Electrochemical pesticide sensors based on electropolymerized metallophthalocyanines

Journal of Electroanalytical Chemistry 804 (2017) 53–63

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Electrochemical pesticide sensors based on electropolymerized metallophthalocyanines

MARK

Duygu Akyüza, Turgut Keleşb, Zekeriya Biyiklioglub,⁎, Atıf Kocac,⁎ a b c

Department of Chemistry, Faculty of Science and Letters, Marmara University, Istanbul, Turkey Department of Chemistry, Faculty of Science, Karadeniz Technical University, Trabzon, Turkey Department of Chemical Engineering, Faculty of Engineering, Marmara University, Istanbul, Turkey

A R T I C L E I N F O

A B S T R A C T

Keywords: Electrochemistry Electropolymerization Selective pesticide sensor Modified electrode Metallophthalocyanine

New metallophthalocyanines (MPcs) were designed with redox active Co(II) (CoPc(ma)), Cl-Mn(III) (Cl-MnPc (ma)), and Ti(IV)O (TiOPc(ma)) metal centers and morpholin and amino bearing substituents (ma). While redox active metal centers enhanced redox activity of the complexes, redox active and electropolymerizable [2-(4{[(1E)-(4-morpholin-4-ylphenyl)methylene]amino}phenyl)ethoxy] substituents triggered the coating of MPcs with the oxidative electropolymerizations. Voltammetry and in situ spectroelectrochemistry techniques were used for the electrochemical characterizations of MPcs. All complexes gave metal based reduction processes in addition to the Pc based processes. Moreover, all complexes were coated on GCE with the oxidations of morpholin and amino moieties of the substituents, thus redox active and conductive GCE/MPc(ma) electrodes were constructed. Modified electrodes were investigated as the potential pesticide sensors. Changing the metal center of the complexes significantly altered their sensing activities. While all complexes showed interaction abilities for chlorophyros, fenitrothion, and methomyl. GCE/CoPc(ma) electrode sensed fenitrothion with good selectivity and sensitivity. A linear range for the fenitrothion sensing with GCE/CoPc(ma) electrode was observed between 1.20 μmoldm− 3 and 42.0 μmoldm− 3 concentrations. Moreover, sensitivity and LOD of the electrode were found as 0.26 Acm− 2 M− 1 and 0.46 μmoldm− 3 respectively. Although GCE/TiOPc(ma) electrode also sensed fenitrothion with a good selectivity, the linear range of this sensing was very narrow. GCE/Cl-MnPc(ma) electrode sensed all pesticides with similar voltammetric responses, thus its selectivity is poorer than the others, although it has good sensitivity for the pesticides.

1. Introduction Due to the highly toxic effects of pesticides on human nervous system, there has been growing interest to develop new pesticide sensors with high sensitivity, ease preparation, low cost, reliability and selectivity [1–3]. The rapid detection of these toxic agents in the environment, public places, or workplaces and the monitoring of individual exposures to chemical warfare agents is crucial for human health. For these purposes, numerous pesticide sensors working with different techniques were published [2,4–6]. Among the techniques such as gas chromatography–mass spectrometry (GC–MS) [7,8], highperformance liquid chromatography (HPLC) [9], optical [10] approaches, and etc., the electrochemical methods were extensively preferred due to the one of the solution for disadvantages of the analyses at centralized laboratories, requiring extensive labor and analytical resources, and often results in a lengthy turnaround time. With the electrochemical methods, long analysis time and extensive sample

⁎

handling could be resolved with portability, rapid turnaround time, and cost-effectiveness. In many of pesticide sensing systems, enzymes, such as acetylcholinesterase enzyme (AChE) is the most studied functional material [5,11–13]. However, AChE based biosensors have especially low stability, difficulties of immobilization, leakage from the substrate surface, and low chemical and thermal stabilities [5,11–13]. Low selectivity is the main drawback of AChE based biosensors. Similar interaction reactions of AChE with the pesticides show very similar electrochemical responses for every pesticides [12,14,15]. In order to solve these mentioned limitations, the studies have been focused on new functional materials, which can be used as instead of enzymebased sensors. Metal oxides and transition metal's organic compounds were investigated as the sensitive and selective enzymeless pesticide sensors [2,3,16,17]. For example, Kang J.F. et al. reported a parathion sensor based on electrodeposition of gold nanoparticles on a multiwalled carbon nanotubes modified glassy carbon electrode [18]. They reported a linear response to parathion in the concentration range from

Corresponding authors. E-mail addresses: [email protected] (Z. Biyiklioglu), [email protected] (A. Koca).

http://dx.doi.org/10.1016/j.jelechem.2017.09.044 Received 11 June 2017; Received in revised form 15 September 2017; Accepted 20 September 2017 Available online 23 September 2017 1572-6657/ © 2017 Elsevier B.V. All rights reserved.

Journal of Electroanalytical Chemistry 804 (2017) 53–63

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6.0 × 10− 5 to 5.0 × 10− 7 M and a detection limit of 1.0 × 10− 7 M. Although they reported reasonable sensitivity, an selectivity analysis was not performed. Similarly Gong J. et al. developed a biosensor based on zirconia nanoparticles decorated graphene nano-sheets (labelled as ZrO2NPs-GNs) for the detection of methyl parathion [19]. The detection limit of 0.6 ng mL− 1 was reported for methyl parathion (MP) in aqueous solutions. Although they reported reasonable sensitivity, selectivity phenomena were not mentioned. To increase the selectivity of the biosensors, molecularly imprinted sensors (MIPs) were generally developed [20–23]. Marx S. and his coworkers developed a molecularly imprinted sol–gel polymer and selectively detect the parathion [24]. Although MIPs sensors were studied as the mimic function of biological receptors with high selectivity, preparation difficulties and stability of the sensors are still important problems for these electrodes. Therefore, the studies based on stable, selective and sensitive pesticide biosensors are still take great attention. One of the functional materials presently studied as the selective and stable sensors are metallophthalocyanines (MPcs) [3,25,26]. Due to the high thermal and chemical stabilities and excellent redox activities, MPcs have been extensively studied in various technological fields as well as sensor applications. Tailoring of MPc with different metal cations in the cavity of Pc and various substituents ensure synthesizing MPcs having desired properties for the target applications [3,27–33]. Various MPcs were reported as pesticide sensors [34–38]. For instance, the formetanate hydrochloride was detected with 9.7 × 10− 8 mol dm− 3 LOD value with a CoPc-fMWCNT/GCE sensor [37]. Similarly, Sibulelo Vilakazi and co-workers constructed CoPc based electrode and sensed dicrotophos pesticide with the LOD value of 1.25 × 10− 7 mol dm− 3 [38]. It was clearly indicated that the metal center and substituents of MPcs and the electrode modification techniques considerably influence the sensing properties of these complexes. In our previous studies, we have reported different MPcs as selective and sensitive pesticide sensors [3,32,35]. Various sensor electrodes based on MPcs by using different modification techniques, such as, Langmuir-Blodgett, self-assembled monolayer thin films, electropolymerization, spin coating and click electrochemistry methods were reported. Here we have constructed new electrodes with the electropolymerization of MPcs having redox active CoII (CoPc(ma)), MnIII (Cl-MnPc(ma)), and TiIVO (TiOPc(ma)), and redox active and electropolymerizable [2-(4-{[(1E)-(4-morpholin-4-ylphenyl)methylene] amino}phenyl)ethoxy] containing substituents. MPcs having morpholin containing substituents was previously reported. Here MPcs having two polymerizable groups (morpholin and amino moieties) have been investigated to alter the redox activity of the electropolymerized films and to increase the selectivity and sensitivity of the sensors.

laser accumulating 50 laser shots using Bruker Microflex LT MALDITOF mass spectrometer Bremen, Germany). Optical spectra in the UV–Vis region were recorded with a Perkin Elmer Lambda 25 spectrophotometer. Melting points were measured on an electrothermal apparatus and are uncorrected. The elemental analyses were performed on a Costech ECS 4010 instrument. 2.2. Synthesis 2.2.1. 2-(4-{[(1E)-(4-Morpholin-4-ylphenyl)methylene]amino}phenyl) ethanol (1) A mixture of 2-(4-aminophenyl)ethanol (1.40 g, 10.4 mmol), 4morpholin-4-ylbenzaldehyde (2.00 g, 10.4 mmol) and six drops of acetic acid were added to ethanol (120.0 mL) and the reaction mixture was stirred at 80 °C under an nitrogen atmosphere for 1 day. Then, the solvent was evaporated under reduced pressure to near dryness. The crude product was purified by recrystallization from ethanol. Yield: 2.60 g (81.0%), m.p. 267–269 °C. IR (ATR), ν/cm− 1: 3213 (OH), 3028 (AreH), 2969–2829 (Alif. CeH), 1609, 1594, 1560, 1517, 1504, 1438, 1383, 1347, 1263, 1234, 1220, 1177, 1112, 1065, 1050, 938, 921, 890, 820, 802. 1H NMR (DMSO‑d6), (δ:ppm): 8.43 (s, 1H, ]CH), 7.75 (d, 2H, AreH), 7.13 (d, 2H, AreH), 7.01 (d, 2H, AreH), 6.79 (d, 2H, AreH), 3.73 (t, 4H, CH2eO), 3.51 (t, 2H, CH2eO), 3.22 (t, 4H, CH2eN), 2.91 (t, 2H, AreCH2), 2.25 (s, 1H, OH). 13C NMR (DMSO‑d6), (δ:ppm): 157.16, 156.08, 153.18, 143.76, 130.05, 127.52, 122.58, 116.10, 114.50, 66.40, 63.22, 47.83, 38.45. MS (ES+), (m/z): 311 [M + H]+. Elemental analysis: (found: C 73.80, H 6.90, N 9.28%, C19H22N2O2 (310) requires C 73.52, H 7.14, N 9.03%). 2.2.2. 4-[2-(4-{[(1E)-(4-Morpholin-4-ylphenyl)methylene]amino}phenyl) ethoxyl] phthalonitrile (3) 4-Nitrophthalonitrile 2 (0.660 g, 3.87 mmol) was dissolved in 20.0 mL dry DMF under N2 4-atmosphere and of 2-(4-{[(1E)-(4-morpholin-4-ylphenyl)methylene]amino}phenyl)ethanol 1 (1.20 g, 3.87 mmol) was added to this mixture. After stirring for 10 min at 50 °C, finely ground anhydrous K2CO3 (1.600 g, 11.61 mmol) was added portion within 2 h. The reaction mixture was stirred under N2 at 50 °C for 4 days. Then, the reaction mixture was poured into water. The crude product was crystallized from ethanol. Yield: 1.31 g (78.0%), m.p. 216–218 °C. IR (ATR), ν/cm− 1: 3074 (AreH), 2969–2849 (Alif. CeH), 2231 (C^N), 1624, 1594, 1563, 1514, 1484, 1447, 1380, 1340, 1307, 1246, 1232, 1192, 1122, 1051, 1030, 952, 923, 838, 803, 722, 651. 1H NMR (DMSO‑d6), (δ:ppm): 8.49 (s, 1H, ]CH), 8.11 (d, 1H, AreH), 7.80 (d, 3H, AreH), 7.40 (d, 1H, AreH), 7.38 (d, 2H, AreH), 7.22 (d, 2H, AreH), 7.05 (d, 2H, AreH), 4.12 (t, 2H, CH2eO), 3.75 (t, 4H, CH2eO), 3.25 (t, 4H, CH2eN), 3.17 (t, 2H, AreCH2). 13C NMR (DMSO‑d6), (δ:ppm): 161.87, 160.69, 153.67, 151.59, 150.12, 136.76, 130.63, 126.82, 123.39, 122.88, 122.18, 121.66, 117.13, 116.40, 115.88, 114.38, 108.43, 71.12, 66.36, 47.66, 38.44. MS (ES+), (m/z): 437 [M + H]+. Elemental analysis: (found: C 74.42, H 5.40, N 12.98%, C27H24N4O2 (436) requires C 74.29, H 5.54, N 12.84%).

2. Experimental 2.1. Materials and equipment The experimental measurements were carried on with high purity chemicals. Dimethylsulfoxide (DMSO) (Merck, K38436331 807, 99.9% purity), dichlorometane (DCM) and ultra-pure water (≥ 18 MΩ, MilliQ, Millipore) were used as solvents. Tetrabutylammonium perchlorate (TBAP) (Fluka Analytical, 86893, ≥ 99% purity) and LiClO4 (Aldrich, 431567-50G) were used as supporting electrolytes in the solutions. Chlorophyros, fenitrothion, and methomyl were purchased from SigmaAldrich and they were used as received. 2-(4-Aminophenyl)ethanol, 4morpholin-4-ylbenzaldehyde, and 4-nitrophthalonitrile were purchased from Aldrich. The IR spectra were recorded on a Perkin Elmer 1600 FT-IR Spectrophotometer, using KBr pellets. 1H and 13C NMR spectra were recorded on a Bruker Avance III 400 MHz spectrometers in CDCl3, DMSO‑d6 and chemical shifts were reported (δ) relative to Me4Si as internal standard. Mass spectra were measured on a Micromass Quatro LC/ULTIMA LC-MS/MS spectrometer. MALDI-MS of complexes were obtained in dihydroxybenzoic acid as MALDI matrix using nitrogen

2.2.3. 2(3),9(10),16(17),23(24)-Tetrakis-[2-(4-{[(1E)-(4-morpholin-4ylphenyl)methylene]amino}phenyl)ethoxy]phthalocyaninato cobalt(II) [CoPc(ma)], (3a) 4-[2-(4-{[(1E)-(4-Morpholin-4-ylphenyl)methylene]amino}phenyl) ethoxy]phthalonitrile 3 (0.150 g, 0.340 mmol), anhydrous cobalt(II) chloride (0.023 g, 0.170 mmol), n-pentanol (2.7 mL) and 1,8-diazabycyclo[5.4.0]undec-7-ene (DBU) (6 drops) was refluxed with stirring for 24 h under N2 at 160 °C. After cooling to room temperature, 50 mL ethanol was added to precipitate the product. The precipitate was filtered and dried in vacuo. The crude product was purified by passing through an aluminum oxide column using CHCl3:CH3OH (100:2) as solvent system. Yield: 0.038 g (25.0%), m.p. > 300 °C. IR (ATR), ν/ cm− 1: 3032 (AreH), 2918–2850 (Alif. CeH), 1601, 1515, 1494, 1466, 1406, 1378, 1332, 1262, 1228, 1196, 1171, 1112, 1093, 1051, 956, 54

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Fig. 1. The synthesis of metallophthalocyanines CoPc(ma) (3a), TiOPc(ma) (3b), MnClPc(ma) (3c). (i) EtOH, acetic acid, 80 °C. (ii) K2CO3, N2, DMF. (iii) n-pentanol, DBU, 160 °C, metal salts.

924, 819, 749. UV–Vis CHCl3), λmaks(logε)nm: 677 (4.99), 620 (4.58), 350 (5.34). MALDI-TOF-MS m/z: 1805 [M + H]+. Elemental analysis: (found: C 72.03, H 5.11, N 12.63%, C108H96N16O8Co (1804) requires C 71.87, H 5.36, N 12.42%).

2.2.5. 2(3),9(10),16(17),23(24)-Tetrakis-[2-(4-{[(1E)-(4-morpholin-4ylphenyl) methylene] amino}phenyl)ethoxy]phthalocyaninato manganese (III) [Cl-MnPc(ma)], (3c) Manganese(III) phthalocyanine was synthesized similarly to 3a by using MnCl2 instead of CoCl2. Yield: 0.104 g (67.0%), m.p. > 300 °C. IR (ATR), ν/cm− 1: 3030 (AreH), 2960–2848 (Alif. CeH), 1600, 1514, 1494, 1465, 1399, 1334, 1262, 1227, 1195, 1171, 1111, 1073, 951, 923, 888, 819, 742. UV–Vis CHCl3), λmaks(logε)nm: 734 (4.97), 672 (4.65), 528 (4.46), 356 (5.39). MALDI-TOF-MS m/z: 1836 [M]+. Elemental analysis: (found: C 70.92, H 5.01, N 12.48%, C108H96N16O8MnCl (1836) requires C 70.64, H 5.27, N 12.20%).

2.2.4. 2(3),9(10),16(17),23(24)-Tetrakis-[2-(4-{[(1E)-(4-morpholin-4ylphenyl)methylene] amino}phenyl)ethoxy] phthalocyaninato titanium(IV) [TiOPc(ma)], (3b) Titanium(IV) phthalocyanine was synthesized similarly to 3a by using Ti(OBu)4 instead of CoCl2. Yield: 0.067 g (44.0%), m.p. > 300 °C. IR (ATR), ν/cm− 1: 3035 (AreH), 2958–2849 (Alif. CeH), 1601, 1514, 1493, 1472, 1379, 1335, 1261, 1227, 1172, 1111, 1097, 1067, 1009, 923, 888, 818, 741. 1H NMR (CDCl3), (δ:ppm): 8.43–8.39 (m, 8H, AreH), 7.82 (bs, 8H, AreH), 7.36–7.28 (m, 20H, AreH), 6.94–6.92 (m, 12H, AreH), 4.19 (m, 8H, CH2eO), 3.87 (m, 16H, CH2eO), 3.25 (m, 16H, CH2eN), 3.12 (m, 8H, AreCH2). 13C NMR (CDCl3), (δ:ppm): 159.22, 159.08, 155.14, 153.22, 148.37, 131.81, 130.45, 127.84, 127.78, 127.70, 122.73, 121.72, 120.76, 120.70, 116.63, 114.48, 113.46, 71.44, 66.72, 48.05, 38.85. UV–Vis CHCl3), λmaks(logε)nm: 705 (5.01), 669 (4.86), 640 (4.60), 354 (5.18). MALDI-TOF-MS m/z: 1810 [M + H]+. Elemental analysis: (found: C 71.90, H 5.05, N 12.57%, C108H96N16O9Ti (1809) requires C 71.67, H 5.35, N 12.38%).

2.3. Electrochemical measurements The electrochemical measurements were carried out with a potentiostat (GAMRY Instruments, Reference 600 Potentiostat/ Galvanostat/ZRA) utilizing a three-electrode configuration at 25 °C by following the procedure in the literature [3]. For cyclic voltammetry (CV) and square wave voltammetry (SWV) techniques were used for electrochemical measurements with was a bare or modified glassy carbon electrodes (GCE) as the working electrodes with a surface area of 0.072 cm2. A Pt wire and Ag/AgCl electrode were used as the counter 55

56

– – – –

[40] [55] [56] [40] [57] [57] [56] [40] 1.56 – –

1.03 – – 1.18 – – – 1.28

– 1.21 0.34

– 0.89 1.00 – 0.99 0.99 – –

120 0.88 0.45 163 – 0.54 0.94 0.91 145 75 181

0.38 (0.51) – 0.67 0.47 – 0.81 0.88 – –

– 1.25 1.21 1.08 – – – – 0.91 – 1.12 – 100 – 0.30 0.86 0.35 95

− 1.47 − 0.95 − 1.34 − 0.71 − 0.78 − 0.74 − 0.68 − 0.80

72 67

85

− 0.48 − 0.40

− 0.30

− 0.38 − 0.62 − 0.50 − 0.44 − 0.37 − 0.32 − 0.06 − 0.23

CoPc(ma) TiOPc(ma)

MnClPc(ma)

CoPc(m) CoPc(t-SA) CoTMPyrPc TiOPc(m) TiOPc(5a) TiOPc(6a) MnTMPyrPc MnClPc(m)

1.38

− 0.90

110

1.12

−1.92 −1.32 −1.89 −1.36 −1.88 – −1.63 −1.93 −1.40 −1.04 −1.05 −1.19 −1.04 0.95 0.66

Epa (V) E1/2 (V) E1/2 (V) ΔEp (mV)

Ip,a/Ip,c

E1/2 (V)

ΔEp (mV)

Ip,a/Ip,c

Red3 Red2 Red1

Redox processes

E1/2 (V)

Fig. 2. UV–Vis spectra of CoPc(ma) (3a), TiOPc(ma) (3b), MnClPc(ma) (3c) in CHCl3.

Complexes

Table 1 Electrochemical data of the complexes in DCM/TBAP solution. All potentials were given versus Ag/AgCl.

ΔEp (mV)

Ip,a/Ip,c

Starting from 4-[2-(4-{[(1E)-(4-morpholin-4-ylphenyl)methylene] amino}phenyl) ethoxy]phthalonitrile 3 general synthetic route for the synthesis of metallophthalocyanines (3a–3c) is given in Fig. 1. Reaction of 4-[2-(4-{[(1E)-(4-morpholin-4-ylphenyl)methylene]amino}phenyl) ethoxy]phthalonitrile 3 with metal salts (CoCl2, Ti(OBu)4, MnCl2) and a N-donor base DBU in n-pentanol led to formation of metallophthalocyanines (3a–3c). The structures of novel compounds were characterized by a combination of IR, 1H NMR, 13C NMR, UV–Vis, MS spectral data and elemental analysis. All characterizations supported the proposed structures and purities of the compounds. The UV/Vis spectra of 3a, 3b, 3c in CHCl3 at room temperature are shown in Fig. 2. UV–Vis spectra exhibit intense single Q band absorption of π → π⁎ transitions at 677, 705 and 734 nm, respectively. B bands of the complexes were observed in the UV region at 350, 354 and 356 nm, respectively. Q band absorption of cobalt phthalocyanine 3a was shown at 677 nm. When we compared spectra of TiOPc(ma) 3b with CoPc(ma) 3a, the Q band absorption of TiOPc(ma) 3b red shifted 28 nm. In the same way, the Q band absorption of Cl-MnPc(ma) 3c red shifted 57 nm with respect to that of CoPc(ma) 3a. These spectral behaviors of the complexes were related with the oxidation states of the central metals. Increasing the oxidation states caused to the shifting of the π → π⁎ transitions towards the longer wavelengths.

85 47

3.1. Synthesis and characterization

− 1.29 − 0.67

Oxd1

3. Result and discussion

E1/2 (V)

ΔEp (mV)

Ip,a/Ip,c

Oxd2

ΔEp (mV)

Ip,a/Ip,c

Modified electrodes based on CoPc, Cl-MnPc and TiOPc (GCE/MPc (ma) and ITO/MPc(ma)) were prepared with electropolymerization of the complexes on the bare GCE and ITO electrodes with repetitive CV technique. 5 repetitive CV cycles were recorded on the optimal potential range of the electrolyte containing 5.0 × 10− 4 mol dm− 3 MPc containing TBAP/DCM solutions. Coated GCE/MPc electrodes were rinsed with DCM to remove the monomeric MPc species and dried under atmospheric conditions then stored in a dark place. All sensing measurements were carried out in 0.10 mol dm− 3 LiClO4 containing PBS solutions at pH 7.0. Before sensing measurements, blank tests of the bare GCE and GCE/MPc working electrodes were carried out in pesticide free PBS solution with SWV. When the blank measurements reached to a steady state, the system was titrated with different pesticides and SWV responses of the modified electrodes were recorded during the titrations. Conductivity of ITO/MPcs (ma) electrodes was measured with a 4-probe conductometer.

0.91 0.85

2.4. Electrode modification and sensor measurements

Epa (V)

Odx. of subs.

tw tw

Ref.

and the reference electrodes respectively. In situ UV–Vis spectroelectrochemical and in situ electrocolorimetric measurements were carried out under potentiostatic control using an Ocean Optics QE65000 diode array spectrophotometer, utilizing a three-electrode configuration of a thin-layer quartz spectroelectrochemical cell by following the procedure in the literature [3].

tw

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electron transfer reactions of the central metal of CoPc, the separation between the first reduction and the first oxidation processes (ΔE1/ 2 = 0.78 V) is smaller than those of MPcs having redox inactive metal centers. The peak currents of the oxidation process at 1.21 V is high than those of the other processes due to the oxidation of tetra substituents. Although morpholin and amino groups of the substituents are oxidized, the complex did not electropolymerized during the repetitive CV measurements in TBAP/DMSO electrolyte. In order to investigate the effects of the metal center to the redox properties and electropolymerization abilities, MPcs bearing Cl-MnIII and TiIVO metal centers are also investigated and the results are compared with those of CoPc(ma). The electrochemistry of TiOPc(ma) complex is determined with CV and SWV methods (Fig. SM1). TiOPc (ma) displays four reduction processes, which appear at Red1 = −0.40 V, Red2 = − 0.67 V, Red3 = − 1.32 V, and Red4 = −1.89 V and three oxidation processes at Oxd1 = 0.86 V, Oxd2 = 1.08 V, and Oxd3 = 1.25 V in the potential window of DMSO/ TBAP electrolyte system. Although these redox processes are not clearly observed with CV measurements, SWVs of the complex shows these processes clearly. Irreversibility of the redox processes may be resulted from the aggregation or releasing the axial oxygen ion of the complex. TiOPc(ma) and CoPc(ma) show different number of the substituentbased oxidation processes. While CoPc(ma) shows an oxidation peak at 1.21 V, two substituent-based oxidation peaks are observed at Oxd2 = 1.08 V and Oxd3 = 1.25 V with TiOPc(ma). These peaks are most probably resulted from the oxidation of amino and morpholin groups respectively. As shown in CV and SWV responses of Cl-MnPc(ma) in Fig. SM2, ClMnPc(ma) gives four quasi-reversible reduction processes, Red1 at − 0.30 V, Red2 at −0.90 V, Red3 at − 1.36 V, Red4 at −1.88 V respectively during cathodic potential scans. While the first and second reduction processes are attributed to the reduction of [Cl-MnIIIPc2 −] to [Cl-MnIIPc2 −]1 − and then [Cl-MnIPc2 −]2 − species, the later processes are attributed to [Cl-MnIPc2 −]2 −/[Cl-MnIPc3 −]3 − and [ClMnIPc3 −]3 −/[Cl-MnIPc4 −]4 − processes. During the anodic potential scans, a split Pc based peak at 0.38 and 0.51 V and substituent based oxidation peaks at 0.88 V at 1.21 V are observed. All of the oxidation processes are electrochemically and chemically irreversible processes.

3.2. Voltammetric measurements Newly synthesized MPcs were electrochemically characterized in detail with CV and SWV techniques to determine practical usage of them. Recorded voltammograms were analyzed in order to derive basic redox parameters including half-wave peak potentials (E1/2), ratio of anodic and cathodic peak currents (Ipa/Ipc), peak potential separations (ΔEp) (Table 1). Electrochemical responses of MPcs (ma) illustrated that the redox activity of Pc ring was enhanced by incorporating redox active CoII, Cl-MnIII, and TiIVO metal centers and redox active and electropolymerizable [2-(4-{[(1E)-(4-morpholin-4-ylphenyl)methylene] amino}phenyl)ethoxy] substituents. Metal based and substituent based redox processes are observed in addition to the Pc based processes [39]. Its well reported that metal free Pcs can only illustrate Pc based electron transfer reactions. Both of morpholin and amino groups of the substituents are well reported as redox active and electropolymerizable groups [40–42], therefore these groups show characteristic oxidation processes and trigger coating of the complexes on the working electrodes with the oxidative electropolymerization reactions. As discussed below, in addition to the redox responses of the substituents, metal centers of the complexes also influence the electrochemical behaviors of them. Changing the metal centers alters the redox activities of the complexes. As shown in Fig. 3, CoPc(ma) displays three reductions and three oxidation processes on the potential window of DMSO/TBAP electrolyte system. All redox properties are electrochemically and chemically quasi reversible with respect to Ipa/Ipc ratios and ΔEp values of each redox couples. These behaviors indicate instabilities of the electrogenerated redox species and complication of the electron transfer reaction with possible chemical reactions. These chemical reactions are most probably aggregation-disaggregation equilibria and the equilibria between the coordinated and non-coordinated species. It is reported that MPcs have highly aggregation tendencies and especially MPcs having redox active metal centers are coordinated with the donor solvents or supporting electrolyte anions [43,44]. Due to the

3.3. Spectroelectrochemical measurements Redox processes and peak assignments of the complexes were determined with in-situ spectroelectrochemical measurements. in situ spectroelectrochemical responses of Cl-MnPc(ma) (Fig. SM3) and TiOPc (ma) (Fig. SM4) were given in Supplementary file for comparisons. Here in situ spectroelectrochemical responses of CoPc(ma) (Fig. 4) measured in TBAP/DMSO electrolyte system is discussed as an example. Spectral changes of CoPc(ma) observed during the first reduction reaction indicate the metal based character of the process. Since observation of a new band at 470 nm and shifting of the Q band from 686 nm to 707 nm are characteristic spectral changes for [CoIIPc2 −]/ [CoIPc2 −]1 − process (Fig. 4a). While the Q band of MPcs decreases without a shift for the Pc based redox processes, the Q band shifts during the metal based processes. Therefore, the spectral changes observed during the first reduction reaction easily assigned to the reduction of CoII center. Moreover, the band at 470 nm is assigned to the metal to ligand charge transition (MLCT) for [CoIPc2 −]1 − species. These spectral changes and redox mechanism of CoPc(ma) are in agreement with the reported CoPcs [45–47]. Observation of broad band at around 550 nm and decreasing the Q band without shifting assign these processes to [CoIPc2 −]1 −/[CoIPc3 −]2 − and [CoIPc3 −]2 −/ [CoIPc4 −]3 − reductions during both of the second and third reduction couples (Fig. 4b). The first oxidation process could be assigned to [CoIIPc2 −]/[CoIIIPc2 −]1 + process (Fig. 4c). Shifting of the Q band towards longer wavelength with slight increase in intensity are characteristic changes for a metal-based oxidation process for CoPc type

Fig. 3. CV and SWVs of CoPc(ma) in TBAP/DMSO electrolyte system on a GCE working electrode at different scan rates.

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Fig. 4. In-situ UV–vis spectral changes of CoPc(ma) in DMSO/TBAP electrolyte. a) Eapp = − 0.60 V. b) Eapp = −1.50 V. c) Eapp = 0.40 V. c) Chromaticity diagram (each symbol represents the color of electrogenerated species; □:[CoIIPc− 2]; ○:[CoIPc− 2]− 1; △: [CoIPc− 3]− 2; ▽: [CoIPc− 4]− 3; ☆:[CoIIIPc− 2]+ 1.

complexes [46,48,49]. Color changes of CoPc complex during reduction processes were recorded with spectrocolorimetric measurement. Cyan color of the neutral CoPc(ma) turns to green, then yellow and finally to light yellow during the reduction reactions respectively. During the oxidation process, a light green color is observed as shown in Fig. 4d.

3.4. Electrode modification and characterization Electrochemical and spectroelectrochemical analyses show the redox richness of the complexes tailored with the redox active metal centers and redox active and polymerizable substituents. In order to use these functional complexes as possible pesticide sensors, they were coated on GCE with the oxidative electropolymerization reactions. Electropolymerizations of the complexes were carried out in acetonitrile or DCM due to the wide positive potential range and more acidity of these solvents. Fig. 5 illustrates the repetitive voltammograms of CoPc(ma) recorded during the electrochemical polymerization on the GCE with the repetitive CV technique. During the first CV cycle, the complex gives a peak at 1.21 V which is assigned to the oxidation of [2(4-{[(1E)-(4-morpholin-4-ylphenyl)methylene]amino}phenyl)ethoxy] substituents. During the second CV, a new peak appears at 0.89 V and this peak shifts to more positive potentials with increasing in the current intensity during the consecutive CV cycles until 4. CV cycle. These data indicate that formation of cationic CoPc(ma) species during the oxidation at 1.21 V triggers the polymerization reaction and the electropolymerized film gives a new redox peak at 0.89 V. Shifting of the peak of the electropolymerized film is most probably resulted from the decreasing the conductivity of the film with increasing of the film thickness. Optimal redox active and conductive film is prepared with the consecutive four CV cycles between − 0.50 and 1.30 V potential range. Modification of GCE with the redox active and conductive polymer film illustrates possible usage of CoPc(ma) modified electrode in different electrochemical fields, such as sensing applications.

Fig. 5. Repetitive CVs of CoPc(ma) in TBAP/DCM electrolyte system on a GCE working electrode at 0.100 mVs− 1 scan rate.

TiOPc(ma) is also electropolymerized in TBAP/DCM solution. Fig. SM5 shows the repetitive CV cycles of TiOPc(ma). During the first CV cycle, an oxidation peak at 1.08 V is observed. During the consecutive CV cycles, these peak increases in current intensity with the positive potential shift. These data illustrate formation of a redox active and conductive poly-TiOPc(ma) film on the working electrode. Like CoPc (ma) and TiOPc(ma), Cl-MnPc(ma) can also be electropolymerized in DCM/TBAP electrolyte system as shown in Fig. SM6. During the first CV cycle, Cl-MnPc(ma) shows two oxidation wave at 0.99 V and 1.31 V for the oxidation of amino and morpholin groups of the substituents. During the second CV, a new peak is observed at 1.17 V for the formation of electropolymerized film. This peak shifts to potentials that are more positive during the consecutive CV cycles. These data illustrate the formation of the redox active and conductive poly-Cl-MnPc (ma) film on the electrode surface. It is documented in the literature that amino containing phenyl and alkyl and morpholin groups can be easily electropolymerized with the oxidation of these groups. Length of 58

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shown in the cross-section views of the films. 3.5. Pesticide sensing measurements Modified GCE/CoPc(ma), GCE/Cl-MnPc(ma), and GCE/TiOPc(ma) electrodes were tested as possible pesticide sensors for chlorophyros, fenitrothion, and methomyl. SWV technique is used for sensing measurements. In order to determine the selectivity of the sensor electrodes, new redox processes and/or disappearance of existence peaks were followed with respect to increasing pesticide concentration. Current changes with respect to the concentration increases were used to determine sensitivity and LOD of the electrodes. Before each titration, blank tests were performed with 10 consecutive SWV excitations without the pesticides to determine stability of the electrodes and to determine their standard deviations. The construction of the electrodes and their usages as the pesticide sensors are illustrated in Scheme 1. It is shown that changing the metal center of the complexes significantly influenced the sensitivity and selectivity of the sensor electrodes. All GCE/MPc(ma) electrodes illustrated specific SWV responses for each pesticide. Sensing parameters were derived from the SWV responses of the electrodes and listed in Table 2. Sensing responses of GCE/CoPc(ma) electrode are represented in Figs. 7 and 8. GCE/CoPc (ma) electrode shows different SWV responses for the different pesticides. As shown in Fig. 7a, without the pesticide addition, GCE/CoPc (ma) electrode gives two peaks at −0.22 V for [CoIIPc2 −]/ [CoIPc2 −]1 − and at 0.20 V for [CoIIPc2 −]/[CoIIIPc2 −]1 + processes. After reaching to a steady state with 10 successive SWVs, chlorophyros is added gradually and SWV responses of GCE/CoPc(ma) electrode are followed. With increasing chlorophyros concentration, the peak at 0.22 V shifts to − 0.020 V with a slight increase in the current intensity until 0.25 μmol dm− 3 chlorophyros addition, then this peak decreases in the intensity during the further chlorophyros additions. As shown in Fig. 7a, while the sensitivity of the electrode is reasonable high, its range of detection is very narrow. Fig. 7b represents SWV responses of GCE/CoPc(ma) electrode with respect to the gradual addition of methomyl. While the peak at −0.22 V disappears immediately the peak at 0.20 V decrease in the current intensity with a potential shift to − 0.090 V with respect to the increasing methomyl concentration. The linear range of methomyl sensing is also very narrow and any selective new peak is observed for methomyl sensing (Fig. 7b). Although sensing responses of GCE/CoPc(ma) electrode for chlorophyros and methomyl are not satisfying, better sensing responses are observed for the fenitrothion sensing. GCE/CoPc(ma) electrode shows completely different SWV response for the fenitrothion sensing as shown in Fig. 8. During the gradual increase of the fenitrothion concentration, two new redox peaks are observed at − 0.25 V and − 0.80 V during the cathodic potential scans and two redox peaks are observed at − 0.24 V and 0.06 V during the anodic potential scans. The currents of

Fig. 6. SWVs of GCE/MPc(ma) in PBS electrolyte at pH 7.

the alkyl chain and substituents of the morpholin groups influenced the electrochemical feature of these compounds [42,50,51]. Therefore, different MPcs bearing amino and morpholin groups showed different polymerization responses due to the different metal centers. Modified GCE/CoPc(ma), GCE/Cl-MnPc(ma), and GCE/TiOPc(ma) electrodes were characterized with SWV, IR, and SEM. Fig. 6 illustrates SWVs of the electrodes in PBS solution at pH 7. All electrodes show good redox activity and these properties of the electrodes vary with changing the metal centers of the complexes. While GCE/CoPc(ma) shows two peaks at −0.21 V and 0.03 V, Cl-MnPc(ma) shows three peaks at − 0.17, 0.18, and 0.94 V. Moreover, GCE/TiOPc(ma) electrode shows also two peaks at − 0.12 and 0.06 V. In order to indicate the conductivities of the electrodes, MPc(ma) complexes were coated on the ITO electrodes and conductivities of these electrodes were derived from the current vs. time responses of ITO/MPc(ma) electrodes (Fig. SM7). ITO/CoPc(ma), ITO/MnClPc(ma), and ITO/TiOPc(ma) electrodes have conductivities of 0.0677, 0.0695, and 0.0785 S cm2 − respectively. These conductivities are sufficient for the practical usage of the electrodes for various electrochemical technologies, e.g. electrochemical sensors. IR spectral characterization of the monomeric MPcs and their electropolymerized films are performed comparatively (Fig. SM8). As shown in the IR spectra, while the aliphatic CeH bands for the monomeric complexes at around 2969–2849 cm− 1 disappear, a new broad band is observed at this region after the polymerizations of the complexes. The bands between 1650 and 800 cm− 1 also decrease in intensity after electropolymerizations. These spectral responses support successive performing of the electropolymerization reactions. SEM image of the electropolymerized films on ITO electrodes are shown in Fig. SM9. As shown in the top view of SEM images, all complexes are homogeneously coated on ITO substrates and they show granular structures. Film thicknesses of the films are between 250 and 400 nm as

Scheme 1. Construction of electrochemical pesticide sensor with electropolymerization and SWV responses of GCE/MPc electrodes, with and without pesticides.

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Table 2 Sensing of different pesticides with different sensors. Sensors

GCE/CoPc(ma)

GCE/MnPc(ma)

GCE/TiOPc(ma) GCE/MnPc(m) GCE/TiOPc(m) rGO/AuNP SiO2/MWCNTs GCE/GO nano-MIP-CP

Pesticides

Fenitrothion Chlorophyros Methomyl Fenitrothion Chlorophyros Methomyl Fenitrothion Methomyl Diazinon Eserine Fenitrothion Fenitrothion Fenitrothion Diazinon

Sensing parameters

Ref.

Linear range (μmoldm− 3)

LOD (μmoldm− 3)

Sensitivity (Acm− 2 M− 1)

Selectivity

Technique

1.20–42.0 0.70–3.5 0.47–24.77 0.15–4.50 0.17–6.78 0.40–5.00 0.52–12.52 0.30–3.50 – 2.0–20.0 0.1–6.25 ng mL− 1 3.0–6.6 1–400 ng mL− 1 01–2.0

0.46 0.61 0.77 0.14 1.20 0.11 0.14 0.78 6.63 10− 3 1.35 0.036 ng mL− 1 0.45 ppm 0.10 ng mL− 1 7.9 10− 4

0.26 2.88 0.24 7.30 0.13 0.17 0.67 1.18 140.3 0.86 – 8.22 Acm− 2 M− 1 – 95.08 μA L μmol− 1

x – – – – – x – x x x x – x

SWV SWV SWV SWV SWV SWV SWV SWV SWV SWV CA SWV SWV

tw

tw

tw [40] [40] [58] [59] [60] [61]

Fig. 7. SWV responses of GCE/CoPc(ma) during the titration with a) chlopyrifos and b) methomyl pesticides in PBS solution at pH 7. Fig. 8. a) SWV responses of GCE/CoPc(ma) during the titration with fenitrothion pesticide in PBS solution at pH 7. b) Calibration line derived from Fig. 8a.

these peaks continuously increase with the increasing fenitrothion concentration. As shown in Fig. 8b, a linear range is observed between 1.20 μmol dm− 3 and 42.0 μmol dm− 3 concentrations. Sensitivity and LOD of the electrode are found as 0.26 Acm− 2 M− 1 and 0.46 μmol dm− 3 respectively. These data indicate usability of the GCE/ CoPc(ma) electrode as the selective and sensitive fenitrothion sensor among the pesticides studied here. These different sensing responses are most probably resulted from the different molecular structure of the pesticides. While methomyl is a carbamate, fenitrothion and chlorophyros are organophosphates (Fig. 9). Main differences between fenitrothion and chlorophyros are presence of nitrophenyl group on the fenitrothion and trichloropyridinyl group on the chlorophyros. Most probably catalytic reduction of nitrophenyl group on the fenitrothion with GCE/CoPc(ma) electrode gives the selective redox processes for this pesticide. In order to investigate effects of the metal center of MPcs to the sensing properties, GCE/Cl-MnPc(ma) and GCE/TiOPc(ma) electrodes are also tested and the results are tabulated in Table 2 and they are compared with those of GCE/CoPc(ma). GCE/MnClPc(ma) showed

similar SWV responses for all pesticides. Fig. 10 represents SWVs of GCE/Cl-MnPc(ma) during titration with the methomyl as an example. GCE/Cl-MnPc(ma) electrode shows two peaks at 0.91 V and 0.06 V without the pesticide addition. When the methomyl concentration gradually increases, the peak at 0.91 V decreases in the current intensity and disappears finally. At the same time, the peak at 0.06 V shifts to the negative potentials with a decrease in the intensity. Same SWV changes are observed for other pesticides studied here, the only current changes, and degree of potential shifts are different from each other's. Therefore, GCE/Cl-MnPc(ma) electrode could not be used as selective pesticide sensor, although its sensitivity is reasonably well. GCE/TiOPc(ma) electrode gives characteristic SWV changes only for the fenitrothion (Fig. 11). GCE/TiOPc(ma) electrode gives a redox couple at 0.06 V without the pesticides in the electrolyte. When the concentration of the fenitrothion gradually increases, a new peak increases at − 0.28 V as a function of the increasing fenitrothion concentration during the anodic potential scans. When compared with the 60

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Fig. 11. a) SWV responses of GCE/TiOPc(ma) during the titration with fenitrothion pesticide in PBS solution at pH 7. b) calibration line derived from Fig. 15a.

Therefore, it is proposed that CoII, MnIII and TiIVO centers of MPc(ma) coordinated with the pesticides and these reactions altered the redox responses of the complexes. The strengths of the coordination bonds of the complexes with the pesticides are different than each other due to the different electron donating behaviors of the pesticides and coordination tendencies of MPcs. These differences influenced the sensing responses of the complexes, thus each complexes gave specific voltammetric responses for different pesticides. The reproducibility and repeatability of the electrodes were tested by SWVs which were carried out in the blank solutions and the solutions containing concentration of 10.0 μmoldm− 3 fenitrothion. Measurements were performed by using five identical electrodes with 10 SWV measurements. Any significant loss of electrocatalytic activity were observed with all electrodes and less than 5.0% relative standard deviations (RSD) were observed for all electrodes. After storage under atmospheric condition, all electrode kept > 92% of their activities. In order to investigate the effects of the interferences, real sample tests of GCE/CoPc(ma) electrode is performed with the fenitrothion contaminated orange juice. The SWV responses of the electrode slightly changed due to the different electrolyte system and the effects of the interferences, but the peaks of GCE/CoPc(ma) electrode at −0.22 V for [CoIIPc2 −]/[CoIPc2 −]1 − and at 0.20 V for [CoIIPc2 −]/[CoIIIPc2 −]1 + process were clearly observed. No voltammetric response corresponding to fenitrothion was observed when the non-contaminated orange juice was analyzed. After addition of fenitrothion to the orange juice, the interaction peaks of the fenitrothion with CoPc(ma) were clearly recorded at −0.25 V and −0.80 V. Standard-additions method was used to estimate the accuracy of the sensor electrode. The measurement results showed that 10.0 μmoldm− 3 fenitrothion in contaminated orange juice was found as 9.78 μmoldm− 3 with recovery of 97.8%. In our previous paper, the pesticide sensing properties of electropolymerized film CoPc(m), MnPc(m), and TiOPc(m) bearing electropolymerizable {[4-(2-morpholin-4-ylethoxy)benzyl]oxy} substituents were reported for eserine, parathion, diazinon, and fenitrothion pesticides [40]. The main differences between the MPcs(m) in the previous paper and the complexes MPc(ma) studied here is the using of two

Fig. 9. Molecular structure of pesticides; a) fenitrothion, b) methomyl and c) chlorophyros.

Fig. 10. SWV responses of GCE/MnClPc(ma) during the titration with methomyl pesticide in PBS solution at pH 7.

results of GCE/Cl-MnPc(ma) electrode, the linear range is very narrow (0.52 μmol dm− 3–12.52 μmol dm− 3) and LOD value (0.14 μmol dm− 3) is small with GCE/TiOPc(ma) electrode. Small LOD value and good selectivity of GCE/TiOPc(ma) electrode illustrate its possible usage as a possible fenitrothion sensor. All sensing results indicated that MPcs were interacted with the pesticides and these interactions influenced the redox activities of the complexes. It is well known that metal center of the MPcs having redox active metal centers, such as CoII, MnIII and TiIVO can be coordinated from the axial positions with the electron donating species [52–54]. 61

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polymerizable groups (morpholin and amino group) now instead of morpholin bearing substituents. When we compared the results of MPcs (m) and MPcs(ma), it is clear that changing the substituents significantly altered the pesticide sensing activities. While CoPcs(m) did not sensed any of eserine, parathion, diazinon and fenitrothion pesticides, MnPcs(m) sensed diazinon and TiOPcs(m) only sensed eserine pesticide. Here CoPc(ma) and TiOPc(ma) sensed the fenitrothion with high sensitivity and selectivity. 4. Conclusions In order to increase redox activity, electropolymerizable ability and sensing properties of MPcs, new MPcs were synthesized by the cyclotetramerization of 4-[2-(4-{[(1E)-(4-morpholin-4-ylphenyl)methylene] amino}phenyl)ethoxy]phthalonitrile 3. The synthesized cobalt(II), titanium(IV), manganese(III) phthalocyanines were characterized by a combination of IR, 1H NMR, 13C NMR, UV–Vis, MS spectral data, and elemental analysis. Electrochemical features of the complexes were performed with CV and SWV techniques. All complexes were coated on GCE electrodes with the oxidation for polymerizable morpholin and amino groups of the substituents of the complexes. Modified electrodes prepared with the oxidative electropolymerization were tested as pesticide sensors for chlorophyros, fenitrothion, and methomyl. GCE/CoPc (ma) electrode behaves as selective and sensitive fenitrothion sensor. Although sensitivity of GCE/TiOPc(ma) selectively sense fenitrothion, its linear range is very narrow. GCE/Cl-MnPc(ma) showed good sensitivity for all pesticide, but its selectivity is poorer than other electrodes. The reproducibility and stability of the GCE/MPc electrodes were good enough for the practical usages. Acknowledgements Authors thank to The Scientific and Technological Research Council of Turkey (TUBITAK, Project Number: 114Z914) and Turkish Academy of Sciences (TUBA) for their financial support. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jelechem.2017.09.044. References [1] H. Chen, X. Zuo, S. Su, Z. Tang, A. Wu, S. Song, D. Zhang, C. Fan, An electrochemical sensor for pesticide assays based on carbon nanotube-enhanced acetycholinesterase activity, Analyst 133 (2008) 1182–1186. [2] J. Gong, X. Miao, T. Zhou, L. Zhang, An enzymeless organophosphate pesticide sensor using Au nanoparticle-decorated graphene hybrid nanosheet as solid-phase extraction, Talanta 85 (2011) 1344–1349. [3] Y. İpek, H. Dinçer, A. Koca, Selective electrochemical pesticide sensor modified with “click electrochemistry” between cobaltphthalocyanine and 4-azidoaniline, J. Electrochem. Soc. 161 (2014) B183–B190. [4] E. Llorent-Martínez, P. Ortega-Barrales, M. Fernández-de Córdova, A. Ruiz-Medina, Trends in flow-based analytical methods applied to pesticide detection: a review, Anal. Chim. Acta 684 (2011) 30–39. [5] S. Viswanathan, H. Radecka, J. Radecki, Electrochemical biosensor for pesticides based on acetylcholinesterase immobilized on polyaniline deposited on vertically assembled carbon nanotubes wrapped with ssDNA, Biosens. Bioelectron. 24 (2009) 2772–2777. [6] R. Bogue, Nanosensors: a review of recent research, Sens. Rev. 29 (2009) 310–315. [7] N. Kim, I.-S. Park, D.-K. Kim, High-sensitivity detection for model organophosphorus and carbamate pesticide with quartz crystal microbalance-precipitation sensor, Biosens. Bioelectron. 22 (2007) 1593–1599. [8] D. Štajnbaher, L. Zupančič-Kralj, Multiresidue method for determination of 90 pesticides in fresh fruits and vegetables using solid-phase extraction and gas chromatography-mass spectrometry, J. Chromatogr. A 1015 (2003) 185–198. [9] R. Bossi, K.V. Vejrup, B.B. Mogensen, W.A. Asman, Analysis of polar pesticides in rainwater in Denmark by liquid chromatography–tandem mass spectrometry, J. Chromatogr. A 957 (2002) 27–36. [10] H. Li, F. Li, C. Han, Z. Cui, G. Xie, A. Zhang, Highly sensitive and selective tryptophan colorimetric sensor based on 4, 4-bipyridine-functionalized silver nanoparticles, Sensors Actuators B Chem. 145 (2010) 194–199.

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