Ordered Mesoporous NiCo2O4 Nanospheres as a Novel


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Ordered mesoporous NiCo2O4 nanospheres as a novel electrocatalyst platform for 1-naphthol and 2-naphthol individual sensing application Jiangjiang Zhang, Qianwen Mei, Yaping Ding, Kai Guo, Xinxin Yang, and Jing-Tai Zhao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08497 • Publication Date (Web): 14 Aug 2017 Downloaded from http://pubs.acs.org on August 14, 2017

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Ordered Mesoporous NiCo2O4 Nanospheres as a Novel Electrocatalyst Platform for 1-Naphthol and 2-Naphthol Individual Sensing Application Jiangjiang Zhang a, Qianwen Mei b, Yaping Ding *b, Kai Guo a, Xinxin Yang *a, Jingtai Zhao *a,c a. School of Materials Science and Engineering, Shanghai University, Shanghai 200444, P. R. China b. Department of Chemistry, College of Sciences, Shanghai University, Shanghai 200444, P. R. China c. State Key Laboratory of Advanced Special Steel, Shanghai University, Shanghai 200444, P. R. China KEYWORDS: mesoporous nanospheres; NiCo2O4; naphthol isomers; amperometry; electrochemical sensor

ABSTRACT

The novel ordered mesoporous NiCo2O4 (meso-NiCo2O4) nanospheres were synthesized by the nanocasting strategy followed by a calcination process for 2-naphthol (2-NAP) and 1-naphthol (1-NAP) individual sensing application. The as-obtained meso-NiCo2O4 material possesses

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mesoporous structure in spinel crystalline type with larger specific surface area than other structures. The meso-NiCo2O4 modified carbon paste electrode exhibited excellent electrocatalytic performance for NAP detection by amperometry measurement. The fabricated sensor of 2-NAP and 1-NAP has wide linear detection range (0.02 - 300 μM and 0.02 - 20 μM) with high sensitivity (1.822 and 1.510 μA μM-1 cm-2) and low limit of detection (0.007 μM and 0.007μM), respectively. In addition, the NAP sensors possess excellent reproducibility, stability, and selectivity.

1. INTRODUCTION Naphthols, especially 2-naphthol (2-NAP) and 1-naphthol (1-NAP) are two types of important industrial raw materials, and widely applied in synthesises of dyes, antiseptic substances, pigment, bactericides, mildew, rubber antioxidants, vermifuge, and so on.

1

Meanwhile, they are also common environmental pollutants with hypertoxicity and strong corrosivity. The naphthol can be absorbed very easily through human skin, then cause serious damages on blood circulation, kidney, cornea, or even cause cancer.

2-3

Hence it is extremely

urgent to develop simple and fast routine methods to detect 1-NAP and 2-NAP for the purpose of public health and environmental protection. So far, various methods have been established for detection of 1-NAP and 2-NAP, such as spectrofluorimetry 4, quartz crystal microbalance(QCM) 5

, gas chromatography-masss spectrometry 6, and high performance liquid chromatography 7.

Although these traditional techniques show good detection performance, they have some disadvantages including the expensive instruments, sophisticated professional operation, and time-consuming procedure. Furthermore, most of them strongly require highly skilled professionals for pre-treating the complex sample, which cannot be easily applied by the public.

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With the development of detection methods, electrochemical methods are considered to be the most promising approach with the advantages of short time, simple operation, low cost, and low limit of detection (LOD) for detecting 2-NAP and 1-NAP. Up to now, many electrochemical sensors modified with nanomaterials have been used for detecting 2-NAP and 1-NAP. For instance, Wang et al. successfully developed MWCNT modified with Pt nanoparticle for detecting 2-NAP.

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Li et al. developed a poly (3-

methylthiophene)-nano-Au modified electrode for detecting naphthol isomers. reported SnO2 modified electrode for detection of 1-NAP,

1

9

Huang et al

and so on. However, in view of

noble metals and Sn in binary metal oxide with the complex processes and the scarce storage resource, the focus of the research has been turned to ternary metal oxide for achieving high conductivity and multiple valence states. Among them, the NiCo2O4 is known as a promising catalyst and has been widely applied in electrochemical application as electrode material due to their low price, simple procedure, and rich resource. 10-12 Nowadays, electrochemical sensors based on NiCo2O4 nanomaterials have been fabricated with many different morphologies including hollow nanospheres

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, nanoparticles

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, porous

structures 15, etc. exhibiting as good catalytic properties as noble metals. However, to the best of our knowledge, NiCo2O4 nanomaterial has never been reported for 2-NAP and 1-NAP sensors. As is well known, the mesoporous structure displays excellent properties in electro-catalytic fields due to its high porosity and huge specific surface area. The large specific area can provide more reactive sites for enhancing the electro-catalytic performance of catalysts. Literatures reported that NiCo2O4 with porous structure demonstrated higher sensitivity than other microstructures. 14, 16

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In this study, ordered mesoporous NiCo2O4 (meso-NiCo2O4) nanospheres have been successfully synthesized by the nanocasting strategy followed by calcination process. Afterwards, meso-NiCo2O4 nanospheres were used as a novel electrochemical platform for the detection of 2NAP and 1-NAP for the first time. The sensor shows simplicity, low price, high sensitivity, low LOD, excellent chemical reproducibility, stability, and selectivity. 2. EXPERIMENTAL SECTION 2.1 Reagents and Apparatus Now list all chemicals of analytical grade quality used in this work: Co(NO3)2•6H2O, NaOH, Ni(NO3)2•6H2O, Na2HPO4, NaH2PO4, H3PO4, dopamine (DA), ethanol, uric acid (UA), and ascorbic acid (AA), were purchased from Sinopharm Chemical Reagent Co., and then used without further purification. 1 M of 2-NAP and 1-NAP stock solutions were prepared through dissolving NAP into ethanol, followed by storing at 277 K in a refrigerator. Diluting the above stock solutions with ethanol was to prepare the working solutions. Phosphate buffer solutions (PBS, 0.1 M) of various pH values were configured with 0.1 M of Na2HPO4, NaH2PO4, H3PO4, and NaOH. The whole experiments were operated in 0.1 M PBS. The 18.2 MΩ cm deionized water was used throughout the whole experiments. The structure and the chemical composition of meso-NiCo2O4 nanospheres were characterized by X-ray diffractometer (XRD) with Cu-Kα radiation (Rigaku D\MAX-2550, λ=1.54178 Å, 18 KW) and X-ray photoelectron spectra (XPS; ESCALAB 250Xi), respectively. The morphology of meso-NiCo2O4 nanospheres was measured by a field-emission scanning electron microscope (JSM-7500F, 15 kV; FESEM), and transmission electron microscope (JEM2100F, 200 kV; FETEM), equipped with energy-dispersive spectroscope and selected-area

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electron diffraction (EDS and SAED). N2 adsorption-desorption isotherm was measured using micrometrics ASAP 2020 sorptometer at 77 K liquid nitrogen temperature. Electrochemical measurements of 2-NAP and 1-NAP were carried out by the CHI842B electrochemical workstation, composing with a traditional three-electrode detection mode, in which mesoNiCo2O4 modified 3 mm diameter carbon paste electrode (CPE) as the working electrode, along with the Pt foil auxiliary and saturated calomel reference electrodes (SCE). The various pH values were checked using a pH-meter. 2.2 Synthesis of meso-NiCo2O4 nanospheres and bulk-NiCo2O4 The KIT-6 hard template, a kind of mesoporous silica, was synthesized as reported previously.

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The KIT-6 microstructure is composed of the interconnected cylindrical pore –

channels with the cubic Ia3d space group to form ordered mesoporous nanosphere. It is often used as template to synthesize mesoporous materials with large surface area. Here, mesoNiCo2O4 nanospheres were prepared with KIT-6 as the hard template by nanocasting strategy followed by calcination process. Typically, the KIT-6 was uniformly dispersed in 5 mL ethanol solution containing 1 mM of Ni(NO3)2•6H2O and 2 mM of Co(NO3)2•6H2O with continuous stirring for the formation of the precursor at 298 K for 1 h. The precursor was heated at 333 K until the ethanol was evaporated. Then the resulting mixture was heated in an oven at 473 K for 7 hours. The nanocasting and heating process were repeated thrice with the same conditions for achieving a higher load. Finally, the KIT-6 was removed using NaOH (2 M) aqueous solution at 298 K, and then calcined the leftovers at 723 K for 5 h in the muffle furnace. For comparison, bulk-NiCo2O4 powders were also prepared in line with above steps without template. One advantage of this method exhibited is that by multiple nancasting and heating as well as tuning

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the filling degree of the metal solution, one can control the nanoparticle sizes and morphology to obtain optimal conditions. 2.3 Preparation of meso-NiCo2O4 / CPE and bulk-NiCo2O4 / CPE The CPE was prepared as described previously.

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The meso-NiCo2O4 / CPE and bulk-

NiCo2O4 / CPE were fabricated as followings: typically, appropriate amounts of meso-NiCo2O4 or bulk-NiCo2O4 were uniformly dispersed in 1 mL deionized water under ultrasonic agitation, and then dropped on the CPE surface, followed by drying under infrared lamp. 3. RESULTS AND DISCUSSION 3.1 Structural and morphology analysis Fig. 1A presents the wide-angle XRD patterns of meso-NiCo2O4 nanospheres (red curve) and bulk-NiCo2O4 (black curve). The major diffraction peaks at 2θ of 31.1, 36.7, 44.6, 59.1, and 64.9o correspond to the (220), (311), (400), (511), and (440) planes, respectively. All the diffraction peaks were in absolute agreement with the NiCo2O4 phase comparing to standard JCPDS NO.73-1702.

19-20

No other diffraction peaks were detected, indicating completely

converted to NiCo2O4 from cobalt nitrate and nickel nitrate after calcination. The low-angle XRD patterns of ordered meso-NiCo2O4 and bulk- NiCo2O4 are illustrated in Fig. 1B. A sharp diffraction peak (2θ = 1o) along with one weak peak (2θ = 1.8o) was observed in meso-NiCo2O4 –

pattern, corresponding to the (211) and (332) reflections of 3D cubic Ia3d ordered mesoporous architecture, indicating successful topological replica from KIT-6 template. were in accordance with previously reported.

24-25

17, 21-23

These results

However, bulk-NiCo2O4 displayed no

diffraction peaks. As shown in Fig. 1C and D, the NiCo2O4 has a face-centered cubic (fcc) spinel

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_

structure with Fd3m symmetry with a = 8.11 Å and a strong (100) orientation. The 8 Ni and 8 Co cations are distributed randomly in the (16d) octahedral sites, while other 8 Co cations occupy the (8a) tetrahedral sites.

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Having such a spinel structure, the NiCo2O4 can enhance

electrocatalytic performance as a sensing material. 27-28

Fig. 1 (A) Wide-angle XRD and (B) Low-angle XRD patterns of meso-NiCo2O4 nanospheres (red curve) and bulk-NiCo2O4 (black curve). (C) – (D) Crystal structure of NiCo2O4 (ICSD No.24211).

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Fig. 2 (A) FESEM, (B – D) FETEM, (E, F) HRTEM images, (G) SAED pattern, and (H) EDS spectrum of meso-NiCo2O4 nanospheres.

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The morphologies of KIT-6, meso-NiCo2O4 nanospheres and bulk-NiCo2O4 were investigated by FESEM and FETEM. As shown in Fig. S1 (see Supporting Information) and Fig. 2A, the images of FESEM present that the KIT-6 and meso-NiCo2O4 reveals uniformly spheroidal morphology with the average diameter of about 180 nm. However, bulk-NiCo2O4 displays an irregular appearance in Fig. S2. Fig. 2B displays most of the as-prepared samples with a highly ordered mesoporous structure. Fig. 2C and D show images of partial enlargement of nanospheres. The microstructure of the meso- NiCo2O4 has nanosphere forms with the similar sizes of those template nanospheres, while these meso- NiCo2O4 nanospheres are constructed by smaller sized nanoparticles of ca.4 nm. We may assume that the sintering process produced these small nanoparticles after the templates were removed, resulting to nanoparticles assembled as an ordered mesoporous architecture. Such a mesoporous architecture possesses lower density and larger surface area than the solid counterparts, which results in more reactive sites available for enhancing electro-catalytic performance. Fig. 2E and F show the high resolution TEM (HRTEM) images, and the interplane spacing of 0.47 and 0.24 nm respectively corresponded to the (111) and (311) planes of NiCo2O4. SAED image (Fig. 2G) illustrates lattice planes of (111), (220), (311), (422), (511) and (440), indicating the polycrystalline properties of NiCo2O4. Furthermore, the chemical composition of the NiCo2O4 was clearly confirmed by EDS and Fig. 2H shows the result. The EDS spectrum indicates the existence of Ni, Co, and O elements with the Ni and Co contents of 23.77 and 50.43 wt%, indicating the formation of NiCo2O4. In addition, other element of Cu shown in the EDS result comes from the copper grid with the sample. This result is in line with the XRD observations and the previous reports. 26

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Fig. 3 XPS spectra of the meso-NiCo2O4 nanospheres: (A) full scan, (B) Ni 2p, (C) Co 2p and (D) O 1s. The valence state and element composition of the meso-NiCo2O4 nanospheres were also evaluated by XPS and corresponding results are illustrated in Fig. 3. From Fig. 3A, the full scan revealed the elemental signal of Ni, Co, O, and C. All binding energies of meso-NiCo2O4 were referenced to the C 1s peak (284.5 eV). The Ni 2p spectrum in Fig. 3B respectively presented two pairs of spin–orbit doublets at 855.1 /872.5 eV and 856.8 / 875.1 eV, corresponding to the characteristic of Ni3+ and Ni2+.

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Similarly, from Fig. 3C, the Co 2p spectra consisted of two

strong spin–orbit doublets was characteristic of Co3+ and Co2+, along with two satellites. The two

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peaks at 779.9 eV and 795.0 eV belonged to Co3+, while others at 781.4 eV and 796.9 eV were attributed to Co2+. 31 The O 1s spectra (Fig. 3D) was consisted of two oxygen peaks, which were marked as O2 (532.7 eV) and O1 (529.5 eV). It was reported that O1 and O2 were respectively attributed to the metal-oxygen bond

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and defective sites with low oxygen coordination.

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In

summary, XPS results indicate that the NiCo2O4 contains electron couples of Ni3+ / Ni2+ and Co3+ / Co2+, which is consistent with the previous reports. 20, 34-35

Fig. 4(A) N2 adsorption-desorption isotherm curve and (B) Pore size distribution of mesoNiCo2O4 nanospheres. The pore size distribution and specific surface area of KIT-6 (Fig. S3) and meso-NiCo2O4 nanospheres (Fig. 4) were characterized by N2 adsorption-desorption isotherm measurements at 77 K. The isotherm of the meso-NiCo2O4 nanospheres in Fig. 4A clearly demonstrates a typical isotherm of type-IV with the hysteresis loop of type H3 at the pressure (P / P0) between 0.40 and 1.0. The appearance of hysteresis loop indicates the existence of mesoporous architecture. Fig. 4B shows pore size distribution of meso-NiCo2O4 nanospheres calculated using the BarrettJoyner-Halenda (BJH) equation from the nitrogen isotherm desorption branch. The main pore

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diameter of meso-NiCo2O4 nanospheres is about 3.8 nm, consistent with the FETEM observations. The specific surface area of meso-NiCo2O4 is 153 m2g-1. Therefore, the mesoporous structure of NiCo2O4 increases the diffusion of electrons in the oxidation reaction of 2-NAP and 1-NAP for enhancing the electro-catalytic activity. 3.2 Electrochemical characterizations

Fig. 5 CV curves of bare CPE (curves a, b), bulk-NiCo2O4 / CPE (curves c, d) and mesoNiCo2O4 / CPE (curves e, f) in PBS (0.1 M, pH=7) at low scan rate of 0.02 V s-1 without or with 500 μM 1-NAP (Fig. A) and 2-NAP (Fig. B), respectively. Electrocatalytic performances of 2-NAP (Fig. 5B) and 1-NAP (Fig. 5A) were respectively studied by cyclic voltammetry (CV) on bare CPE, bulk-NiCo2O4 / CPE and meso-NiCo2O4 / CPE in PBS (0.1 M, pH=7) at low scan rate of 0.02 V s-1. When 500 μΜ of 1-NAP and 2-NAP are injected into the PBS, a completely irreversible oxidation peak signal (Ipa) is respectively captured at 0.422 V for 1-NAP and 0.592 V for 2-NAP on these three electrodes (curves b, d and f), compared to the ones without 1-NAP and 2-NAP (curves a, c and e). Meanwhile, oxidation peak current on meso-NiCo2O4 / CPE (curve f) and bulk-NiCo2O4 / CPE (curve d) shows a larger

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response value than those of on bare CPE (curve b), which is attributed to the presence of multiple oxidation states. Furthermore, the Ipa of meso-NiCo2O4 / CPE (curve f) reveals a higher value than that of bulk-NiCo2O4 / CPE (curve d) since the ordered mesoporous structure will provide huge surface area and more binding sites available for increasing the contact areas between catalysts and analysts, which is helpful to the catalytic performance. One may see that very broad “peaks” appear on the reduction side of the e and f curves for both Fig.5A and 5B, and the difference between e and f is more pronounced for Fig.5B. By considering the irreversible nature of the CV procedure for 1-NAP and 2-NAP, these “peaks” may have come from the reaction of other reactants in solution with the electrode material. The large decrease of the value in f curve for Fig.B might be associated with the polymerization of 2-NAP after oxidation. In order to further investigate catalytic capacities of different modified electrodes for detecting 2-NAP and 1-NAP, the amperometry characterizations were carried out, respectively, at the applied potential of + 0.4 (for 1-NAP) and + 0.6 V (for 2-NAP) by successive injection of NAP with different concentrations (4, 8, 12, 16, and 20 μM)) into 0.1 M PBS (pH=7) under a stirring condition. As shown in Fig. 6C and A, the Ipa of 2-NAP and 1-NAP on meso-NiCo2O4 / CPE (red curve c) is stronger than those of bare CPE (black curve a) and bulk-NiCo2O4 / CPE (blue curve b). Meanwhile, when the amperometric response is steady, calibration curves of response current values against corresponding concentrations are obtained (Fig. 6D, B). The slope of calibration curve can reflect the sensitivity of the modified electrode. It is clear that higher sensitivities for 2-NAP and 1-NAP are respectively obtained on meso-NiCo2O4 / CPE (red curve c) than those on the bare CPE (black curve a) and bulk-NiCo2O4 / CPE (blue curve b). Amperometric results are in agreement with the above CV observations. These good catalytic

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capacities of meso-NiCo2O4 nanospheres are attributed to the following reasons: firstly, mesoNiCo2O4 nanospheres possess large surface area, providing target molecules with more active sites for increasing opportunity of oxidation reaction; secondly, two electron couples Ni2+/Ni3+ and Co2+/Co3+ enhance oxidation reaction of 2-NAP and 1-NAP. All these results show that meso-NiCo2O4 has appropriate property and is promising to be used as the sensing materials for detecting 2-NAP and 1-NAP.

Fig. 6 Amperometric responses of bare CPE (black curve a), bulk-NiCo2O4 / CPE (blue curve b), and meso-NiCo2O4 / CPE (red curve c) with step wise addition of 1-NAP (Fig. A) and 2-NAP (Fig. C) (4, 8, 12, 16, and 20 μM, respectively) into PBS (pH=7, 0.1 M), applied potential of +

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0.4 V and + 0.6 V, separately. Curve plots of the response currents of bare CPE (black curve a), bulk-NiCo2O4 / CPE (blue curve b) and meso-NiCo2O4 / CPE (red curve c) against the concentrations of 1-NAP (Fig. B) and 2-NAP (Fig. D), separately. 3.3 Effect of operational parameters Influence of several operational parameters for detecting 2-NAP and 1-NAP were optimized, including the pH of the PBS, scan rate, as well as the applied potential and concentration of modifier. These results illustrated that the optimal applied potentials are + 0.6 / + 0.4 V for 2NAP and 1-NAP, respectively. Meanwhile, optimal concentrations of modifier are all 2.0 mg mL-1 meso-NiCo2O4 nanospheres (Fig. S4). The influence of the pH of PBS on oxidation peak potential and current (Epa and Ipa) of NAP (500 μM) were investigated by CV on the meso-NiCo2O4 / CPE in PBS (0.1 M) at a scan rate of 0.02 V s-1 (Fig. 7A and C). As shown in Fig. 7B and D, as the pH value of PBS increased from 5 to 9, the Epa of both 1-NAP and 2-NAP shifted toward lower value, which indicated that the proton was involved in the reaction process. 36 The linear equations of the Epa and pH value can be expressed as followings: Epa(1-NAP) = 0.7643 – 0.0501 pH (V, R2 = 0.9919)

(1)

Epa(2-NAP) = 1.004 – 0.0592 pH (V, R2 = 0.9928)

(2)

In accordance with the Nernst equation, Epa = Eθ – (0.059 m/n) pH,

37

slopes of both linear

regression equations were close indeed to the theoretical value of –0.059, indicating that equal number of electrons (n) and protons (m) took part in the electro-oxidation reaction. Meanwhile, with the solution pH from 5 to 9, the maximum Ipa of 1-NAP (Fig. 7B) and 2-NAP (Fig. 7D)

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were respectively obtained in neutral PBS (pH=7), and decreased on both sides, i.e. PBS with pH=7 is the optimal condition for the further electro-analytical experiments.

Fig. 7 CV curves of 500 μM 1-NAP (A) and 2-NAP (C) on meso-NiCo2O4 / CPE in 0.1 M PBS with various pH values (from curves a to e: 5, 6, 7, 8, and 9) at the scan rate of 0.02 V s-1; the plots of the oxidation peak potential (Epa) and current (Ipa) vs. the pH for 1-NAP (B) and 2-NAP (D), separately. The influence of the scan rate on Ipa was investigated by CV methods. Fig. 8C and A show the CV curves of meso-NiCo2O4 / CPE in PBS (pH=7, 0.1 M) containing 500 μM 2-NAP and 1NAP, separately, with various scan rates from 0.005 to 0.09 V s-1. It can be observed that the Ipa gradually increases with increasing scan rate. For Fig. 8D and B, Ipa of 2-NAP and 1-NAP were

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linear relationship with square-root of scan rate (v1/2), respectively. The linear equations can be expressed as: Ipa(1-NAP) = 81.98 v1/2 - 1.264 (μA, V s-1, R2 = 0.9975)

(3)

Ipa(2-NAP) = 136.4 v1/2 – 2.149 (μA, V s-1, R2 = 0.9953)

(4)

respectively, which indicated that electro-oxidation reactions of 2-NAP and 1-NAP at the electrode surface were diffusion-controlled process. Furthermore, for reducing background current to achieve sharp peaks, the scan rate of 0.02 Vs-1 was employed for the further experiments.

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Fig. 8 (A) CV curves of 500 μM 1-NAP (A) and 2-NAP (C) on meso-NiCo2O4 / CPE in 0.1 M PBS (pH=7) with various scan rates of 0.005, 0.02, 0.04, 0.06, 0.08, and 0.09 V/s; the linear plots of Ipa vs. v1/2 for 1-NAP (B) and 2-NAP (D), separately. The plots of Epa vs. naturallogarithm of scan rate (ln v). 3.4 Oxidation mechanism of 2-NAP and 1-NAP on meso-NiCo2O4 / CPE The oxidation mechanism of NAP on the modified electrode is elucidated for a better understanding of higher electrochemical performance of 2-NAP and 1-NAP at meso-NiCo2O4 / CPE. As is well-known, for totally irreversible electrode process, the transferred-electron number was determined from the following equation of Epa and ln v 38: Epa = Eθ + M [0.78 + ln (D1/2 ks-1) − 0.5 ln M] + 0.5M ln v,

(5)

M = RT / [(1-α) n F],

(6)

where Eθ is formal potential; D is diffusion coefficient; v is scan rate; n, ks and α are the electron transfer number, rate constant, and coefficient involving in the rate-controlling process; and F, T and R are Faraday constant, temperature, and gas constant, respectively. As can be seen in Fig. 8B and D, the Epa is positively shifted with increasing the Natural logarithm of the scan rate (ln v) for detecting 2-NAP and 1-NAP. The linear regression equations between Epa and ln v can be described as followings: Epa(1-NAP) = 0.03057 ln v + 0.5395 (V, V s-1, R2 = 0.9969)

(7)

Epa(2-NAP) = 0.03327 ln v + 0.7312 (V, V s-1, R2 = 0.9964)

(8)

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From equations (5)-(8), the values of (1-α) n are calculated respectively to be 0.42 for 1-NAP and 0.39 for 2-NAP. For the completely irreversible electrochemical reaction, α is generally assumed as 0.5.

39

Hence n were calculated to be 0.84 and 0.78, close to 1, which indicated that

both one electron took part in 2-NAP and 1-NAP oxidation reaction, respectively. From the correlation of Epa and pH, we know that equal number of electrons and protons took part in the electro-oxidation reaction. Therefore, we can draw a conclusion that the oxidation reaction of 2NAP and 1-NAP on meso-NiCo2O4 / CPE is conducted by one-proton and one-electron. According to the above conclusion and previous report,

2, 40

the oxidation mechanism of 2-NAP

and 1-NAP on meso-NiCo2O4 / CPE may be presented in Scheme 1, respectively.

Scheme 1 Oxidation Mechanism of 2-NAP and 1-NAP on meso-NiCo2O4 / CPE. 3.5 Amperometric detection of 2-NAP and 1-NAP on meso-NiCo2O4 / CPE Under optimal measurement conditions, amperometry (Fig. 9C, A) was carried out for respectively investigating the oxidation of 2-NAP and 1-NAP with different concentrations in PBS (pH=7, 0.1 M) for obtaining the calibration curves on the fabricated sensor. The mesoNiCo2O4 / CPE achieved 95% steady-state currents within 5 seconds after introduction of the 2NAP and 1-NAP. Furthermore, calibration curves of 2-NAP and 1-NAP were illustrated in Fig.

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9D and B, respectively. Meso-NiCo2O4 / CPE for the detection of 1-NAP showed liner response in the range of 0.02 to 20 μM, which can be expressed as:

Fig. 9 Amperometric responses of meso-NiCo2O4 / CPE with stepwise addition of 1-NAP (Fig. A) (0.02, 0.1, 0.5, 1, 2, 3, 4, 8, 12, 16, and 20 μM) and 2-NAP (Fig. C) (0.02, 0.1, 0.5, 1, 2, 3, 4, 8, 12, 16, 20, 30, 40, 60, 100, 150, 200, 250, and 300 μM) into 0.1M PBS (pH = 7) buffer at +0.4 and +0.6 for 1-NAP and 2-NAP, respectively; Inset: magnification of Fig. C. Curve plots of the response currents of meso-NiCo2O4 / CPE against the concentrations of 1-NAP (Fig. B) and 2NAP (Fig. D), respectively; Inset: magnification of Fig. D. Ipa(1-NAP) = 0.1067 C1-NAP + 0.08227 (μA, μM, R2 = 0.9969)

(9)

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The modified electrode for detecting 2-NAP reveals two-segment linear responses in the wide ranges of 0.02 - 20 μM and 20 - 100 μM, which can be expressed as: Ipa(2-NAP) = 0.1287 C2-NAP + 0.1153 (μA, μM, R2 = 0.9972)

(10)

Ipa(2-NAP) = 0.041 C2-NAP + 1.974 (μA, μM, R2 =0.9982)

(11)

For 1-NAP and 2-NAP, the sensitivities of meso-NiCo2O4 / CPE were calculated to be 1.510 and 1.822 μA μM-1 cm-2, respectively. Moreover, the LODs were both 0.007 μM at signal to noise ratio of 3 (abbreviated as S / N=3). Table 1 Comparison of various electrodes for detection of 1-NAP and 2-NAP. Electrodes MWCNTnanoPt/GCE TFaGCE PAOc/GCE

Linear range (μM) 1-NAP 2-NAP

Detection limit (μM) 1-NAP 2-NAP

Methods References

1.0-800 0.2-3.2

0.5 0.08

Derivative Voltammetry 8 DPVb 41 Linear voltammogram 2 QCM technique and chemometrics 5 Semi-derivative voltammogram 9

1.0-800 0.8-10 -

-CDd/TiO2/QCM 1.0-130 1.0-120 0.8 e P3MT nanoAu/GCE 0.7-150 1.0-150 0.1 mesoNiCo2O4/GCE 0.02-20 0.02-300 0.007 a TF: Tosflex (a perfluoro-anion-exchange membrane) b DPV: Differential pulse voltammetry

0.6 0.2 0.2 0.3

0.007 Amperometry This work d β-CD: β-cyclodextrin e P3MT: Poly (3-methylthiophene)

c PAO: Poly (acridine orange)

As is well known, it is very important to detect the existence of naphthol for environmental pollution monitoring. At present, many methods (listed in Table 1) have been developed for detection of 1-NAP and 2-NAP. These traditional methods display general detection performance, but the expensive instruments, sophisticated professional operation and time-

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consuming procedure will largely hinder their practical application. Although most of them can be achieved for high concentration detection, the LOD has not been achieved below 0.08μM. The amperometry has many advantages such as low cost, short operating time, simple operation procedure and low LOD for 2-NAP and 1-NAP. The meso-NiCo2O4 / CPE fabricated here have high sensitivity and an order of magnitude improvement for the LOD, achieving as low as 0.007 μM for the detection of naphthol. These results indicated that meso-NiCo2O4 / CPE may be used as an appropriate electrocatalyst platform for detection of naphthols, and it was owe to the excellent physicochemical properties of meso-NiCo2O4 nanospheres. 3.6 Reproducibility, stability and selectivity For investigating the reproducibility of fabricated electrode, we prepared five modified electrodes in the same process and then compared the current responses of 20 μM 2-NAP and 1NAP by amperometry under optimal conditions, respectively. After five experiments, we found that the relative standard deviation (RSD) was calculated to be 3.9% and 3.8%, respectively. The result has shown the excellent reproducibility of meso-NiCo2O4 / CPE. The stability of meso-NiCo2O4 / CPE was also evaluated by amperometry. Only 4.3% and 4.7% decreases of the current responses for 2-NAP and 1-NAP were respectively observed. After two weeks for refrigerator storage, all the response currents decreased to 95.4%. The result indicated the good stability of meso-NiCo2O4 / CPE. The meso-NiCo2O4 / CPE also exhibited excellent selectivity for NAP detection. As shown in Fig. S5, the detection of NAP (4 μM) was studied by adding 4 μM DA, AA and UA into 0.1 M PBS, resulting negligible current signal changes.

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4. CONCLUSIONS Spinel meso-NiCo2O4 nanospheres were synthesized by the nanocasting strategy followed by a calcination process. Meso-NiCo2O4 / CPE were fabricated and showed good electrocatalytic activity for detecting 2-NAP and 1-NAP, which were testified by amperometry. The advantage of the modified electrode was attributed to the large specific surface area and multiple oxidation states of meso-NiCo2O4 nanospheres. Both 2-NAP and 1-NAP detections obey the reaction mechanism of one-electron oxidation. Meanwhile the modified electrode reaction is diffusioncontrolled process. The developed electrode possesses low LOD, high sensitivity, excellent reproducibility, excellent stability, and selectivity. Therefore, this work offers a versatile strategy to build simple high performance sensors of 2-NAP and 1-NAP.

ASSOCIATED CONTENT Supporting Information SEM image of KIT-6 and bulk-NiCo2O4. N2 adsorption–desorption isotherm and Pore diameter distribution of KIT-6. Effect of the applied potential and concentration of modifier. The selectivity of meso-NiCo2O4 / CPE for NAP detection.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (J.T. Zhao)

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* E-mail: [email protected] (Y.P. Ding) * E-mail: [email protected] (X.X. Yang) ACKNOWLEDGMENT The authors thank Shanghai Municipal Science and Technology Commission (15DZ2260300), Young Eastern Scholar Project of Shanghai Municipal Education Commission (QD2015031), and the National Natural Science Foundation of China (21671132). REFERENCES 1.

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