparticles into mesoporous silica (-MCM-41) via encapsulation. They used a facile and effective direct synthetic method for the production of Al-MCM-41 capsules with CuO nanoparticles.47 Copper oxide (CuO) as an important p-type semiconductor with low cost, low toxicity, and narrow band-gap (1.2–1.7 eV) absorbs visible light to generate electron–hole pairs (e –h+) with enough life-time to let chemical reactions occur.48,49 Azizi et al. synthesized ZSM-5 NPs from bagasse as a low cost silica source and doped it with silver nanoparticles to study the electrooxidation of oxalic acid.50 Abrishamkar et al. in 2013 reported the synthesis and characterization of template free nano sized ZSM-5 zeolite. Then, they applied it to prepare a new zeolite-modified carbon paste electrode (ZMCPE) based on the Ni-MFI type zeolite for formaldehyde electrocatalytic oxidation.51 Moos et al. in 2006 prepared a selective impedance gas sensor for hydrocarbons using ZSM-5 zeolite films with a chromium(III) oxide interface.52 Srivastava et al. in 2015 prepared NiCo2O4/Nano-ZSM-5 nanocomposite materials by the calcination of a physical mixture of NiCo2O4 and Nano-ZSM-5 and then an electrochemical sensor based on the NiCo2O4/Nano-ZSM-5 nanocomposite constructed for simultaneous determination of ascorbic acid, dopamine, uric acid, and tryptophan.53 Kumar et al. in 2017 developed electrochemically active nonenzymatic glucose sensor probes on the basis of a nickel–cobalt (Ni–Co)/iron oxide (Fe3O4) nanocomposite.54 In this work, we synthesized ZSM-5 nanoparticles according to the method suggested in our previous work and applied it for the first time for the preparation of a CuO/ZSM-5 NP modified CPE (CuO/ZSM-5 NPs/CPE). At first, CuO/ZSM-5 NPs were prepared. In this synthetic method, CuO particles were directly encapsulated into the pores and expected to be available at higher loadings. For this propose, as a first step, copper ions were substituted with sodium cations present in the ZSM-5 network. Then, NaOH solution (0.1 M) was added to adjust the pH to 8 along with vigorous stirring. Afterwards, calcination was done at high temperature for the specified time to prepare

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CuO/ZSM-5 NPs. Afterwards, a CPE was modified with CuO/ZSM-5 NPs and was used for simultaneous electrochemical oxidation of HY and HA using DPV (differential pulse voltammetry) and amperometry techniques. To our best knowledge, there is no reported literature about the preparation of CuO/ZSM-5 NPs/ CPE as a sensor for electrocatalytic oxidation of HY and HA simultaneously.

2. Experimental and methods 2.1.

Materials and apparatus

HY, HA, graphite powder, paraffin, and sodium aluminate (NaAlO2) were purchased from Fluka. Hydrochloric acid (HCl) and sodium hydroxide (NaOH) were all purchased from Merck. In order to characterize the CuO/ZSM-5 NPs, X-ray diffraction (XRD) patterns were recorded on an advance Bruker D8 XRD using Cu Ka radiation (l = 1.5418 Å). Fourier transform infrared (FT-IR) spectra were recorded with a FTIR spectrometer (Tensor 27 Bruker) at room temperature in the range of 400–4000 cm 1. The morphology of the prepared samples was investigated using scanning electron microscopy (SEM; EM-3200, KYKY) followed by determination of the elemental composition by energy dispersive X-ray spectrometry (EDS). The morphology of samples was also investigated with transmission electron microscopy (TEM; EM-10C, ZEISS). The specific surface areas of the samples were determined from the linear portion of the Brunauer–Emmett–Teller (BET, model BELSORP mini 100 instrument) plots. The pore size (diameter DBET) distribution was calculated from nitrogen adsorption data using the conventional Barrett–Joyner–Halenda (BJH) method. A DropSens bipotentiostat/ galvanostat (mSTAT 400) was applied for all the electrochemical measurements. An Ag|AgCl|KCl (3 M) electrode, a platinum wire and CuO/ZSM-5 NPs/CPE were used as reference, auxiliary and working electrodes, respectively. 2.2.

Preparation of ZSM-5 NPs

Nanosilica was extracted from a natural source like in our previous study50 according to the Kalapathy method with some modification.55 In a typical synthesis, nanosilica powder was extracted from BGA ash by refluxing in 2.5 M HCl solution under stirring at 100 1C for 8 h. Then, it was cooled at room temperature. Afterwards, the formed sodium silicate solution was centrifuged and titrated with 1 M HCl to neutralize it for the formation of a silica gel. After filtering of the formed silica gel, it was washed with doubled deionized water several times. Then, the product was dried at 70 1C for 12 h. ZSM-5 NPs were synthesized using silica powder extracted from BGA ash and aluminum isopropoxide as a silica and aluminum source, respectively. Also, TMAOH was applied as a structure directing agent. Reactants were mixed in the molar ratio as follows: Al2O3 : 60.25SiO2 : 0.30Na2O : 15.06TMAOH : 1156.62H2O. At first, 0.12 g of aluminum isopropoxide was dissolved in 8.5 mL of a solution of TMAOH (1 M) with vigorous stirring. Then, 11.4 mL of deionized water and 0.0035 g of NaOH were added to the above solution. Subsequently, 2.3 g of


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was done for 30 min. Afterwards the synthesized gel was transferred into a Teflon-lined stainless steel autoclave and hydrothermally heated at 100 1C for 18 h in an oven. The


silica extracted from BGA was supplemented to obtain a solution under vigorous stirring that was continued for 12 h at ambient temperature. After that, ultrasound pre-treatment

Fig. 1 (A) XRD pattern, (B) (a) SEM image and (b) EDS spectrum, (C) SEM images at two magnifications, 1 mm and 500 nm, of CuO/ZSM-5 NPs, (D) N2 adsorption–desorption isotherm and (E) pore diameter distribution of (a) ZSM-5 NPs and (b) CuO/ZSM-5 NPs.

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obtained ZSM-5 NPs were centrifuged, thoroughly washed several times with deionized water and dried at 105 1C overnight and calcined at 550 1C for 10 h.

filtered, washed with deionized water several times, and dried at 80 1C overnight. Finally, the powder mixture was calcined at 400 1C for 3 h to prepare CuO/ZSM-5 NPs.

2.3.

2.4.

Preparation of CuO/ZSM-5 NPs

At first, 0.605 g of Cu (NO3)23H2O (0.1 M) was dissolved in 25 mL of deionized water. Then, 0.5 g of ZSM-5 (Si/Al = 35) was added to it under vigorous magnetic stirring for 5 h to exchange Cu2+ ions in solution with Na+ ions in ZSM-5. The pH value of the mixture was adjusted to 8.0 by adding a NaOH solution (0.1 M) to the vigorously stirred solution. The gel mixture was stirred for 5 h and aged for 12 h. The resulting product was

Preparation of modified electrodes

To prepare CuO/ZSM-5 NPs/CPE, 1.0 g of CuO/ZSM-5 NPs, 0.1 g of graphite and 6 droplets of paraffin were mixed in a mortar by hand mixing for 20 min until a homogenous paste was obtained. The resulting paste was inserted at the bottom of a glass tube (with internal radius 1.5 mm). The electrical connection was implemented by a copper wire lead fitted into the glass tube. Also, ZSM-5 NPs/CPE was prepared by mixing

Fig. 2 (A) TEM images of (a) ZSM-5 NPs and (b) CuO/ZSM-5 NPs, and (B) CVs of CuO/ZSM-5 NPs/CPE in 0.1 M PBS buffer solution at pH 7.0 at a scan rate of 0.02 V s 1.


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1.0 g of ZSM-5 NPs, 0.1 g of graphite and 6 droplets of paraffin. CPE was prepared by blending 0.1 g of graphite powder and 4 droplets of paraffin oil.

3. Results 3.1.

Characterization

The XRD pattern of CuO/ZSM-5 NP powder in the 2y range of 5–601 is shown in Fig. 1A. As can be seen, the main peaks at 2y = 7.9, 8.9, 23.2 and 24.51 are related to ZSM-5 NPs.56 Also, the peaks at 2y = 36, 38.8, 49 and 591 demonstrate the formation of CuO without impurities indicating that the structure of the ZSM-5 NPs was well-preserved after modification to CuO/ZSM-5 NPs.57 Fig. S1 (ESI†) demonstrates the FT-IR spectrum recorded for the CuO in CuO/ZSM-5 NP powder in the range of 400 to 4000 cm 1. The peak at 603 cm 1 may be attributed to the Cu–O stretching.58 Also, Fig. S1 (ESI†) presents FT-IR spectra of ZSM-5 NPs. The bands situated at 790 and 1100–1230 cm 1 display SiO4 tetrahedron units. The external asymmetric stretching vibration near 1219 cm 1 is as a result of the presence of structures containing four chains of fourmembered rings of ZSM-5 NPs. The band near 790 cm 1 is assigned to the symmetric stretching of external linkages and the one near 540 cm 1 is ascribed to a structure-sensitive vibration brought about by the double four-membered rings of the external linkages. The absorption band near 450 cm 1 is due to the T–O bending vibrations of SiO4 and AlO4 internal tetrahedra. The presence of absorption bands at 540 and 450 cm 1 is characteristic of ZSM-5.59 EDS analysis along with SEM characterization were performed to study the elements present in the CuO/ZSM-5 NP powder (Fig. 1(B-a, b)). As can be seen, the spectrum illustrates the presence of O, Si, Al and Cu elements without detecting any impurities. The presence of Si, Al and O was from the ZSM-5 NP substrate. Hence, the formation of CuO in the ZSM-5 NP structure can be proved from the EDS analysis. The SEM images of the CuO/ZSM-5 NPs sample are displayed in Fig. 1(C-a, b) at two different magnifications. The images represent the regular shape with an accumulated spherical morphology of the ZSM-5 NPs. The average particle size of the material is about 60 nm. With doping of CuO into the pores of the ZSM-5 NPs, the properties of the zeolites change. Zeolites doped with CuO can exhibit smaller pore diameters, thicker pore walls, and enhanced thermal stability.47 The N2 adsorption–desorption isotherms of ZSM-5 NPs exhibit a type 1 isotherm with a H4 hysteresis loop which demonstrate the formation of micro-pore zeolites (Fig. 1D(a)). From the BET equation and t-plot method, the surface area (300 m2 g 1) and total pore volume (0.169 cm3 g 1) are obtained. On the other hand, the direct synthesized CuO/ZSM-5 NP sample (Fig. 1D(b)) retains the same isotherm shapes but the amount of adsorbed N2 molecules decreases and the onset of the capillary condensation step shifts to a smaller relative pressure. The decrease of the absorption amount can be attributed to the reduced surface area, whereas the shift of inflection point of the step to the lower relative pressure P/P0 is caused by the smaller pore size. This can be associated with the pore-filling effect due to doping of the CuO NPs into the micropores of the solid materials.

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Fig. 3 DPVs of (A) CPE, (B) ZSM-5 NPs/CPE and (C) CuO/ZSM-5 NPs/CPE in the (a) absence and (b) presence of HY (0.3 mM), and HA (0.5 mM) in 0.1 M PBS buffer solution at pH 7.0 at a scan rate of 0.02 V s 1 with a pulse amplitude of 50 mV.

For the CuO/ZSM-5 NP sample, according to the BET equation and t-plot method, the surface area (180 m2 g 1) and total pore volume (0.12 cm3 g 1) are obtained. Also, the pore size distributions (Fig. 1E(a and b)) were examined from the adsorption branches of the isotherms using the BJH model. As can be seen from BJH, the pore diameters decrease from 3.86 nm to 2.22 nm for ZSM-5 NPs and CuO/ZSM-5 NPs, respectively. Fig. 2A(a and b) indicate the TEM images of ZSM-5 NPs and CuO/ZSM-5 NPs at various magnifications, respectively. As can


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be seen, with doping of CuO into the ZSM-5 NPs, dark spots appear in the TEM image of the zeolite. This is due to the presence of CuO in the structure of the zeolite particle. In other words, this can prove the incorporation of the CuO particles into the pores of ZSM-5 NPs. In other words, here, CuO was directly encapsulated into the pores and expected to be available at higher loadings. 3.2.

Electrochemical behaviour of the modified electrode

Cyclic voltammetry (CV) of CuO/ZSM-5 NPs/CPE was recorded in 0.1 M PBS buffer solution at pH 7.0 at a scan rate of 0.02 V s 1 (Fig. 2). CuO/ZSM-5 NPs/CPE exhibits two anodic peaks for oxidation of Cu0 to Cu+(I) and Cu+ to Cu2+(II) at 0.04 and 0.29 V vs. Ag|AgCl and two cathodic peaks for reduction of Cu2+ to Cu+(III) and Cu+ to Cu0(IV) at 0.08 and 0.19 V vs. Ag|AgCl, respectively.60–62 These observations can be ascribed to the oxidation/reduction of CuO species inserted into the pores of ZSM-5/CPE during Cu2+ ion exchange with Na+ ions and subsequent calcination at high temperature. 3.3.

Voltammetric studies of HZ and HA on the modified electrode

Fig. 3 displays the comparative behaviour of CPE, ZSM-5 NPs/ CPE, and CuO/ZSM-5 NPs/CPE in the absence and presence of a mixture of HY (0.3 mM) and HA (0.5 mM) in 0.1 M PBS buffer solution at pH 7.0 by the DPV technique at a scan rate of 0.02 V s 1 with a pulse amplitude of 50 mV. Fig. 3A(a and b) compares the DPVs of CPE in the absence and presence of a mixture of HY and HA, respectively. As can be seen, weak, small and broad oxidation peaks with low current intensities and high potentials at 0.67 and 0.90 V vs. Ag|AgCl for HY and HA, respectively, appeared in the mixture. This demonstrates that the CPE is not suitable for the determination of HY and HA. Fig. 3B(a and b) displays the behaviour of ZSM-5 NPs/CPE in the absence and presence of a mixture of HY and HA, respectively. It is seen that almost similar results to the previous electrode are obtained with the difference that the oxidation peak current intensities increase for both HY and HA. Also, the peak potentials that appeared at 0.46 and 0.56 V vs. Ag|AgCl decrease. This matter is due to the presence of ZSM-5 NPs with insulating nature which has no direct role in the oxidation of HY and HA. Also, Fig. 3C(a and b) shows the behaviour of CuO/ZSM-5 NPs/CPE in the absence and presence of a mixture of HY and HA, respectively. Two clear peaks are observed at the potentials of 0.25 and 0.40 V vs. Ag|AgCl displaying an improvement in the oxidation peak currents of HY and HA. Moreover, the oxidation peaks of HY and HA shift to negative potentials, leading to more peak separation between HY and HA (0.15 V). This behaviour is ascribed to the presence of ZSM-5 NPs as a suitable support for the catalytic performance of CuO in the electrode which provides a porous structure with a high surface area for incorporation of catalysts. In other words, the porosity of the ZSM-5 NPs is more than that of CPE. This matter leads to the acceleration of HY and HA oxidation. Based on these results, when CuO/ZSM-5 NPs/CPE was used as the working electrode the detection sensitivity improved considerably and impressive separation of the anodic peaks of HY and HA was attained.

Fig. 4 (A) DPVs of CuO/ZSM-5 NPs/CPE in 0.1 M PBS buffer solution at pH 7.0 in the presence of HY (0.3 mM) and HA (0.5 mM) recorded at various scan rates at the range of 0.02 to 0.1 V s 1 with a pulse amplitude of 50 mV, (B) plot of I vs.n1/2, and (C) plot of Epa vs. log n.

3.4.

Effect of scan rate

The DPVs of CuO/ZSM-5 NPs/CPE in 0.1 M PBS buffer solution at pH 7.0 in the presence of HY (0.2 mM) and HA (0.5 mM) were recorded at various scans rate from 0.02 to 0.1 V s 1 with a pulse amplitude of 50 mV (Fig. 4A). As can be seen, with increasing scan rate, the peak currents increase for both HY and HA. There is a linear correlation between the anodic peak currents and n1/2 proposing that the kinetics of the overall process are controlled by mass transport of HY and HA from the bulk solution to the modified electrode surface (Fig. 4B). It is noteworthy that the oxidation peak potentials (Ep) for HY and HA shift to more positive values with increasing scan rate (Fig. 4C).


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Scheme 1

Mechanism of simultaneous determination of HY and HA at CuO/ZSM-5 NPs/CPE.

The electrooxidation of HY at the CuO/ZSM-5 NPs/CPE occurs according to the following equation with the final product of N2 in aqueous solution: N2H4 + 2Cu2+ - N2 + 2Cu0 + 4H+.

peak currents at the CuO/ZSM-5 NPs/CPE in 0.1 M PBS buffer solution at pH 7.0 at a scan rate of 0.02 V s 1 using a DPV

(1)

Also, in general, the following mechanism can be considered for the electrooxidation of HA to N2O: 2Cu2+ + 2NH2OH - 2Cu0 + N2O + 3H2O.

(2)

The mechanisms for simultaneous determination of HY and HA on the fabricated sensor (CuO/ZSM-5 NPs/CPE) are shown in Scheme 1. 3.5.

Effect of HY and HA concentrations

Fig. 5 shows the effect of different concentrations of both of HY and HA (in the 0.02–0.4 mM range) on the electrooxidation

Fig. 5 DPVs of the mixtures of HY and HA at the CuO/ZSM-5 NPs/CPE electrode in 0.1 M PBS buffer solution at pH 7.0 and a scan rate of 0.02 V s 1 with pulse amplitude 50 mV. Concentrations for the inner to outer curves: HY and HA from a–h: 0.02, 0.1, 0.15, 0.2, 0.25, 0.3, 0.35 and 0.4 mM.

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Fig. 6 DPVs at the CuO/ZSM-5 NPs/CPE electrode (A) containing HA (0.4 mM) and different concentrations of HY (from inner to outer): 0.02, 0.10, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4 and 0.5 mM and (B) containing HY (0.4 mM) and different concentrations of HA (from inner to outer): 0.02, 0.1, 0.15, 0.25, 0.3, 0.35, 0.4, 0.5, 0.6, 0.65 and 0.9 mM in 0.1 M PBS buffer solution at pH 7.0 at a scan rate of 0.02 V s 1 with a pulse amplitude of 50 mV.


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amplitude of 50 mV. The oxidation peaks of these two compounds are distinctly separated from each other. If the concentrations of HY and HA are increased simultaneously, the peak currents at the CuO/ZSM-5 NPs/CPE electrode increase accordingly (Fig. 5). 3.6. Simultaneous oxidation of HY and HA on CuO/ZSM-5 NPs/CPE The electrocatalytic oxidation behaviour of HY and HA (in the range of 0.02–0.5 and 0.02–0.9 mM, respectively) was studied at CuO/ZSM-5 NPs/CPE in 0.1 M PBS buffer solution at pH 7.0 using DPV and at a scan rate of 0.02 V s 1 with a pulse amplitude of 50 mV. In this case, the concentration of one of the species was changed, whereas the other species concentration is kept constant and the results are presented in Fig. 6. Inspection of Fig. 6A displays that the anodic peak current of HY increases with increase in HY concentration when the concentration of HA is kept constant. Although the current was enhanced after oxidation of HY, the peak currents of HY did not change. As shown in Fig. 6B, when the concentration of HY was kept constant, the oxidation peak currents of HA are proportional to its concentration while the current for the other compound does not change. 3.7. Amperometric measurements of HY and HA at CuO/ZSM-5 NPs/CPE Fig. 7A(a and b) shows the amperometric response of CuO/ ZSM-5 NPs/CPE to the consecutive additions of HY and HA separately in a stirring 0.1 M PBS buffer solution at pH 7.0 with an applied practical potential of 0.25 and 0.40 V vs. Ag|AgCl, respectively. The amperometric method is a very sensitive and more effective method than CV for determination and measurement of low concentrations of different analytes. As can be observed in the figures, the sensor displays quick and fast amperometric responses and could obtain 95% of its steady state current within 3 s. Calibration plots of the amperometric current responses versus HY and HA concentrations are depicted in Fig. 7B(a and b), respectively. As can be seen, the current response and HY concentration have linear relationships in the ranges of 25 mM to 0.9 mM and 0.9 mM to 4.5 mM. Also, there are linear relationships in the ranges of 20 mM to 0.9 mM and 0.9 mM to 7.0 mM for HA. Also, low detection limits of 3.6 mM and 3.2 mM (S/N = 3) are achieved for HY and HA, respectively. The analytical parameters of the CuO/ZSM-5 NPs/CPE are compared with some previously reported sensors for HY and HA and the obtained results are summarized in Table 1. These results demonstrate that the suggested electrode shows good and suitable analytical performance for determination of HY and HA. This can be ascribed to the addition of porous zeolite NPs to the electrode followed by incorporation of CuO particles in their structure with effective surface area and nice electron transfer kinetics. CuO/ZSM-5 NPs can provide easier access of analytes into the active sites and facilitate the transport of electrons. As a result, CuO/ZSM-5 NPs/CPE has a rapid response, high sensitivity, low detection limit and wide linear range toward the simultaneous electrooxidation of HY and HA.

Fig. 7 (A) Amperometric responses of the CuO/ZSM-5 NPs/CPE to successive addition of (a) HY at an applied potential of 0.25 and (b) HA at an applied potential of 0.40 V vs. Ag|AgCl in a continuously stirred 0.1 M PBS buffer solution at pH 7.0 and (B) calibration plots of CuO/ZSM-5 NPs/CPE in the presence of various concentrations of (a) HY (25 mM to 4.5 mM) and (b) HA (20 mM to 7.0 mM).


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Table 1

Comparison of analytical parameters for simultaneous determination of HY and HA at CuO/ZSM-5 NPs/CPE with other sensors

Sensor

Electrolyte (pH)

Method

Applied potential HY and HA, respectively (V) vs. Ag|AgCl

MBCPE/DPSPP/ RGO/Fe3O4NPsa Au/PPy/GCEb RuON-GCEc

0.1 M PBS (pH 7.0)

CV and DPV

0.7 and 0.2

0.1 M PBS (pH 7.0) 0.1 M PBS (pH 3.0)

DPV DPV

0.135 and 0.218 0.19 and 0.45

CuO/ZSM-5 NPs/CPE

PBS 0.1 M (7.0)

DPV and Amperometry

0.25 and 0.40

Linear range for HY and HA, respectively (mM) 120.0–600.0 nM and not reported 5 to 200 mM and 10 to 200 mM 2.0–268.3, 268.3–417.3 mM and 4.0–33.8, 3.8–78.3 mM 25–4500 and 20–7000

a Magnetic bar carbon paste electrode modified with reduced graphene oxide/Fe3O4 nanozeolites. modified glassy carbon electrode. c Ruthenium oxides as a bifunctional.

3.8.

Interference study

The selectivity of the suggested sensor for both HY and HA (0.5 mM) determination was examined in the presence of a diverse range of presumably interfering materials including F , Cl , NO3 , Na+, K+, NH4+, Ba2+, Ca2+, Co2+, glucose, sucrose and ascorbic acid and the results are shown in Fig. 8(A and B). As can be seen, the presence of the above-mentioned materials does not result in remarkable amperometric responses for both HY (Fig. 8A) and HA (Fig. 8B), while good amperometric

b

Detection limit (mM)

Ref.

40.0 nM and not reported 0.20 and 0.21 mM 0.15 and 0.45 mM

39

3.6 and 3.2

63 37 This work

Gold nanozeolite–polypyrrole nanowire

responses are seen for both HY and HA in the absence (a) and presence of these materials (b–m). Hence, the sensor exhibits good anti-interference capability for the determination of both HY and HA. Also, it can be concluded that even the presence of several-fold excess concentrations of interfering species cannot interfere with the detection of both HY and HA, which is mostly ascribed to the low applied potential of the sensor during the determination of analytes. 3.9.

Real sample analysis

Water samples were prepared from various sources and used for real sample analysis in order to evaluate the practicability of the sensor toward detection of both HY and HA (Table 2). For analysis of HY and HA in real samples, a standard addition method was employed. For this purpose, standard solutions of HY and HA were added to the sample. Then, the standard addition curves were utilized for the determination of HY and HA concentrations. Nevertheless, the recovery of the suggested sensor was evaluated by spiking of a known amount of HY and HA into the sample. The spiked concentrations were 0.29 and 0.34 mM for HY and 0.80 and 0.68 mM for HA, respectively. The acquired results are reported in Table 2. It can be observed that the measured concentrations are 0.31 and 0.32 mM for HY and 0.76 and 0.70 mM for HA with corresponding RSDs of 1.6 and 2.0 for HY and 3.4 and 3.2 for HA for five-time reduplicative evaluations. In addition, the corresponding recoveries are 106.89 and 94.11% for HY and 95 and 102.94% for HA. Hence, the acceptable recoveries obtained in real sample analysis demonstrate the practical possibility of the suggested sensor for the determination of both HY and HA in water sample analysis.

Table 2 Determination of HA and HY in water samples using CuO/ZSM-5 NPs/CPE

Fig. 8 (A and B) Amperometric response of CuO/ZSM-5 NPs/CPE for the successive injection of 0.5 mM HY and HA (a) in the presence of several-fold excess concentrations of F (b), Cl (c), NO3 (d), Na+ (e), K+ (f), NH4+ (g), Ba2+ (h), Ca2+ (i), Co2+ (j), glucose (k), sucrose (l) and ascorbic acid (m) in 0.1 M PBS buffer solution at pH 7.0 at potentials of 0.25 and 0.40 V vs. Ag|AgCl, respectively, applied for measuring HY and HA depending on their oxidation potential.

13720 | New J. Chem., 2017, 41, 13712--13723

Sample

Analyte

Added (mM)

Found (mM)

RSD (n = 5)

Recovery (%)

Tap water

HY HA

0.29 0.80

0.31 0.76

1.6 3.4

106.89 95

River water

HY HA

0.34 0.68

0.32 0.70

2.0 3.2

94.11 102.94

RSD: relative standard deviation; n: number of reduplicative evaluations


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provides appropriate substrates for CuO particles. This study also illustrates good RSD% and the feasible analytical usefulness of the modified electrode for determination of HY and HA in two tap and river water samples.

Conflicts of interest Fig. 9 Long term current–time curves of CuO/ZSM-5 NPs/CPE obtained in a solution containing 0.5 mM of (A) HY and (B) HA in 0.1 M PBS buffer solution at pH 7.0. The practical potentials of 0.25 (for HY) and 0.40 (for HA) V were applied for 1500 s to the electrode.

There are no conflicts to declare.

References 3.10.

Stability and reproducibility

The long-term stability of CuO/ZSM-5 NPs/CPE was checked by evaluating its response for both HY and HA oxidation after 3 months of keeping in a normal temperature laboratory. Fig. 9(A and B) display long term current-time curves of CuO/ ZSM-5 NPs/CPE toward a practical potential of 0.25 and 0.40 V vs. Ag/AgCl for 1500 s in a solution containing 0.5 mM of HY and HA in 0.1 M PBS buffer solution at pH 7.0. The electrode retains 95% of its initial response, for both HY and HA. As can be seen, the current decline is considerably slower in all experiment ranges, which illustrates its excellent endurance capability toward both HY and HA electrocatalytic oxidation. Also, the reproducibility of the suggested sensor was measured by recording the cyclic voltammograms of 5 various electrodes which were made under similar conditions toward HY and HA electrocatalytic oxidation and a relative standard deviation of 2.5% was acquired. These results demonstrate appropriate reproducibility for the suggested sensor.

4. Conclusions In this study, we introduced a novel electrochemical sensor for simultaneous determination of HY and HA on the basis of CuO/ ZSM-5 NPs/CPE for the first time with the ion exchange method followed by calcination at high temperature. The prepared CuO/ZSM-5 NPs/CPE presents good electrocatalytic behaviour exhibiting a decrease in overpotential of HY and HA oxidation and increase in oxidation peak currents. The modified electrode was used for sensitive determination of HY and HA using DPV and amperometry techniques. In amperometric measurements, the current response and the HY concentration have linear relationships in the ranges of 25 mM to 0.9 mM and 0.9 mM to 4.5 mM. Also, there are two linear relationships for the current–time responses of HA in the ranges of 20 mM to 0.9 mM and 0.9 mM to 7.0 mM. Moreover, low detection limits of 3.6 mM and 3.2 mM (S/N = 3) are achieved for HY and HA, respectively. The CuO/ZSM-5 NPs/CPE presented suitable stability together with lower detection limit, good linear range, high sensitivity and short response time as well as antiinterference capability. This remarkable activity of CuO/ZSM-5 NPs/CPE can be ascribed to the presence of highly porous ZSM-5 NPs with a wide surface area on the electrode surface which

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