Anal. Chem. 1996, 68, 498-502

Determination of Paraquat by Square-Wave Voltammetry at a Perfluorosulfonated Ionomer/ Clay-Modified Electrode Jyh-Myng Zen,*,† Su-Hua Jeng,† and Horng-Ji Chen‡

Departments of Chemistry and Soil Science, National Chung-Hsing University, Taichung, Taiwan 402

A novel Nafion/clay-modified electrode (NCME) was developed for the determination of paraquat by squarewave cathodic stripping voltammetry. The clay that showed the best performance for the fabrication of the NCME is nontronite (SWa-1, ferruginous smectite). The electrochemical behavior of paraquat showed that the cathodic peak at -0.70 V vs Ag/AgCl permits adequate quantification of the analyte. Linear calibration curves are obtained over the 0-80 ppb range, with a detection limit of 0.5 ppb in pH 8 phosphate buffer solution for 4 min preconcentration time. Various factors influencing the determination of paraquat were thoroughly investigated in this study. The practical analytical utility is illustrated by selective measurements of paraquat in real water samples. Paraquat (the 1,1′-dimethyl-4,4′-bipyridylium ion), also known as methyl viologen, has been widely used as a herbicide for more than 10 years. This herbicide is extremely toxic and is often encountered in cases of accidental and suicidal poisonings.1,2 Various techniques have been reported for the determination of paraquat, such as spectrophotometry,3-7 thin-layer chromatography,8,9 gas chromatography,10 liquid chromatography,1,11,12 polarography,13 and radioimmunoassay.14 However, these techniques suffer from either instability or the need for extensive sample pretreatment. Recently, several interesting electrochemical paraquat sensors were developed, based on crown ethers and ion exchangers.15-17 Unfortunately, strong interference from potassium ions is evident, and these electrodes have yet to be demonstrated for use with real samples. The interference from potassium ions was improved by use of a polymeric membrane †

Department of Chemistry. Department of Soil Science. (1) Gill, R.; Qua, S. C.; Moffat, A. C. J. Chromatogr. 1983, 255, 483. (2) Sagar, G. R. Hum. Toxicol. 1987, 6, 7. (3) Calderbank, A.; Yuen, S. H. Analyst 1965, 90, 99. (4) Yuen, S. H.; Bagness, J. E.; Myles, D. Analyst 1967, 92, 375. (5) Ganesan, M.; Natesan, S.; Ranganathan, V. Analyst 1979, 104, 258. (6) Guijarro, E. H.; Sedeno, P. Y.; Diez, L. M. P. Anal. Chim. Acta 1987, 199, 203. (7) Shivhare, P.; Gupta, V. K. Analyst 1991, 116, 391. (8) Tadjer, G. S. J. Forensic Sci. 1967, 12, 549. (9) Crone, H. D.; Smith, E. M. J. Chromatogr. 1973, 77, 234. (10) Cannard, A. J.; Criddle, W. J. Analyst 1975, 100, 848. (11) Kawano, Y.; Audino, J.; Edlund, M. J. Chromatogr. 1975, 115, 289. (12) Paschal, D. C.; Needham, L. H.; Rollen, Z. J.; Liddle, J. A. J. Chromatogr. 1979, 177, 85. (13) Sedeno, P. Y.; Carrazon, J. M. P.; Diez, L. M. P. Mikrochim. Acta 1985, 3, 279. (14) Braithwaite, R. A. Hum. Toxicol. 1987, 6, 83. (15) Moody, G. J.; Owusu, R. K.; Thomas, J. D. R. Analyst 1987, 112, 121. (16) Moody, G. J.; Owusu, R. K.; Thomas, J. D. R. Analyst 1988, 113, 65. (17) Moody, G. J.; Owusu, R. K.; Thomas, J. D. R. Analyst 1987, 112, 1347.

electrode with the incorporation of neutral cyclotetrasiloxanes.18 However, the interference effect of surface-active compounds was not mentioned. Another carbon paste electrode, chemically modified with Amberlite XAD-2 resin, was shown to be very promising for the determination of paraquat by cathodic stripping voltammetry.19 Nevertheless, p-nitrophenol and Triton X-100 were reported to have the strongest interference in the determination of paraquat. Overall, up to now, sensitive and selective methods are still needed to be developed for the detection and determination of paraquat. In this paper, a different way of fabricating the paraquat sensor, taking advantage of both a polymer-modified electrode and a clay-modified electrode (CME), was developed for the selective detection of paraquat. The interaction of paraquat with soils, including clay minerals, has been extensively studied by UV, IR, and X-ray diffraction techniques.20-24 The herbicidal activity was found to decrease significantly when the sample was incorporated into clays. This has been attributed to its strong adsorption on the silicate surface, preventing subsequent uptake by plants. For paraquat, the adsorption of bipyridylium cations by clay minerals is believed to be a major mechanism for its biological inactivation in the soil environment. On the other hand, many electrochemists have also extensively studied the CMEs for the purpose of photoelectrochemical applications. The well-defined layered structure, flexible adsorption properties, and potential as catalyst and/or catalyst support make clay an interesting material for modifying electrode surfaces. Methyl viologen (i.e., paraquat), with its stable chemical structure and good electroactivity, was frequently used as the photoelectroactive species in those studies.25-30 Besides, utilization of clay is promising because spatial constraints and surface chemical effects induced by the material can lead to new patterns



498 Analytical Chemistry, Vol. 68, No. 3, February 1, 1996

(18) Saad, B.; Tahir, M.; Ahmad, M. N.; Saleh, M. I.; Jab, M. S. Anal. Chim. Acta 1994, 285, 271. (19) Alvarez, E.; Sevilla, M.-T.; Pinila, J. M.; Hernandez, L. Anal. Chim. Acta 1992, 260, 19. (20) Summers, L. A. The Bipyridinium Herbicides; Academic Press: New York, 1980; Chapter 6. (21) Theng, B. K. G. The Chemistry of Clay-Organic Reactions; Wiley: New York, 1974; Chapter 4. (22) Knight, B. A. G.; Tomlinson, T. E. J. Soil Sci. 1967, 18, 233. (23) Knight, B. A. G.; Denny, P. J. Weed Res. 1970, 10, 40. (24) Raupach, M.; Emerson, W. W.; Slade, P. G. J. Colloid Interface Sci. 1979, 69, 398. (25) Ghosh, P. K.; Bard, A. J. J. Am. Chem. Soc. 1983, 105, 5691. (26) White, J. R.; Bard, A. J. J. Electroanal. Chem. 1986, 197, 233. (27) Castro-Acuna, C. M.; Fan, F.-R. F.; Bard, A. J. J. Electroanal. Chem. 1987, 234, 347. (28) King, R. D.; Nocera, D. G.; Pinnavaia, T. J. J. Electroanal. Chem. 1987, 236, 43. (29) Brahimi, B.; Labbe, P.; Reverdy, G. J. Electroanal. Chem. 1989, 267, 343. (30) Villemure, G.; Bard, A. J. J. Electroanal. Chem. 1990, 282, 107. 0003-2700/96/0368-0498$12.00/0

© 1996 American Chemical Society

of reactivity and selectivity. For example, montmorillonite CMEs were used in ion-exchange voltammetry and applied to the detection of inorganic cations such as Fe3+ and Ru(NH3)63+.31,32 Moreover, a laponite CME was applied to the detection of neutral and cationic organometallic compounds.33 One of the goals of this study is to demonstrate that the techniques used in the field of CME, together with the knowledge of clay minerals from the clay community, can indeed provide an interesting way to construct a paraquat sensor. We report here the construction of a novel paraquat sensor consisting of a Nafion-coated thin-film electrode containing appropriate amounts of clay. Six representative claysskaolinite (KGa-1), illite, bentonite, nontronite (SWa-1, ferruginous smectite), montmorillonite (SWy-1), and vermiculite (VTx-1)swere chosen for use in fabricating the paraquat sensor. Kaolinite has a welldefined 1:1 layer-lattice structure, formed by sharing O atoms between one silicon tetrahedral sheet and one dioctahedral gibbsite sheet. The rest of the clays chosen all have 2:1 layerlattice structures, formed by combination of an octahedral sheet sandwiched between two tetrahedral sheets. They differ mainly in the extent and location of isomorphous substitution. It is the difference in the type of predominant interlayer cation that gives the five 2:1 clays chosen very distinctive properties. Meanwhile, Nafion was chosen for use in the construction of the paraquat sensor because Nafion has been used for electrode coatings in a variety of electrochemical studies, mostly in conjunction with the immobilization of positively charged redox couples within the film. Polymer-modified electrodes have received considerable attention because they offer many advantages in improving resistance to interferences from surface-active compounds and common ions. The good cation-exchange ability and mechanical stability of the Nafion film has led us to investigate the possibility of making a paraquat sensor from clay and Nafion on the surface of a glassy carbon electrode. In this paper, the preparation of the Nafion/ clay-modified electrode (NCME) and various factors influencing the determination of paraquat are thoroughly investigated. Typical interferences encountered in the determination of paraquat are discussed. The analytical utility of the NCME is demonstrated by application to real water samples. EXPERIMENTAL SECTION Chemicals and Reagents. Nafion perfluorinated ion-exchange powder, 5 wt % solution in a mixture of lower aliphatic alcohols and 10% water, was obtained from the Aldrich Chemical Co. (Milwaukee, WI). Standard clay minerals, kaolinite (KGa-1), illite, bentonite, nontronite (SWa-1, ferruginous smectite), montmorillonite (SWy-1), and vermiculite (VTx-1), were purchased from the Source Clay Minerals Repository (University of Missouri, Columbia, MO). Paraquat (Sigma, St. Louis, MO) and all other compounds (ACS-certified reagent grade) used in this work were prepared without further purification in doubly distilled deionized water. Groundwater and lake water were collected from the campus of Chung-Hsing University (Taichung, Taiwan). The samples were taken in glass bottles, filtered through a 0.45-µm nylon filter, and stored at 4 °C. Apparatus. Electrochemistry was performed on a Bioanalytical Systems (West Lafayett, IN) CV-50W electrochemical (31) Wang, J.; Martinez, T. Electroanalysis 1989, 1, 167. (32) Wielgos, T.; Fitch, A. Electroanalysis 1990, 2, 449. (33) Labbe, P.; Brahimi, B.; Reverdy, G.; Mousty, C.; Blankespoor, R.; Gautier, A.; Degrand, C. J. Electroanal. Chem. 1994, 379, 103.

analyzer. A BAS Model VC-2 electrochemical cell was employed in these experiments. The three-electrode system consists of one of the following working electrodes, glassy carbon electrode, CME, or NCME; a Ag/AgCl reference electrode (Model RE-5, BAS); and a platinum wire auxiliary electrode. Procedure. Clay colloids were prepared in the sodium form generally according to procedures previously described.26-30 In brief, the clay was stirred in 1 M NaCl for 48 h to be converted to sodium form. After centrifugation, it was repeatedly washed first with 50% and then with 95% alcohol until a negative chloride test was obtained. The fractions were then separated by centrifugation and freeze-dried. In the preparation of the CMEs, clay films were prepared by dropping a known volume (100 µL) of a clay colloid (0.5 g/L) onto a clean glassy carbon electrode and dried under ambient conditions, usually ∼0.5-1 h. Uniform films could be cast reproducibly from clays. As for the NCMEs, 4 µL of the coating solution, containing appropriate amounts of Nafion and clay, was spin-coated on the glassy carbon electrode surface at 3000 rpm. A uniform thin film was formed after about 3 min of spinning. The electrodes were subsequently rinsed with water and then transferred to the argon-degassed electrochemical cell for experiments. Unless otherwise stated, a 0.05 M, pH 8 phosphate buffer was used as the supporting electrolyte. The accumulation step proceeded in constantly stirred (300 rpm) paraquat solution, and the voltage scanning step was performed after 2 s of quiet time. RESULTS AND DISCUSSION Electrochemical Behavior of Paraquat on the CMEs. The typical steady-state cyclic voltammograms for paraquat incorporated into the six different CMEs are shown in Figure 1. It takes about 2-3 min of scanning to reach the steady state for the six different CMEs at a paraquat concentration of 1 × 10-3 M, and a longer time is needed for the lower concentration of paraquat. As can be seen, among the clays tested, the steady-state reduction currents (ipc) for paraquat are in the following order: SWa-1 > illite > SWy-1 > bentonite > KGa-1 > VTx-1. Note that all the CMEs obtained a higher ipc than that obtained with a bare glassy carbon electrode. Moreover, the 2:1 CMEs resulted in higher ipc than the 1:1 CME. This is as expected, since KGa-1 has very little isomorphous substitution and so has very low specific surface and cation exchange capacity (CEC).34-36 Taking into account the values of CEC for the six clays, it is reasonable to expect that the order of the measured ipc is related to the CEC. Overall, since SWa-1 possesses the highest ipc measured, it was therefore chosen for use in fabricating the paraquat sensor in subsequent study. Optimization of the Paraquat Signal on the NCMEs. As pointed out, polymer-modified electrodes have received considerable attention because of many the advantages they offer in improving resistance to interferences from surface-active compounds and common ions. The paraquat sensor was thus fabricated by spin-coated a coating solution of SWa-1 and Nafion onto the glassy carbon electrode. It is well known that the sensitivity of square-wave anodic stripping voltammetry (SWASV) of adsorbed species is proportional to the degree of reversibility (34) van Olphen, H. An Introduction to Clay Colloid Chemistry; Wiley: New York, 1963. (35) Grim, R. E. Clay Mineralogy; McGraw-Hill: New York, 1968. (36) Jaynes, W. F.; Bigham, J. M. Clays Clay Miner. 1987, 35, 440.

Analytical Chemistry, Vol. 68, No. 3, February 1, 1996

499

Figure 1. Steady-state cyclic voltammograms of 1 × 10-3 M paraquat solution on bare GCE (A) and six different CMEs: (B) kaolinite, (C) illite, (D) bentonite, (E) nontronite, (F) montmorillonite, and (G) vermiculite. Supporting electrolyte was 0.05 M, pH 8 phosphate buffer; scan rate was 100 mV s-1.

of the electrochemical reaction.37,38 Since the redox paraquat couple showed a reversible behavior at the NCME, a clear advantage of using the NCME in the SW mode with respect to the sensitivity of detection is expected. To arrive at the optimum conditions for paraquat determination, the major factors that should be considered are the Nafion/clay composition, the preconcentration time, the preconcentration potential, and the SW parameters. The Nafion/clay composition directly controls the electrode performance. The optimum composition depends on both the diffusion process of the paraquat ions in the film and the maximum loading that does not affect the adhesion of the film to the glassy carbon surface. The NCMEs prepared from coating solutions that contain 1.7 mL of Nafion and 1 mL of SWa-1 (0.05, 0.1, 0.5, and 1 wt % in ethanol) were examined under identical conditions. As shown in Figure 2A, in all cases, paraquat responses could be obtained, and the peak current reached a maximum when the content of SWa-1 was around 0.1 wt %. Apparently, the accumulation ability of SWa-1 with paraquat functions properly in the NCME. Note that even a Nafion-coated electrode accumulates paraquat efficiently on its own. It has been shown previously that Nafion has a very high affinity for hydrophobic organic cations.39 The condition was further optimized by varying the amount of Nafion in the coating solutions as follows: 2-0.1 mL of Nafion and 1 mL of SWa-1 (0.1 wt % in ethanol). As shown in Figure 2B, the results indicate that the optimum coating solution is 0.25 mL of Nafion and 1 mL of SWa-1 (0.1 wt % in ethanol). Electrodes prepared with the above coating solution composition were therefore used in all subsequent work. (37) Lovric, M.; Branica, M. J. Electroanal. Chem. 1987, 226, 239. (38) Lovric, M.; Komorsky-Lovric, S. J. Electroanal. Chem. 1988, 248, 239. (39) Szentirmay, M. N.; Martin, C. R. Anal. Chem. 1984, 56, 1898.

500 Analytical Chemistry, Vol. 68, No. 3, February 1, 1996

The effect of preconcentration potential on the SW response for paraquat is shown in Figure 3A. As can be seen, the peak current increases as the potential of the electrode becomes more negative between 0 and -0.3 V vs Ag/AgCl. This behavior is explained by the fact that the paraquat ion bears a positive charge; as a result, the adsorption of paraquat is favored at more negative potentials. However, the peak current drops rapidly as the potential becomes more negative than -0.3 V. This is because as the preconcentration potential moves closer to the redox potential of the paraquat couples, more of the deposited paraquat is reduced, causing a decrease in peak current. A preconcentration potential of -0.3 V vs Ag/AgCl was therefore used in all subsequent work. The effect of preconcentration time on the SW response for paraquat is shown in Figure 3B. For 50 ppb of paraquat, the peak current increases as the preconcentration time increases, and starts to level off around 4 min. It takes an even longer time for the peak current to level off for a lower concentration of paraquat. This phenomenon is as expected and further confirms the ion-exchange process between the NCME and the paraquat ions. Therefore, to increase the sensitivity of detection, a longer time is needed for the lower concentration of paraquat. For convenience, a preconcentration time of 4 min was used in most of the subsequent work. The SW parameters that were investigated are the frequency, the pulse height, and the pulse increment. These parameters are interrelated and have a combined effect on the response. The response for paraquat increases with SW frequency, but at frequencies higher than 90 Hz, sloping background current renders the measurement difficult. An increase in the pulse height causes an increase in the paraquat peak for up to 30 mV. The scan increment, together with the frequency, defines an effective

Figure 2. Effect of the coating solution used in preparing the NCME on the peak current of paraquat determination: (A) 1.7 mL of Nafion + 1 mL of SWa-1 (0.05, 0.1, 0.5, and 1 wt % in ethanol); (B) 2-0.1 mL of Nafion + 1 mL of SWa-1 (0.1 wt % in ethanol). [paraquat] ) 50 ppb; preconcentration time (tp) ) 4 min; and preconcentration potential (Pp) ) 0 V vs Ag/AgCl.

scan rate; hence, an increase in either the frequency or the pulse increment results in an increase in the effective scan rate. Overall, the best signal-to-background current characteristics can be obtained with the following instrumental settings: modulation amplitude, 30 mV; modulation frequency, 90 Hz; modulation step, 12 mV. Analytical Characterization of the NCMEs. Calibration data were obtained under the optimum experimental conditions mentioned above. For 4 min preconcentration time, the SWCSV voltammograms indicated that, in all cases, at concentrations between 0 and 80 ppb, a stripping response was observed at a potential in the vicinity of -0.7 V vs Ag/AgCl, as shown in Figure 4. The observed peak currents were then used to construct the calibration plot. Note that fresh sample solutions were used for each individual concentration and preconcentration time, and at least six determinations were made for each data point. For 4-min preconcentration experiments, the RSD was 1-3%. The plot shows a very linear behavior with slope (µA/ppb), intercept (µA), and correlation coefficient of 0.22, 0.02, and 0.9981, respectively. The linear range for 4-min preconcentration is from 0 to 80 ppb of paraquat, and the detection limit is 0.5 ppb (S/N ) 3). The sensitivity started to decrease when the concentration of paraquat was higher than 80 ppb. An even lower detection limit could be achieved for paraquat, provided that the preconcentration time is longer than 4 min.

Figure 3. Effect of preconcentration potential (A) and preconcentration time (B) on the peak current of paraquat determination obtained at the NCME. Conditions: (A) [paraquat] ) 20 ppb, tp ) 5 min; (B) [paraquat] ) 50 ppb, Pp ) -0.3 V vs Ag/AgCl.

Figure 4. Dependence of the SWCSV peak current on increasing paraquat concentrations: (A) 0, (B) 10, (C) 30, (D) 50, and (E) 70 ppb.

Analytical Chemistry, Vol. 68, No. 3, February 1, 1996

501

Table 1. Influence of Other Ions on the Response of Paraquata ions K(I), Cl(I), fulvic acid, p-nitrophenol Ca(II) Pb(II) Zn(II) Cu(II) Triton X-100 uric acid glucose a

concn excess over [paraquat]

Table 2. Determination of Paraquat in Groundwater and Lake Water

contribution (%) [ip(paraquat) ) 100%]

1000×

0

100× 1000× 100× 500× 10× 100× 10× 100× 100× 1000× 100× 1000× 100× 1000×

0 62 0 109 0 -33 0 -23 0 -88 0 -18 0 -16

[paraquat] ) 20 ppb; tp ) 4 min.

Various ions were examined with respect to their interference with the determination of paraquat, as summarized in Table 1. For 20 ppb of paraquat, with 4-min preconcentration time, the results showed that over 1000-fold excess concentration of K(I), Cl(I), fulvic acid, and p-nitrophenol did not influence the paraquat response. Uric acid, glucose, Pb(II), Ca(II), and Triton X-100 were found to interfere at a 1000-fold excess, while Zn(II) and Cu(II) interfered at a 100-fold excess. Note that p-nitrophenol and Triton X-100 were reported to have the strongest interference in the determination of paraquat, with limits of tolerance of 1 ppm and 0.01 ppb, respectively.19 As can be seen, this problem was largely overcome by use of the proposed NCME, since one of the functions of the Nafion membrane coating on the NCME is to prevent the organic interferences from reaching the interface at which the deposition/stripping takes place. The improvement in tolerance was from 1 ppm to 100 ppm and from 0.01 ppb to 1 ppm for p-nitrophenol and Triton X-100, respectively. The analytical utility of the paraquat sensor was assessed by applying it to the determination of paraquat in groundwater and

502 Analytical Chemistry, Vol. 68, No. 3, February 1, 1996

groundwater lake water a

paraquat added (ppb)

paraquat founda (ppb)

recovery (%)

10.00 10.00

9.86 ( 0.35 9.80 ( 0.51

98.6 98.0

Number of samples assayed, 6.

lake water. None of the natural water samples analyzed contained any paraquat, so they had to be spiked with the analyte at a certain concentration, and the results are summarized in Table 2. The paraquat stripping peaks are clearly displayed for both spiked water samples. The recovery of the spiked paraquat was also observed to be good in both water samples. Apparently, the interference effect in these two real water samples is almost negligible. These results provide sufficient evidence of the high feasibility of the NCME employed for determining paraquat in real water samples. CONCLUSIONS The results show that the application of the NCME in the determination of trace paraquat in real water samples is very promising. The recovery of the spiked paraquat was observed to be good in natural water samples. The paraquat sensor not only offers considerably higher resistance to organic interferences and common ions than previous reported sensors but also yields higher sensitivity when used in conjunction with the SW voltammetry. ACKNOWLEDGMENT The authors gratefully acknowledge financial support from the National Science Council of the Republic of China under Grant NSC 85-2113-M-005-014. Received for review June 14, 1995. Accepted November 7, 1995.X AC950590X X

Abstract published in Advance ACS Abstracts, December 15, 1995.

Clay-Modified Electrode

(1) Gill, R.; Qua, S. C.; Moffat, A. C. J. Chromatogr. 1983, 255, 483. ... (4) Yuen, S. H.; Bagness, J. E.; Myles, D. Analyst 1967, 92, 375. ..... Calibration data.

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