Electrochemical Sensors Based on Au-ZnS Hybrid Nanorods with Au

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Electrochemical sensors based on Au-ZnS hybrid nanorods with Au-mediated efficient electron relay Yeonho Kim, Krishnan Giribabu, Jong Guk Kim, Jin Bae Lee, Won G. Hong, Yun Suk Huh, and Hae Jin Kim ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05603 • Publication Date (Web): 18 Dec 2018 Downloaded from http://pubs.acs.org on December 20, 2018

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ACS Sustainable Chemistry & Engineering

Electrochemical Sensors Based on Au-ZnS Hybrid Nanorods with Au-Mediated Efficient Electron Relay Yeonho Kim†, Krishnan Giribabu‡,ǁ, Jong Guk Kim†, Jin Bae Lee†, Won G. Hong†, Yun Suk Huh*, ‡, and Hae Jin Kim,*, † †Electron

Microscopy Research Center, Korea Basic Science Institute, Daejeon 34133, Republic

of Korea ‡Department

of Biological Engineering, Biohybrid Systems Research Center (BSRC), Inha

University, Incheon 22212, Republic of Korea ǁElectrodics

and Electrocatalysis Division, Central Electrochemical Research Institute,

Karaikudi, Tamil Nadu, India

*Corresponding author: [email protected] (Y.S.H.); [email protected] (H.J.K.)

ACS Paragon Plus Environment

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ABSTRACT

Development of a novel and stable electrochemical sensor electrode for the sensitive and reliable determination of p-nitrophenol (p-NP) is of great importance to environment. In the present work, electrocatalysts of Au/ZnS hybrid nanorods were prepared via a facile photo-assisted reduction process for an efficient detection of p-NP. The microscopic analysis revealed the uniform adherence of Au onto ZnS nanorods. As-fabricated AZS nanorods were evaluated for the efficient sensing of p-NP by modifying a glassy carbon electrode (GCE). The cyclic voltammetry analysis revealed the unique oxidative sensing ability of AZS for p-NP at 0.14 V with a low ΔEp (118 mV) when compared to that of bare GCE. Based on the notable sensing ability of AZS, a reliable and sensitive electrochemical method was anticipated for the determination of p-NP. Moreover, the effects of scan rate and pH level were examined to find out the optimized conditions at which higher sensitivity and low detection limit. At optimal conditions, the p-NP oxidation current was found to be follow linear relationships in the concentration range of 150-2000 nM, and the lowest detection limit for p-NP was obtained for 8AZS with a value of 320 nM and a signal-to-noise ratio of 3. The proposed electrochemical method was further evaluated in the presence of other inorganic cations and anions, and it was found that the interference was almost negligible. The real sample analyses confirmed the acceptable recovery levels.

KEYWORDS: Electron relay, Photodeposition, Electrochemical sensing, Electrocatalyst, pNitrophenol


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INTRODUCTION Aromatic nitro-compounds have been considered dangerous environmental pollutants that have been used widely in the manufacture of pharmaceuticals, pesticides, explosives, and dyestuffs.1-9 The massive release of these toxic pollutants into industrial wastewater, freshwater, and the surrounding environment creates a complex problem to the nature, inducing multiple human disorders.2 Many aromatic nitro-compounds have been included in the environmental legislation because they can cause undesirable effects in very low concentrations. In particular, pnitrophenol (p-NP) has been considered as a toxic derivative.1 The purification of wastewater polluted by p-NP is a difficult task due to their high stability in terms of chemical and biological degradation.10 Thus, it is important to fabricate a reliable, rapid, and sensitive electrochemical sensor for the efficient detection of p-NP. There are numerous techniques available for sensing pNP.11-16 Among them, electrochemical analytical techniques have received considerable attention for effective detection of p-NP due to their advantages, such as cost efficiency, fast response, and easy for on-site determination, compared to spectroscopic and chromatographic methods. It is well-known that the sensitivity and selectivity of sensors are dependent on the properties of electrocatalysts. Nanostructured metals and semiconductors have been used as electrode materials to examine the electrochemical behaviors of organic compounds17-23 however, these materials still require better detection performance at the reduction side, which hampers the realtime monitoring of the analyte. Hence, it is necessary to develop an electrochemical sensor composed of novel nanomaterials with an excellent electrocatalytic performances in p-NP detection. Although gold has been used as an important catalytic material with an excellent electrochemical performance in the detection of p-NP, it is not suitable for electrode materials due to the cost inefficiencies for a large-scale synthesis. On the other hand, semiconductors have the


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advantages of cost efficiency and large-scale production. Moreover, low-dimensional systems are not only good candidates for active sensing materials but also fascinating core parts for the application of photocatalytic hydrogen production and solar energy conversion, due to efficient transport of electrons.24-28 The attachment of noble metals to semiconductors provides enhanced optical and electrical properties. The hybridization manner (i.e. heterojunction) can give unique properties compared to corresponding individual materials.29-32 There are a number of ways in which noble metals can be loaded onto semiconductor nanostructures.33-35 Among them, photoassisted reduction is a facile and green approach in which the loading of a noble metal can be achieved directly onto the surface of a semiconductor without using any surfactant or heating process. Moreover, this approach does not produce an insulating organic layer between the metal and semiconductor, which negatively influences electrochemical properties. The aim of this work was to fabricate a novel and stable electrochemical sensor for the sensitive and reliable determination of p-NP. A nanosized Au incorporation on ZnS nanorods (AZS) were prepared via a photodeposition process and used as efficient electron relay for the fabrication of p-NP sensors. To the best of the authors’ knowledge, this is the first time to report the enhancing electrochemical sensing properties based on Au-ZnS nanorods modified GCE with an efficient electron relay. A cyclic voltammetry analysis showed the unique oxidative sensing ability of AZS towards p-NP with a lowering of potential compared to that of bare GCE. The experimental parameters were examined to optimize the conditions for electrodes with a high sensitivity and low detection limit, and the kinetic parameters of the modified electrode were also examined. EXPERIMENTAL SECTION Materials. All raw materials were analytical grade and used as received without further purification: zinc chloride (s, >98%), hydrazine monohydrate (l, 98%), ethylenediamine (l, 98%),


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sulfur (s, 99%), gold (III) chloride trihydrate (s, >99.9%), p-nitrophenol (s, >99%), o-nitrophenol (s, 98%), m-nitrophenol (s, 99%), α-Dinitrophenol (s, >98%), glacial acetic acid (l, 100%), sodium acetate (s, >99%), sodium dihydrogen phosphate (s, >99%), potassium chloride (s,>99%) Ferrous sulphate heptahydrate (s, >99%), sodium hydroxide (s, >98%) from Sigma-Aldrich. Ethanol (l, >99%) from Daejung Chemicals and Ultrapure deionized water (>18 MΩ cm) from a Millipore Milli-Q system was used throughout the experiments. Preparation of Au-ZnS hybrid nanorods. ZnS nanorods were synthesized via a wet-chemical solution process according to the reported method.36 Au-deposited ZnS nanorods were prepared using the photo-assisted reduction process. All the photodeposition reactions were carried out using a Xenon arc lamp (Newport, Model 66902), which illuminated the entire quartz reaction vessel. First step, as-prepared 30 mg of ZnS nanorods are dispersed in 30 mL of water. Subsequently, HAuCl4·3H2O precursor solution was added into the above colloidal solution with magnetic stirring for 5 min. Ultraviolet light were irradiated with a light irradiance of 70 mW cm-2 for 30 min. The weight percent (wt%) of Au to ZnS varied from 2 to 16% which was controlled with adding amounts of Au precursors; the weight ratio of Au to ZnS nanorods was x, the nanocomposite sample of Au-decorated nanorods was designated as xAZS. After injection of Au precursors to the above solution, the color of mixture gradually changed from white to light purple, the irradiation stopped after 30 min. The free electrons from ZnS nanorods during light irradiation were used for reducing agents for Au3+ ion convert to metallic Au nanoparticles onto the ZnS nanorods. The resultant products were centrifuged, washed, and dried in vacuum. Characterization. Transmission electron microscopy (TEM) and Scanning TEM (STEM) images, selected area electron diffraction (SAED), and energy-dispersive X-ray (EDX) elemental mappings were obtained using a JEOL JEM-2100F high-resolution microscope. X-ray diffraction


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(XRD) patterns were recorded with a Bruker D8 DISCOVER diffractometer. X-ray photoelectron spectroscopy (XPS) spectra were obtained by a Thermo Scientific Sigma Probe ESCA spectrometer. Photoluminescence (PL) spectra were taken using an Ocean Optics USB2000+ detector with a Q-switched Quantel Brilliant Nd:YAG laser. Electrochemical experiments. Background electrolytes were prepared by mixing of 0.1 M acetic acid–0.1 M sodium acetate for pH 3-5 and mixing of 0.1 M NaH2PO4 for pH 6-11. A buffer of pH 6 was used as the background electrolyte. A CHI 600 series electrochemical workstation was used for all electrochemical experiments. A cyclic voltammetry was measured in the potential range from -0.8 to 0.6 V with 0.1 mM p-NP at pH 6. A square-wave voltammetry was conducted in the range of -0.1 to 0.6 V with a voltage step of 0.02 V, an amplitude of 0.1 V along with a frequency of 50 Hz using pH 6 as the supporting electrolyte.

RESULTS AND DISCUSSION Figure 1 illustrates a photo-assisted reduction process for the preparation of electrocatalytic materials of Au-ZnS nanocomposites. First step, as-synthesized ZnS nanorods are dispersed in water and exposed to ultraviolet (UV) illumination. Injection of Au precursors to the above solution, the color of mixture solution gradually changed from white to light purple. This implied a successful formation of Au nanoparticles (Au NPs) onto ZnS nanorods. The working electrode of GCE modified with Au-ZnS nanocomposites has been used as an efficient sensing platform. Figure 2 shows typical TEM image of ZnS nanorods, and their distinct one-dimensional structures have remained invariant regardless of the subsequent photo-assisted reduction process. TEM images in Figures 2b-d illustrate that the photodeposition procedure generates homogeneously dispersed Au dots onto the ZnS nanorods. The number density of Au NPs onto


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ZnS surfaces was found to increase with the increment of Au loading amounts, while size and morphology of Au NPs were almost invariant. During the photodeposition process, the Au NPs were decorated onto ZnS surfaces, which was achieved using the following reaction under light irradiation: ZnS → ZnS (e¯ + h+)

(1)

AuCl4¯ + 3e¯ → Au + 4Cl¯

(2)

The conduction band electrons of ZnS were trapped in surface states or vacancy defect-sites with a high reduction potential, which is the driving force converting AuCl4¯ (Au3+) to metallic Au and leading Au NPs growth directly onto ZnS surfaces. This synthetic procedure is a more greenchemical approach compared to conventional reduction methods,34 because photoreduction does not require any ligands, heat treatment, pH adjustment, or strong reductants. The low-magnification TEM image in Figure 3a shows that as-fabricated 4AZS hybrid nanorods have stable and well-defined geometry under the light-irradiation process. As-fabricated ZnS nanorods have an average width of 40 nm and a typical length of 1.6 m. To identify the structure details, the HRTEM image was measured at the edge of the 4AZS, which was presented in Figure 3b. The marked d-spacings of 0.332, 0.312, and 0.298 nm in Figure 3b are in good accordance with the expected separations of the (010), (002), and (011) planes in the reference wurtzite ZnS, respectively.36 The spherical shape of Au NPs with an average diameter of 5 nm can be clearly seen in Figure 3b. The d-spacing of 0.235 nm has been attributed to the (111) plane of the Au, which indicates that Au NPs were selectively deposited onto ZnS surfaces via the photoreduction process. In Figures 3a and b, the clear lattice fringes and SAED patterns show a high crystallinity and single-crystalline characteristics. To further investigate the chemical composition and constitution of the nanocomposites, an elemental analysis was performed with


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the HADDF-STEM mode of EDX. Deposited Au dots onto ZnS surfaces are almost same size of 5 nm and appeared brighter than ZnS because of the higher atomic number of Au (79) compared to Zn (30) and S (16). The distribution maps in Figure 3g indicate that Au nanodots were distributed homogeneously onto ZnS surfaces. As-synthesized ZnS nanorods, without capping any ligand, might lower the barriers for the interfacial electron transfer and result in the subsequent photoreduction of AuCl4– to metallic Au NPs. Thus, most of added gold ions (from precursors) were deposited onto ZnS surfaces as a metallic Au form via the photo-assisted reduction process, which can be confirmed by EDX composition analysis of Figure S1. The crystal structure of AuZnS nanorods, such as ZnS, 4AZS, and 8AZS, was investigated by monitoring HRXRD, which shown in Figure S2. The intense diffraction peaks arising from the samples indicated that the obtained nanocomposites have a high-crystalline structure. In Figure S1, all the peaks were indexed to the highly crystalline ZnS; neither impurity nor remarkable peaks shifts were observed after Au deposition, suggesting that the nanocomposites with the present Au weight ratios have a negligible effect on the expansion the hexagonal wurtzite ZnS lattice. An overview of the XPS spectra was performed to monitor the surface chemical states of the electrocatalysts. Figure 4a shows the full survey XPS spectrum of 4AZS in which the peaks of Zn 2p, S 2p, Au 4f, and C 1s can be clearly observed. Each Zn 2p spectrum in Figure 4b shows a significant spin-orbit doublet, whose 2p1/2 and 2p3/2 binding energies were measured to 1044.5 and 1021.3 eV, respectively, which indicates the existence of chemical state Zn2+ in the ZnS crystal lattice.37-40 The splitting peak values of Zn 2p is 23.2 eV, which indicate the stable valence of 2+ during the photoreduction process. As shown in Figure 4c, each biding energies of S 2p was observed to be 161.6 eV. From the XPS analysis of Figures 4b and c, the peak values of Zn 2p and S 2p were invariant with the direct photo-assisted reduction process. This implies that deposited


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Au NPs did not bond with any atoms of Zn or S but mostly appeared onto ZnS surfaces as a metallic form. A detailed oxidation state and a compositional analysis of the Au 4f peak are presented in Figure 4d. The Au 4f spectrum were deconvoluted into two peaks; the Au0 4f7/2 peak at 83.7 eV is attributed to fully reduced Au (zero-valent state) on ZnS nanorods, and the Au+ 4f7/2 peak at 85.0 eV is assigned to partially reduced Au+ ion. The Au+ ion could have resulted from the formation of gold hydroxide under this synthetic condition. The higher binding energy regions for Au 4f5/2 and Zn 3p can be deconvoluted into four peaks correspond to Au0 4f5/2 (87.1 eV), Au+ 4f5/2 (89.2 eV), Zn 3p1/2 (88.2 eV), and Zn 3p3/2 (91.2 eV), as shown in Figure 4d. The PL spectra in Figure S3 were used to examine the behavior of luminescence quenching of AZS nanocomposites. With the increasing incorporation of Au NPs, the band-edge emission at 325 nm decreases drastically. This quenching phenomenon is associated with the charge-transfer process and clearly supports successful deposition and existence of their interfacial junction with maintaining their physical and optical properties.41 Figures 5a and b show the cyclic voltammetric behavior with and without 0.1 mM p-NP in pH 6 at GCE, ZnS, 1AZS, 4AZS, and 8AZS modified GCE. All the modified electrodes exhibited similar behaviors without p-NP, i.e., no electrochemical active peaks were observed. In the presence of p-NP, all the modified electrodes exhibited oxidation and reduction peaks in the range of 0.15 to -0.07 V. A new reduction peak revealed at -0.76V in the presence of p-NP, which represents to the reduction of p-NP to p-hydroxylaminephenol,42 and it was found that the oxidation peak at 0.14 V corresponds to the oxidation of p-NP to p-quinoneimine (O1). The observed potential could be attributed to the electrocatalysis of modifier (AZS). Considering the responses presented in Figure 5, a possible redox mechanism of p-NP on AZS modified GCE can be expressed42 as follows in Scheme 1:


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Scheme 1. Possible oxidation and reduction process occurring at the AZS modified GCE.

The observed results were consistent with the previous published results.19 The observation of the present study clearly infers that the modified electrode shows a better electrochemical behavior towards p-NP based on the lower ∆Ep values of the modified electrodes 139 mV (GCE and ZnS), 132, 119, and 118 mV for 1AZS, 4AZS, and 8AZS, respectively. In addition, it was observed that the loading of Au onto ZnS becomes critical for the sensing of p-NP, and it was noted that Au loading onto ZnS surfaces improves electrochemical activity towards p-NP up to 4% weight of Au. Beyond this, any increase of Au did not lead to any enhancement in the electrochemical current response in terms of oxidation potential (Epa) or current (Ipa) (i.e., 8% Au). Hence, the high loading of Au on semiconductor nanoparticles should be optimized before conducting any electrochemical sensing studies. The TEM image in Figure S4 indicates that larger Au NPs (~ 13 nm) have been generated onto the ZnS nanorods with increasing the gold content up to 12%. This might be decrease the exposed active surface site and the electron transfer of electrocatalysts. Because a higher loading of Au may not result in better electrochemical performance as anticipated, the optimization of Au loading is crucial for electrochemical studies. As the potential is scanned towards positive direction, electron depletion at the interface of AuZnS nanorods occurs due to the presence of Au.43,44 This effect induces the formation of an additional positively charged region at the electrode surface that in turn, enables the oxidation process of p-NP at a low potential which is observed in the present work. The positive charge at


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the Au-ZnS nanorods junction also leads to formation of Au[OH]ads through the chemisorption of hydroxide anions.45 This chemisorbed hydroxide anions may form anion (adsorbed layer)-π interaction (p-NP contribution), which lead to being favorable for electro-oxidation process of pNP. When the potential is scanned towards negative direction, the surface of the Au-decorated ZnS nanorods is under a regime of negative charge accumulation which enhances the efficiency of pNP reduction.46 In Figure 6a, the pH effect for the oxidation of 0.1 mM p-NP at 8AZS modified GCE was investigated with varying pH range by cyclic voltammetery, and the corresponding voltammograms were presented. The modifier exhibited a better electrocatalytic performance among the modified materials. The oxidation peak current of p-NP in Figure 6b was increased and reached maximum current at pH 6 in the pH range of 3 to 6, thus pH 6 was chosen for further electroanalytical studies. When the pH was increased from 3 to 8, the oxidation potential (Epa) of p-NP shifted towards the lower potential range linearly, which shown in Figure 6c. The linear regression equation for the change in the pH experiment can be expressed as Epa(V) = -0.057pH + 0.5300 (R2 = 0.9935). This indicates that the overall oxidation of p-NP is a proton dependent process, and the electron transfer step was followed by protonation with an equal number of protons and electrons involved in the process.47 According to the Nernst equation,48 Epa = E [(2.303mRT)/(nF)]pH, the ratio of m/n was found to be ~1; where m denotes the number of protons intervening in the oxidation process, n denotes the number of electrons transferred. R, T, and F maintain their usual meanings. Moreover, the number of transfer proton can be calculated if the number of transfer electron is known. In addition, the electron and proton transfer number involved in the p-NP oxidation can be elucidated from the following equation:49 ip = nFQν/4RT, where the value of n was found to be 1. The values of electron transfer and proton transfer were found to be


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same. Based on these values, electro-oxidation of p-NP at modified electrode involves an equal number of electrons and protons in the electrochemical process. Figure 7a and S5 display the influence of scan rate in the range 10 to 500 mV s-1 for the electrooxidation of p-NP. The effect of scan rate was examined to investigate the electrode kinetics of the electrochemical process under study. The cyclic voltammetry of p-NP in pH 6.0 containing 0.1 mM p-NP at different scan rates from 10 to 500 mVs-1 was studied using various modified electrodes, such as ZnS, 1AZS, 4AZS, and 8AZS, respectively. The voltammetry peak indicated that the peak current increased linearly with an increase in the scan rate. The double log plot of log () vs log (Ipa) clearly showed that the electrochemical oxidation process is purely diffusioncontrolled one rather than adsorption-controlled. The observation was based on the slope values of the plot, such as 0.4, 0.36, 0.39, and 0.39 for ZnS, 1AZS, 4AZS, and 8AZS, respectively. Therefore, the Au modified GCE follows a diffusion-controlled process for hydroquinone oxidation, as the electrochemical process was found to be similar to that of the hydroquinone system in the present study.50 Figures 8a and b display the square-wave voltammetry (SWV) curves of p-NP under different p-NP concentrations from 150 to 2000 nM on ZnS and 8AZS modified electrodes, respectively. The oxidation peak currents at 0.12 V increased with an increase in p-NP concentrations. Figures 8c and d show the corresponding calibration curve for p-NP. Figure S6 also indicates the SWV and their corresponding calibration curves for 1AZS and 4AZS modified GCE. From the graphs in Figures 8 and S6, it is clear that the oxidation currents were proportional to the p-NP concentrations in the employed molar ranges. The regression equation, detection range, and sensitivities of the modified electrodes are listed in Table 1. The modified GCE exhibited a lower detection limit than most of the other electrodes which shown in Table 2; Among the comparison,


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graphene-gold nanocomposites exhibited a LOD of 0.01 µM, which is lower than the 8AZS, the reason for this is the fast electron kinetics extended by graphene. Whereas other electrode materials compared in Table 2 showed LOD, which is higher than that of 8AZS. This lower LOD of the 8AZS might be attributed to the heterojunction effect of Au-ZnS. Thus, the modified electrode employed in the work can be used to detect p-NP in solutions with a high sensitivity. The stability and reproducibility of the modified electrodes were examined by evaluating the electrode response for 0.1 mM p-NP each week for four consecutive weeks. The modified electrode was stored in refrigerator when not in use. The fabrication of modified GCE were carried out five times. The fabricated electrode shows the relative standard deviation of current response within 3.1 %, which implies the acceptable reproducibility of the fabricated electrochemical sensor. The oxidation peak current of p-NP was retained at 93% after 7 days and 90% after 30 days of storage. Figure S7 shows that the interference from different inorganic ions at 100-fold excess (Na+, K+, Ca2+, Mg2+, SO42-, Cl-, NO3-) and 40-fold excess (Fe2+, Cu2+, and Zn2+) with respect to p-NP was tested, which did not interfere with p-NP determination. On the other hand, 5-fold increase in the concentration of phenol derivatives (2-chlorophenol, 4-chlorophenol, 3nitrophenol, 2-nitrophenol, and 2,4-dinitrophenol) interfere with p-NP determination. The interference observed was a decrease in current along with a slight positive potential shift for pNP oxidation. Thus, as-fabricated 8AZS modified GCE have potential for a real sample analysis of p-NP. An 8AZS modified GCE was employed for the real sample analysis because this modified electrode exhibited a better sensitivity among the modified electrodes investigated. The p-NP determination was done using the standard addition method. As shown in Table 3, the fabricated electrochemical sensor exhibited recoveries in the range of 98 to 99.7%, which clearly proves that the 8AZS modified GCE sensor should be reliable for p-NP determination. Furthermore, the


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developed sensor showed that the presence of common interferences in water samples were almost insignificant towards the electroanalytical signal. CONCLUSION In summary, Au-ZnS nanocomposites were fabricated and applied to electrochemical sensors. The Au-ZnS electrocatalysts were found to possess the oxidative electrochemical sensing ability of p-NP at a lower potential than the bare GCE. The electro-oxidation of p-NP at modified electrode involves an equal number of electrons and protons, and their electrochemical process is purely diffusion-controlled. The presence of Au NPs enhances electrocatalytic properties via an efficient electron relay from GCE to p-NP. Moreover, Au NPs without capping ligands have an advantage of a low barrier for electron transfer to p-NP. The optimized electrode composed of 8AZS modified GCE shows the high sensitivity (11 nA/nM) and low detection limit (320 nM). The proposed method was applied for a real sample determination of p-NP and the recoveries were found in the range of 98-99%. Furthermore, the method is highly sensitive, reliable, and robust towards the oxidative electrochemical sensing of p-NP.


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FIGURES

Figure 1. Schematic illustration of Au-ZnS nanocomposites with the photo-assisted reduction process and their application for electrochemical sensor.

Figure 2. TEM images of Au-ZnS nanocomposites for (a) 0AZS, (b) 2AZS, (c) 4AZS, (d) and 8AZS. Each scale bars represent 200 nm.


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Figure 3. (a) Low-magnification TEM image, (b) HRTEM image observed from the indicated square regions of the TEM image, (c) the corresponding SAED pattern, (d) HAADF-STEM image, and (e-g) EDX elemental maps of Zn, S, and Au of 4AZS nanocomposites.

Figure 4. (a) Full survey, (b) Zn 2p, (c) S 2p, and (d) Au 4f and Zn 3d XPS spectra of ZnS-Au nanorods.


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Figure 5. Cyclic voltammetric curves of (a) ZnS, 1AZS, 4AZS, and 8AZS in the absence of 0.1 mM p-NP and (b) GCE, ZnS, 1AZS, 4AZS, 8AZS in the presence of 0.1 mM p-NP at pH 6 at a scan rate of 50 mV s−1.

Figure 6. Cyclic voltammetric curves of 8AZS modified GCE (a) at different pH (3–9), (b) effect of pH on Ipa of p-NP and (c) pH vs. Epa.


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Figure 7. Cyclic voltammograms and corresponding double log plot of (υ) vs. (Ipa) of the (a and c) ZnS and (b and d) 8AZS modified GCE with 0.01 mM p-NP in pH 6.0 at different scan rates: (a): 5, 10, 20, 30, 40, 50, 60, 70, 100, 125, 150, 200, 250, 300, 350, 400, 450, and 500 mV s−1.

Figure 8. Square wave voltammograms of p-NP and corresponding calibration curves for (a and c) ZnS and (b and d) 8AZS modified GCE, respectively.


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TABLES Table 1. Comparison of limit of detection for all the synthesized materials Materials

Regression equation

Sensitivity (nA/nM)

Limit of detection (nM)

ZnS

Ipa = 0.0051 [p-NP] + 6.375

5

680

1AZS

Ipa = 0.006 [p-NP] + 4.5502

6

540

4AZS

Ipa = 0.008 [p-NP] + 5.6772

8

400

8AZS

Ipa = 0.011 [p-NP] + 9.6878

11

320

Table 2. Comparison of analytical performance of different electrochemical sensors for pNP Electrode

LOD (µM)

References

HRP-TiO2 nanotube

0.2

17

HA-NPi/GCE

0.6

47

Graphene–gold nanocomposites

0.01

51

Nano-gold/GCE

8

52

PANI nanorods/GCE

3.2

53

Nano-Cu2O/GCE

0.5

54

ZnS

0.680

This work

1AZS

0.540

4AZS

0.400

8AZS

0.320


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Table 3. The recoveries of determination of p-NP in real water samples Sample

Added (X 10-7 M)

I II III

10.0 10.0 10.0

By the present method (X 10-7 M) 9.92 9.84 9.97

RSD (%)

Recovery (%)

2.95 3.20 2.74

99.2 98.4 99.7

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publication website at DOI:

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (Y.S.H.). *E-mail: [email protected] (H.J.K.). Author Contributions Y.K. and K.G. contributed equally to this work. The manuscript was written through contributions of all authors. All authors have given approval to the final version of manuscript. Notes The authors declare no competing financial interest.


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ACKNOWLEDGEMENTS We acknowledge the financial support from the R&D Convergence Program (CAP-15-02-KBSI) of NST (National Research Council of Science & Technology) of Republic of Korea.

REFERENCES

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Table of Contents

Highly efficient and reliable electrochemical sensors based on Au-ZnS hybrid nanorods with Aumediated electron relay


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Figure 1. Schematic illustration of Au-ZnS nanocomposites with the photo-assisted reduction process and their application for electrochemical sensor. 227x63mm (150 x 150 DPI)


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Figure 2. TEM images of Au-ZnS nanocomposites for (a) 0AZS, (b) 2AZS, (c) 4AZS, (d) and 8AZS. Each scale bars represent 200 nm. 173x134mm (96 x 96 DPI)



Figure 3. (a) Low-magnification TEM image, (b) HRTEM image observed from the indicated square regions of the TEM image, (c) the corresponding SAED pattern, (d) HAADF-STEM image, and (e-g) EDX elemental maps of Zn, S, and Au of 4AZS nanocomposites. 249x101mm (96 x 96 DPI)


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Figure 4. (a) Full survey, (b) Zn 2p, (c) S 2p, and (d) Au 4f and Zn 3d XPS spectra of ZnS-Au nanorods. 208x156mm (150 x 150 DPI)



Figure 5. Cyclic voltammetric curves of (a) ZnS, 1AZS, 4AZS, and 8AZS in the absence of 0.1 mM p-NP and (b) GCE, ZnS, 1AZS, 4AZS, 8AZS in the presence of 0.1 mM p-NP at pH 6 at a scan rate of 50 mV s−1. 200x81mm (96 x 96 DPI)


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Figure 6. Cyclic voltammetric curves of 8AZS modified GCE (a) at different pH (3–9), (b) effect of pH on Ipa of p-NP and (c) pH vs. Epa. 230x59mm (121 x 121 DPI)



Figure 7. Cyclic voltammograms and corresponding double log plot of (υ) vs. (Ipa) of the (a and c) ZnS and (b and d) 8AZS modified GCE with 0.01 mM p-NP in pH 6.0 at different scan rates: (a): 5, 10, 20, 30, 40, 50, 60, 70, 100, 125, 150, 200, 250, 300, 350, 400, 450, and 500 mV s−1. 199x157mm (96 x 96 DPI)


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Figure 8. Square wave voltammograms of p-NP and corresponding calibration curves for (a and c) ZnS and (b and d) 8AZS modified GCE, respectively. 253x183mm (150 x 150 DPI)


Electrochemical Sensors Based on Au-ZnS Hybrid Nanorods with Au

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