A derivative photoelectrochemical sensing platform for 4-nitrophenolate contained organophosphates pesticide based on carboxylated perylene sensitized nano-TiO2

Analytica Chimica Acta 766 (2013) 47–52

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A derivative photoelectrochemical sensing platform for 4-nitrophenolate contained organophosphates pesticide based on carboxylated perylene sensitized nano-TiO2 Hongbo Li a,b , Jing Li b , Qin Xu a , Zhanjun Yang a , Xiaoya Hu a,∗ a b

College of Chemistry and Engineering, Yangzhou University, 88 South University Avenue, Yangzhou 225002, PR China College of Chemistry and Biology Engineering, Yancheng Institute of Technology, 9 Yingbin Avenue, Yancheng 224051, PR China

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

 A novel enzymeless photoelectrochemical sensor for 4-nitrophenolate contained OPs.  Sensors have performances of rapid response, good sensitivity and selectivity.  PTCA as sensitizer can form ultrastable thin film and is economic as well.  The strategy extends the application of PTCA for photoelectrochemical sensor.

a r t i c l e

i n f o

Article history: Received 17 October 2012 Received in revised form 17 December 2012 Accepted 20 December 2012 Available online 2 January 2013 Keywords: Photoelectrochemistry Sensor Heterojunction Nano-titania Carboxylated perylene

a b s t r a c t A novel visible light sensitized photoelectrochemical sensing platform was constructed based on the perylene-3,4,9,10-tetracarboxylic acid/titanium dioxide (PTCA/TiO2 ) heterojunction as the photoelectric beacon. PTCA was synthesized via facile steps of hydrolysis and neutralization reaction, and then the PTCA/TiO2 heterojunction was easily prepared by coating PTCA on nano-TiO2 surface. The resulting photoelectric beacon was characterized by transmission electron microscope, scanning electron microscopy, X-ray diffractometry, FTIR spectroscopy, and ultraviolet and visible spectrophotometer. Using parathion-methyl as a model, after a simple hydrolyzation process, p-nitrophenol as the hydrolysate of parathion-methyl could be obtained, the fabricated derivative photoelectrochemical sensor showed good performances with a rapid response, instrument simple and portable, low detection limit (0.08 nmol L−1 ) at a signal-to-noise ratio of 3, and good selectivity against other pesticides and possible interferences. It had been successfully applied to the detection of parathion-methyl in green vegetables and the results agreed well with that by GC–MS. This strategy not only extends the application of PTCA, but also presents a simple, economic and novel methodology for photoelectrochemical sensing. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Organophosphates (OPs) are the most toxic species commonly found in both pesticides and chemical-warfare agents whose rapid and severe effects on human and animal health lie in their ability to

∗ Corresponding author. Tel.: +86 514 87971818; fax: +86 514 87311374. E-mail addresses: [email protected], yz [email protected] (X. Hu). 0003-2670/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.aca.2012.12.038

block the action of acetylcholinesterase (AChE), a critical centralnervous-system enzyme [1,2]. Parathion-methyl (PM) is one kind of OPs that is very toxic, with an LD50 of 3 mg kg−1 in rats, and may be responsible for more deaths among agricultural field workers than any other pesticide [3]. This creates a demand for the development of accurate, sensitive, rapid, easy-to-use and portable method to detect the high toxic PM. Over the past two decades, extensive researches for PM analysis have been developed, such as LC–MS or LC–EC [4–6],

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electrochemical sensor [7,8] or enzyme-based electrochemical biosensor [9–12], fluorescence biosensor [13–15], and molecular imprinted technique (MIT)-based electrochemical sensors [16–19]. In general, each of these methods suffers from at least one undesirable limitation, such as limited selectivity, low sensitivity, operational complexity, lack of portability, or the difficulties of realtime monitoring. Recently, great progresses have been made in applying nanomaterial-based electrochemical sensors/biosensors [20,21] or fluorescent chemosensors [22] development for pesticides, which were well known as two kinds of convenient and simple means of chemical detection. But most of them are still enzyme-based biosensors [12,14,15] or MIT-based sensors for PM [18,19]. Enzyme-based biosensors for the detection of OPs can be categorized into two general classes on the basis of the enzyme employed-acetylcholinesterase (AChE) or organophosphorus hydrolase (OPH) and the later have distinct advantages over AChE-based systems [23]. But neither of them has good specificity to PM, let alone the good stability of the sensors. At this point, MIT-based sensors have excellent specificity but suffer from tedious preparation. Consequently, the development of simple, rapid, economic, selective and sensitive method to detect PM is still a challenge. Photoelectrochemical measurement is potentially as sensitive as electrochemiluminescence (ECL) owing to the complete separation of excitation source (light) and detection signal (photocurrent). It has attracted considerable interests as a newly developed and promising analytical technique [24–35]. Moreover, the utilization of electronic detection makes the photoelectrochemical instruments simpler and low-cost compared with those of the conventional optical methods. During the recent decades, the fascinating inorganic semiconductor titanium dioxide (TiO2 ) has attracted extensive attention in the photocatalytic and photoelectrochemical area due to its nontoxicity, hydrophilicity, cheap availability, stability and against photocorrosion for its suitable flat band potential in addition to its easy supported on various substrates [36–38]. However, the wide band gap of TiO2 (∼3.2 eV, anatase) only allows it to absorb the ultraviolet light (<387 nm) [36]. In order to extend the photoresponse to the visible region and promote the photoelectric conversion efficiency, many modification methods have been applied, such as dye sensitization, metal ion/nonmetal atoms doping, semiconductor coupling, and noble metal deposition [38]. Among the above methods, organic molecule-based photovoltaic materials and devices are attracting more and more attention for the advantages such as low-cost, light-weight, feasibility for largescale device manufacturing [39–42]. Thus, considering of the high electron mobility of nanocrystals as well as the possibility of tuning the optical band gap into visible light region by organic materials, the organic–inorganic heterojunction can fabricate a robust photoelectrochemical sensor. Recently, Ju team [29] has developed a photoelectrochemical biosensor for glutathione based on iron–porphyrin-containing sulfonic group on TiO2 nanoparticles. Considering of the carboxylic-group-containing porphyrins possess a higher solar-energy conversion efficiency than sulfonicgroup-containing porphyrins in dye-sensitized solar cells due to their stronger absorption coefficient [43], they further developed the other photoelectrochemical biosensor for cysteine based on carboxylic-group-containing free-base porphyrins functionalized ZnO nanoparticles, which has nearly the same band-gap as TiO2 [44]. Also, our group have developed two photoelectrochemical sensors for pesticides dichlofenthion and chlorpyrifos based on TiO2 photocatalysis coupled with electrochemical detection, namely a derivative electrochemical sensor and P3HT sensitized TiO2 , respectively [45,46]. Recently, a novel AChE-functionalized photoelectrochemical biosensor for OPs was developed [47] based on bismuth oxyhalides as the photoelectric beacon, which has

Scheme 1. Schematic illustration of proposed photoelectrochemical mechanism for parathion-methyl at PTCA/TiO2 modified GCE.

the band gap of 1.8 eV and the absorption edge is about 680 nm. Although the limit of detection (LOD) for PM was lowered to 0.16 nM, but its specificity for PM is limited since AChE has the similar response to other OPs and carbamate pesticides [1,14,15]. Since neither AChE nor OPH has good specificity to PM, novel enzymeless photoelectrochemical sensor with better sensitivity and selectivity for PM is our dream. Consequently, our eyes were focused on novel photoelectric beacon with excellent performances. Perylene-3,4,9,10-tetracarboxylic acid (PTCA, see Fig. S1) with its conjugated polyaromatic core and the two directly attached active carboxyl groups that facilitate surface anchoring by hydrogen bond formation, may be a good candidate to construct a new heterojunction with TiO2 . It can absorb broad visible light and emit from a singlet state with quantum yields near unity [48], which offers both fundamental and practical advantages for uses as a sensitizer since it has large extinction coefficient and high photostability. As the mother dye of PTCA, perylene together with its derivants has been widely explored in organic electronics [49,50] or as fluorescent agents [51–54]. But surprisingly, there is still no report on their application in photoelectrochemical sensing. Herein, we developed a novel enzymeless derivative photoelectrochemical sensor for PM based on PTCA–TiO2 heterojunction, considering of the structure differences of PM with the above two pesticides. This novel sensor has more advantages over our previous photoelectrochemical sensors for OPs [45,46] and the newly developed photoelectrochemical sensor for OPs [47], such as it is more sensitive, more economic, and more facile to prepare the photo-sensitizer, together with no loss in the good selectivity and stability. PTCA was synthesized via facile steps of hydrolysis of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCD) and neutralization reaction [53,54], and then the PTCA/TiO2 heterojunction was easily prepared by coating PTCA on nano-TiO2 surface. After hydrolysis of PM to p-nitrophenol which has the phenolic hydroxyl as an excellent electron donor [9,55], the fabricated photoelectrochemical sensor was used for the derivative PM (Scheme 1) detection. It had been successfully applied to the detection of PM in green vegetables and the results agreed well with that by GC–MS. 2. Experimental 2.1. Materials and reagents 3,4,9,10-Perylenetetracarboxylic dianhydride (≥99%) (PTCD), TiO2 nanopowder (anatase, <25 nm, 99.7%), chlorpyrifos, dichlofenthion and parathion-methyl (≥98%) were purchased from Sigma–Aldrich (St. Louis, MO). Parathion-ethyl, fenitrothion and dicapthon (≥98%) were purchased from Aladdin (Shanghai Corp.,

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China). All other chemicals were of analytical grade. In this work, 0.1 mol L−1 phosphate buffer solution (PBS) was always employed as the supporting electrolyte after being deaerated with high-purity nitrogen. Aqueous solutions were prepared with twice-distilled water, and the pH of PBS was 7.0 unless indicated otherwise. 2.2. Apparatus Transmission electron micrographs (TEMs) were obtained using a Tecnai 12 TEM (Philips, Netherlands). Scanning electron microscopy (SEM) was performed using a Hitachi S-4800 scanning electron microscope (Hitachi, Tokyo, Japan). X-ray diffraction (XRD) patterns of PTCA/TiO2 nanocomposites were measured in the range of 2 = 10–80◦ by step scanning on the Bruker D8 Advance (super speed) diffractometer (Bruker-AEX, German) with Cu K␣ radiation (k = 0.15406 nm) operated at 40 kV and 100 mA. Fourier-transform infrared (FTIR) spectra were obtained on a Nicolet iS10 instrument (Nicolet, USA). UV–Visible spectra were recorded at room temperature with a Cary 5000 ultraviolet and visible spectrophotometer (Varian, USA). Photoelectrochemical measurements were performed with a home-built photoelectrochemical system. Photocurrent was measured by the current–time curve experimental technique on a CHI760D electrochemical workstation (CH Instruments, Shanghai, China) with a 250 W tungsten halogen lamp light as the irradiation source (simulated sunlight irradiation). All experiments were carried out at room temperature using a conventional three electrode system with a modified glassy carbon electrode (GCE) as the working electrode, a platinum wire as the auxiliary electrode, and a saturated calomel electrode as the reference electrode. The model surveyor apparatus used in this study was a GC/MS. A Thermo Quest TRACE 2000 gas chromatograph coupled to a GCQ (Thermo Quest, Austin, TX) ion trap mass spectrometer was used. The GC was equipped with a split/splitless injector operated in the splitless mode and an autosampler AS-2000. The analytical column used was a 30 m × 0.25 mm i.d., 0.25 ␮m film thickness, Rtx-5 ms (Restek, Bellefonte, PA) coated with a 5% diphenyl–95% dimethylsiloxane stationary phase. The temperature program consisted of 2.0 min hold at 70 ◦ C, ramp at 18 ◦ C min−1 to 150 ◦ C and held for 1 min, 2 ◦ C min−1 to 200 ◦ C, 15 ◦ C min−1 to 280 ◦ C, and a final hold for 5 min. The injector was operated at 200 ◦ C with a split flow of 50 mL min−1 and a splitless time of 0.75 min. The helium carrier gas flow was 1 mL min−1 . The ion source was operated in the CI mode with methane as the reagent gas. The source temperature was set at 200 ◦ C, pressure at 1.2 × 10−4 Torr (1 Torr = 133.322 Pa), and transfer line temperature at 275 ◦ C. The system was tuned in the negative ion chemical ionization mode with heptacosafluorotributylamine(C4F9)3N (FC-43, ULTRA Scientific, North Kingstown) with the electron multiplier set at 1375 V and trap offset at 7 V. 2.3. Preparation of PTCA, PTCA/TiO2 , and modified GCE PTCA was obtained according to the literature with a little modification [53,54]. Briefly, PTCD was dissolved in 5% aqueous solution of KOH under stirring at 65 ◦ C. After cooling to room temperature, 0.1 mol L−1 HCl was added drop-wise under stirring till the pH value was 4.8. The PTCA precipitate formed was filtered, kept from heat, and vacuum dried at room temperature to yield a red powder (yield 96.5%). The PTCA/TiO2 heterojunction was prepared according to the following steps: first, 10.0 mg of TiO2 nanoparticles was added to 10.0 mL of aqueous solution forming 1.0 mg mL−1 suspension. After a GCE had been polished with Al2 O3 (0.3 ␮m), washed with acetone and twice-distilled water, and dried at room temperature, then, 10 ␮L of the TiO2 suspension (1.0 mg mL−1 ) was coated onto the GCE and dried at room temperature to obtain a TiO2 -modified GCE. Following, different concentrations DMF solution of PTCA was

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spreaded onto the TiO2 -modified GCE. Thus the PTCA/TiO2 heterojunction was fabricated until the DMF completely volatilized at room temperature. 2.4. Preparation of parathion-methyl derivant and detection procedure Before the photoelectrochemical measurement of PM, the derivative process was first carried out. According to its structure characteristics, PM is more easily hydrolyzed than other OPs pesticides. Its hydrolyzation is highly pH-dependent and it could be fastly hydrolyzed in basic solution. Its hydrolyzation rate increases rapidly with the increase of solution pH from 7 to 11 with the hydrolysate of p-nitrophenol [55]. Here, we chose 0.05 mol L−1 NaOH solution to hydrolyze PM. That is, 26.3 mg PM was dissolved in 100 mL of 0.05 mol L−1 NaOH solution to form 1 mmol L−1 PM in NaOH solution. A color change from colorless to flavescent solution was observed immediately once PM was added into the NaOH solution and the solution color deepened with the reaction process. In order to enhance the reaction, it needed to be adequately ultrasonic hydrolyzed in 0.05 mol L−1 sodium hydroxide solution for 1 h. Here, 1 h is long enough for 1 mmol L−1 PM hydrolyzation completely according to our experiment results with GC–MS tracking. Then, different volumes of the hydrolyzed PM were injected into the photoelectrochemical cell with 10 mL of PBS (pH 7.0) indicating different concentrations of PM. The fabricated photoelectrochemical sensor was applied to detect different concentration solution of hydrolyzed PM by current–time curve experimental technique with tungsten halogen light excitation. 3. Results and discussion 3.1. Characterization of PTCA/TiO2 The TEM morphologies of neat TiO2 and PTCA/TiO2 nanoparticles are clearly displayed in Figs. 1A and 2B. It can be confirmed that the morphology of the composites is similar to that of neat TiO2 (Fig. 1A). In addition, the modification of PTCA does not significantly change the size of neat TiO2 (Fig. 1B). The mean sizes of both nanoparticles are 20–30 nm approximately. The thickness of the PTCA/TiO2 film on an ITO surface was estimated by SEM to be 5.1 ␮m (Fig. 1C). The homogeneous heterojunction film was beneficial for separation of the photon-generated carrier and then injected into the electrode. To prove the produced PTCA and PTCA/TiO2 heterojunction, the FTIR spectrum of PTCD, PTCA, TiO2 , and PTCA/TiO2 is presented in Fig. 2. The FTIR spectrum of PTCD showed several obvious peaks at 3100, 1800, 1600–1450, and 1300–1030 cm−1 representing the stretching vibrations of C H, C O, C C, and C O, respectively (curve a). Compared with curve a, the peak at 1600 cm−1 is assigned to aromatic v(C C) stretching vibration in PTCA (curve b). The very broad stretching bands v(O H) at about 3000 cm−1 and v(C O) at 1770 cm−1 are assigned to the carboxyl groups of PTCA [56]. Moreover, the transmittance of C O absorption peak remarkably decreased (curves a and b). These implications indicated that PTCA was successfully produced. After PTCA was grinded with TiO2 , the FTIR spectra of PTCA/TiO2 (curve d) showed the absorption peaks of PTCA itself in comparison with that of TiO2 (curve c). However, the absorption peak of C O in PTCA/TiO2 was a little hypsochromic shift owing to the forming of hydrogen bond between COOH of PTCA and OH of TiO2 surface and the steric hindrance of TiO2 consequently, which indicated that the PTCA/TiO2 heterojunction could be handily constructed. The XRD pattern of TiO2 , PTCA, and PTCA/TiO2 is shown in Fig. 3. It can be seen that there were a little differences between curves

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Fig. 2. FTIR spectrum of (a) PTCD, (b) PTCA, (c) TiO2 , and (d) PTCA/TiO2 (4:100) powders.

Fig. 3. X-ray diffraction pattern of TiO2 nanoparticles (a), PTCA (b), and PTCA/TiO2 nanocomposite (c) (4:100).

bathochromic-shift than PTCD in the solid state [57]. Compared with TiO2 alone (Fig. 4, curve a), the absorption wavelength was broaden largely to the visible region when PTCA was coated on it, thus extremely favorable to improve the sensitivity of the fabricated photoelectrochemical sensor.

Fig. 1. TEM images of (A) TiO2 and (B) PTCA/TiO2 nanoparticle suspensions at 1.0 mg mL−1 and (C) SEM section image of PTCA/TiO2 -modified ITO glass.

a and c in shape and position of the diffraction peaks. The amount of PTCA was so small that diffraction peaks only at 2 = 12.4◦ and 27.3◦ of PTCA were observed in curve c. Results implied that the crystalline phase of TiO2 had not been changed by the modification of PTCA. The mean sizes of TiO2 nanoparticles and PTCA/TiO2 nanocomposites, calculated by Scherrer’s formula, were 21 and 26 nm, respectively, which were approximately consistent with the results of TEM analyses. Therefore, the formed PTCA/TiO2 heterojunction was further confirmed. Fig. 4 shows the solid UV–vis absorption spectra of TiO2 , PTCA, and PTCA sensitized TiO2 on ITO glass. The optical band gap of PTCA estimated from the band edge of absorption (∼745 nm) was 1.66 eV, which was well according with Egcal (1.65 eV), significant

Fig. 4. UV–visible diffuse reflectance spectra of (a) TiO2 , (b) PTCA, (c) PTCA/TiO2 (4:100) powders.

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Fig. 5. Photocurrent responses of (a, c) TiO2 and (b, d) PTCA/TiO2 (4:100) modified GCEs in 0.1 mol L−1 pH 7.0 PBS in the (a, b) absence and (c, d) presence of 5 nmol L−1 hydrolyzed PM at a bias voltage of 0.2 V with tungsten halogen light excitation.

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Fig. 6. Photocurrent responses at PTCA/TiO2 modified GCE in 0.1 mol L−1 pH 7.0 PBS in the presence of 0, 0.1, 0.5, 1, 2, 3, 4, 5, 6, 8, and 10 nmol L−1 PM (from bottom to top) at a bias voltage of 0.2 V with tungsten halogen light excitation. Inset: linear calibration curve.

3.2. Photoelectrochemical sensing response and mechanism As to the detection of PM, the hydrolysate could improve the photocurrent response for about 2 times (Fig. S2 in ESI), so we chose the hydrolysate of PM for photoelectrochemical sensing. Upon photoexcitation with tungsten halogen light (simulated sunlight), the TiO2 -modified GCE showed a photocurrent of 16.5 nA at a bias voltage of 0.2 V (Fig. 5, curve a), whereas the PTCA/TiO2 -modified GCE showed a photocurrent of 24.8 nA (curve b), indicating the improvement of the photocurrent conversion efficiency of TiO2 by the addition of PTCA because of the strong electronic coupling between the excited-state PTCA and the conduction band of TiO2 . Furthermore, the improved photocurrent conversion efficiency could be further amplified through the PTCA positive holes (h+ ) digestion process by p-nitrophenol as the hydrolysate of PM. Upon addition of 5 nmol L−1 hydrolyzed PM, the photocurrent of PTCA/TiO2 -modified GCE increased by 47.9 nA (curves b and d), which was 1.8 times the photocurrent increment of 26.1 nA observed at the TiO2 -modified GCE (curves a and c). The results indicated that PTCA can absorb the broad visible light to increase the sensitivity of photoelectrochemical sensing for PM. The relative energy level of PTCA and TiO2 is shown in Scheme 1. On the basis of the results of photoelectrochemical measurements, the photoelectrochemical mechanism for hydrolyzed PM oxidation under tungsten halogen light (simulated sunlight) irradiation can be inferred as follows. PTCA absorbed broad visible light to induce and transport electron from HOMO to LUMO transition. The CB of TiO2 and LUMO of PTCA matched well in energy level, which can cause a synergic effect. On the basis of the synergic effect, the excited-state electrons could be injected into the dorbital (CB) of TiO2 readily and subsequently transferred to the GCE surface, leading to a sharp increase of the photocurrent. At the same time, a positive charged hole (h+ ) may form and migrate onto the modified electrode surface, which can oxidize p-nitrophenol as the hydrolysate of PM. Consequently, the more p-nitrophenol can produce the more amplifying photocurrent response and thus the amount of PM can be determined by the derivative photoelectrochemical measurements. 3.3. Analytical performance The photocurrent–time curve of PTCA/TiO2 -modified GCE clearly illustrated the rapid response of the modified electrode to

the hydrolyzed PM at a bias voltage of 0.2 V with tungsten halogen light irradiation (Fig. 6). Under the optimal conditions (see ESI Fig. S3), the response displayed a linear increase as the concentration of PM increased from 0.1 to 10 nmol L−1 (0.026–2.632 ng mL−1 ) with a detection limit of 0.08 nmol L−1 (21 ng L−1 ). Although the detection limit of 21 ng L−1 was higher than that of LC/APCI–MS (9 ng L−1 ) [5] and AChE assembled on PDMS–PDDA–Au nanocomposite modified electrochemical biosensor (1 ng L−1 ) [12], it was much lower than those of the enzyme-based biosensors (0.5, 5.2, 1.0 ng mL−1 ) [9–11], fluorescent immunosensor (15 ␮g L−1 ) [13], MIP-based electrochemical sensors (5, 3, 0.09, 17.2 ng mL−1 ) [16–19] and LCECD methods (0.5 ␮g L−1 , 60 ng L−1 ) [4,6]. Even if compared with the newly developed layered double hydroxides/graphene hybridbased electrochemical method (0.6 ng mL−1 ) [8] and photoelectrochemical method (40 ng L−1 ) [47], this sensor is still superior in both the sensitivity and selectivity. Moreover, the lower detection limit of 21 ng L−1 is suitable enough for the photoelectrochemical detection of PM in drinking water because it is well below the maximum residue limits set by the European legislation (100 ng L−1 ) [5]. In addition, the enzymeless derivative photoelectrochemical sensor for PM showed good fabrication reproducibility with a relative standard deviation of 4.6% estimated from the slopes of the calibration plots of six freshly prepared PTCA/TiO2 -modified GCEs. When the concentrations of PM were 0.5 and 6.0 nmol L−1 , the photoelectrochemical sensor showed good repeatability with relative standard deviations of 4.2% and 4.8%, respectively (n = 7). When the photoelectrochemical sensor was not in use, it was stored in the shade at room temperature and measured every few weeks. No obvious decrease in the photocurrent response to hydrolyzed PM was observed after two weeks, and 94.5% of the initial photocurrent response was maintained after one month. This implies that the structure of PTCA/TiO2 is efficient for retaining the activity of PTCA and preventing TiO2 from leaking out of the photoelectrochemical sensor. It also had an excellent specificity against other pesticides and vegetable matrixes for the detection of PM (Interference detection in ESI). To demonstrate the feasibility of the derivative photoelectrochemical sensor applied to the real samples, the determination of PM in green vegetables was performed (Real sample detection in ESI). The detected result of 1.28 ± 0.03 ␮mol L−1 (n = 7) with the proposed method was consistent with the 1.24 ± 0.02 ␮mol L−1 obtained by GC/MS, indicating acceptable accuracy of it.

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4. Conclusions A novel enzymeless derivative photoelectrochemical sensing platform for PM was developed using facilely synthesized PTCA/TiO2 heterojunction. Under optimized conditions, the proposed method was applied to the detection of PM in green vegetables and the value was consistent with that obtained by GC/MS, indicating acceptable accuracy of the photoelectrochemical sensor. There are several merits for the photoelectrochemical sensor: firstly, PTCA could extend the photoresponse of TiO2 to nearly all the visible light region and increase the photocurrent therefore. Moreover, Compared to the famous sensitized dyes ruthenium tris(bipyridine), porphyrin or phthalocyanine, and also poly(3hexylthiophene), it can help forming ultrastable thin film and is more economic as well. Secondly, simulated sunlight irradiation without monochromatic light makes the instrument more economic, portable and simpler, which is different from the routine photoelectrochemical sensors. Thirdly, as far as we know, this is the first case for derivative photoelectrochemical sensing. In addition, it would also be applicable to the 4-nitrophenolate contained OPs, which can be hydrolyzed to 4-nitrophenol or its derivatives. Here, using PM as a model, the derivative photoelectrochemical sensor showed good performances such as low bias voltage, rapid response, excellent sensitivity and selectivity, favorable repeatability and reproducibility in addition to its good stability. Lastly, the active carboxyl group on PTCA is favorable for the adsorption of 4nitrophenol and improve the sensitivity and selectivity therefore and also makes it more convenient to connect with other molecules with active groups such as NH2 and OH, thus, the economic PTCA/TiO2 heterojunction as photoelectric beacon opens a new avenue for the construction of photoelectrochemical sensors, especially for bioassays. This strategy not only extends the application of PTCA, but also presents a simple, economic and novel methodology for photoelectrochemical sensing. Acknowledgements We gratefully acknowledge the financial support from the Natural Science Foundation of China (21075107, 21005070, 21275124 and 21275125), Natural Science Foundation of Jiangsu Province (BK2012247), Foundation of Key Laboratory for Advanced Technology in Environ-mental Protection of Jiangsu Province (AE201162), and Talent Introduction Foundation of Yancheng Institute of Technology (kjc2012051). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.aca.2012.12.038. References [1] K. Kim, O.G. Tsay, D.A. Atwood, D.G. Churchill, Chem. Rev. 111 (2011) 5345–5403. [2] G.A. Jamal, Toxicol. Rev. 16 (1997) 133–170. [3] J. Collee, in: D. Greenwood, R. Slack, J. Peutherer (Eds.), Medical Microbiology, 15th ed., Churchill Livingstone, London, 1997. [4] C. Garcia Pinto, J.L. Perez Pavon, B. Moreno Cordero, Anal. Chem. 67 (1995) 2606–2612. [5] S. Lacorte, D. Barceló, Anal. Chem. 68 (1996) 2464–2470. [6] T. Galeano-Daz, A. Guiberteau-Cabanillas, N. Mora-Dez, P. Parrilla-Vzquez, F. Salinas-Lpez, J. Agric. Food Chem. 48 (2000) 4508–4513.

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