Nonenzymatic electrochemical sensor based on CuO-TiO2 for sensitive and selective detection of methyl parathion pesticide in ground water

Accepted Manuscript Title: Nonenzymatic electrochemical sensor based on CuO-TiO2 for sensitive and selective detection of methyl parathion pesticide in ground water Authors: Xike Tian, Lin Liu, Yong Li, Chao Yang, Zhaoxin Zhou, Yulun Nie, Yanxin Wang PII: DOI: Reference:

S0925-4005(17)31966-4 https://doi.org/10.1016/j.snb.2017.10.066 SNB 23370

To appear in:

Sensors and Actuators B

Received date: Revised date: Accepted date:

25-4-2017 12-10-2017 13-10-2017

Please cite this article as: Xike Tian, Lin Liu, Yong Li, Chao Yang, Zhaoxin Zhou, Yulun Nie, Yanxin Wang, Nonenzymatic electrochemical sensor based on CuO-TiO2 for sensitive and selective detection of methyl parathion pesticide in ground water, Sensors and Actuators B: Chemical https://doi.org/10.1016/j.snb.2017.10.066 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Nonenzymatic electrochemical sensor based on CuO-TiO2 for sensitive and selective detection of methyl parathion pesticide in ground water

Xike Tian, Lin Liu, Yong Li*, Chao Yang, Zhaoxin Zhou, Yulun Nie, Yanxin Wang Faculty of Materials Science and Chemistry, China University of Geosciences, Wuhan 430074, PR China *Corresponding author. Tel.: +86 027 67884574. Email address: [email protected]

Graphical abstract:

A new nonenzymatic electrochemical sensor based on CuO-TiO2 nanocomposites has been developed for the sensitive and selective detection of methyl parathion in ground water.

Highlights: 1. 2. 3. 4.

The CuO-TiO2 nanocomposites have successfully synthetized by a simple methods. The nanoparticles showed strong affinity for methyl parathion. The dynamic detection range from 0 ppb to 2000 ppb with a detection limit 1.21 ppb. This electrochemical sensor is applicable for detecting MP in ground water samples.

Abstract: Detection of pesticides in ground water has become a very important and crucial research area due to the rapid expansion of agriculture and stringent environmental protection acts. In this paper, the as-prepared CuO-TiO2 hybrid nanocomposites were decorated on the glass carbon electrode as the nonenzymatic electrochemical nanosensor for the sensitive and selective detection of methyl parathion. The electrochemical behavior of the modified electrode was 1

investigated by cyclic voltammetry, showing that the modified electrode can be used for methyl parathion detection. Differential pulse voltammetry was applied to evaluate methyl parathion detection ability under optimized experimental conditions, and found the modified electrode can detect methyl parathion sensitively in a wide dynamic detection range from 0 ppb to 2000 ppb with a lower limit of detection (LOD) of 1.21 ppb. In addition, other interfering materials have no any influences on the detection of methyl parathion. Furthermore, the modified electrode has also been used for methyl parathion detection in the actual ground water samples, and showed efficient sensing ability. The electrochemical sensor developed would have great potentiality for methyl parathion sensing and provide new insights into the detection of other organophosphorous pesticides in the ground water. Key words: CuO-TiO2 nanocomposites; Methyl parathion; Electrochemical sensor; Differential pulse voltammetry

1. Introduction In modern society, the widespread use of persistent pesticides globally over the last decades has greatly contaminated the groundwater, resulting with serious problems in non-target species such as animals and humans, especially people living in the traditional agricultural areas where pesticides are most often sprayed to preserve the crops from pests [1, 2]. As a group of commonly used pesticides, organophosphorus pesticides (OPs) can exert irreversibly inhibitory effects toward important enzymes in animals and humans body, such as acetylcholinesterase (AChE), and usually cause important negative impacts on the visual system, sensory function, cognitive function, and nervous system even when the concentration of OPs is very low [3-6]. Due to the high toxicity and perniciousness of OPs, an important task of environmental analytical chemistry is to develop the novel and sensitive methods to detect and monitor their concentrations in waters, especially in the ground water. The traditional approaches applied for the detection of OPs in ground water are mainly liquid-solid

extraction

(LSE),

gas

chromatographic

(GC),

high-performance

liquid

chromatography (HPLC), mass spectrometry (MS) and so on [7-10]. Although these methods can detect OPs with high sensitivity and reliability, the shortcomings such as time-consuming, tedious pre-treatment procedures, high expense, and trained personnel requirement have greatly limited their filed applications, thus these detection methods may not suitable for on-site detection of OPs. So the more convenient, direct, quick and sensitive detection methods for OPs in ground water are greatly demanded in the environmental monitoring area. In recent years, electrochemical technique is a very attractive choice for OPs detection because of its unique advantages, such as low cost, on-site detection, high sensitivity, fast response et al [11-14]. Numerous enzyme-based

2

electrochemical biosensors have been designed and developed for the detection of OPs based on the modified electrodes and their inhibition on the enzymatic reaction of AChE [15-18]. However, the requirement of the environment for enzyme is greatly critical. The external conditions for enzymes usage, such as temperature, pressure, storage conditions, and enzyme activity must be strictly controlled, making it is more difficult to work with enzyme-based electrochemical biosensors for OPs detection. Considerable attentions have been focused on the development of nonenzymatic electrochemical sensors. And to modify the electrodes with as-prepared active materials which can absorb OPs onto their surfaces would make OPs detection possible with the nonenzymatic electrochemical sensors. Recently, nanoscale materials have been widely used to modify the electrodes due to their large specific surface area, small size effect, uniform pore structure, and high loading capacity [19-23]. For example, Wang et al. developed an electrochemical nonenzymatic sensor based on CoO nanoparticles for the simultaneous determination of carbofuran and carbaryl pesticides [24]. Qu et al. have modified the glass carbon electrode (GCE) with Au-TiO2/chitosan nanocomposites for the fast detection of trace OPs [25]. Du et al. also described an electrochemical stripping analysis of OPs based on SPE at ZrO2 nanoparticles modified electrode [26]. Though many nanomaterials have also been developed for OPs detection with the electrochemical methods, they still possess some intrinsic drawbacks, such as the relatively low detection sensitivity, precious metal required, and complex preparation process. Therefore, to design and prepare some effective nanomaterials to modify GCE for OPs electrochemical detection with high sensitivity and selectivity is still in need. Copper oxide (CuO), an important p-type semiconducting material with important potential applications in batteries, sensors and solar energy conversions due to its non-toxic, easily produced, inexpensive, readily stored and high specific capacitance properties [27-32]. Recently, more and more CuO based nanomaterials were developed as electrochemical sensors for micropollutants detection. Yang et al. prepared CuO nanoparticles onto multi-walled carbon arrays as the glucose sensor [33]. Huo et al. synthesized a hybrid nanocomposite consisting of CuO nanowires and single walled carbon nanotubes to modify glass carbon electrode for OPs detection [34]. That is because those CuO based nanomaterials can easily bind OPs with high affinity due to the existence of phosphate groups in OPs. In addition, titanium dioxide (TiO2) nanomaterials were also widely reported to fabricate GCE as electrochemical sensors with enhanced sensitivity, stability, and physical rigidity for their strong affinity to phosphate group, photocatalytic effect, porosity, steadiness and high specific surface areas [35-37]. So it may be an applicable choice to hybridize CuO and TiO2 nanomaterials as electrochemical sensors for OPs detection. Herein, CuO-TiO2 nanocomposites were successfully prepared via a simple and facile method,

3

and the as-prepared nanocomposites were used to fabricate GCE for the fast detection of trace OPs. Methyl parathion (MP) is a kind of typical OPs, and used to assess the sensing ability of CuO-TiO2 nanocomposites. The electrochemical characterization of modified eletrode for determination of MP via specific adsorption were evaluated by ultravioler-visible spectrum (UV-vis), cyclic voltammetry (CV) and differential pulse voltammetric (DPV). Under the optimal detection conditions investigated, the developed eletrochemical sensor has shown an excellent performance for trace MP sensing with good sensitivity, selectivity, and a lower LOD. In addition, the electrochemical sensor based on CuO-TiO2 nanocomposites modified GCE was used to detect MP in ground water, and exhibited good effectiveness. The electrochemical sensor we developed here would provide a fast, simple, sensitive, selective and on-site detection approach for OPs detection in ground water.

2. Experimental 2.1 Chemicals and instrumentation All chemicals used in this work were of analytical grade and used as received without any further purifications. Tetrabutyl titanate, copper nitrate (Cu(NO3)2), nitric acid (HNO3, >99.9%), 4-nitrobenzaldehyde, nitrobenzene were purchased from Aladdin. Methyl parathion, Nafion were purchased from Sigma-Aldrich. Ultrapure water (18.25 MΩ·cm-1) was deionized (DI) and doubly distilled prior to use. UV-vis spectra were recorded using a Perkin Elmer Lambda 35 spectrophotometer. The surface morphology of CuO-TiO2 nanocomposites was performed using field scanning electron microscopy (SEM, Hitachi S-4800). Powder X-ray diffraction (XRD) patterns were recorded by a Rigaku Multiflex diffractometer using a powder X-ray beam from Cu Kα radiation (40 kV, 30 mA), over a 2θ range from 10 to 80° at room temperature. X-ray photoelectron spectroscopy (XPS) was obtained with an MULTILAB2000 electron spectrometer from VG Scientific using 300 W Al Kα radiation. Electrochemical measurements were carried out on a CHI-660C workstation (Shanghai, China) with a conventional three-eletrode system. 2.2 Preparation of CuO-TiO2 nanocomposites CuO-TiO2

hybrid

nanocomposites

were

prepared

by

a

simple

and

facile

liquid-control-precipitation method. Firstly, 10 mL of Cu(NO3)2 was dissolved in the mixture solution of 2 mL of HNO3 and 10 mL of anhydrous ethanol, and then 10 mL of tetrabutyl titanate in 40 mL of anhydrous ethanol was slowly added into the above mixture solution with vigorous stirring. After that, the solution was stirred for 3 hours until the transparent glaucous sol was obtained. The sol was kept stewing for 6 hours under room temperature, and then dried at 70 ℃ for 36 hours in a vacuum oven. Finally, the black powder, CuO-TiO2 nanocomposites was obtained by

4

the calcination at 500 ℃ for 2 hours. Pure CuO nanomaterials were prepared following the previous procedures with the substitution of tetrabutyl titanate with ultrapure water, and TiO2 nanomaterials were also prepared following the previous procedures with the substitution of Cu(NO3)2 with ultrapure water. 2.3 Fabrication of CuO-TiO2 modified GCE 0.8 mL of deionized water, 0.2 mL of isopropanol and 30 μL of Nafion were mixed together, and then 4 mg of CuO-TiO2 nanocomposites were added with ultrasonication to form a homogeneous solution. Before the modification of electrodes, GCE (diameter 3 mm) were polished with 1 μm and 0.3 μm alumina slurries sequentially. And then the electrodes were ultrasonicated in anhydrous ethanol, deionized water, and acetone for 3 min, respectively. After that, the electrodes were washed thoroughly with ultrapure water. The electrodes were then activated by being immerged into 0.5 M H2SO4 until their performed CV curves were stabilized. Finally, 5 μL of CuO-TiO2 suspension was dropped onto the clean eletrode. After drying in the air, the CuO-TiO2 modified GCE were used for the future elecrtrochemical sensing of MP. 2.4 Eletrochemical measurements All the electrochemical experiments were performed on a CHI 660C eletrochemical workstation at ambient temperature, using a traditional three-electrode electrochemical cell with CuO-TiO2 modified eletrode as a working electrode, Ag/AgCl (3 M KCl) as a reference electrode, and graphite electrode as a counter electrode. 0.1 M of phosphate buffer solution with an appropriate pH value was applied as the eletrolyte in the whole study. CV curves were recorded between -0.4V and +0.6V potential at a scan rate of 100 mV·s-1. DPV experiments were conducted under the conditions of pulse amplitude 5 mV, pulse width of 100 ms, pulse period of 40 ms, and standing time of 30 s. 2.5 Actual sample detection In order to investigate the feasibility of CuO-TiO2 modified GCE in the actual sensing ability towards MP, the electrochemical detection of MP in the ground water samples was conducted. The ground water was obtained from Jianghan Plain, Hubei, China. The ground water samples were directly detected by DPV with CuO-TiO2 modified GCE without any extraction or preconcentration steps. The concentration of MP in ground water samples were determined from the calibration curve obtained from DPV experiments.

3. Results and discussion 3.1 Characterization of CuO-TiO2 nanocomposites CuO-TiO2 nanocomposites were successfully prepared via a simple and facile liquid-control-precipitation method. The morphological structure of CuO-TiO2 hybird

5

nanocomposites was invesrigated investigated by SEM. As shown in Fig. 1a, we can find a great number of TiO2 nanoparticles with granulates morphology homogeneously stack on the smooth surface of CuO nanosheets. And the TiO2 nanoparticles and CuO nanosheets would provide a large contact aera for subsequent interaction with MP. XRD characterization was further used to investigate the structure and the phase composition of CuO-TiO2 nanocomposites. As illustrated in Fig. 1b, different characteristic diffraction peaks of CuO nanomaterials can be perfectly assigned to (1 1 0), (0 0 2), (1 1 1), (-2 0 2), (0 2 0), (0 2 1), (-2 2 2) crystal planes, respectively. While the chararaction diffraction peaks of TiO2 nanomaterials also can also be assigned to (1 1 0), (1 0 1), (2 0 0), (1 1 1), (2 1 0), (2 1 1), (2 2 0), (0 0 2), (3 1 0), (3 0 1) crystal planes, respectively. For the XRD pattern of CuO-TiO2 nanocomposites, we can easily notice that their characterization characteristic diffraction peaks are completely comprised by the XRD patterns of CuO and TiO2 nanomaterials, indicating that the hybrid nanocomposites are successfully prepared by CuO and TiO2 precursors. And also, no appraent impurity diffraction peaks of CuO-TiO2 nanocomposites can be observed, indicating that the nanocomposites are of high crystalline quality and high purity. XPS measurements were performed to determine the composition and chemical state of as-prepared CuO-TiO2 nanocomposites (Fig. 1c and 1d). As shown in the Fig. 1c, the peaks at 464.5 eV and 458.3 eV for Ti 2p and 2p1/2 can be ascribed to TiO2 species, and the peaks at 952.7 eV, 941.9 eV and 933.8 eV in the Fig. 1d can be assigned to Cu 2p1/2, 2p3/2 and 2p3/2,spa, respectively. The XPS measurements indicated the successful preparation of CuO-TiO2 hybrid nanocomposites.

(Insert here Fig. 1) 3.2 UV-vis spectra absorption effect between CuO-TiO2 nanocompistes and MP UV-vis spectra were used to prove whether MP can be absorbed onto the surface of the modified GCE. As shown in Fig. S1, there is an obious absorbance peak at 265 nm in the presence of MP (100 ppb), which is described as the MP characteristic peak [38]. After the CuO-TiO2/GCE was immersed into MP solution for 5 min, the UV-vis spectra were obtained, and found the characteristic absorbance peak decreased, which is due to that MP molecules were absorbed onto the surface of the electrode resulting the decrease of MP concentration. This phenomenon indicates that CuO-TiO2 nanocomposites can be used to absorb MP, thus it can be served as an effective electrochemical sensor for MP detection. 3.3 Electrochemical behavior of MP on CuO-TiO2/GCE To comprehend the electrochemical behavior of different electrodes, CV measurements were conducted to investigate electrochemical behavior of different nanomaterials modified elecrodes. As shown in Fig. S2, the CVs of TiO2/GCE, CuO/GCE, CuO-TiO2/GCE in 0.1 M pH 6 phosphates

6

buffer solution in the presence of 10 ppb MP were obtained. There were no obvious peaks observed at the TiO2/GCE, indicating that TiO2 are difficult to occur redox reaction. While we can notice the strong anodic peak at +0.2 V and cathodic peak at +0.09 V for CuO/GCE, which can be ascribed to the valence changes of CuO. In additon, for the CV curves of CuO-TiO2 nanomaterials modified GCE, we can find the more prominant anodic and cathodic peaks appeared, and also these two peaks had the positive shifts. That is due to the high active surface area and good biocompatibility of TiO2 nanomaterials, which greatly enhance the electron transfer ability between CuO and the electrode. The CV curves here also demonstrated that the synergistic effect between CuO and TiO2 nanomaterials would increase the electrochemical ability of the CuO-TiO2 nanocomposites, and thus their potential electrochemical sensing ability would also enhance. Fig. 2 demonstrated the effect of electrochemical behavior of the CuO-TiO2 nanocomposites for different concentrations of MP (0, 40, 100 ppb) in 0.1 M phosphate buffer solution by CV. As shown, the anodic and cathode peaks current density simultaneously decreased with the increasing MP concentrations. It was negligible for unconspicuous peak shift in potential, which can be illustrated that the prensence of MP would hinder the electron transfer between CuO and electrode. One also can be inferred that MP had the high binding affinity with CuO-TiO2 nanocomposite, thus the barrier was formed to block the electron transfer for redox reaction of CuO.

(Insert here Fig. 2)

Based on the above experimental results and discussion, the as-prepared CuO-TiO2 nanocomposites modified GCE can be used for MP electrochemical detection. CuO-TiO2 nanocomposites possess the stronger electron transfer ability, and exhibit the higher anodic and cathic peaks current density than the CuO and TiO2 nanomaterials alone. When the nanocomposites modified GCE are used for electrochemical detection of MP, which is easily absorbed onto the large specific surface of CuO-TiO2 nanocomposites, the electron transfer ability between the nanocomposites and electrode would decrease obviously. That is because MP molecules would occupy many actice sites of the nanocomposites, and the redox reaction of CuO would be hindered, thus the anodic and cathdic peaks current density would decrease. To further investigate the redox reaction of CuO, we have calculated the number of electrons transferred, reversibility of the electron transfer, transfer coefficient. Firstly, we can know that the electrochemical behavior of CuO decorated GCE is a typical irreversible process based on the D-value calculation of oxidation and reduction peak potentials (Fig. S3a). And then we have examined the predominate of mass transport, a study was carried out using Linear Sweep

7

Voltammetray to calculate the number of electrons transferred and transfer coefficient. We can see the cathode peak of CuO-TiO2 grew with the scan rate (v) increasing (Fig. S3b). It is observed that the cathode peak potential (Ep,c) was a linear relationship to the logarithm of scan rate in the range of 50-300 mV·s-1, following the equation: Ep,c=0.3295+0.0141×lnv. According to Laviron theory [39], Ep,c = E0 – m×ln(RTKs/αnF) + m×lnv (m=RT/αnF), where α is transfer coefficient, n is the number of electrons transferred. And m has been calculated as 0.0141. According to Bard theory for irreversible process, ｜Ep – Ep/2｜= 47.7/α, α is caculated to be 0.7, and n is about 2, so we can speculate that Cu(II) would transfer to Cu(0) at the cathode in the electrochemical process. Torrents et al. has reported the hydrolysis of MP on the surface of minerals. They confirmed that the metal elements on the surface of the oxide solid (such as Al2O3) can combine with sulfonium to reduce the phosphorus atoms of electron cloud density so as to promote the hydrolysis of MP [40]. Jean et al. also depicted the divalent metal Cu ion-catalyzed hydrolysis of thionate (P=S) organophosphorus pesticides according to that the metal ion coordinates with thionate sulfur to enhance the electrophilicity of the phosphorus electrophilic site [41]. Based on these previous reports, we inferred that CuO can interact with MP through a coordination effect, and the electrochemical sensing of MP by modified GCE is mainly following the subsequent three steps: (1) The electrochemical behavior occurred on the nanocomposites decorated electrode; (2) MP molecules were adsorbed onto the electrode surface through the coordination effect; (3) The peak current decreases after MP adsorption, thus MP can be detected through the electrochemical method. And the detailed sensing mechanism of CuO-TiO2 nanocomposites modified GCE for electrochemical detection of MP is shown in Fig. 3.

(Insert here Fig. 3)

3.4 Optimization detection conditions for MP by CuO-TiO2/GCE Different experimental conditions would influence the MP detection ability by CuO-TiO2/GCE electrochemical sensor, and herein, the important operational parameters, such as the loading capacity of CuO-TiO2, pH values of the phosphate buffer solution, the molar ratio of CuO to TiO2, and the time for MP enrichment have been investigated to obtain the optimal detection conditions. Fig. S4 presents the effect of the total loading capacity of CuO-TiO2 composites on the

8

response of the modified electrode to 100 ppb MP. As shown, the current density increased when the loading capacity of CuO-TiO2 reached to 10 μL from 5 μL, the current density increasedwhich was due to the efficient surface of electrode increasing. And there were nearly almost was no changes when the loading capacity of CuO-TiO2 reached 15 μL because of the efficient surface reaching saturation. And the active real surface was calculated to illustrate the changes. The active surface area A of GCE was calculated by the redox probe ferricyanide ions following the Randles-Sevcik equation: ip = kn3/2AD1/2ν1/2c, where ip is the peak current, n is the number of electrons transferred, D is the diffusion coefficient of ferricyanide ion, ν is the scan rate, and c is the concentration of ferricyanide ions. As seen from Fig. S5, the peak current ip was a linear relationship to the square root of scan rate in the range of 100-800 mV·s-1, and the slope was determined as 1.1027×10-4. The active surface was calculated to be 5.1558×10-4 cm2 which is smaller than the surface area of electrode of 7.0651×10-4 cm2 determined by using the diameter of 3 mm. For the CuO-TiO2 modified electrode, the active surface area was determined by comparing oxidation peak current of the bare GCE with the modified electrode. The active surface areas of the modified electrode with 5 μL, 10 μL and 15 μL of CuO-TiO2 were calculated to be 4.6453×10-4 cm2, 5.0397×10-4 cm2 and 5.0459×10-4 cm2, respectively. Then it is reasonable that the optimum loading capacity is 10 μL because the active surface area almost did not change with the further increase of loading capacity after 10 μL, and the active surface area of 5.0397×10-4 cm2 was used for calculating the current density in the following discussions. The pH effect on the response of CuO-TiO2 modified GCE electrochemical sensor to 40 ppb MP in 0.1 M phosphate buffer solution is shown in Fig. S6. As shown, the current density increased with pH values increasing until arriving at a maximum value of pH 6.0, and then the current density decreased when the pH value still increasing from 6.0 to 6.6. This kind of phenomenon is due to that the organophosphates compounds and H+ are both needed in the irreversible reduction, but the structure of the electrode can be destroyed if the acidity is too low. Therefore, 0.1 M phosphate buffer solution of pH 6.0 was used for the following electrochemical detection experiments. The effect of molar ratios of CuO to TiO2 in CuO-TiO2 nanocomposites varied from 1:10, 1:5, 1:1, 5:1 to 10:1 was also studied on the response of the modified electrode to 10 ppb MP solution (Fig. S7). The electrochemical performances of the nanocomposites modified GCE obviously enhanced with the CuO content increasing until the molar ratio of CuO to TiO2 is 1:1. When the molar ratio of CuO to TiO2 is higher than 1:1, the peak current density weakened greatly due to the the electronic conduction ability would be reduced with the decrease of TiO2. Thus a molar ratio of CuO to TiO2 of 1:1 with the highest anodic peak at +0.20V and cathodic peak at +0.09V has been selected for the subsequent experiments. Except the pH value and molar ratios, the enrichment time is also an important parameter to influcence the detection

9

sensivity due to that the detection prosess is a typical process of adsorption MP onto the modified electrode. As shown in the Fig. S8, the current density initially increased and reached a maximum at 300 s. With the time prolonging, the current density had no any further changes, indicating the adsorption of MP onto CuO-TiO2 nanocomposites reached a saturation after 300s. Therefore, 300s was selected as adsorption time for following detection experiments. 3.5 Differential pulse voltammetric determination of MP Under the above investigated optimal experimental parameters, CuO-TiO2/GCE was used to detect MP with DPV in this work. Fig. 4a displayed the DPV of the CuO-TiO2/GCE in 0.1 M pH 6 phosphate buffer solution at a potential range from 0 V to 0.6 V in the presence of different MP concentrations. As shown, the current density at a well reduction peak of 0.170 V decreased with the MP concentation increasing from 0 ppb to 2000 ppb. It indicated that the MP can hinder the electron transfer ability between CuO-TiO2 nanocomposites and electrode, thus the current density decreased accordingly. The corresponding calibration plot (ΔI/I0 vs. CMP) was shown in Fig. 4b. We can find the plot increased gradually with the concentration of MP increasing till saturation at a higher concentration. And I0 is the peak current density in the presence of 0 ppb MP by DPV, ΔI is the decrease value of peak current density between I0 and the value in the presence of MP with different concentrations. The trend of ΔI/I0 changes with MP concentrations comforms to the Langmuir isothermal theory, and the formula ΔI/I0 = 22.7287CMP^0.2531/(1+0.1505CMP^0.2531) (R2=0.9875) was used to fit the calibrtion curves. A liner fit for the liner range from 10 ppb to 500 ppb was presented in the inset of Fig. 4b, and the corresponding fitting equation was ΔI/I0 = 0.0412CMP + 43.7754 (R2=0.9796). To better understand the effect of CuO and TiO2 in the CuO-TiO2 nanocomposites, we have conducted the same tests with CuO decorated GCE. As shown in Fig. S9a, the according peak current densities of CuO decorated GCE in the absence and presence of MP were obviously weaker than those of CuO-TiO2 decorated GCE, and we noticed that when the concentration of MP is 2000 ppb, the adsorption of MP is 60% for CuO/GCE (Fig. S9b), while the CuO-TiO2/GCE can absorb 80% of MP, indicating that the existence of TiO2 in the nanocomposites can enhance the affinity of CuO and MP due to the high active surface area and good biocompatibility. And the LOD of MP by the CuO-TiO2 decorated electrode can be determined to be 1.21 ppb according to the equation: LOD = 3σ/S, where σ is the standard deviation of the blank, S is the slope of the calibration line. It is much lower than other electrochemical sensors reported, such as 13 ppb of carbon paste electrode [42], 29.17 ppb of gold 10

nanoparticles-multiwalled carbon nanotubes modified glassy carbon electrode [43], 3.30 ppb of AChE/ZrO2/Chitston composites film modified electrode [44], 2 ppb of Au-polypyrrole interlaced network-like nanocomposites acetycholinesterase sensor [45] et al. In addition, some other nonenzymatic nanosensors also have the lower LOD compared with the sensor we reported, such as 0.1 ppb for CuO nanowires-SWCNTs nanocomposite decorated electrode [34], 0.5 ppb for Au-TiO2/chit modified sensor [25]. Such the excellent sensing sentivity comparing with other electroochemical sensors could be attributed to the strong affinity of CuO and TiO2 in the nanocomposites, and the optimal synergistic effect between the two nanomaterials. And the electrochemical nanosensor we developed here would provide a new approach for OPs, such as MP detection with high sensitivity and wide linear concentration range.

(Insert here Fig. 4)

Reproducibility and stability are the important parameters to investigate the actual ability of the electrochemical sensors. Successive measurements using CuO-TiO2/GCE for MP detection in 0.1 M pH 6.0 phosphate buffer solution have been conducted. The relative standard deviation was 2.9% for eight replicate determinations of 10 ppb MP, indicating the developed sensor possess a good reproducibility. Long term storage stability of the sensor has also been studied when the sensor is stored in ultrapure water at room temperature for several days. We find the current density has no siginificant changes after 3 days, and the current responses decreased only by 9.2% for the following half of month, indicating the electrochemical sensor is of a high stability due to their good chemical and thermal stability. Detection selectivity is another important parameter to assess the developed electrochemical sensor.

As

known,

the

interferring

electroactive

nitrophenyl

molecules,

such

as

4-nitrobenzaldehyde (PNT), nitrobenzene (NB) and inorganic ions, such as PO43-, SO42-, NO3-, Fe2+, Ni2+ and K+ may coexist with MP in water. And other pesticides, such as trichlorphon (Tri), caeberidazim (CB) and carbaryl (Car) belong to the major categories of pesticides which were organophosphorus and carbamate pesticides, while MP belongs to containing surful group oganophosphorus, so it is important to conduct the MP selective detection experiments. DPV measurements were performed in 0.1 M pH 6.0 phosphate buffer solution with 100 ppb MP and other pesticides, and 1000 ppb other interferences. As shown in Fig. 5a, ΔI values almost have no changes when MP coexists with other interference, indicating the interferences have no disturbances on the electrochemical detection of MP. Fig. 5b shows that ΔI in the prensence of MP

11

is greatly higher than that of other interferences. Annd other interfering pesticides do not generate significant interference on MP detection, indicating there exists a special coordination between CuO and S=P. All these experiments demonstrate the good selectivity of the sensor on MP electrochemical detection.

(Insert here Fig. 5)

3.6 Actual sample detection The reliability and practicality of the developed electrochemical sensor were carefully studied by sensing MP in the ground water with different concentrations of MP. The ground water is obtained from Jianghan Plain, Hubei, China. The ground water samples were firstly filtered through a filter paper (D = 0.45 μm) to remove all the solid impurities, and then the real water samples were spiked with different amounts of MP (40, 80, 160 and 200 ppb). Each concentration of MP was tested for several times, and the average value was presented with standard deviation. DPV test results showed that the current density gradually decreased with the various concentrations of MP spiked into ground water samples and then their concentrations were determined according to the calibration fitting curves. The detailed experimental results are shown in Table 1. The concentrations of MP detected by the above mentioned method were consistent with the added values and the quantitative spiked recoveries of MP range from 98.80% to 106.70% in the real water samples. And the low relative standard deviations (RSDs), ranging from 2.13% to 3.45%, confirmed the accuracy of this method, showing that the electrochemical sensor has a good reliability for MP detection in real ground water samples.

(Insert here Table 1)

4. Conclusion Herein, a new kind of CuO-TiO2 hybrid nanocomposites was prepared via a simple and facile method, and the as-synthesized nanocomposites were decorated on the glass carbon electrode as the nonenzymatic electrochemical nanosensor for the highly sensitive and selective detection of a typical OPs pesticide, MP. Experimental results indicated that CuO in the nanocomposites was proved to possess strong affinity for MP, and the existence of TiO2 would enhance the electron transfer ability between CuO and the electrode, thus their electrochemical detection ability can be greatly strengthened. The electrochemical behavior of the nanocomposites modified electrode was investigated by cyclic voltammetry and electrochemical impedance spectroscope. And also, the optimal detection conditions, such as pH, molar ratio of CuO to TiO2, and enrichment time were

12

determined for the MP detection. Differential pulse voltammetry measurements were used to evaluate MP detection ability and showed that the modified electrode can detect MP sensitively in a wide dynamic detection range from 0 ppb to 2000 ppb with a lower LOD of 1.21 ppb. In addition, other interfering materials, such as 4-nitrobenzaldehyde, nitrobenzene, PO43-, SO42-, NO3-, Fe2+, Ni2+, K+ and other pesticides have no any influences on the detection of MP. And also experimental studies indicated that the electrochemical sensor possesses good stability and reproducibility for MP detection. Finally, the hybrid nanocomposites modified electrode has been used for MP detection in the actual ground water samples and showed efficient sensing ability. These results presents the developed CuO-TiO2 hybrid nanocomposites sensor is the attractive candidate for fast, simple, sensitive analysis of OPs in ground water.

Ackowlegements This work was supported by the National Natural Science Foundation of China (No. 41773126) and the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (No. 41521001) and International Science & Technology Cooperation Program of China (2014DFA20720) and the “Fundamental Research Funds for the Central Universities”.

Appendix A. Supplementary date The supporting figures, such as UV-vis sepctra and experimental conditions optimazation et al. are in the Supporting information file.

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on

acetylcholinesterase

immobilized

onto

Au-polypyrrole

nanocomposites, Biosens. Bioelectron. 24 (2009) 2285.

17

interlaced

network-like

Author Biographies: Xike Tian is a Professor at the Faculty of Material Science and Chemistry, China University of Geosciences (Wuhan). His current research interests include optical detection and prevention of hazardous substances in the environment. Lin Liu is currently pursuing her master degree in Faculty of Material Science and Chemistry, China University of Geosciences (Wuhan). And her current research interests focused on the design of nonenzymatic electrochemical sensor for organophosphorus pesticides detection. Yong Li is a Lecturer at Faculty of Materials Science and Chemistry, China University of Geosciences (Wuhan). He mainly focuses on the optical detection of environmental pollutants. Chao Yang is an Associate professor at Faculty of Materials Science and Chemistry, China University of Geosciences (Wuhan). His research focused on optical detection technology of environmental pollutants. Zhaoxin Zhou is an Associate professor at Faculty of Materials Science and Chemistry, China University of Geosciences (Wuhan). His research focused on optical detection technology on pollutant in the environment and oilfield sewage disposal. Yulun Nie is a professor at Faculty of Material Science and Chemistry, China University of Geosciences (Wuhan). His research focused on the identification of refractory organic pollutants and their safe transformation and degradation. Yanxin Wang is a Professor of Environmental Hydrogeology and the President of China University of Geosciences at Wuhan. His research has been focused on hydrogeochemistry and groundwater contamination.

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Figure and table captions: Fig. 1 (a) SEM image of CuO-TiO2 nanocomposites; (b) XRD patterns of CuO, TiO2, and CuO-TiO2 nanocomposites; (c) Ti 2p XPS of CuO-TiO2 nanocomposites; (d) Cu 2p XPS of CuO-TiO2 nanocomposites. Fig. 2 The CVs of CuO-TiO2/GCE in the presence of different MP concentrations (0, 40, 100 ppb) at a scan rate of 100mV·s-1. Fig. 3 The schematic electrochemical sensing mechanism of CuO-TiO2/GCE for MP pesticides. Fig. 4 (a) DPV of CuO-TiO2/GCE in 0.1 M pH 6 phosphate buffer solution with different MP concentrations from top to down , 0, 1, 2, 5, 10, 20, 50, 80, 100, 150, 200, 300, 400, 500, 800, 1000, 1500, 2000 ppb, respectively; (b) The corresponding calibration curve with Langmuir adsorption fitting curve (the inset is the liner relevant fit). Fig. 5 (a) Different pulse voltammetric experiments were performed with 0.1 M pH 6 phosphates buffer solution containing 100 ppb MP, Tri, CB, Car, 1000 ppb PNT, NB, PO43-, SO42-, NO3-, Fe2+, Ni2+ and K+, respectively. (b) The current density change with interferences ions and MP, respectively. Table 1 Recovery studies of spiked MP in ground water samples.

19

Fig. 1 (a) SEM image of CuO-TiO2 nanocomposites; (b) XRD patterns of CuO, TiO2, and CuO-TiO2 nanocomposites; (c) Ti 2p XPS of CuO-TiO2 nanocomposites; (d) Cu 2p XPS of CuO-TiO2 nanocomposites.

20

Fig. 2 The CVs of CuO-TiO2/GCE in the presence of different MP concentrations (0, 40, 100 ppb) at a scan rate of 100mV·s-1.

21

Fig. 3 The schematic electrochemical sensing mechanism of CuO-TiO2/GCE for MP pesticides.

22

Fig. 4 (a) DPV of CuO-TiO2/GCE in 0.1 M pH 6 phosphate buffer solution with different MP concentrations from top to down, 0, 1, 2, 5, 10, 20, 50, 80, 100, 150, 200, 300, 400, 500, 800, 1000, 1500, 2000 ppb, respectively; (b) The corresponding calibration curve with Langmuir adsorption fitting curve (the inset is the liner relevant fit).

23

Fig. 5 (a) Different pulse voltammetric experiments were performed with 0.1 M pH 6 phosphates buffer solution containing 100 ppb MP, Tri, CB, Car, 1000 ppb PNT, NB, PO43-, SO42-, NO3-, Fe2+, Ni2+ and K+, respectively. (b) The current density change with interferences ions and MP, respectively.

24

Table 1 Recovery studies of spiked MP in ground water samples.

Spiked concentrations

Found (ppb)

Recovery (%)

RSD (%)

40

42.69

106.72

3.45

80

81.70

102.13

3.09

160

162.68

101.61

2.91

200

197.77

98.80

2.13

(ppb)

25