ARTICLE pubs.acs.org/ac
A Novel Photoelectrochemical Sensor for the Organophosphorus Pesticide Dichlofenthion Based on Nanometer-Sized Titania Coupled with a Screen-Printed Electrode Hongbo Li,†,‡ Jing Li,‡ Zhanjun Yang,† Qin Xu,† and Xiaoya Hu*,† † ‡
College of Chemistry and Engineering, Yangzhou University, 88 South University Avenue, Yangzhou 225002, P. R. China College of Chemistry and Biology Engineering, Yancheng Institute of Technology, 9 Yingbin Avenue, Yancheng 224051, P. R. China ABSTRACT: A novel photoelectrochemical sensor for detection of the organophosphorus pesticide (OP) dichlofenthion using nanometer-sized titania coupled with a screen-printed electrode is presented. Nonelectroactive dichlofenthion can be indirectly determined through the photocatalytical degradation of dichlofenthion with nanometer-sized titania. The electrochemical characterization and anodic stripping voltammetric performance of dichlofenthion were evaluated using cyclic voltammetric (CV) and differential pulse anode stripping voltammetric (DPASV) analysis, respectively. DPASV analysis was used to monitor the amount of dichlofenthion and provide a simple, fast, and facile quantitative method for dichlofenthion. Operational parameters, including the photocatalysis time, pH of buffer solution, deposition potential, and accumulation time have been optimized. The stripping voltammetric response is linear over the 0.020.1 and 0.21.0 μmol/L ranges with a detection limit of 2.0 nmol/L. The assay result of dichlofenthion in green vegetable with the proposed method was in acceptable agreement with that of the gas chromatographmass spectrometer (GCMS) method. The promising sensor opens a new opportunity for fast, portable, and sensitive analysis of OPs in environmental samples.
O
rganophosphate pesticides (OPs), one group of the most commonly used pesticides in agriculture, can disrupt cholinesterase and lead to cholinergic dysfunction and death, which endanger the health of both humans and animals.16 In concern over health and the environment, limiting the amount of OPs in agriculture and prohibiting the use of hypertoxic OPs are critical worldwide. Recently, some hypertoxic OPs, such as parathion and methyl parathion, were banned or severely restricted in agriculture by the United Nations Environment Programme (UNEP) and the Food and Agricultural Organization (FAO).6 In view of the major concerns regarding the toxicity of OPs, the addition of OPs residue determination in environment samples and agricultural products is urgently needed. Although, instrumental methods such as GCMS and liquid chromatogram mass spectrometer (LCMS) offer high sensitivity and specificity and have the potential for simultaneous determination of multiple analogues,711 the associated high costs and timeconsuming labor requirements are necessary. Enzyme-based biosensors have emerged in the past few years as the most promising alternative for direct monitoring of pesticides.12,13 Various inhibition and noninhibition biosensor systems, based on the immobilization of acetylcholinesterase (AChE) or OP hydrolase (OPH) onto various electrochemical or optical transducers, have been proposed for field screening of OP neurotoxins.1425 However, the method needs long analysis time and extensive sample handling with multiple washing steps. In recent r 2011 American Chemical Society
years, OP pesticide kits have become commercially available that offer advantages, including portability, rapid turnaround time, and cost-effectiveness. Drawbacks of these test kits include the complicated handling procedure and often a lack of sensitivity (only ppm level) and precision. Moreover, in most cases, these tests are qualitative or semiquantitative and show false positive and negative results. To avoid the use of enzymes and antibodies, molecular imprint technologies with high selectivity toward specific OP species have been developed and applied to the detection of pesticides in environmental samples.26,27 Electrochemical detection of nitroaromatic OPs showed great promise when it was coupled with different separation technologies, such as high-performance liquid chromatography28 or capillary electrophoresis.29 Lin and co-workers fabricated an electrochemical sensor for nitroaromatic OPs based on a gold electrode modified with zirconia nanoparticles.4 Electrochemical detections combined with chromatography are used as reference methods but present some drawbacks that are similar to GCMS or LCMS methods, and the reproducibility and stability of the modified electrode are not easily ensured for long-term assay. Therefore, these methods are also restricted to a limited analyte spectrum. Received: March 19, 2011 Accepted: May 23, 2011 Published: May 23, 2011 5290
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Analytical Chemistry Searching for a new, simple, low-cost, portable, and sensitive analytical method is of considerable interest. Titanium dioxide has been demonstrated to be a very efficient catalyst and especially suitable to work by solar UV light.30 The mechanism of the photocatalytic decomposition of various organic molecules on TiO2 surfaces has been reported elsewhere.3135 When TiO2 is irradiated by photons whose energy exceeds the TiO2 band gap (3.2 eV), electrons are excited from the valence band to the conduction band, resulting in the formation of charge carrier pairs, that is, a hole (hþ) and an electron (e). These charge carriers can migrate to the surface and react with adsorbed molecules, unless recombination occurs first. The hole typically oxidizes adsorbed water to a hydroxyl radical (•OH), whereas the electron reduces adsorbed molecular oxygen to a superoxide radical anion (O2•). These oxidizing radicals react with adsorbed organic molecules, inducing electroactive product production and oxidative degradation to carbon dioxide and water finally.36,37 The OPs insecticides are comprised within the 10 most widely used pesticides, such as dichlofenthion, bromophos ethyl, and parathion ethyl, which can be changed into electroactive hydroxysubstituted aromatic ring compounds by photocatalytic degradation.38 However, to the best of our knowledge, photocatalytic degradation combined with electrochemistry for determination of OPs insecticide residues has not been reported at present. In this paper, we describe a novel and simple photoelectrochemical sensor for dichlofenthion (Figure 1) based on nanometer-sized titania photocatalysis coupled with a screen-printed carbon electrode (SPCE). The proposed method is promising for detecting dichlofenthion residue in field-screening scenarios.
’ EXPERIMENTAL SECTION Reagents and Solutions. The UV light (20 W, λ = 254 nm) was obtained from Wisbay M&E Co., Ltd. (Shenzhen, China). Dichlofenthion for calibration was purchased from SigmaAldrich, and a 0.003 mmol/L stock solution was prepared with pH 4.0 phosphate buffer solutions (PBS), which was used as the supporting electrolyte. The working solutions were prepared by diluting the corresponding standard stock solution by PBS. Titanium dioxide (Degussa P25), a known mixture of 65% anatase and 35% rutile form with an average particle size of 30 nm that was nonporous with a reactive surface area (BET) of 50 ( 15 m2/g, was used as received for all photocatalytic experiments, and the 0.125 mmol/L solution was also prepared with pH 4.0 PBS. Other reagents were commercially available and were of analytical reagent grade. Solutions were prepared with ultrapure water from a Millipore Milli-Q water purification system (Billerica, MA). Apparatus. The cyclic voltammograms were recorded between 0.0 and 1.2 V at a scan rate of 100 mV/s and differential pulse anode stripping voltammograms with an amplitude of 50 mV, a pulse width of 0.2 s, and a pulse period of 0.5 s. Measurements were performed using an electrochemical analyzer (CHI760D, CH Instruments, Shanghai, China) connected to a personal computer. SPCEs were obtained from Gwent group (Torfaen, United Kingdom) that included a three-electrode configuration printed on the same strip. The strips had a 2 mm diameter disk screen-printed carbon working electrode, a counter electrode, and a Ag/AgCl pseudoreference electrode, an insulating layer serves to define the working electrode area. Electrochemical experiments were carried out at room temperature (25 °C). The model surveyor apparatus used in this study
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Figure 1. Structure of dichlofenthion.
was a GCMS [QP 5000 Shimadzu equipped with capillary column (DB-1701, 30 m 0.25 mm 0.25 μm)]. For GCMS, the column temperature was maintained at 40 °C for 1 min and then programmed to 110 °C at 30 °C/min, to 230 °C at 5 °C/ min, and then to 280 °C at 10 °C/min, and held for 5 min. The carrier gas at a rate of 1.2 mL/min was helium, with purity of 99.999%. The injection port temperature was 280 °C, and the splitless mode was used for injection of 1 μL volume with the valve opened for 30 s. The ionization energy was 70 eV. The ion source and the GCMS interface were kept at 220 and 280 °C, respectively. Photocatalysis and Electrochemical Detection. Ten milliliters of the dichlofenthion solution containing the TiO2 powder (0.1 mmol/L) were magnetically stirred before the illumination. The suspension was left for 30 min in the dark in order to achieve the maximum adsorption of pesticides onto the semiconductor surface. Subsequently, a drop of suspension was dripped into the fixed cylindrical cell and then was irradiated under the UV radiation for 20 min with the distance of 10 cm between the UV light and liquid level. Afterward, the photocatalytic degradation compound of dichlofenthion was detected by differential pulse anode stripping voltammetric (DPASV) analysis, and the oxidation peak current was proportional to the concentration of dichlofenthion. The procedure is shown in Figure 2. DPASV experiments comprised an electrochemical deposition step at 0.2 V for 180 s, an equilibration period of 2.0 s, and a differential pulse voltammetric (DPV) stripping scan usually from 0.6 to 0.9 V. Before each measurement a preconditioning step (for cleaning of the electrode) at a potential of 1.0 V was applied for 60 s. Preparation and Detection of Green Vegetable Samples. The green vegetable samples were prepared as follows: 4 g of 5291
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Figure 2. Scheme of the photoelectrochemical sensor for dichlofenthion: (A) diagram of the SPCE, (B) a drop of suspension was dripped into the cylindrical cell fixed of the SPCE, (C) the suspension was irradiated by UV, and (D) differential pulse anode stripping voltammogram of the irradiated dichlofenthion.
green vegetable leaves were chopped into small pieces and then equably sprayed with a known amount of dichlofenthion stock solution. After placed in a glass disk for 8 h, the contaminated sample was taken out and washed with water three times. Subsequently, the residual dichlofenthion on the leaves was extracted with hexane. All the extract liquid was collected for concentration and then diluted with anhydrous ethanol. Afterward, the liquid was filtered through a 0.45 μm membrane filter, transferred into a 10 mL volumetric flask that was brought to volume with ethanol, and reserved for GCMS assay. For electrochemical measurement, 10.0 μL samples were added to 10.0 mL of PBS medium and then analyzed by the same DPASV procedure as the dichlofenthion standard solution. Then, a quantitative dichlofenthion stock solution was added into the above measured sample system, and DPASV was started under the same condition.
’ RESULTS AND DISCUSSION Choice of Extractant. Green vegetables contain large amounts of ascorbic acid and other components. It is necessary to separate ascorbic acid from the matrix because much ascorbic acid can interfere with the electrochemical determination of dichlofenthion. Ascorbic acid is easily dissolved in water but it is difficult to dissolve in low-polarity organic solution; the opposite is true for dichlofenthion. So, the low-polarity organic solvent hexane was chosen as the extractant for dichlofenthion. Electrochemical Behavior of Dichlofenthion and Its Photolysis Compound. The electrochemical behaviors of dichlofenthion and its photolysis compound were studied, respectively. As can be seen in Figure 3A, there is no oxidation peak at curve a or b in PBS (pH 4.0), which indicates that the oxidation reaction of PBS or dichlofenthion does not take place at the potential window from 0.0 to 1.2 V. However, one oxidation peak markedly appears at curve c for the 10.0 μmol/L photocatalyzed dichlofenthion, which demonstrates that the photolysis compound of dichlofenthion has good electrochemical activity. The photodegradation products of dichlofenthion have been reported by Konstantinou’s team, and 2,4-dichlorophenol and p-chlorophenol were two of the species.38 Moreover, 2,4-dichlorophenol and p-chlorophenol are electroactive species, and their
Figure 3. (A) CV curves of photocatalysis before and after in 0.1 mol/L pH 4.0 PBS containing 10.0 μmol/L dichlofenthion at SPCE: (a) blank buffer, (b) in the presence of dichlofenthion, and (c) in the presence of photocatalyzed dichlofenthion. (B) The cyclic voltammograms of 10.0 μmol/L of 2,4-dichlorophenol (a) and p-chlorophenol (b) in 0.1 mol/L pH 4.0 PBS.
circulation voltammetric behaviors are demonstrated in Figure 3B. As can be seen from the oxidation peak of 2,4-dichlorophenol, p-chlorophenol, and the photolysis compound, three oxidation peaks nearly appeared at 0.83 V, which confirms that the curve c in Figure 3A can be attributed to the oxidation of 2,4-dichlorophenol and p-chlorophenol. Therefore, dichlofenthion can be indirectly detected by its photolysis compounds. Optimization of the Experimental Conditions for Dichlofenthion Measurement. Since the proposed method for the determination of dichlofenthion is based on the electrochemical signal being proportional to the concentration of photocatalyzed dichlofenthion, it is more important to improve the sensitivity of the proposed method. The effect factors for measurements concerning photocatalysis time, pH of buffer solution, deposition potential, and accumulation time were studied in detail. Optimal Conditions for Photocatalysis Time. The analytical performance of the photoanalysis was related to the concentration of electroactive hydroxy-substituted aromatic ring compounds in the measuring system. As can be seen in Figure 4, the DPV peak current increased with the increasing photocatalysis time from 2 to 25 min, and the photoelectrochemical sensor showed the increasing response until a photocatalysis time of 20 min (inset of Figure 4). Longer photocatalysis time did not obviously improve the response. It can be explained that more photodegraded species chlorophenols were produced as 5292
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Figure 4. DPV responses of the photoelectrochemical sensor with increasing photocatalysis time in 0.1 mol/L pH 4.0 PBS containing 10.0 μmol/L dichlofenthion (ag). The inset shows the effects of photocatalysis time of dichlofenthion on the amperometric response of the photoelectrochemical sensor.
Figure 6. Effects of deposition potential (A) and accumulation time (B) on DPASV peak current of photocatalyzed dichlofenthion, the other conditions being the same as in Figure 5.
Figure 5. Effect of PBS buffer solution pH on the DPV peak current of photocatalyzed dichlofenthion, the other conditions are the same as in Figure 4.
the photodegradation progresses with the increase of the time. Meanwhile, some of other photodegraded species may be absorbed on the titanium dioxide surface and prevent the process of photodegradation; i.e., the titanium dioxide catalyst may be poisoned. In addition, the resulting charge carrier pairs, a hole (hþ) and an electron (e), recombined first, resulting in fewer hydroxyl radicals and superoxide radical anions being obtained, so that it was very difficult to completely photodegrade dichlofenthion. Considering the optimal sensitivity and analysis efficiency, therefore, the photocatalysis time of 20 min was chosen as the optimal photocatalysis condition for the detection of dichlofenthion. Optimal Conditions for pH. Variation of the peak current as a function of solution pH in the range of 3.08.0 is shown in Figure 5. Due to the acidic hydroxyl group of 2,4-dichlorophenol and p-chlorophenol, the photoproducts of dichlofenthion,38 the pKa value of them is about 7.9 and 9.4, respectively.39 Therefore, the DPV peak current for chlorophenol is expected for a pH less than 9.4. In fact, under the optimal condition of photocatalysis time, it was clear that the DPV peak current of chlorophenol increased gradually with the increasing buffer solution pH and
reached maximum at pH 4.0. Afterward, it gradually decreased up to pH 7.0 and then decreased rapidly at higher pH. It can be explained that the concentration of neutral chlorophenol reached the maximum at pH 4.0 and the more neutral chlorophenol can be oxidated on the SPCE. Therefore, pH 4.0 was chosen as the optimum pH in the subsequent measurements. Optimum of Deposition Potential and Accumulation Time. Under aforesaid optimal conditions, the more sensitive electrochemical response depends on the accumulation of photocatalyzed dichlofenthion at the SPCE. As shown in Figure 6A, when the accumulation time was first fixed at 60 s and the deposition potential changed from 0.6 to 0.2 V, the peak current for 10.0 μmol/L photocatalyzed dichlofenthion first increased rapidly until it reached maximum at 0.2 V and then gradually decreased. It may be explained that the concentration of neutral chlorophenol reached a maximum in pH 4.0 buffer solutions and the deposition potential of 0.2 V was close to the zero charge potential. Therefore, more or less than 0.2 V would result in gathering of more negative or positive charge on the surface of SPCE and further prevent the neutral chlorophenol from reaching on the surface of the SPCE. So a negative potential of 0.2 V was favorable to the deposition of photocatalyzed dichlofenthion. Then, 0.2 V was fixed during the optimizing of accumulation time. With the increasing accumulation time, the peak height dramatically increased until it reached a maximum at 180 s and then remained a constant value (Figure 6B), indicating that the accumulation of photocatalyzed dichlofenthion on the electrode surface nearly reached a saturation state at 180 s. 5293
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Table 1. Measurement Results of Dichlofenthion in Green Vegetable Sample (n = 3)a GCMS this method
a
Figure 7. DPASV analysis of increasing dichlofenthion concentration, from bottom to top, 0.02, 0.04, 0.06, 0.08, 0.1, 0.2, 0.4, 0.6, 0.8, and 1.0 μmol/L, respectively. The inset shows the calibration curve. Electrochemical stripping detection condition is the same as in Figure 6.
Thus, 0.2 V and 180 s were selected as the optimal deposition potential and accumulation time, respectively. Analytical Performance. The DPASV analysis of dichlofenthion showed a strong signal under optimum experimental conditions. Well-defined peaks, proportional to the concentration of the corresponding dichlofenthion, were observed. A linear relationship between the stripping current and the concentration of dichlofenthion was obtained covering the concentration ranges from 0.02 to 0.1 and 0.2 to 1.0 μmol/L, the linear regression equations being I (μA) = 0.5210C þ 0.6009 and I (μA) = 2.111C þ 0.3944, with two correlation coefficients of 0.9993 and 0.9967, respectively (Figure 7). The detection limit of 2.0 nmol/L was determined at a signal-to-noise ratio of 3, which was lower than that reported (4 ng/mL, 12.7 nmol/L) so far with a polyclonal antibody-based biosensor.40 The assay precision of the photoelectrochemical sensor was evaluated by assaying dichlofenthion at two levels for nine replicative measurements. The assay variation coefficients with this method were 2.6% and 3.5% at 0.05 and 0.8 μmol/L concentrations of dichlofenthion, respectively, showing a good repeatability. The assay variation coefficients at these concentrations on nine photoelectrochemical sensors made independently were 3.9% and 5.7%, respectively, indicating acceptable fabrication reproducibility. Interference of Foreign Species. Under the optimal conditions, each interferent was mixed with TiO2 and irradiated. The effects of various coexisting substances were studied by introducing each one to the buffer solution containing 0.1 μmol/L dichlofenthion. Each one was considered as an interfering agent, when the electrochemical signal exhibited a deviation more than (5%. The interference of some substances on the stripping voltammetric measurements was examined. A 103-fold mass ratio of NH4þ, Kþ, Naþ, Zn2þ, Ca2þ, Mg2þ, Ba2þ, Al3þ, Cu2þ, SO42-, Cl, NO3, and PO43; 100-fold mass ratio of amino acid, glucose, and sucrose, 50-fold mass ratio of Fe3þ, Fe2þ, Co2þ, and Ni2þ; 10-fold mass ratio of parathion methyl, parathion, and chlorpyrifos; and 5-fold ascorbic acid had no influence on the determination of dichlofenthion. However, 20 nmol/L phenol and chlorophenol could increase the current response and brought positive interference for 0.1 μmol/L dichlofenthion measurement. Analysis of Real Sample. To demonstrate the feasibility of the sensor applied to the practical samples, the determination of dichlofenthion in green vegetable was performed. The green
added
found
(μmol/L) (μmol/L)
recovery
analyte
(μmol/L)
(μmol/L)
(%)
dichlofenthion
0.078 0.075
0.076 0.072
0.1 0.4
0.172 0.493
96.0 105.3
0.081
0.079
0.8
0.871
99.0
n is the repetitive measurements number.
vegetable samples were pretreated as mentioned in section 2.4 and diluted with ethanol to the required volume. The sample solution was injected directly into 10 mL of PBS (pH 4.0), and the residual dichlofenthion content was detected by means of DPASV analysis using the dichlofenthion sensor. Subsequently, an amount of standard dichlofenthion solution was added into the above solution, and the recovery of the added dichlofenthion was observed to be good for three determinations. The results obtained by the proposed method also accorded very well with those obtained by GCMS. The results were summarized in Table 1. These results implied that the sensor was capable of practical applications.
’ CONCLUSIONS We have demonstrated a sensitive, simple, fast, and portable photoelectrochemical sensing protocol for the organophosphorus pesticide dichlofenthion based on the use of nanometersized titania coupled with SPCE. The promising DPASV characteristics provide a facile quantitative method for dichlofenthion. The results obtained from this work implied that the combination of a disposable SPCE with a portable electrochemical instrument would benefit the field monitoring of dichlofenthion. The proposed photoelectrochemical sensing technology is thus expected to monitor the other nonelectroactive OPs and opens new opportunities for detecting OPs in the environment, public places, or workplaces and for monitoring the exposures of individuals to chemical warfare agents. ’ AUTHOR INFORMATION Corresponding Author
*Tel.: þ86 514 87971818. Fax: þ86 514 87311374. E-mail:
[email protected].
’ ACKNOWLEDGMENT We gratefully acknowledge the financial support from the Natural Science Foundation (no. 20705030, no. 20875081, no. 21075107, no. 21005070) of China, projects of the 863 Plan (2009AA03Z331), and the Natural Science Foundation (XKY2009009) of Yancheng Institute of Technology. ’ REFERENCES (1) Fennouh, S.; Casimiri, V.; Burstein, C. Biosens. Bioelectron. 1997, 12, 97–104. (2) Valdez, S. B.; Garcia, D. E. I.; Wiener, M. S. Rev. Environ. Health 2000, 15, 399–412. (3) Guerrieri, A.; Monaci, L.; Quinto, M.; Palmisano, F. Analyst 2002, 127, 5–7. (4) Liu, G. D.; Lin, Y. H. Anal. Chem. 2005, 77, 5894–5901. (5) Lu, C. S.; Barr, D. B.; Pearson, M. A.; Waller, L. A. Environ. Health Perspect. 2008, 116, 537–542. 5294
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