Glyphosate and glufosinate detection at electrogenerated NiAl-LDH thin films

Analytica Chimica Acta 654 (2009) 97–102

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Glyphosate and glufosinate detection at electrogenerated NiAl-LDH thin films Aicha Khenifi a,c , Zoubir Derriche c , Claude Forano a , Vanessa Prevot a , Christine Mousty a,∗ , Erika Scavetta b,∗∗ , Barbara Ballarin b , Lorella Guadagnini b , Domenica Tonelli b a

Laboratoire des Matériaux Inorganiques, UMR CNRS 6002, Université Blaise Pascal, Clermont-Ferrand, France Laboratorio di Chimica Analitica, Dipartimento di Chimica Fisica ed Inorganica, Università degli Studi di Bologna, Italy c Laboratoire de physico-chimie des matériaux, catalyse et environnement Usto, Oran, El M’nouar, Algeria b

a r t i c l e

i n f o

Article history: Received 25 June 2009 Received in revised form 9 September 2009 Accepted 17 September 2009 Available online 20 September 2009 Keywords: Glyphosate Glufosinate Sensor Layered double hydroxides Amperometry Adsorption

a b s t r a c t An amperometric sensor based on Ni1−x Alx (OH)2 NO3x ·nH2 O layered double hydroxide (LDH) has been developed for the electrochemical analysis in one step of two herbicides: glyphosate (N(phosphonomethyl)glycine, Glyp) and glufosinate ((DL-homoalanine-4-yl)-methylphosphinic acid, Gluf). NiAl-LDH was prepared by coprecipitation or by electrodeposition at the Pt electrode surface. Inorganic films were fully characterized by X-ray diffraction, Raman spectroscopy and scanning electron microscopy. Adsorption isotherms of Glyp onto this inorganic lamellar material have been established. Electrocatalytic oxidation of Glyp and Gluf is possible at the Ni3+ centres of the structure. The electrochemical responses of the NiAl-LDH modified electrode were obtained by cyclic voltammetry and chronoamperometry at 0.49 V/SCE as a function of herbicide concentration in 0.1 M NaOH solution. The electrocatalytic response showed a linear dependence on the Glyp concentration ranging between 0.01 and 0.9 mM with a detection limit of 1 ␮M and sensitivity 287 mA/M cm2 . The sensitivity found for Gluf was lower (178 mA/M cm2 ). © 2009 Elsevier B.V. All rights reserved.

1. Introduction Organophosphate and organophosphonate compounds (OP) constitute one family of the most commonly applied pesticides in agriculture. For instance glyphosate (N-(phosphonomethyl)glycine, Glyp) and glufosinate ((DL-homoalanine-4-yl)-methylphosphinic acid, Gluf) (Scheme 1) are employed as non-selective, post-emergence contact herbicides. The intensive use of these herbicides generates concerns regarding their impact in environment and their possible health hazards. The analytical method employed for this group of chemicals was reviewed by [1]. They consist mainly in gas chromatography, liquid chromatography, capillary electrophoresis and immunoassay. Most of these methods developed for analysis of Glyp and Gluf need pre or posttreatments to functionalize the analytes to allow their detection. Indeed, liquid chromatography techniques, using either UV or fluorimetric detectors, are limited to the analysis of pesticides bearing chromophore groups which is not the case with Glyp and Gluf. Electrochemical sensors represent then a good and innovative alternative in term of on-site detection, sensitivity,

∗ Corresponding author. Tel.: +33 473 407 598; fax: +33 473 407 108. ∗∗ Corresponding author. Fax: +39 512 093 690. E-mail addresses: [email protected] (C. Mousty), [email protected] (E. Scavetta). 0003-2670/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2009.09.023

rapid response, and miniature size for the determination of these pesticides [2–5]. Sensors and biosensors based on clay modified electrodes have been used for the determination of pollutants, such as pesticides, phenol derivatives, cyanide, metals, etc. [6]. In particular, layered double hydroxides (LDH) with the general formula: q− [MIII 1–x MIII (OH)2 ]x+ [Xx/q · nH2 O] (abbreviated as MII MIII -X) are solids showing 2D structural arrangement also referred as anionic clays or hydrotalcite like compounds and they have been used for the development of different types of electrochemical sensors or biosensors. For instance, LDH containing a transition metal like CoII , NiII , MnII , which undergoes a reversible redox reaction in a useful potential range, have been described as electrode coating materials due to their properties of charge transport and redox catalysts especially in basic solution. NiAl-X-LDH modified electrodes have been applied to the electrocatalytic detection of mono and polyhydric alcohols (such as methanol, ethanol, propanol, glucose, fructose, sucrose, etc.) and aliphatic or aromatic amines [7–9]. LDH matrices can be easily prepared in laboratory compared to cationic clays. Different synthetic methods have been reported in the literature [10,11] such as, for instance, pH constant coprecipitation, coprecipitation using retardant base such as urea [12] or polyol process preparation [13]. Electrosynthesis of LDH based on electrochemical generation of hydroxyl by cathodic reduction of nitrate ions was also described by different authors as an efficient method to prepare LDH thin films suitable for sensing applications [14–17].

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Scheme 1. Chemical formula of Glyp and Gluf.

In the present paper, modified electrodes based on electrodeposited Ni1−x Alx (OH)2 NO3x ·nH2 O LDH have been applied to the electrochemical analysis in one step of Glyp and Gluf. NiAl-LDH are interesting materials because they combine good electroactive properties based on the Ni2+ /Ni3+ redox process and suitable structural and textural properties for the adsorption of negatively charged herbicides. To get better insight on interaction between herbicides and LDH materials, isotherm establishment was carried out. 2. Materials and methods 2.1. Products Glyphosate, glufosinate and metal salts were purchased from Sigma–Aldrich and used without further purification. 2.2. Apparatus The X-ray diffraction (XRD) patterns of the powders and thin films of samples were recorded using a Philips X’Pert Pro diffractometer with a Cu K␣ radiation ( = 0.15415 nm) in the 2◦ –70◦ 2 range, in steps of 0.02◦ with a counting time per step of 75 s. Scanning electron microscopy (SEM) characteristics of the samples were imaged by a JEOL 5190 microscope operated at 15 keV at the CASIMIR Laboratory (Clermont-Ferrand). Raman spectroscopy measurements were performed using a Jobin Yvon Horiba T64000 equipped with a liquid N2 cooled CCD detector. An Ar laser line at 515.5 nm was used as excitation source with a power of 20 mM. Specular reflectance-FTIR spectra were recorded on a Nicolet 5700 (thermo electron corporation) spectrometer. Cyclic voltammetry and chronoamperometry experiments were carried out with potentiostat/galvanostat Autolab PGSTAT100 (Ecochemie), in air, using a single compartment three electrode cell. Electrode potentials were measured with respect to an aqueous saturated calomel electrode (SCE), a Pt wire was used as counter electrode and a Pt disk as working electrode (A = 0.196 cm2 ).

2.3.2. Adsorption experiments Adsorption isotherms and kinetic studies were measured at 25 ◦ C using the batch equilibrium method in open bottles. Each experiment was repeated at least three times. The suspensions were kept in a vessel with continuous shaking. To get a homogeneous dispersion, the samples were dispersed in 25 mL of deionised/decarbonated water and stirred for 24 h before the Glyp or Gluf molecules were added. The pH value was adjusted (pH 13) by NaOH (0.1 M). Fifty milligrams of the LDH mass was used for the different batch equilibrium experiments. After a contact time of 24 h, the suspensions were centrifuged. The amount of Glyp or Gluf adsorbed by NiAl-LDH (Qe ) was determined from the difference between the initial (Ci ) and the final equilibrium concentration (Ce ) per gram of adsorbent. The amount of Glyp and Gluf present in the supernatant was measured as elementary phosphorus by ICP (inductively coupled plasma) emission spectrometry with a Perkin-Elmer Optima 3000XL atomic emission spectrometer. 2.3.3. Electrochemical characterization Pt electrodes were polished with alumina particles (0.05 ␮m) then they were cleaned by ultrasonic treatment in water and ethanol and finally rinsed with water and dried. Thin films of NiAl-NO3 -LDH were deposited on the Pt surface (A = 0.196 cm2 ) by cathodic reduction of a solution containing 0.0225 M Ni(NO3 )2 , 0.0075 M Al(NO3 )3 and 0.3 M KNO3 . The electrodeposition was carried out at −0.9 V/SCE for different times: 10, 20, 60, 90 and 120 s. The electroactivity of the resulting NiAl-LDH modified electrodes was characterized by cyclic voltammetry at 10 mV/s in 0.1 M NaOH, in absence and in the presence of Glyp and Gluf. Amperometric detections of herbicides were performed at 0.49 V/SCE with a rotating disk electrode (500 rpm). 3. Results and discussion 3.1. Physical characterization of NiAl-NO3 -LDH films NiAl-NO3 -LDH films were formed on Pt electrodes by the optimized electrodeposition method as described previously by [15]. Three electrodeposition times (ts ) were used, 60, 120 and 200 s. XRD patterns were recorded on films and compared to the coprecipitated NiAl-NO3 -LDH reference material (Fig. 1). The characteristic

2.3. Procedures 2.3.1. NiAl-LDH coprecipitation To perform adsorption experiments, Ni2 Al-NO3 LDH was prepared by pH constant coprecipitation as previously reported [10]. Typically, an aqueous solution of Ni and Al nitrate with Ni/Al molar ratio equal to 2 and total metal ion concentration of 1 M was added drop-wise to a flask containing 100 mL of deionised water, pH being maintained constant at 10.0 ± 0.1 by the addition of a sodium hydroxide solution (2 M). The coprecipitation was carried out under nitrogen atmosphere in order to minimise the contamination with atmospheric CO2 and vigorous magnetic stirring. Finally, the precipitate was washed by three dispersion and centrifugation cycles in deionised water, and air-dried.

Fig. 1. X-ray diffraction of (a) coprecipitated NiAl-NO3 -LDH powder, (b) NiAl-LDH powder obtained after 10 different electrodepositions for 200 s and thin films for electrodeposition time of (c) 60 s, (d) 120 s, (e) 200 s ( impurities of KNO3 electrolyte, * Pt substrate).

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Fig. 2. Raman spectrum (A) and specular reflectance-FTIR (B) of (a) coprecipitated NiAl-NO3 -LDH and (b) electrodeposited NiAl-LDH film (ts = 1000 s) on Pt electrode.

diffraction lines of the LDH structure (hexagonal lattice with a R-3m rhombohedral symmetry), are clearly visible on the powder X-ray diffractogram of the reference material (Fig. 1a) with the characteristic cell parameters of a = 0.1499 nm, c = 2.5689 nm and a basal spacing of d = 0.8563 nm. These diffraction lines did not appear on the XRD patterns of deposits, even for 200 s electrodeposition time

(Fig. 1c–d). Thin films obtained at such low ts are mainly amorphous NiAl-NO3 -LDH and formed by very small particles, which generate a high diffusion of X-ray at low 2 angle. Electrodeposition was repeated ten times for an electrolysis time of 200 s and powder was scratched from Pt substrate and collected in order to record powder X-ray diffractograms (Fig. 1b). XRD of this sample is typi-

Fig. 3. SEM images of NiAl-LDH thin films for electrodeposition time of 60 s (a), (b), (c) 120 s, (d), (e), (f), 200 s (g), (h) and (i) of coprecipitated NiAl-NO3 -LDH, (j) NiAl-NO3 -LDH after Glyp adsorption (k), and NiAl-NO3 -LDH after Gluf adsorption.

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Fig. 4. (A) Adsorption kinetics of Glyp and Gluf by coprecipitated NiAl-NO3 -LDH (Ci = 200 mg/L; LDH/H2 O = 50 mg/50 mL). (B) Adsorption isotherms of Glyp and Gluf by coprecipitated NiAl-NO3 -LDH.

cal of a NiAl-LDH with a low cristallinity as previously mentioned in the literature [15]. EDX analyses of the various deposits display always the presence of both Ni and Al elements. The Raman and FTIR spectra of the electrodeposited LDH film (ts = 1000 s) are compared to that of NiAl-NO3 -LDH prepared by coprecipitation (Fig. 2A and B). Both samples display the characteristic vibration bands of the NiAl-LDH lattice, with typical translation and stretching modes of NiAl–O planes below 610 cm−1 superimposed to the vibration of the D3h NO3 − intercalated anion, 1 (NO3 ) at 1055 and 1044 cm−1 (Fig. 2A), 2 (NO3 ) at 823 and 825 cm−1 (Fig. 2B), respectively for the reference and the electrodeposited materials. Note that the 4 (NO3 ) is absent (Fig. 2A) for the NiAl thin film, this extinction may be due to a preferred orientation of the NiAl-LDH platelets at the surface of the Pt substrate. The band at 981 cm−1 has not yet been identified. The textural properties of the different deposits were investigated by SEM. Fig. 3 displays the SEM images of the films obtained at three different times (ts = 60, 120, 200 s). Obviously, the time of electrodeposition has a great effect on the morphology of the films. At 60 s, a dense and homogeneous membrane made of nanoparticles with size lower than 50 nm connected in a gel-like manner is obtained. The highest magnified image (Fig. 3a) shows the presence of mesopores dispersed in the continuous film. The thickness of this dense film (<150 nm) may be estimated from Fig. 3c. Electrogeneration of OH− all over the surface of the electrode favours the formation of this dense film. When ts is increased (120 s), the film becomes less compact; platelets are growing homogeneously, in a perpendicular direction to the electrode leading to a network of interconnected sheets covering the all surface (Fig. 3d and e). Platelets tend to aggregate together to give pseudo-spherical secondary particles. Finally, at ts = 200 s, spherical particles, made of platelets aggregation in a sand roses like morphology, are formed all over the surface of the dense NiAl-LDH membrane (Fig. 3i). Strong similarity with the morphology of coprecipitated NiAl-NO3 -LDH particles (Fig. 3j and k) are then evidenced. Electrogenerated coprecipitation of the NiAl-LDH is no more localized at the surface of the Pt electrode and the growth of LDH platelets is more similar to that of standard coprecipitation. 3.2. Adsorption of herbicides on NiAl-NO3 -LDH The adsorption isotherms of both Glyp and Gluf by NiAl-NO3 LDH reference material (Fig. 4) were measured in order to quantify the anion adsorption and to determine either adsorption or intercalation may affect the electrochemical response. Indeed, Glyp (pKa1 0.78 (1st phosphonic), pKa2 2.29 (carboxylate), pKa3 5.96 (2nd phosphonic), and pKa4 10.98 (amine)) and Gluf (pKa1 2.0 (phosphonic), pKa2 2.9 (carboxylate) and pKa4 9.8 (amine)) are negatively charged under pH condition of electrochemical analysis (pH 13.0), respectively in the Glyp3− and Gluf2− forms. Consequently, Glyp3− and

Gluf2− may be adsorbed in the electrogenerated films during their electrochemical detection due to the anion-exchange properties of NiAl-NO3 -LDH. Adsorption of glyphosate and glufosinate on soils [18–21] and synthetic minerals [22–24] has been reported in literature. Kinetics of adsorption of both herbicides was first measured (Fig. 4A) to determine the equilibrium time needed to reach complete adsorption. No adsorption of Gluf was observed at pH 13.0 while up to 170 mg/g of LDH was reached at pH 7.0 (data not shown). On the opposite, Glyp is clearly adsorbed with an equilibrium time of adsorption of 175 min (Fig. 4A). Glyp isotherm (Fig. 4B) was satisfactorily fitted (R2 = 0.97) using the Langmuir model in its linear form (Ce /qe = 1/(bqmax ) + Ce /qmax , with qmax the adsorption capacity and b the affinity coefficient. qmax and b were calculated to be respectively equal to 25 mg/g and 0.043. However this maximum adsorbed amount of Glyp3− corresponds only to 5.6% of the anion-exchange capacity of Ni2 Al-NO3 -LDH (aec = 290 mq/g). As it can be seen, a second step of adsorption is evidenced at equilibrium concentration of Glyp greater than 100 mg/L. This smooth increase evidences the presence of less energetic adsorption sites. Under such basic conditions (pH 13), the basal spacing of NiAl-NO3 decreased to 0.751 nm whatever the equilibrium concentration of Glyp. NiAl-NO3 -LDH undergoes an anion exchange by both CO3 2− and OH− anions which display the greater affinity for LDH phases. This competing effect confines the adsorption of Glyp at the surface of the NiAl platelets and prevents any intercalation in between the layers. Contrarily, under lower pH conditions, Glyp2− has been reported to intercalate in LDH compounds leading to a structural expansion [22]. As evidenced on the SEM images (Fig. 3k and l) of NiAl-LDH after Glyp and Gluf adsorption, the spherical aggregates are maintained during the adsorption process. 3.3. Electrochemical detection of herbicides Modified electrodes prepared by electrodeposition of NiAl-LDH on Pt electrode have been characterized by cyclic voltammetry in 0.1 M NaOH aqueous solution (pH 12.8). Cyclic voltammograms displayed a typical reversible signal in the potential range between 0.3 and 0.6 V (Fig. 5A), relative to the redox couple Ni(II/III) involved in the electrochemical process described in Eq. (1). The current peaks increased with the deposition time. The number of electroactive Ni centres in the LDH coatings was estimated by integrating the anodic peak at low scan rate (10 mV/s). The anodic charge (Qa ) increases rapidly until 60 s then tends to stabilize at higher deposition times (Fig. 5B, curve a). The increase of current recorded when the electrodeposition time was prolonged from 10 to 60 s is due to an increase in the Ni centres involved the electrochemical process. When the electrodeposition time was further increased (ts > 60 s), the NiAl-LDH film becomes thicker and less homoge-

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Fig. 5. (A) Cyclic voltammograms of NiAl-LDH/Pt electrodes in 0.1 M NaOH as function of the electrodeposition time (a) 10 s, (b) 20 s, (c) 30 s, (d) 60 s, (e) 120 s (v = 10 mV/s). (B) Variation of the anodic charge corresponding to the Ni(II) oxidation (a) and oxidation current of Glyp (0.9 mM) (b) as function of the electrodeposition time (each point corresponds to a new electrode).

neous as shown in SEM images (Fig. 3). The transportation of charges and hydroxyl ions, necessary to ensure the electroneutrality, was hindered through a thicker coating, limiting the proportion of electroactive Ni centres available in the film. This observation can be related to morphology changes observed in SEM, and evidences the interest of the preparation of homogeneous thin films of NiAl-LDH by electrodeposition at the electrode surface compared to thicker coatings obtained at longer deposition times or by sedimentation of NiAl-LDH prepared by the coprecipitation method. When Glyp was added into the electrolyte solution, an increase of anodic current in the cyclic voltammetric curves recorded at NiAl-LDH electrode was observed and the intensity of the anodic peak depended on the Glyp concentration (Fig. 6A). We have verified that direct oxidation of Glyp did not occur within this potential range at a bare Pt electrode (Fig. 6B). Based on previous results relative to the electrooxidation of aliphatic and aromatic amines at NiAl-LDH electrogenerated on a Pt electrode [8,25], the increase of the anodic peak current relative to Ni(II) centres suggests an electrocatalytic oxidation of Glyp, according to the following reactions: Ni(II)Al-LDH + OH−  Ni(III)Al(OH)-LDH + e−

(1)

Hebred + Ni(III)Al(OH)-LDH → Hebox + Ni(II)Al-LDH

(2)

where reaction (2), which is the rate limiting step, permits the regeneration of Ni(II) during the oxidation of amine compounds. Glyp and Gluf determinations were therefore realized under hydrodynamic conditions at a rotating disk electrode (500 rpm) at Eapp = 0.49 V with successive additions of herbicides into the electrolyte solution. The sensor presented a very stable and rapid response with an average response time of 20 s (Fig. 7A). Calibra-

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Fig. 6. (A) Cyclic voltammograms of NiAl-LDH/Pt electrode (ts = 60 s) in (a) 0.1 M NaOH, (b) 0.250 mM Gyp, (c) 0.500 mM Glyp, (d) 0.750 mM Gyp (v = 10 mV/s). (B) Cyclic voltammograms of bare Pt electrode in 0.1 M NaOH and in the presence of 0.200 mM Glyp (v = 50 mV/s).

tion curves were obtained by plotting the steady state current, measured after subtraction of the base line current, vs. substrate concentration. Modified electrodes prepared at five different electrodeposition times were tested (ts = 10, 20, 60, 90, 120 s). As shown in Fig. 5B (curve b), the current corresponding to a Glyp concentration of 0.9 mM follows the same tendency than Qa as a function of electrodeposition time. This confirms the role of electroactive Ni centres in the electroxidation of Glyp. NiAl-LDH modified electrodes prepared at 60 s were chosen for reproducibility experiments, since this film configuration was found to be the most reproducible and the stable in time [8]. Three different electrodes were prepared under this optimized condition and the reproducibility and repeatability of the sensors were examined. For a given electrode, the relative standard deviation (RSD) was 7.1% for nine additions of 0.0125 mM Glyp. These sensors presented a linear range between 0.01 and 0.9 mM and a mean sensitivity of 287 ± 12 mA/M cm2 (Table 1). The limit of detection (LoD), determined at a signal-to-noise ratio of 3, was 1 ␮M. The same procedure was applied for the detection of Gluf. In this case the sensitivity decreased to 178 mA/M cm2 over a linear range between 0.01 and 0.3 mM and the LoD was 5 ␮M. As structural feature, Glyp and Gluf are very similar compounds containing primary or secondary amine groups (Scheme 1). Their electro-detection is based on the oxidation of amine in their structure [2,25]. However, the adsorption behaviours of both herbices at NiAl-LDH surface are quite different, which probably modifies the electrocatalytic efficiency of Ni centres. Interestingly, an opposite tendency with a higher sensitivity for Gluf than for Glyp was reported using gold electrode to detect these herbicides by an adsorption/desorption electrocatalytic process [2], evidencing the different materials affinity over organic pollutants. Due to the structural similarity

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trode has been recently tested for the electroxidation of glyphosate [26]. 4. Conclusion In this study, NiAl-LDH modified electrodes were prepared by electrodeposition and their efficiency for the electrocatalytic detection of Glyp and Gluf was evaluated. Their electro-detection is based on the oxidation of amine group in their structure by Ni(III) centres. The morphology of the as-obtained LDH films, and their electrocatalytic efficiency as well, appear to be strongly dependent on the electrodeposition time. Under pH condition of electrochemical analysis (pH 13.0), the adsorption of herbicides on LDH remains very weak, the intercalation of Glyp3− is prevented by the competition with carbonate and Gluf2− is not adsorbed. A detailed adsorption study of both herbicides on NiAl-LDH at various pH is currently under investigation in our laboratory. Since the modification of electrode surfaces with these electroactive NiAl-LDH films is very simple and rapid, their use for the electrochemical determination of these amino compounds offers a valid alternative to UV or fluorescence detection for which derivation procedures are needed and they open new possibility in remediation process of water polluted with toxic organic compounds. Acknowledgement This work 2008/17559SB.

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References

Fig. 7. (A) Current responses for seven successive additions of 0.125 mM Glyp in 0.1 M NaOH at NiAl-LDH/Pt electrode, inset shows the LoD of Glyp (ts = 60 s, Eappl = 0.49 V). (B) Glyp and Gluf calibration curves obtained at NiAl-LDH/Pt electrodes (ts = 60 s).

Table 1 Sensors characteristics for herbicides determination. Herbicides

Sensitivity (mA/M cm2 )

Glyp Gluf

287 178

Linear range (mM) 0.01–0.9 0.05–0.3

LoD (␮M)

R2 (n)

1 5

0.9994 (17) 0.9902 (14)

of these pesticides to naturally occurring plant materials, such as amino acids, interferences can occur with these direct electroxidation processes. As suggested by Sato et al for Au electrode [2], this problem can be overcome using an anion-exchange chromatography coupled with these electrochemical detection devices. In this case, the use of preconcentrated solutions would certainly improve the detection limit. Obviously, we know that the present analysis method is less sensitive compare to the new amperometric biosensors, published in 2009, and based on the inhibition of horseradish peroxidase activity [4,5]. The limit of detection of this biosensor for Glyp was 0.16 ␮g L−1 , which is very close to EU limit of any pesticides in drinking water (0.1 ␮g L−1 ). However the electrooxidation of herbicides at NiAl-LDH electrode seems to be efficient and it opens new possibility in remediation process of water polluted with toxic organic compounds. Indeed, NiAl-LDH modified electrodes can certainly be a cheaper alternative to dimensionally stable anode (DSA) containing expensive metal oxide RuO2 and IrO2 . This later elec-

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