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Is the boron-doped diamond electrode a suitable substitute for mercury in pesticide analyses? A comparative study of 4-nitrophenol quantification in pure and natural waters
Journal of
Electroanalytical Chemistry Journal of Electroanalytical Chemistry 573 (2004) 11–18 www.elsevier.com/locate/jelechem
Is the boron-doped diamond electrode a suitable substitute for mercury in pesticide analyses? A comparative study of 4-nitrophenol quantification in pure and natural waters Valber A. Pedrosa, Lucia Codognoto, Sergio A.S. Machado, Luis A. Avaca
*
Instituto de Quı´mica de Sa˜o Carlos, Universidade de Sa˜o Paulo C.P. 780, 13560-970 Sa˜o Carlos, SP, Brazil Received 24 February 2004; received in revised form 25 March 2004; accepted 1 June 2004 Available online 27 July 2004
Abstract A comparison between the analytical performance of a hanging mercury drop electrode (HMDE) and a boron-doped diamond (BDD) electrode for the quantification of 4-nitrophenol (4-NP) in spiked pure and natural waters is reported in this work. Square wave voltammetry (SWV) was chosen as the electroanalytical technique and Britton–Robinson buffer as the electrolyte. For the reduction process, the quantification limits varied between 5.7 and 66.0 lg l1 for the HMDE and between 14.1 and 61.3 lg l1 for the BDD electrode for water samples with increasing degree of contamination. The oxidation of 4-NP on BDD was also used for analytical purposes and the quantification limits in this case varied from 9.4 to 53.1 lg l1. In all cases, the detection limits followed the same trend. These latter results illustrate the advantages arising from the possibility of using an oxidation process on BDD electrodes for analytical purposes in contaminated matrices. The recovery experiments showed values above 95% for spiked samples thus indicating the feasibility of the electroanalytical methodology to quantify 4-NP in pure or natural waters. HPLC measurements were also performed for comparison and confirmed the values measured by SWV. 2004 Elsevier B.V. All rights reserved. Keywords: Electroanalysis; Boron-doped diamond; Mercury electrodes; 4-Nitrophenol; Square wave voltammetry
1. Introduction Since the pioneering works of Hance [1] in the 1970s, electroanalytical methods have been intensively used to quantify pesticides in different matrices and two completely different approaches are normally followed. In the first one, solid electrodes have their surfaces modified with enzymes such as cholinesterase [2–4] or peroxidase [5,6] or with antibodies [7,8] thus becoming pesticide sensors or biosensors. These are indirect measurement methods that usually determine the degree of inhibition of the enzyme or the co-enzyme activation on the substrate. Although such methods are highly spe*
Corresponding author: Tel.: +55-16-3373-9943; fax: +55-16-33739982/9943. E-mail address: [email protected] (L.A. Avaca). 0022-0728/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2004.06.014
cific they show several problems related to interference by the matrix components, i.e., the deactivation of the enzymes can be caused by a variety of reasons and this is a major problem to be solved. On the other hand, direct determination methods using voltammetric techniques are becoming increasingly more popular [9–12]. In such methods, the electroanalytical measurements are usually based on reduction processes occurring on mercury surfaces. Mercury electrodes have some unique advantages that make them very appropriate for the determinations of pesticides. The low interactions of the Hg surface with organic molecules ensure minimisation of the adsorption processes that usually block the surface of solid electrodes thus hindering their utilisation. Moreover, the mercury drop is easily renewed and each new surface is identical to the previous one. However, the hazardous effects of mercury compounds in human
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health as well as the possibility of developing remote sensors for the environment have driven the efforts of researchers all over the world to find a harmless substitute. A good alternative was found by using carbon paste electrodes modified with enzymes [13], active catalysts such as phthalocyanine complexes [14,15] or other compounds. Such electrode systems have the advantage of easily renewable surfaces and are, in general, simple to construct and use. As drawbacks, the existence of ohmic drop as well as the solubility of the binding component in organic solvents frequently used to dissolve pesticides molecules must be pointed out. Boron-doped diamond (BDD) electrodes have been reported quite recently as another excellent choice [16– 18]. The outstanding electrochemical features of this material, such as the wide potential window in aqueous solutions [19,20], very low background current [21], weak adsorption for most types of organic molecules [22] and high stability of response [23,24] make this new material a promising one for electroanalytical applications. Recent work reported in the literature has shown that several inorganic, organic and bio-molecules can be satisfactorily determined with BDD electrodes [25–30]. On these lines, Rao et al. [25] used BDD electrodes for the chromatographic analysis with electrochemical detection of selected carbamate pesticides, reporting a superior performance when compared with other carbon-type electrodes. The same conclusion was reached by Terashima et al. [26] for the oxidation of disulphides, thiols and methione in aqueous acid media. Moreover, the use of BDD in the electroanalytical determinations of several chemical species whose detection is complicated when using conventional electrodes such as glassy carbon or metals was successfully carried out by Rao et al. [27]. In all cases, the BDD surface was conditioned by anodic polarisation prior to the measurements since the authors proposed an intrinsic relationship between the catalytic activity and the surface termination, clearly seen in the electroanalytical determination of DNA and glutathione [27]. Previous work from this laboratory has shown the effectiveness of BDD electrodes for the direct electroanalytical determination of pentachlorophenol in pure and contaminated waters [28] as well as in a polluted soil [29] and in pure water for 4-nitrophenol (4-NP) [30] and 4-chlorophenol [31] using square wave voltammetry (SWV). Meanwhile, it should be stressed that, in these cases, a cathodic polarisation was found necessary for conditioning the BDD surface prior to the electroanalytical determinations. Such pre-treatment improved the voltammetric response of the BDD surface resulting in very low quantification limits and high data reproducibility [32]. The reasons for this behaviour of the BDD electrodes are not clear at the moment but the subject has attracted some research in recent times [33,34].
To compare the performance of the BDD electrode with the traditional HDME, the behaviour of 4-NP on these surfaces was selected as the probe reaction using pure and contaminated waters. The importance of 4NP determination is related to the degradation pathway of several organophosphorous pesticides such as fenitrothion, methyl-parathion, ethyl-parathion, etc., that are decomposed in soils and waters producing 4-NP as an intermediate or final product of the reaction [35,36]. Additionally, 4-NP can be formed by reaction of phenol with nitrite ions in environmental waters [37]. The 4-NP is a hazardous substance that has a high environmental impact due to its toxicity and persistence. According to the Environmental Protection Agency (EPA) the allowed level of 4-NP for waters in general is 30 lg l1 [38]. Various non-electrochemical procedures have been reported for its determination, such as gas chromatography [39], high performance liquid chromatography [40], liquidchromatography associated with mass spectroscopy [41] and capillary electrophoresis [42,43]. On the other hand, electroanalytical methods have been proposed for 4-NP determination at the hanging mercury drop electrode (HMDE) electrode [44,45] based on its wellknown four-electron reduction process [46,47]. The aim of this work is to make a detailed comparative study between the performances of HMDE and BDD electrodes for the determination of 4-NP by a reduction process using SWV in spiked electrolytes prepared with either pure or contaminated waters as well as to report the results obtained by oxidation of the same molecule on BDD electrodes.
2. Experimental A 1.0 · 103 mol l1 stock solution of 4-NP (Aldrich 99%) was prepared using Milli-Q (Millipore) water. For the measurements with BDD, a 0.1 mol l1 Britton–Robinson (BR) buffer at pH 6.0 (adjusted by adding the appropriate amount of a 0.1 mol l1 NaOH solution) was used as the supporting electrolyte while in the case of the HMDE the same buffer but with pH 3.0 was the electrolyte. These pH values were established after optimisation of the corresponding analytical responses as shown in Fig. 1 for the 4-NP reduction process on BDD and HMDE while, for the oxidation on BDD, an analogous study has been published already [30]. Acetonitrile (HPLC and pesticide grade, UltimAR) was obtained from Mallinckrodt. All other reagents were analytical grade and purified nitrogen was used for de-aeration of the solutions. A conventional three-electrode cell with the AgjAgCl system and a Pt wire as the reference and auxiliary electrodes, respectively, was used. The BDD working electrode was a 0.62 · 1.0 cm2 single-faced plate, kindly provided by Eng. W. Haenni from CSEM, Neuchaˆtel,
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Fig. 1. Influence of the pH on the SWV peak potential (j) and peak current density (m) for the reduction in 0.1 mol l1 BR buffer of 1.5 · 105 mol l1 4-NP on the HMDE (with f = 120 s1, a = 40 mV, DEs = 2 mV) and for 3.0 · 105 mol l1 4-NP on the BDD electrode (with f = 100 s1, a = 60 mV, DEs = 2 mV).
Switzerland. Details of the preparation and physical characterization of the CSEM diamond electrodes used in this work can be found in the literature [18]. Prior to the experiments, the electrode was polarised at +3.0 V vs. AgjAgCl 13 M KCl for 30 min to remove the hydrophobic film that covers the surface and then for another 30 min at –3.0 V for surface conditioning [32]. The HMDE used was a model. MDE 303 EG&G PAR system with a 0.2 cm2 geometric area. Before each experiment, a stream of N2 was passed through the solution for ca. 10 min. Square wave voltammetry measurements were carried out with a model. 273A EG&G PARC potentiostat with M270 software for the BDD electrode and with a model. 394 from EG&G PARC for experiments with the HMDE. In all cases, the experimental SWV parameters such as square wave frequency (f), amplitude (a) and scan increment (DEs) were optimised before the measurements and these results were also used for the characterization of the electrode process via the SWV diagnostic criteria. Qualitative and quantitative determinations of 4-NP were also carried out by HPLC-UV. These analyses were carried out on a model. SCL-10Avp Shimadzu system equipped with a model. LC-10ATVP pumping unit and a spectrophotometric UV/Vis detector (SPD-
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10AVP). Data were processed on a Shimadzu LC WorkStation Class LC-10. The HPLC conditions were: a LiChrosorb RP-18 column (250 mm · 4.6 mm, 5 lm, Merck), with a RP-18 pre-column (30 mm · 4 mm, 5 lm, Merck); the mobile phase was 70/30 v/v acetonitrile/water with 1% v/v acetic acid, at a flow-rate of 1.0 ml min1. The injection volume was 20 ll and detection was performed at a wavelength of 310 nm. Analytical curves were obtained for each technique by the standard addition method. The effect of interference was evaluated using natural water samples taken from a local creek (Monjolinho) at two different points of Sa˜o Carlos city, namely, before (point 1) and after crossing town (point 2). The water samples were collected in glass bottles during the dry season and kept under refrigeration (4 C) for no longer than one week. The degree of contamination in the natural waters increases from point 1 to point 2 as indicated by the measured values of the oxygen demand, i.e., BOD (6.0 and 12.0 mg l1) and COD (19.0 and 33.0 mg l1), respectively. The electrolytes were prepared by dissolving the salts necessary for the BR buffer in either pure or natural waters and the measurements were performed without pre-treatment of the solutions but the pH was adjusted appropriately to the desired value in each case. The standard deviation of the mean value of currents (SB) measured at the 4-NP reduction potential for 10 voltammograms of the blank solution in electrolytes prepared with the different water samples was used [48,49] for the determination of the quantification and detection limits (QL and DL, respectively) together with the slope of the straight line of the analytical curves (b) and Eq. (1) 10S B 3S B QL ¼ ; DL ¼ : ð1Þ b b The recovery experiments were carried out by adding a known amount of 4-NP to the supporting electrolyte followed by standard additions from the 4-NP stock solution and plotting the resulting analytical curve. All measurements were performed in triplicate. The recovery efficiencies (R%) for the different systems under investigation were calculated using Eq. (2) where the value [4-NP] found refers to the concentration obtained by extrapolation of the analytical curve in the corresponding spiked water samples ½4 NP found %R ¼ 100 : ð2Þ ½4 NP added
3. Results and discussion 3.1. Hanging mercury drop electrode and HPLC validation The SWV curves obtained on the HMDE with a f of 120 s1, a of 40 mV and DEs of 2 mV in a 0.1 mol l1 pH
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3.0 BR buffer prepared with purified water show a single, diffusion-controlled reduction peak at approximately 0.4 V. These results are presented in Fig. 2 for concentrations of 4-NP added to the electrolyte up to 4.0 · 105 mol l1. The dependence of the peak current density with concentration resulted in the linear plot shown in the inset of Fig. 2. The corresponding linear equation was determined as j p = 0.152 + 0.550 · [4-NP] with r = 0.9998 while the QL value for these experiments was calculated using Eq. (1) and the methodology described in Section 2. The value obtained for pure water solutions using the HMDE was 5.7 g l1 (or 4.16 · 108 mol l1) and is included in Table 1 together with the corresponding DL value. To validate the previous electroanalytical results, HPLC quantification experiments were carried out in electrolytes prepared with pure water. Thus, 4-NP additions were made to a BR buffer solution prepared with purified water and with the pH adjusted to 6. Aliquots of these spiked solutions in the same concentration range as before (0.5–4.0 · 105 mol l1) were injected
Fig. 2. SWV responses on the HMDE for the reduction of different 4NP concentrations: 0 (1); 0.5 (2); 0.75 (3); 1.0 (4); 1.25 (5); 1.5 (6); 2.0 (7); 3.0 (8) and 4.0 · 105mol l1 (9) in 0.1 mol l1 BR buffer, pH 3.0, with f = 120 s1, a = 40 mV, DEs = 2 mV. Inset: linear dependence of jp with 4-NP concentration.
into the chromatograph and a peak with a retention time of 3 min, corresponding to 4-NP was detected at 310 nm. To calculate the QL value a different approach was followed since, in chromatographic experiments, the blank response does not present the same deviations observed in electroanalyses. In this case, the quantification limit was obtained using the standard deviation of the yintercept of 10 analytical curves obtained with the chromatographic procedure above. This parameter replaces the SB term in Eq. (1) [48,49]. The resulting QL value was 2.0 · 108 mol l1. This value is approximately only one-half of that obtained from the electroanalytical experiments on the HMDE, a fact probably related to the pre-concentration of the 4-NP molecules in the chromatographic column during the HPLC experiments, and confirms the validity of the previous SWV results. The same electroanalytical methodology described above for pure water solutions was then applied to quantify 4-NP in the natural water samples described in Section 2. Here, the samples collected from the Monjolinho creek were used to prepare the supporting electrolyte and then spiked with the minimum detectable amount of 4-NP followed by further additions of the standard solution in the range 0.5–4.0 · 105 mol l1. The resulting voltammograms at different concentrations were practically identical to those in Fig. 2 except for the jp values that were smaller than the equivalent values in pure water solutions. The corresponding analytical curves are presented in Fig. 3 together with that obtained in pure water, for comparison. The straight lines correspond to the equations: jp = 0.091 + 0.500 · [4-NP], r = 0.99907 for point 1 and jp = 0.044 + 0.485 · [4-NP], r = 0.99883 for point 2 samples, respectively. As can be observed from the slopes of the analytical curves, the determination of 4-NP in contaminated waters has a lower sensitivity when compared to pure water. This behaviour has frequently been observed and related to the organic matter dissolved in natural waters, mainly humic and fulvic acids [28,50,51]. The values of QL and DL in these natural water samples were again evaluated using Eq. (1) and are also collected in Table 1. Moreover, recovery experi-
Table 1 Analytical parameters for 4-NP determinations on HMDE and BDD electrodes in pure and contaminated water samples Sample
b/A l mol1 cm2
r
SB/lA cm2
QL/lg l1
DL/lg l1
Recovery/%
Reduction on HMDE
Milli-Q Point 1 Point 2
0.550 0.500 0.485
0.9998 0.9990 0.9988
0.002 0.025 0.022
5.7 63.2 66.0
1.7 18.9 19.8
98.0 ± 2.3 95.1 ± 3.2 93.2 ± 2.7
Reduction on BDD
Milli-Q Point 1 Point 2
0.756 0.500 0.548
0.9989 0.9990 0.9920
0.022 0.020 0.024
14.1 58.9 61.3
4.2 17.0 18.5
97.3 ± 2.0 95.2 ± 1.9 94.5 ± 2.1
Oxidation on BDD
Milli-Q Point 1 Point 2
0.629 0.580 0.532
0.9998 0.9995 0.9960
0.127 0.014 0.016
9.4 36.1 53.1
2.8 10.8 16.0
98.0 ± 1.1 96.0 ± 2.2 97.1 ± 1.8
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Fig. 3. Analytical curves for the reduction of 4-NP on the HMDE in solutions prepared with: Milli-Q water (j), point 1 (d) and point 2 (m) water samples from the Monjolinho creek.
ments were carried out for the quantification of 4-NP in the different water samples following the procedure presented in Section 2. Here, values of 98.0%, 95.1% and 93.2% were obtained for pure water and for the samples collected at points 1 and 2, respectively (Table 1). The results obtained with the HMDE demonstrate that the electroanalytical procedure used here is suitable for 4NP quantification in the matrices analysed. However, the significant increase of the QL values for the contaminated waters reveals the high sensitive of mercury towards organic molecules contained in the samples. 3.2. Electrochemical reduction on the boron-doped diamond electrode Aiming to evaluate the behaviour of BDD electrodes in comparison to the HMDE, the same electroanalytical methodology describe above was applied to this new electrode material and the electrochemical reduction of 4-NP was again studied in pure and natural water samples by experiments completely analogous to those described before. The SWV curves for 4-NP reduction on BDD electrodes at different concentrations in pure water solutions are presented in Fig. 4 and again show a unique, diffusion-controlled peak at 0.8 V vs. AgjAgCl in BR buffer with pH 6.0. In this case, the following optimised voltammetric conditions were used: f of 100 s1, a of 60 mV and DEs of 2 mV. The relatively large negative shift in the peak potential when compared to the HMDE experiments (Fig. 2) is due to the strong dependence of the NO2 group reduction on pH [47]. Meanwhile, the current density values are very similar to each other suggesting that the same four-electron reduction process is occurring on the BDD electrode.
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Fig. 4. SWV responses on the BDD electrode for the reduction of different 4-NP concentrations: 0 (1); 0.5 (2); 0.7 (3); 1.0 (4); 1.2 (5); 1.5 (6); 2.0 (7); 3.0 (8) and 4.0 · 105mol l1(9) in 0.1 mol l1 BR buffer, pH 6.0, with f = 100 s1, a = 60 mV, DEs = 2 mV. Inset: linear dependence of jp with 4-NP concentration.
The analytical curve for 4-NP in the electrolyte prepared with pure water is also shown in Fig. 4 as an inset. The slope of this straight line is the sensitivity of the method and was used to calculate the QL value with the help of Eq. (1). The value found here, 14.1 lg l1 (or 10.3 · 108 mol l1) is somewhat higher than that obtained for the HMDE electrode but of the same order of magnitude. This parameter is also included in Table 1 together with the corresponding DL value. Recovery experiments performed with the same procedure used for the HMDE furnished a value of 97.3% (Table 1), which is practically identical to that obtained before for mercury (98.0%). Subsequently, the electroanalytical methodology was applied to the quantification of 4-NP in contaminated waters where, as before, the electrolyte (BR buffer, pH 6.0) was prepared with samples collected from two points of the urban creek. As in the previous case, only one voltammetric peak was found at practically the same potential value recorded for pure water solutions. The dependence of jp with 4-NP concentration also produces linear analytical curves, as shown in Fig. 5 together with that obtained in pure water solutions, for comparison. Meanwhile, the overall reduction processes on BDD electrodes in contaminated waters seem to be much more complex than on mercury and the analytical curves have a considerable current contribution at zero 4-NP concentration. In this case, the calculated value of QL was 58.9 lg l1 (or 41.6 108 mol l1) for point 1 and 61.3 lg l1 (or 45.3 · 108 mol l1) for point 2 samples, respectively. These values are slightly lower than those obtained on the mercury electrode under the same conditions (see Table 1) suggesting that the BDD electrode works more efficiently in contaminated waters. Such behaviour could be associated with the very low interactions existing between diamond surfaces and organic molecules thus minimising the inhibition of the pesticide reduction by blockage of the electrode surface.
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Fig. 5. Analytical curves for 4-NP reduction on the BDD electrode in solutions prepared with: Milli-Q water (j), point 1 (d) and point 2 (m) water samples from the Monjolinho creek.
Recovering experiments were performed in the natural waters yielding the values of 95.2% and 94.5% for points 1 and 2, respectively (Table 1). These values were a little smaller than that obtained in pure water solutions (97.3%) but also point to the convenience of this method for 4-NP quantification. 3.3. Electrochemical oxidation on the boron-doped diamond electrode A further development of the electroanalytical methodology described here is the use of the 4-NP oxidation process on BDD electrodes for its quantification in different water solutions. According to the literature, 4-NP can be oxidised to the corresponding o-benzoquinone [52,53] following a mechanism similar to that determined for the oxidation of pentachlorophenol [54], that is, an electrochemical reaction involving the transference of four electrons. To use such an electrode process to quantify 4-NP, SWV experiments were performed between 0.8 and 1.15 V vs. AgjAgCl using the following optimised parameters, f = 100 s1, a = 50 mV and DEs = 2 mV in pH 6.0 BR buffer prepared with purified water. The results obtained for several 4-NP concentrations in solution (0.6 · 105–5.0 · 105 mol l1) are shown in Fig. 6 where the linear relationship between jp and [4-NP] appears in the inset. In this case, the SWV diagnostic criteria indicate that the process is controlled by adsorption of the reagent and/or the products. The slope of the analytical curve was used to calculate the QL value using Eq. (1) as in the previous cases and the result obtained here was 9.4 lg l1(or 6.9 · 108 mol l1). This value, also collected in Table 1, is somewhat higher than that obtained for 4-NP reduction on the HMDE but smaller than that for reduction on the BDD electrode. This is probably related to the existence of fewer interactions between the diamond surface and adsorbed molecules in the positive branch of the
Fig. 6. SWV responses on the BDD electrode for the oxidation of different 4-NP concentrations: 0 (1); 0.6 (2); 1.0 (3); 1.5 (4); 2.0 (5); 3.0 (6); 4.0 (7); 4.5 (8) and 5.0 · 105mol l1 (9) in 0.1 mol l1 BR buffer, pH 6.0, with f = 100 s1, a = 50 mV, DEs = 2 mV. Inset: linear dependence of the peak current density with 4-NP concentration.
potential range but this assumption will require further studies. In addition, recovery experiments furnished a value of 98.0% for the present conditions and this is an indication that the oxidation methodology is capable of detecting all 4-NP present in solution. The same experiments were carried out in pH 6.0 BR buffer prepared with the contaminated water samples and the voltammograms were similar to those of Fig. 6. The linear relationships between the jp and 4-NP concentrations are presented in Fig. 7 together with the analytical curve obtained in pure water solutions, for comparison. The slopes of all the lines in Fig. 7 (Table 1) are larger than those measured for the HMDE indicating that the process of 4-NP oxidation on BDD has higher sensitivity than the reduction on mercury surfaces. The resulting QL value was 36.1 lg l1(or 25.9 · 108 mol l1) for point 1 and 53.1 lg l1(or 38.1 · 108 mol l1) for point 2 samples, respectively, and are included in Table 1. These results are a clear indication that the contaminants are less active on BDD
Fig. 7. Analytical curves for 4-NP oxidation on the DDB electrode in solutions prepared with: Milli-Q water (j), point 1 (d) and point 2 (m) water samples of the Monjolinho creek.
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electrodes in the positive range of potentials used. This is probably due to a decrease in the adsorption of organic molecules as well as a minimal contribution to the electrode process by cations present in the contaminated waters. Finally, the recovery experiments yielded values of 96% and 97% for samples from points 1 and 2, respectively, thus confirming the suitability of this methodology to quantify 4-NP by means of its oxidation reaction on BDD electrodes. 4. Conclusions The results of this investigation have clearly revealed several advantages from the use of BDD electrodes instead of the traditional HMDE for the electroanalytical determination of 4-NP under different experimental conditions. First of all, the handling of a non-toxic material facilitates the experimental procedures and enables future developments of remote sensors for the environment. This can be achieved with the BDD without great losses in performance and with even better QL and recovery results than those obtained on mercury when dealing with heavily contaminated matrices. This previous statement applies both to the reduction and the oxidation processes studied here on BDD electrodes where the QL values were initially larger than those of the HMDE in pure water solutions but increased at the lower rate when the contamination of the samples become more intense. On the other hand, the use of an oxidation reaction on BDD electrodes for the quantification of 4-NP and other undesired organic molecules in environmental samples such as wastewater cannot be achieved on mercury due to its electrochemical instability under those conditions. Such an oxidation process proved to be more efficient than reduction and the reasons for that might be linked to a diminished adsorption of organic contaminants on the BDD surface as well as to the fact that many inorganic pollutants are not electroactive in such a potential region. Acknowledgements The authors thank CNPq and FAPESP (03/00710-6), Brazil, for scholarships received and Eng. W. Haenni from CSEM, Neuchaˆtel, Switzerland, for providing the diamond electrodes. References [1] R.J. Hance, Pestic. Sci. 01 (1979) 112. [2] A.L. Hart, W.A. Collier, D. Janssen, Biosens. Bioelectron. 12 (1997) 645.
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[3] G.A. Evtugyn, H.C. Budnikov, E.B. Nikolskaya, Talanta 4 (1998) 465. [4] M.P. Xavier, B. Vallejo, M.D. Marazuela, M.C. Moreno-Bondi, F. Baldini, A. Falai, Biosens. Bioelectron. 14 (2000) 895. [5] J. Diehl-Faxon, A.L. Ghindilis, P. Atanasov, E. Wilkins, Sens. Actuat. B 36 (1996) 448. [6] N.G. Karousos, S. Aouabdi, A.S. Way, S.M. Reddy, Anal. Chim. Acta 469 (2002) 189. [7] M.C. Hennion, D. Barcelo, Anal. Chim. Acta 362 (1998) 3. [8] G.S. Nunes, M.P. Marco, M.L. Ribeiro, D. Barcelo, J. Chromotogr. A 823 (1998) 109. [9] L. Herna´ndez, P. Herna´ndez, J. Vicente, Fresenius J. Anal. Chem. 345 (1993) 712. [10] M. Kotoucek, M. Opravilova´, Anal. Chim. Acta 329 (1996) 73. [11] A. Guiberteau, T. Galeano, N. Mora, P. Parrilla, F. Salinas, Talanta 53 (2001) 943. [12] L.D.O. dos Santos, G. Abate, J.C. Masini, Talanta 62 (2004) 667. [13] G.S. Nunes, D. Barcelo´, B.S. Grabaric, J.M. Daz-Cruz, M.L. Ribeiro, Anal. Chim. Acta 399 (1999) 37. [14] M.D.P.T. Sotomayor, A.A. Tanaka, L.T. Kubota, Anal. Chim. Acta 455 (2002) 215. [15] A.A. Ciucu, C. Negulescu, R.P. Baldwin, Biosens. Bioelectron. 18 (2003) 303. [16] Y.V. Pleskov, Russ. J. Electrochem. 38 (2002) 1411. [17] Y.V. Pleskov, J. Anal. Chem. 55 (2000) 1045. [18] D. Gandini, P. Michaud, I. Duo, E. Mache´, W. Haenni, A. Perret, C. Comninellis, New Diamond Front. Carbon Technol. 9 (1999) 303. [19] J.W. Strojek, M.C. Granger, T. Dallas, M.W. Holtz, G.M. Swain, Anal. Chem. 68 (1996) 2031. [20] J. Xu, G.M. Swain, Anal. Chem. 70 (1998) 1502. [21] Y. Yano, D.A. Tryk, K. Hashimoto, A. Fujishima, J. Electrochem. Soc. 145 (1998) 1870. [22] N. Vinokur, B. Miller, Y. Avyigal, R. Kalisk, J. Electrochem. Soc. 143 (1996) L238. [23] T.N. Rao, Y. Yagi, T. Miwa, D.A. Tryk, A. Fujishima, Anal. Chem. 71 (1999) 2506. [24] E. Popa, H. Notsu, T. Miwa, D.A. Tryk, A. Fujishima, Electrochem. Solid-State Lett. 2 (1999) 49. [25] T.N. Rao, B.H. Loo, B.V. Sarada, C. Terashima, A. Fujishima, Anal. Chem. 74 (2002) 1578. [26] C. Terashima, T.N. Rao, B.V. Sarada, Y. Kubota, A. Fujishima, Anal. Chem. 75 (2003) 1564. [27] T.N. Rao, T.A. Ivandini, C. Terashima, B.V. Sarada, A. Fujishima, New Diamond Front. Carbon Technol. 13 (2003) 79. [28] L. Codognoto, S.A.S. Machado, L.A. Avaca, Diamond Relat. Mater. 11 (2002) 1670. [29] L. Codognoto, V.G. Zuin, D. de Souza, J.H. Yariwake, S.A.S. Machado, L.A. Avaca, Microchem. J. 77 (2004) 177. [30] V.A. Pedrosa, L. Codognoto, L.A. Avaca, J. Braz. Chem. Soc. 14 (2003) 530. [31] V.A. Pedrosa, L. Codognoto, L.A. Avaca, Quim. Nova 26 (2003) 844. [32] H.B. Suffredini, V.A. Pedrosa, L. Codognoto, S.A.S. Machado, L.A. Avaca, Electrochim. Acta 49 (2004) 4021. [33] R. Sato, T. Kondo, K. Shimizu, K. Honda, Y. Shibayama, K. Shirahama, A. Fujishima, Y. Einaga, Chem. Lett. 32 (2003) 972. [34] L.C. Hian, K.J. Grehan, R.G. Compton, J.S. Foord, F. Marken, Diamond Relat. Mater. 12 (2003) 590. [35] M. Castillo, R. Domingues, M.F. Alpendurada, D. Barcelo`, Anal. Chim. Acta 353 (1997) 133. [36] S.V. Dzyadevych, J.M. Chovelon, Mater. Sci. Eng. C 21 (2002) 55. [37] P. Patnaik, J.N. Khoury, Water Res. 38 (2004) 206. [38] S. Environmental Protection Agency, Fer. Regist., 44 (1979) 233. [39] S.P. Wang, H.J. Chen, J. Chromatogr. A 979 (2002) 439.
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Electroanalytical Chemistry Journal of Electroanalytical Chemistry 573 (2004) 11–18 www.elsevier.com/locate/jelechem
Is the boron-doped diamond electrode a suitable substitute for mercury in pesticide analyses? A comparative study of 4-nitrophenol quantification in pure and natural waters Valber A. Pedrosa, Lucia Codognoto, Sergio A.S. Machado, Luis A. Avaca
*
Instituto de Quı´mica de Sa˜o Carlos, Universidade de Sa˜o Paulo C.P. 780, 13560-970 Sa˜o Carlos, SP, Brazil Received 24 February 2004; received in revised form 25 March 2004; accepted 1 June 2004 Available online 27 July 2004
Abstract A comparison between the analytical performance of a hanging mercury drop electrode (HMDE) and a boron-doped diamond (BDD) electrode for the quantification of 4-nitrophenol (4-NP) in spiked pure and natural waters is reported in this work. Square wave voltammetry (SWV) was chosen as the electroanalytical technique and Britton–Robinson buffer as the electrolyte. For the reduction process, the quantification limits varied between 5.7 and 66.0 lg l1 for the HMDE and between 14.1 and 61.3 lg l1 for the BDD electrode for water samples with increasing degree of contamination. The oxidation of 4-NP on BDD was also used for analytical purposes and the quantification limits in this case varied from 9.4 to 53.1 lg l1. In all cases, the detection limits followed the same trend. These latter results illustrate the advantages arising from the possibility of using an oxidation process on BDD electrodes for analytical purposes in contaminated matrices. The recovery experiments showed values above 95% for spiked samples thus indicating the feasibility of the electroanalytical methodology to quantify 4-NP in pure or natural waters. HPLC measurements were also performed for comparison and confirmed the values measured by SWV. 2004 Elsevier B.V. All rights reserved. Keywords: Electroanalysis; Boron-doped diamond; Mercury electrodes; 4-Nitrophenol; Square wave voltammetry
1. Introduction Since the pioneering works of Hance [1] in the 1970s, electroanalytical methods have been intensively used to quantify pesticides in different matrices and two completely different approaches are normally followed. In the first one, solid electrodes have their surfaces modified with enzymes such as cholinesterase [2–4] or peroxidase [5,6] or with antibodies [7,8] thus becoming pesticide sensors or biosensors. These are indirect measurement methods that usually determine the degree of inhibition of the enzyme or the co-enzyme activation on the substrate. Although such methods are highly spe*
Corresponding author: Tel.: +55-16-3373-9943; fax: +55-16-33739982/9943. E-mail address: [email protected] (L.A. Avaca). 0022-0728/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2004.06.014
cific they show several problems related to interference by the matrix components, i.e., the deactivation of the enzymes can be caused by a variety of reasons and this is a major problem to be solved. On the other hand, direct determination methods using voltammetric techniques are becoming increasingly more popular [9–12]. In such methods, the electroanalytical measurements are usually based on reduction processes occurring on mercury surfaces. Mercury electrodes have some unique advantages that make them very appropriate for the determinations of pesticides. The low interactions of the Hg surface with organic molecules ensure minimisation of the adsorption processes that usually block the surface of solid electrodes thus hindering their utilisation. Moreover, the mercury drop is easily renewed and each new surface is identical to the previous one. However, the hazardous effects of mercury compounds in human
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health as well as the possibility of developing remote sensors for the environment have driven the efforts of researchers all over the world to find a harmless substitute. A good alternative was found by using carbon paste electrodes modified with enzymes [13], active catalysts such as phthalocyanine complexes [14,15] or other compounds. Such electrode systems have the advantage of easily renewable surfaces and are, in general, simple to construct and use. As drawbacks, the existence of ohmic drop as well as the solubility of the binding component in organic solvents frequently used to dissolve pesticides molecules must be pointed out. Boron-doped diamond (BDD) electrodes have been reported quite recently as another excellent choice [16– 18]. The outstanding electrochemical features of this material, such as the wide potential window in aqueous solutions [19,20], very low background current [21], weak adsorption for most types of organic molecules [22] and high stability of response [23,24] make this new material a promising one for electroanalytical applications. Recent work reported in the literature has shown that several inorganic, organic and bio-molecules can be satisfactorily determined with BDD electrodes [25–30]. On these lines, Rao et al. [25] used BDD electrodes for the chromatographic analysis with electrochemical detection of selected carbamate pesticides, reporting a superior performance when compared with other carbon-type electrodes. The same conclusion was reached by Terashima et al. [26] for the oxidation of disulphides, thiols and methione in aqueous acid media. Moreover, the use of BDD in the electroanalytical determinations of several chemical species whose detection is complicated when using conventional electrodes such as glassy carbon or metals was successfully carried out by Rao et al. [27]. In all cases, the BDD surface was conditioned by anodic polarisation prior to the measurements since the authors proposed an intrinsic relationship between the catalytic activity and the surface termination, clearly seen in the electroanalytical determination of DNA and glutathione [27]. Previous work from this laboratory has shown the effectiveness of BDD electrodes for the direct electroanalytical determination of pentachlorophenol in pure and contaminated waters [28] as well as in a polluted soil [29] and in pure water for 4-nitrophenol (4-NP) [30] and 4-chlorophenol [31] using square wave voltammetry (SWV). Meanwhile, it should be stressed that, in these cases, a cathodic polarisation was found necessary for conditioning the BDD surface prior to the electroanalytical determinations. Such pre-treatment improved the voltammetric response of the BDD surface resulting in very low quantification limits and high data reproducibility [32]. The reasons for this behaviour of the BDD electrodes are not clear at the moment but the subject has attracted some research in recent times [33,34].
To compare the performance of the BDD electrode with the traditional HDME, the behaviour of 4-NP on these surfaces was selected as the probe reaction using pure and contaminated waters. The importance of 4NP determination is related to the degradation pathway of several organophosphorous pesticides such as fenitrothion, methyl-parathion, ethyl-parathion, etc., that are decomposed in soils and waters producing 4-NP as an intermediate or final product of the reaction [35,36]. Additionally, 4-NP can be formed by reaction of phenol with nitrite ions in environmental waters [37]. The 4-NP is a hazardous substance that has a high environmental impact due to its toxicity and persistence. According to the Environmental Protection Agency (EPA) the allowed level of 4-NP for waters in general is 30 lg l1 [38]. Various non-electrochemical procedures have been reported for its determination, such as gas chromatography [39], high performance liquid chromatography [40], liquidchromatography associated with mass spectroscopy [41] and capillary electrophoresis [42,43]. On the other hand, electroanalytical methods have been proposed for 4-NP determination at the hanging mercury drop electrode (HMDE) electrode [44,45] based on its wellknown four-electron reduction process [46,47]. The aim of this work is to make a detailed comparative study between the performances of HMDE and BDD electrodes for the determination of 4-NP by a reduction process using SWV in spiked electrolytes prepared with either pure or contaminated waters as well as to report the results obtained by oxidation of the same molecule on BDD electrodes.
2. Experimental A 1.0 · 103 mol l1 stock solution of 4-NP (Aldrich 99%) was prepared using Milli-Q (Millipore) water. For the measurements with BDD, a 0.1 mol l1 Britton–Robinson (BR) buffer at pH 6.0 (adjusted by adding the appropriate amount of a 0.1 mol l1 NaOH solution) was used as the supporting electrolyte while in the case of the HMDE the same buffer but with pH 3.0 was the electrolyte. These pH values were established after optimisation of the corresponding analytical responses as shown in Fig. 1 for the 4-NP reduction process on BDD and HMDE while, for the oxidation on BDD, an analogous study has been published already [30]. Acetonitrile (HPLC and pesticide grade, UltimAR) was obtained from Mallinckrodt. All other reagents were analytical grade and purified nitrogen was used for de-aeration of the solutions. A conventional three-electrode cell with the AgjAgCl system and a Pt wire as the reference and auxiliary electrodes, respectively, was used. The BDD working electrode was a 0.62 · 1.0 cm2 single-faced plate, kindly provided by Eng. W. Haenni from CSEM, Neuchaˆtel,
V.A. Pedrosa et al. / Journal of Electroanalytical Chemistry 573 (2004) 11–18
Fig. 1. Influence of the pH on the SWV peak potential (j) and peak current density (m) for the reduction in 0.1 mol l1 BR buffer of 1.5 · 105 mol l1 4-NP on the HMDE (with f = 120 s1, a = 40 mV, DEs = 2 mV) and for 3.0 · 105 mol l1 4-NP on the BDD electrode (with f = 100 s1, a = 60 mV, DEs = 2 mV).
Switzerland. Details of the preparation and physical characterization of the CSEM diamond electrodes used in this work can be found in the literature [18]. Prior to the experiments, the electrode was polarised at +3.0 V vs. AgjAgCl 13 M KCl for 30 min to remove the hydrophobic film that covers the surface and then for another 30 min at –3.0 V for surface conditioning [32]. The HMDE used was a model. MDE 303 EG&G PAR system with a 0.2 cm2 geometric area. Before each experiment, a stream of N2 was passed through the solution for ca. 10 min. Square wave voltammetry measurements were carried out with a model. 273A EG&G PARC potentiostat with M270 software for the BDD electrode and with a model. 394 from EG&G PARC for experiments with the HMDE. In all cases, the experimental SWV parameters such as square wave frequency (f), amplitude (a) and scan increment (DEs) were optimised before the measurements and these results were also used for the characterization of the electrode process via the SWV diagnostic criteria. Qualitative and quantitative determinations of 4-NP were also carried out by HPLC-UV. These analyses were carried out on a model. SCL-10Avp Shimadzu system equipped with a model. LC-10ATVP pumping unit and a spectrophotometric UV/Vis detector (SPD-
13
10AVP). Data were processed on a Shimadzu LC WorkStation Class LC-10. The HPLC conditions were: a LiChrosorb RP-18 column (250 mm · 4.6 mm, 5 lm, Merck), with a RP-18 pre-column (30 mm · 4 mm, 5 lm, Merck); the mobile phase was 70/30 v/v acetonitrile/water with 1% v/v acetic acid, at a flow-rate of 1.0 ml min1. The injection volume was 20 ll and detection was performed at a wavelength of 310 nm. Analytical curves were obtained for each technique by the standard addition method. The effect of interference was evaluated using natural water samples taken from a local creek (Monjolinho) at two different points of Sa˜o Carlos city, namely, before (point 1) and after crossing town (point 2). The water samples were collected in glass bottles during the dry season and kept under refrigeration (4 C) for no longer than one week. The degree of contamination in the natural waters increases from point 1 to point 2 as indicated by the measured values of the oxygen demand, i.e., BOD (6.0 and 12.0 mg l1) and COD (19.0 and 33.0 mg l1), respectively. The electrolytes were prepared by dissolving the salts necessary for the BR buffer in either pure or natural waters and the measurements were performed without pre-treatment of the solutions but the pH was adjusted appropriately to the desired value in each case. The standard deviation of the mean value of currents (SB) measured at the 4-NP reduction potential for 10 voltammograms of the blank solution in electrolytes prepared with the different water samples was used [48,49] for the determination of the quantification and detection limits (QL and DL, respectively) together with the slope of the straight line of the analytical curves (b) and Eq. (1) 10S B 3S B QL ¼ ; DL ¼ : ð1Þ b b The recovery experiments were carried out by adding a known amount of 4-NP to the supporting electrolyte followed by standard additions from the 4-NP stock solution and plotting the resulting analytical curve. All measurements were performed in triplicate. The recovery efficiencies (R%) for the different systems under investigation were calculated using Eq. (2) where the value [4-NP] found refers to the concentration obtained by extrapolation of the analytical curve in the corresponding spiked water samples ½4 NP found %R ¼ 100 : ð2Þ ½4 NP added
3. Results and discussion 3.1. Hanging mercury drop electrode and HPLC validation The SWV curves obtained on the HMDE with a f of 120 s1, a of 40 mV and DEs of 2 mV in a 0.1 mol l1 pH
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3.0 BR buffer prepared with purified water show a single, diffusion-controlled reduction peak at approximately 0.4 V. These results are presented in Fig. 2 for concentrations of 4-NP added to the electrolyte up to 4.0 · 105 mol l1. The dependence of the peak current density with concentration resulted in the linear plot shown in the inset of Fig. 2. The corresponding linear equation was determined as j p = 0.152 + 0.550 · [4-NP] with r = 0.9998 while the QL value for these experiments was calculated using Eq. (1) and the methodology described in Section 2. The value obtained for pure water solutions using the HMDE was 5.7 g l1 (or 4.16 · 108 mol l1) and is included in Table 1 together with the corresponding DL value. To validate the previous electroanalytical results, HPLC quantification experiments were carried out in electrolytes prepared with pure water. Thus, 4-NP additions were made to a BR buffer solution prepared with purified water and with the pH adjusted to 6. Aliquots of these spiked solutions in the same concentration range as before (0.5–4.0 · 105 mol l1) were injected
Fig. 2. SWV responses on the HMDE for the reduction of different 4NP concentrations: 0 (1); 0.5 (2); 0.75 (3); 1.0 (4); 1.25 (5); 1.5 (6); 2.0 (7); 3.0 (8) and 4.0 · 105mol l1 (9) in 0.1 mol l1 BR buffer, pH 3.0, with f = 120 s1, a = 40 mV, DEs = 2 mV. Inset: linear dependence of jp with 4-NP concentration.
into the chromatograph and a peak with a retention time of 3 min, corresponding to 4-NP was detected at 310 nm. To calculate the QL value a different approach was followed since, in chromatographic experiments, the blank response does not present the same deviations observed in electroanalyses. In this case, the quantification limit was obtained using the standard deviation of the yintercept of 10 analytical curves obtained with the chromatographic procedure above. This parameter replaces the SB term in Eq. (1) [48,49]. The resulting QL value was 2.0 · 108 mol l1. This value is approximately only one-half of that obtained from the electroanalytical experiments on the HMDE, a fact probably related to the pre-concentration of the 4-NP molecules in the chromatographic column during the HPLC experiments, and confirms the validity of the previous SWV results. The same electroanalytical methodology described above for pure water solutions was then applied to quantify 4-NP in the natural water samples described in Section 2. Here, the samples collected from the Monjolinho creek were used to prepare the supporting electrolyte and then spiked with the minimum detectable amount of 4-NP followed by further additions of the standard solution in the range 0.5–4.0 · 105 mol l1. The resulting voltammograms at different concentrations were practically identical to those in Fig. 2 except for the jp values that were smaller than the equivalent values in pure water solutions. The corresponding analytical curves are presented in Fig. 3 together with that obtained in pure water, for comparison. The straight lines correspond to the equations: jp = 0.091 + 0.500 · [4-NP], r = 0.99907 for point 1 and jp = 0.044 + 0.485 · [4-NP], r = 0.99883 for point 2 samples, respectively. As can be observed from the slopes of the analytical curves, the determination of 4-NP in contaminated waters has a lower sensitivity when compared to pure water. This behaviour has frequently been observed and related to the organic matter dissolved in natural waters, mainly humic and fulvic acids [28,50,51]. The values of QL and DL in these natural water samples were again evaluated using Eq. (1) and are also collected in Table 1. Moreover, recovery experi-
Table 1 Analytical parameters for 4-NP determinations on HMDE and BDD electrodes in pure and contaminated water samples Sample
b/A l mol1 cm2
r
SB/lA cm2
QL/lg l1
DL/lg l1
Recovery/%
Reduction on HMDE
Milli-Q Point 1 Point 2
0.550 0.500 0.485
0.9998 0.9990 0.9988
0.002 0.025 0.022
5.7 63.2 66.0
1.7 18.9 19.8
98.0 ± 2.3 95.1 ± 3.2 93.2 ± 2.7
Reduction on BDD
Milli-Q Point 1 Point 2
0.756 0.500 0.548
0.9989 0.9990 0.9920
0.022 0.020 0.024
14.1 58.9 61.3
4.2 17.0 18.5
97.3 ± 2.0 95.2 ± 1.9 94.5 ± 2.1
Oxidation on BDD
Milli-Q Point 1 Point 2
0.629 0.580 0.532
0.9998 0.9995 0.9960
0.127 0.014 0.016
9.4 36.1 53.1
2.8 10.8 16.0
98.0 ± 1.1 96.0 ± 2.2 97.1 ± 1.8
V.A. Pedrosa et al. / Journal of Electroanalytical Chemistry 573 (2004) 11–18
Fig. 3. Analytical curves for the reduction of 4-NP on the HMDE in solutions prepared with: Milli-Q water (j), point 1 (d) and point 2 (m) water samples from the Monjolinho creek.
ments were carried out for the quantification of 4-NP in the different water samples following the procedure presented in Section 2. Here, values of 98.0%, 95.1% and 93.2% were obtained for pure water and for the samples collected at points 1 and 2, respectively (Table 1). The results obtained with the HMDE demonstrate that the electroanalytical procedure used here is suitable for 4NP quantification in the matrices analysed. However, the significant increase of the QL values for the contaminated waters reveals the high sensitive of mercury towards organic molecules contained in the samples. 3.2. Electrochemical reduction on the boron-doped diamond electrode Aiming to evaluate the behaviour of BDD electrodes in comparison to the HMDE, the same electroanalytical methodology describe above was applied to this new electrode material and the electrochemical reduction of 4-NP was again studied in pure and natural water samples by experiments completely analogous to those described before. The SWV curves for 4-NP reduction on BDD electrodes at different concentrations in pure water solutions are presented in Fig. 4 and again show a unique, diffusion-controlled peak at 0.8 V vs. AgjAgCl in BR buffer with pH 6.0. In this case, the following optimised voltammetric conditions were used: f of 100 s1, a of 60 mV and DEs of 2 mV. The relatively large negative shift in the peak potential when compared to the HMDE experiments (Fig. 2) is due to the strong dependence of the NO2 group reduction on pH [47]. Meanwhile, the current density values are very similar to each other suggesting that the same four-electron reduction process is occurring on the BDD electrode.
15
Fig. 4. SWV responses on the BDD electrode for the reduction of different 4-NP concentrations: 0 (1); 0.5 (2); 0.7 (3); 1.0 (4); 1.2 (5); 1.5 (6); 2.0 (7); 3.0 (8) and 4.0 · 105mol l1(9) in 0.1 mol l1 BR buffer, pH 6.0, with f = 100 s1, a = 60 mV, DEs = 2 mV. Inset: linear dependence of jp with 4-NP concentration.
The analytical curve for 4-NP in the electrolyte prepared with pure water is also shown in Fig. 4 as an inset. The slope of this straight line is the sensitivity of the method and was used to calculate the QL value with the help of Eq. (1). The value found here, 14.1 lg l1 (or 10.3 · 108 mol l1) is somewhat higher than that obtained for the HMDE electrode but of the same order of magnitude. This parameter is also included in Table 1 together with the corresponding DL value. Recovery experiments performed with the same procedure used for the HMDE furnished a value of 97.3% (Table 1), which is practically identical to that obtained before for mercury (98.0%). Subsequently, the electroanalytical methodology was applied to the quantification of 4-NP in contaminated waters where, as before, the electrolyte (BR buffer, pH 6.0) was prepared with samples collected from two points of the urban creek. As in the previous case, only one voltammetric peak was found at practically the same potential value recorded for pure water solutions. The dependence of jp with 4-NP concentration also produces linear analytical curves, as shown in Fig. 5 together with that obtained in pure water solutions, for comparison. Meanwhile, the overall reduction processes on BDD electrodes in contaminated waters seem to be much more complex than on mercury and the analytical curves have a considerable current contribution at zero 4-NP concentration. In this case, the calculated value of QL was 58.9 lg l1 (or 41.6 108 mol l1) for point 1 and 61.3 lg l1 (or 45.3 · 108 mol l1) for point 2 samples, respectively. These values are slightly lower than those obtained on the mercury electrode under the same conditions (see Table 1) suggesting that the BDD electrode works more efficiently in contaminated waters. Such behaviour could be associated with the very low interactions existing between diamond surfaces and organic molecules thus minimising the inhibition of the pesticide reduction by blockage of the electrode surface.
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Fig. 5. Analytical curves for 4-NP reduction on the BDD electrode in solutions prepared with: Milli-Q water (j), point 1 (d) and point 2 (m) water samples from the Monjolinho creek.
Recovering experiments were performed in the natural waters yielding the values of 95.2% and 94.5% for points 1 and 2, respectively (Table 1). These values were a little smaller than that obtained in pure water solutions (97.3%) but also point to the convenience of this method for 4-NP quantification. 3.3. Electrochemical oxidation on the boron-doped diamond electrode A further development of the electroanalytical methodology described here is the use of the 4-NP oxidation process on BDD electrodes for its quantification in different water solutions. According to the literature, 4-NP can be oxidised to the corresponding o-benzoquinone [52,53] following a mechanism similar to that determined for the oxidation of pentachlorophenol [54], that is, an electrochemical reaction involving the transference of four electrons. To use such an electrode process to quantify 4-NP, SWV experiments were performed between 0.8 and 1.15 V vs. AgjAgCl using the following optimised parameters, f = 100 s1, a = 50 mV and DEs = 2 mV in pH 6.0 BR buffer prepared with purified water. The results obtained for several 4-NP concentrations in solution (0.6 · 105–5.0 · 105 mol l1) are shown in Fig. 6 where the linear relationship between jp and [4-NP] appears in the inset. In this case, the SWV diagnostic criteria indicate that the process is controlled by adsorption of the reagent and/or the products. The slope of the analytical curve was used to calculate the QL value using Eq. (1) as in the previous cases and the result obtained here was 9.4 lg l1(or 6.9 · 108 mol l1). This value, also collected in Table 1, is somewhat higher than that obtained for 4-NP reduction on the HMDE but smaller than that for reduction on the BDD electrode. This is probably related to the existence of fewer interactions between the diamond surface and adsorbed molecules in the positive branch of the
Fig. 6. SWV responses on the BDD electrode for the oxidation of different 4-NP concentrations: 0 (1); 0.6 (2); 1.0 (3); 1.5 (4); 2.0 (5); 3.0 (6); 4.0 (7); 4.5 (8) and 5.0 · 105mol l1 (9) in 0.1 mol l1 BR buffer, pH 6.0, with f = 100 s1, a = 50 mV, DEs = 2 mV. Inset: linear dependence of the peak current density with 4-NP concentration.
potential range but this assumption will require further studies. In addition, recovery experiments furnished a value of 98.0% for the present conditions and this is an indication that the oxidation methodology is capable of detecting all 4-NP present in solution. The same experiments were carried out in pH 6.0 BR buffer prepared with the contaminated water samples and the voltammograms were similar to those of Fig. 6. The linear relationships between the jp and 4-NP concentrations are presented in Fig. 7 together with the analytical curve obtained in pure water solutions, for comparison. The slopes of all the lines in Fig. 7 (Table 1) are larger than those measured for the HMDE indicating that the process of 4-NP oxidation on BDD has higher sensitivity than the reduction on mercury surfaces. The resulting QL value was 36.1 lg l1(or 25.9 · 108 mol l1) for point 1 and 53.1 lg l1(or 38.1 · 108 mol l1) for point 2 samples, respectively, and are included in Table 1. These results are a clear indication that the contaminants are less active on BDD
Fig. 7. Analytical curves for 4-NP oxidation on the DDB electrode in solutions prepared with: Milli-Q water (j), point 1 (d) and point 2 (m) water samples of the Monjolinho creek.
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electrodes in the positive range of potentials used. This is probably due to a decrease in the adsorption of organic molecules as well as a minimal contribution to the electrode process by cations present in the contaminated waters. Finally, the recovery experiments yielded values of 96% and 97% for samples from points 1 and 2, respectively, thus confirming the suitability of this methodology to quantify 4-NP by means of its oxidation reaction on BDD electrodes. 4. Conclusions The results of this investigation have clearly revealed several advantages from the use of BDD electrodes instead of the traditional HMDE for the electroanalytical determination of 4-NP under different experimental conditions. First of all, the handling of a non-toxic material facilitates the experimental procedures and enables future developments of remote sensors for the environment. This can be achieved with the BDD without great losses in performance and with even better QL and recovery results than those obtained on mercury when dealing with heavily contaminated matrices. This previous statement applies both to the reduction and the oxidation processes studied here on BDD electrodes where the QL values were initially larger than those of the HMDE in pure water solutions but increased at the lower rate when the contamination of the samples become more intense. On the other hand, the use of an oxidation reaction on BDD electrodes for the quantification of 4-NP and other undesired organic molecules in environmental samples such as wastewater cannot be achieved on mercury due to its electrochemical instability under those conditions. Such an oxidation process proved to be more efficient than reduction and the reasons for that might be linked to a diminished adsorption of organic contaminants on the BDD surface as well as to the fact that many inorganic pollutants are not electroactive in such a potential region. Acknowledgements The authors thank CNPq and FAPESP (03/00710-6), Brazil, for scholarships received and Eng. W. Haenni from CSEM, Neuchaˆtel, Switzerland, for providing the diamond electrodes. References [1] R.J. Hance, Pestic. Sci. 01 (1979) 112. [2] A.L. Hart, W.A. Collier, D. Janssen, Biosens. Bioelectron. 12 (1997) 645.
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