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A voltammetric sensor for diazinon pesticide based on electrode modified with TiO2 nanoparticles covered multi walled carbon nanotube nanocomposite
Journal of Electroanalytical Chemistry 807 (2017) 1–9
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A voltammetric sensor for diazinon pesticide based on electrode modified with TiO2 nanoparticles covered multi walled carbon nanotube nanocomposite Javad Ghodsi, Amir Abbas Rafati
MARK
⁎
Department of Physical Chemistry, Faculty of Chemistry, Bu-Ali Sina University, P.O. Box 65174, Hamedan, Iran
A R T I C L E I N F O
A B S T R A C T
Keywords: Voltammetric sensor Diazinon MWCNTs/TiO2NPs nanocomposite Synergic electrocatalytical reduction
This work describes the development of a voltammetric sensor for diazinon pesticide determination based on diazinon reduction on glassy carbon electrode (GCE) surface modified with multi walled carbon nanotubes covered by TiO2 nanoparticles (MWCNTs/TiO2NPs). Voltammetric investigations were carried out by cyclic voltammetry (CV), linear sweep voltammetry (LSV), differential pulse voltammetry (DPV) and square wave voltammetry (SWV). Prepared nano-composite of MWCNTs/TiO2NPs was characterized with scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy dispersive X-ray analysis (EDX) techniques. Nanocomposite of MWCNTs and TiO2NPs showed suitable synergic electrocatalytical properties in diazinon reduction which resulted the completely sensitive determination of diazinon in laboratory samples and real samples including city piped water and agricultural well water. Linear range, limit of detection (LoD) and limit of quantitation (LoQ) obtained by developed sensor were 11–8360 nM, 3 nM and 10 nM successively which were quite satisfying in compared to other works reported about diazinon determination. Response time, Repeatability and stability of sensor were examined and found completely suitable. Moreover electrode modification method was very fast, inexpensive and easy to follow.
1. Introduction Various pesticides are commonly used for the augmentation of food production. The increasingly applying of pesticides in public health and agriculture has caused important environmental pollution and potential health risk and therefore is cause of concern. Organophosphates (OPs), highly used class of pesticides, are very dangerous and detrimental because of their noxious nature. These pesticides have their own toxic effects by irreversible inhibition of the enzyme acethylcholinesterase (AChE) which is an essential enzyme for the function of central nervous system [1–3]. diazinon (O,O-diethylO-2-isopropyl6-methylpyrimidin-4-yl phosphorothioate), as an organophosphorus insecticide and an extremely toxic insecticide, often used widely on cultivation of various types of crops such as apple, rice, grapes, pistachios, palm, citrus and pear. Diazinon is used to control a wide range of pests such as, cockroaches, mites, saprophytes, chewing insects, fleas and so on. Diazinon is also poisonous for aquatics, mammals, and some non-target insects. High doses of diazinon can cause muscle tremors, nausea, dimness of vision and difficult breathing. It can cause harmful effects on skin, lungs,
⁎
gastrointestinal tract and nervous system. Moreover, diazinon has been identified as a potential chemical mutagens and it is proved that diazinon is highly toxic to fish and water organisms [2–6]. According to what was explained qualitative and quantitative analysis methods for OPs and especially for diazinon (as an important OP model) is very serious for human health and environmental control. Common techniques applied for OPs detection are gas chromatography and high-performance liquid chromatography (HPLC), mass spectrometry method, spectrophotometry, infrared spectroscopy and an enzyme immunoassay [1,3,7]. Though these methods are suitably sensitive and highly reproducible and show low detection limits, because of some problems such as very expensive techniques, time-consuming methods, complicated laboratory equipment, low speed analysis under field conditions and requiring trained man power, developing of new, simple, inexpensive and sensitive quantifying methods is still a critical subject in analytical chemistry [3,5,8]. For solving above mentioned disadvantages of OPs determination methods, some other approaches have been reported, such as multi syringe flow injection analysis (MSFIA), bioanalytical methods based on enzyme inhibition and immunoassay and electrochemical methods [1,8]. Among the all reported methods,
Corresponding author. E-mail address: [email protected] (A.A. Rafati).
https://doi.org/10.1016/j.jelechem.2017.11.003 Received 3 August 2017; Received in revised form 27 October 2017; Accepted 1 November 2017 1572-6657/ © 2017 Elsevier B.V. All rights reserved.
Journal of Electroanalytical Chemistry 807 (2017) 1–9
J. Ghodsi, A.A. Rafati
Fig. 1. SEM image of MWCNTs/TiO2NPs nano-composite on GCE surface.
O K• 1500
Fig. 2. EDX analysis of MWCNTs/TiO2NPs nanocomposite on GCE surface.
TiK•
C K•
TiL• N K•
1000
500
TiK•
0
keV 0
5
10
new nano materials for application in fields of electrochemical sensors, owing to unparalleled unit surface, the high sensitivity of the electronic properties of nanotubes to molecules adsorbed on their surface and their interesting electrical, mechanical and catalytical properties [9–11]. Titanium dioxide (TiO2), n-type semiconductor and one of the most important transition metal oxides, is the common semiconductor applied for the oxidation or reduction of inorganic and organic species [12,13]. Its nontoxicity, long-term stability, low cost and other remarkable chemical and physical properties, make it suitable for various applications such as photocatalysis, photo-chromic devices,
those that are based on electrochemical techniques have been more interested because of their several advantages such as ease of use, cost effectiveness, fast measurement, high sensitivity and capable of application in field conditions and so on [8]. As far as we know, up to now a few electrochemical methods have been reported for determination of diazinon such as nafion modified glassy carbon electrode, biosensors and DNA-composed carbon nanotube electrode [2]. This work provides a novel voltammetric sensor based on electrode modification with MWCNTs/TiO2NPs which supplies a simple, sensitive and easy to fabricate sensor. Carbon nanotubes (CNTs) are promising 2
Journal of Electroanalytical Chemistry 807 (2017) 1–9
J. Ghodsi, A.A. Rafati
16
2. Experimental 25.28°
14
2.1. Materials
Intensity
12
MWCNTs with the average diameter of 10–40 nm were purchased from Neutrino Co. (Iran). Tetraethyl orthotitanate [(C₂H₅O)₄Ti] and sodium dodecyl sulfate (SDS) were purchased from Sigma-Aldrich and were used as received. Acetone was from Merck. Diazinon (O,O-diethylO-(2-isopropyl-6-methylpyrimidin-4-yl)-monothiophosphate) was purchased from Kavosh Kimia Co. (Iran) and was used as received. Titanium (IV) sulfate (Ti(SO4)2) and H2O2 were from Sigma-Aldrich. All other chemicals were of analytical grade. Phosphate buffers (PB) (50 mM) were prepared from H3PO4, NaH2PO4 and Na2HPO4 and pH values were adjusted by HCl and NaOH solutions. All phosphate buffers used in the experiments were in pH 7, unless otherwise stated. The solutions were prepared using deionized water and deoxygenated by bubbling high purity (99.99%) nitrogen gas through them for 15 min prior to the experiments. All experiments were carried out at room temperature.
10 8 6 4 27.42°
2 0 15
25
35
45
55
65
75
2θ Fig. 3. XRD spectrum of MWCNTs/TiO2NPs nano-composite on GCE surface.
electrochemical sensors, electronic devices and catalytic applications and so on. Also due to the high affinity of nano TiO2 on phosphate groups, it can be used in electrochemical sensor development for organophosphate pesticides like as fenitrothion and parathion [13–16]. CNTs coated with metal oxides hybrids will lead to new composite materials which are expected to show different physical properties than those of each neat component or even a synergistic effect. These materials have received great interest in recent years because of their exclusive electrical, optical, mechanical and thermal properties, so enabling the application in catalytic and electrochemical technologies [9,13,14]. CNTs/titanium dioxide (TiO2) composite materials have attracted many attentions in various research fields. It was verified incorporation of MWCNTs with nanostructured TiO2 significantly improved electrocatalytic properties for sensor application [14,17].
H3C
N
Voltammetric measurements were carried out using a three-electrode system, including the modified electrode (GCE/MWCNTs/ TiO2NPs) as working electrode, an Ag/AgCl (3.0 M KCl) as reference electrode, and a platinum foil as counter electrode. Voltammetric measurements were carried out using a computer-controlled μ-Autolab modular electrochemical system (PGSTAT101, the Netherlands), driven with NOVA Software (upgrade 1.10). All electrochemical experiments in current work were performed by CV, LSV, DPV and SWV techniques. X-ray powder diffraction (XRD) spectra were taken on a Bruker D8advance x-ray diffractometer with Cu Kα radiation. Size, morphology and percent elemental composition of MWCNTs/TiO2NPs were investigated by using field emission scanning electron microscopy (FESEM, TSCAN, and S-300000).
CH3
H3C
N
S
CH3
O
N
S
CH3
P
O O
CH3
HN
2e- + 2H+
P H3C
2.2. Apparatus
H3C
CH3
O
O O
CH3
2e- + 2H+
H3C
CH3
HN
NH
S
CH3
P H3C
O
O O
Scheme 1. Schematic representation of two successive electrochemical reduction of diazinon at electrode surface.
3
CH3
Journal of Electroanalytical Chemistry 807 (2017) 1–9
J. Ghodsi, A.A. Rafati
2.3. MWCNTs/TiO2NPs nanocomposite synthesis
2.4. GCE/MWCNTs/TiO2NPs preparation
Prior to MWCNTs/TiO2NPs nanocomposite synthesis, MWCNTs were refluxed by HCl 0.5 M for 2 h for removing impurities. Then MWCNTs/TiO2NPs nanocomposite was synthesized as reported in ref. [18]. Summarily 0.1 g of HCl treated MWCNTs was dispersed in acetone/tetraethyl orthotitanate solution by ultrasonication then 0.04 g of SDS, as a shape controller, was further added. Then, hydrolysis was initiated by adding some deionized water in to the mixture; freshly formed MWCNTs/TiO2 nanocomposites filtered and dried in a vacuum oven at 77 °C for at least 20 h; Finally, MWCNTs/TiO2 nanocomposites powder was heated at a heating rate of 1 °C/min to at temperature of 400 °C in an oven under ambient condition, maintaining at this temperature for 30 min.
GCE (diameter 3 mm) was polished with alumina slurry and followed by ultrasonically cleaning with ethanol/double distilled water mixture and then dried with acetone. 10 mg of MWCNTs/TiO2NPs nanocomposite was ultrasonicated in 10 mL acetone and 10 μL of resulted MWCNTs/TiO2NPs/acetone mixture was dropped on GCE surface and dried at room temperature.
2.5. GCE/electrodeposited TiO2NPs preparation To better represent the MWCNTs/TiO2NPs nanocomposites role in electrocatalytic reduction of diazinon, we also compared GCE/ MWCNTs/TiO2NPs response with GCE/electrodeposited TiO2NPs in
Fig. 4. SW voltammograms of 2 μM diazinon in buffer obtained by a) GCE, b) GCE/MWCNTs, c) GCE/TiO2NPs and d) GCE/MWCNTs/ TiO2NPs as working electrodes.
10 9 8 7
Current (µA)
6
(d)
5
(c)
4
(b) 3
(a)
2 1 0 -0.4
-0.6
-0.8
-1 Potential (V)
-1.2
-1.4
-1.6
Fig. 5. CVs of 10 μM diazinon in buffer obtained by a) GCE, b) GCE/ MWCNTs, c) GCE/TiO2NPs and d) GCE/MWCNTs/TiO2NPs as working electrodes.
Potential (V) 40 -1.6
-1.4
-1.2
-1
-0.8
-0.6
-0.4 -10
(a) -60
-110
(c) -160
-210
(d) -260
-310
4
Current (µA)
(b)
Journal of Electroanalytical Chemistry 807 (2017) 1–9
J. Ghodsi, A.A. Rafati
3. Results and discussion
addition to other electrodes. For TiO2NPs electrodepositing on GCE surface, Firstly GCE was polished and cleaned like as above mentioned; then it was immersed into electrolyte consisting of 3 M KCl, 10 mM H2O2 and 10 mM Ti(SO4)2 and constant potential of −0.1 V was applied for 30 min [14]. Modified GCE was washed with double distilled water tree times and dried at room temperature.
3.1. Characterization of GCE/MWCNTs/TiO2NPs with SEM, EDX and XRD Fig. 1 shows the SEM image of GCE/MWCNTs/TiO2NPs. As can be seen the walls of the nanotubes are densely entirely covered with TiO2NPs and rarely TiO2NPs apart from walls are observed. Nanoscale
Fig. 6. SW voltammograms of GCE/MWCNTs/TiO2NPs in buffer (a) and buffer containing 4 μM diazinon (b).
14 12
Current (μA)
10 8
(b)
6
(a) 4 2
-0.4
0
-0.6
-0.8
-1
-1.2
-1.4
-1.6
Potential (V)
Fig. 7. (a) SWV, (b) DPV, (c) CV and (d) LSV response of developed sensor to 2 μM diazinon.
Potential (V) 15 -0.3
-0.5
-0.7
-0.9
-1.1
-1.3
-1.5
10
(a) 5
(b)
Current (µA)
0 -5
(d)
(c)
-10 -15 -20 -25 -30 -35 5
-1.7
Journal of Electroanalytical Chemistry 807 (2017) 1–9
J. Ghodsi, A.A. Rafati
16
(a)
(b)
14
10
8360 nM Current (µA)
10
Current (µA)
y = 1.1753x + 2.7349 R² = 0.9964
12
12
8 6
8 6
11 nM
4
Fig. 8. a) SW voltammograms for successive addition of diazinon to phosphate buffer in concentration range of 11 nM–8360 nM diazinon and b) Its resulted standard calibration curve.
14
4 2
2 0 -0.5
0
-0.8
-1.1
-1.4
0
2
4
6
8
10
Diazinon / µM
Potential (V)
(a)
4
6
pH
Fig. 9. a) SW voltammograms of 2 μM diazinon in pHs of 4–9 (only first diazinon reduction peak is showed); b) linear plot for diazinon potential reduction with pH and its corresponding linear regression equation and c) diazinon first reduction peak current vs. pH.
9
Current (µA)
5 4 3 2 1 -0.3
-0.5
-0.7
-0.9
-1.1
-1.3
Potential (V)
pH
(b)
(c)
-0.4
3
5
7
-0.6 -0.7 -0.8 -0.9 -1 -1.1
9
4 Reduction current (µA)
Reduction potential (V)
-0.5
y = -0.0866x - 0.2056 R² = 0.9885
4.5
3.5
3
2.5
2
3
5
7
9
pH
diameters of some MWCNTs/TiO2NPs are detected in the figure. GCE/ MWCNTs/TiO2NPs surface also was analyzed by EDX and respective result is showed in Fig. 2. As figure shows and expectedly, the nanocomposite is mainly composed of Ti, C and O elements. XRD pattern of the MWCNTs/TiO2NPs on GCE surface was obtained using Cu Kα radiation and showed in Fig. 3. The (101) peak (2θ = 25.28°) of anatase and the (110) peak (2θ = 27.42°) of rutile can be seen in this pattern. Regarding to these two peak intensities, most of TiO2NPs deposited on MWCNTs walls have anatase structure.
3.2. Electrochemical investigations In this work diazinon was measured with electrocatalytically reduction on MWCNTs/TiO2NPs nanocomposite surface. Diazinon can undergo two successive reductions according to Scheme 1. For better verifying MWCNTs/TiO2NPs nanocomposite role in electrocatalytically reduction of diazinon than each neat of MWCNTs and TiO2NPs, SWV response of different electrodes to a fixed amount of diazinon were investigated and obtained results were showed in Fig. 4. Fig. 4 shows 6
Journal of Electroanalytical Chemistry 807 (2017) 1–9
J. Ghodsi, A.A. Rafati
(a)
0
20 -1.2
-1
-0.8
-0.6
0
-0.4 0
-10
-20
-20
-40
10 mV. s-1
-60
scan rate
-80
4430 mV. s-1
Current (µA)
-1.4
Current (µA)
-1.6
(Potential scan rate)1/2 / V1/2.s-1/2
(b)
Potential (V)
0.5
1
1.5
2
2.5
-30 -40 -50
-100 -120
-60
-140
-70
y = -27.874x - 0.3812 R² = 0.9961
Fig. 10. a) CVs of 3 μM diazinon in potential scan rates of 10–4430 mV/s and b) Linear plot for reduction peak current of diazinon vs. square root of potential scan rate.
LSV, DPV, CV and SWV response of developed sensor to a 2 μM diazinon were compared in Fig. 7. SWV, DPV, CV and LSV voltammograms are labeled as a to d successively. As the figure shows, most sensitivity of sensor is corresponded to when SWV technique was applied. Therefore diazinon calibration curve and next diazinon determination in real samples was obtained by applying SWV technique. Fig. 8a shows the SWV voltammograms for successive addition of diazinon to phosphate buffer which resulted the standard calibration curve (Fig. 8b) in really vast concentration range of 11 nM–8360 nM. Also LoD and LoQ were obtained as 3 nM and 10 nM which are calculated by 3sb/m and 10sb/m successively. Where sb is the standard deviation of the blank and m is the slope of calibration curve [3]. pH effect on sensor response also was investigated. Fig. 9a shows the SWV voltammograms of 2 μM diazinon in pHs of 4–9 but to avoid sloppy voltammograms, only first diazinon reduction peak is regarded. As can be seen diazinon reduction potential negatively shifts when pH increases from 4 to 9. Linear plot for diazinon reduction potential with pH and its corresponding linear regression equation is shown in Fig. 9b. As voltammograms of Fig. 9a show, reduction current of diazinon increases with pH to 6 and then decreases with more pH increasing to 9. Fig. 9c represents reduction current vs. pH. This increasing and decreasing regime for reduction current with pH can be explained as follows: in high pHs, lack of proton is a barrier to diazinon reduction (proton was consumed during diazinon reduction as showed in Scheme 1) and in low pHs hydrogen generation due to the decomposition of water became the strong hurdle against the electrochemical reduction of diazinon. As another effective parameter, potential scan rate effect on sensor response to diazinon was also studied. Fig. 10a shows CV voltammograms of 3 μM diazinon in potential scan rates of 10–4430 mV/s. As voltammograms show, reduction peak current of diazinon was increased with scan rate and also reduction potential shifts to more negative ones. Fig. 10b shows linear plot for reduction peak current of diazinon vs. square root of potential scan rate which resulted from CVs of Fig. 10a. Regarding to linear proportional of reduction current vs. square root of scan rate, it can be concluded diazinon reduction at electrode surface is mainly controlled under the diffusion step. To investigate the applicability of developed sensor in real samples, two real water samples including agricultural well water and city piped water were tested with this sensor by standard addition method. Water samples freshly received and were spiked with three amounts of diazinon (1, 2 and 3 μM). Obtained Results are showed at Table 1. As Table 1 shows, relative percentage of recovery amounts are between 97.5 and 105.5 which are very satisfying.
Table 1 Diazinon determination by developed sensor in two real samples of water. Sample
Added (μM)
Expected (μM)
Found (μM) - the mean of three measured values
Recovery (%)
Well watera
1.0 2.0 3.0 1.0 2.0 3.0
1.0 2.0 3.0 1.0 2.0 3.0
1.03 1.95 3.14 1.02 2.11 3.08
103.0 97.5 104.7 102.0 105.5 102.7
Tap waterb
a b
Sampled from a well water used for agricultural in Karaj city (Iran). From city piped water of Bu-Ali University (Hamadan-Iran).
SWV voltammograms of a 2 μM diazinon in buffer obtained by a) GCE, b) GCE/MWCNTs, GCE/TiO2NPs and GCE/MWCNTs/TiO2NPs as working electrodes. Diazinon successive reduction peak currents are seen in SWV of GCE/MWCNTs/TiO2NPs at about − 0.78 V and − 0.97 V. As clearly seen, these two reduction peaks appeared at potentials which are lower than that of similar peaks in other SWVs. Furthermore SWV of GCE/MWCNTs/TiO2NPs shows significantly more sensitive response to diazinon. It's easy to deduce from SWVs of b-d that the MWCNTs and TiO2NPs have significant synergic effect in diazinon reductions at electrode surface. SWVs of curves d and c show another reduction peak at about − 1.2 V which is corresponded to reduction of Ti4 + + e− → Ti3 + corresponded to TiO2NPs deposited on MWSNTs [19]. Due to the larger reduction current of first peak, all next measurements in this work are based on the first reduction peak of diazinon. Above experiment was repeated by CV measurements and related results were showed in Fig. 5. Fig. 5 shows CVs of 10 μM diazinon in buffer obtained by a) GCE, b) GCE/MWCNTs, GCE/TiO2NPs and GCE/MWCNTs/TiO2NPs as working electrodes. It can be concluded from mentioned SWVs, lowest reduction potential and most sensitive response was corresponded to GCE/MWCNTs/TiO2NPs. It must be said, CVs in Fig. 5 were separated from each other in order to prevent overlapping voltammograms and to better comparison between CVs. As other electrochemical test, GCE/MWCNTs/TiO2NPs SWV response to bulk (phosphate buffer) and buffer containing diazinon ware compared and result was shown in Fig. 6. Fig. 6 shows the SWV voltammograms of GCE/MWCNTs/TiO2NPs in buffer (a) and buffer containing 4 μM diazinon (b). It is most probably concluded from this figure that reduction peak around − 1.2 V is mainly corresponded to Ti4 + + e− → Ti3 + reduction of TiO2NPs structures and as can be seen are quite similar in two SWVs. 7
Journal of Electroanalytical Chemistry 807 (2017) 1–9
J. Ghodsi, A.A. Rafati
8
(a)
Table 2 Comparison of our developed sensor performance with some other reported methods.
(1)
7
(2)
6 (3)
Current (µA)
5 4 3 2 1 0 -0.5
-0.7
-0.9
-1.1
-1.3
Potential (V)
Technique
Linear range (nM)
LoD (nM)
Ref.
Dispersive liquid–liquid microextraction HPLC Micro IrOx potentiometric sensor HMDEa/SWASVb c MIP modified CPE/CV and SWV Nafion coated GCE/CV, LSV and SWV GCE/MWCNTs/TiO2NPs/ CV and SWV
9.9–1971.4
3.3–6.6
[20]
657.1–6571.4 100–10,000 and 10,000–1,000,000 40–390 1–100 and 100–2000
328.6 3000
[21] [22]
11 0.786
[2] [23]
0–5000
75
[7]
11–8360
3
Present work
a b
7
(b)
c
(1)
6
(2)
which referred to complete repeatable sensor responses. Also response time of developed sensor was completely short, so that it reached 93% of its maximum signal after 5 s. Current sensor was quite stable and maintained about 89% of its first day signal after 28 days in room temperature. Finally our developed sensor performance was compared with some other works reported about diazinon determination and results were summarized at Table 2. Since it has not been reported a lot of electrochemical works about diazinon determination up to now, some of non-electrochemical works also were mentioned in Table 2. According to the table our developed sensor shows acceptable performance in compared to other reported works.
(3)
Current (µA)
5 4 3 2 1 0 -0.5
-0.7
-0.9
-1.1
Hanging mercury dropping electrode. Square wave adsorptive stripping voltammetry. Molecularly imprinted polymer.
-1.3
Potential (V) 4. Conclusions 8
(c)
7
This work reports a voltammetric sensor for diazinon which uses MWCNTs/TiO2NPs nanocomposite to modify electrode surface. Diazinon was determined by applying suitable synergic electrocatalytical effect of MWCNTs with TiO2NPs in reduction of diazinon. Developed sensor showed good sensitivity, vast linear range, low LoD and low LoQ. The sensor was successfully examined for diazinon determination in real water samples including agricultural well water and city piped water and obtained results showed acceptable recovery amounts. Stability, repeatability and response time of the sensor also were tested and found satisfying results. Current sensor performance was compared to some electrochemical and non-electrochemical works about diazinon determination and showed the good performance of our sensor. Finally developed sensor suggests a fast and easy to follow method to sensor fabrication for other important organophosphorus insecticides.
(1)
6
Current (µA)
5 4 3
(3)
(2)
2 1 0 -0.5
-0.7
-0.9
-1.1
-1.3
Potential (V) Fig. 11. SW voltammograms of 1) a sample from well water spiked with 1 μM of diazinon, 2) a sample from tap water spiked with 1 μM of diazinon and 3) phosphate buffer containing 1 μM of diazinon obtained by GCE/MWCNTs/TiO2NPs. Panels b and c show same SWVs with 2 μM and 3 μM of diazinon respectively.
Acknowledgments This work was supported by the ‘Iran National Science Foundation’ [Grant No.95004691].
Moreover the detected results from the real samples were compared with the reference method. Corresponded results were showed in Fig. 11. Fig. 11a shows SWV voltammograms of 1) a sample from well water spiked with 1 μM of diazinon, 2) a sample from tap water spiked with 1 μM of diazinon and 3) phosphate buffer containing 1 μM of diazinon obtained by GCE/MWCNTs/TiO2NPs. Fig. 11b and c show same SWVs with 2 μM and 3 μM of diazinon respectively. As can be clearly seen, SWV responses to reference sample show high accordance with SWVs of tap and well water in three above mentioned diazinon concentrations. This clearly verifies the high selectivity of developed sensor that only response to diazinon in different real samples of water. Repeatability of developed sensor was tested by 20 successive determination of a fixed amount of diazinon and relative standard deviation (RSD) of obtained diazinon amounts was calculated as 3.8%
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[5]
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988–993. [15] R. Ramachandran, V. Mani, S.-M. Chen, G. Gnana-kumar, M. Govindasamy, Recent developments in electrode materials and methods for pesticide analysis—an overview, Int. J. Electrochem. Sci. 10 (2015) 859–869. [16] A. Kumaravel, M. Chandrasekaran, Electrochemical determination of chlorpyrifos on a nano-TiO2/cellulose acetate composite modified glassy carbon electrode, J. Agric. Food Chem. 63 (2015) 6150–6156. [17] S. Abbasi, S.M. Zebarjad, S.H. Noie Baghban, Decorating and filling of multi-walled carbon nanotubes with TiO2 nanoparticles via wet chemical method, ENG 5 (2013) 207–212. [18] E. Khosravifard, M. Salavati-Niasari, M. Dadkhah, G. Sodeifian, Synthesis and characterization of TiO2-CNTs nanocomposite and investigation of viscosity and thermal conductivity of a new nanofluid, J. Nanostruct. 2 (2012) 191–197. [19] A. Kumaravel, M. Chandrasekaran, Electrochemical determination of imidacloprid using nanosilver Nafion®/nanoTiO2 Nafion® composite modified glassy carbon electrode, Sensors Actuators B Chem. 158 (2011) 319–326. [20] M. Pirsaheb, N. Fattahi, S. Pourhaghighat, M. Shamsipur, K. Sharafi, Simultaneous determination of imidacloprid and diazinon in apple and pear samples using sonication and dispersive liquid–liquid microextraction, LWT- Food Sci. Technol. 60 (2015) 825–831. [21] A.W. Abu-Qare, M.B. Abou-Donia, Determination of diazinon, chlorpyrifos, and their metabolites in rat plasma and urine by high-performance liquid chromatography, J. Chromatogr. Sci. 39 (2001) 200–204. [22] J. Wang, M. Yokokawa, T. Satake, H. Suzuki, A micro IrOx potentiometric sensor for direct determination of organophosphate pesticides, Sensors Actuators B Chem. 220 (2015) 859–863. [23] A. Motaharian, F. Motaharian, K. Abnous, M.R.M. Hosseini, M. HassanzadehKhayyat, Molecularly imprinted polymer nanoparticles-based electrochemical sensor for determination of diazinon pesticide in well water and apple fruit samples, Anal. Bioanal. Chem. 408 (2016) 6769–6779.
DLLME-GC-FID method for diazinon determination, Arab. J. Sci. Eng. 40 (2015) 3041–3046. X. Wang, S. Dong, T. Hou, L. Liu, X. Liu, F. Li, Exonuclease I-aided homogeneous electrochemical strategy for organophosphorus pesticide detection based on enzyme inhibition integrated with a DNA conformational switch, Analyst 141 (2016) 1830–1836. S.R. Mirmasoomi, M.M. Ghazi, M. Galedari, Photocatalytic degradation of diazinon under visible light using TiO2/Fe2O3 nanocomposite synthesized by ultrasonic-assisted impregnation method, Sep. Purif. Technol. 175 (2017) 418–427. G. Erdoğdu, A sensitive voltammetric method for the determination of diazinon insecticide, J. Anal. Chem. 58 (2003) 569–572. A. Gahlaut, A. Gothwal, A.K. Chhillar, V. Hooda, Electrochemical biosensors for determination of organophosphorus compounds, Open J. Appl. Biosens. 1 (2012) 1–8. S.H. Sharif Zein, A.R. Boccaccini, Synthesis and characterization of TiO2 coated multiwalled carbon nanotubes using a sol gel method, Ind. Eng. Chem. Res. 47 (2008) 6598–6606. I.V. Zaporotskova, N.P. Boroznina, Y.N. Parkhomenko, L.V. Kozhitov, Carbon nanotubes: sensor properties. A review, Mod. Electron. Mater. 2 (2016) 95–105. J. Ghodsi, A.A. Rafati, Y. Shoja, First report on electrocatalytic oxidation of oxytetracycline by horse radish peroxidase: application in developing a biosensor to oxytetracycline determination, Sensors Actuators B Chem. 224 (2016) 692–699. B. Gao, G.Z. Chen, G.L. Puma, Carbon nanotubes/titanium dioxide (CNTs/TiO2) nanocomposites prepared by conventional and novel surfactant wrapping sol–gel methods exhibiting enhanced photocatalytic activity, Appl. Catal., B 89 (2009) 503–509. D. Eder, A.H. Windle, Carbon–inorganic hybrid materials: the carbon-nanotube/ TiO2 interface, Adv. Mater. 20 (2008) 1787–1793. L.C. Jiang, W.D. Zhang, Electrodeposition of TiO2 nanoparticles on multiwalled carbon nanotube arrays for hydrogen peroxide sensing, Electroanalysis 21 (2009)
9
Contents lists available at ScienceDirect
Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem
A voltammetric sensor for diazinon pesticide based on electrode modified with TiO2 nanoparticles covered multi walled carbon nanotube nanocomposite Javad Ghodsi, Amir Abbas Rafati
MARK
⁎
Department of Physical Chemistry, Faculty of Chemistry, Bu-Ali Sina University, P.O. Box 65174, Hamedan, Iran
A R T I C L E I N F O
A B S T R A C T
Keywords: Voltammetric sensor Diazinon MWCNTs/TiO2NPs nanocomposite Synergic electrocatalytical reduction
This work describes the development of a voltammetric sensor for diazinon pesticide determination based on diazinon reduction on glassy carbon electrode (GCE) surface modified with multi walled carbon nanotubes covered by TiO2 nanoparticles (MWCNTs/TiO2NPs). Voltammetric investigations were carried out by cyclic voltammetry (CV), linear sweep voltammetry (LSV), differential pulse voltammetry (DPV) and square wave voltammetry (SWV). Prepared nano-composite of MWCNTs/TiO2NPs was characterized with scanning electron microscopy (SEM), X-ray diffraction (XRD) and energy dispersive X-ray analysis (EDX) techniques. Nanocomposite of MWCNTs and TiO2NPs showed suitable synergic electrocatalytical properties in diazinon reduction which resulted the completely sensitive determination of diazinon in laboratory samples and real samples including city piped water and agricultural well water. Linear range, limit of detection (LoD) and limit of quantitation (LoQ) obtained by developed sensor were 11–8360 nM, 3 nM and 10 nM successively which were quite satisfying in compared to other works reported about diazinon determination. Response time, Repeatability and stability of sensor were examined and found completely suitable. Moreover electrode modification method was very fast, inexpensive and easy to follow.
1. Introduction Various pesticides are commonly used for the augmentation of food production. The increasingly applying of pesticides in public health and agriculture has caused important environmental pollution and potential health risk and therefore is cause of concern. Organophosphates (OPs), highly used class of pesticides, are very dangerous and detrimental because of their noxious nature. These pesticides have their own toxic effects by irreversible inhibition of the enzyme acethylcholinesterase (AChE) which is an essential enzyme for the function of central nervous system [1–3]. diazinon (O,O-diethylO-2-isopropyl6-methylpyrimidin-4-yl phosphorothioate), as an organophosphorus insecticide and an extremely toxic insecticide, often used widely on cultivation of various types of crops such as apple, rice, grapes, pistachios, palm, citrus and pear. Diazinon is used to control a wide range of pests such as, cockroaches, mites, saprophytes, chewing insects, fleas and so on. Diazinon is also poisonous for aquatics, mammals, and some non-target insects. High doses of diazinon can cause muscle tremors, nausea, dimness of vision and difficult breathing. It can cause harmful effects on skin, lungs,
⁎
gastrointestinal tract and nervous system. Moreover, diazinon has been identified as a potential chemical mutagens and it is proved that diazinon is highly toxic to fish and water organisms [2–6]. According to what was explained qualitative and quantitative analysis methods for OPs and especially for diazinon (as an important OP model) is very serious for human health and environmental control. Common techniques applied for OPs detection are gas chromatography and high-performance liquid chromatography (HPLC), mass spectrometry method, spectrophotometry, infrared spectroscopy and an enzyme immunoassay [1,3,7]. Though these methods are suitably sensitive and highly reproducible and show low detection limits, because of some problems such as very expensive techniques, time-consuming methods, complicated laboratory equipment, low speed analysis under field conditions and requiring trained man power, developing of new, simple, inexpensive and sensitive quantifying methods is still a critical subject in analytical chemistry [3,5,8]. For solving above mentioned disadvantages of OPs determination methods, some other approaches have been reported, such as multi syringe flow injection analysis (MSFIA), bioanalytical methods based on enzyme inhibition and immunoassay and electrochemical methods [1,8]. Among the all reported methods,
Corresponding author. E-mail address: [email protected] (A.A. Rafati).
https://doi.org/10.1016/j.jelechem.2017.11.003 Received 3 August 2017; Received in revised form 27 October 2017; Accepted 1 November 2017 1572-6657/ © 2017 Elsevier B.V. All rights reserved.
Journal of Electroanalytical Chemistry 807 (2017) 1–9
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Fig. 1. SEM image of MWCNTs/TiO2NPs nano-composite on GCE surface.
O K• 1500
Fig. 2. EDX analysis of MWCNTs/TiO2NPs nanocomposite on GCE surface.
TiK•
C K•
TiL• N K•
1000
500
TiK•
0
keV 0
5
10
new nano materials for application in fields of electrochemical sensors, owing to unparalleled unit surface, the high sensitivity of the electronic properties of nanotubes to molecules adsorbed on their surface and their interesting electrical, mechanical and catalytical properties [9–11]. Titanium dioxide (TiO2), n-type semiconductor and one of the most important transition metal oxides, is the common semiconductor applied for the oxidation or reduction of inorganic and organic species [12,13]. Its nontoxicity, long-term stability, low cost and other remarkable chemical and physical properties, make it suitable for various applications such as photocatalysis, photo-chromic devices,
those that are based on electrochemical techniques have been more interested because of their several advantages such as ease of use, cost effectiveness, fast measurement, high sensitivity and capable of application in field conditions and so on [8]. As far as we know, up to now a few electrochemical methods have been reported for determination of diazinon such as nafion modified glassy carbon electrode, biosensors and DNA-composed carbon nanotube electrode [2]. This work provides a novel voltammetric sensor based on electrode modification with MWCNTs/TiO2NPs which supplies a simple, sensitive and easy to fabricate sensor. Carbon nanotubes (CNTs) are promising 2
Journal of Electroanalytical Chemistry 807 (2017) 1–9
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16
2. Experimental 25.28°
14
2.1. Materials
Intensity
12
MWCNTs with the average diameter of 10–40 nm were purchased from Neutrino Co. (Iran). Tetraethyl orthotitanate [(C₂H₅O)₄Ti] and sodium dodecyl sulfate (SDS) were purchased from Sigma-Aldrich and were used as received. Acetone was from Merck. Diazinon (O,O-diethylO-(2-isopropyl-6-methylpyrimidin-4-yl)-monothiophosphate) was purchased from Kavosh Kimia Co. (Iran) and was used as received. Titanium (IV) sulfate (Ti(SO4)2) and H2O2 were from Sigma-Aldrich. All other chemicals were of analytical grade. Phosphate buffers (PB) (50 mM) were prepared from H3PO4, NaH2PO4 and Na2HPO4 and pH values were adjusted by HCl and NaOH solutions. All phosphate buffers used in the experiments were in pH 7, unless otherwise stated. The solutions were prepared using deionized water and deoxygenated by bubbling high purity (99.99%) nitrogen gas through them for 15 min prior to the experiments. All experiments were carried out at room temperature.
10 8 6 4 27.42°
2 0 15
25
35
45
55
65
75
2θ Fig. 3. XRD spectrum of MWCNTs/TiO2NPs nano-composite on GCE surface.
electrochemical sensors, electronic devices and catalytic applications and so on. Also due to the high affinity of nano TiO2 on phosphate groups, it can be used in electrochemical sensor development for organophosphate pesticides like as fenitrothion and parathion [13–16]. CNTs coated with metal oxides hybrids will lead to new composite materials which are expected to show different physical properties than those of each neat component or even a synergistic effect. These materials have received great interest in recent years because of their exclusive electrical, optical, mechanical and thermal properties, so enabling the application in catalytic and electrochemical technologies [9,13,14]. CNTs/titanium dioxide (TiO2) composite materials have attracted many attentions in various research fields. It was verified incorporation of MWCNTs with nanostructured TiO2 significantly improved electrocatalytic properties for sensor application [14,17].
H3C
N
Voltammetric measurements were carried out using a three-electrode system, including the modified electrode (GCE/MWCNTs/ TiO2NPs) as working electrode, an Ag/AgCl (3.0 M KCl) as reference electrode, and a platinum foil as counter electrode. Voltammetric measurements were carried out using a computer-controlled μ-Autolab modular electrochemical system (PGSTAT101, the Netherlands), driven with NOVA Software (upgrade 1.10). All electrochemical experiments in current work were performed by CV, LSV, DPV and SWV techniques. X-ray powder diffraction (XRD) spectra were taken on a Bruker D8advance x-ray diffractometer with Cu Kα radiation. Size, morphology and percent elemental composition of MWCNTs/TiO2NPs were investigated by using field emission scanning electron microscopy (FESEM, TSCAN, and S-300000).
CH3
H3C
N
S
CH3
O
N
S
CH3
P
O O
CH3
HN
2e- + 2H+
P H3C
2.2. Apparatus
H3C
CH3
O
O O
CH3
2e- + 2H+
H3C
CH3
HN
NH
S
CH3
P H3C
O
O O
Scheme 1. Schematic representation of two successive electrochemical reduction of diazinon at electrode surface.
3
CH3
Journal of Electroanalytical Chemistry 807 (2017) 1–9
J. Ghodsi, A.A. Rafati
2.3. MWCNTs/TiO2NPs nanocomposite synthesis
2.4. GCE/MWCNTs/TiO2NPs preparation
Prior to MWCNTs/TiO2NPs nanocomposite synthesis, MWCNTs were refluxed by HCl 0.5 M for 2 h for removing impurities. Then MWCNTs/TiO2NPs nanocomposite was synthesized as reported in ref. [18]. Summarily 0.1 g of HCl treated MWCNTs was dispersed in acetone/tetraethyl orthotitanate solution by ultrasonication then 0.04 g of SDS, as a shape controller, was further added. Then, hydrolysis was initiated by adding some deionized water in to the mixture; freshly formed MWCNTs/TiO2 nanocomposites filtered and dried in a vacuum oven at 77 °C for at least 20 h; Finally, MWCNTs/TiO2 nanocomposites powder was heated at a heating rate of 1 °C/min to at temperature of 400 °C in an oven under ambient condition, maintaining at this temperature for 30 min.
GCE (diameter 3 mm) was polished with alumina slurry and followed by ultrasonically cleaning with ethanol/double distilled water mixture and then dried with acetone. 10 mg of MWCNTs/TiO2NPs nanocomposite was ultrasonicated in 10 mL acetone and 10 μL of resulted MWCNTs/TiO2NPs/acetone mixture was dropped on GCE surface and dried at room temperature.
2.5. GCE/electrodeposited TiO2NPs preparation To better represent the MWCNTs/TiO2NPs nanocomposites role in electrocatalytic reduction of diazinon, we also compared GCE/ MWCNTs/TiO2NPs response with GCE/electrodeposited TiO2NPs in
Fig. 4. SW voltammograms of 2 μM diazinon in buffer obtained by a) GCE, b) GCE/MWCNTs, c) GCE/TiO2NPs and d) GCE/MWCNTs/ TiO2NPs as working electrodes.
10 9 8 7
Current (µA)
6
(d)
5
(c)
4
(b) 3
(a)
2 1 0 -0.4
-0.6
-0.8
-1 Potential (V)
-1.2
-1.4
-1.6
Fig. 5. CVs of 10 μM diazinon in buffer obtained by a) GCE, b) GCE/ MWCNTs, c) GCE/TiO2NPs and d) GCE/MWCNTs/TiO2NPs as working electrodes.
Potential (V) 40 -1.6
-1.4
-1.2
-1
-0.8
-0.6
-0.4 -10
(a) -60
-110
(c) -160
-210
(d) -260
-310
4
Current (µA)
(b)
Journal of Electroanalytical Chemistry 807 (2017) 1–9
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3. Results and discussion
addition to other electrodes. For TiO2NPs electrodepositing on GCE surface, Firstly GCE was polished and cleaned like as above mentioned; then it was immersed into electrolyte consisting of 3 M KCl, 10 mM H2O2 and 10 mM Ti(SO4)2 and constant potential of −0.1 V was applied for 30 min [14]. Modified GCE was washed with double distilled water tree times and dried at room temperature.
3.1. Characterization of GCE/MWCNTs/TiO2NPs with SEM, EDX and XRD Fig. 1 shows the SEM image of GCE/MWCNTs/TiO2NPs. As can be seen the walls of the nanotubes are densely entirely covered with TiO2NPs and rarely TiO2NPs apart from walls are observed. Nanoscale
Fig. 6. SW voltammograms of GCE/MWCNTs/TiO2NPs in buffer (a) and buffer containing 4 μM diazinon (b).
14 12
Current (μA)
10 8
(b)
6
(a) 4 2
-0.4
0
-0.6
-0.8
-1
-1.2
-1.4
-1.6
Potential (V)
Fig. 7. (a) SWV, (b) DPV, (c) CV and (d) LSV response of developed sensor to 2 μM diazinon.
Potential (V) 15 -0.3
-0.5
-0.7
-0.9
-1.1
-1.3
-1.5
10
(a) 5
(b)
Current (µA)
0 -5
(d)
(c)
-10 -15 -20 -25 -30 -35 5
-1.7
Journal of Electroanalytical Chemistry 807 (2017) 1–9
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16
(a)
(b)
14
10
8360 nM Current (µA)
10
Current (µA)
y = 1.1753x + 2.7349 R² = 0.9964
12
12
8 6
8 6
11 nM
4
Fig. 8. a) SW voltammograms for successive addition of diazinon to phosphate buffer in concentration range of 11 nM–8360 nM diazinon and b) Its resulted standard calibration curve.
14
4 2
2 0 -0.5
0
-0.8
-1.1
-1.4
0
2
4
6
8
10
Diazinon / µM
Potential (V)
(a)
4
6
pH
Fig. 9. a) SW voltammograms of 2 μM diazinon in pHs of 4–9 (only first diazinon reduction peak is showed); b) linear plot for diazinon potential reduction with pH and its corresponding linear regression equation and c) diazinon first reduction peak current vs. pH.
9
Current (µA)
5 4 3 2 1 -0.3
-0.5
-0.7
-0.9
-1.1
-1.3
Potential (V)
pH
(b)
(c)
-0.4
3
5
7
-0.6 -0.7 -0.8 -0.9 -1 -1.1
9
4 Reduction current (µA)
Reduction potential (V)
-0.5
y = -0.0866x - 0.2056 R² = 0.9885
4.5
3.5
3
2.5
2
3
5
7
9
pH
diameters of some MWCNTs/TiO2NPs are detected in the figure. GCE/ MWCNTs/TiO2NPs surface also was analyzed by EDX and respective result is showed in Fig. 2. As figure shows and expectedly, the nanocomposite is mainly composed of Ti, C and O elements. XRD pattern of the MWCNTs/TiO2NPs on GCE surface was obtained using Cu Kα radiation and showed in Fig. 3. The (101) peak (2θ = 25.28°) of anatase and the (110) peak (2θ = 27.42°) of rutile can be seen in this pattern. Regarding to these two peak intensities, most of TiO2NPs deposited on MWCNTs walls have anatase structure.
3.2. Electrochemical investigations In this work diazinon was measured with electrocatalytically reduction on MWCNTs/TiO2NPs nanocomposite surface. Diazinon can undergo two successive reductions according to Scheme 1. For better verifying MWCNTs/TiO2NPs nanocomposite role in electrocatalytically reduction of diazinon than each neat of MWCNTs and TiO2NPs, SWV response of different electrodes to a fixed amount of diazinon were investigated and obtained results were showed in Fig. 4. Fig. 4 shows 6
Journal of Electroanalytical Chemistry 807 (2017) 1–9
J. Ghodsi, A.A. Rafati
(a)
0
20 -1.2
-1
-0.8
-0.6
0
-0.4 0
-10
-20
-20
-40
10 mV. s-1
-60
scan rate
-80
4430 mV. s-1
Current (µA)
-1.4
Current (µA)
-1.6
(Potential scan rate)1/2 / V1/2.s-1/2
(b)
Potential (V)
0.5
1
1.5
2
2.5
-30 -40 -50
-100 -120
-60
-140
-70
y = -27.874x - 0.3812 R² = 0.9961
Fig. 10. a) CVs of 3 μM diazinon in potential scan rates of 10–4430 mV/s and b) Linear plot for reduction peak current of diazinon vs. square root of potential scan rate.
LSV, DPV, CV and SWV response of developed sensor to a 2 μM diazinon were compared in Fig. 7. SWV, DPV, CV and LSV voltammograms are labeled as a to d successively. As the figure shows, most sensitivity of sensor is corresponded to when SWV technique was applied. Therefore diazinon calibration curve and next diazinon determination in real samples was obtained by applying SWV technique. Fig. 8a shows the SWV voltammograms for successive addition of diazinon to phosphate buffer which resulted the standard calibration curve (Fig. 8b) in really vast concentration range of 11 nM–8360 nM. Also LoD and LoQ were obtained as 3 nM and 10 nM which are calculated by 3sb/m and 10sb/m successively. Where sb is the standard deviation of the blank and m is the slope of calibration curve [3]. pH effect on sensor response also was investigated. Fig. 9a shows the SWV voltammograms of 2 μM diazinon in pHs of 4–9 but to avoid sloppy voltammograms, only first diazinon reduction peak is regarded. As can be seen diazinon reduction potential negatively shifts when pH increases from 4 to 9. Linear plot for diazinon reduction potential with pH and its corresponding linear regression equation is shown in Fig. 9b. As voltammograms of Fig. 9a show, reduction current of diazinon increases with pH to 6 and then decreases with more pH increasing to 9. Fig. 9c represents reduction current vs. pH. This increasing and decreasing regime for reduction current with pH can be explained as follows: in high pHs, lack of proton is a barrier to diazinon reduction (proton was consumed during diazinon reduction as showed in Scheme 1) and in low pHs hydrogen generation due to the decomposition of water became the strong hurdle against the electrochemical reduction of diazinon. As another effective parameter, potential scan rate effect on sensor response to diazinon was also studied. Fig. 10a shows CV voltammograms of 3 μM diazinon in potential scan rates of 10–4430 mV/s. As voltammograms show, reduction peak current of diazinon was increased with scan rate and also reduction potential shifts to more negative ones. Fig. 10b shows linear plot for reduction peak current of diazinon vs. square root of potential scan rate which resulted from CVs of Fig. 10a. Regarding to linear proportional of reduction current vs. square root of scan rate, it can be concluded diazinon reduction at electrode surface is mainly controlled under the diffusion step. To investigate the applicability of developed sensor in real samples, two real water samples including agricultural well water and city piped water were tested with this sensor by standard addition method. Water samples freshly received and were spiked with three amounts of diazinon (1, 2 and 3 μM). Obtained Results are showed at Table 1. As Table 1 shows, relative percentage of recovery amounts are between 97.5 and 105.5 which are very satisfying.
Table 1 Diazinon determination by developed sensor in two real samples of water. Sample
Added (μM)
Expected (μM)
Found (μM) - the mean of three measured values
Recovery (%)
Well watera
1.0 2.0 3.0 1.0 2.0 3.0
1.0 2.0 3.0 1.0 2.0 3.0
1.03 1.95 3.14 1.02 2.11 3.08
103.0 97.5 104.7 102.0 105.5 102.7
Tap waterb
a b
Sampled from a well water used for agricultural in Karaj city (Iran). From city piped water of Bu-Ali University (Hamadan-Iran).
SWV voltammograms of a 2 μM diazinon in buffer obtained by a) GCE, b) GCE/MWCNTs, GCE/TiO2NPs and GCE/MWCNTs/TiO2NPs as working electrodes. Diazinon successive reduction peak currents are seen in SWV of GCE/MWCNTs/TiO2NPs at about − 0.78 V and − 0.97 V. As clearly seen, these two reduction peaks appeared at potentials which are lower than that of similar peaks in other SWVs. Furthermore SWV of GCE/MWCNTs/TiO2NPs shows significantly more sensitive response to diazinon. It's easy to deduce from SWVs of b-d that the MWCNTs and TiO2NPs have significant synergic effect in diazinon reductions at electrode surface. SWVs of curves d and c show another reduction peak at about − 1.2 V which is corresponded to reduction of Ti4 + + e− → Ti3 + corresponded to TiO2NPs deposited on MWSNTs [19]. Due to the larger reduction current of first peak, all next measurements in this work are based on the first reduction peak of diazinon. Above experiment was repeated by CV measurements and related results were showed in Fig. 5. Fig. 5 shows CVs of 10 μM diazinon in buffer obtained by a) GCE, b) GCE/MWCNTs, GCE/TiO2NPs and GCE/MWCNTs/TiO2NPs as working electrodes. It can be concluded from mentioned SWVs, lowest reduction potential and most sensitive response was corresponded to GCE/MWCNTs/TiO2NPs. It must be said, CVs in Fig. 5 were separated from each other in order to prevent overlapping voltammograms and to better comparison between CVs. As other electrochemical test, GCE/MWCNTs/TiO2NPs SWV response to bulk (phosphate buffer) and buffer containing diazinon ware compared and result was shown in Fig. 6. Fig. 6 shows the SWV voltammograms of GCE/MWCNTs/TiO2NPs in buffer (a) and buffer containing 4 μM diazinon (b). It is most probably concluded from this figure that reduction peak around − 1.2 V is mainly corresponded to Ti4 + + e− → Ti3 + reduction of TiO2NPs structures and as can be seen are quite similar in two SWVs. 7
Journal of Electroanalytical Chemistry 807 (2017) 1–9
J. Ghodsi, A.A. Rafati
8
(a)
Table 2 Comparison of our developed sensor performance with some other reported methods.
(1)
7
(2)
6 (3)
Current (µA)
5 4 3 2 1 0 -0.5
-0.7
-0.9
-1.1
-1.3
Potential (V)
Technique
Linear range (nM)
LoD (nM)
Ref.
Dispersive liquid–liquid microextraction HPLC Micro IrOx potentiometric sensor HMDEa/SWASVb c MIP modified CPE/CV and SWV Nafion coated GCE/CV, LSV and SWV GCE/MWCNTs/TiO2NPs/ CV and SWV
9.9–1971.4
3.3–6.6
[20]
657.1–6571.4 100–10,000 and 10,000–1,000,000 40–390 1–100 and 100–2000
328.6 3000
[21] [22]
11 0.786
[2] [23]
0–5000
75
[7]
11–8360
3
Present work
a b
7
(b)
c
(1)
6
(2)
which referred to complete repeatable sensor responses. Also response time of developed sensor was completely short, so that it reached 93% of its maximum signal after 5 s. Current sensor was quite stable and maintained about 89% of its first day signal after 28 days in room temperature. Finally our developed sensor performance was compared with some other works reported about diazinon determination and results were summarized at Table 2. Since it has not been reported a lot of electrochemical works about diazinon determination up to now, some of non-electrochemical works also were mentioned in Table 2. According to the table our developed sensor shows acceptable performance in compared to other reported works.
(3)
Current (µA)
5 4 3 2 1 0 -0.5
-0.7
-0.9
-1.1
Hanging mercury dropping electrode. Square wave adsorptive stripping voltammetry. Molecularly imprinted polymer.
-1.3
Potential (V) 4. Conclusions 8
(c)
7
This work reports a voltammetric sensor for diazinon which uses MWCNTs/TiO2NPs nanocomposite to modify electrode surface. Diazinon was determined by applying suitable synergic electrocatalytical effect of MWCNTs with TiO2NPs in reduction of diazinon. Developed sensor showed good sensitivity, vast linear range, low LoD and low LoQ. The sensor was successfully examined for diazinon determination in real water samples including agricultural well water and city piped water and obtained results showed acceptable recovery amounts. Stability, repeatability and response time of the sensor also were tested and found satisfying results. Current sensor performance was compared to some electrochemical and non-electrochemical works about diazinon determination and showed the good performance of our sensor. Finally developed sensor suggests a fast and easy to follow method to sensor fabrication for other important organophosphorus insecticides.
(1)
6
Current (µA)
5 4 3
(3)
(2)
2 1 0 -0.5
-0.7
-0.9
-1.1
-1.3
Potential (V) Fig. 11. SW voltammograms of 1) a sample from well water spiked with 1 μM of diazinon, 2) a sample from tap water spiked with 1 μM of diazinon and 3) phosphate buffer containing 1 μM of diazinon obtained by GCE/MWCNTs/TiO2NPs. Panels b and c show same SWVs with 2 μM and 3 μM of diazinon respectively.
Acknowledgments This work was supported by the ‘Iran National Science Foundation’ [Grant No.95004691].
Moreover the detected results from the real samples were compared with the reference method. Corresponded results were showed in Fig. 11. Fig. 11a shows SWV voltammograms of 1) a sample from well water spiked with 1 μM of diazinon, 2) a sample from tap water spiked with 1 μM of diazinon and 3) phosphate buffer containing 1 μM of diazinon obtained by GCE/MWCNTs/TiO2NPs. Fig. 11b and c show same SWVs with 2 μM and 3 μM of diazinon respectively. As can be clearly seen, SWV responses to reference sample show high accordance with SWVs of tap and well water in three above mentioned diazinon concentrations. This clearly verifies the high selectivity of developed sensor that only response to diazinon in different real samples of water. Repeatability of developed sensor was tested by 20 successive determination of a fixed amount of diazinon and relative standard deviation (RSD) of obtained diazinon amounts was calculated as 3.8%
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