Effect of Ce doping into ZnO nanostructures to enhance the phenolic sensor performance

Abstract

Various Ce-doped ZnO nanostructures (Ce/ZnO NSs) were prepared by a facile wet chemical method using reducing agents in alkaline medium. The Ce/ZnO NSs were characterized by UV/vis, FT-IR, field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (XEDS), X-ray photoelectron spectroscopy (XPS), and X-ray powder diffraction (XRD). The Ce/ZnO NSs were deposited onto flat glassy carbon electrode (GCE) with conducting Nafion binders to produce a sensor that has a fast response towards selective 3-methoxyphenol (3MP). Characteristics including higher sensitivity, lower detection limit, better reliability, good reproducibility, ease of integration, long-term stability, high selectivity, and enhanced electrochemical performance were investigated in detail at room conditions. The calibration plot is linear (r² = 0.9879) over a large concentration range (0.9 nM to 0.9 mM). The sensitivity and detection limit was calculated as ∼94.937 μA cm⁻² μM⁻¹ and 11.5 ± 0.2 pM (at a signal-to-noise-ratio [SNR] of 3), respectively. Finally, the efficiency of the proposed chemisensor can be applied and effectively utilized for the detection of various toxic chemical compounds in the environment with acceptable and reasonable results.

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Introduction

Resorcinol monomethyl ether, m-methoxyphenol, and 3-hydroxyanisol are synonyms of 3-methoxyphenol (3MP), a phenol derivative used in catalysts and in the synthesis of other organic compounds, such as antioxidants. Methoxyphenols are produced from the pyrolysis of lignin, and they also can be produced selectively by varying the reaction conditions and catalysis to obtain 3MP.¹ The National Institute for Occupational Safety and Health (NIOSH) safety data sheet for 3MP shows that it is a dangerous and toxic compound causing respiratory irritation, eye damage, and skin irritation.² Contamination of the environment with 3MP from industry and biomass combustion, such wood smoke, can affect the atmosphere and water resources.³ Different methods have been reported for the determination of methoxyphenols (i.e., 3MP) from these techniques, such as amperometric electrochemical approach, gas chromatography, and electrocatalysis.⁴ Detecting very low concentrations of environmental pollutants has become essential. Doped nanomaterials have superior properties compared to other undoped substances, such as mechanical strength, thermal stability, catalytic activity, electrical conductivity, magnetic properties, and optical properties. The development of chemical sensors based on doped semiconductor metal oxide nanoparticles and the use of nanocomposites for the detection of various toxic elements and chemicals due to their unique large surface area comprise a major field of study.⁵ Safety is a main concern from the perspective of the environment and health, which makes the study of chemical sensors for the detection of poisonous chemical via a well-recognized technique an important issue. Semiconductor nanostructure material is very efficient and sensitive because of its highly active surface area and various spherical morphologies relative to the volume ratio in comparison with the typical diameters in micro to nano ranges. Recently, the nanostructure of metal oxides has gotten a great deal of attention due to their fascinating properties such active surface area, higher porosity, permeability, quantum confinement effect, nontoxicity, and stability.⁶ Sensor-based metal oxides are widely used to monitor carcinogenic chemical constituents, air–water contamination, and toxic chemical agents in the environment,⁷ with their properties of high response, large surface area, lower charge, mesoporous nature, and portability.⁸ Detection and separation of toxic chemicals from industrial wastewater is one of the major issues in the biological and environmental field.

Metal oxide nanomaterials have attracted much attention because of their exceptional properties and potential applications in every area of science and technology.⁹ The easy synthetic route of doped material is probably self-aggregation, in which ordered aggregates are formed by a spontaneous chemical process in the presence of reactive reactant precursors.¹⁰ However, it is still a big challenge to develop a simple and reliable method for a low-dimensional doped metal oxide with designed chemical components and controlled morphology using inorganic transition doping materials, which strongly affect the properties of the host nanomaterials.¹¹ Recently, nano-sized nanostructures of paramagnetic iron oxides coupled with various materials have been investigated extensively due to their wide applications in nano-fields such as ferro-fluids,¹² magneto-caloric refrigeration,¹³ biotechnology and biomedical field,¹⁴ in controlled drug delivery systems, as contrast agent of magnetic resonance imaging, and in tissue repair and the detoxification of biological fluids.¹⁵ It also has been extensively studied in diverse fields including catalysis,⁷ environmental protection,¹⁶ magnetic storage devices,¹⁷ clinical diagnosis, and treatment.¹⁸ Nanomaterials may also be utilized in different technological fields, refrigeration systems, medical imaging, drug targeting, biological applications, and electrocatalysis.¹⁹ Since the reduction–oxidation reactions are activated by the active surface area, reducing the crystal dimension and enhancing the surface area of the sensing materials provide optional approaches to improving the responses of nanoparticles.²⁰ ZnO is a multifunctional nanomaterial because of its unique physical and chemical properties that reflect its numerous chemical and physical applications. Industries such as rubber, pharmaceutics, cosmetics, textile, electronic, and electro-technology use ZnO in many of their products, beside its use in electrocatalysis and photocatalysis.²¹ ZnO doped with different metals and nonmetals show enhanced photocatalytic activity; the band-gap energy of ZnO nanostructure is ∼3.37 eV, which can be lowered by maximizing the valence band, minimizing the conduction band, or introducing mid-band-gap energy levels.²² Besides that, doping also results in high surface-to-volume ratio, crystal defects, and the initiation of charge carrier traps.²³ CeO₂ nanoparticles with high ionic conductivity, negligible cytotoxicity, and good redox activity extend ZnO's application in many ways.²⁴

Different types of nanoparticles and metallic colloids have been used to improve patient acquiescence and therapeutic efficacy of applicable medicines in the medical field. Ferro-fluids are stable dispersions of magnetic iron-oxide nanoparticles in water and have been studied in biomedical sciences as efficient devices for in vitro diagnosis, cell separation, immunoassays, and nucleic acid concentration.²⁵ In chemistry, iron oxide nanoparticles have been used in NO reduction,²⁶ as adsorbents for heavy metals,²⁷ as pigments in cosmetic powders,²⁸ as anodes in lithium-ion batteries,²⁹ in the detection of hydrogen peroxide,³⁰ in the polymer coating of supra-magnetic nanoparticles,³¹ in magnetic resonance imaging,³² in biomedical applications,³³ as imaging agents,³⁴ in photocatalysis,³⁵ for removing inorganic and organic pollutants,³⁶ in glycerol hydrogenolysis,³⁷ in the hydrogenation of nitrobenzene,³⁸ in high-performance supercapacitors,³⁹ in catalytic oxidation,⁴⁰ in water treatment,⁴¹ in the separation of acid dye,⁴² in antibody functionalization,⁴³ in biosensor applications,⁴⁴ in the hybridization of nanotube,⁴⁵ in oil spill removal,⁴⁶ in bio-distribution studies,⁴⁷ etc. Substituted and unsubstituted phenols are common byproducts of the industrial process and are highly toxic. They are frequent contaminants in food and in freshwater and wastewater⁴⁸ environments. 3MP is toxic chemical that greatly effects the environment and health. Due to the many demerits of phenols, it is very urgent to develop an appropriate analytical method that is reliable, robust, cheap, and effective for the accurate quantification and sensitive detection of 3MP. Various sensing techniques have been already introduced and published in the scientific reports to detect phenolic compounds by electrochemical methods, HPLC, and spectrometry. Among several detection methods, the electrochemical current vs. voltage (I–V) technique is cheap, portable, fast, robust, and easy to handle. Therefore, numerous chemically modified electrodes based on different nanostructured materials, semiconductor-doped nanomaterials, transition metal oxides, and electrocatalytic moieties (electron-mediator species) have been developed for the detection of 3MP.⁴⁹

In this study, Ce/ZnO nanostructures were synthesized by a wet chemical process using different ratios by volume, which yielded a various structures and morphologies of the doped transition metal. The Ce/ZnO NSs allow very sensitive recognition and transduction during the chemical interaction, thus changing the electrochemical properties. Finally, Ce/ZnO NSs were fabricated at a suitable volume-to-volume ratio and applied onto polished GCE surface to make a simple, reliable, and efficient chemical sensor for 3MP at room conditions. To the best of our knowledge, this is the first report of the highly sensitive detection of 3MP with Ce/ZnO NSs using the simple and reliable I–V method with a short response time.

Experimental sections

Materials and method

Cerium(III)sulfate (Ce₂(SO₄)), sodium hydroxide (NaOH), disodium phosphate, monosodium phosphate, 1,2 dichlorobenzene, benzaldehyde, chloroform, hydrazine, n-hexane, pyridine, 3-methoxy phenol, phenol, ethanol, methanol, ammonium hydroxide, 4-aminophenol, 3-aminophenol, 2-nitrophenol, and all other chemicals used were of analytical grade and purchased from Sigma-Aldrich Company. The dried Ce/ZnO NSs were investigated with UV/visible spectroscopy (Lamda-950, PerkinElmer, Germany). FT-IR spectra were measured for the sample Ce/ZnO NSs with a spectrophotometer (Spectrum-100 FT-IR) in the mid-IR range, obtained from PerkinElmer, Germany. The XPS measurement of Ce/ZnO NSs was measured on a Thermo Scientific K-Alpha KA1066 spectrometer. A monochromatic Al Kα X-ray radiation source was used as excitation source, and the beam spot size was kept at 300.0 μm. The spectra were recorded in the fixed analyzer transmission mode, where pass energy was kept at 200.0 eV. The scanning of the spectra was performed at pressures less than 10⁻⁸ Torr. The powder X-ray diffraction (XRD) prototypes were evaluated with an X-ray diffractometer (XRD, X'Pert Explorer, PANalytical diffractometer) prepared with Cu Kα₁ radiation (λ = 1.5406 nm) using a generator voltage of 40.0 kV and current of 35.0 mA applied for the measurement. The morphology of Ce/ZnO NSs was examined on a FE-SEM instrument (FESEM, JSM-7600F, Japan). Elemental analysis was investigated using EDS from JEOL, Japan. I–V measurement was employed with an electrometer (Keithley, 6517A, Electrometer, USA). In the I–V system, two electrodes were used as working and counter electrodes connected directly to the electrometer. The resultant current was measured against the applied potential in the fabricated Ce/ZnO NSs sensor for selective 3MP detection.

Preparation of Ce/ZnO NSs

Solutions of 0.1 M Ce₂(SO₄)₃ and 0.1 M ZnCl₂ were prepared in deionized water separately under vigorous magnetic stirring at room conditions. Four ratios of various volumes of prepared precursor samples were mixed to prepare the mother solutions (Ce : Zn) separately as (1 : 1, 2 : 1, 3 : 1, 4 : 1). The reaction was carried out in different sets of 250.0 ml Erlenmeyer flasks. The pH of each mixture was slowly adjusted using 2.0 M sodium hydroxide by dropwise addition into the reactant mixtures (pH > 10). The sample solutions in the flasks were kept under stirring and heating condition at over 80.0 °C for 6 h. After cooling all reaction mixtures at room conditions, they were washed thoroughly with acetone, ethanol, and water, consecutively, to remove organic and inorganic contaminants. The as-grown final products of various Ce/ZnO compositions were dried in the oven at 60.0 °C for 24 h. The final products were characterized in detail in terms of their morphological, structural, optical, and elemental evaluation with various conventional methods. The following reactions ((i)–(v)) summarize the formation of Ce/ZnO nanostructures:

NaOH_(s) → Na_(aq)⁺ + OH_(aq)⁻ (i)

Ce₂(SO₄)_3(s) → 2Ce_(aq)³⁺ + 3SO_4(aq)²⁻ (ii)

ZnCl_2(s) → Zn_(aq)²⁺ + 2Cl_(aq)⁻ (iii)

8OH_(aq)⁻ + 2Ce_(aq)³⁺ + Zn_(aq)²⁺ → Zn(OH)_2(aq)↓ + 2Ce(OH)_3(aq)↓ (iv)

Zn(OH)_2(aq) + Ce(OH)_3(aq) → Ce/ZnO_(s)↓ + H₂O_(l) (v)

In Ce/ZnO NSs growth method, initial ZnO and CeO₂ nucleus growth takes place by self- and mutual-aggregation when two individual reactant precursors [Ce₂(SO₄)₃ and ZnCl₂] are mixed by vigorous magnetic stirring at lower temperature. Then, nanocrystals of reactant precursors [Zn(OH)₂ and Ce(OH)₃] reaggregated and formed aggregated ZnO and CeO₂ nanocrystals by releasing water molecules into the reaction medium via the Ostwald ripening method at a higher temperature. Nanocrystals are crystallized and re-aggregated (ZnO and CeO₂), with each counter-heterogeneous part in the presence of disperse reactant precursors through van der Waals forces. Finally, Ce/ZnO nanostructure morphology is re-formed at the calcination temperature, as presented in Scheme 1.

Scheme 1 Schematic representation of the growth mechanism of Ce/ZnO nanostructures by a wet chemical process.

Fabrication of GCE with Ce/ZnO NSs

Phosphate buffer solution (PBS, 0.1 M, pH 7.0) was prepared by mixing 0.2 M Na₂HPO₄ and 0.2 M NaH₂PO₄ solutions in 100.0 ml of deionized water. The glassy carbon electrode (GCE, surface area ∼ 0.0316 cm²) was fabricated with Ce/ZnO NSs using conducting coating binders (5% Nafion in ethanol). Then, it was transferred into the oven at 40.0 °C for 12 h until the film was uniform and completely dried. An electrochemical cell was constructed with Ce/ZnO NS-coated GCE and Pd wire as working and counter electrodes, respectively. 3MP (0.09 M stock solution) was diluted at different concentrations in DI water and used as a target chemical. The amount of 0.1 M PBS was kept constant in the beaker at 10.0 ml throughout the chemical analysis. The analyte solution (25.0 μl) was dropped into 10.0 ml PBS solution systematically from lower to higher concentrations of target 3MP solution (0.09 nM to 0.09 M). The sensitivity was calculated from the slope of voltage versus current against the slope of the calibration plot by considering the surface area of sensors. An electrometer was used as a voltage source for the I–V method in the two-electrode system.

Results and discussions

Characterization of Ce/ZnO NSs

The Ce/ZnO NSs were examined with respect to atomic and molecular vibrations to determine the functional nature of the nanostructures by FT-IR. Here, the spectrum was recorded in the region of 4000–400 cm⁻¹ under room conditions. Fig. 1 shows the FTIR spectra of Ce/ZnO (2 : 1) NSs. The spectra of Ce/ZnO NSs were observed at 760, 1120, 1660, and 3712 cm⁻¹, corresponding to Ce–O and Zn–O stretching vibrations from the formation of metal–oxygen bonds, C–O stretching, C=O stretching, and O–H band stretching vibrations, respectively.⁵⁰

Fig. 1 FT-IR spectrum for Ce/ZnO nanostructures.

In UV/visible principle, the outer electrons of the atom absorb radiant energy and then shift to the higher energy levels. The spectrum including the band-gap energy of the metal oxide is achieved due to optical absorption. The UV/vis spectra of the Ce/ZnO NSs was recorded in the visible range. Absorption spectrum in the visible range between 300.0 and 800.0 nm for Ce/ZnO NSs is presented in Fig. 2a, which shows a broad absorption band at 375.0 nm for the selective Ce/ZnO (2 : 1) ratio. This absorption coefficient value is related to the optical band-gap energy in accordance with the expression α = (hν − E_g)^1/2, where h is Planck's constant and ν is the frequency of incident photons. The intercept on the energy axis is obtained by extrapolating the linear portion of Tauc's plot (αhν)² vs. hν, as shown in Fig. 2b. The band-gap energy (E_g) of the doped Ce/ZnO (2 : 1 ) is ∼2.85 eV. The band-gap energy is calculated at a lower magnitude for Ce/ZnO NSs compared to only zinc oxide or cerium oxide.⁵¹

Fig. 2 (a) UV/visible spectra for Ce/ZnO (2 : 1) NSs and (b) band-gap energy (E_g) comparison.

XPS is a quantitative spectroscopic technique used to indicate the chemical nature of the elements existing in a particular material. XPS spectra are recorded by irradiating the doped nanomaterial with an X-ray beam, and kinetic energy, including the electron number of the doped sample might be determined consecutively. Here, the XPS spectra for the binding energy of Ce/ZnO NSs are confirmed in the presence of CeO₂, as presented in Fig. 3a. The blank ZnO and CeO₂ peaks appear at 882.4 eV for Ce3d [Fig. 3b], 1022.6 eV for Zn2p [Fig. 3c], and 532 eV and 537 eV for O1s peaks in the two undoped samples [Fig. 3d]. The shifting of binding energy could be a result of the Ce doping at different ratios, where the values are obtained in good agreement with the literature.^52,53 Table 1 summarizes the binding energy and band-gap energy values for various compositions of Ce/ZnO, ZnO, and CeO₂.

Fig. 3 XPS spectra of (a) undoped CeO₂, ZnO, and doped Ce : ZnO (2 : 1) nanoparticles, (b) Ce3d, (c) Zn2p, and (d) O1s level. X-ray beam spot size is 400.0 μm; pass-energy is 200.0 eV; pressures less than 10⁻⁸ Torr.

Table 1 Comparison of calculated binding energy and E_bg of various compositions of ZnO, CeO₂, and Ce/ZnO materials

Sample		Binding energy (eV)			Ebg (eV)
		Zn2p	O1s	Ce3d
ZnO		1022.6	532	—	3.25
Ce : Zn	1 : 1	1022.3	532	883.7	3.0
	2 : 1	1022.2	533.2	882.8	2.85
	3 : 1	1022.0	531	883.2	2.96
	4 : 1	1022.2	532	883.6	2.96
CeO2		—	537	882.4	3.1

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X-ray powder diffraction was employed to measure the Ce/ZnO nanomaterial phases by comparing with the standard value of lattice parameters, crystal structures, and crystallinity in the Joint Committee on Powder Diffraction Standards (JCPDS) of ZnO and CeO₂. XRD spectra in Fig. 4 reveal that all of the blank ZnO peaks (*) match the Bragg reflections of the standard hexagonal phase structure of space group P6₃mc (186), a = b = 3.249 Å, c = 5.207 Å, (wurtzite, JCSPD 36-1451), for the diffraction planes at (100), (002), (101), (102), (110), (103), (200), (112), (201), (004), and (202). The blank cerium oxide match CeO₂ peaks (#) for [JCPDS No. 65-5923] face-centered cubic structure at reflections planes of (111), (200), (220), (311), (222), (400), (331), and (422). In comparison of the two blank pattern with the Ce/ZnO spectra, it shows the peaks (+) which indicate Ce-doping with ZnO and change of intensities in some peaks. Although the XRD profile of the doping samples specially (2 : 1, 3 : 1, 4 : 1) resemble the CeO₂ which gives indication that the Zn doped in CeO₂ structure, this might be because of large ionic radii of Ce and difference of oxidation numbers.^51,54,55

Fig. 4 XRD spectra of CeO₂, ZnO, and Ce/ZnO nanomaterials.

FESEM is one of the prominent methods for studying the nanostructure morphology of nanomaterials. The morphology of the prepared Ce/ZnO NSs were measured using FESEM-coupled XEDS. Fig. 5a and b shows typical shapes of Ce/ZnO nanostructures for 2 : 1 ratio sample at low and high magnification.

Fig. 5 FESEM images of (2 : 1) Ce/ZnO sample (a) low magnification and (b) high magnification​.

Energy-dispersive X-ray spectroscopy (EDS) gives the quantitative presence of all elements in the prepared doped Ce/ZnO nanomaterial samples. Fig. 6a and b for Ce/ZnO indicates the presence of oxygen (31.21% wt), zinc (9.58% wt), and cerium (59.21% wt).

Fig. 6 EDS analysis of 2 : 1 Ce/ZnO nanostructures. (a) Morphological and (b) elemental analysis.

Applications: detection of 3-methoxyphenol with Ce/ZnO nanomaterials

The potential application of Ce/ZnO NSs as chemical sensors for measuring and detecting hazardous chemicals was explored using 3MP as a target molecule. The use of nanostructured materials as chemical sensors for selective detection is reported elsewhere with different nanocomposites or undoped or doped materials.^56,57 The Ce/ZnO NSs sensors have several advantages, such as consistency in air, nontoxicity, chemical stability, large surface area, electrochemical activity, simple to assemble or construct, and bio-safe characteristics. As in the case of 3MP sensors, the main mechanism is that the current response in the I–V method of Ce/ZnO NSs significantly changes when aqueous 3MP is adsorbed as the target analyte. The fabrication process and detection techniques are presented in the schematic diagram (Fig. 7); the fabricated surface of the Ce/ZnO NSs sensor was prepared with conducting coating binders on the polycrystalline GCE surface, which is presented in Fig. 7a. The fabricated GCE was kept in air for 12 h to make it smooth, dry, stable, and with a totally uniform surface. Theoretical I–V signal of the selective chemical sensor with doped thin film is expected to be a function of current vs. potential for 3MP, as presented in Fig. 7b. The electrical responses of target 3MP is investigated by a simple and reliable I–V technique using Ce/ZnO NS-fabricated GCE film, which is presented in Fig. 7c. The time holding of the electrometer was set at 1.0 s. A considerable amplification in the current response with applied potential was noticeably confirmed. The possible reaction mechanism for the simple and reliable detection of 3MP on Ce/ZnO NSs sensor surfaces by I–V method is generalized and presented in Fig. 7d.

Fig. 7 Schematic representation of (a) fabrication of GCE with coating agents, (b) I–V detection method (theoretical), (c) outcomes of I–V experimental results, and (d) proposed reaction mechanisms of 3MP detection in the presence of Ce/ZnO NSs/GCE nanomaterials.

Fig. 8a displays the current responses for bare GCE and for that with Ce/ZnO NS coating on the working electrode surface. The current response of the bare GCE was greater than that of the one coated with doped nanomaterials. The response of the resultant cell current (Fig. 8b) is shown for the Ce/ZnO (1 : 1) NSs implemented for the detection of a group of various analytes to study the selectivity in the presence of different analytes. This study shows the suitability of using Ce/ZnO (1 : 1) NSs as a good selective sensor towards 3MP compared to all other chemicals, even phenolic derivatives.

Fig. 8 I–V responses of (a) bare GCE and coated ZnO/GCE, CeO₂/GCE, and Ce/ZnO NSs/GCE (0.1 M PBS) system. (b) Selectivity is studied with various analytes using Ce/ZnO (1 : 1) nanostructure materials in similar analyte concentration in 0.1 M PBS system.

A set of Ce/ZnO nanostructure samples was prepared with different ratios by volume as 1 : 1, 2 : 1, 3 : 1, 4 : 1. The electrodes fabricated with these sample were tested with 3MP to study the effect of doping ratios, where the highest current response for 3MP was found to be Ce/ZnO with 2 : 1 ratio (Fig. 9a). Then, the fabricated electrode was used for concentration and repeatability studies for measuring and calculating the other sensor parameters. The concentration of 3MP was varied from 0.09 mM to 0.09 nM by adding deionized water at various proportions. A significant increase in the current value with applied potential was clearly demonstrated for the fabrication of thin NS film on flat GCE, as shown in Fig. 9c. It is observed that the current of NSs/GCE as a function of 3MP concentration at room conditions gradually increases from a low to high value of current with the increasing concentration of the target analyte. Fig. 9d shows the repeatability (R1 to R7) testing at 9.0 μM with a good consistent profile for the total run-set. A calibration curve is plotted (at +1.2 V) from the electrochemical responses in various 3MP concentrations and presented in Fig. 9e. The sensitivity and detection limit was calculated from the calibration curve based on the active surface area of fabricated GCE electrodes. The calibration plot is linear (r² = 0.9879) over the 0.09 nM to 0.09 mM 3MP concentration range. The sensitivity and detection limit is found as 94.937 μA cm⁻² μM⁻² and ∼11.5 ± 0.2 pM (based on signal-to-noise-ratio of 3), respectively. The fabricated 3MP sensor also exhibits good sensitivity, a good detection range of 0.9 nM to 9.0 mM, and long-term stability as well as enhanced electrochemical response towards the target analyte. The response time was approximately 10.0 s for the Ce/ZnO NSs-coated GCE to achieve saturated steady-state current. The prominent sensitivity of the 3MP sensor could be attributed to good absorption (porous surfaces fabricated with conducting binders), adsorption ability (large surface area), and high catalytic activity. The sensitivity of Ce/ZnO NSs affords good electron communication features, which enhances the direct electron transfer between the active sites of NSs and sensor electrode surfaces. The modified thin NSs/GCE has a better reliability as well as stability compared to those reported in literature.^58–60

Fig. 9 (a) I–V responses of various analytes; (b) current responses of fabricated electrodes (presented in percentage), potential range: 0 to +1.5 V; delay time: 1.0 s; (c) concentration variations of 3MP (0.09 nM to 0.09 M); (d) repeatability runs for 9.0 μM of 3MP; and (e) calibration plot (at +1.2 V) of Ce/ZnO/Nafion/GCE.

3MP is mostly converted to benzoquinone (BQ) and methanol in the presence of doped semiconductor nanomaterials. Here, oxygen (dissolved) is chemisorbed onto the Ce/ZnO/GCE NSs surfaces, while the NS-coated film electrode is immersed into the PBS system. During chemisorption, the dissolved oxygen is converted to ionic species (such as O₂⁻ and O⁻) by gaining electrons from the conduction band of aggregated Ce/ZnO NSs, which improve and enhance the current responses against potential during the I–V measurement at room conditions^61,62 (eqn (6) & (7)). The aqueous 3MP sensing mechanism of Ce/ZnO/GCE NS sensors is presented here based on the semiconductor oxides, due to oxidation or reduction of the semiconductor oxide itself, according to the dissolved O₂ in bulk solution, the surface/air interface, and the surrounding atmosphere.

e⁻(Ce/ZnO/GCE) + O₂ → O₂⁻ (6)

e⁻(Ce/ZnO/GCE) + O₂⁻ → 2O⁻ (7)

These reactions take place in the bulk solution, air/liquid interface, or surrounding air due to the low carrier concentration, which increases the resistance. 3MP sensitivity toward Ce/ZnO/GCE NSs (MO_x) could be attributed to the high oxygen deficiency and defect density, leads to increased oxygen adsorption. The larger the amount of oxygen adsorbed on the surface, the larger the oxidizing capability and the faster the oxidation of 3MP would be. The reactivity of 3MP would have been very large as compared to other chemicals with the surface under the same condition. 3MP reacts with the adsorbed O₂ on the doped surface of the film, liberating free electrons in the conduction band as could be expressed through the following reactions:

MO_x + 3MP + O₂⁻ → M(3MP_(OX)) + H⁺ + 2e⁻ (8)

M(3MP_(OX)) + H₂O → BQ + CH₃OH + H⁺ (9)

These reactions correspond to oxidation of the reducing carriers. These processes are increased with the carrier concentration of analyte and consequently reduce the resistance on contact with reducing liquids. At room conditions, the contact of Ce/ZnO NS surface with reducing liquid/analytes results in a surface-mediated chemical process. The elimination of ionosorbed oxygen enhances the electron density as well as the surface conductance of the Ce/ZnO NSs/GCE film. The reducing analyte 3MP donates electrons to Ce/ZnO NSs/GCE surface. Therefore, resistance decreases and hence, conductance increases. This causes the 3MP analyte response (current response) to increase with increasing potential (increasing the respective current), as proposed in Fig. 10. Thus, the produced electrons cause rapid enhancement in conductance of the large surface area of Ce/ZnO NSs. The Ce/ZnO NS eccentric regions dispersed on the surface would enhance the ability of the material to absorb more oxygen species, giving high resistance in ambient air.

Fig. 10 Mechanism of Ce/ZnO/GCE NS chemical sensors at ambient conditions. (a) Before and (b and c) after injecting target 3MP analytes.

The sensor response time was first set at 10 s for the Ce/ZnO/GCE NSs to reach saturated steady-state current. The higher sensitivity of the coated film could be attributed to the good absorption (porous surfaces fabricated with coating), adsorption ability, high catalytic activity, and good biocompatibility of the Ce/ZnO nanostructure materials. The estimated sensitivity of the fabricated sensor is relatively high for 3MP based on other composite or materials modified electrodes. Due to their large surface area, the nanomaterials provided a favorable nano-environment for the chemical detection with good quantity. The higher sensitivity of the Ce/ZnO/Nafion/GCE assembly provides better electron communication features, which improved the direct electron transfer between the active sites of Ce/ZnO NS materials and the GCE. The modified thin film has good stability due to high specific surface area, where the nanomaterials of Ce/ZnO NSs imparted a favorable environment for 3MP detection (by adsorption) with a large quantity. Ce/ZnO/GCE reveals several advantages for this approach in providing chemical-based sensors, encouraging the progress executed in this research. As for the doped nanomaterials, Ce/ZnO NS provides a path to a new generation of chemical sensors. Further research effort is required for the expansion of doped nanostructures in large-scale applications and to gain access to individual selective and sensitive chemical sensors. Reliable methods for fabricating, assembling and integrating the building blocks on sensitive chemical sensors need to be explored. Table 2 presents selected applications of nanocomposites or nanomaterials for sensing phenol compounds by electrochemical approaches.

Table 2 Comparison of phenolic sensor performance based on various electrochemical approaches by different electrode assemblies

Sensing layer	Analyte	Transduction	Performances	Ref.
MnO2/screen-printed sensor	Phenol	Voltammogram	Sensitivity: 0.123 mA μM^−1 cm^−2, DL: 0.64 μM, LDR: up to 716 μM	63
SiO2/Nb2O5 sol–gel	Phenol	Potentiometry	Sensitivity: 3.2 nA dm^3, DL: 0.5 μmol dm^−3, LDR: 5–25 μmol dm^−3	64
Ag2S QD/GC electrode	Phenol	Amperometry	Sensitivity: 0.0612 μA μM^−1 cm^−2, DL: 0.015 μM, LDR: 1 μM to 16 mM	65
Graphene–polyaniline/GCE	Aminophenol	DPV	Sensitivity: 1.776042 μA μM^−1 cm^−2, DL: 0.065 μM, LDR: 0.2–20, 20–100 μM	66
RGO/P-L-GSH/GCE	Aminophenol	Amperometry	Sensitivity: 0.0272 mA μM^−1 cm^−2, DL: 0.03 μM, LDR: 0.4–200 μM	67
POAS–Ag/MWCNT/GCE	Methoxyphenol	I–V	Sensitivity: 3.829 μA μM^−1 cm^−2, DL: 0.36 nM, LDR: 0.4 nM to 40.0 mM	49
Ce/ZnO NSs	3-Methoxy phenol	I–V method	Sensitivity: 94.937 μA cm^−2 μM^−1, DL: 11.5 pM (at SNR of 3), LDR: 0.9 nM to 9.0 mM, linearity, r^2 = 0.9879, response time: 10 s	This work

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Conclusion

Finally, we have successfully fabricated a 3MP chemical sensor based on low-dimensional Ce/ZnO NSs with suitable doping ratio by immobilizing flat GCE with conducting coating Nafion binders. Facile Ce/ZnO NSs were prepared by a wet chemical method using reducing agents in an aqueous alkaline system at low temperature—a simple, convenient, and economical approach. The 3MP sensor was studied by simple and reliable I–V method at room conditions to investigate the analytical performance thoroughly in terms of sensitivity, detection limit, response time, selectivity, storage stability, and reproducibility. The present work provides an extensive research activity that convened on the synthesis, total characterization, and 3MP sensing application of Ce/ZnO NSs on GCE by a simple and reliable I–V method. Finally, the proposed I–V method provides reasonable and reliable results for the selective determination of 3MP as a hazardous analyte. Thus, the method presented may play an important role for using this Ce/ZnO-NSs/GCE assembly for the selective detection of 3MP by I–V method at room conditions. This novel approach introduces a well-organized route of efficient phenolic chemical sensor development for the detection of selected 3MP chemicals in environmental fields at a broad scale.

Acknowledgements

Center of Excellence for Advanced Materials Research (CEAMR), King Abdulaziz University, Saudi Arabia is highly acknowledged for the financial and instrumental facilities.

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