Uniform and Vertically Oriented ZnO Nanosheets Based on Thin

Research Article pubs.acs.org/journal/ascecg

Uniform and Vertically Oriented ZnO Nanosheets Based on ThinLayered MoS2: Synthesis and High-Sensing Ability Tao Yang,† Yanan Cui,† Meijing Chen,† Renzhong Yu,† Shizhong Luo,† Weihua Li,*,‡ and Kui Jiao† †

Key Laboratory of Sensor Analysis of Tumor Marker of Education Ministry, College of Chemistry and Molecular Engineering, Qingdao University of Science and Technology, 53# Zhengzhou Road, Qingdao 266042, P.R. China ‡ Institute of Oceanology, Chinese Academy of Sciences, 7# Nanhai Road, Qingdao 266071, P.R. China S Supporting Information *

ABSTRACT: Recently, a lot of nanomaterial-based composites have attracted wide interests in the field of electrochemical biosensing. Especially, the nanostructured ZnO (such as nanobelts, nanowires, and nanosheets) has emerged as a unique family of nanomaterials. In this Research Article, the uniform and vertically oriented ZnO nanosheets were electrodeposited on the preobtained MoS2 scaffold via cyclic voltammetry method further as a sensitively sensing platform to detect some molecules containing aromatic or conjugated rings. The morphology and electrochemical behaviors of ZnOMoS2 nanosheets were investigated by scanning electron microscopy (SEM), X-ray powder diffraction (XRD), and some electrochemical methods. Taking the nitro-substituted aromatic explosive 2,4,6-trinitrotoluene (TNT) as an example, the parallel experiments were made to prove that the uniform ZnO-MoS2 nanosheets composite was a worthy electrochemical monitoring platform toward the TNT detection. Compared with individual ZnO and individual MoS2, the ZnO-MoS2 showed the most obvious electrochemical signal, lowest limit of detection, and superior linear relation in the low concentration range. Moreover, the detections of other molecules containing aromatic or conjugated rings, such as 2′-deoxyaguanosine-5′triphosphate trisodium (dGTP) and riboflavin (Vb), were also investigated. KEYWORDS: MoS2, ZnO, cyclic voltammetry, Electrodeposition, Electrochemical sensing application, TNT, dGTP, Aromatic molecules

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INTRODUCTION Recently, ZnO nanomaterials with various morphologies and structures, such as nanowires,1 nanobelts,2 nanowalls,3 and nanonails,4 have attracted significant attention. And, nanostructured ZnO materials have many advantages in the aspect of biosensors, like large surface-to-volume ratio, excellent biocompatibility, high electron-transfer rates, and biosafety.5 Meanwhile, a series of synthetic routes can be adopted to obtain different ZnO nanostructures. For examples, Vayssieres et al.6 successfully adopted a hydrothermal method for controlled fabrication of ZnO nanowires. Besides, ZnO nanorod array could be synthesized via metal organic vapor deposition.7 Additionally, there are also other methods including electrochemical method,8 chemical vapor transfer,9 and so on. Among them, electrodeposition of semiconductor oxides has generated wide interests due to its outstanding merits, for instance, low cost, high deposition rate, and easy to control.10 For instance, the electrochemically synthesized ZnO nanowires remained interesting for constructing new device architectures.11 Recently, ZnO nanosheets were electrochemically prepared with the thickness of 50−100 nm and demonstrated higher photocatalytic activity.12 © 2017 American Chemical Society

Except for fabrication methods, ZnO was often integrated with other functional materials to extend its performances, such as graphene,13 CdS,14 and TiO2.15 According to Park’s report,13 smooth ZnO thin films were obtained on graphene layers adopting a low-pressure metal−organic vapor-phase epitaxy system. In 2008, it was reported CdS-cap layer has obvious influence on the optical and photoelectrical properties of ZnO nanowalls,14 where CdS served as a passivation layer to limit the adverse surface states of ZnO nanowalls. During the process of preparing ZnO−TiO2 nanocomposites, the formation of flower-like ZnO nanorods, consisting of many aggregative nanorods, required the presence of TiO2 nanoparticles act as the seed layer.15 Meanwhile, flourishing researches of thin-layered molybdenum disulfide (MoS2, as another semiconductor) have also emerged in many areas due to its unique sandwich-structure.16 It has been confirmed that MoS2 can adsorb some molecules containing aromatic or conjugated rings in their molecular Received: July 20, 2016 Revised: December 30, 2016 Published: January 6, 2017 1332

DOI: 10.1021/acssuschemeng.6b01699 ACS Sustainable Chem. Eng. 2017, 5, 1332−1338

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ACS Sustainable Chemistry & Engineering

Scheme 1. Schematic Preparation and Comparison of ZnO-MoS2 with Sole MoS2 and Sole ZnO toward the Detection of TNT

structures, such as aptamer and so on.16,17 Our group has also obtained thin-layered MoS2 via ultrasonication, which was simple, green and high efficiency. And a poly(xanthurenic acid, Xa) film has been electrochemically grown on a thin-layered MoS2 substrate to obtain a novel electroactive biosensing interface.17 Until now, the 2D MoS2-based materials are still worth to further investigate.18 It has been widely known that electrochemical activity and sensitivity are highly structuredependent.3,5,12 For example, Li presented a novel strategy to prepare electrochemical biosensors using MoS2 sheets as base materials. The MoS2-thionin composite with layered morphology was adopted to detect circulating DNA from healthy human serum.19 However, there is still little report about nanostructured ZnO directly grown on MoS2 substrate. It is expected that the integration of MoS2 with the nanostructured ZnO may brings synergistic effect, such as obviously enhanced sensing ability. So, in this Research Article, the uniform and vertically oriented ZnO nanosheets were electrodeposited on the preobtained MoS2 scaffold via cyclic voltammetry (CV) method to improve the sensing ability of ZnO nanosheets. Due to electrostatic interactions, the formed Zn2+ ion tended to adsorb on the negative MoS2 layers and the active nucleation sites nearly produced on the MoS2 scaffold,20,21 which were favorable to the subsequent generation of ZnO nanosheets on the MoS2 scaffold.22 The resulting ZnO can adsorb some molecules containing aromatic or conjugated rings, such as dopamine.23 Taking the nitro-substituted aromatic explosive 2,4,6-trinitrotoluene (TNT) served as a model molecule in this experiment, compared with sole ZnO and sole MoS2, the uniform ZnO-MoS2 nanosheets composite was a worthy electrochemical sensing platform toward the detection of TNT (Scheme 1). Besides, the detections of 2′-deoxyaguanosine-5′-triphosphate trisodium (dGTP) and riboflavin (Vb) were also investigated.

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electrode (SCE), and a platinum wire counter electrode. The obtained composite was characterized by scanning electron microscopy (SEM, JSM-6700F machine, JEOL, Tokyo, Japan). X-ray powder diffraction (XRD) spectra were obtained on Bruker D8 advanced X-ray diffractometer using Cu Kα radiation, being performed at 40 kV and 30 mA. Reagents. Zinc nitrate (Zn(NO3)2·6H2O) was purchased from Tianjin city, BASF Chemical Co. Ltd. Bulk molybdenum disulfide (MoS2, 99.0%) was obtained from SIGMA-Aldrich, Co., 3050 Spruce Street, (St. Louis, Mo, USA); N,N-dimethylformamide (DMF, 99.5%) was obtained from Fuyu Fine Chemical Co., Ltd. (Tianjin, China). TNT was purchased from Shanghai Jingchun Bioengineering Co. Ltd., China, and dGTP was supplied by Thermo Scientific (Lithuania). Vb was obtained from Tianjin Basifu Chemical Industry Co. Ltd., China. Three stock solutions (TNT, dGTP, Vb) were, respectively, prepared by dissolving them into 0.3 M phosphate buffer solution (PBS) solution. PBS was prepared by mixing the stock solutions of 0.1 M NaH2PO4 and 0.1 M Na2HPO4, and the pH value was adjusted by HCl. All the reagents used were of analytical grade and all solutions were prepared by ultrapure water from an Aquapro ultrapure water system (Ever Young Enterprises Development. Co. Ltd., Chongqing, China). Preparation of Modified Electrodes. Thin-layered MoS2 was obtained on the basis of bulk MoS2 according to our previous report,17 and it was ultrasonicated for 5 h under room temperature to obtain a black suspension with the concentration of 1 g/L. Then 20 μL of the dispersion solution was dripped on the GCE tip and naturally dried. The prepared electrode was named as MoS2/GCE. ZnO was electrodeposited by CV (from 0.8 V to −1.0 V) in 0.1 M Zn(NO3)2 under 80 °C on the above MoS2/GCE or bare GCE to obtain the ZnO-MoS2/GCE or ZnO/GCE through a water bath.12 Electrochemical Measurements. CV measurement was carried out in 1.0 mM [Fe(CN)6]3−/4− (1:1) solution at a certain scan rate (50 mV/s) from 0.6 to −0.3 V. Electrochemical impedance spectroscopy (EIS) measurement was carried out in the same supporting electrolyte, with the voltage frequencies from 104 Hz to 0.1 Hz and the applied potential 0.172 V vs SCE. Differential pulse voltammetry (DPV) parameters: increment potential, 0.004 V; pulse width, 0.06 s; pulse amplitude, 0.05 V; pulse period, 0.2 s; quiet time, 2 s. The reported result in this research for every electrode was the average value of three parallel measurements.

EXPERIMENTAL SECTION

Apparatus. Electrochemical measurements were performed with a CHI 660B electrochemical workstation (Shanghai CH Instrument Company, China), which contained a glassy carbon electrode (GCE) or a modified GCE as working electrode, a saturated calomel reference 1333


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Figure 1. SEM images of (A) MoS2, (B) ZnO, (C) ZnO-MoS2, and (D) magnified view of ZnO-MoS2. The XRD results of (E) MoS2, (F) ZnO, and (G) ZnO-MoS2..

Figure 2. CVs results (A) and Nyquist plots (B) of 1.0 mM [Fe(CN)6]3−/4− in 0.1 M NaCl recorded at different electrodes: (a) ZnO-MoS2, (b) MoS2, and (c) ZnO.

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RESULTS AND DISCUSSION Characterization of MoS2, ZnO, and ZnO-MoS2. The various morphologies of sole MoS2, sole ZnO, and the ZnOMoS2 were shown in SEM images (Figure 1). Figure 1A shows that thin-layered MoS2 presents pieces and smooth of morphology. Interestingly, the morphology of sole ZnO looks like teleneurons in Figure 1B. However, ZnO-MoS2 composite seems to show large-scale and leaves-like nanostructure in Figure 1C. Its part magnified view (Figure 1D) is consisted of smooth nanosheets and irregular lattices. In the process of electrodepositing, due to dissociation of Zn(NO3)2 salt in the acid solution, Zn2+ appears in the solution as illustrated in followed eq 1. Formation of OH− is due to electroreduction of NO3− catalyzed by the presence of Zn2+ ions in eq 2. MoS2 scaffold owned abundant negative charged S edges, which could be benefit for adsorbing the positively charged Zn2+ ion. The weak electrostatic force induces mutual attraction. So, Zn2+ gathered around MoS2 layers and the active nucleation sites nearly formed on MoS2 surface, which had a lead influence on the subsequent growth of ZnO on the MoS2 substrates. Then, on the thin-layered MoS2 modified electrodes, the reaction between OH− ions and Zn2+ ions would induce the generation of Zn(OH)2 (eq 3). Afterward, Zn(OH)2 would easily dehydrated into ZnO under around 80 °C (eq 4).24 The total procedure are listed as follows:12,15 Zn(NO3)2 → Zn 2 + + 2NO3−

(1)

NO3− + H 2O + 2e− → NO2− + 2OH−

(2)

Zn 2 + + 2OH− → Zn(OH)2

(3)

Zn(OH)2 → ZnO + H 2O

(4)

The formation of ZnO nanosheets is listed as follows. First, ZnO seed layer occurs in the above restrained nucleation and growth process. Then the large-scale growth of ZnO began. In the following electrodeposition process with extended time (for more details, please see subsequent the Choice of Deposition Time section), these smaller clusters agglomerated to grow bigger clusters tardily. At last, the sheet-like structures were fabricated to form uniform and vertically oriented nanosheets under the acceleration of the active sites.12 The sole MoS2, sole ZnO, and ZnO-MoS2 were also researched by XRD spectra as illustrated in Figure 1(E−G). In Figure 1E and G, a reflection peak centered about 14.08° indicates the existence of MoS2.25 Moreover, in Figure 1F and G, the main four peaks appearing at 2θ values of 32.2, 36.2, 47.1, 56.4, corresponded to the crystal planes (100), (101), (102), (103) of ZnO, respectively.26 All the peaks were consistent with the wurtzite structured ZnO, which could confirm the successful preparation of ZnO−MoS2. MoS2 owned abundant negative charged S edges,27 which could be benefit for adsorbing the positively charged Zn2+ ion. And the electrostatic forces between them improved the electrodeposition efficiency.28 Characteristics of the Modified Electrodes. The electrodes modified by three different materials were also characterized using CV technique in [Fe(CN)6]3−/4− (1:1) and the results were presented in Figure 2A. The ZnO/GCE shows weak redox peak currents owing to its weak conductivity. The redox peak currents of thin-layered MoS2 are larger, but the enhancement is not obvious, which was ascribed that MoS2 is still semiconductor because of its weaker electron transfer between the interlayers. The obtained ZnO-MoS2/GCE (curve 1334


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Figure 3. (A) DPVs of TNT recorded at different electrodes: (a) ZnO-MoS2, (b) MoS2, and (c) ZnO. (B) Schematic of the adsorbed molecules containing aromatic or conjugated rings and the mechanism (TNT as a model molecule).

Figure 4. EIS of [Fe(CN)6]3−/4− (A) and DPVs of TNT (B) at the electrodes of ZnO nanomaterials grown with different deposition times: (a) 400, (b) 600, and (c) 800 s. The SEM images of ZnO-MoS2 obtained at different deposition times (panel C, 400 s; panel D, 600 s; panel E, 800 s).

a) exhibits obviously enhanced redox peak currents, which demonstrated that the obtained ZnO-MoS2 owns excellent electrochemical activity. The EIS technique is used to further demonstrate the above results of CV, Normally, the results of EIS can be demonstrated through the corresponding Nyquist plot. In the Nyquist plot, the semicircle portion observed in high frequency regions often presents the electron transfer limiting process shown in Figure 2B. After ZnO-MoS2 being modified on the surface of GCE, the semicircle obviously decreases, while compared with sole ZnO and sole MoS2. It is known that ZnO can adsorb some molecules containing aromatic or conjugated rings.23 Adopting the nitro-substituted aromatic TNT as a model molecule, we investigate the synergistic effect of ZnO-MoS2 for sensing application as shown in Figure 3A. The detailed detection mechanism as listed as follow (Figure 3B): Initially, the nitro group of TNT was electro-reduced via a two-electron transfer process. Then, a 2-electron transfer process occurred consecutively to reduce the nitrous groups. Finally, the 2-electron transfer process

promoted the reduction of hydroxylamine group into an aromatic amine.29 Some experiment conditions for optimal detection are further investigated.11,30 Choice of Deposition Temperature. The deposition temperature has a very important effect on the electrochemical performances of ZnO−MoS2. The Figure S1 presents the redox signals of [Fe(CN)6]3−/4− (1:1) at ZnO−MoS2 obtained under different electrodeposition temperature, the electrochemical signal increased with the increase of the deposition temperature. However, after 80 °C, the signal decreased, indicating that 80 °C is the optimal electrodeposition temperature. Choice of Deposition Time. According to Yang’s report, after a certain electrodeposition time, the degradation of ZnO nanosheets comes to maximum of 90%.12 Here, electrodeposition time is a very important factor being worth to deeply investigate for pursuing their optimal electrochemical properties. As shown in Figure 4C, for the growth of 400 s, part nanosheet structures began to grow vertically on the substrate, but large-scale, uniform, and oriented ZnO nanosheets have not 1335


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Figure 5. EIS of 1.0 mM [Fe(CN)6]3−/4− (A) and DPVs of TNT (B) in PBS (pH 4.0) recorded at ZnO−MoS2, the concentrations of Zn(NO3)2: a (0.05 M), b (0.1 M), and c (0.15 M). The SEM images of ZnO−MoS2 prepared in Zn(NO3)2 solution of different concentrations (panel C, 0.05 M; panel D, 0.1 M; panel E, 0.15 M).

Figure 6. DPVs of various concentrations of TNT at (A) ZnO−MoS2/GCE (from a to i, 1, 0.6, 0.4, 0.2, 0.1, 5 × 10−2, 1 × 10−2, 1 × 10−3, 1 × 10−4 ppm), (B) MoS2/GCE (from a to h, 1, 0.6, 0.4, 0.2, 0.1, 5 × 10−2, 1 × 10−2, 1 × 10−3 ppm), and (C) ZnO/GCE (from a to g, 1, 0.6, 0.4, 0.2, 0.1, 5 × 10−2, 1 × 10−2 ppm). Calibration plots of the peak current versus different concentrations of TNT at ZnO-MoS2/GCE (D), MoS2/GCE (E), and ZnO/GCE (F).

formed.31 At 600 s, the large-scale rigid nanosheets (Figure 4D) on the platform of MoS2 appeared, which could be proved by the impedance change in Figure 4A. Further increase of the deposition time to 800 s could cause the collapse of the oriented nanosheet structures (Figure 4E) to destroy its electrochemical ability as shown in Figure 4A (curve c). The result was consistent with the DPV results of TNT detection shown in Figure 4B, with the appropriate electrodeposition time (600 s), the formed ZnO-MoS2 could monitor TNT high sensitively. Choice of the Concentration of Zn(NO3)2. In this system, Zn(NO3)2 also plays an important role during the electrodesposition process, so it is necessary to select the optium concentration of Zn(NO3)2. As depicted in Figure 5A, accompanied by the addition of Zn(NO3)2 in a certain range (0.05−0.1 M), the resistance of the obtained nanocomposite always decreased. The low concentration of Zn(NO3)2 was not enough to form clear ZnO nanosheets as shown in Figure 5C, maybe ascribed to the weak electrostatic force between MoS2 and ZnO. In the Figure 5A, when the concentration was above

0.1 M, the resistance obviously increased (0.15 M, curve c). The related SEM image was shown in Figure 5E, large amount ZnO nanoparticles gathered like balls on the surface of MoS2. The stack limit their electrochemical performances just as illustrated in Figure 5B.32 The strongest TNT reduction peak observed at 0.1 M, which is consistent with the result of EIS (Figure 5A) and SEM (Figure 5C−E). pH Value Optimization of the Detection Solution. Aiming at investigating the influence of pH values of the detection solution on electrochemical behaviors of TNT at ZnO−MoS2, DPVs results recorded in the different pH values were shown in Figure S2A. The corresponding line relationships of the peak currents of TNT vs different pH values of buffer solution are shown in Figure S2B. Clearly, all of reduction peak current reach the highest at pH 4.0. Comparison of TNT Determination Recorded at Different Electrodes. The electrochemical detections of TNT on sole ZnO, sole MoS2, and ZnO−MoS2 were carried out to compare the sensing ability, shown in Figure 6A−C. From calibration plots of the peak currents in Figure 6D−F, 1336



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obviously, the sole ZnO and sole MoS2 were helplessness during the TNT detection in low concentration range.33 According to the derived calibration curve in Figure 6D, the detection result of ZnO−MoS2/GCE showed a linear range between 1 × 10−4 ppm and 0.2 ppm with regression equation as Ip = 1.5467C + 0.8167 (R2 = 0.9889), revealing a detection limit of 5.2 × 10−5 ppm (3σ). The ZnO−MoS2 had the maximum electrochemical signal, lowest limit of detection, and superior linear relation in the low concentration range. What is more, this detection method is simple and sensitive, and it can be performed within a few minutes. Determination of dGTP Recorded at ZnO-MoS2/GCE. In our earlier reports,34 it was found that the current differences of dGTP between the pre-PCR sample mixture and post-PCR sample mixture can be utilized for the simple, cost-effective and qualitative detection of sequence-specific DNA.35 ZnO maybe also adsorb conjugated dGTP, which inspire us adopt ZnOMoS2 hybrid electrode for direct detection of dGTP shown in Figure S3.36 The possible oxidation mechanism is illustrated in the inset of Figure S3. The detection range for low concentration of dGTP was from 6 × 10−6 M to 1 × 10−8 M (detection limit = 2.2 × 10−8 M using 3σ) with a good linear relationship, which was Ip = 5.4785C + 0.04038 (R2 = 0.9811). Determination of Riboflavin (Vb) Recorded at ZnOMoS2/GCE. Vitamins are important organic compounds existed in foods, which absence in humans can cause various diseases. The Vb also owns the aromatic ring.37,38 So, the different concentrations of Vb were detected on our ZnO−MoS2 in Figure S4. The result exhibited that this Vb detection assay achieved a detection limit of 4.8 × 10−8 M with a linear range (4 × 10−5 M to 5 × 10−8 M). The equation of linear regression is Ip = 0.4245C + 0.0158 (R2 = 0.9865). These results indicated that Vb could be also determined sensitively by ZnO−MoS2 nanocomposites in a wide range.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +86-532-84022665. Fax: +86-532-84023927. ORCID

Tao Yang: 0000-0001-6677-3207 Weihua Li: 0000-0002-1648-6266 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 41476083, 21675092, 21275084, 51525903), 863 program (No. 2015AA034404), National Basic Research Program of China (No. 2014CB643304), and Marine science and technology projects of Huangdao district (2014-41).

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CONCLUSIONS The results from morphology, surface and electrochemical characterization revealed that the uniform and vertically oriented ZnO-MoS2 nanosheets directly grown on MoS2 substrate could be employed as a sensing platform for detecting molecules owning aromatic or conjugated rings, such as TNT, dGTP, and Vb. Especially, for the detection of TNT, ZnO− MoS2 exhibits highest sensing ability compared with the sole MoS2 or ZnO. The ZnO−MoS2 owns the maximum electrochemical signal, lowest limit of detection, and superior linear relation, especially in the low concentration range, which is worth pursuing the extended application of MoS2 in the field of electrochemical sensing.

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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.6b01699. Optimization of the deposition temperature, the optimization of the pH values of TNT detection solution, the determination of dGTP recorded at ZnOMoS2/GCE, and the determination of riboflavin (Vb) recorded at ZnO-MoS2/GCE (PDF) 1337


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Uniform and Vertically Oriented ZnO Nanosheets Based on Thin

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