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School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou. 225002, China. *Corresponding author. Email: [email protected], ...
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Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 2360−2367

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Facile Synthesis of Ultrathin Nickel−Cobalt Phosphate 2D Nanosheets with Enhanced Electrocatalytic Activity for Glucose Oxidation Yun Shu, Bing Li, Jingyuan Chen, Qin Xu, Huan Pang,* and Xiaoya Hu* School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou 225002, China

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S Supporting Information *

ABSTRACT: Two-dimensional (2D) ultrathin nickel−cobalt phosphate nanosheets were synthesized using a simple one-step hydrothermal method. The morphology and structure of nanomaterials synthesized under different Ni/Co ratios were investigated by transmission electron microscopy, scanning electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. Moreover, the influence of nanomaterials’ structure on the electrochemical performance for glucose oxidation was investigated. It is found that the thinnest nickel−cobalt phosphate nanosheets synthesized with a Ni/Co ratio of 2:5 showed the best electrocatalytic activity for glucose oxidation. Also, the ultrathin nickel− cobalt phosphate nanosheet was used as an electrode material to construct a nonenzymatic electrochemical glucose sensor. The sensor showed a wide linear range (2−4470 μM) and a low detection limit (0.4 μM) with a high sensitivity of 302.99 μA·mM−1·cm−2. Furthermore, the application of the as-prepared sensor in detection of glucose in human serum was successfully demonstrated. These superior performances prove that ultrathin 2D nickel−cobalt phosphate nanosheets are promising materials in the field of electrochemical sensing. KEYWORDS: nickel−cobalt phosphate, nanosheets, electrocatalytic activity, glucose, human serum

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INTRODUCTION So far, various two-dimensional (2D) nanomaterials, such as graphene, transition metal dichalcogenides, metal organic framework, and metal oxides and hydroxides, have been widely explored.1−7 In particular, 2D ultrathin nanosheets have received great attention in recent years. Their 2D morphology and ultrathin thickness make them possess unique chemical and physical properties, such as many highly accessible active sites on their surface, ultrahigh specific surface area, and maximum mechanical flexibility.4 These characteristics make them extensively used in the fields of electrochemical energy storage, catalytic, sensing, and optoelectronic devices.8−11 To date, various chemical methods have been developed to synthesize 2D nanosheets; however, it still remains a great challenge to synthesize uniform ultrathin 2D nanosheets with highly accessible surface active sites, especially for synthesis of multicomponent compounds with ultrathin nanosheet morphologies and significant functionalities. Transition metal (Co, Ni) phosphates have received great interest over the past decades and extensively applied in the areas of electrochemical energy storage and electrocatalysts for splitting water due to their high electrochemical activities.12−14 Minakshi et al. reported the synthesis of porous sodium nickel phosphate nanosheets at different temperatures and evaluated their performance for supercapacitor applications.15 In our © 2018 American Chemical Society

previous work, ultrathin cobalt phosphate nanosheets are successfully synthesized and applied as an electroactive material for supercapacitors.16 However, the applications of multicomponent transition metal phosphate nanosheets in the field of sensors have been rarely reported. Accurate detection of glucose is important in clinical diagnostics, food industry, and biotechnology. Over the past few decades, various techniques have been developed to detect glucose. The electrochemical method has drawn great attention, attributed to its unique advantages, such as easy operation process, simple instrumentation, low cost, and excellent sensitivity. Conventional electrochemical glucose biosensors are based on glucose oxidase, which shows high selectivity and high sensitivity. However, the high cost and poor stability of the enzyme limit the widespread applications of these enzymatic glucose biosensors. Therefore, great attention has been paid to the establishment of nonenzymatic glucose sensors. Previous research shows that because of the novel physical and chemical properties of nanomaterials, nanomaterials exhibit electrocatalytic activity that can be electrocatalytic oxidation of glucose. However, the electrode material is considered as a Received: November 8, 2017 Accepted: January 2, 2018 Published: January 2, 2018 2360

DOI: 10.1021/acsami.7b17005 ACS Appl. Mater. Interfaces 2018, 10, 2360−2367

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ACS Applied Materials & Interfaces

acetate was denoted P1, and the product synthesized with addition of 0.4 g of nickel acetate was denoted P3. Material Characterization. The morphology of nanomaterials was observed with a JEM-2100 transmission electron microscope and field-emission scanning electron microscope (Supra 55, Zeiss). Highresolution transmission electron microscopy (HRTEM), scanning transmission electron microscopy, and energy-dispersive spectrometry mapping images were obtained on a Tecnai G2 F30 transmission electron microscope (at an acceleration voltage of 300 kV). Atomic force microscopy (AFM) images were captured with MFP-3D-SA (Asylum Research). XRD characterization was performed on a Bruker AXS D8 Advance diffractometer. XPS analysis was carried out using a Thermo Scientific ESCALAB 250Xi X-ray photoelectron spectrometer. Preparation of Modified Electrodes. First, a glassy carbon electrode (GCE, diameter 3 mm) was carefully polished with 0.3 μm Al2O3 slurry, cleaned by brief ultrasonication with deionized water, and then dried with high-purity nitrogen. For surface modification, 3 mg of nickel−cobalt phosphate nanosheets was dispersed in 100 μL of 1% Nafion solution (1 wt % in ethanol), followed by ultrasound treatment of the suspension for 20 min. A certain volume (5 μL) of suspension was added onto the surface of the precleaned GCE, and it was dried in air to be used as the working electrode. All experiments were performed in a three-electrode cell system with the saturated calomel electrode acting as the reference electrode and where platinum (Pt) wire acts as a counter electrode using the electrochemical workstation (CHI760D, CHI Incorporation, Shanghai). Glucose sensing measurements were performed by the cyclic voltammetry (CV) and amperometry (i−t) techniques. Detection of Glucose in Human Serum Samples. Human whole blood samples were obtained from the hospital of Yangzhou University. Blood samples were centrifuged at 3000g for 5 min to remove cells and cellular debris from the serum. Then, glucose, serum, glucose and the mixture of serum with glucose were successively spiked into 10 mL of N2-saturated 0.1 M NaOH solution. The current responses were recorded. The glucose levels in serum samples and recovered ratios of glucose were calculated.

very important factor affecting the performances of nonenzymatic electrochemical glucose sensors. Herein, in this work, ultrathin nickel−cobalt phosphate nanosheets were prepared using a simple and mild hydrothermal method. Furthermore, we investigated the effect of Ni/ Co ratio on the growth of nanomaterials. The morphology and structure of as-prepared nanomaterials were characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The nanosheets synthesized under different conditions were used for electrocatalytic oxidation of glucose (Scheme 1). The thinnest nickel−cobalt Scheme 1. Schematic Illustrationa

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a

(A) Illustration of the morphological changes for products synthesized under different Ni/Co ratios. (B) Schematic of nickel− cobalt phosphate-modified GCE for electrocatalytic oxidation of glucose.

RESULTS AND DISCUSSION Characterization of the Synthesized Materials. Nanomaterials were synthesized by a simple hydrothermal method using cobalt acetate, nickel acetate (absent in sample P1), and sodium pyrophosphate as reactants. Figure 1A−C shows the SEM images of as-prepared nanomaterials. Rectangular nanosheet-like nanomaterials were synthesized without Ni(CH 3 COO) 2 (Figure 1A). When the amount of Ni(CH3COO)2 increases, the morphology of nanomaterials changes from rounded rectangular nanosheets (Figure 1B) to elliptical nanosheets (Figure 1C). XRD (Figure 1D) is used to identify the crystal structure of the nanomaterials. In the XRD pattern, the reaction product is Co2P2O7 (JCPDS No. 79-0825) when there is no Ni(CH3COO)2 as the reactant. With the continuous increase of Ni(CH3COO)2, the diffraction peaks of Co 2 P 2 O 7 gradually disappear and diffraction peaks of Co3(PO4)2 (JCPDS No. 80-1997) become gradually stronger. Major diffraction peaks at 17.9, 26.1, 26.9, and 27.5° can be indexed to the (111), (112), (202), and (021) facets and other weak diffraction peaks at 13.4, 34.1, 38.5, 48.5, and 53.7° can be indexed to the (101), (121), (031), (402), and (224) facets of the Co3(PO4)2 phases. Meanwhile, diffraction peaks of the Ni3(PO4)2 phases (JCPDS No. 70-1796) can also be observed. Thus, reaction products P2 and P3 are nickel−cobalt phosphate nanosheets. The TEM, HRTEM, and AFM images further identified the synthesized products (sample P2) as uniform 2D nanosheets (Figure 2A−C). The length and width of a nanosheet is about

phosphate nanosheets showed the best electrochemical performance for glucose oxidation. The high electrocatalytic activities of nickel−cobalt phosphate nanosheets could be attributed to their ultrathin thickness (∼4 nm) that endows them with sufficient electroactive sites. It demonstrates that the nickel−cobalt phosphate nanosheets could be expected to be applied in practical glucose detection.

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EXPERIMENTAL SECTION

Reagents and Apparatus. Nafion solution was bought from DuPont. All other reagents were purchased from Sinopharm Chemical Reagent Shanghai Co., Ltd., and 0.1 M NaOH solution was used as the supporting electrolyte. Synthesis of Nickel−Cobalt Phosphate Nanosheets. The nanomaterials were synthesized using a simple hydrothermal method we reported previously with some variations.17 Nickel acetate (0.2 g), cobalt acetate (0.5 g), and sodium pyrophosphate (0.5 g) were mixed in 10 mL of distilled water; after stirring the mixture for 30 min, the mixture was then heated to 160 °C and reacted for 8 h. Finally, the product was washed by distilled water and ethanol three times. The sample was denoted P2. Synthesis of other nickel−cobalt phosphate samples was similar to the above procedures except the variation of the amounts of nickel acetate. The detailed synthesis parameters are shown in Table S1. The product synthesized without addition of nickel 2361

DOI: 10.1021/acsami.7b17005 ACS Appl. Mater. Interfaces 2018, 10, 2360−2367

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Figure 1. SEM images of synthesized cobalt pyrophosphate (A) and nickel−cobalt phosphate sample P2 (B) sample P3 (C). (D) (a) XRD pattern of cobalt pyrophosphate (sample P1 without addition of Ni(CH3COO)2). (b, c) XRD patterns of nickel−cobalt phosphate (samples P2−P3).

phosphate (sample P3)-based GCE possesses a pair of redox peaks with the anodic peak and cathodic peak located at +0.52 and +0.43 V, respectively (Figures 3A (inset) and S4). The appearance of redox peaks could be attributed to the redox reactions of the nickel- and cobalt-based composites in NaOH electrolyte and their corresponding mechanisms are given by18

200−400 and 80 nm, respectively. The AFM analysis was conducted in tapping mode to collect phase and height data of the as-prepared nickel−cobalt phosphate nanosheets simultaneously (Figure 2C). The AFM height image indicates that the average thickness of the nickel−cobalt phosphate nanosheets is about 4 nm, suggesting that ultrathin nickel−cobalt phosphate nanosheets are synthesized. Figure S1 shows the AFM image and TEM image of sample P3, which was synthesized while continuously increasing the amount of Ni(CH3COO)2. The morphology makes some changes, while changing from rounded rectangular nanosheets to elliptical nanosheets. However, they are still nanosheets with the thickness of about 20 nm. Elemental mapping of nickel−cobalt phosphate demonstrates that Co, Ni, P, and O elements are homogeneously distributed within nickel−cobalt phosphate (Figure 2D). XPS analysis is used to identify the valence state of the Co, Ni, P, O elements. According to the XPS analysis (Figure 2E−H), the binding energies for Ni 2p3/2 and Ni 2p1/2 are centered at around 856.6 and 874.2 eV with two shake-up satellite peaks (861.2 and 880.3 eV), which verifies the presence of Ni(II) (Figure 2F). The Co 2p spectrum can be best-fitted by Co 2p3/2 and Co 2p1/2 peaks located at around 781.4 and 797.8 eV, yielding characteristics of the Co(II) state (Figure 2G). The peaks located at 133.4 and 134.3 eV correspond to the characteristic P 2p3/2 peaks of P(V). The XPS results further show that the as-synthesized product is nickel−cobalt phosphate. Figures S2−S3 show the XPS spectra of samples P1 and P3. Co(II) and P(V) existed in sample P1, which further suggests that the product is cobalt pyrophosphate. Simultaneously, the presence of Ni(II), Co(II), and P(V) in sample P3 further suggests that the product is nickel−cobalt phosphate. Electrochemical Behavior of Nanomaterials. The electrochemical behavior was analyzed in 0.1 M NaOH electrolyte with the (CV) technique. Nickel−cobalt phosphate (sample P2)-based GCE exhibits a pair of redox peaks (Figure 3A (inset), B). The anodic peak and cathodic peak are located at +0.56 and +0.38 V, respectively. Also, the nickel−cobalt

NiCo2O4 + OH− + H 2O → NiO(OH) + 2CoO(OH) + e−

CoO(OH) + OH− → CoO2 + H 2O + e−

(1) (2)

However, cobalt pyrophosphate (sample P1) has no redox peaks (Figure 3A, inset). The reason may be that the high amount of pyrophosphate blocks the electrochemical activities of cobalt ions. Figure 3B displays the peak current and potential change as scan rates increase. The anodic and cathodic peak currents of the nickel−cobalt phosphate/GCE increase linearly with the scan rates ranging from 10 to 100 mV·s−1, suggesting a surface-controlled electrochemical process of nickel−cobalt phosphate.19 Figure 3A shows the electrochemical behavior of different samples in a 1.6 mM glucose solution. It illustrates that the oxidation and reduction peaks of sample P2- and P3-modified GCEs are much higher than those of bare GCE and sample P1modified GCE under the same conditions. Furthermore, it can be observed that oxidation current responses increase with the concentration of glucose in sample P2- and P3-modified GCEs (Figure 3C,D), demonstrating that glucose can be easily oxidized on the surface of nickel−cobalt phosphate (samples P2 and P3) over a wide concentration window. It indicates that samples P2−P3 on the surface of electrode show better electrochemical behavior because of high conductivity and high capacitive current. The enhancement of anodic current can be ascribed to the oxidation of glucose to gluconolactone. The irreversible glucose catalytic oxidation process is accompanied by the conversion of Ni(III) to Ni(II) and Co(IV) to Co(III) 2362

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Figure 2. (A) TEM image of nickel−cobalt phosphate (sample P2). (B) HRTEM image of nickel−cobalt phosphate (sample P2). (C) AFM image of nickel−cobalt phosphate (sample P2). Inset: the height profile of nickel−cobalt phosphate. (D) Elemental mapping images of nickel−cobalt phosphate. (E) XPS spectra of the as-prepared nickel−cobalt phosphate (sample P2). (F)−(H) XPS spectra of Ni 2p, Co 2p, and P 2p, respectively, for nickel−cobalt phosphate.

the following experiments. The typical i−t curves of different sample-modified GCEs for the gradual injection of different amounts of glucose at 0.55 V are displayed in Figure S7A. Compared with sample P1- and P3-modified GCEs, P2modified GCE exhibits the maximum current response when adding the same concentration of glucose. The corresponding current versus concentration calibration plots clearly show that the P2-modified GCE displays the highest sensitivity (Figure S7B). Therefore, the analytical performance of the nonenzymatic glucose electrochemical sensor based on nickel− cobalt phosphate (sample P2) is investigated in detail. Figure 4A further shows the current response of nickel− cobalt phosphate (sample P2)-modified GCEs with successive addition of glucose (a wider concentration window) in 0.1 M NaOH solution. With the addition of glucose, the current increases rapidly, indicating its excellent glucose detection performance. The corresponding calibration curve for glucose detection is plotted in Figure 4B. The sensor exhibits a linear range from 2 to 4470 μM glucose with a correlation coefficient

Co(IV) + Ni(III) + glucose → Co(III) + Ni(II) + gluconolactone

CVs of the nickel−cobalt phosphate GCE in the presence of glucose at different scan rates were performed to further study the electrocatalytic activities of the nickel−cobalt phosphate for glucose oxidation (Figure S5). A good linearity is obtained by plotting the peak currents versus square root of scan rates, suggesting that the oxidation of glucose is confined by diffusion of glucose molecules to the electrolyte/electrode interface.20,21 Amperometric Detection of Glucose at the Nickel− Cobalt Phosphate GCE. The detection sensitivity on the relevant modified electrodes is further explored through amperometric study. Figure S6 displays the influence of the applied potential on the current response of nickel−cobalt phosphate-modified GCE toward 15 μM glucose. It can be seen that the maximum current response is obtained when the potential is 0.55 V. Thus, the potential is chosen as 0.55 V in 2363

DOI: 10.1021/acsami.7b17005 ACS Appl. Mater. Interfaces 2018, 10, 2360−2367

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ACS Applied Materials & Interfaces

Figure 3. (A) CV curves of cobalt pyrophosphate (sample P1)- and nickel−cobalt phosphate (sample P2−P3)-modified GCEs and bare GCE in N2saturated 0.1 M NaOH with 1.6 mM glucose and without glucose (inset) at a scan rate of 100 mV·s−1. (B) CV curves of nickel−cobalt phosphatemodified GCEs in 0.1 M NaOH at different scan rates (sample P2). Scan rates (a−j): 10, 20, 30, 40, 50, 60, 70, 80, 90, and 100 mV·s−1. Inset: the plot of cathodic and anodic peak currents vs scan rates. (C, D) Typical CV curves of nickel−cobalt phosphate-modified GCEs (samples P2 and P3) in the presence of different concentrations of glucose in 0.1 M NaOH at a scan rate of 100 mV·s−1.

Figure 4. (A) Amperometric i−t curve of the response of nickel−cobalt phosphate (sample P2)-modified GCE to glucose with successive additions of glucose in N2-saturated 0.1 M NaOH at 0.55 V. Inset: i−t curve of the response of modified GCE to glucose with successive additions of 4, 8, 16, 20, and 40 μM glucose. (B) Corresponding calibration curve of current vs glucose concentration for modified GCE. Error bars are the standard error of the mean (five parallel electrodes). (C) Amperometric i−t response of nickel−cobalt phosphate (sample P2)-modified GCE at 0.55 V in N2saturated 0.1 M NaOH with the successive addition of 65 μM glucose, 0.1 mM KCl, 0.1 mM ascorbic acid (AA), 0.1 mM dopamine (DA), and a second addition of 65 μM glucose. (D) Amperometric responses recorded at the nickel−cobalt phosphate (sample P2) GCE on successive addition of glucose, serum, glucose and the mixture of serum with glucose.

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ACS Applied Materials & Interfaces (R2) of 0.9925. The sensitivity is calculated to be 302.99 μA· mM−1·cm−2. When the concentration of glucose increases to 5.67 mM, it is found that the current response reaches saturation gradually, indicating that all active sites of the electrode are covered with reaction intermediates at high concentration of glucose. The detection limit for the sensor is estimated to be 0.4 μM (signal-to-noise ratio S/N = 3). Therefore, nickel−cobalt phosphate is excellent as a promising candidate for glucose sensing with a low detection limit, wide linear range, and high sensitivity. Its analytical performance is compared to that of reported glucose nonenzymatic and enzymatic electrochemical sensors (Table 1), showing that our analytical parameters are satisfactory and even better than those of previous sensors. The high electrocatalytic performance is due to its high electrical conductivity and many electroactive sites, efficiently accelerating the electron transfer rate between the electrode and electrolyte during glucose oxidation. Also, the amperometric i−t and corresponding current versus concentration calibration curves for sample P3-modified GCEs are displayed in Figure S8. A linear range from 1 to 3000 μM is established, and the detection limit is estimated to be 0.5 μM. The sensitivity is calculated to be 142.35 μA·mM−1·cm−2. Anti-Interference, Reproducibility, and Stability Studies of the Sensor. The selectivity of the sensor is tested with various electroactive species such as KCl, ascorbic acid (AA), and dopamine (DA). Figure 4C shows that there are no obvious current changes except with the addition of glucose. Results indicate that the sensor possesses good antiinterference. For the reproducibility test, five different nickel− cobalt phosphate-modified GCEs were tested with 50 μM glucose and the relative standard deviation (RSD) is 2.3%. Long-time stability is performed by measuring the current response of 1 mM glucose for a month, revealing that the modified GCE maintained 97% of its original value (Figure S9). The above results demonstrate that the nickel−cobalt phosphate-based sensor has good reproducibility and longtime stability. Detection of Glucose in Human Serum. Furthermore, the nickel−cobalt phosphate-modified electrode is utilized to test the glucose levels in human blood serum samples using the standard addition method, two additions of standard glucose samples, two additions of serum samples, two additions of standard glucose samples, and one addition of the mixed (serum and glucose) sample were performed respectively. The results are shown in Figure 4D. It indicates an acceptable recovery ranging from 98.3 to 101.6% in human serum samples (Table 2). The glucose levels are determined to be 5.08 and 4.96 mM for two human serum samples, respectively, which are close to the values of 5.02 and 5.35 mM obtained by enzyme catalytic spectrophotometry.

Table 1. Comparison with Previously Reported Nonenzymatic and Enzymatic Electrochemical Sensors4,13,19,22−32

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CONCLUSIONS Two-dimensional nickel−cobalt phosphate nanosheets were synthesized using a simple, low-cost, and efficient hydrothermal method. By increasing nickel acetate concentrations, the morphology of nanosheets has changed from rectangular nanosheets to elliptical nanosheets. AFM results indicated that the thickness of nickel−cobalt phosphate nanosheets synthesized with a Ni/Co ratio of 2:5 was only about 4 nm. The electrochemical behaviors of different products were compared, and the results indicated that the thinnest nanosheets exhibited the highest electrocatalytic activity for glucose oxidation, mainly attributed to its high specific surface

area and multiple electroactive sites. The as-fabricated nickel− cobalt phosphate nanosheet-based nonenzymatic electrochemical sensor exhibited high performance, such as low detection limit, high sensitivity, wide linear range, long-term stability, and good reproducibility. Furthermore, the glucose levels in human serum samples were determined using the fabricated sensor and satisfactory results were obtained, suggesting that the 2D nickel−cobalt phosphate nanosheets are promising materials in clinical diagnosis and food and environmental chemistry fields. 2365

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Table 2. Analytical Results for the Determination of Glucose in Human Serum Samples

sample 1

sample 2

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added (μM)

found (μM)

mean recovery (%)

RSD (%, n = 3)

0 25 50 75 0 25 50 75

33.02 57.95 83.32 108.25 32.24 57.65 82.15 107.63

0 99.7 100.6 100.3 0 101.6 98.3 100.5

0 1.5 2.3 1.8 0 1.6 2.1 2.5

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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/acsami.7b17005. AFM and TEM images of nickel−cobalt phosphate (sample P3); XPS spectra of cobalt pyrophosphate (sample P1); XPS spectra of sample P3; CV curve of sample P3-modified GCEs in NaOH solution without glucose; CV curves of P2- and P3-modified GCEs in NaOH with glucose at different scan rates; influence of the applied potential on the current response of sample P2-modified GCE; amperometric i−t curves of the responses of sample P1-, P2-, and P3-modified GCEs with successive additions of glucose and corresponding calibration curves of current versus glucose concentrations; amperometric i−t curve of the response of sample P3-modified GCE with successive additions of glucose and corresponding calibration curve of current versus glucose concentrations for P3-modified GCE; and variation of the current response to glucose for nickel− cobalt phosphate-modified GCE versus storage time (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.P.). *E-mail: [email protected] (X.H.). ORCID

Huan Pang: 0000-0002-5319-0480 Xiaoya Hu: 0000-0002-8061-5165 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support from the NSFC (Nos. 21705141, 21275124, 21275125, 21575124, 21675140, 21201010, and 21671170), Jiangsu Planned Projects for Postdoctoral Research Funds (1601075C), Postdoctoral Science Foundation of China (2016M601897), the Program for New Century Excellent Talents in University (grant no. NCET-13-0645), PAPD and TAPP of Jiangsu Higher Education Institutions, Graduate Innovation Project Foundation of Jiangsu province (KYLX 1333 and KYLX 1334), Natural Science Research Projects of Jiangsu Higher Education (16KJB150044), and Yangzhou University Innovation and Cultivation Fund (2016CXJ014). 2366

DOI: 10.1021/acsami.7b17005 ACS Appl. Mater. Interfaces 2018, 10, 2360−2367

Research Article

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DOI: 10.1021/acsami.7b17005 ACS Appl. Mater. Interfaces 2018, 10, 2360−2367