Potential application of mixed metal oxide nanoparticle-embedded glassy carbon electrode as a selective 1,4-dioxane chemical sensor probe by an electrochemical approach

Here, low-dimensional mixed metal oxide (ZnO/NiO/MnO2) nanoparticles (NPs) were prepared to develop a selective, efficient and ultra-sensitive 1,4-dioxane sensor by using the wet-chemical method (co-precipitation) in alkaline medium at low temperature. Detailed characterization of the prepared calcined NPs was achieved via conventional methods, including X-ray diffraction, field emission scanning electron microscopy, and X-ray photoelectron, UV-vis, Fourier-transform infrared and energy dispersive X-ray spectroscopies. To develop a thin layer of nanomaterial on the fabricated electrode, a slurry of prepared NPs was used to coat the glassy carbon electrode (GCE) with conductive Nafion (5% in ethanol) binder. The fabricated electrochemical sensor showed good sensitivity (1.0417 μA μM−1 cm−2), a wide linear dynamic range (0.12 nM to 1.2 mM), lower detection limit (9.14 ± 4.55 pM), short response time, good reproducibility, and long-term stability to selectively detect 1,4-dioxane in the optimized buffer system. Thus, this work presents a reliable alternative approach over existing methods to selectively detect hazardous chemicals in large scale for safety in the environmental and healthcare fields.


Introduction
The cyclic diether 1,4-dioxane is a well-known nonbiodegradable organic compound that is used as solvent in various industrial processes, including the production of paints, inks, dyes, varnish, pharmaceuticals, paper, fabric cleaners and electronics. 1,2 Also, it is used extensively to produce consumer and personal care items. Besides this, 1,4-dioxane is obtained as a by-product during the manufacture of ethylene oxide and ethylene glycol. 3 However, the excessive industrial application and high solubility of 1,4-dioxane has led to the pollution of ground and surface water. 4,5 Therefore, 1,4-dioxane contamination of water is found to be signicantly increased in the industrial regions of the world. [6][7][8] Due to the high toxicity of 1,4dioxane, these industrial effluents are a great threat to humans and environmental ecosystems. 9 Therefore, much research has been done on the detection of 1,4-dioxane, a possible carcinogen. The International Agency for Research on Cancer (IARC) has listed 1,4-dioxane as a probable human carcinogen. 10,11 Furthermore, it is suspected that 1,4-dioxane is also responsible for damage to the nervous system and failure of kidney, lungs, and liver in humans. 12,13 To increase public awareness, a trustworthy detection method for 1,4-dioxane is essential. To date, a number of methods have been applied to detect 1,4dioxane, such as high-performance liquid chromatography (HPLC), 14 solid phase extraction-gas chromatography/mass spectrometry (SPE-GC/MS), 15,16 and thermal desorption (TD) gas chromatography (GC). 17 However, these methods are limited in the trace detection of 1,4-dioxane. Therefore, a simple and efficient method for the detection of 1,4-dioxane in aqueous medium is required. Recently, the electrochemical approach has become prevalent due to its reliable and effective trace detection of toxic chemicals in aqueous media. 18,19 Due to their excellent electrochemical properties, nanostructured semiconductors and transition metal oxides have been utilized as sensing elements in numerous chemical sensing applications. 20,21 Among the transition metal oxides, a ZnO p-type semiconductor, with a band-gap energy of 3.3 eV, has been investigated to detect xanthine, 22 2-nitrophenol, 23 1,2dichlorobenzene 24 and hydrazine 25 in aqueous media. Another p-type semi-conductor, NiO, with a band-gap energy of 4.0 eV, has been reported as efficient and reliable in sensor applications to detect 4-aminophenol 26 and 4-methoxyphenol. 27 Therefore, p-type semi-conductive nanostructured metal oxides with wide band-gap energies are suitable as sensing materials in the eld of electrochemical sensor development. Furthermore, MnO 2 is a promising semi-conductive metal oxide with low resistance, large specic surface area, and high catalytic activity, along with an attractive electrochemical response. 28 Additionally, due to its promising optical-electrical properties, SnO 2 composited with various metal and metal oxides has exhibited potential as a sensing element to detect gas. [29][30][31] Thus, the approach of this research is to develop a consistent 1,4dioxane chemical sensor using a ternary mixture of the semiconductive nanostructured metal oxides ZnO, NiO, and MnO 2 .
In this study, an electrochemical sensor with ternary metal oxide ZnO/NiO/MnO 2 nanoparticles on GCE was developed and applied to selectively detect 1,4-dioxane in aqueous medium by electrochemical method. The proposed 1,4-dioxane chemical sensor was found to exhibit higher sensitivity, broad linear dynamic range, short response time, lower detection limit, excellent reproducibility, and long-term stability.

Synthesis of mixed metal oxide NPs
The preparation of the ZnO/NiO/MnO 2 NPs in this study was found to be a sensitive task. A co-precipitation (wet-chemical method) technique was implemented to synthesize ZnO/NiO/ MnO 2 NPs in alkaline medium. Wet-chemical techniques, particularly co-precipitation, are the oldest methods used to synthesize metal oxides or composites. Presently, the solvothermal and sol-gel methods are extensively implemented to prepare various metal oxides and composites. [32][33][34][35][36][37][38] To perform the wet-chemical (co-precipitation) process, 0.1 M solutions of Zn(NO 3 ) 2 $6H 2 O, NiCl 2 $6H 2 O and MnSO 4 $H 2 O were prepared in three individual 100 mL round-bottom asks using deionized water. Then, 50.0 mL of each prepared solution was poured into a 250 mL conical ask and allowed to heat at 85.0 C using a hot plate with continuous magnetic stirring. To gradually increase the pH of the solution to 10.5, an ammonium hydroxide solution of 0.1 M in concentration was added dropwise, and all the metal ions were co-precipitated in the form of metal hydroxides [Zn(OH) 2 , Ni(OH) 2 and Mn(OH) 2 ]. Under these conditions, the conical ask was kept for several hours on the hot plate with magnetic stirring. The proposed reaction scheme in aqueous (alkaline) medium is presented below.
Zn (aq) 2+ + Mn (aq) 2+ + Ni (aq) 2+ + OH (aq) In the wet-chemical method, precipitation of metal hydroxides depends on the value of the solubility product constant, K s . The reported K s values are 3 Â 10 À27 , 5.48 Â 10 À16 , and 1.9 Â 10 À13 , corresponding to Zn(OH) 2 , Ni(OH) 2 and Mn(OH) 2 , in alkaline pH medium. 39 Due to the dropwise addition of NH 4 OH solution, the pH of the reaction medium slowly increases. Since Zn(OH) 2 has a lower K s value, it starts to precipitate rst and forms the nuclei of crystal formation. Then, the resulting crystallites aggregate to form larger crystallites. With increasing pH value, Ni(OH) 2 starts to precipitate and attach to crystallites of Zn(OH) 2. Similarly, the last semi-conductive metal hydroxide (Mn(OH) 2 ) is precipitated out. Thus, at pH 10.5, all metal ions are precipitated out quantitatively. The obtained metal hydroxides were separated from the aqueous medium and consequently washed with acetone and deionized water, successively. Next, the precipitate was placed inside a lowtemperature oven and allowed to dry at 110.0 C overnight. To obtain the metal oxide from the metal hydroxide, the resultant dry sample was subjected to calcination in a high-temperature muffle furnace at 500.0 C for 5 h. In this process, all the metal hydroxides are transformed to their oxide forms with higher oxidation number in the presence of atmospheric O 2 . The corresponding reaction inside the muffle furnace is given below.

Instruments
The following instruments and methods were used to complete the characterization, electrode preparation, and sensor application of the NPs. To investigate the photosensitivity of the calcined ZnO/NiO/MnO 2 NPs, a 300 UV/visible spectrophotometer (Thermo Scientic) was implemented to measure the UVvis spectra, and a NICOLET iS50 FTIR spectrometer (Madison, WI, USA) was used in the range of 400-4000 cm À1 to identify the functional groups. The oxidation states and binding energies of species were measured by X-ray photoelectron spectroscopic (XPS) analysis using a Ka1 spectrometer (Thermo Scientic, Ka 1066). Beside this, the morphology of the prepared ZnO/NiO/ MnO 2 NPs was inspected by eld emission scanning electron microscopy (FESEM, JEOL, JSM-7600, Japan), and the respective elemental analysis was executed by energy dispersive X-ray spectroscopy (EDS, JEOL, Japan). Moreover, the crystallinity of synthesized NPs was analyzed by X-ray diffraction. Electrochemical investigations were carried out using an electrometer (651-Keithley Electrometer, USA).

Fabrication of GCE electrode with ZnO/NiO/MnO 2 NPs
The dominant part of this electrochemical sensor is the working electrode. To prepare such working electrode, a glassy carbon electrode (GCE) with a surface area of 0.0316 cm 2 was coated with a slurry of ZnO/NiO/MnO 2 NPs and dried under ambient conditions. Then, a drop of Naon (5% in ethanol) was added, and the modied GCE was dried inside an oven at 35.0 C for an hour to completely dry the thin lm of ZnO/NiO/MnO 2 NPs. Naon is a co-polymer, and it acts as a conducting binder. As a result, the fabricated electrode shows long-term stability in aqueous medium as well as improved electron transfer rate of the electrode. 40,41 To accomplish electrochemical analysis via an electrochemical method, an electrochemical sensor was assembled with a Keithley Electrometer, ZnO/NiO/MnO 2 NPs/ binder/GCE and Pt-wire. Several 1,4-dioxane solutions with varying concentrations ranging from 0.012 M to 0.12 nM were prepared and used as analyte in the assembled electrochemical sensor. The slope of the calibration curve, which resulted from plotting the concentration of 1,4-dioxane vs. the current, was used to measure the sensitivity, detection limit (DL) and linear dynamic range (LDR) of the proposed electrochemical sensor. During electrochemical measurements, the buffer solution in the inspecting beaker was kept constant at 10.0 mL for the entire experiment.

Results and discussion
Morphological and elemental analysis of ZnO/NiO/MnO 2 NPs The morphology of ZnO/NiO/MnO 2 nanoparticles was examined and is presented in Fig. 1(a and b). As observed from Fig. 1, the low and high magnication FESEM show the aggregation of irregularly shaped NPs, which are mesoporous in nature. Thus, it is concluded that the synthesized nanomaterials formed particles, as identied earlier. 42,43 To conrm the elemental composition of the prepared ZnO/NiO/MnO 2 NPs, EDS analysis was executed, as illustrated in Fig. 1(c and d). From the pattern shown in Fig. 1 Binding energy and oxidation states XPS was performed for the detailed study of the binding energy and oxidation states of ZnO/NiO/MnO 2 NPs prepared using the wet-chemical technique. The resulting XPS spectra are presented in Fig. 2. The core level spectrum of Zn 2p is presented in Fig. 2(a), and it shows two identical peaks at 1022.5 and 1046.0 eV, corresponding to the Zn 2p 3/2 and Zn 2p 1/2 spin orbits, respectively, with a 23.5 eV spin-orbit splitting. This observation has been reported as Zn 2+ in ZnO. [44][45][46][47] As shown in Fig. 2(b), the O 1s XPS peak is positioned at 532.0 eV and is associated with the absorption of -OH group on the surface of ZnO/NiO/MnO 2 NPs. [48][49][50][51][52] Moreover, the XPS spectrum of Ni 2p is displayed in Fig. 2(c), in which the two main XPS peaks of Ni 2p are situated at 856.5 and 874.0 eV, associated with Ni 2p 3/2 and Ni 2p 1/2 spin-orbits, respectively, and the spin energy separation between these two peaks is equal to 17.5 eV. This value ts with the oxidation of Ni 2+ in NiO. Furthermore, the core level spectrum of Ni 2p shows two satellite peaks located at 863.0 and 880.5 eV, related to Ni 2p 3/2 and Ni 2p 1/2 with spin energy separation of 17.5 eV. Therefore, both the main and satellite peaks of Ni 2p are associated with equal oxidation of Ni 2+ in NiO. [53][54][55][56] According to Fig. 2(d), the Mn 2p orbits show two peaks

Optical and phase crystallinity analyses
To conrm the functional groups present in the synthesized ZnO/NiO/MnO 2 NPs, FTIR investigation was executed. The resultant FTIR spectrum is illustrated in Fig. 3(a); it demonstrates absorption bands at 435, 1100, 1620, and 3400 cm À1 . The broad and intense absorption band at 435 cm À1 is associated with the stretching vibration mode of Zn-O or Mn-O. [61][62][63] The resultant absorption band at 1100 cm À1 resembles the coordination of Mn by O-H bond, 64,65 as seen in Fig. 3(a). Here, E bg is band-gap energy, and l is the maximum absorption wavelength.
To analyse the phase crystallinity, powder X-ray diffraction analysis was carried out on the synthesized ZnO/NiO/MnO 2 NPs, with the applied range of 2q ¼ 16-76 . The resultant XRD pattern is given in Fig. 3(c). A number of typical diffraction peaks of ZnO, designated as q, are assigned to the (100), (002),  [76][77][78] Moreover, the studied XRD shows some diffracted peaks of NiO directories, designated as m, of (111), (200), and (220). This nding is supported by JCPDS no. 0047-1049 and other reports. 79,80 The Scherer equation [eqn (vii)] is used to measure the crystalline particle size at the highest b peak of (200), 81 and it is found to be 14.28 nm.
Here, l is the wavelength of X-ray radiation (1.5418Å), and b is full width at half maximum (FWHM) of the peak at diffraction angle q.
Additionally, photoluminescence (PL) spectroscopy is a powerful tool to investigate the quality, purity and optical properties of the nanostructured materials. Owing to the excellent light absorption properties of nanomaterials, they might be very fascinating for further research in electrocatalytic as well as in photocatalytic devices. The PL emission spectrum of the synthesized ZnO/NiO/MnO 2 NPs at excitation wavelength gives a peak in the visible region at 468 nm, corresponding to blue emission (Fig. 4). The emission band in the blue region is due to transition vacancies of oxygen and interstitial oxygen. The band corresponding to blue emission is also due to a singly ionized oxygen vacancy. This ZnO/NiO/MnO 2 NPs show strong blue emission band in the visible region, which is in good consensus with the literature. 82,83 Optimization and evaluation of the analytical performance of the sensor The desired chemical sensor was achieved using ZnO/NiO/ MnO 2 NPs coated as a thin layer on a GCE. The binding strength between the NPs and GCE was boosted by adding a drop of Naon, a conducting binder. To conrm the appropriate activity of the ZnO/NiO/MnO 2 NPs/binder/GCE sensor, several phosphate buffer media, with pH values ranging from 5.7 to 8.0, were tested, and the sensor's performance was examined at an applied potential of 0 to +1.5 V. As is apparent from Fig. 5(a), the electrochemical sensor gives the highest electrochemical response in a buffer with a pH value of 5.7. The selectivity of this  predictable chemical sensor was assessed by the analysis of environmental toxic chemicals at micro-level concentrations and an applied potential of 0 to +1.5 V. The electrochemical responses towards zimtaldehyde, paracetamol, acetonitrile, xanthine, ethylene glycol, benzyl chloride, diethyl malonate, 1,4-dioxane, 3-methylaniline, and chlorobenzene are exempli-ed in Fig. 5(b), and perceptibly, 1,4-dioxane shows the maximum activity toward the probable ZnO/NiO/MnO 2 NPs/ binder/GCE chemical sensor in the optimized buffer system at pH 5.7. The analytical-grade 0.12 M 1,4-dioxane has been diluted in deionized water to prepare a number of solutions ranging in concentration from 0.012 M to 0.12 nM, and the corresponding electrochemical response of each 1,4-dioxane solution is plotted in Fig. 5(c). As seen in the gure, the electrochemical responses are distinct from lower to higher concentrations of 1,4-dioxane; this performance was measured at an applied potential of 0 to +1.5 V. To estimate the analytical parameters of the 1,4-dioxane chemical sensor, current data were collected from Fig. 5(c) at an applied potential of +1.5 V and plotted as the concentration of 1,4-dioxane vs. the current, as illustrated in Fig. 5(d), denoted as the calibration curve. The sensitivity of the anticipated 1,4-dioxane chemical sensor was assessed from the slope of the calibration curve and surface area of the GCE (0.0316 cm 2 ), and was found to be 1.0417 mA mM À1 cm À2 , a result that is realistically high. As observed from Fig. 5(d), the current data are homogeneously scattered along a linear plot over the concentration range of 0.12 nM to 1.2 mM 1,4-dioxane, giving a linear dynamic range (LDR) that evidences the reliability of our method. To explore the linearity of the calibration plot within the linear dynamic range (0.12 nM to 1.2 mM), the current data are plotted against concentration in logarithmic scale, as shown in Fig. 5(e). The current data are coordinated with a regression coefficient r 2 ¼ 0.9939. In short, it is apparent that the 1,4-dioxane chemical sensor is able to detect 1,4-dioxane in a real application. The detection limit of the 1,4-dioxane chemical sensor was determined from the slope of calibration curve using a signal-to-noise ratio of 3, and it is equal to 9.14 AE 4.55 pM, a value that is actually very low.
As can be seen from Fig. 5(c), the electrochemical response of the 1,4-dioxane chemical sensor is amplied with a boost in 1,4-dioxane concentration in the sensing medium. A proposed pictorial representation of the 1,4-dioxane detection process is revealed in Scheme 1. The 1,4-dioxane itself is a Lewis base, and it is able to donate electrons at pH 5.7. 84,85 Therefore, during the  electrochemical sensing of 1,4-dioxane in an optimized buffer system with a pH of 5.7, 1,4-dioxane is oxidized to generate formic acid and oxalic acid. Simultaneously, the density of electrons in the sensing medium is found to be enhanced, which is responsible for the high conductivity of the sensing medium. As a result, the electrochemical responses vary with the concentration of 1,4-dioxane. A similar oxidation of 1,4dioxane has been reported by previous authors. [86][87][88][89][90] Reliability and stability of the electrochemical method The reproducibility of the chemical sensor is a key analytical parameter in sensor development. Therefore, the reproducibility of the anticipated 1,4-dioxane electrochemical sensor was tested at 0.012 mM 1,4-dioxane and the potential of 0 to +1.5 V, and the results are demonstrated in Fig. 6(a). As seen in Fig. 6(a), the seven replicated runs are basically indistinguishable, and the electrochemical responses are not altered, even with washing of the electrode aer each trial. Thus, it can be settled that the proposed chemical will be consistent in real-eld application to detect 1,4-dioxane in aqueous medium. To explore the precision of the 1,4-dioxane chemical sensor, the relative standard deviation (RSD) of the current data at an applied potential of +1.5 V was calculated, and the measured RSD is found to be 1.29%, a result showing little appreciable differences. The analogous reproducibility test of the 1,4dioxane chemical sensor was performed over four days, and the ndings are represented in Fig. 6(a). Therefore, from Fig. 6(a), it can be predicted that the 1,4-dioxane chemical sensor has longterm stability with consistency in performance, under identical conditions. Fig. 6(b) shows the comparison of electrochemical responses between 1,4-dioxane, chlorobenzene and acetonitrile. It can be observed that 1,4-dioxane demonstrates the highest electrochemical response compared to other toxins at 0.012 mM concentration. Fig. 6(c) shows the electrochemical responses towards 1,4-dioxane only, 1,4-dioxane & chlorobenzene, and 1,4dioxane & acetonitrile. These electrochemical responses are completely indistinguishable. Thus, it can be supposed that the demonstrated 1,4-dioxane chemical sensor is selective to 1,4dioxane, and no interference effect is found in the presence of other toxins. The response time is another indicator of the analytical performance of the chemical sensor. As presented in Fig. 6(d), the obtained response time for the 1,4-dioxane chemical sensor is around 18.0 s; this experiment was executed at 0.012 mM concentration of 1,4-dioxane solution. Therefore, considering the sensor analytical performance in terms of sensitivity, linear dynamic range (LDR), detection limit (DL), response time, reproducibility, and accuracy of the electrochemical responses, the 1,4-dioxane chemical sensor exhibits highly signicant performance.
To investigate the validity of the study, a comparison with similar research using various nanocomposites or nanomaterials was explored 89,90 and is presented in Table 1. The analytical parameters, such as sensitivity, LDR and DL, were found to be quite high and satisfactory.

Application of sensor in real samples using the recovery method
The ultimate target of the development of this chemical sensor is its application in the real environmental eld. Therefore, the anticipated 1,4-dioxane chemical sensor probe has been applied to analyze real environmental samples for the selective detection of 1,4-dioxane. The recovery method is a process in which a known concentration of the sensing target (e.g., 1,4dioxane) is added to a real water sample, and subsequently, the concentration is measured by the fabricated chemical sensor. Here, this detection process is executed by recovery method. The real samples have been obtained from various sources, including sea water (Red Sea, Jeddah, KSA), extracts from a PCbaby bottle, PVC-food packaging bag, and PVC-water bottle, and waste effluent from the Jeddah industrial zone. The analysis data are represented in Table 2 and appear to be quite satisfactory.

Conclusion
In this research, a selective 1,4-dioxane chemical sensor was developed based on wet-chemically synthesized lowdimensional mixed metal oxide (ZnO/NiO/MnO 2 ) NPs coated onto a GCE sensor probe with 5% Naon conducting binder. The prepared ZnO/NiO/MnO 2 NPs were fully characterized using XRD, XPS, FESEM, EDS, FTIR and UV-vis analysis. The proposed 1,4-dioxane chemical sensor exhibits reliable and substantial performance, as revealed by considering its excellent sensing parameters, such as higher sensitivity, wide linear dynamic range, short response time, ultra-low detection limit, and reproducibility with high accuracy. Therefore, this research method might pave the way for the development of a selective chemical sensor for the large-scale detection of unsafe environmental contaminants in the environmental and healthcare elds.

Conflicts of interest
There are no conicts to declare.