4-Hexylresorcinol sensor development based on wet

Journal of Industrial and Engineering Chemistry 66 (2018) 446–455

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4-Hexylresorcinol sensor development based on wet-chemically prepared Co3O4@Er2O3 nanorods: A practical approach Tahir Ali Sheikha,b , Mohammed M. Rahmana,b,* , Abdullah M. Asiria,b , Hadi M. Marwania,b , Md. Rabiul Awualc a b c

Chemistry Department, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia Center of Excellence for Advanced Materials Research, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia Depertment of Chemical Engineering, Curtin University, GPO Box U1987, Perth, WA 6845, Australia

A R T I C L E I N F O

Article history: Received 26 September 2017 Received in revised form 29 May 2018 Accepted 13 June 2018 Available online 21 June 2018 Keywords: Co3O4@Er2O3 nanorods 4-Hexyl resorcinol Sensitivity Glassy carbon electrode Environmental safety

A B S T R A C T

In this approach, Co3O4@Er2O3 nanorods (NRs) were prepared by a wet-chemical method using reducing agents in alkaline medium. The resulting nanoparticles were characterized in details by UV/Vis and FT-IR spectroscopy, X-ray powder diffraction, Elemental dispersive analysis (EDS) coupled with field-emission scanning electron microscopy (FESEM). Co3O4@Er2O3NRs were deposited on a glassy carbon electrode (GCE) to give a selective sensor with a fast response toward 4-hexyl resorcinol (4-HR) in phosphate buffer phase (PBS) by electrochemical approach. The 4-HR sensor also displays good sensitivity, large linear dynamic range, lowest detection limit, and long-term stability, and enhanced electrochemical response. The calibration plot is linear over the 0.1 nM–0.01 M 4-HR concentration range. The sensitivity is
14.765 mAmM1cm2, and the detection limit is 64.29 pM (signal-to-noise ratio, at a SNR of 3). We also discuss possible future prospective uses of this doped metal oxide semiconductor nanomaterial in terms of chemical sensing. © 2018 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.

Introduction Nanoscience and nanotechnology have great impact not only on the growth of modern science but also living beings of this universe. These are the emerging field of this era with wealth of applications in various fields such as biological [1], pharmaceutical [2], food industry [3], energy storage & conversion devices [4], toxic chemicals sensing [5] and many other industries. The growth of these fields relies on the production and application of different morphologies. Semiconductor nanomaterials of metal oxides have very impressive physical and chemical properties than their bulk substances such as enhanced electrical conductivity, optical property, structural and magnetic properties, mechanical strength, catalytic activity and thermal stability [6–11]. Lately, advance researches on semiconductor nanomaterials have revealed that doped metal oxides have increased in popularity owing to their

* Corresponding author at: Center of Excellence for Advanced Materials Research & Chemistry Department, Faculty of Science, King Abdulaziz University, Jeddah 21589, P.O. Box 80203, Saudi Arabia. E-mail addresses: [email protected], [email protected] (M.M. Rahman).

excellent physio-chemical propertiessuch as mechanical strength, heat tolerance, electrical conductance, electro-magnetic property, and photo-catalytic property etc. [12]. As we know that rare earth elements have the high electric conductivity and doping with semiconductor transition metals can further improve its electrical performance. So in this approach erbium in combination with cobalt was used to synthesized the doped semiconductor nanostructure material with improve electrical performance. Similarly, among the transition metal oxides, cobalt oxide (Co3O4) is found to be very important with respect of its distinctive commercial applications such as gas sensing [13–16], catalysis [17], energy storage [18], magnetic resonance imaging (MRI) and drug delivery [19]. Herein the synthesis of Co3O4@Er2O3 NRs reported by facile wet chemical method for electrochemical detection of toxic chemicals in aqueous solutions. These days, a vast and unwanted amount of hazard substances/chemicals are being released in our ecosystem due to rapid industrialization and urbanization. Among the hazards chemicals phenolic compounds and their derivative are ubiquitous contaminants in our environment and have very toxic effect in our ecosystem. The main sources of these ubiquitous contaminants are municipal, pharmaceutical, food and agriculture industries because of their wide range of uses in dyes, paint, polymers, drugs, and other organic substances [20]. 4-HR is well

https://doi.org/10.1016/j.jiec.2018.06.012 1226-086X/© 2018 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.

T.A. Sheikh et al. / Journal of Industrial and Engineering Chemistry 66 (2018) 446–455

known derivative of phenol and has many commercial uses in food, pharmaceutical and cosmetic industries [21]. In some pharmaceutical preparations, it develops the bacteriocidal [22], antiseptic [23] and anesthetic [24] properties in addition to the vermifuges activity [25]. In food industries it is used as an additive/ preservative in frozen foods to increase their shelf life. It act as an anti-browning agent during the storage of apple slices. In frozen sea food such as shrimps, crabs and prawns it is used as melanosis inhibitor in combination with EDTA and sodium pyrophosphate and also increase their shelf life [26–28]. In cosmetic industries, it is also used in anti-aging, skin whiting & lightening creams and hair dyes [29,30]. 4-HR is also known as cosmetic biocides because it reduces the lines and wrinkles on skin, protects it from ultraviolet A & B rays and also inhibits the growth of microbes on skin by killing them [31]. Due to the vast use of 4-HR in our daily life it is easily absorbed through skin and gastric tract, which causes the thyroid dysfunction, hematuria, hypothermia, dyspnea, CNS effects, altered relative adrenal gland weights, irritation in skin, eyes, nose, throat and upper respiratory system. The National Institute for Occupational Safety and Health (NIOSH), World Health Organization (WHO) and New Jersey Department of Health, USA, have declared it as hazardous substance [32–34]. Owing to its acute toxicity different analytical methods such as high performance liquid chromatography coupled with different detectors such as UV [35], fluorescence [28] and mass spectrometry [36], thin layer chromatography (TLC) [37], spectrophotometry [38], amperometric [39] and cyclic voltammetric [21,40] methods have been reported for the determination of 4-hexylresorcinol. Multi-walled carbon nanotubes (MWCNT) and activated carbon cloth (ACC) have been used as adsorbent material for the adsorption of 4-HR and resorcinol from aqueous solution [21,41,42]. These reported methods are not very effective because they are very expensive and complicated. So it has become our need to design the cheap and reliable method for the sensitive, selective and accurate determination of 4-HR. GCE modified with nanomaterials as chemical sensors have been reported for the detection of toxic chemicals [43,44]. In this study GCE, fabricated with Co3O4@Er2O3 NRs, was designed to develop the ultrasensitive and selective sensor for the detection of 4-HR among the various toxic chemicals by electrochemical approach using electrochemical technique. This nanomaterial displays the very good structural and morphological metal oxide nanostructure in addition to the very sensitive transduction in liquid-surface interactions to modify the electrochemical properties which was investigated by reliable electrochemical method under normal

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condition. It offers an effective, cheap, reliable and rapid detection of 4-HR qualitatively and quantitatively. As per our knowledge, this is the first selective study report for the detection of 4-HR qualitatively and quantitatively using electrochemical technique in short response time based on Co3O4@Er2O3 NRs which were coated with conducting binder nafion onto GCE. Experimental section Materials and methods The analytical grade chemicals such as erbium(III) chloride (ErCl3), cobalt nitrate hexahydrate (Co(NO3)26H2O), sodium hydroxide, thiourea, nafion (5% ethanolic solution), 2, 4-dinitrophenol (2,4-DNP), 2-aminophenol (2-AP), 3-methoxy phenylhydrazine HCL (3-MP HDN), 3-methoxyphenol (3-MP), 4hexylresorcinol (4-HR), 4-nitrophenyl hydrazine (4-NP HDN), bisphenol A (BPA), M-tolyl hydrazine (M-Tol HDN), phenylhydrazine (Ph-HDN) and p-nitrophenol (p-NP), monosodium phosphate and disodium phosphate were purchased from the Sigma-Aldrich company and used without any further modification and refinement. Spectroscopic characterization such as FTIR and UV/Vis spectra of the synthesized Co3O4@Er2O3 NRs were investigated on Thermo scientific NICOLET iS50 FTIR spectrometer (Madison, WI, USA), and 300 UV/Vis spectrophotometer (Thermo scientific), respectively. FESEM (JEOL, JSM-7600F, Japan) equipped EDS was also used to study the optical categorizations such as elemental analysis, molecular arrangement and morphology of the Co3O4@Er2O3 NRs. In addition to these above stipulated analytical tools crystallinity of the Co3O4@Er2O3 NRs were observed by the implementation of XRD analysis under ambient conditions. Trace detection of 4-HR was carried out by Keithley electrometer (6517A, USA) and the designed Co3O4@Er2O3 NRs/Nafion/GCE was used as a working electrode with conducting binder (ethanolic 5% nafion). A renowned electrochemical technique such as electrochemical method was applied at selective potential for probing the 4-HR qualitatively and quantitatively. De-ionized water was used in this whole study for the solutions preparation. Synthesis of Co3O4@Er2O3 NRs by wet-chemical method Co3O4@Er2O3 NRs were synthesized by wet-chemical method, Scheme 1. In this method Erbium(III)chloride (ErCl3), cobalt nitrate hexahydrate (Co(NO3)26H2O) and NaOH were used as reacting precursors. Equimolar solutions of ErCl3 (50 ml) and (Co

Scheme 1. Schematic diagram of wet-chemical process for the preparation of Co3O4@Er2O3nanorods.

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(NO3)26H2O) (50.0 ml) were taken in Erlenmeyer flask and mix them properly. Same volume (50 ml) of equimolar solution of thiourea was also added to the above flask and shake well to homogenize it. Surfactant plays a vital role for de-agglomeration of precursors and product (nanoparticles) as well. It also acts as a stabilizing agent and by some kind of attraction like chemisorption or electrostatic attraction it physically attached to their surface so as to make them separate in comparatively dilute solution. High concentration of the surfactant has high tendency for selfassembling into ordered form during depositing on solid surface. After the addition of thiourea, pH of the medium was shifted to alkaline range and adjusted at 10 by the drop wise addition of (2.0 M) NaOH. Then flask was kept in an oven with continuous stirring at 80  C for 6 h. As a final point Co3O4@Er2O3 formed as a co-precipitation, filtered and then washed with water and acetone to remove the remove the undesired impurities if present. After washing it was dried at 80  C in an oven for 24 h to make the sample moisture free. After complete drying it was grinded and then shifted in muffle furnace (Barnstead Thermolyne, 6000 Furnace, USA) for calcination at 600  C for 6 h continuously in order to get the Co3O4@Er2O3 NRs. After calcination. Following reactions occur during the formation Co3O4@Er2O3 NRs [45,46]. NaOH(s) → Na+(aq) + OH(aq)

(1)

ErCl3 → Er3+(aq) + 3Cl(aq)

(2)

Co(NO3)26H2O → Co2+(aq) + 2NO3(aq) + 6H+(aq) + 6OH(aq)

(3)

Na+(aq) + 5OH(aq) + Co2+(aq) + Er2+(aq) + Cl(aq) + NO3 → Co(OH)2 +   (4) (aq) + Er(OH)3(aq) + Na (aq) + Cl (aq) + NO 3(aq)

4Co(OH)2(aq) + O2(aq) → 4CoOOH + 2H2O

(5)

2Er(OH)3(aq) + 2CoOOH + Co(OH)2(aq) → Co3O4@Er2O3(s)# + 5 H2O(aq)

(6)

The alkaline pH plays a key role for the synthesis of Co3O4@Er2O3nanocrystals. For alkaline pH NaOH was used, because it behaves like a buffer to keep pH of the system at alkaline range during the whole process by contributing the OH to the system. ErCl3 and Co(NO3)26H2O are then hydrolyzed into their respective unstable hydroxides Er(OH)3 and Co(OH)2) by NaOH at particular pH. When concentration of Co2+ and OH reached at critical value then Co(OH)2 nuclei begin to precipitate. High concentration of Er3+ also exist in the system besides the OH ion, so nucleation of Co(OH)2 becomes easy due to the lower activation energy barrier of heterogeneous nucleation. As we know

that higher concentration of Er3+ exist in the system, a number of larger aggregated precipitation formed among the nanomaterials as Co3O4@Er2O3. Co3O4@Er2O3 nanocrystals begin to grow according to Ostwald ripening method phenomena. Initially Co3O4@Er2O3 nucleus begin to grow by self and mutual aggregation of nanocrystals then it reaggregated and form the aggregated Co3O4@Er2O3 nanocrystals. Nano crystal crystallize and re-aggregates with each other counterpart through Van der Waal forces and form the Co3O4@Er2O3 NRs with a porous morphology, Scheme 2. After the synthesis of calcined Co3O4@Er2O3 NRs, advance analytical tools like FTIR, UV/Vis, XRD, EDS, and FESEM equipped with EDS were used to study the optical, morphological and structural properties. After the corroboration of the nanostructure of our nanomaterial it was applied for the sensing of 4-HR by simple and reliable I–V method for the first time. Fabrication of GCE with Co3O4@Er2O3 NRs Very simple and easy protocol was applied for the fabrication of GCE by Co3O4@Er2O3 NRs. First of all slurry was made by the mixing of Co3O4@Er2O3 NRs with ethanol and then applied on to the glassy carbon electrode (GCE) with 5% ethanolic nafion solution as an adhesive conducting binder. After coating, it was shifted into the oven to get the completely dried and evenly coated stable Co3O4@Er2O3 NRs/nafion/GCE for 45 min at 40  C. So in this way a lab made electrochemical cell was formed by Pt-wire (1.5 mm in diameter) as a counter electrode and Co3O4@Er2O3 NRs/nafion/GCE as a working electrode to measure the I–V response. The phosphate Buffer solution (PBS) of 6.5 pH was made by mixing the equimolar (0.2 M) solutions of Na2HPO4 (39.0 ml) and NaH2PO4 (61.0 ml) in a 100.0 ml measuring cylinder. The amount of (0.2 M) PBS was kept constant throughout the whole trial in a beaker as 10 ml. The stock solution of 4-HR was used to make the different concentrations of 4-HR (full concentration range: 0.01 nM
0.1 M) in DI water and used as our target analyte. From the Slope of calibration curve (from current versus concentration plot) linear dynamic range (LDR), regression coefficient r2, sensitivity, Limit of Detection (LOD) (at S/3N), and Limit of Quantification (LOQ) for 4-HR was calculated. Electrometer was used as a constant voltage sources for I–V measurement in simple two electrodes system. Results and discussion Optical and structural characteristics of Co3O4@Er2O3 NRs UV/Vis and FTIR analysis were carried out for the determination of atomic and molecular vibrations in addition to the optical properties of the synthesized Co3O4@Er2O3 NRs. UV/Vis spectrophotometer was run to record the optical absorption spectrum in the range of 200–800 nm in order to determine the maximum absorption (lmax). The maximum absorption was found to be

Scheme 2. Schematic representation of growth mechanism of Co3O4@Er2O3 nanorods by facile wet-chemical process.

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449

Fig. 1. Characterization of Co3O4@Er2O3 NRs: (a) UV spectrum, (b) band-gap energy plot, and (c) FTIR spectrum.

292.0 nm and from the spectrum band-gap energy 2.33 eV was also calculated using the Tauc’s rule (direct band-gap rule) Fig. 1a and b [43]. In FTIR spectra, the frequencies of atomic and molecular vibrations before and after the calcinations were recorded in the range of 400–4000 cm1 in order to identify the structural and functional nature of the synthesized doped nanomaterials. In Fig. 1c (as grown curve), the very intensive and sharp peaks at 3630 cm1 and 1653 cm1were observed and corresponded to the O H stretching which indicate the presence of Co(OH)2. As well as bending peak at 511 cm1 was correlated with CoOH stretching [47]. The peaks at 1089 cm1, 1130 cm1, 1380 cm1, and 1512 cm1were predicted to be combined peaks of Co(OH)2 in association with Er(OH)3. Further, FTIR spectrum of the calcined doped nanomaterials in the same range as mentioned above is also shown in Fig. 1c (calcined curve). The very sharp and prominent peaks at different positions were observed after calcinations, which indicate the formation of doped metal oxides. Peaks at 3451 cm1and1653 cm1belongs to O H stretching may be due to water adsorption at surface of doped metal oxides. Intensive peaks at 664 cm1 and 564 cm1 belong to the bending vibration of Er O and Co O respectively. In the same way stretching at 1512 cm1 and 1418 cm1 are associated with Er O bands [48,49]. The peaks at 1380 cm1 and 1130 cm1 belong to CoO stretching, and 1089 cm1 belongs to Er O, become little bit sharp and displaced from their original positions due to inter atomic vibrations and appeared at 1365 cm1, 1160 cm1 and 1121 cm1 in calcined curved of Fig. 1c respectively, which strongly indicates the formation doped nanostructure materials. In material chemistry, crystalline nature is the good indication of the metal oxygen framework. The crystalline pattern of the synthesized doped nanostructure material was investigated by XRD analysis in the range of (2u) of 10–80 , Figs. 2 and 3. The diffraction peaks for 2u value were observed at (211), (222), (411),

Fig. 2. Powder XRD pattern of the Co3O4@Er2O3 nanorodes.

(440), and (622), which were in good consensus with respect to the reported Er2O3 JCPDF file (No. 77-0464 and 77-0777) [50,51]. Other diffraction plans at (220), (311), (400) and (511) from the same spectrum in addition to the Er2O3 were also observed and these were also correlated to the previously reported Co3O4(JCPDF No. 42-1467) [52]. Some Peaks intensities and positions after calcination change to some extent which reflects the incorporation of the both metal oxides and it is also the indication of corroboration of the doped nanomaterial formation. Scherrer equation was used to calculate the average diameter of the crystalline nanomaterial as an individual particle and found to be 7.0 nm. D¼

0:94l ðbcosu Þ

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Fig. 3. Morphological and elemental evaluation. (a-b) Low-high resolution FESEM images & (c–d) EDS spectrum for the calcined Co3O4@Er2O3 NRs.

Where, l=wavelength of X-ray radiation; β = full width at half maximum (FWHM) of the peaks at diffracting angle; and u = Bragg angle. Morphological and elemental analyzes The morphology of the Co3O4@Er2O3 NRs was investigated by FESEM. Typical morphological information of the calcined Co3O4@Er2O3 NRs from low to high-magnified images is presented in Fig. 4(a–b) . Calcined Co3O4@Er2O3 NRs has an average aggregated nanorods diameter 35.6 nm in the range of 20.0– 70 nm. It is also analyzes the TEM and HR-TEM and presented in the ESM (Fig. S3). This exceptional rod-shape structure of Co3O4@Er2O3 provides a large surface area and increases the electronic transport and conductivity of doped materials. Chemical compositions of the calcined Co3O4@Er2O3 NRs were determined by EDS analysis and presented in Fig. 4(c–d), which confirms the presence of Co, Er and O. Composition (wt. %) of cobalt, erbium, and oxygen in the Co3O4@Er2O3 NRs was 2.21%, 65.12%, and 32.67%, respectively. Elemental mapping (Co, Er, and O elements) is measured of Co3O4@Er2O3 NRs and presented in ESM (Fig. S4). BET analysis The N2 adsorption–desorption isotherms were carried out by the 3Flex analyzer (Micromeritics, USA) at 77.0 K. The specific surface area (SBET) was calculated using multi-point adsorption

Fig. 4. Analysis of BET for Co3O4@Er2O3 NRs.

data from the linear segment of the N2 adsorption isotherms using BET theory. The surface characterization of the Co3O4@Er2O3 NRs based on the N2 adsorption-desorption isotherms showed the IVtype isotherm with hysteresis loop, which was representative of the mesoporous ordered framework textural pores as judged in Fig. 4. In addition, the mesoporous Co3O4@Er2O3 NRs exhibited significant high surface area (SBET), pore volume, and pore diameters according to the data analysis. Also the N2 isotherm

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indicated the uniform channels with well pore size having surface area of 11.27 m2/g, and high pore volume of 0.026 cm3/g and pore diameter of 2.87 nm. The high surface area of the mesoporous Co3O4@Er2O3 NRs exhibited case cavities from reduction of reactant precursors using alkaline medium. Application: 4-Hexylresorcinol detection by electrochemical method The main application of the Co3O4@Er2O3 NRs in this study is the electrochemical sensing of toxic chemicals in aqueous solution. Sensor based on this chemically inert nanomaterial was found to be very selective and sensitive for 4-HR among the various toxic chemicals in aqueous solution. It has many advantages such as easy to assemble, good current response, large surface area, stability in air, non-toxic and bio-safe characteristics. Fig. 5ashows the current response of coated and non-coated (bare) GCE. It shows that current response increase remarkably in GCE coated with Co3O4@Er2O3 NRs in 0.1 M Phosphate buffer solution. After that selectivity study was carried out in the presence of various toxic chemicals such as 2,4-DNP, 2-AP, 3-MP HDN, 3-MP, 4-HR, 4-NP HDN, BPA, M-Tol HDN, Ph-HDN, and p-NP and found to be selective only for 4-HR as compared with other toxic chemicals in Fig. 5b. Statistical approach was also applied to study the interference effect and effect of these above toxic chemicals in the presence of 4-HR on Co3O4@Er2O3 NRs/Nafion/GCE was also observed, Fig. 5c

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and Table S1 presented in ESM. It was found that this newly designed GCE is very sensitive and selective only for the detection of 4-HR and does not reveal any significant change in current response. Similarly in Fig. 5d, change in current response of in the presence and absence of 4-HR was also observed which illustrates that current response increases in the presence of analyte (4-HR) with large surface area coverage with better adsorption and absorption of target analyte (4-HR) onto surface of porous nanomaterial coated on GCE. Similarly, control experiment was also conducted with other metal oxides such as Fe2O3, ZnO, Ag2O, and NiO in order to validate the newly synthesized Co3O4@Er2O3 NRs selective and sensitive only for 4-HR. For this purpose the change in current responses of these above metal oxides in addition to the Co3O4 and Er2O3 were investigated and compared with Co3O4@Er2O3 NRs in the presence of 4-HR, Fig. S1 and Fig. S2 presented in ESM. It was noticed that change in current response of GCE coated with Co3O4@Er2O3 NRs was higher as compared to other metal oxides and found to be very selective and sensitive to only for 4-HR. Similarly, it was also noticed that change in current responses of single Co3O4 and Er2O3 were comparatively higher than the other metal oxides but on the contrary their current responses were lower than the newly synthesized Co3O4@Er2O3 NRs. Therefore, this experiment also enlightens us that electrical conductivity increased significantly in doped semiconductor nanostructure material.

Fig. 5. (a) I–V response of bare and coated electrode, potential range: 0–+1.5 V, (b) selectivity study with various analytes using semiconductor Co3O4@Er2O3 NRs, (c) current response of analytes at +0.8 V and analyte concentration was taken at 0.1 mM, and (d) current response of Co3O4@Er2O3 NRs/Nafion/GCE with and without target 4HRanalytes.

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The ultra-sensitive detection of 4-HR by Co3O4@Er2O3 NRs/ Nafion/GCE has been shown in Scheme 3 with I–V graphical responses. The whole process of fabrication of GCE has been stipulated in experimental section. Current response of coated GCE in the absence and presence of target analyte (4-HR) are shown in Scheme 3a and b respectively, with their comparisons Scheme 3c. The simple and probable reaction mechanism for the determination of 4-HR is also shown in Scheme 3d.

The chemistry of sensing mechanism for 4-HR relies on the oxidation or reduction of the semiconductor Co3O4@Er2O3 NRs fabricated on to the GCE. When coated GCE is immersed into the PBS, the dissolved oxygen (O2) is chemisorbed on the porous surface of Co3O4@Er2O3 NRs and converted into ionic species such as (O2) and (O) after gaining the electrons from their conduction bands. As a result of chemisorptions of dissolved oxygen on the porous surface of semiconductor Co3O4@Er2O3 NRs, the I–V response of coated GCE increases, as shown in Scheme 3a [53–55].

Scheme 3. (a) I–V response of coated GCE without 4-HR, (b) I–V response of coated GCE with 4-HR, (c) comparison of I–V responses, and (d) probable detection mechanism of 4-HR on Co3O4@Er2O3 NRs/Nafion/GCE in the presence of target analyte.

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e (Co3O4@Er2O3 NRs/GCE) + O2 → O2

(7)

e (Co3O4@Er2O3 NRs/GCE) + O2 → 2O

(8)

The reaction (8) continues and leads to the formation of free hydroxyl radical in excess because of the presence of surface adsorbed water [56]. The overall reaction is as follow: e + O2 → O2 + e → 2O + H2O → H2O2 → OH

(9)

Co3O4@Er2O3 NRs/Nafion/GCE sensitivity toward the 4-HR is corresponded to higher deficiency of oxygen which in turns enhances the oxygen adsorption. The free hydroxyl radicals oxidize the 4-HRs with the release of electrons in conduction band. It causes the change in current response against the potential applied during the I–V measurement of Co3O4@Er2O3 NRs/nafion/GCE under ambient condition, Scheme 3b and d. It was also observed that the change in current response was higher as compared to the current response in the absence of 4-HR as shown in Scheme 3c. Due to accessibility of excess hydroxyl radicals it further break into aliphatic carboxlic acids and finally carbon dioxide and water [21,40,57]. These oxidation reactions take place in bulk system on the porous surface of semiconductor Co3O4@Er2O3 NRs. Larger the amount of oxygen adsorbed then larger would be the oxidizing capability which results faster the oxidation of 4-HRs which in turns increase current response due to the release of electrons in conduction band. I-V responses of this designed Co3O4@Er2O3 NRs/nafion/GCE, at different concentration of 4-HR from 0.1 M to 0.1 nM in aqueous solution, were observed. A systematic increase in current response of designed fabricated electrode from lower to higher value as a function of 4-HR concentration was noticed at normal condition, Fig. 6a. Linear dynamic range (LDR) 0.1 nM  0.01 M, regression coefficient r2 = 0.9490, sensitivity 14.765104 mAmM1cm2 and, limit of detection 64.29 pM at signal to noise ratio 3 were

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calculated from linear calibration curve which was plotted at +0.8 V from various concentration of 4-HR, Fig. 6b. In addition to this repeatability tests, Fig. 6c, were also carried out at 0.1 mM and excellent reproducible responses were found with Co3O4@Er2O3 NRs/nafion/GCE under the identical conditions with a sequence of nine to ten successive measurement. Fig. 6d shows the plot of response time of the Co3O4@Er2O3 NRs/nafion/GCE toward the 4HR, which was approximately 10 sec to achieve the saturated steady state current and it was drawn from concentration variation graph. Electrochemical responses of this newly designed fabricated GCE were examined for up to 2 weeks in order to find out its stability and reproducibility. It was found to be very stable with no significant change in current response in addition to electrode fouling and poisoning during the detection of 4-HR. The experiments were conducted at normal condition and sensitivity remained almost same as the initial response after washing for each experiment. The higher sensitivity of the synthesized nanomaterial coated on GCE is due to its high electron communication feature between the active site of Co3O4@Er2O3 NRs and surface of GCE in addition to its good stability, catalytic activity and bio compatibility. Large surface area of the Co3O4@Er2O3 NRs offers the excellent nanoenvironment for the detection of 4-HR with better adsorption and absorption of the 4-HR onto the porous nano surface of the Co3O4@Er2O3 NRs fabricated onto the GCE. As far as this study enlighten us that this is the initial report for the sensitive and selective detection of 4-HR by I–V technique based on the Co3O4@Er2O3 NRs/nafion/GCE as compared to other previously reported methods. Table 1, shows the assessment of proposed I–V method for the detection of 4-HR with other previously reported methods. It also indicates that this outstanding proposed method, with our designed Co3O4@Er2O3 NRs/nafion/GCE as a sensor, for probing the 4-HR is more sensitive and efficient than previously reported methods (Table 2).

Fig. 6. Optimization of 4-HR sensor. (a) Concentration variation of 4-HR (0.1 M to 0.01 nM), (b) calibration plot (at +0.8 V) of Co3O4@Er2O3 NRs/nafion/GCE, (c) sensor response time, and (d) repeatability test (1–10 runs) at 0.1 mM.

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Table 1 Comparison of proposed method with different previously reported analytical methods for the detection of 4-HR. Methods

Analytes

Sensitivity

LDR

LOD

LOQ

Ref.

AdsSV(MWCNT-BPPGE) AdsSV(MWCNT-SPE) HPLC-FLD

4-HR 4-HR 4-HR (uncooked shrimp) (cooked shrimp) (meat crab) 4-HR Resorcinol Resorcinol Resorcinol 4-HR

– – –

(3.0 mM–50 mM) (0.5 mM–10 mM)

2 mM 0.14 mM

– –

0.00412 mM 0.00823 mM 0.00721 mM 0.015 mM 0.45 mM 0.6 mM 0.02977 mM 64.29 pM

0.01287 mM 0.02522 mM 0.02161 mM – – – – 0.214 nM

[21] [21] [28]

HPLC/TOF-MS UV Amperometric MWCNTs-mETD array Voltametry pDMDT–Au/CPE I-V method Co3O4@Er2O3 NRs/Nafion/GCE

(0.026 mM–5.15 mM) – – – 0.019 mAnM1 4.666108 mAnM1

(0.257 mM–10.29 mM) (0.91 mM–18.16 mM) (6.0 mM–100 mM) – (0.1 nM–0.01 M)

[36] [38] [39] [40] This work

AdsSV = adsorptive stripping Voltammetry; (MWCNT-BPPGE) = multiwalled carbon nanotube modified basal plane pyrolytic graphite electrode; (-SPE) = modified screenprinted electrode; HPLC-FLD = High performance liquid chromatography fluorescence detector; TOF-MS = time of flight mass spectrometry; DMDT–Au/CPE = 2,5-dimercapto1,3,4-thiadiazole with gold carbon paste electrode; and GCE = glassy carbon electrode.

Table 2 Real sample analysis for 4-HR by Co3O4@Er2O3 NRs/nafion/GCE sensor in various environmental samples. Real samples

Industrial effluent PC bottle PVC packaging

Calibrated concentration range

0.1 nM–0.1 M 0.1 nM–0.1 M 0.1 nM–0.1 M

Measured current (mA) R1

R2

R3

0.6382 1.2494 0.5682

0.6048 1.2607 0.5821

0.6187 1.236 0.587

Avg. measured current (mA)

Respective concentration (nM)

0.6206 1.2487 0.5791

0.060 0.121 0.056

Analysis of real environmental samples

References

Standard addition method was applied for the concentration determination of 4-HR in real (seawater, industrial effluents, PC bottle, and well water) in order to validate the proposed electrochemical method using Co3O4@Er2O3 NRs/nafion/GCE. A fixed amount of 25.0 mL of each sample was analyzed in (0.2 M) PBS. Results regarding the concentration of 4-HR in real samples was in good agreement with the proposed electrochemical method which is reliable, satisfactory and stable for analyzing the real samples with designed Co3O4@Er2O3 NRs/nafion/GCE.

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Conclusions Finally, we have successfully fabricated 4-HR chemical sensor based on low-dimensional Co3O4@Er2O3 NRs immobilized GCE with conducting binders for the first time. Nanomaterials are selectively prepared by solution method with reducing agents in aqueous alkaline system, which represents a simple, convenient, and economical approach. 4-HR sensor is studied by simple I–V technique at room conditions, and investigated the analytical performances thoroughly in terms of sensitivity, detection limit, quantification limit, response time, and stability as well as reproducibility. This report provided an extensive research activities that convened on the synthesis, characterization and 4-HR sensing application of Co3O4@Er2O3 NRs. This new approach is introduced as a new route for efficient selective chemical sensor development for detection of toxic chemicals in healthcare and environmental fields. Acknowledgments Center of Excellence for Advanced Materials Research, King Abdulaziz University, Jeddah, Saudi Arabia is highly acknowledged for financial supports and research facilities. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.jiec.2018.06.012.

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