Synthesis of Palladium, ZnFe2O4 Functionalized Reduced Graphene

Mar 28, 2017 - ... Qingbo Chang , Xiaobin Fan , Guoliang Zhang , Fengbao Zhang , Wenchao Peng , Yang Li. Ceramics International 2018 44 (5), 5250-5256...
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Synthesis of Palladium, ZnFe2O4 Functionalized Reduced Graphene Oxide Nanocomposites as H2O2 Detector Lingyun Ning, Yizhe Liu, Jingwen Ma, Xiaobin Fan, Guoliang Zhang, Fengbao Zhang, Wenchao Peng, and Yang Li* Lab of Advanced Nano-structures & Transfer Processes, Department of Chemical Engineering, Tianjin University, Tianjin 300354, China S Supporting Information *

ABSTRACT: In the present work, a highly efficient catalyst toward the reduction of hydrogen peroxide (H2O2) was synthesized on account of reduced graphene oxide (rGO). The rGO were coated with ZnFe2O4 and Pd nanoparticles (Pd/ ZnFe2O4/rGO), which were used to modify a glassy carbon electrode to detect H2O2. The Pd/ZnFe2O4/rGO nanocomposites were characterized by scanning electron microscopy, transmission electron microscopy, X-ray powder diffraction, and X-ray photoelectron spectroscopy. Cyclic voltammetry and amperometry measurements were used to characterize and optimize the property of as-prepared H2O2 sensor. The proposed H2O2 sensor displayed a wide linear range from 25 μM to 10.2 mM with an enhanced sensitivity of 621.64 μA mM−1 cm−2.

1. INTRODUCTION The analysis of H2O2 is extremely necessary because of its presence in various fields such as industrial, food, clinical, pharmaceutical, and environmental analysis.1,2 Conventional H2O2 detection methods such as titrimetry,3 chemiluminescence,4 fluorescence,5 and spectrometry6 are expensive, complex, time-consuming, and related to sources of interference, whereas the electrochemical method used to detect H2O2 has many superiorities such as simply operation, fast response, low cost, and high sensitivity.7−9 Sensing applications based on metal nanoparticles-modified electrodes are very important, as they can greatly enhance surface area, mass transport rate, and catalytic efficiency. Despite these unique advantages, the agglomeration of metal particles is still an urgent problem needing to be solved. Graphene materials are often used as carriers in the method to solve this problem.10 Graphene attracts extensive interest because of its superior properties, such as high specific surface area, good mechanical strength, superior electric conductivity, and so on.11 The reduced graphene oxide (rGO) incorporates some functional groups, such as hydroxyl group, carboxyl group, epoxy group, and so on, which are easily blended with other functional materials to improve their dispersibility and processability.12,13 In the preparation of the nanocomposites, the functional groups on the graphene surfaces are also helpful for nucleation and formation of metal nanoparticles.14,15 The integration of rGO and metal nanoparticles may increase the electrochemically active surface area and electronic transmission speed, resulting in current magnification and sensitivity improvement. At present, graphene-based materials are making progress © XXXX American Chemical Society

toward excellent nanomaterials for producing electrochemical H2O2 biosensors.16−18 Because of their small energy band gap, spinel ZnFe2O4 nanoparticles (NPs) have been applied in various applications, such as electronic, catalytic, and biomedical. Especially, ZnFe2O4 NPs possess intrinsic peroxidase-like activity to catalyze the reduction of H2O2.19 In addition, Pd NPs possess good stability, excellent selectivity, and high catalytic efficiency, and have been widely used as electrochemical electrode materials in the construction of biosensors.20,21 Moreover, Pd NPs can readily reduce H2O2. The catalytic activity of ZnFe2O4/rGO NPs to the reduction of H2O2 has a stable response but low sensitivity,22 while Pd NPs have high sensitivity but an unstable response. 23 In consideration of the preceding analysis, the combination of graphene, ZnFe2O4 NPs, and Pd NPs is expected to show unique performance in the ability to overcome the existing limitations. Despite there being some report of using similar graphene-based materials as an effective peroxidase mimic that has a good catalytic effect,24 finding a more environmentally friendly and simpler method is still an urgent task. ZnFe2O4/rGO nanocomposite was synthesized through an easy and environmentally friendly solvothermal process, in which the formation of ZnFe2O4 NPs, the reduction of graphene oxide (GO), and the mixing of the two substances were completed in one step. The newly synthesized nanoReceived: Revised: Accepted: Published: A

December 23, 2016 February 24, 2017 March 28, 2017 March 28, 2017 DOI: 10.1021/acs.iecr.6b04964 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Scheme 1. Schematic of Pd/ZnFe2O4/rGO-GCE Used for Detecting H2O2

was centrifuged at 14000 rpm, then washed with distilled water and absolute ethanol. At last, the obtained hybrids were dried in a vacuum oven at 60 °C for 12 h. For comparison, pure ZnFe2O4 nanospheres were also prepared with the same method without adding GO. 2.3. Preparation of the Modified Electrodes. The surface of glassy carbon electrode (GCE, 3 mm in diameter) was polished with 0.3 and 0.05 μm alumina slurry on a polishing cloth to remove traces and create a mirror surface. The electrode surface was cleaned by sonication in ethanol and deionized water for 3 min and then dried under nitrogen. Catalyst ink was prepared by mixing 1 mL of ZnFe2O4/rGO suspension (2 mg/mL) with 10 μL of 5 wt % Nafion solution, then the mixture was ultrasonicated for 1 h to get a uniform solution. The viscosity of Nafion makes it act as an adhesive, increasing the adhesion between catalysts and substrate. Nafion can be used as a protector to keep the catalyst from separating.26 A 5 μL ZnFe2O4/rGO suspension was casted onto the surface of GCE as working electrode. The modified electrode was dried in the air before use. Then the Pd NPs were deposited on ZnFe2O4/rGO surface through amperometry (i− t) at the potential of −0.4 V for 1500 s in 2 mM K2PdCl4 solution. The modified GCE was cleaned by doubly distilled water to eliminate the remaining K2PdCl4 and dried at room temperature. Phosphate buffer solution (PBS, 0.1 M, pH 7.4) containing NaH2PO4 and Na2HPO4 was deoxygenated by bubbling with high-purity N2 for 20 min.

composite denoted as Pd/ZnFe2O4/rGO was introduced through electrodeposition. This method has received widespread attention because not only it is simple and versatile, but also it can ensure the high purity of hybrids and the selective position of Pd on the surface.25 The proposed Pd/ZnFe2O4/ rGO-GCE was characterized as a biosensor for H2O2 (Scheme 1). The Pd/ZnFe2O4/rGO-GCE has a great sensitivity value of 621.64 μA mM−1 cm−2 and good discrimination ability. This modified electrode has increased electron transport, stable response, and high sensitivity. Therefore, it is meaningful to research the electrocatalytic activity of H2O2 electroreduction on the Pd/ZnFe2O4/rGO nanocomposites modified electrode interface.

2. EXPERIMENTAL SECTION 2.1. Reagents and Apparatus. All chemicals, including iron(III) chloride (FeCl3), zinc acetate (Zn(Ac)2·2H2O), sodium acetate (NaAc), ethylene glycol (EG), potassium tetrachloropalladate (K2PdCl4), hydrogen peroxide (H2O2), ethanol (C2H5OH, 99.7% purity), D-(+)-glucose, uric acid (UA), and ascorbic acid (AA) are of analytical grade and used as received. Double distilled water was used in the experiments. The morphology and microstructure were characterized by scanning electron microscopy (SEM, Hitachi S-4800), highresolution transmission electron microscopy (TEM, Philips Tecnai G2 F20). Compositional analysis was performed by energy dispersive X-ray spectroscopy (EDX, Hitachi S-4800). The crystallinity of the obtained products were performed by X-ray powder diffraction (XRD, Cu Kα radiation, BrukerNonius D8 FOCUS). Elemental analysis was carried out using X-ray photoelectron spectroscopy (XPS, PerkinElmer, PHI 1600 spectrometer). Electrochemical measurements were characterized on a CHI660E electrochemical workstation using a three-electrode cell. Platinum wire and Ag|AgCl electrode were used as counter electrode and reference electrode, respectively, in aqueous phosphate buffer solutions (PBS, 0.1 M, pH 7.4). 2.2. Synthesis of ZnFe2O4/rGO Hybrids. GO was prepared by the Hummers method. ZnFe2O4/rGO with different graphene contents (8, 9, 11.5, 14.7, 20.6 wt %) were prepared via the solvothermal method. The synthesis process of ZnFe2O4/rGO with 14.7 wt % graphene is as follows: 62.5 mg of GO was dissolved in 50 mL of EG with sonication for 30 min. After that, 3 mmol FeCl3, 1.5 mmol Zn(Ac)2·2H2O, and 1.2 g of NaAc were dispersed into the GO with stirring for 30 min at room temperature, forming a stable dark-brown homogeneous solution. The resulting mixture was transferred into a 100 mL Teflon-lined stainless steel autoclave and heated to 200 °C for 24 h under autogenous pressure. After the mixture was slowly cooled to room temperature, the precipitate

3. RESULTS AND DISCUSSION 3.1. Characterization of ZnFe2O4/rGO and Pd NPs on the Electrode Surface. The morphologies and microstructures of the obtained hybrids were investigated by SEM and TEM. Figure 1a and 1b show SEM images of the obtained ZnFe2O4/rGO and Pd/ZnFe2O4/rGO nanocomposites, respectively. As shown in Figure 1a, the rGO are uniformly decorated with ZnFe2O4 with much reduced aggregation and dispersiveness with an average size of about 224 nm. The inset of Figure 1a is the SEM image of ZnFe2O4 NPs, showing that ZnFe2O4 NPs are spherical. Figure 1b displays Pd/ZnFe2O4/ rGO modified on the GCE and the surface of Pd/ZnFe2O4/ rGO seems to be rough. The SEM results indicate that the rGO is flaky with wrinkles, and the reduction progress did not change the configuration of graphene (Figure S1). The ethylene glycol (EG) also worked as a reducing agent to reduce GO to rGO effectively during the solvothermal process. Figure 1c is a TEM image of obtained Pd/ZnFe2O4/rGO. The regional distribution of light and shade is shown in Figure 1c, which proves that there is a large number of internal pores in the sample. It indicates that the ZnFe2O4 nanospheres have a porous structure, which is expected to let the electrolyte enter B

DOI: 10.1021/acs.iecr.6b04964 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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(311), (400), (422), (511), and (440) planes of ZnFe2O4, respectively. Compared with the JCPDS (No. 22-1012) data of ZnFe2O4, the latter three peaks are slightly shifted (Figure S3), probably indicating the formation of an alloy.19 No characteristic diffraction peaks of Pd are observed because of its lower loading content and weak crystallization, on the other hand, also implying good dispersion of the very small Pd NPs on the ZnFe2O4/rGO surface. XPS was applied to confirm the composition of the Pd/ ZnFe2O4/rGO catalysts and their valence of the various functionalities. Figure 2 panels b, c, and d show the XPS survey spectra of the Zn 2p, Fe 2p, and Pd 3d, respectively, which confirm the chemical composition without any impurity. In the XPS analysis of Zn 2p, a prominent peak corresponds to Zn 2p3/2 at 1020.7 eV (Figure 2b). For the Figure 2c, two peaks at their respective position are related to Fe 2p1/2 at 724.6 eV and Fe 2p3/2 at 710.7 eV. The binding energy of Fe 2p3/2 shifts slightly, from 711.3 to 710.7 eV. This implies a portion of Fe3+ in ZnFe2O4 was converted into Fe2+, which leads to the gradual decrease in the binding energy for Fe 2p3/2.27 The other peak was characterized as satellite peak at 720 eV due to shakeup process.28 In Figure 2d, the binding energy of Pd 3d3/2 (339.8 eV) shows an obvious shift of 0.5 eV toward low binding energy while the Pd 3d5/2 peak (334.5 eV) shows a shift of 0.3 eV. The presence of Zn and Fe may result in a decrease in the oxidation state of Pd, as well as an increase in the metal state.26,29 XPS studies of relevant nanocomposites (ZnFe2O4/ rGO and Pd/ZnFe 2 O 4 /rGO) can also determine the corresponding components of the resulting products (Figure S4). 3.2. Electrocatalytic Reduction of H 2O2 on Pd/ ZnFe2O4/rGO Nanocomposites Modified Electrode. Electrochemical impedance spectroscopic (EIS) measurements were carried out to measure the charge transfer resistance of Pd, ZnFe2O4/rGO, and Pd/ZnFe2O4/rGO nanocomposite (Figure 3a). The typical impedance spectrum includes a semicircle portion at higher frequencies that corresponds to

Figure 1. SEM images of ZnFe2O4/rGO (a) and Pd/ZnFe2O4/rGO nanocomposites (b). The inset in panel a is the magnified SEM image of ZnFe2O4 NPs. TEM image of Pd/ZnFe2O4/rGO nanocomposites (c). TEM image of Pd/ZnFe2O4/rGO (d) and corresponding elemental mapping images of C, O, Zn, Fe, and Pd.

the inner pores of the nanosphere, increasing the contact area between the electrode material and the electrolyte, as well as allowing the escape of O2 produced by H2O2 hydrolysis from the electrode. The Pd NPs are attached on both ZnFe2O4 nanospheres and the rGO surface (Figure S2), which should contribute to the enhancement of electrocatalytic performance by the synergetic effect. EDX mapping was used to determine the primary composites of Pd/ZnFe2O4/rGO nanocomposites (shown in Figure 1d). The corresponding elemental mapping images show homogeneous distribution of Pd in the entire range, especially on the surfaces of ZnFe2O4 nanospheres. Porous structure of ZnFe2O4 may increase the utilization of palladium. Figure 2a presents the XRD pattern of the Pd/ZnFe2O4/ rGO. The first peak located at 23°, is related to the rGO support. Another six peaks of the nanocomposite at 30.2°, 35.5°, 42.9°, 51.0°, 55.1°, and 60.5° can be assigned to (220),

Figure 3. Nyquist plots (a) of different electrodes in 5.0 mM [Fe(CN)6]4−/3− solution containing 0.2 M KCl. CVs (b) of bare, Pd, ZnFe2O4/rGO, and Pd/ZnFe2O4/rGO modified GCEs in 0.1 M pH 7.4 PBS. CVs (c) of Pd/ZnFe2O4/rGO-GCE at different concentrations of H2O2 from a to f: 0, 0.5, 1, 1.5, 2, and 2.5 mM in 0.1 M pH 7.4 PBS. CVs (d) of Pd/ZnFe2O4/rGO in 0.1 M PBS at a scan rate of (from inner to outer): 10, 20, 50, 100, 120, and 150 mV/s, respectively.

Figure 2. XRD images of GO (a). X-ray photoelectron spectroscopy of Zn 2p (b), Fe 2p (c), and Pd 3d (d). C

DOI: 10.1021/acs.iecr.6b04964 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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the meantime, the peak currents of the anode and cathode increased as the scan rate increased from 10 to 150 mV/s. The calibration equation is Ipa (μA) = −7.0276υ (mV s−1) + 3.5284 (R2 = 0.998) (Figure S8), which demonstrates that the electron transfer on Pd/ZnFe2O4/rGO-GCE surface for the reduction of H2O2 is a predominantly adsorption-controlled process.31−34 It can be obtained that the Pd/ZnFe2O4 complex was successfully dispersed on the rGO surface and the conductivity of Pd/ZnFe2O4/rGO nanocomposite is good. 3.3. Amperometric Response to H2O2 Detection. Amperometric detection is one of the most common electrochemical techniques on account of the measurement of oxidation or reduction current at a given potential and a fixed period of time.10 As shown in Figure 4a, there is only weak or

the electron-transfer resistance and a linear part at lower frequencies that corresponds to the diffusion process.30 A large semicircle is observed in ZnFe2O4/rGO (324.9 Ω), which proves the high resistance of the ferrite nanoparticles. The semicircle is reduced to 156.4 Ω for Pd/ZnFe2O4/rGO owing to the presence of Pd. Pd (140.4 Ω) may reduce the resistance of the semiconductor material because of its excellent conducting nature. To investigate the electrocatalytic activity of the nanointerface of Pd/ZnFe2O4/rGO nanocomposites for a H2O2 sensor, cyclic voltammetry was employed in a range from −0.8 V to +0.6 V at a scan rate of 50 mV/s in 0.1 M PBS on Pd/ ZnFe2O4/rGO-GCE. As shown in Figure 3b, no obvious reduction peak is observed at bare GCE, which means GCE itself is not active, while it can be seen that there is coupled reduction−oxidation peaks around −0.44 V and −0.22 V after the GCE was modified with ZnFe2O4/rGO. The reduction peak current of ZnFe2O4/rGO is −53.73 μA. Also, there is a couple of reduction−oxidation peaks belonging to the reduction and oxidation of Pd after modification with Pd NPs. The reduction of Pd starts from 0.17 V then remains stable until about −0.33 V. The CV of the Pd/ZnFe2O4/rGO electrode has two couples of reduction−oxidation peaks. The electrode reactions are mainly based on the following equations:

Figure 4. (a) Amperometric response of Pd/ZnFe2O4/rGO-GCE, ZnFe2O4/rGO-GCE, and Pd-GCE to successive additions of H2O2 at −0.2 V in 0.1 M PBS. The inset in panel a is a linear relationship between the current response and the concentration of H2O2. (b) Amperometric response of Pd/ZnFe2O4/rGO-GCE to successive additions of H2O2 at −0.2 V in 0.1 M PBS (add H2O2 to the arrows). The inset in panel b is the calibration curve for H2O2 detection.

ZnFe2O4 + OH− + H 2O ↔ ZnOOH + 2FeOOH + e−

FeOOH + OH− ↔ FeO2 + H 2O + e− Pd + 2OH− ↔ PdO + H 2O + 2e−

The reduction peak current of ZnFe2O4 is −92.26 μA at −0.404 V, and the oxidation peak current is 86.41 μA at −0.236 V. The reduction and oxidation peak currents after Pd modification are −84.7 μA at 0.034 V and 87.77 μA at −0.527 V, respectively. The entire current is dramatically enhanced. Therefore, it can be inferred that the nanoporous structure is favorable for exposing abundant and accessible active sites by effectively preventing the aggregation of the catalyst and thus enhancing activity. To obtain the optimum content of graphene, with the increase of graphene, the response current increases, but the current of the catalyst decreases with the excess graphene (Figures S5−S7). Among all the nanocomposites with different graphene oxide contents, Pd/ZnFe2O4/rGO (14.7 wt %) show the highest reduction current of −84.7 μA. Therefore, this nanocomposite is chosen for the further research. Figure 3c shows the CVs of Pd/ZnFe2O4/rGO-GCE in the presence of H2O2 at different concentrations from 0 to 2.5 mM. As shown in Figure 3c, an apparent reduction peak at 0.075 V shows the presence of 0.5 mM H2O2. As the concentration of H2O2 increases, the cathode current is significantly enhanced while the anode current decreases. The reduction peak becomes sharp and slightly turns to the negative potential, which is probably due to the torpid electron transfer kinetics.13 A significant reduction current peak indicates that H2O2 can easily be reduced by this modified electrode over a wide range of concentrations. The charge transfer properties of Pd/ZnFe2O4/rGO nanocomposite at different scan rates can be measured by the CV technique. As the scan rate increases, the slight peak potential varies from 0.077 to 0.054 V when the scan rate reaches 100 mV/s and then is maintained stably at 0.054 V (Figure 3d). In

relatively high amperometric response to the addition of H2O2 on the ZnFe2O4/rGO-GCE (the black curve) and Pd-GCE (the blue curve). Compared with ZnFe2O4/rGO-GCE and PdGCE, Pd/ZnFe2O4/rGO-GCE proves to have an enhanced amperometric response for H2O2 detection (the red curve). As the obtained Pd/ZnFe2O4/rGO nanocomposite has excellent electrocatalytic activity, it can be applied as a nonenzymatic sensor to detect H2O2. The biosensor responded rapidly when H2O2 was added and reached a steady state (95% of the maximum value) within 4 s, indicating a fast diffusion of the substrate in the hybrid film modified on the electrode. It probably owes to the porous structure which provides the effective exposure of active sites and facilitates mass-transfer. The inset of Figure 4a exhibits the corresponding calibration curve for the H2O2 sensor. The sensitivity of electrodes are in the tendency of ZnFe2O4/rGO-GCE (49.76 μA mM−1 cm−2) < Pd-GCE (467.42 μA mM−1 cm−2) < Pd/ZnFe2O4/rGO-GCE (621.64 μA mM−1 cm−2), indicating that Pd/ZnFe2O4/rGOGCE has a great increased sensitivity. The Pd/ZnFe2O4/rGOGCE possesses higher sensitivity which is contributed to the unique properties of rGO and the synergistic effect of Pd and ZnFe2O4. The sensitivity of Pd/ZnFe2O4/rGO-GCE is also higher than some previously reported values of other electrodes in Table 1.35−40 Figure 4b shows the amperometric response of Pd/ZnFe2O4/rGO to the successive addition of H2O2 at different concentrations under the applied potential of −0.2 V. The Pd/ZnFe2O4/rGO can detect H2O2 in a wide linear range from 25 μM to 10.2 mM. The linear regression equation corresponding to Pd/ZnFe 2 O 4 /rGO electrode is y = −43.9412x−17.66315 [y(μA); x(mM)] with a correlation coefficient of R2 = 0.998. The detection limit of the modified electrode was calculated to 2.12 μM at a signal/noise ratio of 3. D

DOI: 10.1021/acs.iecr.6b04964 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Table 1. Comparison of the Present Pd/ZnFe2O4/rGO Nanocomposite H2O2 Sensor With Other Sensors Based on Different Materials electrode

detection limit

sensitivity (μA mM−1 cm−2)

linear range

ref

(HRP-Pd)/f-graphene-Gr Au/Cu2O graphene−AgNPLs Ag/FeOOH/PGE α-Fe2O3−ITO δ-MnO2/CNTs Pd/ZnFe2O4/rGO

0.05 μM 1.05 μM 3 μM 22.8 μM 10 μM 1 μM 2.12 μM

92.82 292.89 183.5 8.07 7.16 243.9 621.64

0.025−3.5 mM 0.025−11.2 mM 0.02−10 mM 0.03−15 mM 0.1−5.5 mM 0.05−22 mM 0.025−10.2 mM

35 36 37 38 39 40 this work

environmentally friendly, and easily fabricated sensor exhibits great electrocatalytic activity with a wide range of detection concentrations of 0.025−10.2 mM for H2O2 detection. The Pd/ZnFe2O4/rGO electrode has great sensitivity value of 621.64 μA mM−1 cm−2 and good discrimination ability. It is suggested that the Pd/ZnFe2O4/rGO composite would be a good candidate for nonenzymatic H2O2 sensor.

All these results suggest that Pd/ZnFe2O4/rGO has excellent performance of a wide detection linear range, low detection limit, and high sensitivity. Those may be due to the high surface-to-volume ratio and good electrochemical activities of rGO and Pd/ZnFe2O4 nanocomposites with lots of electroactive sites and large surface area for H2O2 molecules to adsorb and react. This makes Pd/ZnFe2O4/rGO a promising material for nonenzymatic H2O2 sensing. An important factor in the application of electrochemical analysis is the interference of electrochemical signals. The effect of some electroactive substances on the current response of H2O2 sensor can be measured by amperometry. As shown in Figure 5, 0.1 mM H2O2, 0.1 mM uric acid (UA), 0.1 mM

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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/acs.iecr.6b04964. SEM and HRTEM of GO and Pd NPs, XRD and XPS studies of relevant nanocomposites, and CVs of Pd/ ZnFe2O4/rGO nanocomposite electrodes (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86-22-27890090. ORCID

Xiaobin Fan: 0000-0002-9615-3866 Wenchao Peng: 0000-0002-1515-8287 Yang Li: 0000-0003-3003-9857 Notes

Figure 5. Amperometric response to successive additions of 0.1 mM H2O2, 0.1 mM UA, 0.1 mM AA, 0.1 mM GLU, and a second 0.1 mM H2O2 of Pd/ZnFe2O4/rGO at −0.2 V in 0.1 M PBS.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the Specialized Research Funds for the Doctoral Program of Higher Education of China (No. 20120032110025, 20130032120017), the National Natural Science Foundation of China (No. 21506157), the Foundation for the Author of National Excellent Doctoral Dissertation of China (No. 201251), the Natural Science Foundation of Tianjin (No. 14JCQNJC05800), and the Program of Introducing Talents of Discipline to Universities (No. B06006).

ascorbic acid (AA), 0.1 mM glucose (GLU), and 0.1 mM H2O2 were injected into PBS to test the discrimination ability of the Pd/ZnFe2O4/rGO nonenzymatic sensor. After adding UA, AA, and GLU, there was no obvious current response, indicating that these species had no significant current response to the Pd/ZnFe2O4/rGO-GCE when measuring H2O2. The high selectivity of the sensor is mainly due to the suitable potential, as well as the synergistic catalytic effect among Pd NPs, ZnFe2O4 NPs, and rGO nanosheets. The Pd/ZnFe2O4/rGOGCE also shows good stability. After a two-week storage at 4 °C, it retains 94.7% of its initial response (Figure S9). The slow decrease in response may be related to the gradual deactivation of the composite.19

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REFERENCES

(1) Shan, G.; Zheng, S.; Chen, S.; Chen, Y.; Liu. Detection of labelfree H2O2 based on sensitive Au nanorods as sensor. Colloids Surf., B 2013, 102, 327−330. (2) Mokrushina, A. V.; Heim, M.; Karyakina, E. E.; Kuhn, A.; Karyakin, A. A. Enhanced hydrogen peroxide sensing based on prussian blue modified macroporous microelectrodes. Electrochem. Commun. 2013, 29, 78−80. (3) Hurdis, E. C.; Romeyn, H. Accuracy of determination of hydrogen peroxide by cerate oxidimetry. Anal. Chem. 1954, 26, 320− 325. (4) Greenway, G. M.; Leelasattarathkul, T.; Liawruangrath, S.; Wheatley, R. A.; Youngvises, N. Ultrasound-enhanced flow injection chemiluminescence for determination of hydrogen peroxide. Analyst 2006, 131, 501−508.

4. CONCLUSIONS In summary, Pd/ZnFe2O4/rGO-GCE (14.7 wt %) was developed as a highly active sensor for H2O2 via a simple solvothermal method and electrodeposition route. The existence of rGO allows the formation of well-dispersed ZnFe2O4 nanospheres and Pd nanoparticles. Nanoporous structures in ZnFe2O4 nanospheres can cause high specific surface area and exposing sufficient catalytic sites. This low cost, E

DOI: 10.1021/acs.iecr.6b04964 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

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DOI: 10.1021/acs.iecr.6b04964 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research (40) Begum, H.; Ahmed, M. S.; Jeon, S. A novel δ-MnO2 with carbon nanotubes nanocomposite as an enzyme-free sensor for hydrogen peroxide electrosensing. RSC Adv. 2016, 6, 50572−50580.

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DOI: 10.1021/acs.iecr.6b04964 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX