Electrochemical stripping analysis of organophosphate pesticides and nerve agents

Electrochemical stripping analysis of organophosphate pesticides and nerve agents

Electrochemistry Communications 7 (2005) 339–343 www.elsevier.com/locate/elecom Electrochemical stripping analysis of organophosphate pesticides and ...

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Electrochemistry Communications 7 (2005) 339–343 www.elsevier.com/locate/elecom

Electrochemical stripping analysis of organophosphate pesticides and nerve agents Guodong Liu, Yuehe Lin

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Pacific Northwest National Laboratory, 902, Battelle Boulevard, Richland, WA 99352, USA Received 4 January 2005; received in revised form 1 February 2005; accepted 1 February 2005 Available online 19 February 2005

Abstract A sensitive electrochemical stripping voltammetric method for analyzing organophosphate (OP) compounds was developed using a carbon paste electrochemical (CPE) transducer. OPs strongly adsorb on a CPE surface and provide facile electrochemical quantitative methods for electroactive OP compounds. Operational parameters have been optimized, and the stripping voltammetric performance has been studied using square wave voltammetry. The adsorptive stripping voltammetric response is highly linear over the 1–60 lM methyl parathion range examined (2-min adsorption), with a detection limit of 0.05 lmol/L (10-min adsorption) and good precision (RSD = 3.2%, n = 10). These findings can lead to a widespread use of electrochemical sensors to detect OP contaminates. Ó 2005 Elsevier B.V. All rights reserved. Keywords: Organophosphate compounds; Nerve agents; Pesticides; Carbon paste; Adsorptive stripping voltammetry

1. Introduction Organophosphate (OP) compounds are significant environmental and food chain pollutants [1] because they are used intensively as pesticides, insecticides, and chemical-warfare agents. Because of the high toxicity of OPs, the rapid detection of these toxic agents in the environment, public places, or workplaces and the biomonitoring of individualÕs exposures to chemical warfare agents become increasingly important for homeland security and health protection [2–5]. Analyzing OPs in environmental and biological samples is routinely carried out using analytical techniques such as gas or liquid chromatography and mass spectrometry [6]. Such analysis is generally performed at centralized laboratories, requiring extensive labor and analytical re-

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Corresponding author. Tel.: +1 509 376 0529; fax: +1 509 376 5106. E-mail address: [email protected] (Y. Lin).

1388-2481/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2005.02.002

sources, and often results in a lengthy turn-around time. However, these analysis methods have a number of disadvantages, limiting their applications primarily to laboratory settings and prohibiting their use for rapid analyses under field conditions. To meet these requirements of rapid warning and field deployment, more compact low-cost instruments, coupled to smaller sensing probes, are highly desirable for facilitating the task of on-site monitoring of OP compounds. Various inhibition and non-inhibition biosensor systems, based on the immobilization of acetylcholinesterase or organophosphorus hydrolase onto various electrochemical or optical transducers, have been proposed for field screening of OP neurotoxins [7–11]. Specific antibodies against OP pesticides have been recently developed for enzyme linked immunoassay [12]. Amperometric electrochemical detection of OPs was developed by applying constant potential in connection with high-performance liquid chromatography [13] or capillary electrophoresis [14]. Martinez

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et al. [13] developed an automated high-performance liquid chromatographic method for the determination of organophosphorus pesticides in waters with dual electrochemical (reductive–oxidative) detection. Recently, a capillary electrophoresis microchip for separation and amperometric detection of OP nerve agents was developed by Wang et al. [14]. Surprisingly, little attention has been given to electrochemical sensing of nitroaromatic OP compounds, despite their inherent redox activity and the compact nature of electrochemical instruments. In this paper, we describe the electrochemical stripping analysis of OPs on a carbon paste transducer. OPs strongly adsorb on the surface of the carbon paste transducer and provide a facile voltammetric quantitative method for some electroactive OPs. The electrochemical characterization and anodic stripping voltammetric performance of adsorbed electroactive OP compounds were evaluated using cyclic voltammetric and square-wave voltammetric (SWV) analysis. The promising stripping voltammetric performances open new opportunities for analyzing OPs. A disposable screen-printed electrode and portable electrochemical instrument would benefit the field monitoring of OPs.

2.2. Reagents Paraoxon, methyl parathion, and fenitrothion were purchased from Sigma–Aldrich, and their 10,000 mg/L stock solutions were prepared in acetonitrile. A 0.2 M of acetate buffer (pH 5.2) was used as the supporting electrolyte and also served as the adsorption medium during the adsorption experiments. Other chemicals were obtained from Sigma–Aldrich (St. Louis, MO, USA). Deionized water was obtained from a Millipore Milli-Q water purification system (Billerica, MA, USA). 2.3. Procedure A carbon paste electrode was dipped into a stirring acetate buffer containing the desired concentration of OP pesticide compounds for 2 min and then washed with distilled water carefully and transferred to a 2-mL electrochemical cell. SWV scanning was performed from 0.8 to +0.2 V with a step potential of 4 mV, amplitude of 20 mV, and frequency of 25 Hz. Cyclic voltammetric measurements were performed under batch conditions. The cyclic voltammogram was recorded between 0.8 and +1.0 V at a scan rate of 100 mV/s. All measurements were performed at room temperature.

3. Results and discussion 2. Experimental 2.1. Apparatus Cyclic voltammetric and SWV measurements were performed using an electrochemical analyzer CHI 621A (CH Instruments, Austin, TX) connected to a personal computer. A three-electrode configuration was employed, consisting of a carbon paste electrode (3 mm diameter) serving as a working electrode, while Ag/AgCl/(3 M KCl) and platinum wire served as the reference and counter electrode, respectively. Electrochemical experiments were carried out in a 2-mL voltammetric cell at room temperature (25 °C). All potentials are referred to the Ag/AgCl reference electrode (CH Instruments). The carbon paste electrode was prepared by thoroughly hand mixing 120 mg mineral oil and 280 mg graphite powder. The resulting paste was squeezed into a home-made electrode hold with a polyvinyl chloride tube of 3-mm ID to a depth of 1 cm. Inside the tube, the mass was in contact with a conducting graphite rod, which was in turn connected to an electric wire to complete the measurement circuit. The carbon electrode surface was renewed by turning the nut to extrude a 0.1-mm thick outer paste layer and by polishing with a weighting paper to produce a smooth shiny surface [15].

3.1. Voltammetric characteristics of adsorbed methyl parathion on carbon paste electrode Fig. 1 shows the voltammogram of adsorbed methyl parathion on a carbon paste electrode surface at 0.2 M acetate buffer (pH 5.2). A pair of rather well defined

Fig. 1. Cyclic voltammograms of methyl parathion adsorbed carbon paste electrode (a) and blank carbon paste electrode (b) in 0.2 M acetate buffer (pH 5.2). Potential scanning rate: 100 mV/s. Methyl parathion adsorbed carbon paste electrode was prepared by dipping the blank CPE electrode in stirring acetate buffer containing 30 lM methyl parathion for 2 min and carefully washing with distilled water before electrochemical measurement.

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redox peaks (Epa, 0.08 V and Epc, 0.0 V) and irreversible reduction peak (Epc, 0.61 V) were observed in the potential range from 0.8 to 1.0 V (Fig. 1(a)). The irreversible reduction peak corresponds to the reduction of the nitro group to hydroxylamine group (Reaction (1)) and the reversible redox peaks are attributed to two-electron-transfer process (Reactions (2) and (3)), as shown below.

ð1Þ

ð2Þ

ð3Þ These profiles are consistent with those described elsewhere for OP pesticides and nitroaromatic compounds [14,16–18]. A control experiment was performed under the same conditions in the absence of methyl parathion; no redox peak was observed (Fig. 1(b)). The another two electroactive OPs, paraoxon and fenitrothion, which possess similar structure with methyl parathion, exhibit similar cyclic voltammetric characteristics (not shown) after they adsorb on the carbon paste electrochemical (CPE) surface, but peak potentials shift slightly. The mixture of three identical concentrations of OP compounds shows the same electrochemical characteristics, the peak current increase around three times compared with an individual OP. Fig. 2 presents the voltammograms of methyl parathion with an increasing potential scanning rate. Also, the inset shows the

Fig. 2. Cyclic voltammograms of methyl parathion adsorbed CPE at increasing potential scanning rate, from curve a to g, scan rates are 20, 40, 60, 80, 100, 200, and 300 mV/s, respectively; inset also shows the relationship between scan rate and reduction peak current of methyl parathion. Other conditions were the same as in Fig. 1.

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relationship between the oxidation peak current and the scanning rate. The peak currents exhibit a linear dependence on the potential scanning rate, ranging from 20 to 300 mV/s, indicating that the electrode reaction is controlled by a non-diffusion process. 3.2. Square-wave voltammetric characteristics The attracting cyclic voltammetric characteristics of OP on the CPE transducer provide a facile electrochemical quantitative method for analyzing OP. SWV analysis has higher sensitivity than other electrochemical technologies, such as cyclic voltammetry and differential pulse voltammetry. Fig. 3 shows a typical SWV voltammogram of adsorbed methyl parathion in 0.2 M acetate buffer (pH 5.2). A very sharp and well defined peak was obtained at the potential range from 0.8 V to 0.2 V. The peak potential of the oxidation peak (0.06 V) shifts 20 mV to a negative potential direction compared with a cyclic voltammogram. Our experiment results show that the initial scanning potential has a strong effect on the stripping peak current (Fig. 4(a)). The initial scanning potential between 0.2 and 0.9 V was selected to study the effect. No stripping peak was observed when the initial potential located between 0.2 and 0.5 V. Peak current increases with the decrease of the initial potential after 0.5 V and then starts to level at 0.8 V. All subsequent work was thus carried out using an initial potential of 0.8 V. The effect of the initial potential on stripping voltammetric performance comes from the reduction reaction of nitroaromatic compound on electrode surface. The reduction reaction (1) of the nitroaromatic compound occurs at a more negative potential, which is less than 0.5 V. When the initial scanning potential is more than 0.5 V, the reduction reaction can not take place, which prevents the

Fig. 3. Typical stripping voltammograms of methyl parathion adsorbed carbon past electrode in 0.2 M acetate buffer (pH 5.2). Methyl parathion adsorbed carbon paste electrode was prepared by dipping the CPE in 0.2 M acetate buffer containing 9 lM methyl parathion and stirring for 2 min, and then carefully washing with distilled water. Scanning potential range: from 0.8 to 0.2 V; frequency: 25 Hz; increasing potential: 4 mV.

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ditions. All subsequent work was thus carried out using a frequency of 25 Hz with 2 min for adsorbing and a scanning potential range from 0.8 to 0.2 V.

Current (µA)

(a) 7.8

5.8 3.8

3.3. Analytical performance

1.8

-0.2 -1100

-900

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-500

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a (mV) Initiative scanning potential

Current (µA)

(b)

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7

6

5 0

10

20

30

0 40

5 50

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Frequency (HZ) (c)

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Current (µA)

8 6 4 2 0 0

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Time (s) Fig. 4. Effect of initial scanning potential (a), frequency (b) and adsorbing time (c) on the stripping peak current. Frequency (a) 25 Hz; initial scanning potential (b), 800 mV; the concentration of methyl parathion: 60 lM. Other conditions were the same as in Fig. 3.

following electron transfer from occurring. The effect of different variables of the SWV waveform was explored for further optimization. Fig. 4(b) examines the influence of the square-wave frequency upon the response to 60 lM methyl parathion. The peak current rises with the frequency at first up to 25 Hz, and then it decays gradually. The effect of accumulation time on the stripping peak current was investigated (Fig. 4(c)). The peak currents increase rapidly with the accumulation time at first and then more slowly and level at 2 min. The resulting current vs. time plot (Fig. 4(c)) displays a curvature consistent with adsorption processes. No such surface accumulation is indicated in analogous measurements at the bare glassy carbon surface (not shown). We also observed the accumulation of methyl parathion under constant potential conditions (not shown); there is no significant increasing of stripping peak current. Further experiments were thus employed under open-circuit con-

Fig. 5 displays the SWV response of adsorbed methyl parathion at CPE for increasing concentrations in adsorbing solution under the optimum experimental conditions (only show the curves at the potential range between 0.2 and 0.2 V). Well-defined peaks, proportional to the concentration of the corresponding methyl parathion, were observed. A linear relationship between the stripping current and methyl parathion concentration was obtained covering the concentration range from 1 to 60 lM, the linear regression equation being I (lA) = 0.14C  0.33, with a correlation coefficient of 0.9963. The detection limit was improved significantly by increasing the accumulation time. A detection limit of 0.05 lM was estimated on the basis of an s/n = 3 characteristic of the 0.1 lM data points in connection with a 600-s accumulation time. The detection limit obtained is comparable with that reported so far with an enzyme electrode [10]. A series of 10 repetitive measurements of a solution containing 5 lM yielded reproducible peak currents with relative standard deviations of 3.2. Interferences arising from the other nitrophenyl derivatives that are expected to co-exist in solution were used to evaluate the selectivity of stripping voltammetric analysis. Adsorbing experiments of 30 lM methyl parathion were performed in the presence of p-nitrophenol or nitrobenzene (30 lM), the stripping peak current varies slightly, and no new stripping peak was recorded at the selective potential range. The selectivity for stripping voltammetric detection of OP is probably due to stronger adsorption effect of OP compound on the surface of carbon paste electrode over p-nitrophenol or nitrobenzene. The stripping peak potential of OPs is around 0.1 V, which also avoids the interferences from other

Fig. 5. Stripping voltammograms of increasing methyl parathion concentration, from bottom to top, 0, 3, 6, 9, 15, 30, and 60 lM, respectively; also, the inset shows the calibration curve.

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phenol compounds, whose oxidation potentials are more than 0.3 V. An electrochemical stripping analysis used in conjunction with a carbon paste transducer thus holds great promise for direct analysis of relevant water samples without any prior separation or pretreatment.

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Research and located at PNNL. PNNL is operated by Battelle for DOE under Contract DE-AC0576RL01830.

References 4. Conclusion A sensitive method for electrochemical stripping analysis of OPs has been demonstrated. OPs strongly adsorb on the carbon paste electrode surface and provide a facile quantitative method. An anodic stripping analysis with a very low stripping peak potential avoids interferences from other nitrophenyl derivatives and phenol compounds. The promising stripping voltammetric performance opens new opportunities for analyzing OP pesticides and nerve agents. The results obtained from this work imply that a combination of a disposable screen-printed electrode with a portable electrochemical instrument would benefit the field monitoring of OPs.

Acknowledgments The work is supported by a laboratory directed research and development program at Pacific Northwest National Laboratory (PNNL). The research described in this paper was performed at the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the US Department of EnergyÕs (DOEÕs) Office of Biological and Environmental

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