Research Article pubs.acs.org/journal/ascecg
Pigeon-Excreta-Mediated Synthesis of Reduced Graphene Oxide (rGO)/CuFe2O4 Nanocomposite and Its Catalytic Activity toward Sensitive and Selective Hydrogen Peroxide Detection C. Karthikeyan,† K. Ramachandran,† Sunirmal Sheet,‡ Dong Jin Yoo,*,§ Yang Soo Lee,‡ Y. Satish kumar,‡ Ae Rhan Kim,⊥ and G. Gnana kumar*,† †
Department of Physical Chemistry, School of Chemistry, Madurai Kamaraj University, Madurai-625021, Tamil Nadu, India Department of Forest Science and Technology, College of Agriculture and Life Sciences, Chonbuk National University, 567 Baekje-daero, Jeonju-si, Jeollabuk-do, Republic of Korea § Department of Life Science, Graduate School of Department of Energy Storage/Conversion Engineering, and Hydrogen and Fuel Cell Research Center, Chonbuk National University, Jeonju-si, Jeollabuk-do 54896, Republic of Korea ⊥ R&D Center for Canutech, Business Incubation Center, Chonbuk National University, Jeonju-si, Jeollabuk-do 54896, Republic of Korea ‡
S Supporting Information *
ABSTRACT: A copper ferrite (CuFe2O4)/reduced graphene oxide (rGO) nanocomposite was developed via a one-pot strategy by using environmentally favorable pigeon excreta as a reducing and stabilization agent. The obtained micrographs substantiated that the spherical-shaped CuFe2O4 nanostructures were uniformly anchored over the rGO sheets. The mechanism involved in the simultaneous reduction of GO sheets and Cu2+/Fe2+ ions by using the pigeon excreta is explicated with the number of structural characterizations. The electrocatalytic activities of as-prepared nanostructures for nonenzymatic H2O2 detection were evaluated under the neutral conditions. The as-prepared rGO/CuFe2O4 nanocomposite exhibited the high sensitivity of 265.57 μA mM−1 cm−2, low detection limit of 0.35 μM and wide linear range from 1 μM to 11 mM toward H2O2 sensing, because of the systematic arrangement of metallic active sites supported via the active rGO support. The robust structures developed in the prepared composite exhibited the excellent selectivity and stability, which allowed the reproducible assessement of H2O2 in human urine samples. These findings have not only showered salient insights on the environmentally favorable preparation of rGO-based composites but have also provided promising features for the prepared catalysts in nonenzymatic H2O2 sensors. KEYWORDS: Pigeon excreta, Electrical conductivity, Green synthesis, Interference, Metallic active sites
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INTRODUCTION Graphene, which is a carbonaceous one-atom thickened graphitic material, is considered as an attractive platform in material science scenarios, because of its unique features, including high theoretical surface area (2630 m2 g−1), electrical (7200 S cm−1), and thermal (500−600 W m−1 K−1) conductivities, flexibility, chemical inertness, and biocompatibility.1,2 The extensive applications of graphene in many electrochemical devices captivated the utilization of various techniques, including chemical vapor deposition (CVD), electric arc discharge, solution-based chemical reduction, epitaxial growth, etc., for the preparation of graphene with first-grade quality.3−5 Although the synthesis of first-grade graphene is guaranteed from the aforementioned techniques, it exhibited certain hindrances including lower yield, high cost, sophisticated instrument setup, high temperature, etc., which © 2017 American Chemical Society
pronounced an effectual alternative technique for the large-scale preparation of graphene. Hence, the progressive steps have been brought forth toward the preparation of graphene derivates such as graphene oxide (GO) and reduced graphene oxide (rGO). The restoration of electrical conjugation in GO sheets is highly essential for deriving the comparable electrical conductivity and surface area with graphene6 and is usually achieved with the strong reducing agents such as hydrazine hydrate, hydroquinone, sodium borohydride, hydrogen sulfide, etc.,7,8 However, the toxic surface of rGO sheets generated via the aforementioned reducing agents hindered their biological applications. Hence, Received: January 31, 2017 Revised: March 29, 2017 Published: April 13, 2017 4897
DOI: 10.1021/acssuschemeng.7b00314 ACS Sustainable Chem. Eng. 2017, 5, 4897−4905
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Figure 1. TEM images of (a, b) rGO, (c) rGO/Cu, and (d−f) rGO/CuFe2O4 nanostructures. (Inset in panel (f) shows the corresponding SAED pattern.)
electronics equipped with integrated circuits, the poor dielectric loss and high density of magnetic nanostructures limited their extensive applications. The previously discussed limitations of the aforementioned individual nanostructures could be overwhelmed by combining the materials of interest and its composite formation with the active and biocompatible rGO support.20−22 Pigeons are commonly found birds in all the corners of world and generally take shelter in building roofs, churches, public places, home roofs, etc.,23 The droppings of pigeon excreta (PE) spills over the buildings and public places and spreads Chlamydophila psittati and Cryptococcus neoformans pathogens in the environment, which may propagate many diseases, including histoplasmosis.24 Furthermore, the PE deposition of PE on buildings causes several types of damage, such as scarring fabrics, spoiling appearance, and degradation of building blocks, which may collectively decrease the life span of the buildings.25 Hence, the proper disposal of these droppings has become a tedious issue for the municipal and public health council, requiring extensive labor work and have an economical impact. If proper utilization of the above excreta is achieved, it may effectively trounce the practical difficulties of these disposals. Hence, this research effort is intended to explore the novel and environmentally benign PE as a reducing agent for the preparation of rGO-based metal nanocomposites. This research effort is also intended to explore the influences of active carbon support and metallic active sites on the nonenzymatic H2O2 sensor performances.
the utilization of environmentally benign reducing agents, including microorganisms, ionic liquids, critical temperature fluids, biomass, etc., has holds outstanding appeal in the reduction of GO sheets.9 Although the prepared rGO sheets guaranteed its biocompatible surface, certain constraints, including the utilization of external stabilizers/capping agents to avoid the restacking behavior of rGO sheets under the aqueous medium and traces of autotroph extract in active carbon support limited the exposure of active sites and accessible surface area, specifying that the development of novel environmentally benign reducing agents is still under exploration. Hydrogen peroxide (H2O2), as an intracellular messenger, plays a significant role in cell growth, aging, redox signaling, differentiation and function of cell and essential intermediate in cellular reactions.10 In a physiological system of living organisms, H2O2 is secreted at an adequate number of cells including phagocytic leukocytes, nonphagocytic, adipose, thyroid gland, and oxidase enzymes.11 Although H2O2 is considered to be innocuous, the unstable nature of peroxide leads to a homolytic cleavage reaction (HO···OH), which would end up in cytotoxic OH° formation.12 This reactive oxygen species production in cellular system in abnormal quantity (≥50 μM) causes a malfunction in deoxyribonucleic acid (DNA), proteins, and lipids, leading to many deadly diseases (cancer, Alzheimer’s disease, diabetes, myocardial infarction, atherosclerosis, etc.),13 which necessitated the immense need for the exploration of high-performance nonenzymatic H2O2 sensors.14−16 Although the utilization of rGO sheets could lower the overpotential of bare electrodes, the effectual electroreduction of H2O2 could be achieved only with the metallic active sites, in which the attention on copper (Cu) and iron (Fe) nanostructures is significant. Hence, copper oxide (CuO) nanoleaves,17 α-iron oxide (Fe2O3),18 and Feenriched natural zeolite19 were exploited as electrochemical probes for the nonenzymatic H2O2 detection. However, the large volume changes, poor surface stability, lower electrical conductivity, and high agglomeration of the aforementioned nanostructures limited the H 2O 2 sensor performances. Although the utilization of magnetic nanostructures effectively shielded the electromagnetic radiation/interference (EMI) of
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EXPERIMENTAL SECTION
Preparation of PE Extract. The PE collected from the local environment was dried and well-ground, and 10 wt % PE extract was prepared by using the sterile deionized water at 80 °C for 10 min. The as-prepared PE extract was filtered by using Whatmann filter paper and was cooled at 4 °C overnight prior to use. Preparation of rGO Sheets. The modified Hummer’s method was exploited to prepare GO from graphite powder, as per the procedure detailed elsewhere.26 To the GO dispersion (0.5 mg mL−1), 1.25 mL of PE extract was gradually added and refluxed at 100 °C for 12 h. The resultant rGO then was centrifuged at 12 000 rpm and carefully washed with deionized water and dried at 80 °C. 4898
DOI: 10.1021/acssuschemeng.7b00314 ACS Sustainable Chem. Eng. 2017, 5, 4897−4905
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Figure 2. Schematic illustration of (i) the preparation of the rGO/CuFe2O4 nanocomposite and (ii) the electrocatalytic reduction of H2O2 at rGO/ CuFe2O4/GCE. Preparation of rGO/Cu Nanocomposite. 0.01 M Cu(NO3)2· 3H2O was gradually added into the GO dispersion (0.5 mg mL−1) and the mixture was magnetically stirred. To the above solution, 1.25 mL of PE extract was gradually added and subjected for reflux condensation at 100 °C for 12 h. The resultant rGO/Cu composite was collected via centrifugation at 12 000 rpm and dried at 80 °C for 12 h. Preparation of rGO/CuFe2O4 Nanocomposite. To the GO dispersion (0.5 mg mL−1), a mixture of 0.005 M Cu(NO3)2·3H2O and 0.005 M FeSO4·7H2O was gradually added and magnetically stirred. 1.25 mL of PE was gradually added into the above solution and the above mixture was refluxed for 12 h at 100 °C. The obtained rGO/ CuFe2O4 nanocomposite was centrifuged at 12 000 rpm and dried at 80 °C for 12 h.
Characterizations. The as-prepared nanostructures were characterized by transmission electron microscopy (JEOL, Model JEM7610F), powder X-ray diffractometry (XRD) (Rigaku, Model D-max2500), Fourier transform infrared (FT-IR) spectroscopy (PerkinElmer), Raman spectrometry (Horiba, Model LabRAM-HR), and X-ray photoelectron spectroscopy (XPS) (Kratos Analytical). Electrode Modification. The glassy carbon electrode (GCE) (Φ = 3 mm and area = 0.07 cm2) was polished as per the procedure described elsewhere.27 A quantity of 6 μL of the as-prepared nanostructure dispersion in dimethylformamide (DMF) solvent (5 mg mL−1) was placed over the GCE surface and dried at room temperature. To fix the electrocatalyst on GCE surface, 6 μL Nafion solution was dropped on the catalyst-modified GCE surface and dried at room temperature. The rGO, rGO/Cu, and rGO/CuFe 2O4 4899
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Figure 3. (i) XRD patterns of GO (spectrum (a)), rGO (spectrum (b)), rGO/Cu (spectrum (c)), and rGO/CuFe2O4 nanostructures (spectrum (d)); (ii) FT-IR spectra of PE extract (spectrum (a)), GO (spectrum (b)), rGO (spectrum (c)), rGO/Cu (spectrum (d)), and rGO/CuFe2O4 nanostructures (spectrum (e)); and (iii) Raman spectra of GO (spectrum (a)), rGO (spectrum (b)), rGO/Cu (spectrum (c)), and rGO/CuFe2O4 nanostructures (spectrum (d)). nanostructure-modified GCEs are denoted as rGO/GCE, rGO/Cu/ GCE, and rGO/CuFe2O4/GCE, respectively. Electrochemical Characterizations. The electrochemical experiments were evaluated according to the procedure described elsewhere.16 The cyclic voltammograms of studied electrodes were recorded at a potential range of −0.8 to 0.0 V vs Ag/AgCl in 0.1 M PBS solution at a scan rate of 20 mV s−1 under the presence and absence of 10 mM H2O2. The amperometric current−time (i−t) curves were measured at an applied potential of −0.4 V vs Ag/AgCl to monitor the current variations during the successive injections of H2O2.
carbonyl, epoxy, and hydroxyl functional groups and stimulated the uniform adherence of Cu2+/Fe2+ ions over the rGO sheets.28 The PE extract simultaneously reduced GO sheets and Cu2+/Fe2+ ions into rGO and CuFe2O4 nanostructures, respectively, leading to the formation of the rGO/CuFe2O4 nanocomposite29,30 and the mechanism involved in the formation of the rGO/CuFe2O4 nanocomposite is schematically illustrated in Figure 2(i). The elemental compositions of prepared nanostructures were ensured from the energydispersive X-ray spectroscopy (EDAX) patterns (see Figure S1 in the Supporting Information). XRD Studies. The unique diffraction peak of graphite powder generally recognized at 26° disappeared for GO and a new diffraction peak was observed at 10.43°, corresponding to the (001) reflection plane (spectrum (a) in Figure 3(i)). The reduction of GO sheets achieved via PE extract was confirmed from the disappearance of (001) reflection plane and the appearance of (002) reflection plane at 24.68° (spectrum (b) in Figure 3(i)). Along with the characteristic diffraction peak of rGO, the rGO/Cu composite exhibited additional diffraction peaks at 43.25° and 50.48° (spectrum (c) in Figure 3(i)), which are ascribed to the (111) and (200) reflection planes, respectively, of the cubic crystalline structure of Cu nanoparticles (JCPDS No. 04-0836).31,32 The rGO/CuFe2O4 nanocomposite exhibited the characteristic (220), (311), (400), (511), and (440) reflection planes (spectrum (d) in
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RESULTS AND DISCUSSION Morphological Studies. Ultrathin, multilayered, and wrinkled sheetlike nanostructures were observed for the rGO sheets (Figures 1a and 1b). From Figure 1c, it is clear that Cu nanostructures with the spherical morphology were orderly anchored over the rGO sheets with a narrow size distribution, and the average particle size of the Cu nanoparticles is found to be 12 nm (Figures 1d−f). Ordered lattice fringes with the same orientation were observed for the CuFe2O4 nanoparticles, and the selected angle electron diffraction (SAED) pattern obtained is indexed to the polycrystalline structure of CuFe 2 O 4 nanoparticles in rGO/CuFe2O4 nanocomposite. The positively charged Cu2+/Fe2+ ions were electrostatically interacted with the negatively charged GO sheets containing carboxyl, 4900
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Figure 4. (i) Wide-scan XPS spectrum and (i−iv) narrow scan XPS spectra of (ii) C 1s, (iii) Fe 2p, and (iv) Cu 2p of rGO/CuFe2O4 nanocomposite.
and ID/IG ratio for GO is found to be 0.89.37 The aforesaid carbonaceous modes of rGO were shifted to 1332 and 1602 cm−1 (spectrum (b) in Figure 3(iii)), respectively, and the ID/ IG ratio is increased to 1.0. The 2D and D+G bands observed at 2645 and 2920 cm−1 (see spectrum (a) in Figure S2(i)), respectively, ensured the reduction of GO sheets. Along with the aforementioned carbonaceous modes, the rGO/Cu nanocomposite exhibited an additional band at 618 cm−1 (see spectrum (c) in Figure 3(iii) and (Figure S2(ii) in the Supporting Information), which is related to the Bg mode of the Cu nanostructure.37 The rGO/CuFe2O4 nanocomposite exhibited the characteristic CuFe2O4 modes of A1g, F2g, and T2g at 687, 538, and 470 cm−1 (see spectrum (d) in Figure 3(iii)), as well as Figure S2(ii)), respectively, representing the composite formation of CuFe2O4 nanostructures with rGO sheets.38 The ID/IG ratio of rGO/CuFe2O4 nanocomposite is measured to be 1.3, which is accountable for the enhanced electrical conductivity and number of structural defects. XPS Studies. The full-scan XPS survey spectrum of the rGO/CuFe2O4 nanocomposite (Figure 4(i)) exhibited the Cu 2p, Fe 2p, O 1s, and C 1s peaks. The graphitic carbon of rGO sheets is confirmed from the strong C 1s peak found at 284.5 eV (Figure 4(ii)), ascribing to the C−C bonding. The other observed weaker peaks are assigned to the oxygenated carbons, revealing the reduction of GO into rGO sheets with the left-out trace level oxygen labile functional groups. The Fe 2p core level spectrum displayed the pronounced Fe 2p3/2 and Fe 2p1/2 spin−orbit splitting peaks at 711.5 and 724.6 eV, respectively (Figure 4(iii)). The weak satellite peak observed at 719.8 eV corresponds to the fingerprint of the electronic structure of Fe3+.39,40 The Cu 2p spectrum exhibited two distinctive peaks at 933.7 and 956.1 eV, representing the Cu 2p3/2 and Cu 2p1/2, respectively, and its satellite peak was found at 944.9 eV (Figure 4(iv)), enunciating the existence of Cu2+ in the rGO/CuFe2O4
Figure 3(i)), which are ascribed to the cubic structure of CuFe2O4 (JCPDS No. 25-0283).33 FT-IR Studies. The FT-IR spectrum of the PE extract (spectrum (a) in Figure 3(ii)) exhibited the characteristic band at 3485 cm−1, which is attributed to the −OH stretching vibrations. The characteristic bands observed at 2358 and 1645 cm−1 are attributed to the −CO or NH stretching vibration and amide I bands of proteins, respectively. The bands found at 1039 and 1118 cm−1 correspond to the glycogen and DNA/ RNA, respectively. The FT-IR bands of GO (spectrum (b) in Figure 3(ii)) manifested the −OH stretching vibrations, CO stretching vibration of carboxylic group, and C−O stretching vibrations of carboxyl and alkoxy groups at 3408, 1728, 1396, and 1096 cm−1, respectively. The incorporation of PE extract effectively reduced the GO sheets, as evidenced from the reduced intensities of CO and C−O stretching vibrations of rGO (spectrum (c) in Figure 3(ii)), specifying the removal of oxygen labile functional groups. The characteristic amide band observed at 1647 cm−1 for the PE extract was red-shifted to 1628 cm−1 for the prepared rGO and its composites, specifying that amide I bond of proteins generated from the carbonyl stretching in proteins is responsible for the reduction of GO sheets and Cu2+/Fe2+ ions ((ii) in Figures 3d and 3e).34 The existence of Cu nanoparticles in the rGO/Cu nanocomposite was identified from the stretching vibration found at 665 cm−1 (spectrum (d) in Figure 3(ii)).35 The rGO/CuFe2O4 nanocomposite exhibited the characteristic carbonaceous modes, along with the Cu and Fe−O stretching vibrations at 665 and 469 cm−1, respectively36 (see spectrum (e) in Figure 3(ii)), which ensured the composite formation. Raman Studies. The D and G carbonaceous modes found for GO sheets at 1342 and 1610 cm−1 (spectrum (a) in Figure 3(iii)) are ascribed to the breathing mode of k-point photons of A1g symmetry and E2g phonon of sp2 carbon atom, respectively, 4901
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Figure 5. (i) Cyclic voltammetry (CV) responses of studied electrodes under the (dashed lines) absence and (straight lines) presence of 10 mM H2O2 in 0.1 M PBS at a scan rate of 20 mVs−1, (ii) CV responses of rGO/CuFe2O4/GCE as a function of different concentrations of H2O2 in 0.1 M PBS at a scan rate of 20 mV s−1, and (iii) CV responses of rGO/CuFe2O4/GCE under the presence of 10 mM of H2O2 in 0.1 M PBS as a function of scan rates ranging from 20 to 200 mV s−1 (inset shows a calibration plot of the peak current (Ipc) versus the square root of the scan rate).
nanocomposite.41 Thus, the XPS spectra obtained ensured the composite formation of rGO with CuFe2O4 nanostructures. Electrochemical Reduction of H2O2 Using Different Modified GCEs. The cyclic voltammetry (CV) technique was exploited to identify the electrocatalytic activities of as-prepared nansotructures toward nonenzymatic H2O2 detection. The CVs obtained for the studied GCEs in the absence and presence of H2O2 in 0.1 M PBS solution (pH 7.2) at a scan rate of 20 mV s−1 in the potential range from −0.8 V to 0 V vs Ag/AgCl are depicted in Figure 5(i). The legible capacitive background currents were observed for all of the studied electrodes in 0.1 M PBS solution, in which the maximum background current was observed at rGO/CuFe2O4/GCE, because of its large surface area and high electrical conductivity. In contrast, all of the studied GCEs exhibited the reduction behavior in 10 mM H2O2 in 0.1 M PBS solution (pH 7.2). The limited electrical conductivity and deficiency of active sites of bare GCE provided the weak reduction behavior toward H2O2 (Figure 5(i)), and a slight improvement in H2O2 electroreduction behavior was observed at rGO/GCE, which was due to the improved electrical conductivity and considerable surface area. The significance of metallic active sites toward the electroreduction of H2O2 was clearly observed from the pronounced Ipc of −58.32 μA at −0.46 V vs Ag/AgCl at rGO/Cu/GCE and CuFe2O4/GCE exhibited an inferior electroreduction, in comparison to rGO/Cu/GCE, specifying the influence of the rGO support. rGO/CuFe2O4/GCE exhibited the maximum Ipc of −106.21 μA at −0.4 V vs Ag/
AgCl toward H2O2 reduction and the obtained Ipc is almost twice that of rGO/Cu/GCE and is associated with the positively shifted peak potential (Epc). Generally, MFe2O4 exhibits an inverse spinel structure, in which O forms the face-centered cubic (packing), whereas M(II) occupies the octahedral interstitial sites and Fe(III) is evenly distributed in the O and tetrahedral sites. The aforementioned arrangement is expected to provide good electrical conductivity, because of the electron hopping process between the different valence states of metals in O-sites, and has also provided sufficient surface active sites for the adsorption and activation of electroactive species,33 which collectively enlarged the adsorption and reduction of H2O2. Furthermore, rGO sheets provided several defective sites with the structural disorders, higher conjugated sp2 aromatic ring, and expanded interlayer spacing, which were useful in trapping the active materials. The rGO sheets proficiently offered a large surface area to uniformly accommodate the CuFe2O4 nanoparticles and simultaneously prohibited the irreversible agglomeration of nanoparticles with the enhanced stability. The extended number of catalytically active sites available in CuFe2O4 nanoparticles with the contribution of rGO sheets persuaded the effectual electroreduction of H2O2. The existence of uniform metallic active sites of rGO/CuFe2O4 nanocomposite electrocatalytically reduced H2O2 to form H2O molecules with the aid of electrons and H+ ions; the involved electrochemical mechanism is schematically illustrated in Figure 2(ii). 4902
DOI: 10.1021/acssuschemeng.7b00314 ACS Sustainable Chem. Eng. 2017, 5, 4897−4905
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Figure 6. (i) Amperometric responses of rGO/CuFe2O4/GCE with the successive additions of different concentrations of H2O2 in 0.1 M PBS at an applied potential of −0.4 V vs Ag/AgCl (inset shows amperometric responses to the successive additions of 1 μM to 50 μM H2O2 into the test cell) and (ii) the corresponding calibration curves of H2O2 concentration versus peak current and (iii) the interference tests of rGO/CuFe2O4/GCE with the successive addition of 1.5 mM H2O2 and other interferences in 0.1 M PBS at an applied potential of −0.4 V vs Ag/AgCl.
of calibration curves, the sensitivity and detection limit of rGO/ CuFe2O4/GCE were measured to be 265.57 μA mM−1 cm−2 and 0.35 μM, respectively, where the detection limit was calculated by using the formula
The voltammetric responses of rGO/CuFe2O4/GCE toward H2O2 reduction, as a function of H2O2 concentration ranging from 2 to 10 mM at 20 mV s−1 in 0.1 M PBS (pH 7.2), were evaluated (Figure 5(ii)). It is clearly envisioned that rGO/ CuFe2O4/GCE exhibited the gradually escalated Ipc values with an increase in the H2O2 concentrations without any fouling effect, which provided an attractive platform for the construction of amperometric i−t sensors. Figure 5(iii) displays the CV responses of rGO/CuFe2O4/ GCE in 10 mM H2O2 under 0.1 M PBS solution (pH 7.2), as a function of scan rate in the range of 20−200 mV s−1. The Ipc that corresponds to H2O2 reduction was gradually increased as the scan rate increased with a negative Epc shift. Furthermore, the square root of scan rate and Ipc displayed good linearity with a high correlation coefficient (R) of 0.994 (see inset in Figure 5(iii)). Amperometric Performances. The amperometric i−t responses observed for rGO/CuFe2O4/GCE with the ensuing inclusions of different concentrations of H2O2 at a regular time interval into the constantly stirred 0.1 M PBS (pH 7.2) at an applied potential of −0.4 V (vs Ag/AgCl) is provided in Figure 6(i). Well-defined stairlike amperometric response currents were observed for rGO-CuFe2O4/GCE (see Figure 6(i) and the inset) and the receptive amperometric current reached its constant status in 5 s, specifying the rapid response time toward H2O2 reduction. Furthermore, an excellent linearity was observed between the amperometric currents and concentration of H2O2 ranging from 1 μM to 11 mM with a high correlation coefficient of R = 0.997 (Figure 6(ii)). With the aid
detection limit = 3 × sb × S −1
where sb is the standard deviation of the background current and S is the slope of the calibration plot. The nonenzymatic H2O2 electrochemical performances obtained for rGO/ CuFe2O4/GCE were suitably compared with the relevant H2O2 sensors and are provided in Table S1 in the Supporting Information. rGO/CuFe2O4/GCE exhibited high sensitivity, a wide linear range, and a low detection limit over the Cu- and Fe-based nanocatalysts, and its performances are comparable with the heteratom-doped rGO/Fe3O4 composite. The combinative properties of active carbon support and CuFe2O4 nanostructures provided the large electrochemically available surface area and high electrical conductivity for the rGO/ CuFe2O4 composite. The active rGO sheets provided the proper orientation of CuFe2O4 nanoparticles, which aided the proper exposure of aligned active sites toward H2O2 reduction. Thus, the environmentally favorable and cost-efficient approach associated with the considerable amperometric performances organized the probable applications of rGO/CuFe2O4 nanocomposite in nonenzymatic H2O2 sensors. Interference Studies. The selectivity studies of rGO/ CuFe2O4/GCE was accomplished against the number of interferences including 1.5 mM ascorbic acid (AA), dopamine (DA), citric acid (CA), urea (U), acetaminophen (AP), uric 4903
DOI: 10.1021/acssuschemeng.7b00314 ACS Sustainable Chem. Eng. 2017, 5, 4897−4905
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ACS Sustainable Chemistry & Engineering acid (UA), sodium chloride (NaCl), and glucose (Glu) in 0.1 M PBS (pH 7.2) at an applied potential of −0.4 V vs Ag/AgCl (Figure 6(iii)). It is clearly observed from the amperograms that rGO/CuFe2O4/GCE has not significantly responded against the interferences. However, it exhibited an obvious current response toward 1.5 mM H2O2 and the existence of interferences has not influenced the amperometric response of rGO/CuFe2O4/GCE toward H2O2. Stability and Reproducibility. The amperometric current responses obtained for the identically fabricated five rGO/ CuFe2O4/GCEs in 1.5 mM H2O2 in 0.1 M PBS solution at an applied potential of −0.4 V vs Ag/AgCl fairly demonstrated the excellent reproducibility of as-fabricated rGO/CuFe2O4/GCE with the relative standard deviation (RSD) of 3.21%. Moreover, the excellent stability of rGO/CuFe2O4/GCE was witnessed from the retention of 95.12% current response from its initial current response after the 28th day of storage (Figure S3 in the Supporting Information), bidding an excellent platform for practical applications. Real Sample Analysis. The real sample analysis was achieved for the as-fabricated rGO/CuFe2O4/GCE in 100× diluted human urine derived from a healthy volunteer. The amperometric current responses of rGO/CuFe2O4/GCE were evaluated for the obvious concentrations of H2O2 at an applied potential of −0.4 V vs Ag/AgCl. The as-fabricated rGO/ CuFe2O4/GCE exhibited the prompt recovery in the range of 97.76%−101.88% with a relative standard deviation (RSD) of 2.69%−3.32% (see Table 1 in the Supporting Information), which attested to its practical applications in real biological samples.
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AUTHOR INFORMATION
Corresponding Authors
*Fax: +82632703608. E-mail:
[email protected] (D. J. Yoo). *Tel.: +91 9585752997. E-mail:
[email protected] (G. Gnana kumar). ORCID
G. Gnana kumar: 0000-0001-7011-3498 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This study was supported by the Science and Engineering Research Board, New Delhi, India, major Project Grant No. EMR/2015/000912. This work was supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (No. 20164030201070).
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CONCLUSIONS In summary, a facile and one-pot reduction technique involving pigeon excreta (PE) extract as a reduction and stabilization agent has been achieved for the preparation of rGO/CuFe2O4 nanocomposite. The positive influence of active rGO support toward the nonenzymatic H2O2 sensors, in comparison with the metallic counterparts, was visualized by the number of electrochemical measurements. The amperometric characterizations divulged that rGO/CuFe2O4/GCE exhibited the rapid response time, high sensitivity, low detection limit, and longterm stability under the neutral conditions. Furthermore, rGO/ CuFe2O4/GCE was employed in selective quantification of H2O2, even in the presence of other electroactive species, and has also exhibited the excellent recovery rates in the real sample analysis. Thus, the proposed strategy provided a promising platform for the nonenzymatic detection of H2O2 and a proper focus on the above may pave its potential function in clinical diagnostics.
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variation in current response of rGO/CuFe2O4/GCE (Figure S3); comparison of the electroanalytical performances of different amperometric H2O2 sensors (Table S1); quantification of H2O2 in human urine sample at rGO/CuFe2O4/GCE (Table 1); and associated references (PDF)
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00314. Discussion of the experimental section; EDAX patterns of (i) rGO, (ii) rGO/Cu, and (iii) rGO/CuFe2O4 nanostructures (Figure S1); discussion of EDAX studies; projected (i) 2D band related Raman spectra of (a) rGO, (b) rGO/Cu and (c) rGO/CuFe2O4 nanostructures and (ii) metal structure related Raman spectra of (a) rGO/ Cu and (b) rGO/CuFe2O4 composites (Figure S2); 4904
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Research Article
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