3D Flower-Like Gadolinium Molybdate Catalyst for Efficient Detection

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Energy, Environmental, and Catalysis Applications

3D Flower-Like Gadolinium Molybdate Catalyst for Efficient Detection and Degradation of Organophosphate Pesticide (Fenitrothion) Jeyaraj Vinoth Kumar, Raj Karthik, Shen-Ming Chen, Karikalan Natarajan, Chelladurai Karuppiah, Chun-Chen Yang, and Velluchamy Muthuraj ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 19 Apr 2018 Downloaded from http://pubs.acs.org on April 19, 2018

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ACS Applied Materials & Interfaces

3D Flower-Like Gadolinium Molybdate Catalyst for Efficient Detection and Degradation of Organophosphate Pesticide (Fenitrothion) Jeyaraj Vinoth Kumara,b, Raj Karthika, Shen-Ming Chena*, Karikalan Natarajana, Chelladurai Karuppiahc, Chun-Chen Yangc and Velluchamy Muthurajb* a

Electroanalysis and Bioelectrochemistry Lab, Department of Chemical Engineering and

Biotechnology, National Taipei University of Technology, No. 1, Section 3, Chung-Hsiao East Road, Taipei 106, Taiwan, ROC b c

Department of Chemistry, VHNSN College, Virudhunagar-626001, Tamil Nadu, India

Battery Research Center of Green Energy, Ming Chi University of Technology, New Taipei

City, 24301, Taiwan, ROC

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ABSTRACT: Three-dimensional (3D) nanostructured materials have received enormous curiosity in energy and environment remediation applications. Herein, we developed a novel 3D flower-like gadolinium molybdate (Gd2MoO6; GdM) and used as a bi-functional catalyst for the electrochemical detection and photocatalytic degradation of organophosphate pesticide fenitrothion (FNT). The flower-like GdM catalyst was prepared via simple sol-gel technique with the assistance of urea and ethylene glycol. The properties of GdM were confirmed by various spectroscopic and analytical techniques. The GdM catalyst played significant role to electrochemical reduction of FNT, results very low detection limit (5 nM), wide linear ranges (0.02-123; 173-1823 µM) and good sensitivity (1.36 µA µM−1 cm−2). Interestingly, the GdM electrocatalyst had good recoveries to FNT in soil and water samples analysis. In addition to trace level detection, the flower-like GdM was used as photocatalyst which portrayed excellent photocatalytic degradation behavior to eliminate the FNT in aqueous system. The GdM photocatalyst could degrade above 99% of FNT under UV light irradiation with good stability even after five cycles.

KEYWORDS: Catalyst, Pollutant, Pesticides, Fenitrothion, Gd2MoO6, Electrocatalyst, Photocatalyst


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1. INTRODUCTION Organophosphate compounds are widely used as pesticides throughout the world (approximately 36%) due to their higher efficiencies to kill/control the pests/insects in household and

agricultural

fields1.

Particularly,

fenitrothion

[O,O-dimethyl-O-(4-nitro-m-tolyl

phosphorothioate); FNT] is an important member of organophosphate compound that have been utilized since 1959 to control against chewing and sucking of pests/insects on vegetables, rice, stored grains, fruits, cotton, cereals (for agricultural), cockroach and mosquito control (for house hold). Conversely, FNT can strongly persistent in soil and aquatic environment due to their low water solubility nature. The low concentration of FNT residues can imposes the damage to nervous system, sensory function, visual system and cognitive function of the both human and animals2-4. Therefore, European countries has now been banned FNT but still being used in Russia, United States and some other developing countries5, 6. Moreover, FNT residues in natural water can easily undergo photolysis and produce highly toxic metabolites (i.e., P=S converted into P=O), which is more toxic than parent compound7, 8. For these reasons, trace level detection and complete detoxification of FNT from the environment (i.e., soil and water) is very important concern for health protection and homeland security9-11. Up to now, a number of analytical techniques have been developed and used for the very low level detection of FNT including high-performance liquid chromatography (HPLC)12, gas-chromatography13, mass spectrometry14 enzyme-linked immunosorbent assay (ELISA)15, paper bio-chromatography16, molecular imprinting17, electrochemical sensor and biosensor18. Among all, the electrochemical techniques offered many advantages such as fast response, quick fabrication, good sensitivity, high selectivity, easy to operate, low-cost and portability when compared to other analytical techniques19. On the other hand, photocatalytic technique as a green, eco-friendly and most adoptable method for the removal of toxic organic pollutants into non-toxic compounds20-24. Recently, metal molybdates (AMoxOy; A= Ni, Co, Cu, Bi, Fe, etc.,) have enormous attention in supercapacitors, humidity sensors, lithium-ion batteries, photoluminescence, scintillators, optical fibers, electrochemical sensors, light emitting diodes, photocatalyst and catalyst due to their interesting physicochemical properties including magnetic, electronic and optical properties25-30. However, rare-earth metal molybdates (RExMoyOz, RE = Ce, Pr, Nd, Gd and Yb) have paid tremendous interest in the field of laser host material, optical fibers, phosphors, scintillators, photocatalysis and electrocatalysis owing to their excellent 3 ACS Paragon Plus Environment


physiochemical properties31,20. Especially, gadolinium molybdate (Gd2MoO6; GdM) has superior electrical, mechanical, chemical stability and excellent thermal properties, thus, it has been extensively investigated as ferroelectric-ferroelastic material and phosphors32-34. On the other hand, there is no literature reports are available for the electrochemical and photocatalytic behavior of GdM used as a catalyst. As well known that, the surface morphology and crystalline purity of the material plays a significant role for electrochemical and photocatalytic properties35, 36

. Very few studies have been explored on the morphologies of Gd2MoO6 such as nanoparticles,

block-like, sphere-like and rod-like structures were synthesized via sol-gel, solid-state, pechini and molten salt synthesis method, respectively37-40. However, three-dimensional (3D) hierarchical flower-like structure made from two dimensional (2D) nanosheets possess great interest to the researchers in terms of their less agglomeration nature, excellent charge transportation, high specific surface area, good conductivity and interconnected porous channels41. By motivating the above facts, the development of hierarchical 3D flower-like GdM with good crystallinity through simple and eco-friendly synthesis technique is an important concern. In this article, a novel flower-like GdM was synthesized via a simple sol-gel method and studied for their dual-functional catalytic performances towards the determination and detoxification of hazardous pesticide FNT. The as-prepared flower-like GdM was characterized by various spectroscopic and analytical techniques such as X-ray diffraction (XRD), Raman, Fourier transform infrared spectroscopy (FT-IR), Scanning electron microscopy (SEM), Energydispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), UV-visible diffuse reflectance spectroscopy (UV-DRS) and Brunauer–Emmett–Teller (BET) studies. Furthermore, as-developed flower-like GdM modified glassy carbon electrode (GCE) revealed excellent electrocatalytic activity toward FNT sensing and showed outstanding electroanalytical performance for FNT determination. Besides, the GdM modified GCE sensor can able to detect FNT in contaminated soil and water samples even at very low concentration. In addition, the flower-like GdM utilized as a photocatalyst for the degradation of FNT under UV light irradiation with high photodegradation efficiency (99%). To the best of our knowledge, there is no report available for the detection and degradation of FNT by using flower-like GdM as a catalyst.


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2. MATERIALS AND METHODS 2.1 Chemicals and Apparatus. Gadolinium (III) nitrate (Gd(NO3)3), urea (CH4N2O), ethylene glycol (C2H6O2), sodium molybdate (Na2MoO4) and all other chemicals were purchased from Sigma-Aldrich, Fluka, Alfa Aesar and Merck companies and used without further purification. The phosphate buffer solution (PBS, 0.05 M) was prepared by mixing dipotassium phosphate (K2HPO4) and monopotassium phosphate (KH2PO4). All the solvents and reagents were of analytical grade and used without further purification. All the required solution was prepared by using double distilled (DD) water. All the electrochemical measurements were performed (cyclic voltammetry (CV) and differential pulse voltammetry (DPV)) using CHI 405a and CHI 900 electrochemical workstation (CH Instruments Company, made in U.S.A) with a conventional three electrode cell system comprised of an GCE as working electrode (working area = 0.07 cm2), platinum wire as auxiliary electrode and Ag/AgCl (saturated KCl) as reference electrode. XRD study was conducted by PANalytical X’Pert PRO diffractometer measured with Cu−Kα radiation (λ = 1.54178 Å) in the 2θ range of 5-100°. FTIR spectra were recorded on a FT/IR-6600 spectrophotometer (Shimadzu, Japan). Raman spectra were taken by Horiba HR 800UV confocal Raman spectrophotometer. XPS results were performed using Thermo ESCALAB 250 instrument. SEM was investigated by Hitachi s-4300 SE/N resolution schottky analytical VP SEM instrument attached with the energy dispersive spectroscopy (EDS). BET results were carried out using the Micromeritics, ASAP 2020M instrument. The degradation experiments of FNT were monitored by using Jasco V-770 spectrophotometer and UV-DRS spectra was recorded using Shimadzu UV-2600 spectrophotometer. Total organic carbon (TOC) was performed on TOC analyzer (Multi N/C 2100/2100 S, Analytik Jena) and 1H-NMR analysis was performed Bruker Avance 400 MHZ spectrometer. 2.2. Synthesis of Flower-like Gd2MoO6. The synthesis procedure of flower-like GdM was followed by our previously reported article with slight modification42. In brief, each 0.1 M of Na2MoO4 and Gd(NO3)3 were dissolved in 35 mL DD water separately. Then, the homogeneous suspension was mixed in a 250 mL beaker under constant stirring. After that, 0.5 g (10 mL H2O) of urea and ethylene glycol (7 mL) was added into the above suspension and stirred for an hour. Later, the obtained white precipitate was washed with copious amount of DD water/ethanol and



dried at 80 oC for 12 h. Finally, the collected white precipitate was calcined at 600 oC for 4 h. The overall synthesis procedure and the applications of flower-like GdM as shown in Scheme 1. 2.3. Preparation of Flower-like Gd2MoO6 Modified Electrode. Prior to GCE surface modification, the GCE was well polished with 0.05 µm alumina slurry and washed with DD water to remove the alumina particles on the GCE surface. The flower-like GdM was dispersed in DD water at a concentration of 5 mg/mL and then sonicated for 15 min to get the homogeneous suspension. About 8 µL (optimized concentration, see supporting information section 1.1. (Figure S2)) of flower-like GdM suspension was drop coated on the mirror polished GCE surface and allowed to dry at room temperature. After that, the dried GCE was gently washed with DD water to remove the loosely attached GdM particles on the GCE surface. The obtained flower-like GdM modified GCE was used for further electrochemical measurements. 2.4. Photocatalytic Experiments. The photocatalytic degradation of FNT over flower-like GdM was performed under UV light irradiation. In a typical procedure, 50 mg of the catalyst was dispersed in 100 mL (20 mg/L) of FNT aqueous solution under UV light irradiation. Heber photoreactor that contains 16 W mercury lamp (λ = 300 nm) was used as the UV light source. In earlier, the reaction suspension was stirred vigorously under dark condition for 1 h to attain adsorption-desorption equilibrium of the photocatalyst and FNT solution. At every 10 min, the concentration of FNT was analyzed by UV-vis spectrophotometer. 3. RESULTS AND DISCUSSION 3.1. Characterization of Flower-like Gd2MoO6. XRD analysis was performed to investigate the crystallographic nature and phase purity of as-prepared flower-like GdM as represented in Figure.1A. The observed major diffraction peaks in the 2θ range at 28.74, 31.36, 34.35, 38.47, 45.18, 46.65, 48.79, 53.68, 57.71 59.47 and 66.7˚ were assigned to the (321), (231), (600), (312), (512), (042), (602), (123), (921), (642) and (080) miller indices plane of monoclinic Gd2MoO6, respectively. The determined lattice parameters are a= 15.6Å, b= 11.1Å, c= 5.4Å with specified space group of l2/a (15) and this is in well matched to their standard JCPDS No. 24-0423. No other redundant peaks such as GdO2, Gd2O3 or MoO3 were observed which implying that the high purity of the Gd2MoO6. Figure.1B portrays the Raman spectrum of as-prepared flower-like GdM which displays the high intense peaks at 915 and 1065 cm-1 were corresponds to the terminal symmetric stretching vibrations of Mo=O in Gd2MoO6. The low intense peaks at 313 and 384 cm-1 were assigned to the asymmetric Mo=O bending vibrations43. FTIR spectroscopy is 6 ACS Paragon Plus Environment

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an efficient technique to identify the presence of functional groups in the flower-like GdM. As seen in Figure.1C, the obtained FTIR peaks at 1631 and 3364 cm-1 were related to the O-H bending and stretching vibrations, respectively44. The appearance of peak at 664 cm-1 was ascribed to the Gd-O symmetric stretching vibration. In addition, the anti-symmetric stretching vibrations of Mo-O vibrations were observed at the wavenumber of 764, 845 and 915 cm-1 45. The surface topography of the as-prepared GdM was observed by SEM analysis and it depicted in Figure.2. The low and high magnification SEM images of Figure.2 (A-D) displays the 3D hierarchical flower-like structure of GdM which are randomly arranged one another with the average diameter of 500 nm. As observed from the Figure.2 (A, B), flower-like structures which buildup of numerous amounts of petals attached together with the thickness of ~20-30 nm. Furthermore, the elemental composition and their distribution of the flower-like GdM were investigated by EDS and elemental mapping analysis, respectively. The EDS spectrum in Figure.2E portrays the appearance of gadolinium (Gd), molybdenum (Mo) and oxygen (O) elements without any other discernible impurities. Moreover, the elemental mapping results in Figure.2F(b-d) revealed that the Gd (red color), Mo (green color) and O (dark blue color) elements have same locations and similar shapes to the representing scanning area in Figure.2F(a), which suggested that the homogeneous allocation of the elements obtained in the flower-like GdM. Furthermore, XPS analysis was also performed to determine the detailed elemental composition and their accurate oxidation of the as-prepared flower-like GdM and the results are demonstrated in Figure.3. The wide scan XPS survey spectrum in Figure.3A, clearly revealed that the existence of Gd, Mo and O elements in the flower-like GdM and the results are well matched to the EDS reports. The appearance of carbon (C) in the survey spectrum is due to the presence of hydrocarbon in XPS instrument itself. The high-magnification XPS spectrum of Gd 4d (Figure.3B) located the binding energies of 142.2 and 147.4 eV were ascribed to the Gd 4d5/2 and Gd 4d3/2 spin-orbit of Gd 3+ oxidation state46. From the Figure.3C, the observed peaks at the binding energies of 232.1 and 235.2 eV were assigned to the Mo 3d5/2 and Mo 3d5/2 core-level Mo 3d spin-orbit, implying the Mo 6+ state. The O 1s XPS spectrum in Figure.3D portrays the broad peak in the binding energy ranges of 529.5-532.3 eV, suggesting the existence of O2oxidation state in Gd2MoO6 47.



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The requirement of suitable light source for flower-like GdM was analyzed by using DRS UV-vis spectroscopy as shown in Figure.4A. It observed that the absorption spectrum of GdM falls in the region of 270-405 nm. In addition, the energy gap value was determined by the following Tauc's equation,

(α ) =

A −1 / 2 hν (hν − E g )

(1)

Where, Eg is the energy gap, α is the absorption coefficient, A is a constant, h is Planck’s constant and ν is the frequency of light. The estimated energy gap of GdM is to be 3.36 eV (Figure 4B). Specific surface area of the catalyst plays a vital role in the photocatalytic as well as electrocatalytic activity48. N2 adsorption-desorption isotherms were scrutinized to determine the specific surface area and pore-size distribution of the as-prepared flower-like GdM. Figure.4 (C&D) showed the BET adsorption-desorption isotherm and Barrett–Joyner–Halenda (BJH) pattern of flower-like GdM. From the Figure.4C, the specific surface area of flower-like GdM is found to be 19.72 m2g-1. The BJH pore-size distribution results in Figure.4D portrayed that flower-like GdM has mesoporous category of 27 nm. 3.2. Electrochemical Impedance Spectroscopy. Electrochemical impedance spectroscopy (EIS) was used to examine the charge transfer properties between electrode surface and electrolytes, which could offer beneficial information about the interfacial properties of modified electrodes. Therefore, we evaluate the electrical conductivities of the flower-like GdM modified GCE (Figure.S1(b)) and unmodified GCE Figure.S1(a) containing 5 mM [Fe(CN)6]3-/4- in 0.1 M potassium chloride (KCl). The obtained EIS results for the GdM/GCE and bare GCE are shown in Figure.S1. The plots (a) and (b) display the two different regions viz. a linear part at low frequency region and a semicircle at high frequency region. From the Figure.S1, the bare GCE displays a semicircle with a larger diameter due to the sluggish electron transfer behavior of the bare GCE. Fascinatingly, the GCE modified with flower-like GdM exhibited a significant decrease in the semicircle diameter when compared to bare GCE, suggested that the charge transfer resistance (Rct) decreased upon the GdM modification. The bare GCE and flower-like GdM modified GCE had Rct value of 791 and 421 (Ω), respectively. It should be noted that the ct value of the GdM modified GCE is lower than that of the bare GCE, which is mainly attributed to the large surface area and excellent electron conductivity on the surface of the GdM. 8 ACS Paragon Plus Environment

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The results clearly demonstrate that the GdM catalyst is a promising candidate for electrocatalysis and photocatalysis applications. 3.3. Electrochemical Behavior of FNT at Flower-like GdM Modified GCE. To study the electrocatalytic activity, CVs was performed for the reduction of FNT at the surfaces of flowerlike GdM modified GCE and unmodified GCE in the presence and absence of 200 µM FNT in 0.05 M phosphate buffer solution (PBS) (pH =7) at a scan rate of 50 mV/s as it can be seen in Figure.5A. The unmodified GCE in the presence of 200 µM FNT (Figure.5A (b)), there is a little broad irreversible reduction peak current was observed at the longer negative potential of -0.72 V. In contrast, the usage of flower-like GdM (Figure.5A (c)) as a modifier at GCE surface led to shifting the reduction and oxidation peaks of FNT to lower potentials with enhancing reduction and oxidation peak current. The obtained CVs result confirms that the lower reduction potential and enhanced reduction peak current of FNT were obtained from the excellent electrocatalytic activity of flower-like GdM catalyst. The overall electrochemical mechanism of FNT on electrode surface was given in Scheme 2

49

. A sharp and well-defined cathodic peak (R1) was

observed at the potential of -0.67 V, which is due to the direct reduction (irreversible) of nitro group (scheme 2 (a)) to hydroxylamine (scheme 2 (b)) with four electrons and four protons transfer process. At the reverse scan, there is no oxidation peak was observed corresponds to the reduction peak (R1) which confirms the irreversible reduction behavior of FNT. Whereas, the redox couple was observed at the potential of -0.09 and -0.01 V, attributed to the electrochemical conversion of hydroxylamine to nitroso derivatives (scheme 2 (c)) with two protons and two electrons transfer process and it denoted as R2 (reduction;-0.09 V) and O1 (oxidation; -0.01 V), respectively. Moreover, the obtained cathodic peak (R1) current of FNT is 1.8 fold higher and 50 mV lower overpotential, when compared to unmodified GCE. The enhanced peak current and peak potential shift confirmed the superior electrochemical behavior of flower-like GdM modified GCE for the detection of FNT. Further, the electrocatalytic activity of flower-like GdM catalyst was compared with other rare-earth molybdates such as cerium molybdate (Ce2(MoO4)3; CeM), praseodymium molybdate (Pr6MoO12; PrM), neodymium molybdate (Nd2Mo3O9; NdM) and samarium molybdate (Sm6MoO12; SmM) in the presence of 200 µM FNT containing 0.05 M PBS at a scan rate of 50 mV/s as shown in Figure.S3. It observed that the cathodic peak current of FNT for CeM (14 µA), NdM (13.3 µA) and SmM (13.3 µA) was observed and confirmed, which are 9 ACS Paragon Plus Environment


lower peak current when compared to GdM/GCE (14.4 µA). Besides, the peak potential is also higher for CeM (80 mV), NdM (90 mV), PrM (90 mV) and SmM (80 mV) when compared to flower-like GdM/GCE. The obtained results clearly suggest that the flower-like GdM exhibited higher electrocatalytic activity towards FNT sensing when compared to other rare-earth molybdates. The superior electrocatalytic activity of GdM could be attributed to the unique 3D flower-like structure, good crystallinity and large surface area. In addition, the electrocatalytic behavior of GdM catalyst was further confirmed by adding various concentrations of FNT (Figure.5B, curve a-e; 0-200 µM), the reduction and redox couple peak current gradually increases with increasing the concentration, reveals the excellent electrocatalytic activity. The overall result confirms that the GdM flower-like structure is act as an excellent electron mediator and it could be provide good platform for the detection of FNT by contributing surplus electroactive sites and large surface area of GdM catalyst, thus it improve the electrocatalytic activity for the detection of FNT. 3.4. Effect of Scan Rate. The peak potential shift and the reduction peak current enhancement validated the synergistic manner of the flower-like GdM/GCE for the FNT reduction. Therefore, the synergetic effect of the GdM modified electrode was further investigated by recording CVs of FNT at the surface of the modified electrode with various scan rates ranging from 10-200 mV/s (10-200; a-j) in the presence of 200 µM FNT containing 0.05 M PBS (pH =7). As showed in Figure. 5C, the reduction peak current of FNT was linearly increases with increasing the scan rate of 10-200 mV/s and the linear relationship was obtained between reduction (R1) peak current vs. scan rate (Figure. 5D) with a linear regression equation of Ipc (µA) = 0.08 (mV/s) + 5.79 and correlation coefficient of R2 = 0.991. The above obtained result suggests that the electrochemical performance of FNT at flower-like GdM modified GCE is a typical-adsorption controlled process50. 3.5. Effect of pH. In electrochemical studies, the peak shape, peak potential and peak current of the CVs might be changed while changing the pH of the electrolyte, which can affect the electrochemical response of FNT. Furthermore, the pH study is very useful to understand the assessment of proton to electron ratio participated in electrode reaction. In order to investigated the electrochemical reduction of FNT (presence of 200 µM) using CV in various pH buffer solution such as pH 3, 5, 7, 9 and 11 at a scan rate of 50 mV/s. As mentioned before, change in pH value the peak potential, current and shape of FNT was changed and it can be seen in 10 ACS Paragon Plus Environment

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Figure.6A. It was clearly seen that the peak potential shifted to more negative potential while increasing the pH value from 3 to 11, suggesting the existence of proton in the electrochemical reduction of FNT. A linear relationship was obtained between cathodic peak potential (Ep; R1) vs. different pH (Figure. 6C) with a slope value of -50 mV pH-1 and the linear regression equation of Ep (V) = -0.05x -0.29. According to the Nernst equation, the calculated slope value 50 mV pH-1 is very close to theoretical value of -59 mV pH-1, suggesting the equal number of protons and electrons involved in the electrochemical reaction. Furthermore, from the plot of reduction peak current vs. different pH (Figure. 6B), it can be clearly seen that the reduction peak current increases with increasing the pH value from 3 to 7 and decreased while increasing the pH value above 7 and maximum reduction peak current was obtained at pH 7. Hence, pH 7 was considered as the favorable pH for interaction between flower-like GdM modified electrode surface and FNT. Thus, we have chosen pH 7 for the further electrochemical measurements. 3.6. Determination of FNT at Flower-like GdM Modified GCE. DPV is highly sensitive and better resolution technique when compared to conventional CVs. Hence, we have chosen DPV to detect very low level concentration of FNT and the obtained results are shown in Figure.7A. The linear relationship was obtained between concentration and reduction peak current of FNT. As seen, when increasing the concentrations of FNT (from lower to higher; 0.02 -1883 µM), the reduction peak currents were linearly increased. At lower concentration of FNT, the reduction peak current of FNT is directly proportional to the concentration. In this case, the linearity of FNT detection observed to 0.02 - 123 µM with a detection limit of 5 nM (Figure.7B). However, another linear range was observed while further increasing the FNT concentration, the obtained response range is 173-1823 µM with the correlation coefficient of R2 = 0.985. The sensitivity was calculated to be 1.36 µA µM−1 cm−2 from the slope of lower linear response range of FNT detection. The excellent sensitivity, wide linear response range and the lowest detection limit may afford from the higher conductivity and large surface area of flower-like GdM modified GCE. Moreover, the obtained analytical parameters such as sensitivity, LOD and linear response range are provided in Table 1 51-61. The obtained analytical performances can be compared with previously reported modified electrodes for the detection of FNT. As compared with various modified electrodes, our reported flower-like GdM modified GCE offers good analytical performances such as good sensitivity, lower detection limit and excellent linear response range for FNT sensor. 11 ACS Paragon Plus Environment


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Table 1 Comparison of the analytical parameters of FNT detection with other reports. Modified Electrode

Technique

Linear range (µM)

LOD (µM)

Ref.

CeO2/rGO/GCE

DPV

0.02-2

0.003

51

MWCNTs/GCE

SWV

0.2-60

0.08

52

DME

SWV

0.04-0.2

0.006

53

AuNPs@GMIP/GCE

DPV

0.01-5

0.008

54

Nano-TiO2 polymer

SWV

0.02-10

0.01

55

Nano-TiO2/Nafion composite

DPV

0.2-4

0.09

56

GCE

SWV

0.4-50

0.08

57

SiO2/MWCNTs/RuPc

DPV

3.0-66

1.6

58

SPCE-MIP

SWV

3-100

0.8

59

PANI/GCE

AdSV

0.01-100

0.007

60

RGO/DPA/PGE

SWV

0.10–1.91

0.003

61

Flower-like GdM/GCE

DPV

0.02-123

0.005

This

173-1823

work

3.7. Selective Determination of FNT using Flower-like GdM/GCE. For interference study, DPV was performed using flower-like GdM/GC electrode for the detection of 200 µM in the presence of various common metal ions such as Na+, Zn2+, Ba2+, Mg2+, Fe2+, Co2+, K+, Ca2+, Cu2+, Br-, NO3-, I-; biological compounds such as glucose (Glu), uric acid (UA), ascorbic acid (AA), dopamine (DA); different organic (nitroaromatic) compounds such as nitrobenzene (NB), 4-nitrophenol (4-NP), nilutamide (Nil), and some known pesticides such as diuron (Diu), chlorpyrifos (Chl) and carbofuran (CAF) been used. The interference effect of FNT detection was investigated individually and its effect on the reduction peak (R1) current response was scrutinized (Figure.7C). Figure.7D shows the relative error bar (%) for the detection of FNT in the presence of selected various cations and anions, biological and nitroaromatic species. Most metal ions (50 fold excess concentrations of Na+, Zn2+, Ba2+, Mg2+, Fe2+, Co2+, K+, Ca2+, Cu2+, Br-, NO3-, I-), pesticides (20 fold excess concentrations of Chl and Diu), biological compounds (50 fold excess concentrations of Glu, UA, AA, DA, Nil) have an negative interfering effect of ∼1-9 % on the FNT detection. The electrochemical reduction of FNT current response changed by ∼2-7 % in the presence of 5 fold excess concentrations of NB, 4-NP and CAF compounds. 12 ACS Paragon Plus Environment

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Further, we examined the interference properties of SO42-, SO32- CO32-, PO43-, As3+ and humic acid that shows no interference signals with the FNT detection (Figure S4). The obtained result suggests that the flower-like GdM modified GC electrode is very suitable for the selective detection of FNT for aforementioned common metal ions, biological compounds, pesticides and nitroaromatic group containing compounds. For that excellent selectivity behavior of flower-like GdM, it could be used for the real sample analysis in soil and water samples (Table S1; the detailed information and discussion are provided in supporting information section 1.2). From the Table S1, the observed recovery values are 94 - 99 %, suggesting that the flower-like GdM had great practical viability for the determination of FNT in real sample analysis. Moreover, we have compared the electrochemical method with the HPLC method which exhibited almost similar results. Therefore, this method can be applicable to the practical applications. 3.8. Stability, Repeatability and Reproducibility. The stability of the flower-like GdM modified electrode was investigated in the presence of 5 µM FNT containing 0.05 M PBS (pH 7) by using DPV technique. The reduction peak current at flower-like GdM modified electrode toward 5 µM FNT hold about 94.32 % of stable response for one week, suggests good storage stability of the modified electrode for the detection of FNT. The repeatability of flower-like GdM modified electrode was investigated by using the similar electrode for 5 repeated analysis in 5 µM FNT containing solution with a relative standard deviation (RSD) of 3.32 %. For the reproducibility studies, we have chosen 3 independent flower-like GdM modified GCEs and showed appreciable reproducibility with RSD of 3.14 % for 5 µM FNT in 0.05 M PBS (pH 7). 3.9. Photocatalytic Activity. The potential efficiency of flower-like GdM was tested against the photodegradation of FNT under UV light irradiation. The absorption peak (267 nm) of FNT was monitored in the photodegradation process as given in Figure. 8A. The constant decrement of main absorption peak of FNT was observed which suggested that the constant photodegradation. After 80 min of irradiation, the major absorption peak was completely decreased almost equal to zero which clearly indicated that the complete photodegradation of FNT. There was no other redundant shift or peaks corresponding to the intermediates/ions were observed in Figure. 8A, which vividly confirms the flower-like GdM do not alter the FNT photodegradation pathway62. Figure.8B portrays the concentration changes of FNT with respect to irradiation time in the presence of commercial TiO2 and different rare-earth molybdates such as CeM, PrM, NdM and SmM. From the Figure.8B, there was no significant degradation of FNT were observed in the 13 ACS Paragon Plus Environment


direct photolysis (without catalyst) or in dark condition (without light). Fascinatingly, flower-like GdM exhibited an excellent photodegradation efficacy (99%) compared to commercial TiO2 (23%), CeM (71%), PrM (67%), NdM (53%) and SmM (77%). The influence of operational parameters were essential for the photocatalytic reaction, hence, it was necessary to optimize the operational parameters viz amount of photocatalyst loading and effect of FNT concentration. The amount of catalyst loading is vital in the photocatalytic reaction because optimizing the catalyst amount may leads to minimize the wastages of catalyst. In the present study, the amount of flower-like GdM photocatalyst was varied from 20 to 75 mg/mL while the FNT concentration and light sources were maintained constant and the results are shown in Figure. 8C. In general, the photocatalytic reaction rate is increased while increasing the amount of photocatalyst loading. However, the rate of photodegradation efficiency was increased up to 50 mg of flowerlike GdM loading which may be due to the adequate number of reactive oxidative species (ROS) generated on the surface of the GdM. Overloading of flower-like GdM (above 50 mg) is decreased the photocatalytic reaction rate; it is based on the agglomeration of the photocatalyst. In the present case, beyond 50 mg of GdM loading, the rate of photodegradation was decreased. The rate of photodegradation process also depends on the initial concentration of FNT solution which was studied in the concentration ranges from 15 to 30 mg/L. As shown in Figure. 8D, the degradation efficiency increases with increasing the concentration from 15 to 20 mg/L. After that, photodegradation efficiency of flower-like GdM decreases when the concentration of FNT more than 20 mg/L. At higher concentrations, the more number of FNT molecules can adsorb on the surface of the photocatalyst which leads to reduce the number of photons arrival on the surface of the catalyst. On the other hand, the absorbed photons by the FNT and the excitation of the photocatalyst particles by photons were reduced. The role of reactive oxidative species such as holes (h+), superoxide (.) and hydroxyl radicals (• ) during the photodegradation of FNT in the presence of flower-like GdM under UV light irradiation was investigated by using radical trapping experiments and the results are represented in Figure.8E. In this system, ammonium oxalate (AO), acryl amide (AA) and isopropyl alcohol (IPA) was utilized as the scavengers of h+, . and • radicals, respectively. The photodegradation efficiency of FNT was found to be 99% without the addition of any scavengers. In contrary, the photodegradation performances were greatly reduced from 99 to 31 % and 99 to 24 % after the addition of IPA and AA, respectively, which implying that . and 14 ACS Paragon Plus Environment

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• radicals played a key role on the photodegradation reaction. Furthermore, only 12 % of degradation was decreased behind the addition of AO suggest the minor participation of h+ in the photocatalytic reaction system. Generally, the degradation of organic compound by photocatalysis technique leads to the transformation of all its organic carbon atoms into harmless gaseous CO2 and their recalcitrant salts63-65. Therefore, total organic carbon (TOC) analysis was performed in order to inspect the mineralization efficiency of flower-like GdM towards FNT. Figure.S6 demonstrates the photodegradation and TOC removal efficiency of FNT at different time intervals. It obvious that, both degradation and TOC removal efficiency increases with increasing the irradiation time. After 80 min, degradation and TOC removal efficiency was observed 99% and 78% respectively. This substantial dissimilarity (21%) between the degradation and mineralization efficiency suggested that the FNT degradation products such as inorganic carbon (CO2) and that of sulphur, phosphorous, nitrogen heteroatoms into inorganic anions such as SO42−, PO43− and NO2− are persisted in the degraded solution, fortunately, which are less toxic to the environment66. Furthermore, 1H-NMR spectroscopy was employed to determine the photodegradation products of FNT and the results are shown in Figure.S7. (FNT was dissolved in D2O solvent (signal peak appear at 4.69 ppm) and analyzed at room temperature). The 1H NMR spectrum of parent FNT in Figure.S7A displays the peaks at 6.81-6.90 and 7.78 ppm which correspond to the aromatic protons (3H, m, Ar-H). Protons from methylene group adjacent to the aromatic group appear at 2.29 ppm (3H, s, Ar-CH3) and the signal at 3.62 ppm (6H, d, 2OCH3) is due to the methoxy group of FNT. After 80 min of UV light illumination, the signals of parent FNT completely disappeared and two new signals (Figure. S7B) were appeared in the region of 2.05 (singlet) and 8.09 ppm (singlet), suggested that the formation of CH3 group from acetic acid and OCH3 from formic acid. By a further involvement of hydroxyl radicals, acetic acid and formic acid can decomposed into CO2 and water. From the 1H NMR results and enthused from previous literature67, we proposed a possible degradation pathway in scheme 3. The results revealed that the hazardous FNT can degrade into CO2, H2O and inorganic anions such as SO42−, PO43− and NO2−.



3.10. Plausible mechanism Based on the results from the trapping experiments, the postulated degradation mechanism of FNT by GdM can thus be proposed, and the relevant reactions are expressed by equation (2 to7). + ℎ → + ℎ 2 + → + . 3 . + → • 4 ℎ + → + • 5 • ,• , . + → • + 6 • ,• , . + • → + + !" + #"$ + 7 When Gd2MoO6 was irradiated by UV light, it can generate electrons (e-) in the conduction band (CB) and holes (h+) in valence band (VB) (eq-2). The e- in the CB can react with atmospheric oxygen (O2), produce the superoxide anion radical species (O2•−) (eq-3). The photogenerated h+ react with adsorbed surface hydroxyl group or H2O at the surface of the .

catalyst to afford OH radicals, while the O2•− react with H+ to further generate the HO•2 (eq-5 and 4). Lastly, the FNT was degraded into CO2, H2O and their corresponding heteroatoms (S, P and N) into inorganic anions such as SO42−, PO43− and NO2− through the highly oxidizing species .

( OH, O2•−) (eq-6 and 7). In addition to the photocatalytic performances, the stability and reusability of the photocatalyst are the important concern for their practical function. Figure.8F showed the reusability of the flower-like GdM on the photodegradation of FNT after five repeated experiments. It is obvious that the flower-like GdM demonstrated excellent photodegradation ability towards the FNT solution could possess 92% even after four repeated cycles. Furthermore, the surface morphology and elemental composition of the flower-like GdM after five photodegradation reactions was characterized by SEM and EDS analysis and represented in Figure.S8. The SEM image (Figure.S8A) clearly portrayed that the 3D flower-like structure of GdM remain unchanged even after five repeated cycle runs. The EDS spectrum in Figure.S8B displayed the appearance of Gd, Mo, O and C elements in the reused GdM. The presence of C element is due to the adsorption of FNT intermediate products on the surface of the GdM.


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4. CONCLUSIONS In summary, the inexpensive and active flower-like GdM catalyst was prepared and its efficient electrocatalytic and photodegradation activity were studied toward FNT pesticides. The catalyst was prepared through sol-gel method using urea and ethylene glycol. The GdM catalyst showed higher electrocatalytic activity and efficient electron mediator for the sensitive detection of FNT pesticides. Moreover, the flower-like GdM modified GCE exhibited wide linear response range, lower detection limit, high sensitivity and selectivity. The GdM modified electrode also revealed the excellent stability and reproducibility towards the analysis of FNT. In addition, the flower-like GdM modified GCE achieved well recoveries to determine FNT in various soil and water samples. On the other hand, as-prepared flower-like GdM showed an excellent photocatalytic activity for the degradation of FNT under UV light illumination with high degradation rate of above 99% after 80 min. For these excellent bifunctional activities of flowerlike GdM, it could be applied for the fabrication of electrochemical sensor and degradation of FNT from the contaminated soil and waste water treatment.



■ ASSOCIATED CONTENT Supporting Information EIS spectra of bare GCE and GdM/GCE, Optimization of amount of electrocatalyst, CV comparison of GdM and other rare-earth molybdates towards FNT detection, Interference study, Real sample analysis, Effect of temperature on FNT photodegradation, TOC, 1H-NMR spectrum, SEM and EDS spectrum of reused flower-like GdM.

■ AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. (S-M Chen) Phone: +886-2270-17147. Fax: +886-227025238. *E-mail: [email protected], (V. Muthuraj) Tel: +919940965228. ORCID Shen-Ming Chen: 0000-0002-8605-643X Notes The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS This project was supported by the Ministry of Science and Technology (MOST 106-2113-M027-003 and MOST 106-2811-M-027-004), Taiwan, ROC.


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60. Sreedhar, N. Y.; Reddy, P.; Reddy, C. N.; Prasad, K. S. Electrochemical Analysis of FNT in Human Urine Samples Using Polyaniline Nanosensor Based Electrode. Nanosci. Nanotechnol. Int. J. 2011, 1, 6–11. 61. Ozge, S.; Gulcin, B.; Serdar, A. Electrochemical Behavior and Voltammetric Detection of FNT Based on a Pencil Graphite Electrode Modified with Reduced Graphene Oxide (RGO)/Poly(E)-1-(4-((4-(Phenylamino)Phenyl)Diazenyl)Phenyl)Ethanone

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Scheme 1. The overall synthesis procedure of flower-like GdM and it electrocatalytic and photocatalytic applications.


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Figure 1. XRD pattern (A), Raman (B) and FT-IR spectrum (C) of flower-like GdM.



Figure 2. Different magnification SEM micrographs of flower-like GdM 500 nm (A), 1 µm (B), 3 µm (C), 5 µm (D), EDS spectrum (E) and elemental mapping analysis (F (a-d)).


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Figure 3. XPS survey spectrum of flower-like GdM (A) and high magnification XPS spectra of Gd 4d (B), Mo 3d (C) and O 1s (D).



Figure 4. DRS UV-visible spectra (A) and Energy gap spectrum (B), BET nitrogen adsorption/desorption isotherms (C) and their corresponding BJH pore size distribution (D) of flower-like GdM.


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Figure 5. CVs obtained at (a) flower-like GdM modified GCE (absence of 200 µM FNT) and presence of 200 µM FNT (b) bare GCE and (c) flower-like GdM/GCE in PBS (pH = 7) at a scan rate of 50 mV/s (A). CVs recorded for flower-like GdM/GCE in the absence (a) and presence of different concentration of FNT (50-200 µM; b-e) into the 0.05 M PBS (B). CVs of 200 µM FNT in PBS (pH =7) at various scan rates 10-200 mV/s (a-j) at GdM/GCE (C). A plot of cathodic peak (R1) current vs. scan rate (D).



Figure 6. CVs response for 200 µM FNT at flower-like GdM/GCE in various pH solutions from 3 to 11 at a scan rate of 50 mV/s (A). The plot between cathodic peak current (R1) vs. pH (B), and Ep (R1) vs. pH (C)


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Scheme 2. Electrochemical reduction of FNT (a) to FNTred (b) and oxidation of produced FNTred to FNTox (c).



Figure 7. DPV response of the flower-like GdM/GCE to consecutive addition of FNT from 0.02 to 1883 µM in 0.05 M PBS (pH=7) solution (A). The calibration plot for the cathodic peak current vs. concentrations of FNT (B). The DPV responses of FNT reduction in the presence of co-interfering compounds; Na+, Zn2+, Ba2+, Mg2+, Fe2+, Co2+, K+, Ca2+, Cu2+, Br-, NO3-, I-; Glu, UA, AA, DA, NB, 4-NP, Nil, Diu, Chl, and CAF (C). Relative error bar for the interfering species. The x axis indicates the tested interfering compounds and the y axis indicates interference effect in percentage (%) (D).


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Figure 8. Time-resolved UV-visible spectrum of FNT over flower-like GdM under UV-light irradiation (A), Photodegradation of FNT with different photocatalysts (B), Effect of flower-like GdM dosage on the photodegradation of FNT (C), Effect of initial concentration of FNT (D),



Active species trapping experiment on the photodegradation of FNT (E) and the reusability of flower-like GdM (F).

Scheme 3. A possible photodegradation pathway of FNT in the presence of flower-like GdM under UV light irradiation.


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Table of Contents Graphic


3D Flower-Like Gadolinium Molybdate Catalyst for Efficient Detection

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