Design of Novel Ytterbium Molybdate Nanoflakes Anchored Carbon

Research Article Cite This: ACS Sustainable Chem. Eng. 2018, 6, 8615−8630

pubs.acs.org/journal/ascecg

Design of Novel Ytterbium Molybdate Nanoflakes Anchored Carbon Nanofibers: Challenging Sustainable Catalyst for the Detection and Degradation of Assassination Weapon (Paraoxon-Ethyl) Raj Karthik,† Jeyaraj Vinoth Kumar,‡ Shen-Ming Chen,*,† Thangavelu Kokulnathan,† Han-Yu Yang,† and Velluchamy Muthuraj‡

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†

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 ‡ Department of Chemistry, VHNSN College, 3/151-1, College Road, Virudhunagar, Tamil Nadu 626001, India S Supporting Information *

ABSTRACT: Design of resourceful and sustainable catalyst for the trace level identification as well as detoxification of toxic pollutants into the environment is a major concern to researchers. In view of that, we developed novel flakelike ytterbium molybdate (YbMoO4; YbM) anchored on carbon nanofibers (YbM/f-CNFs) nanocomposite via simple wet-chemical route followed by a sonication process. The physicochemical properties of as-prepared YbM/f-CNFs were carried out by several spectroscopic techniques. The YbM/f-CNFs nanocomposite exhibited excellent electrocatalyst as well as photocatalyst for the detection and detoxification of chemical warfare agent paraoxonethyl (PTL). Interestingly, the electrochemical results illustrated that the YbM/f-CNFs nanocomposite exhibited an excellent electrocatalytic activity in terms of enhanced cathodic peak current and lower peak potential when compared with other modified electrodes. Furthermore, the YbM/f-CNFs modified electrode showed more extended linear response ranges (0.01−12 and 14−406 μM), lower detection limit (2 nM), good sensitivity (2.8 μAμM−1cm−2), and excellent selectivity for the PTL sensing. Besides, the YbM/f-CNFs catalyst had good recovery to PTL in soil and water sample analysis. In addition, the YbM/f-CNFs nanocomposite possesses remarkable photocatalytic activity and stability toward the degradation and mineralization of PTL under visible light irradiation. Furthermore, a possible detection and degradation mechanism was proposed toward PTL. This study provides a novel idea for the design of proficient and stable bifunctional catalyst for the real-time identification and remediation of lethal pollutants. KEYWORDS: Chemical weapon, Organophosphate, Paraoxon-ethyl, Detection, Degradation

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INTRODUCTION Nowadays, the invention of diseases and demises increases everyday due to the long-term occurrence of pesticides and insecticides in the food, fruits, agri-food, aquatic, and soil environment. Among them, organophosphate compounds are widely used as a chemical weapon during the world war and also utilized as pesticides to control the pests/insects in agricultural and household areas.1 In particular, paraoxon-ethyl (PTL) is a widely utilized organophosphate pesticide in the cultivation of cotton, rice, garlic, beans, wheat, and fruits.2 As a result, a large quantity of PTL can enter into the air, soil, and aquatic origins and finally migrates into the food chain. Moreover, PTL can easily penetrate the human body via inhalation to create dangerous health risks, including tremor, diarrhea, vomiting, poor visualization, headaches, lung-edema, and unconsciousness. Furthermore, it has great capability to restrict the acetylcholinesterase enzyme and leads to damage of the central nervous system.3,4 In addition, PTL has nearly 70% similar chemical properties of nerve agent sarin, and therefore, © 2018 American Chemical Society

U.S Environment Protection Agency (EPA) stated that PTL is an extremely toxic substance and produces chronic neurological diseases in both animal and human beings. Furthermore, PTL can persist in the environment for a prolonged time, owing to its high resistance to self-generated hydrolysis nature.5−7 For these reasons, the trace level identification and complete detoxification of PTL from the food, water, and soil samples is a challenging task to the researchers. Until now, different analytical methods including liquid chromatography, gas chromatography, fluorescence, and voltammetric techniques have been successfully developed and utilized for the accurate detection of PTL. Among them, electrochemical (voltammetric) techniques could offer many advantages such as simplicity, low-cost, rapid response, high selectivity, and good sensitivity, when compared to the other analytical methods.8 Recently, Received: February 27, 2018 Revised: May 5, 2018 Published: May 29, 2018 8615

DOI: 10.1021/acssuschemeng.8b00936 ACS Sustainable Chem. Eng. 2018, 6, 8615−8630

Research Article

ACS Sustainable Chemistry & Engineering

phase. Therefore, the development of a simple and efficient technique to synthesize the size- and shape-controlled scheelitetype YbM and its composite is quite essential. On the other hand, carbonaceous materials, including carbon nanofibers (CNFs), mesoporous carbon, carbon black, graphene, and carbon nanotubes (CNTs) have drawn considerable attention because of their exclusive physical properties and wide range of potential applications.52−59 Among them, CNFs can be considered as an excellent supporting matrix for the incorporation of nanomaterials on its surface due to their high conductivity, high mechanical strength, chemical stability, adsorption capacity, extremely high surface area, and porosity. Therefore, CNFs are potentially applied to the various applications such as sensors, photocatalyst, low-temperature fuel cells, secondary batteries, supercapacitors, and solar cells.60−62 However, the pristine CNFs (insoluble in water) have inert surfaces, and so, a pretreatment is required to succeed desirable interaction with metals/nanomaterials on the CNFs surface. Hence, the functionalization of CNFs (f-CNFs) surface is a major concern to improve the hydrophilicity and active surface area. Moreover, the optical, electrical, and mechanical properties of f-CNFs are quite different from the pristine CNFs. Therefore, we have chosen f-CNFs as a supporting matrix for the flakelike YbM to boost the electroand photocatalytic activities. In this strategy, we sought an attempt for the fabrication of novel flakelike YbM anchored f-CNFs composite through a simple wet-chemical route followed by a sonication process, and their physicochemical properties were characterized by various spectroscopic techniques. To the best of our knowledge, there are no reports available on the design of flakelike YbM anchored f-CNFs and their potential dual-functional catalytic applications for the detection and detoxification of PTL. Interestingly, the YbM/f-CNFs composite modified glassy carbon electrodes (GCE) demonstrate superior electrocatalytic performances toward PTL detection with high sensitivity and excellent selectivity. The YbM/f-CNFs composite displays excellent photodegradation and mineralization performances for the detoxification of PTL with good stability. The electrocatalytic and photocatalytic mechanisms were discussed in detail.

many researchers have developed an electrochemical biosensor for the selective detection of PTL by using an enzyme immobilized on the electrode surface. However, the short-term stability of enzyme suffers its practical environment applications.9 To overcome these problems, it is essential to develop a nonenzymatic electrochemical sensor for the detection of PTL in the practical applications. In addition to detection, detoxification of PTL is another primary concern to protect living things and the eco-system. For remediation progress, heterogeneous photocatalysis is a simple, green, and costeffective technique, which converts toxic pollutants into nontoxic compounds like CO2, H2O, H3PO4, etc.10,11 Therefore, the discovery of novel and efficient nanomaterials for both detection and detoxification of hazardous PTL is reasonably crucial. In up-to-date chemistry and material science, the physical and chemical properties of functional materials containing either inorganic/organic hybrids or inorganic compounds are fundamentally associated with their dimensionality, shape, and size.12−15 Hence, the controlled synthesis of well-defined shape, small size, and good dimensionality of the material has become a significant research issue in more modern years.16 Until now, many research endeavors have concentrated on the different ways to control the dimensionality, shape, and size of nanomaterials. Hitherto, a wide variety of inorganic nanomaterials such as tungstates, hydrates, molybdates, metals, metal oxides, and borates have been successfully developed with controlled shape and size using various methodologies, and the materials were applied to various important applications including catalysis, energy storage devices, sensors, photocatalysis, and water treatments due to their extraordinary physicochemical properties.17−23 Though, the simple and sensible synthetic method for the controlled synthesis of nanostructured material through chemical self-assembly is quite drastic and a hot research topic. Among the various nanomaterials, metal molybdates are essential inorganic materials that have been widely investigated in the field of catalysis,24 photoluminescence,25 humidity sensor,26 lasers,27 illumination,28 heavy metal disposal,29 and phytoremediation.30 However, scheelite-type (ABO4) compounds have been widely utilized in scintillation crystal, photoluminescence, Raman lasers, laser-host materials, ion conductors, catalyst, medical fields, energy storage devices, and numerous technological applications because of their excellent optical, electrical, luminescence properties, and large X-ray absorption nature.31−42 Especially, the scheelite-type molybdates (AMoO4, A = divalent cation containing element) have been broadly investigated, owing to their superior chemical and thermal stability, excellent strength, and high decomposition temperature which makes them amenable to the wide variety of applications, including scintillators in medical devices, sensors, solid state lasers, fiber optic communications, anode material for the Li-ion batteries, and photocatalysis.43−49 However, ytterbium molybdate (YbMoO4; YbM) with scheelite-type structure has received slight consideration. Recently, Wu et al. reported that the coprecipitation synthesis of 0−100 mol % Er3+ doped YbMoO4 for the investigation on temperature quenching and sensing properties.50 Followed by Volkov et al., who described the growth, structure, and evaluation of laser properties based on the lithium-doped Yb(MoO4)2 single crystal.51 Until now, there are no reports available on the electrocatalytic and photocatalytic studies of YbM- and YbMbased composite due to the difficult-to-synthesize scheelite-type

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EXPERIMENTAL SECTION

Materials and Apparatus. Ytterbium(III) nitrate pentahydrate (Yb(NO3)3·5H2O), carbon nanofibers (CNFs, D × L = 100 nm × 20−200 μm, average pore volume = 0.075 cm3/g), sodium molybdate (Na2MoO4), paraoxon-ethyl (O2NC6H4OP(O)(OC2H5)2), and all other chemicals were purchased from Sigma-Aldrich and used without further purification. The phosphate buffer solution (PB solution, 0.05 M) was prepared by mixing monobasic sodium phosphate (NaH2PO4) and sodium hydrogen phosphate (Na2HPO4). All the reagents and solvents were of analytical grade and used without further purification. All the required solutions were prepared by using double-distilled (DD) water. The powder X-ray diffraction analysis was investigated in a XRD, XPERT-PRO spectrometer (PANalytical B.V., The Netherlands) with Cu Kα radiation (λ = 1.5406 AÅ). A Raman spectrum was measured using an NT-MDT, NTEGRA SPECTRA spectrometer. The scanning electron microscopy (SEM) and energy-dispersive X-ray (EDS) results were observed using a Hitachi S-3000 H microscope (SEM Tech Solutions) attached with HORIBA EMAX X-ACT. The TEM images were examined on a Shimadzu JEM-1200 EX with an accelerating voltage of 100 kV. The X-ray photoelectron spectroscopy (XPS) results were collected from Thermo ESCALAB 250 instrument. The Micromeritics, ASAP 2020 M instrument was used to determine the 8616


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ACS Sustainable Chemistry & Engineering Scheme 1. Overall Synthesis Procedure and the Applications of YbM/f-CNFs Nanocomposite

Figure 1. (A) XRD and (B) Raman spectra of the as-synthesized YbM/f-CNFs nanocomposite. obtained f-CNFs were dried at 60 °C for overnight and used for composite preparation. Synthesis of YbM/f-CNFs Nanocomposite. In a typical recipe, 20 mg of f-CNFs and 50 mg of flakelike YbM were redispersed in 100 mL DD water and ultrasonicated for 120 min. Finally, the collected products were dried at 60 °C for 12 h and denoted as YbM/f-CNFs nanocomposite. The overall synthesis procedure and the applications of YbM/f-CNFs nanocomposite are shown in Scheme 1. Fabrication of YbM/f-CNFs/GCE Structure. Prior to GCE surface modification, the GCE was well-polished with 0.05 μm alumina slurry and washed with DD water and ethanol to remove the alumina particles from the GCE surface. The as-prepared YbM/f-CNFs nanocomposite was redispersed in DD water at a concentration of 5 mg/mL and then sonicated for 30 min to get a homogeneous suspension. About 8 μL (optimized concentration) of YbM/f-CNFs nanocomposite suspension was drop-coated on the mirror-polished GCE surface, and it was allowed to dry at room temperature. After that, the dried YbM/f-CNFs modified GCE was gently washed with DD water to remove the loosely attached composite molecules on the GCE surface. The obtained YbM/f-CNFs/GCE was used for further electrochemical measurements. Photocatalytic Activity. The photocatalytic performances of the as-prepared YbM and YbM/f-CNFs nanocomposite were determined by the degradation of PTL aqueous suspension under visible light irradiation. In a typical recipe, 0.5 g catalyst was dispersed in 100 L of PTL solution (10 mg/L) and stirred for 1 h in dark conditions to ensure the adsorption−desorption equilibrium of the reactants, and

specific surface area and pore size distribution of the material. All the electrochemical measurements were performed [cyclic voltammetry (CV) and amperometric (i−t)] using CHI 405a electrochemical workstation (CH Instruments Company) with a conventional threeelectrode cell system comprised of an glassy carbon electrode and rotating disc glassy carbon electrode (GCE and RDGCE) as a working electrode [working area = 0.07 cm2 (GCE); 0.2 cm2 (RDGCE)], platinum wire as auxiliary electrode, and Ag/AgCl (saturated KCl) as reference electrode, respectively. Synthesis of FlakeLike Ytterbium Molybdate. The synthesis procedure of YbM was followed by that described in our previous article with slight modification.10 In brief, each 0.4 M of Na2MoO4 and 0.2 M Yb(NO3)3 were dissolved in 70 mL of DD water separately and the homogeneous solution was mixed together in a 250 mL beaker and allowed for continuous stirring. Afterward, 0.5 g (in 10 mL H2O) of urea was added into the above suspension and stirred for an hour. The obtained products were washed with copious amount of water/ethanol and dried at 80 °C overnight. Finally, the collected products were calcined at 650 °C for 4 h in air. Functionalization of CNFs. The surface functionalization process of CNFs was followed by a route proposed in the previously reported article with slight modification.63 In a typical experiment, 1 g of CNFs were added to the freshly prepared 50 mL HNO3/H2SO4 (1:3 ratio) acid mixture and it was allowed to reflux for 10 h at 50 °C. After that, the resultant f-CNFs was centrifuged and washed with copious amounts of DD water until the pH value reached 7.0. Finally, the 8617


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Figure 2. SEM micrographs of the (A) pristine YbM, (B) f-CNFs, (C) YbM/f-CNFs; TEM images of (D) pristine YbM, (E) f-CNFs, (F) YbM/fCNFs; and EDS spectra of (G) pristine YbM, (H) f-CNFs and (I) YbM/f-CNFs nanocomposite.

Figure 3. (A) XPS survey of YbM/f-CNFs nanocomposite, and high-resolution XPS spectra of (B) Yb 4d, (C) C 1s, (D) Mo 3d, and (E) O 1s.

the initial concentration of PTL was noted. Afterward, the above suspension was placed in a photoreactor (Heber Company) for

photodegradation experiments. A 300 W tungsten incandescent lamp was applied as a visible light source. At every 10 min, 5 mL of the 8618


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Figure 4. N2 adsorption/desorption isotherms of (A) YbM, (B) YbM/f-CNFs, (C) DRS UV−vis diffuse reflectance spectra, and (D) energy gap spectra of YbM and YbM/f-CNFs nanocomposite.

ranges of 400−700 cm −1 were ascribed to the MoO 4 tetrahedrons and the high intense peaks (700−1000 cm−1) were related to the stretched MoOn polyhedra vibrations, which was aroused from the appearance of several distorted and independent MoO4 tetrahedrons.65−67 In brief, the Raman modes at 458 and 547 cm−1 were ascribed to the MoO bending modes and Mo−O−Mo symmetric stretching modes, respectively. The high intense Raman peak centered at 795 cm−1 corresponded to the asymmetric stretching vibrations of MoO42− tetrahedrons in YbMoO4.68 The Raman spectrum of fCNFs in curve (b) displays the two peaks at 1343 and 1584 cm−1, attributed to the D band disordered structure of sp3 hybridized carbon and G band ordered sp2 hybridized carbon, respectively.69 As similar to the XRD pattern, both the YbM and f-CNFs Raman modes were presented in the YbM/f-CNFs nanocomposite (curve c), which undoubtedly confirmed the embedment of YbM on the f-CNFs surfaces. The surface microstructures of as-prepared YbM, f-CNFs and YbM/f-CNFs nanocomposite were evaluated by SEM and TEM analysis, and the results can be seen in Figure 2. The SEM micrographs in Figure 2A displays the large amount of narrow sized aggregated flakelike structure of YbM70,71 and the corresponding EDS spectrum (Figure 2G) indicates the presence of Yb, Mo, and O elements. From Figure 2B, it is evident that the pristine f-CNFs demonstrates that the randomly attached fiberlike microstructure and the relevant EDS (Figure 2H) spectrum shows the presence of C and O

aliquot was collected and centrifuged to remove the catalyst particles. Then, the concentration changes of PTL were observed by a Jasco V770 UV−visible spectrophotometer. The degradation percentage of PTL was determined by the following equation,

D(%) = (C0 − C /C0) × 100

(1)

Whereas, D is the photodegradation percentage, C0 and C represent the intensity values of the PTL before visible light irradiation (t = 0) and at selected time interval, respectively.

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RESULTS AND DISCUSSION Characterization of YbM/f-CNFs Nanocomposite. The crystallographic properties of the as-prepared YbM, f-CNFs and YbM/f-CNFs were determined by XRD analysis, as shown in Figure 1A. The distinctive sharp peaks (curve a) at 2θ angles equal to 19.16, 29.63, 32.9, 35.13, 48.88, 50.3, 56.62, 59.5, and 61.5° were attributed to the (101), (112), (004), (200), (204), (220), (116), (132), and (224) planes of tetragonal YbMoO4, respectively, with the space group of l41/a (88) [JCPDS no. 35−1471].50 From the curve (b), the broad intense peak at 2θ = 25° is related to the (002) plane of f-CNFs.64 The XRD pattern of YbM/f-CNFs (curve c) portrayed the appearance of both YbM and f-CNFs diffraction peaks without any other extra peaks. The results suggested the successful fabrication of YbM/ f-CNFs nanocomposite. Furthermore, Raman spectroscopy is a valuable tool to probe the structural information about the carbon -based composite materials. In Figure 1B (curve a), the pristine YbMoO4 portrayed that the low intensity peaks in the 8619


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Figure 5. (A) CVs response of (a) bare GCE, (b) CNFs/GCE, (c) YbM/GCE (d) f-CNFs/GCE, and (e) YbM/f-CNFs/GCE in 5 mM [Fe(CN)6]3−/4− containing 0.1 M KCl at scan rate of 50 mVs−1. (B) CVs response of YbM/f-CNFs/GCE in 5 mM [Fe(CN)6]3−/4− containing 0.1 M KCl by varying scan rate from 10 to 100 mV s−1 and (C) their corresponding linear calibration plot. (D) Different catalyst amount of YbM/fCNFs loading (4 to 10 μL) on the GCE surface.

indexed to epoxy or hydroxyl carbon (C−O) and the Gaussianfittings at the binding energies of 284.8 and 287.8 eV correspondent to amorphous carbon and carboxyl carbon (O−C = O) species, respectively.73 The high-resolution Mo 3d (Figure 3D) core-level XPS spectrum depicts the peaks at 232.3 and 235.6 eV, which were attributed to the Mo 3d5/2 and Mo 3d3/2 of the Mo6+ state.74−77 The O 1s band (Figure 3E) is a superposition of three-fitted peaks with the binding energies 530.6, 531.2, and 532.1 eV, and the components were assigned to lattice oxygen, chemisorbed oxygen, and adsorbed water molecules on the surface of YbM/f-CNFs nanocomposite, respectively.78 The electrocatalytic and photocatalytic performances of a catalyst are strongly dependent on specific surface area and pore-size volume. Therefore, the textural assessment of asprepared YbM and YbM/f-CNFs composite was evaluated using N2 adsorption/desorption isotherm analysis. In Figure 4, the B.E.T isotherm linear plot and the inset demonstrate their corresponding pore-size distribution value [which was gained by the Barrett−Joyner−Halenda (BJH) pattern] of YbM and YbM/f-CNFs nanocomposite. As seen in Figure 4 (panels A and B), the determined specific surface area of YbM/f-CNFs was 47 m2 g−1, which is higher than that of pristine YbM (21 m2 g−1). Moreover, the pore-size distribution of YbM and YbM/f-CNFs (inset of Figure 4, panels A and B) showed mesoporous ranges of 24 and 37 nm, respectively. Thus, the specific surface area and pore volume were increased with the

elements, which confirms the successful functionalization of CNFs. The SEM image of YbM/f-CNFs shown in Figure 2C confirms that the YbM flakelike structures randomly covered the CNFs surface. Furthermore, the TEM studies could provide exhaustive structural information about the as-prepared materials. The TEM image of YbM, as displayed in Figure 2D, comprised epyirregular flakes with the diameter of ∼70 nm, whereas f-CNFs in Figure 2E portrays a networklike structure of nanofibers with a relatively smooth and porous surface. As for YbM/f-CNFs in Figure 2F unambiguously revealed that the YbM still maintained its flakelike structure and uniformly occupied the carbon nanofiber matrix, which leads to the establishment of an interlinked network between YbM and fCNFs. The EDS spectrum in Figure 2I revealed the presence of Yb, Mo, O, and C in the YbM/f-CNFs nanocomposite without discernible components of impurity elements. In addition, the surface elemental compositions and their accurate oxidation states were gathered by using XPS measurements. The survey XPS spectrum of YbM/f-CNFs depicts the signals of C 1s, Yb 4d, Mo 3d, and O 1s elements (Figure 3A), which perfectly matched with the EDS report. The enlarged view of Yb 4d XPS spectrum displays (Figure 3B) the doublet peaks at the binding energies of 182.5 and 191.31 eV attributed to the Yb 4d5/2 and Yb 4d3/2 spin−orbit components, which is characteristic to the divalent state of Yb in the YbM/fCNFs composite.72 From Figure 3C, the high-resolution C 1s spectrum showed the high intense peak at 286.8 eV which was 8620


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Figure 6. CVs of (a) bare GCE, (b) flake-like YbM/GCE, (c) CNFs/GCE, (d) f-CNFs/GCE and (e) YbM/f-CNFs/GCE in the absence (A; a−e) and presence (B; f−j) of 400 μM PTL containing 0.05 M PB solution at a scan rate of 50 mV s−1. (C) The CV curves of YbM/f-CNFs/GCE in 0.05 M PB solution containing 400 μM PTL for the different scan rates ranging from 10 to 100 mV s−1 and (D) the corresponding linear plot of cathodic peak current vs scan rate. (E) The CV responses of the reduction of PTL (400 μM) at YbM/f-CNFs/GCE in various pHs ranging from 3.0 to 11.0 and (F) the corresponding calibration plot of pH vs cathodic peak current.

The optical band gap was determined by Tauc’s formula,11 and the obtained results are shown in Figure 4D. The estimated band gaps of YbM and YbM/f-CNFs are 2.90 and 2.83 eV, respectively. Electrocatalytic Activity of YbM/f-CNFs Nanocomposite. To evaluate the electrochemical properties of different modified electrodes and the synergistic effects between flakelike YbM and f-CNFs on the GCE surface, the CV were implemented, as can be seen in Figure 5A. From Figure 5A, a pair of broad redox couple was clearly observed as related to the electrochemical response of [Fe(CN)6]3−/4− with the bare

introduction of f-CNFs, which helps to generate the large number of reactive sites for the electrocatalytic and photocatalytic systems. DRS UV−visible spectroscopy is an essential tool to identify the suitable kind of light sources (UV or visible) which one needs for the YbM and YbM/f-CNFs nanocomposite. As found in the Figure 4C, the YbM/f-CNFs nanocomposite demonstrated a red shift in the visible light region, when compared to pristine YbM. The appearance of red shift in YbM/f-CNFs nanocomposite might be due to the powerful electronic interfacial contact between the YbM and fCNFs, which greatly induces the diffusion of intense photons. 8621


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Scheme 2. Overall Electrochemical Reduction/Oxidation Mechanism of PTL on the YbM/f-CNFs Nanocomposite

Figure 7. (A) Amperometric (i−t) response at YbM/f-CNFs/RDGCE for the addition of different concentrations (0.01 to 1576 μM) of PTL into the frequently stirred 0.05 M PB solution (pH 7.0); applied potential = −0.62 V; rotation speed = 1200 rpm; inset-enlarged view of lower concentration and current response of PTL. (B) A linear plot for concentrations of PTL vs cathodic current response. (C) Amperometric (i−t) response of the electrochemical sensor for the addition of PTL (a), malathion (b), carbofuran (c), diuron (d), chlorpyrifos (e) glucose (f), uric acid (g), ascorbic acid (h), dopamine (i) metronidazole (j), nitrofurantoin (k), fenitrothion (l), flutamide (m), 4-nitrophenol (n), methyl parathion (o), and nitrobenzene (p) into the frequently stirred 0.05 M PB solution pH 7.0. (D) Steady-state current response at YbM/f-CNFs/RDGCE PTL sensor for the addition of 50 μM PTL in 0.05 M PB solution (pH 7.0) up to 1400 s.

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ACS Sustainable Chemistry & Engineering Table 1. Comparison of the Analytical Parameters of PTL Detection with Other Reportsa electrode

linear range (μM)

LOD (μM)

sensitivity (μA μM−1 cm−2)

refs

CP/OPH GCE/MWCNTs/OPH GCE/SWCNTs/OPH GCE/CB/MC/OPH CP/OPH GCE/SWCNTs/OPH SPE/PBNPs/BuChE GCE/PPy-CNTs/AChE SPE/AChE GCE/YbM/f-CNFs

4.6−46.0 0.5−2.0 0.25−4.0 0.2−8.0 0.02−18 0.5−8.5 0.007−0.01 − − 0.01−12 14−406

0.9 0.31 0.15 0.12 0.02 0.01 0.004 0.003 1.8 0.002

0.0014 0.0259 0.025 0.1980 0.012 0.002 − − − 2.87

81 82 83 84 85 86 87 88 89 this work

a

CP, carbon paste; OPH, organophosphorus hydrolase; GCE, glassy carbon electrode; CNTs, carbon nanotubes; SWCNTs, single wall carbon nanotubes; CB, carbon black; MC, mesoporous carbon; SPE, screen-printed electrodes; PBNPs, prussian blue nanoparticles; BuChE, butyrylcholinesterase; PPy, poly(pyrrole); AChE, acetylcholinesterase.

GCE (a). Later, CNFs (b), YbM (c), f-CNFs (d), and YbM/fCNFs (e) were modified on the bare GCE surface, and the redox peak current response became greater with a smaller peak-to-peak separation (ΔEP) such as 0.356 V (bare GCE), 0.117 V (CNFs), 0.128 V (YbM), 0.139 V ( f-CNFs) and 0.111 V (YbM/f-CNFs nanocomposite), respectively. This phenomenon might be distinct evidence of the fact that YbM can significantly enhance the electron transfer when composite with f-CNFs. Moreover, the prepared flakelike YbM/f-CNFs

Table 2. Results of Determination of PTL in Soil and Water Samples sample soil

water

added (μM)

found (μM)

recovery (%)

0.0 10.0 20.0 0.0 10.0 20.0

− 9.75 19.86 − 9.97 19.91

− 97.5 99.3 − 99.7 99.5

Figure 8. Time-dependent absorption spectrum of PTL in the presence of (A) YbM and (B) YbM/f-CNFs composite under visible light irradiation. (C) Effect of different catalysts on the photodegradation of PTL. (D) Kinetic curves of PTL photodegradation using YbM and YbM/f-CNFs nanocomposite. 8623


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Figure 9. (A) Effects of different amount of YbM/f-CNFs dosage. (B) Effect of various scavengers on photodegradation of PTL over YbM/f-CNFs. (C) TOC removal and (D) reusability study of YbM/f-CNFs on the photodegradation of PTL.

the detection of PTL were affected by the loading of catalyst concentration on the electrode surface. Hence, the optimization of the loading level of YbM/f-CNFs on the GCE surface has a major role on the voltammetric responses. The different amounts of YbM/f-CNFs suspension such as 4, 6, 8, and 10 μL were drop-casted on the GCE surface, and their electrochemical responses (calibration plot) of PTL are shown in Figure 5D. It observed that the reduction peak current increases gradually while increasing the loading amount from 4 to 8 μL of YbM/fCNFs suspension on the GCE surface. Upon further increasing the concentration (above 8 μL), the reduction peak current of PTL decreased, suggesting that the limited amount of YbM/fCNFs suspension on the GCE surface can effectively detect the PTL. Notably, the higher composite suspension amount on the GCE surface may block (agglomeration of composite) the active area on the electrode surface, and thus, the peak current and sensitivity of the electrode were decreased. In this case, the maximal reduction peak current of PTL was observed in 8 μL of YbM/f-CNFs suspension. Therefore, the 8 μL of YbM/fCNFs/GCE suspension was chosen as the optimum modification for the PTL sensing. Electrochemical Behavior of the Flakelike YbM/f-CNFs Nanocomposite Based PTL Sensor. The electrochemical properties of PTL on different-modified electrodes were evaluated using CV. Figure 6 (panels A and B) shows the electrochemical performance on bare GCE (a), flakelike YbM/ GCE (b), CNFs/GCE (c), f-CNFs/GCE (d), and YbM/fCNFs (e) nanocomposite modified GC electrodes in the absence (Figure 6A = a, b, c, d, and e) and presence (Figure 6B

exhibited fast electron transfer (lower peak-to-peak separation) and higher redox peak current, which clearly suggests the unique electronic structures and electrochemical properties of flakelike YbM/f-CNFs that enhanced and improved electrochemical activity by higher electroactive area when compared to other modified and unmodified electrodes. Furthermore, the electroactive surface area of bare GCE, CNFs/GCE, YbM/GCE, f-CNFs/GCE, and YbM/f-CNFs nanocomposite was estimated using CV results. Figure 5B shows the CVs response of flakelike YbM/f-CNFs modified GCE containing 5 mM [Fe(CN)6]3−/4− in 0.1 M KCl with various potential scan rate from 10 to 100 mV/s. The electroactive surface area (A) was calculated using Randles− Sevick equation as follows.79 Ip = 2.69 × 105ACn3/2D1/2v1/2

(2)

where Ip is the obtained cathodic and anodic peak current, v is the potential scan rate, D is the diffusion coefficient of Fe(CN)63−/4− (7.6 × 10−6 cm2 s−1), n is the number of involved electrons, and C is the concentration of Fe(CN)63−/4− (5 mM). The linear plot for square root of scan rate (v1/2) versus peak current is shown in Figure 5C. The obtained electroactive surface of YbM/f-CNFs/GCE was 0.112 cm2, which was larger than that of other modified electrodes such as bare GCE (0.034 cm2), CNFs/GCE (0.049 cm2), YbM/GCE (0.082 cm2), and f-CNFs/GCE (0.091 cm2). Optimization of Amount of the Modifier. The electrochemical response and sensitivity of the modified electrode for 8624


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ACS Sustainable Chemistry & Engineering Scheme 3. Plausible Photodegradation Mechanism of PTL through the •OH Radicals

lower potential of flakelike YbM/f-CNFs nanocomposite modified GCE toward PTL is attributed to the strong interaction between the PTL and f-CNFs and more active sites of flakelike YbM. The overall result demonstrates that the proposed YbM/f-CNFs nanocomposite acts as an excellent electron mediator between the electrolyte and electrode surface, and it could offer a fabulous platform for the PTL sensing by contributing surplus electroactive sites and availability of large surface area on the surfaces of YbM and fCNFs. Thus, it improves the electrocatalytic activity for the PTL detection. Influence of Scan Rate. The effect of scan rate on flakelike YbM/f-CNFs nanocomposite modified GCE for the detection of PTL was investigated by CV at various scan rates ranging over 10−100 mVs−1 (Figure 6C) in order to study the electrontransfer process (selected molecules; PTL) between the electrolyte solution and electrode surface. In Figure 6C, it can be clearly seen that the cathodic peak (R1) and redox peak (Q1 and Q2) currents increased linearly with increasing scan rate, and it offered a linear relationship between scan rate and cathodic peak current (R1) of PTL (Figure 6D). The linear regression equation was obtained as follows:

= f, g, h, i, and j) of 400 μM PTL containing 0.05 M PB solution at a scan rate of 50 mVs−1. The PTL absence is obvious in Figure 6A. There is no oxidation/reduction peak observed for the modified and unmodified electrodes, whereas a sharp, enhanced, and irreversible cathodic peak current (R1) was observed in the presence of 400 μM PTL [Figure 6B (j)]. The observed cathodic peak (R1) is related to the direct reduction of PTL to phenylhydroxylamine with four electrons (e−) and four protons (H+) transfer process (system I). Furthermore, two more peaks (redox couple) were observed, and they were denoted as Q1 and Q2. The observed Q1 and Q2 (reversible) peaks are related to the formation of hydroxylamine to nitroso group with two electrons (e−) and two protons (H+) transfer process (system II).80 Besides, the obtained cathodic peak (R1) current of PTL is much higher when compared with Q2, which designates the PTL reduction at flakelike YbM/f-CNFs/GCE is more favored to form a phenylhydroxylamine in neutral medium or more alkaline medium. The overall electrochemical mechanism of PTL was well-documented previously, and it can be seen in Scheme 2. Conversely, the obtained cathodic peak potential at flakelike YbM/f-CNFs/GCE (−0.62 V) is shifted to a more positive side when compared to other modified electrodes such as bare GCE (−0.73 V), flakelike YbM/GCE (−0.67 V), and CNFs/GCE (−0.64 V). In addition, the observed cathodic peak current of PTL at YbM/f-CNFs/GCE (j) is 9.7 [bare GCE (f)], 8.5 [YbM/GCE (g)], 2.09 [CNFs/GCE (h)], and 2.7 [f-CNFs/ GCE (i)]-fold higher when compared to the aforementioned modified electrodes. These results suggest that the decrease in

Ipc = −1.62(mV) − 64.07 (R2 = 0.990) [for PTL reduction (R1)]

(3)

indicated that the reduction of PTL was a typically adsorptioncontrolled process. Influence of pH. The electrochemical response of PTL must be affected by changing the pH solution (values) due to 8625


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fold excess concentrations of pesticides malathion (b), carbofuran (c), diuron (d), chlorpyrifos (e); 50 fold excess of concentrations of physiological compounds glucose (f), uric acid (g), ascorbic acid (h), dopamine (i); and 20-fold excess concentrations of nitro-aromatic-containing drugs, metronidazole (j), nitrofurantoin (k), fenitrothion (l), flutamide (m), 4nitrophenol (n), methyl parathion (o), and nitrobenzene (p) in a constantly stirred 0.05 M PB solution (pH 7.0). The welldefined peak current response was observed for the addition of 50 μM PTL (a) into the PB solution, and further injection of 50 μM PTL in the next step with a sample interval of 50 s, the PTL current response was increased and steady state current reached within 5 s. Moreover, there is no considerable peak current response observed for the presence of pesticides, physiological compounds, and nitro-aromatic containing drugs (b−p) in the same solution. However, addition of 50 μM PTL induces the same current response [curve (a)] even in the presence of aforementioned cointerfering compounds. These results clearly suggest that the determination of PTL is possible even in the presence of 50-, 50-, and 20-fold excess concentrations of pesticides, physiological compounds, and nitro-aromatic containing drugs, respectively. Stability, Repeatability, and Reproducibility. To study the operational stability of the PTL sensor, the amperometric current response of flakelike YbM/f-CNFs/RDGCE to 50 μM PTL was investigated at an applied potential of −0.62 V over a period of 1400 s (Figure 7D). The reduction current still remained about 92% of the initial response after 1400 s, indicating the good operational stability of the sensor. To demonstrate the accuracy and feasibility of the proposed method, the repeatability of YbM/f-CNFs/GCE was evaluated by measuring the 50 μM PTL solution for 5 successive times with the same modified electrode. The obtained results suggested that the modified electrode holds an acceptable repeatability with relative standard deviation (RSD) of 3.43%. The reproducibility of the sensor was investigated using three independent YbM/f-CNFs modified electrodes with (50 μM PTL) RSD of 3.54%, suggesting that the proposed sensor has good reproducibility. Practical Application. To evaluate the practical viability of the proposed method for the PTL determination in soil and water samples, these two samples were analyzed under optimized (amperometric) conditions. The water and soil samples were collected from around Taipei, Taiwan. For the analysis, soil and water samples was directly used. However, no PTL was detected in soil and water samples. Therefore, the known concentration of PTL was added to both samples, and they were taken for the analysis using standard addition method. The obtained recoveries are displayed in Table 2, and the values were quite satisfactory and acceptable. Photocatalytic Activity. The photocatalytic study of asprepared flakelike YbM and YbM/f-CNFs nanocomposite was evaluated for the degradation of as-detected hazardous pesticide PTL under visible light irradiation. The UV−visible absorption spectroscopy was used to monitor the time-dependent concentration changes of PTL aqueous suspension in the presence of pristine YbM (Figure 8A) and YbM/f-CNFs nanocomposite (Figure 8B) under visible light irradiation. As seen in Figure 8B, the major characteristic absorption peak of PTL at 273 nm was diminished continuously with increasing irradiation time. After visible light exposure for 100 min, the intensity of the major peak decreases to nearly zero, which confirms the complete PTL decomposition by YbM/f-CNFs

the contribution of protons in the overall electrode reaction. The effect of pH (pH values = 3.0, 4.0, 7.0, 9.0, and 11.0) on flakelike YbM/f-CNFs nanocomposite modified GCE was investigated in the presence of 400 μM PTL at a scan rate of 50 mVs−1 by CV. As can be seen in Figure 6E, the cathodic peak current of PTL gradually increases with increasing pH values from 3.0 to 7.0. When the pH value is above 7.0, the cathodic peak current of PTL was gradually decreased. The maximal cathodic peak current was observed at pH 7.0 (Figure 6F), and therefore, the pH 7.0 was chosen as an optimal level for PTL detection. Besides, with the pH increasing from 3.0 to 11.0 in solution, the cathodic peak potential (Epc) is shifted toward more negative potential. The linear plot was obtained between the pH versus Epc as shown in Figure S1. From the linear plot, the linear regression equation can be expressed as follows: Epc(V) = −0.047 pH − 0.277 (R2 = 0.971)

(4)

In accordance with the Nernstian equation: dEpc /dpH = (2.303mRT )/(nF )

(5)

where n is the number of electron, m is the number of proton, and other symbols such as R, T, and F were consistent with their usual meanings. Accordingly, the proton transfer number (m) was calculated to be 2, indicating that the electrochemical reduction process of PTL on the flakelike YbM/f-CNFs/GCE is a transfer process with equal number of protons and electrons. Amperometric Determination of PTL. In Figure 7A, the amperometric (i−t) curve is shown for the flakelike YbM/fCNFs nanocomposite modified RDGCE with consecutive additions of PTL (0.01−1576 μM) in 0.05 M PB solution (pH 7.0) at an applied potential of −0.62 V (vs Ag/AgCl) under stirring (1200 rpm). It is seen that YbM/f-CNFs/ RDGCE exhibited tremendous amperometric current response with increasing PTL concentration. The linear relationship was obtained between the PTL concentration and current signal for YbM/f-CNFs/RDGCE, as depicted in Figure 7B. As a result, two linear ranges were observed in Figure 7B. The first linear response is found over the PTL concentration range of 0.01− 12 μM with linear regression equation of Ip (μA) = 0.39 + 0.20 (R2 = 0.997). The second linear response is over the PTL concentration range of 14−406 μM with the linear regression equation of Ip (μA) = 0.13 + 8.30 (R2 = 0.985). The enlarged view of amperometric current response of PTL at low concentrations is shown in Figure 7A (inset). The sensitivity (2.87 μA μM−1 cm−2) and limit of detection (LOD = 2 nM) were calculated from the lower linear response range of PTL with the signal-to-noise ratio of three (S/N = 3). A comparison of LOD, sensitivity, and wide linear response ranges for YbM/fCNFs/RDGCE and other previously reported PTL sensor can be observed in Table 1. The obtained analytical parameters (linear range, LOD, and sensitivity) for YbM/f-CNFs/RDGCE are very comparable and even superior to those obtained for various electrodes reported on recently. Interference Studies. The anti-interference ability of the YbM/f-CNFs/RDGC electrode was investigated for the detection of PTL in the existence of nitro-aromatic containing drugs, physiological compounds, and pesticides using the amperometric (i−t) technique. Figure 7C reveals the amperometric current response for PTL and other interfering compounds at YbM/f-CNFs/RDGCE in the presence of 508626


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provides inhibitory results of only 9 and 16% degradation of PTL, respectively, implying the minor involvement of h+ and O2•− radicals. Conversely, 76% degradation efficiency was induced by the IPA addition, suggesting that the PTL degradation was mostly persuaded by the •OH radicals. Furthermore, minor involvement of O2•− and major role of •OH radicals during the photodegradation reaction were determined by using nitro blue tetrazolium (NBT) and terepthalic acid (TA) tests, respectively.94−96 Figure S2 portrays the photoluminescence (PL) intensity peak of TAOH at 426 nm greatly increasing with increasing irradiation time, suggesting the generation of a large amount of •OH radicals during the reaction. Conversely, the slight decrement of the NBT absorption spectrum in Figure S3 was observed, which designates the production of a minimum amount of O2•− radicals in the photodegradation of PTL. The results indicate that the •OH radicals generated on the YbM/f-CNFs nanocomposite surface play a significant role in the PTL photodegradation. On the basis of the trapping experiment results and enthused from a previous report,97 we have provided a plausible degradation mechanism of PTL, as shown in Scheme 3. It can be clearly seen that the strong electrophilic •OH radicals can cleave the P−O−C, P−O, and C−C bonds and oxidizes the P and C atoms of PTL. Therefore, the structure of PTL saturated into para-nitrophenol, diethyl phosphoric acid ester, and acetaldehyde. Finally, these compounds were further decomposed into phosphoric acid, carbon dioxide, and water. Furthermore, the mineralization efficiency of YbM/f-CNFs nanocomposite toward the PTL degradation was determined by TOC analysis under optimized conditions. Figure 9C represents that the TOC removal efficiency increases with the irradiation time increasing. After 100 min, the TOC removal efficiency reaches about 71%, indicating the superior mineralization performance of the YbM/f-CNFs nanocomposite. Nevertheless, TOC removal efficiency was 28% less than the degradation efficiency, which might belong to the existence of recalcitrant and stable salts in the solution. For practical implementation of the catalyst, the reusability and stability are essential factors. Consequently, the stability of YbM/f-CNFs nanocomposite catalyst was evaluated by repeating the PTL degradation process six times, and the results are given in Figure 9D. At each degradation process end, the catalyst was recovered by filtration and dried and used in the next cycle. As shown in Figure 9D, the YbM/f-CNFs nanocomposite catalyst still maintained 92% degradation efficiency even after fiverepeated cycles that evidently verifies superior stability and reusability.

nanocomposite. There were no extra peaks corresponding to the intermediates, and this indicated that YbM/f-CNFs nanocomposite does not affect the photodegradation pathway of PTL. Before the photodegradation experiments, the blank (presence of YbM and YbM/f-CNFs alone) and dark (presence of light alone) tests were performed for the PTL removal. As evident in Figure 8C, there was only 6%, 9%, and 14% degradation observed in the presence of light alone, pristine YbM alone, and YbM/f-CNFs nanocomposite alone, respectively, which suggests that the catalyst/light combination is needed for the efficient removal of PTL from the environment. Moreover, the as-prepared YbM/f-CNFs nanocomposite exhibited an excellent photocatalytic performance toward PTL, could degrade above 98% after 100 min visible light irradiation, while pristine YbM demonstrates only 52%. The enhanced degradation efficiency of YbM/f-CNFs was attributed to larger surface area and interfacial interaction between flakelike YbM and f-CNFs, which helps to ameliorate the rapid electron transfer and decelerate the photogenerated charge carrier recombination. Furthermore, the photodegradation kinetic rate of PTL using YbM and YbM/f-CNFs nanocomposite was elucidated by pseudo-first-order kinetic equation as follows, ln C /C0 = −kt

(6)

where C0 denotes the initial concentration of PTL (t = 0), C is the concentration of PTL at various reaction time, and k is the rate constant. Figure 8D depicts that the kinetic rate constant of YbM/f-CNFs composite is to be 0.17369 min−1, which is 2.9 times higher than that of pristine YbM (k = 0.05988 min−1). The amount of catalyst loading plays a key role in the photodegradation of organic pollutants. The optimized amount of YbM/f-CNFs catalyst dosage is an important concern for the effective degradation of PTL, and it also avoids the catalyst wastage. Therefore, we carried out the influence of a different amount of catalyst dosage ranging from 0.1 to 0.7 g on the photodegradation of PTL, while other parameters are kept constant (light sources and PTL concentration). As shown in Figure 9A, the photodegradation efficiency gradually increases with increasing the catalyst amount up to 0.5 g, which might be due to the propagations of sufficient level of photogenerated charge carriers needed for the PTL degradation. At higher dosage (beyond 0.5 g), the catalyst leads to aggregation (particle−particle interaction), which prevents the visible light diffusion into the reaction solution. As a result, the intimate contact between the PTL solution and the photogenerated active species from the catalyst surface was diminished; therefore, the degradation efficiency was decreased. The results suggested that the 0.5 g catalyst is enough for the proficient degradation of PTL. The photodegradation reaction was mainly conducted by the reactive oxidative species (ROS) such as holes (h+), superoxide radical anions (O2•−), and hydroxyl radicals (•OH).90−93 The major contributions of ROS for the PTL degradation were identified by the radical trapping experiments using ethylenediaminetetraacetic acid (EDTA), acrylamide (AA), and isopropyl alcohol (IPA) as a scavengers for h+, O2•−, and •OH, respectively. The control experiment (without scavenger) was performed while catalyst amount, initial PTL concentration, and light source were kept constant. The impacts of different scavengers on the photodegradation of PTL over YbM/f-CNFs nanocomposite are displayed in Figure 9B. From Figure 9B, the introduction of EDTA or AA into the photodegradation system

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CONCLUSIONS In conclusion, the novel and efficient flakelike YbM anchored on carbon nanofibers nanocomposite was successfully prepared by simple wet-chemical route followed by a sonication process, and the product was characterized in detail. The electrocatalytic and photocatalytic performances of the as-prepared YbM/fCNFs nanocomposite were scrutinized toward the detection and degradation of environmentally toxic PTL pesticide. Interestingly, the flakelike YbM/f-CNFs nanocomposite exhibited a tremendous electrocatalytic activity as effective and excellent electron mediator for the PTL detection. Also, the flakelike YbM/f-CNFs nanocomposite modified GCE showed lower LOD, good sensitivity, wide linear response ranges, and good selectivity even in the existence of potentially 8627


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(10) Kumar, J. V.; Karthik, R.; Chen, S. M.; Muthuraj, V.; Karuppiah, C. Fabrication of Potato-Like Silver Molybdate Microstructures for Photocatalytic Degradation of Chronic Toxicity Ciprofloxacin and Highly Selective Electrochemical Detection of H2O2. Sci. Rep. 2016, 6, 1−13. (11) Karthik, R.; Vinoth Kumar, J.; Chen, S. M.; Seerangan, K.; Karuppiah, C.; Chen, T. W.; Muthuraj, V. Investigation on the Electrocatalytic Determination and Photocatalytic Degradation of Neurotoxicity Drug Clioquinol by Sn(Moo4)2 Nanoplates. ACS Appl. Mater. Interfaces 2017, 9, 26582−26592. (12) Chen, A.; Peng, X.; Koczkur, K.; Miller, B. Super-hydrophobic Tin Oxide Nanoflowers. Chem. Commun. 2004, 17, 1964−1965. (13) Golovnev, N. N.; Molokeev, M. S.; Lesnikov, M. K.; Atuchin, V. V. Two Salts and the Salt Co-crystal of Ciprofloxacin with Thiobarbituric and Barbituric Acids: The Structure and Properties. J. Phys. Org. Chem. 2018, 31, No. e3773. (14) Lim, C. S.; Atuchin, V. V. Cyclic MAM Synthesis of SPION/ BaMoO4:Er3+,Yb3+ Composite and Its Optical Properties. Proc. SPIE 2013, 8771, 877110. (15) Dorozhkin, K. V.; Dunaevsky, G. E.; Sarkisov, S. Y.; Suslyaev, V. I.; Tolbanov, O. P.; Zhuravlev, V. A.; Sarkisov, Y. S.; Kuznetsov, V. L.; Moseenkov, S. I.; Semikolenova, N. V.; Zakharov, V. A.; Atuchin, V. V. Terahertz Dielectric Properties of Multiwalled Carbon Nanotube/ Polyethylene Composites. Mater. Res. Express 2017, 4, 106201. (16) Deng, H.; Li, X. J.; Peng, Q.; Wang, X.; Chen, J. P.; Li, Y. D. Monodisperse Magnetic Single-Crystal Ferrite Microspheres. Angew. Chem. 2005, 117, 2842−2845. (17) Ding, Y.; Yu, S. H.; Liu, C.; Zang, Z. A. 3D Architectures of Iron Molybdate: Phase Selective Synthesis, Growth Mechanism, and Magnetic Properties. Chem. - Eur. J. 2007, 13, 746−753. (18) Yang, J.; Lin, C. K.; Wang, Z. L.; Lin, J. In(OH)3 and In2O3 Nanorod Bundles and Spheres: Microemulsion-Mediated Hydrothermal Synthesis and Luminescence Properties. Inorg. Chem. 2006, 45, 8973−8979. (19) Zhang, N.; Bu, W. B.; Xu, Y. P.; Jiang, D. Y.; Shi, J. L. Selfassembled Flowerlike Europium-Doped Lanthanide Molybdate Microarchitectures and Their Photoluminescence Properties. J. Phys. Chem. C 2007, 111, 5014−5019. (20) Jiang, C. L.; Zhang, W. Q.; Liu, Y. K.; Qian, Y. T. Self-assembled Copper Nanowalls into Microstructures with Different Shapes: A Facile Aqueous Approach. Cryst. Growth Des. 2006, 6, 2603−2606. (21) Bai, P.; Wu, P. P.; Yan, Z. F.; Zhou, J. L.; Zhao, X. S. Selfassembly of Clewlike ZnO Superstructures in the Presence of Copolymer. J. Phys. Chem. C 2007, 111, 9729−9733. (22) Zhang, J.; Lin, J. Vaterite-type YBO3: Eu3+ Crystals: Hydrothermal Synthesis, Morphology and Photoluminescence Properties. J. Cryst. Growth 2004, 271, 207−215. (23) Wang, X.; Li, Y. Rare-earth-compound Nanowires, Nanotubes, and Fullerene-like Nanoparticles: Synthesis, Characterization, and Properties. Chem. - Eur. J. 2003, 9, 5627−5635. (24) Pan, L.; Li, L.; Chen, Y. J. Synthesis and Electrocatalytic Properties of Microsized Ag2WO4 and Nanoscaled MWO4 (M= Co, Mn). J. Sol-Gel Sci. Technol. 2013, 66, 330−336. (25) Longo, E.; Volanti, D. P.; Longo, V. M.; Gracia, L.; Nogueira, I. C.; Almeida, M. A. P.; Pinheiro, A. N.; Ferrer, M. M.; Cavalcante, L. S.; Andres, J. Toward an Understanding of the Growth of Ag Filaments on α-Ag2WO4 and Their Photoluminescent Properties: A Combined Experimental and Theoretical Study. J. Phys. Chem. C 2014, 118, 1229−1239. (26) Sundaram, R.; Nagaraja, K. S. Solid State Electrical Conductivity and Humidity Sensing Studies on Metal Molybdate−Molybdenum Trioxide Composites (M= Ni2+, Cu2+ and Pb2+. Sens. Actuators, B 2004, 101, 353−360. (27) Bradler, M.; Baum, P.; Riedle, E. Femtosecond Continuum Generation in Bulk Laser Host Materials with Sub-μJ Pump Pulses. Appl. Phys. B: Lasers Opt. 2009, 97, 561−574. (28) Kumar, A.; Kumar, J. Perspective on Europium Activated FineGrained Metal Molybdate Phosphors for Solid State Illumination. J. Mater. Chem. 2011, 21, 3788−3795.

cointerfering compounds. Besides, YbM/f-CNFs/GCE attained acceptable recoveries to determine PTL in soil and water samples. As a photocatalyst, YbM/f-CNFs nanocomposite could degrade above 98% and mineralize 71% of PTL under visible light irradiation. For those admirable bifunctional activities of flakelike YbM decorated on the f-CNFs surface, it could be applied for the fabrication of electrochemical sensor and degradation of PTL to the contaminated soil and wastewater treatment.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b00936. The linear plot for pH vs cathodic peak potential, PL spectrum of YbM/f-CNFs, and determination of O2•− radical using nitro blue tetrazolium (NBT) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +886-2270-17147. Fax: +886-2270-25238. ORCID

Shen-Ming Chen: 0000-0002-9305-8513 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was supported by the Ministry of Science and Technology (MOST 106-2113-M-027-003 and MOST 1062811-M-027-004), Taiwan, ROC. J.V.K. and V.M. sincerely thank the College managing board, Principal and Head of the Department, VHNSN College, for providing the necessary research facilities.

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REFERENCES

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