ZrO₂ supported Nano-ZSM-5 nanocomposite material for the nanomolar electrochemical detection of metol and bisphenol A

Abstract

ZrO₂ decorated nanocrystalline ZSM-5 nanocomposites with different weight ratios were prepared by the calcination of a physical mixture of nanocrystalline ZrO₂ and Nano-ZSM-5. The material was characterized by the complementary combination of X-ray diffraction, N₂-adsorption, and transmission/scanning electron microscopic techniques. An electrochemical sensor based on the ZrO₂ supported nanocrystalline zeolite was developed for the nanomolar detection of organic water pollutants metol and bisphenol A with high electrocatalytic activity, stability, sensitivity, and selectivity. Under the optimum conditions, a wide linear range was obtained from 4 nM to 800 μM and 6 nM to 600 μM with a limit of detection of 1 nM and 3 nM for metol and bisphenol A, respectively. The analytical performance of the developed sensor was demonstrated for the determination of metol and bisphenol A in different real samples such as river water, metol in photographic solution, and bisphenol A in baby bottles with satisfactory results.

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Introduction

Organic water pollutants have attracted much attention from researchers and scientists in the last decade because of their toxicity and persistence in the environment. The major sources of water pollution are sewage and industrial waste discharged into the rivers. One such important industrial organic compound is bisphenol A (BPA, 2,2-bis(4-hydroxyphenyl)propane).¹ BPA is one of the highest yield chemicals produced worldwide.² BPA is a major monomeric material used in the synthesis of polycarbonate plastics and epoxy resins that are widely used as food storage and packaging materials such as baby bottles, water bottles, water pipes, tableware, and inner coatings of beverage cans etc.^3–5 The widespread use of BPA and its permeation into food and environment from the storage and packaging materials is a serious concern with respect to human health.^6,7 BPA is also released into the environment via waste water from plastic producing industrial sites. The U.S. environmental protection agency in 2010 reported that over one million pounds of BPA are released into the environment annually.⁸ BPA is considered as endocrine disrupting chemicals (EDCs), which mimics the function of hormone estradiol, binds and activates estrogen receptor, and disrupts the endocrine system of human beings.^9,10 Exposure to BPA is considered to be associated with many adverse effects including cardiovascular diseases, obesity, carcinogenicity, neurotoxicity and developmental problems.^11–13 Studies have shown that a very low level exposure of BPA may results in the disruption of cell function and thyroid hormone action.^8,14 Moreover, due to the negative impact of BPA especially on children and infants, the use of BPA in baby bottles has been banned in many countries.⁸ Similar to BPA, metol (N-methyl-p-aminophenol sulfate) is an organic contaminant found in water bodies. Metol is commonly used as monochrome photographic chemical.^15,16 Metol is released into the environment in waste water. Metol is toxic and may cause an allergic skin reaction, serious eye irritation, and damage to the organs through prolonged or repeated exposure.^15,16 It is very toxic to aquatic life with long lasting effects. Hence, it is very important to develop the detection methods for highly sensitive and rapid monitoring of these organic water pollutants.

The electrochemical detection techniques have been proved impressive because of their simplicity, cost-effectiveness, rapid analysis, high sensitivity & selectivity, and possibility of in situ analysis. The direct determination of these organic pollutants using electrochemical sensor is not feasible due to their poor response at traditional electrodes. In order to solve this problem, novel sensing materials with high stability, good catalytic activity, and high sensitivity are required. A large number of electrochemical sensors based on noble metal nanoparticles supported carbon materials have been developed for the detection of bisphenol A.^1,9,17–21 However, only a few electrochemical sensors, namely LiCoO₂ and 1-ethyl-3-methylimidazolium tetrafluoroborate based electrodes are known for the detection of metol.^15,16 Therefore, development of electrode materials for the determination of these organic water pollutants with low limit of detection and high sensitivity is still a challenge.

Our research is mainly focused on the development of nanocomposite materials based on mesoporous zeolites, metal oxides/mixed metal oxides and find their applications in the fabrication of efficient electrochemical sensors.^22–24 In the recent years, we have demonstrated the application of nanocrystalline zeolites based electrodes for the electrochemical detection of important organic/inorganic water pollutants.^25–29 Transition metal oxides and mixed metal oxides exhibit tunable redox and conducting properties, which are responsible for their wide applications in electrocatalysis. Among these transition metal oxides, zirconium oxide commonly known as zirconia has been used in electrocatalysis. Only a handful of references are available which deals with ZrO₂ based electrochemical sensors. This includes electrochemical sensors for organophosphate pesticides, ractopamine & salbutamol, and dopamine & paracetamol.^30–32 Therefore, in this study effort was made to develop ZrO₂ based electrochemical sensors for the detection of BPA and metol. In this study, zirconia-nanocrystalline zeolite nanocomposite materials were synthesized and explored in the electrochemical determination of BPA and metol.

In the present work, a novel ZrO₂ supported nanocrystalline ZSM-5 zeolite (ZrO₂/Nano-ZSM-5) nanocomposite material was synthesized. Impressively high electrocatalytic activity was achieved at ZrO₂/Nano-ZSM-5 modified glassy carbon electrode (GCE) in the determination of BPA and metol when compared with the bare GCE, ZrO₂ or Nano-ZSM-5 modified GCEs. To the best of our knowledge, this is the first report, which deals with the nanomolar determination of BPA and metol using ZrO₂/Nano-ZSM-5 nanocomposite as an electrode material.

Experimental section

Materials

All chemicals were of analytical reagent grade and used as received without further purification. Tetraethylorthosilicate (TEOS, 98%), tetrapropylammonium hydroxide (TPAOH), propyltriethoxy silane (PrTES, 97%), and Pluronic F-127 (HO(CH₂CH₂O)₁₀₆(CH₂CH(CH₃)O)₇₀(CH₂CH₂O)₁₀₆H, designated as EO₁₀₆PO₇₀EO₁₀₆) were purchased from Sigma Aldrich, India. Zirconylnitrate hydrate (ZrO(NO₃)₂·H₂O) was obtained from Loba Chemie Pvt. Ltd., India. Phloroglucinol was obtained from Spectrochem Pvt. Ltd., India and formaldehyde was purchased from SD Fine Chemical Ltd., India. Deionized water from Millipore Milli-Q system (resistivity 18 MΩ cm) was used in the electrochemical studies. Electrochemical measurements were performed in 0.1 M phosphate buffer (Sorenson's buffer) solution, which was prepared by mixing potassium monohydrogen phosphate (Na₂HPO₄) and potassium dihydrogen phosphate (NaH₂PO₄). The standard phosphate buffer solutions (PBS) with different pH values (lower or higher) were prepared by adding 0.1 M aqueous H₃PO₄ or NaOH solution to 0.1 M aqueous PBS, while magnetically stirring until the pH of the aqueous solution reached the desired value. All electrochemical experiments were performed in 0.1 M PBS at pH 7, unless specified otherwise.

Synthesis of ZrO₂/Nano-ZSM-5 nanocomposite

Nanocrystalline ZSM-5 zeolite (Nano-ZSM-5) was prepared using molar composition 90 TEOS/10 PrTES/2.5 Al₂O₃/3.3 Na₂O/25 TPAOH/2500 H₂O by following the reported procedure.³³

Nanocrystalline ZrO₂ was synthesized by following our reported procedure.³⁴ In a typical synthesis, 1.62 g of phloroglucinol and 4.86 g of F-127 were dissolved in 25 mL of absolute ethanol. After dissolving the solid, 2.31 g of zirconylnitrate hydrate was added and stirred at ambient temperature for half an hour to ensure the complete dissolution. 0.15 g of HNO₃ (65%) was added to the above reaction mixture. The resultant solution was stirred at ambient temperature for half an hour. Subsequently, 1.6 g of formaldehyde (37%) was added to the above solution. The reaction mixture was stirred for further 4 h at ambient temperature and then transferred to a Petri dish and ethanol was evaporated at ambient temperature. It was then cured at 373 K for 24 h before carbonization. Material was carbonized at 1073 K under nitrogen atmosphere via heating ramp of 1 deg per min and kept at 1073 K for 3 h. The resultant carbon–metal composite was further calcined at 773 K for 4 h.

For the synthesis of ZrO₂/Nano-ZSM-5 nanocomposite materials, ZrO₂ and Nano-ZSM-5 with different weight ratios (10, 20, and 30, denoted as ZrO₂(10%)/Nano-ZSM-5, ZrO₂(20%)/Nano-ZSM-5 and ZrO₂(30%)/Nano-ZSM-5, respectively) were grounded uniformly with ethanol using mortar and pastel. Ethanol was slowly removed during the mixing process. The mixture was heated at 473 K in air for 45 min to remove the residual ethanol, followed by calcination at 773 K for 12 h in air to obtain ZrO₂/Nano-ZSM-5 nanocomposite.

Instrumentation

X-ray diffraction (XRD) patterns were recorded in the 2θ range of 5–70° with a scan speed of 2° min⁻¹ on a PANalytical X’PERT PRO diffractometer, using Cu Kα radiation (λ = 0.1542 nm, 40 kV, 40 mA) and a proportional counter detector. Nitrogen adsorption measurements were performed at 77 K by Quantachrome Instruments, Autosorb-IQ volumetric adsorption analyzer. Sample was out-gassed at 573 K for 3 h in the degas port of the adsorption apparatus. The specific surface area of material was calculated from the adsorption data points obtained at P/P₀ between 0.05 and 0.3 using the Brunauer–Emmett–Teller (BET) equation. The pore diameter was estimated using the Barret–Joyner–Halenda (BJH) method. TEM investigations were carried out using a FEI, Tecnai G² F30-ST microscope operating at 300 kV. The microscope is equipped with a scanning unit and a HAADF detector from Fischione (model 3000). High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and energy dispersive X-ray spectroscopy (EDS, EDAX Inc.) was performed using the same microscope. The sample was dispersed in ethanol using ultrasonic bath, and dispersed sample was mounted on a carbon coated Cu grid, dried, and used for TEM measurement. Scanning electron microscopy (SEM) measurements were carried out on a JEOL JSM-6610LV.

Electrode fabrication

Cyclic voltammetry (CV), differential pulse voltammetry (DPV), and chronoamperometry studies were performed using Potentiostat-Galvanostat BASi EPSILON, USA. A three-electrode electrochemical cell was employed with Ag/AgCl as the reference electrode (3 M KCl), ZrO₂/Nano-ZSM-5 mounted glassy carbon (3 mm diameter) as the working electrode and Pt foil as the counter electrode. Before modification, the glassy carbon electrode (GCE) was first polished to a mirror like surface with alumina slurry and then ultrasonicated in ethanol and deionized water for 5 min, respectively. 10 μL aliquot of ZrO₂/Nano-ZSM-5 suspension (a homogenous sonicated solution of 2 mg of ZrO₂/Nano-ZSM-5 nanocomposite, 10 μL of Nafion and 1 mL of deionized water) was placed onto the GCE surface. The electrode was dried in air leaving the material mounted onto the GC surface. For comparison, the other modified glassy carbon electrodes were also fabricated in a similar way. Electrochemical impedance spectroscopy (EIS) was performed using Autolab PGSTAT302N.

Real sample preparation

The river water sample (Sutlej River, Rupnagar, Punjab, India) was used for analysis just after filtration to remove the supernatant. The photographic solution was purchased from the local Photoshop, Rupnagar, Punjab. The baby bottle (Bonne Care Super Polycarbonate Feeder) was comminuted into small pieces and weighed (29.3 g) into a 500 mL beaker. Then, 150 mL of chloroform was added into the beaker and sealed. The sample was ultrasonicated in an ultrasonic bath for 30 min. After the sample was dissolved, 160 mL of 0.5 M sodium hydroxide solution was added as extraction solvent. The extraction process was repeated three times, and the extract solutions were collected into a 500 mL volumetric flask. The extract solution was diluted to volume with deionized water for further use.

Results and discussion

A direct, one step synthetic protocol was adopted to prepare nanocrystalline ZrO₂ using F127 tri-block copolymer (as structure-directing agent), a mixture of phloroglucinol and formaldehyde (as carbon source), and zirconylnitrate (as metal source) under mild acidic condition.³⁴ In this method, the obtained ethanolic synthesis mixture was dried under ambient condition in flat Petri disc, cured at 373 K, carbonized at 1073 K and then calcined at 773 K to obtain nanocrystalline ZrO₂. Importance of this method is the direct use of the self-assembly of block copolymer as template for the generation of porous metal–carbon structure, without the extra step required for the generation of templating silica structure. The phloroglucinol and block co-polymer based synthetic route promotes the cooperative assembly of inorganic/co-polymer mesostructure with the help of hydrogen bonds at the inorganic/organic interface, yielding thermally stable nanocrystalline ZrO₂.

Nano-ZSM-5 was synthesized in the presence of propyltriethoxysilane as an additive.³³ ZrO₂/Nano-ZSM-5 nanocomposite materials with different weight ratio were prepared by the calcination of physical mixture of ZrO₂ and Nano-ZSM-5 at 773 K. Calcination was important for the uniform dispersion of ZrO₂ nanoparticles on the surface of Nano-ZSM-5. Surface silanol groups of Nano-ZSM-5 create interface between catalytic active ZrO₂ and Nano-ZSM-5 during the calcination process. The details of physico-chemical characterization are provided in the following section. The electrocatalytic activity of the synthesized material was investigated in the electrochemical determination of organic pollutants BPA and metol. In-depth investigation was made using ZrO₂(20%)/Nano-ZSM-5 because of its higher electrocatalytic activity as described in the later part of the manuscript.

Physico-chemical characterization

Nano-ZSM-5 exhibits XRD pattern corresponding to a highly crystalline MFI framework structure with high phase purity (Fig. 1a). MFI is a three letter code suggested by the International Zeolite Association for ZSM-5 framework topology. XRD pattern of Nano-ZSM-5 is broad, confirming the nanocrystalline nature of the material. The diffraction peaks for ZrO₂ can be indexed to the tetragonal phase of crystalline ZrO₂ (Fig. 1a). Intense peaks at 2θ = 30.2°, 50.3°, 60.2° can be assigned to (011), (112), (121) reflections of the tetragonal ZrO₂ (Fig. 1a). The XRD pattern of ZrO₂(20%)/Nano-ZSM-5 shows the diffraction peaks corresponding to both, ZrO₂ and Nano-ZSM-5 phases (Fig. 1a). Calcination is favorable for the dispersion process, which appears to cause the dispersion of ZrO₂ on the surface of Nano-ZSM-5. XRD patterns of ZrO₂(10%)/Nano-ZSM-5 and ZrO₂(30%)/Nano-ZSM-5 are similar to that of ZrO₂(20%)/Nano-ZSM-5 (Fig. S1, ESI†). The textural properties of the synthesized materials are investigated by N₂-sorption measurements. ZrO₂ exhibits type IV isotherm with H2 hysteresis loop (Fig. 1b). ZrO₂ shows a pore size distribution in the range of 3–9 nm (Fig. 1b, inset). N₂-adsorption isotherms for Nano-ZSM-5 and ZrO₂(20%)/Nano-ZSM-5 exhibit type-IV isotherm similar to that of mesoporous materials (Fig. 1b). The distinct increase of N₂ adsorption for Nano-ZSM-5 in the region 0.4 < P/P₀ < 0.9 is interpreted as a capillary condensation in the intercrystalline mesopore void spaces. The mesopores show a pore size distribution in the range of 2–10 nm for Nano-ZSM-5. ZrO₂(20%)/Nano-ZSM-5 shows a pore size distribution in the range of 3–9 nm. Adsorbed volume for ZrO₂(20%)/Nano-ZSM-5 is found to be less when compared to Nano-ZSM-5, which signify that nanocrystalline ZrO₂ envelop some of the inter-crystalline mesopores and form interconnected ZrO₂-NanoZSM-5 composite during the calcination process. Textural properties obtained from N₂-adsorption study for different materials investigated in this study are summarized in Table 1. Table 1 shows that the BET surface area for ZrO₂/Nano-ZSM-5 samples decreases with increase in the ZrO₂ contents.

Fig. 1 (a) XRD patterns; and (b) N₂-adsorption isotherms of ZrO₂, Nano-ZSM-5, and ZrO₂(20%)/Nano-ZSM-5 materials. Inset shows the pore size distribution.

Table 1 Physico-chemical characteristics of various materials investigated in this study

S. No.	Sample	Total surface area SBET (m^2 g^−1)	External surface area (m^2 g^−1)	Total pore volume (cm^3 g^−1)
1	ZrO2	150	145	0.26
2	Nano-ZSM-5	542	364	0.56
3	ZrO2(10%)/Nano-ZSM-5	526	301	0.51
4	ZrO2(20%)/Nano-ZSM-5	503	290	0.49
5	ZrO2(30%)/Nano-ZSM-5	455	254	0.43

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Detailed structural characterization of ZrO₂(20%)/Nano-ZSM-5 nanocomposite was carried out using TEM. Spheroid aggregated morphology (having dimension of 300–500 nm) of Nano-ZSM-5 is actually composed of very small size nanocrystals of 10–20 nm as observed in the high resolution TEM micrograph (Fig. S2, ESI†). Mesopores are created by the crystal packing of these zeolite nanocrystals. Fig. 2a and b shows the TEM images of ZrO₂(20%)/Nano-ZSM-5. TEM image indeed shows needle like ZrO₂ crystal phase (length 30–50 nm, width 15–20 nm) in the sample. The high resolution TEM image of one such needle like ZrO₂ nanoparticle can be seen in Fig. 2c. Lattice fringes obtained from the HRTEM image is very close to the inter-planar spacing of ZrO₂. The chemical composition of the hybrid nanostructures was investigated using STEM-HAADF-EDX imaging technique. Fig. 3a depicts the STEM-HAADF image of ZrO₂(20%)/Nano-ZSM-5 nanocomposite. The spatial distributions of the atomic contents across ZrO₂(20%)/Nano-ZSM-5 nanocomposite are obtained using drift corrected EDX imaging. The STEM-HAADF image and the corresponding chemical maps for O, Al, Si and Zr from the area marked in Fig. 3a are presented in Fig. 3b–f. EDX maps confirm the decoration of Nano-ZSM-5 with ZrO₂ nanoparticles. Energy dispersive X-ray spectroscopy (EDX) spectrum is shown in Fig. 3g, which clearly shows O, Al, Si and Zr. It may be noted that Cu signals observed in the EDX mapping are due to the copper grid used in the analysis. Uniform Zr content throughout the sample also evidence for the formation of Si–O–Zr–O–Si nanocomposites.

Fig. 2 (a and b) Low magnification TEM images of ZrO₂(20%)/Nano-ZSM-5 and (c) HRTEM image of ZrO₂ particles.

Fig. 3 (a) STEM-HAADF image of ZrO₂(20%)/Nano-ZSM-5 nanocomposite, (b–f) STEM-HAADF-EDX images taken from the marked area 2 in (a), indicating the locations of different elements across the structure, and (g) EDX spectrum from a region marked by 1 in (a).

The presence of ZrO₂ on GCE surface was examined using Scanning electron microscopy (SEM) before and after the electrochemical measurements. During the SEM investigation, energy-dispersive X-ray spectroscopy (EDS) was used to determine the incorporation of ZrO₂ on the electrode surface. Fig. S3, ESI† shows the EDS mapping of ZrO₂(20%)/Nano-ZSM-5 nanocomposite before the coating on the glassy carbon electrode. Zr, Si, Al, O, and Na signals confirm the incorporation of ZrO₂/Nano-ZSM-5 nanocomposite. After the electrochemical measurements, modified electrode was washed several times with deionized water and dried. Fig. S4, ESI† shows the EDS mapping of ZrO₂(20%)/Nano-ZSM-5 nanocomposite after the electrochemical measurements. Zr, Si, Al, O, and Na signals confirm the existence of ZrO₂ and Nano-ZSM-5 on the GCE surface. S and F signals in the EDS mapping are due to the binder nafion used in the coating of ZrO₂(20%)/Nano-ZSM-5 on GCE.

Electrochemical characteristics of modified electrode

The electrochemical behavior of ZrO₂(20%)/Nano-ZSM-5/GCE and ZrO₂/GCE was investigated using CV in 0.1 M PBS (pH 7) at a scan rate 50 mV s⁻¹ (Fig. S5, ESI†). CVs of ZrO₂(20%)/Nano-ZSM-5/GCE and ZrO₂/GCE show a couple of redox peaks at 0.35 V (peak i) and −0.05 V (peak ii) corresponding to the oxidation and reduction of Zr⁴⁺ species.^35,36 The cathodic peak at −0.05 V corresponds to the reduction of Zr⁴⁺ to Zr³⁺ and the anodic peak at 0.35 V corresponds to the subsequent oxidation of Zr³⁺ to Zr⁴⁺. It may be noted that the current response for ZrO₂(20%)/Nano-ZSM-5/GCE is better than ZrO₂/GCE, implying the better electrochemical activity of ZrO₂(20%)/Nano-ZSM-5/GCE.

The electrochemical behavior of different modified electrodes is also investigated using potassium ferricyanide as electrochemical probe by CV. Study is performed in 0.1 M KCl solution containing 1 mM K₃[Fe(CN)₆]/K₄[Fe(CN)₆] at a scan rate of 10 mV s⁻¹ at different modified electrodes (ZrO₂(20%)/Nano-ZSM-5/GCE, ZrO₂/GCE, Nano-ZSM-5/GCE) and bare GCE. The CVs of various electrodes exhibit a pair of redox peaks corresponding to Fe(CN)₆^3−/4− redox couple (Fig. S6, ESI†). The modified electrodes exhibit higher peak current when compared to bare GCE. Fig. S6, ESI† shows that ZrO₂(20%)/Nano-ZSM-5/GCE exhibits the highest peak current and the lowest peak potential among the various electrodes investigated. The electroactive surface area of the different modified electrodes and bare GCE is calculated according to the Randles–Sevcik equation.³⁷ The effective surface area is calculated to be 0.32, 0.44, 0.52, and 0.83 cm² for bare GCE, Nano-ZSM-5/GCE, ZrO₂/GCE, and ZrO₂(20%)/Nano-ZSM-5/GCE. The electroactive surface area of ZrO₂(20%)/Nano-ZSM-5/GCE is 2.6 times higher than that of bare GCE. The high electroactive surface area of ZrO₂(20%)/Nano-ZSM-5/GCE is responsible for its higher electrocatalytic activity.

EIS was employed to further investigate the interface properties of various modified electrodes. Fig. 4 shows the Nyquist plot of impedance spectra recorded in 0.1 M KCl solution containing 1 mM K₃[Fe(CN)₆]/K₄[Fe(CN)₆] over a frequency range of 0.1 Hz to 10⁵ Hz with the AC signal amplitude of 5 mV at an applied potential of 0.3 V. In a typical Nyquist plot, the semicircle portion corresponds to the electron-transfer resistance (R_et) at higher frequency range whereas a linear part at lower frequency range represents the diffusion limited process. Nano-ZSM-5/GCE exhibits reduced semicircular domain when compared to bare GCE. In this study, Nano-ZSM-5 with Si/Al ratio 20 is prepared, which has sufficient number of Na⁺ ions in its matrix. Due to the conducting nature of Nano-ZSM-5 (because of Na⁺ ions present in the matrix), high surface area, and intercrystalline mesopores, diffusion of the analyte is facilitated. Therefore, R_et for Nano-ZSM-5 is somewhat smaller than that of bare GCE. The modification of GCE with ZrO₂/GCE and ZrO₂(20%)/Nano-ZSM-5/GCE displays an almost straight line indicating least electron transfer resistance at these modified electrodes. Hence, ZrO₂(20%)/Nano-ZSM-5/GCE significantly facilitates the electron transfer rate and improves the diffusion of ferricyanide towards the electrode interface.

Fig. 4 Nyquist plots of impedance spectra at various modified electrodes (Nano-ZSM-5/GCE, ZrO₂-GCE, ZrO₂(20%)/Nano-ZSM-5/GCE) and bare GCE in 0.1 M KCl solution containing 1 mM [Fe(CN)₆]^3−/4− over the frequency range from 0.1 Hz to 10⁵ Hz at an applied potential of 0.3 V.

Voltammetric studies of metol and bisphenol A

The electrochemical behavior of metol and BPA was investigated in detail at ZrO₂(20%)/Nano-ZSM-5/GCE because of its higher electrocatalytic activity as discussed in the above section. Fig. 5 shows a comparison of CVs of metol and BPA at ZrO₂(20%)/Nano-ZSM-5/GCE, ZrO₂/GCE, and bare GCE in 0.1 M PBS (pH 7) at a scan rate of 50 mV s⁻¹. The CV of metol exhibits a pair of well defined redox peaks at ZrO₂(20%)/Nano-ZSM-5/GCE, indicating a reversible electrode process (Fig. 5a). A well defined oxidation peak corresponding to BPA is observed at ZrO₂(20%)/Nano-ZSM-5/GCE (Fig. 5b). In the case of BPA, no corresponding reduction peak is obtained in the reverse scan, indicating that the oxidation of BPA is totally irreversible process (Fig. 5b). ZrO₂/GCE also exhibits oxidation peaks corresponding to metol and BPA oxidation (Fig. 5). At bare GCE, metol and BPA exhibit broad and ill defined oxidation peaks with a poor electrochemical response. This may be because of the sluggish electron transfer kinetics at bare GCE. ZrO₂(20%)/Nano-ZSM-5/GCE exhibits much higher oxidation peak currents for metol and BPA, respectively, when compared to ZrO₂/GCE and bare GCE. Furthermore, the oxidation potential shifts negatively at ZrO₂(20%)/Nano-ZSM-5/GCE compared to bare GCE. The higher redox current and shift in the peak potential (decrease in over-potential) indicate that ZrO₂(20%)/Nano-ZSM-5/GCE exhibits catalytic effect in the oxidation of metol and BPA which may be attributed to its faster electron transfer rate, higher electrochemical activity, and larger electroactive surface area. These results clearly indicate that ZrO₂(20%)/Nano-ZSM-5 nanocomposite can act as a promising electrode material for the electrochemical oxidation of metol and BPA.

Fig. 5 Comparison of CVs in a 0.1 M PBS (pH 7) containing (a) metol (10 μM) and (b) BPA (10 μM) at ZrO₂(20%)/Nano-ZSM-5/GCE, ZrO₂/GCE, and bare GCE at a scan rate of 50 mV s⁻¹.

The effect of scan rate on the electrochemical oxidation of metol and BPA was investigated using CV. Fig. S7, ESI† shows the CVs at ZrO₂(20%)/Nano-ZSM-5/GCE containing metol and BPA in 0.1 M PBS (pH 7) at various scan rates (10–600 mV s⁻¹). The CV results show that the peak currents increases with the increase in the scan rate (for metol and BPA). The plot of the oxidation peak current against the square root of scan rate (10–600 mV s⁻¹) shows a linear relationship for both the analytes, indicating a diffusion controlled process. The Semerano coefficient can be calculated from the logarithmic dependence of the electrooxidation peak current on the scan rate (tan α = Δlog I_p/Δlog ν, where I_p is the oxidation peak current and ν is the scan rate).³⁸ The plots of logarithm of anodic peak current vs. logarithm of scan rate provide a straight line with the linear regression equation log I_p = 0.462 log v + 0.354 (R² = 0.992) and log I_p = 0.531 log v − 0.758 (R² = 0.993) for metol and BPA, respectively (Fig. S7c and d, ESI†). Slopes obtain from the linear behavior between the logarithm of anodic peak current vs. logarithm of scan rate for metol and BPA are 0.462 and 0.531, which are close to the theoretical value of 0.5 for a diffusion controlled process. Thus, the electrocatalytic oxidation of metol and BPA is mainly dominated by diffusion process at ZrO₂(20%)/Nano-ZSM-5/GCE. For a reversible wave, the total number of electrons (n) involved in an electrode reaction can be calculated as ΔE_p = E_pa − E_pc = 22.5RT/nF = (59/n) mV at 25 °C, where, E_pa and E_pc are the anodic and cathodic peak potentials.³⁹ The electron number, n = 1.93 is obtained for metol, suggesting that two electrons transfer is involved in the metol oxidation.

In the case of a totally irreversible electrode process, the relationship between the potential (E_pa) and logarithm of scan rate (ln v) can be expressed by the equation E_pa = E° + (RT/αnF)ln(RTK°/αnF) + (RT/αnF)ln v, where E° is formal potential, n is electron transfer number, and K° is standard rate constant of the reaction. R, T, and F have their usual meanings (R = 8.314 J mol⁻¹ K⁻¹, T = 298 K, F = 96 485 C mol⁻¹).^10,40,41 Generally, transfer coefficient (α) is assumed to be 0.5 for a totally irreversible electrode process. A linear relationship is observed between E_pa and log v for BPA with an equation E_pa = 0.060 log v + 0.523 (R² = 0.988) (Fig. S8, ESI†), the slope of which is then used to calculate the number of electrons transferred. The value of n is 1.96 for BPA suggesting that the two-electron transfer is involved in the oxidation of BPA.

Chronoamperometry was used to calculate the diffusion coefficient and rate constant for the electrocatalytic oxidation of metol and BPA at ZrO₂(20%)/Nano-ZSM-5/GCE (Fig. S9 and S10, ESI† includes experimental details along with a brief discussion). The diffusion coefficients are 18.4 × 10⁻⁶ and 6.7 × 10⁻⁶ cm² s⁻¹ for metol and BPA, respectively. The rate constant values for electrocatalytic oxidation of metol and BPA are 2.5 × 10⁴ and 1.1 × 10⁴ s⁻¹ M, respectively. This chronoamperometric data indicates that the electrochemical reaction rate for metol and BPA at ZrO₂(20%)/Nano-ZSM-5/GCE is fast and the electro-oxidation is a typical diffusion controlled process.

The effect of pH on the electrochemical oxidation of metol and BPA was investigated in phosphate buffer solution with different pH values (1–10) at ZrO₂(20%)/Nano-ZSM-5/GCE using DPV. The anodic peak current increases with the increase in pH of the medium and exhibits a maximum anodic peak current at pH 7 (Fig. S11a, ESI†). After pH 7, it was further decreased for both the analytes. Metol and BPA are phenolic compounds. It is known that in basic medium, OH⁻ abstracts –H from the phenol and phenoxide species are formed. Metol and BPA may also be susceptible to form phenoxide ions at pH > 7 that would influence the electrochemical characteristics of metol and BPA and influence the voltammetric current response. At low pH (with high H⁺ concentration), the hydroxyl group of metol and BPA can be protonated (to form OH₂⁺) and thus influence the oxidation mechanism. Investigation shows that the highest anodic peak current and separation of peak potentials is obtained at pH 7. Therefore, in order to obtain high sensitivity, pH 7 is selected for the determination of metol and BPA. Fig. S11b, ESI† shows the effect of pH on the oxidation peak potentials for the analytes. The oxidation peak potentials for metol and BPA shift negatively and vary linearly with increase in pH of the solution, indicating that protons take part in the electrode reaction processes (Fig. S11b, ESI†).

E_p(metol) = −0.061pH + 0.502 (R² = 0.989)

E_p(BPA) = −0.057pH + 0.919 (R² = 0.975)

The slopes of the plot (E_p vs. pH) for metol and BPA are found to be 0.061 and 0.057 V pH⁻¹, which are close to the Nernstian value (0.059 V pH⁻¹) for a two electrons/two protons process.⁴² This suggests that the uptake of electrons is accompanied by equal number of protons. These pH results are in accordance with the results obtained in the previous section for the number of electrons calculated, confirming that two electrons are involved in the electrochemical oxidation of metol and BPA.

Based on the above results and the literature reports, the mechanism for the electrochemical oxidation of metol and BPA at ZrO₂(20%)/Nano-ZSM-5/GCE is proposed (Scheme S1†).^15,43–45 At the electrode surface, Zr⁴⁺ facilitates the oxidation of metol and BPA at a specific potential and itself undergoes reduction to yield Zr³⁺. In the aqueous environment, Zr³⁺ is instantly oxidized to regenerate Zr⁴⁺.³⁶ Hence, Zr ions act as redox mediator and favor the electrocatalytic oxidation of metol and BPA.

Individual electrocatalytic oxidation of metol and bisphenol A

The individual electrocatalytic oxidation of metol and BPA at ZrO₂(20%)/Nano-ZSM-5/GCE is carried out using DPV in 0.1 M PBS (pH 7) at a scan rate 20 mV s⁻¹ (Fig. 6). The DPV results show that both the analytes are oxidized with well-defined and distinguishable sharp oxidation peaks with peak potentials at 51 and 548 mV for metol and BPA, respectively, at ZrO₂(20%)/Nano-ZSM-5/GCE (Fig. 6). A linear dynamic range from 3 nM to 800 μM with a calibration equation of I_metol (μA) = 5.394 + 0.075C_metol (μM) (R² = 0.998) is obtained for metol (Fig. 6a, inset). A linear calibration for BPA is in the range of 4 nM to 700 μM, with a calibration equation of I_BPA (μA) = 3.039 + 0.039C_BPA (μM) (R² = 0.997) (Fig. 6b, inset). These results indicate that ZrO₂(20%)/Nano-ZSM-5/GCE can be used for the individual electrochemical oxidation of metol and BPA with significantly high electrocatalytic activity.

Fig. 6 DPVs at ZrO₂(20%)/Nano-ZSM-5/GCE in 0.1 M PBS (pH 7) by varying the concentrations of (a) metol and (b) BPA. DPV parameters were selected as: pulse amplitude: 50 mV, pulse width: 50 ms, scan rate: 20 mV s⁻¹. Inset shows the calibration plot.

Simultaneous electrochemical determination of metol and BPA

DPV is employed for the simultaneous determination of metol and BPA at ZrO₂(20%)/Nano-ZSM-5/GCE in 0.1 M PBS (pH 7) at a scan rate 20 mV s⁻¹ (Fig. 7). Fig. 7 shows that two distinguished and sharp anodic peaks at potentials 51 and 548 mV corresponding to the oxidation of metol and BPA are obtained in the simultaneous determination of metol and BPA. These peak potentials match well with their individual anodic peak potentials as discussed in the previous section. The voltammograms for the binary mixture are well separated from each other with a potential difference of ΔE_metol–BPA = 497 mV. When the concentration of analytes is increased in the electrochemical cell, a significant increase in the oxidation peak current is observed for both the analytes. The anodic peak current is linearly dependent on the concentration of analytes in the range of 4 nM to 800 μM (R² = 0.997) for metol and 6 nM to 600 μM (R² = 0.996) for BPA with the sensitivity of 1.2 and 0.7 μA μM⁻¹ cm⁻² and the limit of detection of 1 nM and 3 nM for metol and BPA, respectively (Fig. 7, inset). These results demonstrate that ZrO₂(20%)/Nano-ZSM-5/GCE can be used as an effective candidate for the electrochemical determination of metol and BPA.

Fig. 7 DPVs of the binary mixture containing varying concentrations of metol and BPA at ZrO₂(20%)/Nano-ZSM-5/GCE in 0.1 M PBS (pH 7). DPV parameters were selected as: pulse amplitude: 50 mV, pulse width: 50 ms, scan rate: 20 mV s⁻¹. Inset shows the calibration plot for metol and BPA.

A comparison for the electrochemical oxidation of metol and BPA at ZrO₂ supported Nano-ZSM-5 nanocomposite modified GCE with different weight ratio is provided in Fig. S12, ESI.† Among ZrO₂/Nano-ZSM-5 materials with different weight ratio (10, 20, and 30), ZrO₂(20%)/Nano-ZSM-5/GCE exhibits the highest current response (Fig. S12, ESI†). Low activity of ZrO₂(10%)/Nano-ZSM-5/GCE compared with ZrO₂(20%)/Nano-ZSM-5/GCE clearly shows that higher amount of ZrO₂ loading in nanocomposite material is better for the high electrocatalytic activity. However, with further increase in ZrO₂ content (ZrO₂(30%)/Nano-ZSM-5/GCE), the electrocatalytic activity decreases (Fig. S12, ESI†). This can be correlated to the textural properties of nanocomposite materials. Surface area and pore volume for ZrO₂(20%)/Nano-ZSM-5 are more when compared to that of ZrO₂(30%)/Nano-ZSM-5. The high surface area and pore volume are favorable for the diffusion of analytes/products to and from the active sites. Therefore, ZrO₂(20%)/Nano-ZSM-5/GCE is chosen for the electrochemical oxidation of metol and BPA. A comparison of the electrocatalytic activity at ZrO₂(20%)/Nano-ZSM-5/GCE, ZrO₂/GCE, Nano-ZSM-5/GCE, and bare GCE in the simultaneous determination of metol and BPA is shown (Fig. S13 and S14, ESI†). Bare GCE exhibits a broad and ill defined oxidation peaks with a very low current response in the simultaneous determination of metol and BPA (Fig. S13, ESI†). Hence, bare GCE cannot be applied for the simultaneous determination of metol and BPA. Fig. S13, ESI† also shows that ZrO₂/GCE and Nano-ZSM-5/GCE are able to oxidize metol and BPA but with lower current response compared to ZrO₂(20%)/Nano-ZSM-5/GCE. These results clearly show that ZrO₂(20%)/Nano-ZSM-5/GCE exhibits the highest electrocatalytic activity and sensitivity in the electrochemical oxidation of metol and BPA among the various electrodes investigated in this study. The comparison of results shown in this paper with literature reports are provided in Table S1, ESI.† It is clear from Table S1† that the present sensor is able to detect metol and BPA simultaneously with a wide linear range and low limit of detection when compared to literature reports.

Nano-ZSM-5 is playing an important role in measuring the electrochemical behavior of different modified electrodes using [Fe(CN)₆]^3−/4− as electrochemical probe as shown in Fig. S6.† The response current observed in the CV measurement was higher at Nano-ZSM-5 modified electrode when compared to bare GCE, which demonstrate that the [Fe(CN)₆]^3−/4− was easily diffused through this material. Individual, Nano-ZSM-5/GCE and ZrO₂/GCE also oxidizes metol and BPA at a specific potential as shown in Fig. S13.† The current responses observed at ZrO₂/GCE and Nano-ZSM-5/GCE (Fig. S6 and S13†) clearly show that the redox property of ZrO₂ as well as diffusion of reactant/product from the inter-crystalline mesopores of Nano-ZSM-5 at the applied potential, both were important for the determination of metol and bisphenol A. Therefore, in-order to achieve high sensitivity, ZrO₂ was loaded on the external surface of Nano-ZSM-5. Fig. S12† clearly shows that the electrochemical activity is significantly increased after dispersing 20% ZrO₂ on the external surface of Nano-ZSM-5. Highly dispersed isolated active ZrO₂ centre present in ZrO₂(20%)/Nano-ZSM-5 is responsible for the high current response observed in Fig. S12 and S13.† Nano-ZSM-5 plays an important role as suitable support material to incorporate ZrO₂ on the large external surface of Nano-ZSM-5. In this case, isolated active ZrO₂ metal centers are easily accessible to the reactant molecule to exhibit the electro catalytic oxidation of metol and BPA. Furthermore, the abundant surface Si–OH and Al–OH groups present on Nano-ZSM-5 could interact well with the hydroxyl group of metol and BPA due to hydrogen bonding and ZrO₂ electrocatalyze the oxidation of metol and BPA as shown in Scheme S1.† The inter-crystalline mesoporosity in Nano-ZSM-5 provides an efficient transport path for reactant/product molecules because of the short diffusion length. In conclusion, isolated active ZrO₂ centre present on the large external surface of Nano-ZSM-5 along with facile diffusion of reactant/products through inter-crystalline mesopores provided by the Nano-ZSM-5 are responsible for the high sensitivity of ZrO₂(20%)/Nano-ZSM-5 investigated in this study.

Reproducibility, stability, and anti-interference property of the sensor

The reproducibility and stability of the developed sensor was evaluated in the sensing studies. Five, ZrO₂(20%)/Nano-ZSM-5/GCE were constructed and their current responses to 1 μM concentration of metol and BPA were investigated (Fig. S15, ESI†). The relative standard deviation (RSD) was 1.7% and 1.3% for metol and BPA, respectively confirming that the fabrication method is highly reproducible. The stability of ZrO₂(20%)/Nano-ZSM-5/GCE was examined by recording repetitive CVs for 100 scans in PBS (pH 7) at a scan rate 50 mV s⁻¹ in the presence of metol and BPA (Fig. S16, ESI†). No obvious change in the peak current is observed after 100 cycles, which confirms that ZrO₂(20%)/Nano-ZSM-5/GCE is highly stable. The long term stability of the sensor was evaluated by measuring its sensitivity toward 1 μM concentration of metol and BPA for 30 days (Fig. S17, ESI†). The sensor was stored in refrigerator at 278 K and its sensitivity was tested at the interval of 5 days. Prior to each measurement, electrode was washed with 0.1 M PBS and repetitive CV were run in blank 0.1 M PBS (pH 7). The results confirm that the DPV response of the electrode to the same concentration of metol and BPA remained almost same with RSD 2.5% and 2.8% for metol and BPA respectively, indicating that the modified electrode has excellent stability. In order to investigate the selectivity of ZrO₂(20%)/Nano-ZSM-5/GCE toward determination of metol and BPA; DPV measurements were performed in the presence of various interfering agents. The possible interferences of some inorganic ions and organic compounds were investigated by adding a suitable concentration of additive in binary mixture solution containing 1 μM of metol and BPA. The results show that 150 fold excess of K⁺/Na⁺/Cl⁻/HPO₄²⁻/H₂PO₄⁻/Ca²⁺/Mn²⁺/Fe³⁺/Cu²⁺/Zn²⁺/Mg²⁺ has no effect on the peak currents for the oxidation of metol and BPA. The presence of other common interferents (80 fold) such as nitrite, nitrate, ammonia, 2,4-dinitroaniline, nitrophenol isomers, and phenol do not show any change in the peak current response confirming that no interference for these common species occurred.

Determination of metol and BPA in real samples

To demonstrate the practical application of the developed sensor; experiments were performed to determine the concentration of metol and BPA in river water samples. The proposed method was further applied for determination of metol in photographic solution and BPA in baby bottle. The contents of metol and BPA were detected using the standard addition method and the results are summarized in Table 2. The values of recovery are in the range from 99 to 101%, suggesting the accuracy of ZrO₂(20%)/Nano-ZSM-5/GCE based sensor. These results confirm that proposed sensor is reliable and sensitive enough for the determination of metol and BPA in different real samples and can be employed for the routine determination of metol and BPA.

Table 2 Determination of metol and BPA in different real samples at ZrO₂(20%)/Nano-ZSM-5/GCE

Sample	Analyte	Solution^a added (nM)	Spiked^b (nM)	Found (nM)	RSD^c (%)	Recovery (%)
a Required volume of photographic solution/baby bottle solution was added to obtain the value given in nM (based on the calibration curve obtained for commercial metol/BPA).b Required volume of 0.1 M aqueous stock solution of commercial metol/BPA was added to obtain the value given in nM.c Average value of five determinations.
Sutlej river water	Metol	—	300	298	2.5	99.3
	BPA	—	300	304	2.8	101.3
Photographic solution	Metol	400	300	703	1.9	100.7
	Metol	500	300	801	2.3	100.2
	Metol	700	300	997	2.4	99.6
Baby bottle	BPA	100	300	401	2.7	101
	BPA	200	300	498	3.0	99
	BPA	500	300	802	2.9	100.4

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Conclusions

In summary, ZrO₂/Nano-ZSM-5 nanocomposite materials with different weight ratios were synthesized by the calcination of the physical mixture of ZrO₂ and Nano-ZSM-5. An electrochemical sensor based on ZrO₂/Nano-ZSM-5 modified glassy carbon electrode was fabricated for the simultaneous determination of organic pollutants metol and bisphenol A. The results demonstrated that the developed sensor exhibited high electrocatalytic activity, sensitivity, and stability in the detection of metol and bisphenol A. The high activity of ZrO₂/Nano-ZSM-5 can be attributed to the highly dispersed ZrO₂ active centers on the large surface area mesoporous Nano-ZSM-5. The comparison of results showed in this paper with literature reports confirmed that the present sensor was able to detect metol and BPA simultaneously, with a wide linear range and low limit of detection when compared to literature reports. The analytical performance of the developed sensor was extended in the determination of these organic pollutants in different real samples such as river water, metol in photographic solution, and bisphenol A in baby bottle with satisfactory results. The proposed methodology is simple, rapid and provides a potentially new analytical platform for the detection of metol and BPA in different real samples.

Acknowledgements

Authors thank Department of Science and Technology, New Delhi for financial assistance (SR/S1/PC-91/2012). BK is grateful to CSIR, New Delhi for SRF fellowship.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra08391a

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