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Voltammetric sensor based on Pt nanoparticles suported MWCNT for determination of pesticide clomazone in water samples
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Voltammetric sensor based on Pt nanoparticles suported MWCNT for determination of pesticide clomazone in water samples ´ c´ b, Marjan S. Ran d¯elovic´ a, Milan Z. Momcˇ ilovic´ b,∗, Jelena S. Milicevi c d ´ cˇ ev , Sajjad S. Mofarah , Charles C. Sorrel d Rada D. Đurovic-Pej a
Faculty of Science and Mathematics, University of Niš, 18000 Niš, Serbia “Vincˇ a” Institute of Nuclear Sciences, University of Belgrade, 11000 Belgrade, Serbia Institute of Pesticides and Environmental Protection, Banatska 31b, 11080 Belgrade, Serbia d School of Materials Science and Engineering, University of New South Wales, Sydney NSW 2052, Australia b c
a r t i c l e
i n f o
Article history: Received 17 July 2019 Revised 9 October 2019 Accepted 16 October 2019 Available online xxx Keywords: Sensor Clomazone Glassy carbon Differential pulse voltammetry
a b s t r a c t Novel electrochemical sensor based on Pt supported multiwalled carbon nanotubes is used for determination of pesticide clomazone in aqueous media via differential pulse stripping voltammetry (DPSV). Since clomazone is stable and readily soluble in water, it is often found in water sources. Hence, its determination in the environment is of utmost importance. Herein, clomazone is determined in 0.1 M phosphate buffer solution at pH 7.0 in the concentration range of 0.61–20.56 ng cm−3 , with LOQ = 0.61 and LOD = 0.38. These results are in the same range with HPLC/DAD, which is used as comparative method. It is shown that DPSV is a facile and efficient way for determination of clomazone in contrast to precise but field-impractical HPLC. Mechanistic approach in explaining electrode processes is correlated to structural aspects of the synthesized sensor. HRTEM data reveals a uniform distribution of Pt nanoparticles on the MWCNT support as a source of crucial, structural and electronic changes. Furthermore, characterisation of Raman results indicates the existence of structural defects, which is believed to be the leading reason for improvement in sensing response. © 2019 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction Pesticides are widely used in agriculture to improve crop yield and quality by preventing, repelling, eliminating or mitigating various sorts of pests (insects, rodents, fungi, unwanted plants). It is estimated that one third of crop production in the world is secured by pesticide utilisation [1]. However, pesticides are declared to be noteworthy environmental pollutants due to drainage of the treated fields, their improper disposal, accidental spills, and poor delivery to the intended target. The numerous negative health effects associated with exposure to pesticides, including dermatological, gastrointestinal, neurological, carcinogenic, respiratory, reproductive, and endocrine effects [2], attracted global concern and became an alarming challenge in preserving health of living beings. Clomazone (2-[(2-chlorophenyl)methyl]−4,4-dimethyl-3isoxazolidinone) is a selective soil-applied herbicide used for the control of broad-leaved and weeds in crops of oilseed rape, cotton, tobacco, soybeans, rice, maize, sugar cane and vegetables
∗
Corresponding author. ´ E-mail address: [email protected] (M.Z. Momcˇ ilovic).
like peas, beans, carrots and potatoes [3]. It is reported that clomazone (CLM) interferes with chloroplast development and reduces or prevents accumulation of plastid pigments in susceptible species [4]. CLM is highly water-soluble (1100 mg dm−3 ), minimally volatile, resistant to hydrolysis under a wide range of pH values, and weakly sorptive to soil [5]. Due to its long half-life dissipation and excellent water solubility, it can cause groundwater contamination [6]. CLM residues have been detected in many fish [7] and water samples collected from rivers in rice-growing regions, but also in wastewater, ranging from 0.03 to 10 0 0 μg dm−3 [8]. Determination of CLM in environmental water samples is mostly done by the means of high performance liquid chromatography (HPLC) [9]. Accurate assessment of pesticide status in air, soil, and water is of critical importance to their proper management. Up to now, several analytical methods have been exploited to analyze pesticides including gas chromatography-mass spectrometry [10], high performance liquid chromatography [11], capillary electrophoresis [12], enzyme immunoassays [13], Raman spectroscopy [14], and fluorescence methods [15], etc. Although these techniques offer high sensitivity and selectivity at low detection limits, their on-site application is limited by the fact that they often demand expensive
https://doi.org/10.1016/j.jtice.2019.10.013 1876-1070/© 2019 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
´ M.Z. Momcˇ ilovic´ and J.S. Milicevi ´ c´ et al., Voltammetric sensor based on Pt nanoparticles Please cite this article as: M.S. Ran d¯elovic, suported MWCNT for determination of pesticide clomazone in water samples, Journal of the Taiwan Institute of Chemical Engineers, https://doi.org/10.1016/j.jtice.2019.10.013
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sophisticated equipment, a lot of time for sample preparation, a high skilled workforce and use of many chemicals [16]. Analytical principles based on electrochemical sensors receive great attention in modern science. New amperometric and enzymatic sensors for pesticide detection are designed for rapid, simple and low-cost operations in any field, with portable instruments, and with high sensitivity and selectivity of the methods. There is a wide class of biosensors based on the inhibition of acetylcholinesterase (AChE) enzyme [17,18]. Their main shortcomings are related to their low chemical and thermal stability, difficulty of immobilization, and lifetimes that are limited by denaturation of the biological material present on the electrode surface [19,20]. On the other side, a diverse class of non-enzymatic sensors based on noble metal [21] and molecularly imprinted [22] nanoparticles, metal oxides [23], polymers [24], pencil graphite electrodes [25], zeolites [26], graphene oxides [27], etc. is described in scientific literature. It is acknowledged that precious metals can greatly enhance the catalytic diffusion and electron transfer on electrochemical signal with a high sensitivity values [28]. Novel functional materials with enhanced sensing characteristics are required. Nano particles (NP) have gained growing importance in the design of non-enzymatic electrochemical sensors for environmental monitoring. Especially metal and metal-oxide NP have received much attention due to their auspicious electrocatalytic properties, often notably different from bulk materials. The carbon supported metal and metal oxide NPs enable fast electron transfer kinetics and mass transport, increase the electro-active surface area, and reduce overpotential, leading to electrochemical sensors with lower limits of detection (LODs). Multiwalled carbon nanotubes (MWCNTs) showed enormous potential for applications in fields of electrochemical sensors because of their unique electrical, mechanical and catalytical features. Moreover, large surface areas and wide useful potential ranges, commonly from –1.0 to +1.0 V are very important for their application in electroanalyses. A special ability of this nanomaterial to promote electron transfer in electrochemical reactions is attributed to the activity of edge-plane-like graphite sites at the CNT ends [29]. Large edge plane/basal plane ratio, and enhancement of electrode’s surface area and porosity are responsible for higher sensitivity, a lower detection limit, and faster electron transfer kinetics when compared to the traditional carbon electrodes [30]. Recently, considerable efforts have been made to investigate Pt/CNT catalysts for methanol oxidation and hydrogen generation [31–33]. To our best knowledge, there is no report about the electrochemical behaviour of Pt/CNT catalyst for the determination of pesticide in aqueous media by striping voltammetry. Remarkable sensitivity of stripping techniques is attributed to the preconcentration step which is followed then by the stripping step. In the preconcentration step, the target analyte is accumulated onto or into the working electrode. In the stripping step, the accumulated analyte is oxidized or reduced back into the solution. The response, recorded during this step, is proportional to the concentration of that analyte in the sample solution. Commonly, mercury electrodes are employed in stripping analysis. The two mercury electrodes most frequently used are the hanging mercury drop electrode (HMDE) and the mercury film electrode (MFE) [34]. However, disadvantages of the dropping mercury electrode and the hanging mercury drop electrode, such as risk of mercury toxicity and environmental considerations, instability of a mercury drop in flow systems, handling problems as well as high consumption of mercury, are responsible for intensive researches in the field of new electrode materials for voltametric determination of pesticides such as MWCNT-based composites [35,36]. Differential pulse stripping voltammetry (DPSV) is a remarkably sensitive electrochemical technique where the analyte of interest is in preconcentration step electroplated onto the working electrode
and then removed („stripped“) by applying an oxidizing potential in a series of voltage pulses of increasing amplitude. During removal, the cell current is measured as a function of time and as a function of the potential between the indicator and reference electrodes. The current is sampled before and after each voltage pulse. The response, recorded during this step, is proportional to the concentration of analyte in the sample solution. This technique evolved as a successor of cumbersome stripping techniques based on dropping mercury electrodes. In this paper, novel electrochemical sensor based on Pt supported multiwalled carbon nanotubes (Pt-MWCNT) used for sensitive and selective determination of pesticide CLM via DPSV is presented. In contrast to previous studies, our research presents novel method for the synthesis of Pt supported multiwalled carbon nanotubes which assumes in situ reduction of chloroplatinic acid hydrate over MWCNT in water/ethanol medium. Performances of DPSV were compared to HPLC method. In addition, the electrochemical characteristics of Pt-MWCNT was thoroughly examined by using K4 [Fe(CN)6 ] as a model compound to elucidate the electrocatalytic ability of the modified GC electrode. The study is based on the fact that immobilization of metallic nanoparticles onto carbon support can boost the advances of electrochemical sensors and lead to enhancements of their performance and applications. 2. Materials and methods 2.1. Synthesis of material For the functionalization of the carbon support, 50 mg of MWCNT was dispersed in 20 cm3 mixture of ethanol and water (50v/v%). Then, 0.5 cm3 of aniline was introduced into suspension and sonicated for 90 min. In a small cuvette, 26.7 mg of chloroplatinic acid hydrate (H2 PtCl6 · xH2 O; Pt, 38–40%) was dissolved in 2 cm3 of water and added to suspension which was then sonicated for the next 5 min and acidified afterwards with 0.5 cm3 of 5 M HNO3 . For the reduction of ions of platinum to elemental state, excess of 20 mg of sodium borohydride was dissolved in 2 cm3 of water and then added to the suspension. It was stirred for a while and left for the next 24 h, then filtered and rinsed with deionized water to neutral pH. The obtained material was dried at 90 °C for a couple of hours and designated as Pt-MWCNT. 2.2. Characterization The structural characteristics and the order of graphitization of Pt-MWCNT samples were explored by using Micro-Raman spectroscopy (Renishaw inVia 2 Raman Microscope) applying a laser source with a wavelength of 532 nm. The morphology of Pt nanoparticles-supported MWCNT was investigated by the field emission scanning electron microscopy (FE-SEM) by using microscope JEOL JXA-8500F. Distribution of particles over the MWCNT structure was analyzed by high resolution transmission electron microscopy (HR-TEM) by the means of JEOL JEM-F200. Powder XRD data were obtained with a diffractometer (Rigaku Corporation, Japan), with CuKα radiation (l = 1.5406 A) and a scan speed of 5° min−1 , in the range between 0 and 60° 2θ , with a step 0.02°. 2.3. Electrode preparation To use it as electrode material, 5 mg of Pt-MWCNT was dispersed in 1 cm3 of ethanol/water mixture (40v/v%), and the suspension was homogenized in the ultrasonic bath for 30 min. The surface of the glassy carbon (GC) electrode was polished on fine leather containing alumina powder (purchased from Metrohm)
´ M.Z. Momcˇ ilovic´ and J.S. Milicevi ´ c´ et al., Voltammetric sensor based on Pt nanoparticles Please cite this article as: M.S. Ran d¯elovic, suported MWCNT for determination of pesticide clomazone in water samples, Journal of the Taiwan Institute of Chemical Engineers, https://doi.org/10.1016/j.jtice.2019.10.013
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Fig. 1. SEM images of Pt-supported MWCNT at magnifications of (a) 10 0 0X (b) 10 0 0 0 0X (c) 30 0 0 0 0X.
which is part of the instrument polishing kit. A drop of suspension of 15 μL was transferred onto the GC electrode (working area was 0.04 cm2 ) and dried under N2 stream. After the thin carbon layer was dried, it was covered with 10 μL of 0.05 wt.% Nafion in ethanol. The solvent was removed by evaporation. 2.4. Electrochemical analysis CLM was analyzed by the means of differential pulse stripping voltammetry (DPSV). Working solutions with CLM in 0.1 M phosphate buffer were anayzed in a conventional voltammetric cell with operating volume of 10 cm3 by using 797 VA Computrace analyzer (Metrohm, Switzerland) controlled by 797 VA Computrace software (version 1.2). A three-electrode system consisted of Ag/AgCl electrode (saturated with KCl) as a reference, platinum wire as auxiliary and Pt-MWCNT modified GCE as working electrode. Prior to each run, the working electrode was electrochemically activated by potential cycling in the range from –0.8 to –0.1 V with speed 0.1 V s−1 , for 10 cycles. The background voltammograms of the supporting electrolyte were recorded under the same conditions. The blank probe containing only the supporting electrolyte was recorded under the same conditions. Before starting a new set of measurements, the supporting electrolyte was deaerated by suprapure nitrogen for 5 min and then volumes from CLM stock solution were added to probes in order to make desired concentrations. The parameters for DPSV measurement were as follows: initial potential –0.8 V, end potential –0.1 V, accumulation potential –0.25 V, accumulation time 120 s, and the scan rate 100 mV s−1 . Differential pulse cathodic adsorptive stripping voltammetry (DPCASV) was carried out at an accumulation potential of –0.25 V and an accumulation period of 120 s. The stirring was then stopped and time the voltammogram was recorded by applying a differential pulse voltammetry from –0.1 to −0.8 V with 100 mV s−1 scan rate and 50 mV pulse amplitude. All the measurements were performed at room temperature (23 ± 1 °C). As comparative analysis, CLM was assayed by Shimadzu Prominence high performance liquid chromatography (HPLC) system (Shimadzu, Japan) equipped with pump model LC-20AD and diode array detector (DAD) model SPD-M20A. For chromatographic separation by Eclipse XDB-C18 column (4.6 × 150 mm, 3.5 μm), acetonitrile and water (70:30, v/v) were used as mobile phase at flow rate of 1 cm3 min−1 in an oven at 40 °C with 20 μl injection volume, and quantification at 214 nm. 2.5. Chemicals MWCNT (carbon˃95%, OD x L 6–9 nm x 5 μm) and chloroplatinic acid hexahydrate which were used for synthesis of Pt-MWCNT were purchased from Sigma-Aldrich (USA). Analytical standard solution of CLM (Dr Ehrenstorfer, Germany) with concentration of 2 g dm−3 was prepared in methanol (Sigma-Aldrich) and kept in dark at –4 °C. Buffer solutions were prepared by mixing solutions
of 1 mol dm−3 K2 HPO4 (Merck, Germany) and 1 mol dm−3 KH2 PO4 (Merck) and used as supporting electrolytes. The pH values of the CLM solutions were adjusted by using 0.1 M phosphate buffer solution ranging from pH 5.8 to 8.0. Potassium ferrocyanide and potassium chloride were purchased from Sigma-Aldrich (USA). Deionized water was obtained in laboratory by using a Millipore purification system (USA). 3. Results and discussion 3.1. Structural characterization The microstructure of the materials can be seen in the SEM micrographs (Fig. 1) at three different magnifications. Morphology typical for MWCNT noticed at all three images is consistent for all examined samples showing that the MWCNT bundles have been randomly orientated. Diameter of carbon nanotubes is in range of 10 nm while the length can reach more than several micrometers. However, the exact length is difficult to be measured because bundle is always in the meandering form. Pt nanoparticles are observed as light spots or clusters on the surface of MWCNT with diameter of up to 30 nm. In order to acquire detailed information about size, distribution and morphology of Pt nanoparticles on the MWCNT surfaces, further in-depth examination was achieved by TEM. Fig. 2 shows Transition Electron Microscopy (TEM) images of representative structure. The TEM image in Fig. 2a indicates that Pt nanoparticles are well dispersed and successfully deposited on the CNT support. There is no evidence that the interior of the MWCNT is filled by Pt phase. Additionally, a higher magnification TEM image reveals that Pt nanoparticles intrinsically agglomerate and form aggregates with an average diameter of ∼10 nm (Fig. 2b). It should be noted that Pt nanoparticles are firmly attached to the surface of CNT since there is no single observation of freely (detached) Pt nanoparticles after analysis a large numbers of samples. The HRTEM image, as shown in Fig. 2c, reveals a cluster of nanoparticles, where a D-space of 0.225 nm, which corresponds to the space of (111) planes, is identified. The selected area diffraction (SAED) patterns of the Pt NPs were indexed to three main planes of (111), (220), and (222) that illustrates a random orientation of the Pt nanoparticles (Fig. 2d). The powder XRD technique was used to investigate the crystal structure of the Pt-MWCNT samples (Fig. 3). The XRD profile contains peaks for MWCNTs at 26 and 43° 2θ which correspond to the planes of (220), (301) and (002), respectively, as indexed from (ICDD card no. 01- 083–3673). The peaks represented the graphitic reflections. Raman analysis was used to investigate the structural properties of Pt-supported MWCNT. The corresponding spectrum is given in Fig. 4. Three characteristic bands are noticed. The first one is located at ca. 1576 cm−1 and corresponds to the first order G mode (in-plane optical mode), which originates from the flat
´ M.Z. Momcˇ ilovic´ and J.S. Milicevi ´ c´ et al., Voltammetric sensor based on Pt nanoparticles Please cite this article as: M.S. Ran d¯elovic, suported MWCNT for determination of pesticide clomazone in water samples, Journal of the Taiwan Institute of Chemical Engineers, https://doi.org/10.1016/j.jtice.2019.10.013
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Fig. 2. (a,b). TEM images of Pt-supported MWCNT. (c) HRTEM image showing a cluster of Pt nanoparticles (d) SAED pattern of Pt-supported MWCNT.
Fig. 3. XRD pattern for Pt-MWCNT.
´ M.Z. Momcˇ ilovic´ and J.S. Milicevi ´ c´ et al., Voltammetric sensor based on Pt nanoparticles Please cite this article as: M.S. Ran d¯elovic, suported MWCNT for determination of pesticide clomazone in water samples, Journal of the Taiwan Institute of Chemical Engineers, https://doi.org/10.1016/j.jtice.2019.10.013
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Fig. 4. Raman spectrum of Pt-MWCNT.
geometric structure of sp2 graphitic network [37,38]. It is caused by the Raman active E2g phonon. The second band is the D mode (A1g symmetry) around 1339 cm−1 which corresponds to sp3 hybridized carbon [39,40]. It is attributed to the first order of zone boundary phonons and depends strongly on the amount of defects and on the CNT surface. Some previous studies showed that the presence of structural defects and edge planes are responsible for improvements on sensing applications of CNT [41,42]. The third band centered at around 2697 cm−1 is denoted as 2D. This peak arises due to the second order of the zone boundary phonons and is related to the number of graphene layers in CNT [43,44]. The ID /IG ratio shows the disorder degree of the sp2 bonded carbon that can be applied as graphitization degree indicator. According to the general rule, the graphitization degree is inversely proportional to the ID /IG ratio. Such ratio was calculated to be 1.21. 3.2. Cyclic voltammetry The electrochemical performance of Pt-MWCNT modified GC and bare CG electrode was initially investigated by cyclic voltammetry. A electrode reaction of potassium ferrocyanide (K4 [Fe(CN)6 ]) in a stagnant electrolyte was used as a common benchmark redox systems due to its “surface sensitive” electrochemical response onto examined electrodes. The electrode reaction can be formulated as given below:
Fe(CN )36− + e− Fe(CN )46−
(1)
Cyclic voltammograms at scan rate of 100 mV s−1 for K4 [Fe(CN)6 ] at GC and Pt-MWCNT modified GC electrode are illustrated in Fig. 5. The cyclic voltammogram at bare GC electrode (dotted line) indicates low peak current intensity (ip ) and high peak-to-peak separation (potential difference between reduction and oxidation peaks) – ࢞Ep . However, the voltammetric response is apparently improved at Pt-MWCNT modified GC electrode (solid line). Electrocatalytic effect of later electrode is reflected by the enlargement of the peak currents (ip ) and the reduction of the ࢞Ep value. Compared to bare GCE, the peak-to-peak separation (࢞Ep ) was lowered by 98 mV for Pt-MWCNT/GC electrode. Since the peak-to-peak separation is directly correlated to the kinetics of electron transfer, these findings show a significant improvement in the rate of electron transfer for Pt-MWCNT/GC electrode.
In addition, the peak current ratio (Ipa /Ipc ) for the redox reactions at GC electrode was measured to be 0.90, while such value was 0.93 for Pt-MWCNT/GC electrode. The closer the value of the peak current ratio to the value of 1 indicates faster kinetics of electrode reaction which corresponds to a reversible system. Therefore, the redox reaction at Pt-MWCNT/GC (0.93) is dominant on the surface with a higher rate of kinetics compared to that which occurred at the blank GC electrode. The half-wave potentials (E1/2 ) were calculated from the anodic (Epa ) and cathodic (Epc ) peak potentials by using the expression (Epa +Epc )/2. The obtained preliminary results of electrochemical characterization reveal superior electrochemical behavior of PtMWCNT for the redox couple [Fe(CN)6 ]3/4− . All voltammetric parameters for the two electrochemical systems are listed in Table 1. The morphology and distribution of Pt nanoparticles over a fast electron-transport MWCNT network is very important for electrocatalytic behavior of Pt-MWCNT based nanohybrid. Moreover, Pt nanoparticles provide high electrocatalytic activity because of their large surface area, which additionally contributes to a large reduction in electrode cost due to decrease of required Pt amount compared to commercial electrodes. Hence, application of MWCNT as supporters for Pt nanoparticles is a reliable concept to improve electrocatalytic activity and conductivity as well as to reduce the amount of Pt in these heterojunction structures (to around 17%mass). Electrode process can occur on the surface of Pt nanoparticle or at the contact of MWCNT and ultrafine dispersed Pt. This provides an auspicious structure for an efficient electron transfer. 3.3. Differential pulse stripping voltammetry (DPSV) For the determination of CLM at the low ppb level, the DPSV mode was selected. The cathodic response of CLM was evaluated with respect to the several supporting electrolytes including potassium chloride, Britton–Robinson buffer, and phosphate buffer. The later was found to be the most suitable medium. Subsequently, the DPSV curves were recorded in 0.1 M phosphate buffer (pH range 5.8 – 8.0) to study the effect of pH on the voltammetric behavior of CLM. As clearly noticeable (Fig. 6), there is no linear relationship between DP peak potential and pH. It is indicated that mechanism of CLM reduction is complex with no common electrochemical behaviour involving addition of
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Fig. 5. Comparison between cyclic voltammograms obtained by Pt-MWCNT/GC and commercial GC electrode as a working electrode (scan rate 100 mV s-1, 0.2 mM K4[Fe(CN)6] in 0.1 M KCl). Table 1 Electrochemical parameters obtained from voltammograms for a redox couple [Fe(CN)6 ]3/4− on GC and Pt-MWCNT modified GC electrodes. Electrode
Epa (V)
Epc (V)
E (V)
E1/2 (V)
Ipa (μA)
Ipc (μA)
Ipa / Ipc
GCE Pt-MWCNT/GCE
0.390 0.320
0.083 0.111
0.307 0.209
0.236 0.215
33.27 77.89
−36.89 −83.42
0.90 0.93
Fig. 6. DPSV curves 10.2 ng cm-3 of CLM at Pt-MWCNT in 0.1 M phosphate buffer solution (pH 5.8 - 8.0, initial potential −0.8 V, end potential −0.1 V, accumulation potential −0.25 V, accumulation time 120 s, and the scan rate 100 mV s-1).
hydrogen. General mechanism of clomazone reduction is proposed at Fig. 7. It has been ascertained from experimental data that the best peak shape and the maximum peaks current occurred in the 0.1 M phosphate buffer at pH 7.0. This is the reason why it was chosen for further investigation as optimal parameter from the analytical point of view. For comparison, there was no electrochem-
ical response of CLM pesticide with bare GC or MWCNT/GC electrodes. The current intensity of the signal of interest was found to be dependent upon the deposition potential, (E was between +0.15 and –0.65 V). As shown in Fig. 8a, the peak current increased up to a potential of –0.25 V and then started to decrease. When accumu-
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Fig. 7. Mechanism of clomazone reduction.
Fig. 8. Key parameter optimized for differential pulse striping voltammetric determination of CLM (10.2 ng cm-3) in the 0.1 M phosphate buffer solution pH 7.0: (a) Influence of deposition potential, E on I values t = 120 s; and (b) Effect of deposition time, t at E –0.25 V at a scan rate 100 mV s-1.
lation at –0.25 V was carried out, the reduction peaks were higher than those obtained without accumulation as a result of the enhanced adsorption of CLM. The optimal value of –0.25 V was chosen for deposition potential which coincides with the reduction potential for CLM at selected conditions (Fig. 8a). This interesting observation indicates that products of electrochemical reduction of CLM pesticide in a certain way influence maximum peak current in DPSV. In accordance with the experimental data obtained, we assume that the electrochemical reduction process presented by the following reaction is the most probable. The loss of keto group due to the reduction of CLM at electrode is the main reason why the reduced form (rCLM) has an increased tendency to interact with low polar surface of carbon nanotubes [45]. The hydrophobic surfaces of carbon nanotubes adsorb rCLM by hydrophobic interactions and van der Waals interactions. Hence, the Pt-MWCNT/GC electrode which was modified by adsorption of rCLM in pretreatment step provides better adsorption surface for CLM molecules in analyzed solution. Namely, CLM shows a more pronounced tendency to associate with adsorbed rCLM molecules on electrode through van der Waals interactions in comparison to the surface of bare electrode. Therefore, enhanced adsorption and accumulation of CLM on the Pt-MWCNT/GC electrode after its pretreatment at –0.25 V can be exploited here to significantly increase sensitivity of DPSV. Fig. 8b shows that the signal has hyperbolic shape with prolongation of the deposition time to 900 s. The peak current increased rapidly with the increasing of deposition time and almost reached a platform after 120 s. For analytical purpose, Pt-MWC/GC electrode was initially pretreated during 900 s into the solution of 10.2 ng cm−3 of CLM 0.1 M phosphate buffer solution before it was
used for measuring concentration of CLM to achieve down to the low ppb level. The additional preconcentration time of 120 s was employed before each measurement. Selected pretreatment time is enough to obtain a stable analytical signal. In addition, it should be emphasized that Pt NPs are electrocatalyst which in synergism with developed surface area of MWCNTs enables successful application of this electrode for CLM determination. The calibration graph for an accumulation time of 120 s was linear in the range from 0.61 to 20.56 ng ml−1 and obeyed the equation y = 1.09 x + 0.73, where y and x are the peak current (nA) and CLM concentration (ng ml−1 ), respectively (Fig. 9). The relative standard deviation was 1.85% (for 7 determinations). The limit of detection, estimated from 3 times the standard deviation, was 0.38 ng ml−1 . The quantitative DPSV determination of CLM is based on linear relationship between the peak current intensity at –0.2 V and CLM concentration. As can be seen, CLM could be determined by DPSV in the concentration range of 0.61 – 20.56 ng cm−3 (Fig. 6). Thereby, the relative standard deviation does not exceed 1.85%. The analytical parameters of the developed DPSV method and the comparative HPLC/DAD measurements are shown in the Table 2. By comparing the parameteres presented for two analytical methods, it can be seen that limits of detection (LOD) and limits of quantification (LOQ) are very close. Although HPLC gave just a slightly better results in experimental set up applied herein, significant potential of usability of DPSV as an easy, fast and reliable method for determinations of CLM is noticeable. This low-cost electrochemical method certainly imposes itself as a solution to many problems of chromatographic analyses, especially those related to applications on the field. In a similar study, it is confirmed that materials based
´ M.Z. Momcˇ ilovic´ and J.S. Milicevi ´ c´ et al., Voltammetric sensor based on Pt nanoparticles Please cite this article as: M.S. Ran d¯elovic, suported MWCNT for determination of pesticide clomazone in water samples, Journal of the Taiwan Institute of Chemical Engineers, https://doi.org/10.1016/j.jtice.2019.10.013
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Fig. 9. Differential pulse stripping voltammograms recorded at Pt-MWCNT/GCE for different concentrations of CLM in 0.1 M phosphate buffer solution pH 7.0 and the corresponding calibration plot. Accumulation potential –0.25 V, accumulation time 120 s, and the scan rate 100 mV s-1. Table 2 Analytical parameters of the DPSV and HPLC/DAD determination of CLM. Method Parameter Concentration interval [ng cm−3 ] Intercept Slope Correlation coefficient, r LOD [ng cm−3 ] LOQ [ng cm−3 ] RSD [%] (n = 7)
DPSV
HPLC/DAD
0.61 − 20.56 0.73 μA 1.09 μA cm3 ng−1 0.998 0.38 0.61 1.85
0.46 – 92.44 −1234.632 mAU min 7000.832 mAU cm3 ng−1 0.999 0.32 0.46 1.81
on MWCNT can be stable, selective and can achieve ultra sensitivity in electroanalytical detections of pesticides [46]. The interference of some common ions (Ca2+ , Na+ , Ag+ , K+ , Cl− , HCO3− , CO3 2− , NO3− ) and pesticides (linuron, imidacloprid and tebufenozide) on the determination of CLM was also tested in this study. Firstly, DPSV signal of CLM (10.2 ng cm−3 ) was recorded using the Pt-MWCNT/GCE, then either selected ion was added in a concentration that is 50 times higher than CLM or selected pesticide was added in the same final concentration as CLM. The solutions were analyzed by the proposed method and all investigated ions or pesticides neither influenced peak potential nor peak current intensity of CLM significantly. 4. Conclusions Pt-MWCNT heterojunction structures can be successfully synthesised in water/ethanol suspension, containing MWCNT and H2 PtCl6 by using sodium borohydride as reduction agent. The obtained nanohybrid material possesses a favorable morphology with fast electron-transport MWCNT network decorated by Pt nanoparticles enabling advantageous electrocatalytic behavior toward K4 [Fe(CN)6 ] and CLM reduction. The morphology and structure of the material were thoroughly examined by TEM and SAED method indicating nanostructure of Pt which were firmly attached onto MCWNT support. Additional structural information was provided by Raman spectroscopy which are in accordance with the results of previous analysis. The obtained differential pulse strip-
ping voltammetry have shown to be fairly efficient technique for rapid and sensitive determination of pesticide CLM by glassy carbon electrode improved with Pt supported multiwalled carbon nanotubes. Sensing features of Pt-MWCNT toward CLM were found to be superior in comparison to the bare glassy carbon electrode. Electrochemical reduction of CLM with the loss of keto group increases the tendency of reduced analogue to be adsorb onto poorly polar surface of carbon nanotubes in pre-treatment step followed by enhanced sensitivity in subsequent accumulation step. DPSV attained relatively low limits of detection and quantification which in comparison to concurent HPLC method leeds to conclusion that DPSV can be acknowledged as handy and fast alternative for CLM determination especially for on-site applications. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This study was carried out as part of the national scientific projects TR340 08, III 450 06, TR 31043, and III 43009 which are financed by the Ministry of Education, Science and Technological Development of the Republic of Serbia.
´ M.Z. Momcˇ ilovic´ and J.S. Milicevi ´ c´ et al., Voltammetric sensor based on Pt nanoparticles Please cite this article as: M.S. Ran d¯elovic, suported MWCNT for determination of pesticide clomazone in water samples, Journal of the Taiwan Institute of Chemical Engineers, https://doi.org/10.1016/j.jtice.2019.10.013
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´ M.Z. Momcˇ ilovic´ and J.S. Milicevi ´ c´ et al., Voltammetric sensor based on Pt nanoparticles Please cite this article as: M.S. Ran d¯elovic, suported MWCNT for determination of pesticide clomazone in water samples, Journal of the Taiwan Institute of Chemical Engineers, https://doi.org/10.1016/j.jtice.2019.10.013
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Voltammetric sensor based on Pt nanoparticles suported MWCNT for determination of pesticide clomazone in water samples ´ c´ b, Marjan S. Ran d¯elovic´ a, Milan Z. Momcˇ ilovic´ b,∗, Jelena S. Milicevi c d ´ cˇ ev , Sajjad S. Mofarah , Charles C. Sorrel d Rada D. Đurovic-Pej a
Faculty of Science and Mathematics, University of Niš, 18000 Niš, Serbia “Vincˇ a” Institute of Nuclear Sciences, University of Belgrade, 11000 Belgrade, Serbia Institute of Pesticides and Environmental Protection, Banatska 31b, 11080 Belgrade, Serbia d School of Materials Science and Engineering, University of New South Wales, Sydney NSW 2052, Australia b c
a r t i c l e
i n f o
Article history: Received 17 July 2019 Revised 9 October 2019 Accepted 16 October 2019 Available online xxx Keywords: Sensor Clomazone Glassy carbon Differential pulse voltammetry
a b s t r a c t Novel electrochemical sensor based on Pt supported multiwalled carbon nanotubes is used for determination of pesticide clomazone in aqueous media via differential pulse stripping voltammetry (DPSV). Since clomazone is stable and readily soluble in water, it is often found in water sources. Hence, its determination in the environment is of utmost importance. Herein, clomazone is determined in 0.1 M phosphate buffer solution at pH 7.0 in the concentration range of 0.61–20.56 ng cm−3 , with LOQ = 0.61 and LOD = 0.38. These results are in the same range with HPLC/DAD, which is used as comparative method. It is shown that DPSV is a facile and efficient way for determination of clomazone in contrast to precise but field-impractical HPLC. Mechanistic approach in explaining electrode processes is correlated to structural aspects of the synthesized sensor. HRTEM data reveals a uniform distribution of Pt nanoparticles on the MWCNT support as a source of crucial, structural and electronic changes. Furthermore, characterisation of Raman results indicates the existence of structural defects, which is believed to be the leading reason for improvement in sensing response. © 2019 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction Pesticides are widely used in agriculture to improve crop yield and quality by preventing, repelling, eliminating or mitigating various sorts of pests (insects, rodents, fungi, unwanted plants). It is estimated that one third of crop production in the world is secured by pesticide utilisation [1]. However, pesticides are declared to be noteworthy environmental pollutants due to drainage of the treated fields, their improper disposal, accidental spills, and poor delivery to the intended target. The numerous negative health effects associated with exposure to pesticides, including dermatological, gastrointestinal, neurological, carcinogenic, respiratory, reproductive, and endocrine effects [2], attracted global concern and became an alarming challenge in preserving health of living beings. Clomazone (2-[(2-chlorophenyl)methyl]−4,4-dimethyl-3isoxazolidinone) is a selective soil-applied herbicide used for the control of broad-leaved and weeds in crops of oilseed rape, cotton, tobacco, soybeans, rice, maize, sugar cane and vegetables
∗
Corresponding author. ´ E-mail address: [email protected] (M.Z. Momcˇ ilovic).
like peas, beans, carrots and potatoes [3]. It is reported that clomazone (CLM) interferes with chloroplast development and reduces or prevents accumulation of plastid pigments in susceptible species [4]. CLM is highly water-soluble (1100 mg dm−3 ), minimally volatile, resistant to hydrolysis under a wide range of pH values, and weakly sorptive to soil [5]. Due to its long half-life dissipation and excellent water solubility, it can cause groundwater contamination [6]. CLM residues have been detected in many fish [7] and water samples collected from rivers in rice-growing regions, but also in wastewater, ranging from 0.03 to 10 0 0 μg dm−3 [8]. Determination of CLM in environmental water samples is mostly done by the means of high performance liquid chromatography (HPLC) [9]. Accurate assessment of pesticide status in air, soil, and water is of critical importance to their proper management. Up to now, several analytical methods have been exploited to analyze pesticides including gas chromatography-mass spectrometry [10], high performance liquid chromatography [11], capillary electrophoresis [12], enzyme immunoassays [13], Raman spectroscopy [14], and fluorescence methods [15], etc. Although these techniques offer high sensitivity and selectivity at low detection limits, their on-site application is limited by the fact that they often demand expensive
https://doi.org/10.1016/j.jtice.2019.10.013 1876-1070/© 2019 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
´ M.Z. Momcˇ ilovic´ and J.S. Milicevi ´ c´ et al., Voltammetric sensor based on Pt nanoparticles Please cite this article as: M.S. Ran d¯elovic, suported MWCNT for determination of pesticide clomazone in water samples, Journal of the Taiwan Institute of Chemical Engineers, https://doi.org/10.1016/j.jtice.2019.10.013
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sophisticated equipment, a lot of time for sample preparation, a high skilled workforce and use of many chemicals [16]. Analytical principles based on electrochemical sensors receive great attention in modern science. New amperometric and enzymatic sensors for pesticide detection are designed for rapid, simple and low-cost operations in any field, with portable instruments, and with high sensitivity and selectivity of the methods. There is a wide class of biosensors based on the inhibition of acetylcholinesterase (AChE) enzyme [17,18]. Their main shortcomings are related to their low chemical and thermal stability, difficulty of immobilization, and lifetimes that are limited by denaturation of the biological material present on the electrode surface [19,20]. On the other side, a diverse class of non-enzymatic sensors based on noble metal [21] and molecularly imprinted [22] nanoparticles, metal oxides [23], polymers [24], pencil graphite electrodes [25], zeolites [26], graphene oxides [27], etc. is described in scientific literature. It is acknowledged that precious metals can greatly enhance the catalytic diffusion and electron transfer on electrochemical signal with a high sensitivity values [28]. Novel functional materials with enhanced sensing characteristics are required. Nano particles (NP) have gained growing importance in the design of non-enzymatic electrochemical sensors for environmental monitoring. Especially metal and metal-oxide NP have received much attention due to their auspicious electrocatalytic properties, often notably different from bulk materials. The carbon supported metal and metal oxide NPs enable fast electron transfer kinetics and mass transport, increase the electro-active surface area, and reduce overpotential, leading to electrochemical sensors with lower limits of detection (LODs). Multiwalled carbon nanotubes (MWCNTs) showed enormous potential for applications in fields of electrochemical sensors because of their unique electrical, mechanical and catalytical features. Moreover, large surface areas and wide useful potential ranges, commonly from –1.0 to +1.0 V are very important for their application in electroanalyses. A special ability of this nanomaterial to promote electron transfer in electrochemical reactions is attributed to the activity of edge-plane-like graphite sites at the CNT ends [29]. Large edge plane/basal plane ratio, and enhancement of electrode’s surface area and porosity are responsible for higher sensitivity, a lower detection limit, and faster electron transfer kinetics when compared to the traditional carbon electrodes [30]. Recently, considerable efforts have been made to investigate Pt/CNT catalysts for methanol oxidation and hydrogen generation [31–33]. To our best knowledge, there is no report about the electrochemical behaviour of Pt/CNT catalyst for the determination of pesticide in aqueous media by striping voltammetry. Remarkable sensitivity of stripping techniques is attributed to the preconcentration step which is followed then by the stripping step. In the preconcentration step, the target analyte is accumulated onto or into the working electrode. In the stripping step, the accumulated analyte is oxidized or reduced back into the solution. The response, recorded during this step, is proportional to the concentration of that analyte in the sample solution. Commonly, mercury electrodes are employed in stripping analysis. The two mercury electrodes most frequently used are the hanging mercury drop electrode (HMDE) and the mercury film electrode (MFE) [34]. However, disadvantages of the dropping mercury electrode and the hanging mercury drop electrode, such as risk of mercury toxicity and environmental considerations, instability of a mercury drop in flow systems, handling problems as well as high consumption of mercury, are responsible for intensive researches in the field of new electrode materials for voltametric determination of pesticides such as MWCNT-based composites [35,36]. Differential pulse stripping voltammetry (DPSV) is a remarkably sensitive electrochemical technique where the analyte of interest is in preconcentration step electroplated onto the working electrode
and then removed („stripped“) by applying an oxidizing potential in a series of voltage pulses of increasing amplitude. During removal, the cell current is measured as a function of time and as a function of the potential between the indicator and reference electrodes. The current is sampled before and after each voltage pulse. The response, recorded during this step, is proportional to the concentration of analyte in the sample solution. This technique evolved as a successor of cumbersome stripping techniques based on dropping mercury electrodes. In this paper, novel electrochemical sensor based on Pt supported multiwalled carbon nanotubes (Pt-MWCNT) used for sensitive and selective determination of pesticide CLM via DPSV is presented. In contrast to previous studies, our research presents novel method for the synthesis of Pt supported multiwalled carbon nanotubes which assumes in situ reduction of chloroplatinic acid hydrate over MWCNT in water/ethanol medium. Performances of DPSV were compared to HPLC method. In addition, the electrochemical characteristics of Pt-MWCNT was thoroughly examined by using K4 [Fe(CN)6 ] as a model compound to elucidate the electrocatalytic ability of the modified GC electrode. The study is based on the fact that immobilization of metallic nanoparticles onto carbon support can boost the advances of electrochemical sensors and lead to enhancements of their performance and applications. 2. Materials and methods 2.1. Synthesis of material For the functionalization of the carbon support, 50 mg of MWCNT was dispersed in 20 cm3 mixture of ethanol and water (50v/v%). Then, 0.5 cm3 of aniline was introduced into suspension and sonicated for 90 min. In a small cuvette, 26.7 mg of chloroplatinic acid hydrate (H2 PtCl6 · xH2 O; Pt, 38–40%) was dissolved in 2 cm3 of water and added to suspension which was then sonicated for the next 5 min and acidified afterwards with 0.5 cm3 of 5 M HNO3 . For the reduction of ions of platinum to elemental state, excess of 20 mg of sodium borohydride was dissolved in 2 cm3 of water and then added to the suspension. It was stirred for a while and left for the next 24 h, then filtered and rinsed with deionized water to neutral pH. The obtained material was dried at 90 °C for a couple of hours and designated as Pt-MWCNT. 2.2. Characterization The structural characteristics and the order of graphitization of Pt-MWCNT samples were explored by using Micro-Raman spectroscopy (Renishaw inVia 2 Raman Microscope) applying a laser source with a wavelength of 532 nm. The morphology of Pt nanoparticles-supported MWCNT was investigated by the field emission scanning electron microscopy (FE-SEM) by using microscope JEOL JXA-8500F. Distribution of particles over the MWCNT structure was analyzed by high resolution transmission electron microscopy (HR-TEM) by the means of JEOL JEM-F200. Powder XRD data were obtained with a diffractometer (Rigaku Corporation, Japan), with CuKα radiation (l = 1.5406 A) and a scan speed of 5° min−1 , in the range between 0 and 60° 2θ , with a step 0.02°. 2.3. Electrode preparation To use it as electrode material, 5 mg of Pt-MWCNT was dispersed in 1 cm3 of ethanol/water mixture (40v/v%), and the suspension was homogenized in the ultrasonic bath for 30 min. The surface of the glassy carbon (GC) electrode was polished on fine leather containing alumina powder (purchased from Metrohm)
´ M.Z. Momcˇ ilovic´ and J.S. Milicevi ´ c´ et al., Voltammetric sensor based on Pt nanoparticles Please cite this article as: M.S. Ran d¯elovic, suported MWCNT for determination of pesticide clomazone in water samples, Journal of the Taiwan Institute of Chemical Engineers, https://doi.org/10.1016/j.jtice.2019.10.013
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3
Fig. 1. SEM images of Pt-supported MWCNT at magnifications of (a) 10 0 0X (b) 10 0 0 0 0X (c) 30 0 0 0 0X.
which is part of the instrument polishing kit. A drop of suspension of 15 μL was transferred onto the GC electrode (working area was 0.04 cm2 ) and dried under N2 stream. After the thin carbon layer was dried, it was covered with 10 μL of 0.05 wt.% Nafion in ethanol. The solvent was removed by evaporation. 2.4. Electrochemical analysis CLM was analyzed by the means of differential pulse stripping voltammetry (DPSV). Working solutions with CLM in 0.1 M phosphate buffer were anayzed in a conventional voltammetric cell with operating volume of 10 cm3 by using 797 VA Computrace analyzer (Metrohm, Switzerland) controlled by 797 VA Computrace software (version 1.2). A three-electrode system consisted of Ag/AgCl electrode (saturated with KCl) as a reference, platinum wire as auxiliary and Pt-MWCNT modified GCE as working electrode. Prior to each run, the working electrode was electrochemically activated by potential cycling in the range from –0.8 to –0.1 V with speed 0.1 V s−1 , for 10 cycles. The background voltammograms of the supporting electrolyte were recorded under the same conditions. The blank probe containing only the supporting electrolyte was recorded under the same conditions. Before starting a new set of measurements, the supporting electrolyte was deaerated by suprapure nitrogen for 5 min and then volumes from CLM stock solution were added to probes in order to make desired concentrations. The parameters for DPSV measurement were as follows: initial potential –0.8 V, end potential –0.1 V, accumulation potential –0.25 V, accumulation time 120 s, and the scan rate 100 mV s−1 . Differential pulse cathodic adsorptive stripping voltammetry (DPCASV) was carried out at an accumulation potential of –0.25 V and an accumulation period of 120 s. The stirring was then stopped and time the voltammogram was recorded by applying a differential pulse voltammetry from –0.1 to −0.8 V with 100 mV s−1 scan rate and 50 mV pulse amplitude. All the measurements were performed at room temperature (23 ± 1 °C). As comparative analysis, CLM was assayed by Shimadzu Prominence high performance liquid chromatography (HPLC) system (Shimadzu, Japan) equipped with pump model LC-20AD and diode array detector (DAD) model SPD-M20A. For chromatographic separation by Eclipse XDB-C18 column (4.6 × 150 mm, 3.5 μm), acetonitrile and water (70:30, v/v) were used as mobile phase at flow rate of 1 cm3 min−1 in an oven at 40 °C with 20 μl injection volume, and quantification at 214 nm. 2.5. Chemicals MWCNT (carbon˃95%, OD x L 6–9 nm x 5 μm) and chloroplatinic acid hexahydrate which were used for synthesis of Pt-MWCNT were purchased from Sigma-Aldrich (USA). Analytical standard solution of CLM (Dr Ehrenstorfer, Germany) with concentration of 2 g dm−3 was prepared in methanol (Sigma-Aldrich) and kept in dark at –4 °C. Buffer solutions were prepared by mixing solutions
of 1 mol dm−3 K2 HPO4 (Merck, Germany) and 1 mol dm−3 KH2 PO4 (Merck) and used as supporting electrolytes. The pH values of the CLM solutions were adjusted by using 0.1 M phosphate buffer solution ranging from pH 5.8 to 8.0. Potassium ferrocyanide and potassium chloride were purchased from Sigma-Aldrich (USA). Deionized water was obtained in laboratory by using a Millipore purification system (USA). 3. Results and discussion 3.1. Structural characterization The microstructure of the materials can be seen in the SEM micrographs (Fig. 1) at three different magnifications. Morphology typical for MWCNT noticed at all three images is consistent for all examined samples showing that the MWCNT bundles have been randomly orientated. Diameter of carbon nanotubes is in range of 10 nm while the length can reach more than several micrometers. However, the exact length is difficult to be measured because bundle is always in the meandering form. Pt nanoparticles are observed as light spots or clusters on the surface of MWCNT with diameter of up to 30 nm. In order to acquire detailed information about size, distribution and morphology of Pt nanoparticles on the MWCNT surfaces, further in-depth examination was achieved by TEM. Fig. 2 shows Transition Electron Microscopy (TEM) images of representative structure. The TEM image in Fig. 2a indicates that Pt nanoparticles are well dispersed and successfully deposited on the CNT support. There is no evidence that the interior of the MWCNT is filled by Pt phase. Additionally, a higher magnification TEM image reveals that Pt nanoparticles intrinsically agglomerate and form aggregates with an average diameter of ∼10 nm (Fig. 2b). It should be noted that Pt nanoparticles are firmly attached to the surface of CNT since there is no single observation of freely (detached) Pt nanoparticles after analysis a large numbers of samples. The HRTEM image, as shown in Fig. 2c, reveals a cluster of nanoparticles, where a D-space of 0.225 nm, which corresponds to the space of (111) planes, is identified. The selected area diffraction (SAED) patterns of the Pt NPs were indexed to three main planes of (111), (220), and (222) that illustrates a random orientation of the Pt nanoparticles (Fig. 2d). The powder XRD technique was used to investigate the crystal structure of the Pt-MWCNT samples (Fig. 3). The XRD profile contains peaks for MWCNTs at 26 and 43° 2θ which correspond to the planes of (220), (301) and (002), respectively, as indexed from (ICDD card no. 01- 083–3673). The peaks represented the graphitic reflections. Raman analysis was used to investigate the structural properties of Pt-supported MWCNT. The corresponding spectrum is given in Fig. 4. Three characteristic bands are noticed. The first one is located at ca. 1576 cm−1 and corresponds to the first order G mode (in-plane optical mode), which originates from the flat
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Fig. 2. (a,b). TEM images of Pt-supported MWCNT. (c) HRTEM image showing a cluster of Pt nanoparticles (d) SAED pattern of Pt-supported MWCNT.
Fig. 3. XRD pattern for Pt-MWCNT.
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Fig. 4. Raman spectrum of Pt-MWCNT.
geometric structure of sp2 graphitic network [37,38]. It is caused by the Raman active E2g phonon. The second band is the D mode (A1g symmetry) around 1339 cm−1 which corresponds to sp3 hybridized carbon [39,40]. It is attributed to the first order of zone boundary phonons and depends strongly on the amount of defects and on the CNT surface. Some previous studies showed that the presence of structural defects and edge planes are responsible for improvements on sensing applications of CNT [41,42]. The third band centered at around 2697 cm−1 is denoted as 2D. This peak arises due to the second order of the zone boundary phonons and is related to the number of graphene layers in CNT [43,44]. The ID /IG ratio shows the disorder degree of the sp2 bonded carbon that can be applied as graphitization degree indicator. According to the general rule, the graphitization degree is inversely proportional to the ID /IG ratio. Such ratio was calculated to be 1.21. 3.2. Cyclic voltammetry The electrochemical performance of Pt-MWCNT modified GC and bare CG electrode was initially investigated by cyclic voltammetry. A electrode reaction of potassium ferrocyanide (K4 [Fe(CN)6 ]) in a stagnant electrolyte was used as a common benchmark redox systems due to its “surface sensitive” electrochemical response onto examined electrodes. The electrode reaction can be formulated as given below:
Fe(CN )36− + e− Fe(CN )46−
(1)
Cyclic voltammograms at scan rate of 100 mV s−1 for K4 [Fe(CN)6 ] at GC and Pt-MWCNT modified GC electrode are illustrated in Fig. 5. The cyclic voltammogram at bare GC electrode (dotted line) indicates low peak current intensity (ip ) and high peak-to-peak separation (potential difference between reduction and oxidation peaks) – ࢞Ep . However, the voltammetric response is apparently improved at Pt-MWCNT modified GC electrode (solid line). Electrocatalytic effect of later electrode is reflected by the enlargement of the peak currents (ip ) and the reduction of the ࢞Ep value. Compared to bare GCE, the peak-to-peak separation (࢞Ep ) was lowered by 98 mV for Pt-MWCNT/GC electrode. Since the peak-to-peak separation is directly correlated to the kinetics of electron transfer, these findings show a significant improvement in the rate of electron transfer for Pt-MWCNT/GC electrode.
In addition, the peak current ratio (Ipa /Ipc ) for the redox reactions at GC electrode was measured to be 0.90, while such value was 0.93 for Pt-MWCNT/GC electrode. The closer the value of the peak current ratio to the value of 1 indicates faster kinetics of electrode reaction which corresponds to a reversible system. Therefore, the redox reaction at Pt-MWCNT/GC (0.93) is dominant on the surface with a higher rate of kinetics compared to that which occurred at the blank GC electrode. The half-wave potentials (E1/2 ) were calculated from the anodic (Epa ) and cathodic (Epc ) peak potentials by using the expression (Epa +Epc )/2. The obtained preliminary results of electrochemical characterization reveal superior electrochemical behavior of PtMWCNT for the redox couple [Fe(CN)6 ]3/4− . All voltammetric parameters for the two electrochemical systems are listed in Table 1. The morphology and distribution of Pt nanoparticles over a fast electron-transport MWCNT network is very important for electrocatalytic behavior of Pt-MWCNT based nanohybrid. Moreover, Pt nanoparticles provide high electrocatalytic activity because of their large surface area, which additionally contributes to a large reduction in electrode cost due to decrease of required Pt amount compared to commercial electrodes. Hence, application of MWCNT as supporters for Pt nanoparticles is a reliable concept to improve electrocatalytic activity and conductivity as well as to reduce the amount of Pt in these heterojunction structures (to around 17%mass). Electrode process can occur on the surface of Pt nanoparticle or at the contact of MWCNT and ultrafine dispersed Pt. This provides an auspicious structure for an efficient electron transfer. 3.3. Differential pulse stripping voltammetry (DPSV) For the determination of CLM at the low ppb level, the DPSV mode was selected. The cathodic response of CLM was evaluated with respect to the several supporting electrolytes including potassium chloride, Britton–Robinson buffer, and phosphate buffer. The later was found to be the most suitable medium. Subsequently, the DPSV curves were recorded in 0.1 M phosphate buffer (pH range 5.8 – 8.0) to study the effect of pH on the voltammetric behavior of CLM. As clearly noticeable (Fig. 6), there is no linear relationship between DP peak potential and pH. It is indicated that mechanism of CLM reduction is complex with no common electrochemical behaviour involving addition of
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Fig. 5. Comparison between cyclic voltammograms obtained by Pt-MWCNT/GC and commercial GC electrode as a working electrode (scan rate 100 mV s-1, 0.2 mM K4[Fe(CN)6] in 0.1 M KCl). Table 1 Electrochemical parameters obtained from voltammograms for a redox couple [Fe(CN)6 ]3/4− on GC and Pt-MWCNT modified GC electrodes. Electrode
Epa (V)
Epc (V)
E (V)
E1/2 (V)
Ipa (μA)
Ipc (μA)
Ipa / Ipc
GCE Pt-MWCNT/GCE
0.390 0.320
0.083 0.111
0.307 0.209
0.236 0.215
33.27 77.89
−36.89 −83.42
0.90 0.93
Fig. 6. DPSV curves 10.2 ng cm-3 of CLM at Pt-MWCNT in 0.1 M phosphate buffer solution (pH 5.8 - 8.0, initial potential −0.8 V, end potential −0.1 V, accumulation potential −0.25 V, accumulation time 120 s, and the scan rate 100 mV s-1).
hydrogen. General mechanism of clomazone reduction is proposed at Fig. 7. It has been ascertained from experimental data that the best peak shape and the maximum peaks current occurred in the 0.1 M phosphate buffer at pH 7.0. This is the reason why it was chosen for further investigation as optimal parameter from the analytical point of view. For comparison, there was no electrochem-
ical response of CLM pesticide with bare GC or MWCNT/GC electrodes. The current intensity of the signal of interest was found to be dependent upon the deposition potential, (E was between +0.15 and –0.65 V). As shown in Fig. 8a, the peak current increased up to a potential of –0.25 V and then started to decrease. When accumu-
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Fig. 7. Mechanism of clomazone reduction.
Fig. 8. Key parameter optimized for differential pulse striping voltammetric determination of CLM (10.2 ng cm-3) in the 0.1 M phosphate buffer solution pH 7.0: (a) Influence of deposition potential, E on I values t = 120 s; and (b) Effect of deposition time, t at E –0.25 V at a scan rate 100 mV s-1.
lation at –0.25 V was carried out, the reduction peaks were higher than those obtained without accumulation as a result of the enhanced adsorption of CLM. The optimal value of –0.25 V was chosen for deposition potential which coincides with the reduction potential for CLM at selected conditions (Fig. 8a). This interesting observation indicates that products of electrochemical reduction of CLM pesticide in a certain way influence maximum peak current in DPSV. In accordance with the experimental data obtained, we assume that the electrochemical reduction process presented by the following reaction is the most probable. The loss of keto group due to the reduction of CLM at electrode is the main reason why the reduced form (rCLM) has an increased tendency to interact with low polar surface of carbon nanotubes [45]. The hydrophobic surfaces of carbon nanotubes adsorb rCLM by hydrophobic interactions and van der Waals interactions. Hence, the Pt-MWCNT/GC electrode which was modified by adsorption of rCLM in pretreatment step provides better adsorption surface for CLM molecules in analyzed solution. Namely, CLM shows a more pronounced tendency to associate with adsorbed rCLM molecules on electrode through van der Waals interactions in comparison to the surface of bare electrode. Therefore, enhanced adsorption and accumulation of CLM on the Pt-MWCNT/GC electrode after its pretreatment at –0.25 V can be exploited here to significantly increase sensitivity of DPSV. Fig. 8b shows that the signal has hyperbolic shape with prolongation of the deposition time to 900 s. The peak current increased rapidly with the increasing of deposition time and almost reached a platform after 120 s. For analytical purpose, Pt-MWC/GC electrode was initially pretreated during 900 s into the solution of 10.2 ng cm−3 of CLM 0.1 M phosphate buffer solution before it was
used for measuring concentration of CLM to achieve down to the low ppb level. The additional preconcentration time of 120 s was employed before each measurement. Selected pretreatment time is enough to obtain a stable analytical signal. In addition, it should be emphasized that Pt NPs are electrocatalyst which in synergism with developed surface area of MWCNTs enables successful application of this electrode for CLM determination. The calibration graph for an accumulation time of 120 s was linear in the range from 0.61 to 20.56 ng ml−1 and obeyed the equation y = 1.09 x + 0.73, where y and x are the peak current (nA) and CLM concentration (ng ml−1 ), respectively (Fig. 9). The relative standard deviation was 1.85% (for 7 determinations). The limit of detection, estimated from 3 times the standard deviation, was 0.38 ng ml−1 . The quantitative DPSV determination of CLM is based on linear relationship between the peak current intensity at –0.2 V and CLM concentration. As can be seen, CLM could be determined by DPSV in the concentration range of 0.61 – 20.56 ng cm−3 (Fig. 6). Thereby, the relative standard deviation does not exceed 1.85%. The analytical parameters of the developed DPSV method and the comparative HPLC/DAD measurements are shown in the Table 2. By comparing the parameteres presented for two analytical methods, it can be seen that limits of detection (LOD) and limits of quantification (LOQ) are very close. Although HPLC gave just a slightly better results in experimental set up applied herein, significant potential of usability of DPSV as an easy, fast and reliable method for determinations of CLM is noticeable. This low-cost electrochemical method certainly imposes itself as a solution to many problems of chromatographic analyses, especially those related to applications on the field. In a similar study, it is confirmed that materials based
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Fig. 9. Differential pulse stripping voltammograms recorded at Pt-MWCNT/GCE for different concentrations of CLM in 0.1 M phosphate buffer solution pH 7.0 and the corresponding calibration plot. Accumulation potential –0.25 V, accumulation time 120 s, and the scan rate 100 mV s-1. Table 2 Analytical parameters of the DPSV and HPLC/DAD determination of CLM. Method Parameter Concentration interval [ng cm−3 ] Intercept Slope Correlation coefficient, r LOD [ng cm−3 ] LOQ [ng cm−3 ] RSD [%] (n = 7)
DPSV
HPLC/DAD
0.61 − 20.56 0.73 μA 1.09 μA cm3 ng−1 0.998 0.38 0.61 1.85
0.46 – 92.44 −1234.632 mAU min 7000.832 mAU cm3 ng−1 0.999 0.32 0.46 1.81
on MWCNT can be stable, selective and can achieve ultra sensitivity in electroanalytical detections of pesticides [46]. The interference of some common ions (Ca2+ , Na+ , Ag+ , K+ , Cl− , HCO3− , CO3 2− , NO3− ) and pesticides (linuron, imidacloprid and tebufenozide) on the determination of CLM was also tested in this study. Firstly, DPSV signal of CLM (10.2 ng cm−3 ) was recorded using the Pt-MWCNT/GCE, then either selected ion was added in a concentration that is 50 times higher than CLM or selected pesticide was added in the same final concentration as CLM. The solutions were analyzed by the proposed method and all investigated ions or pesticides neither influenced peak potential nor peak current intensity of CLM significantly. 4. Conclusions Pt-MWCNT heterojunction structures can be successfully synthesised in water/ethanol suspension, containing MWCNT and H2 PtCl6 by using sodium borohydride as reduction agent. The obtained nanohybrid material possesses a favorable morphology with fast electron-transport MWCNT network decorated by Pt nanoparticles enabling advantageous electrocatalytic behavior toward K4 [Fe(CN)6 ] and CLM reduction. The morphology and structure of the material were thoroughly examined by TEM and SAED method indicating nanostructure of Pt which were firmly attached onto MCWNT support. Additional structural information was provided by Raman spectroscopy which are in accordance with the results of previous analysis. The obtained differential pulse strip-
ping voltammetry have shown to be fairly efficient technique for rapid and sensitive determination of pesticide CLM by glassy carbon electrode improved with Pt supported multiwalled carbon nanotubes. Sensing features of Pt-MWCNT toward CLM were found to be superior in comparison to the bare glassy carbon electrode. Electrochemical reduction of CLM with the loss of keto group increases the tendency of reduced analogue to be adsorb onto poorly polar surface of carbon nanotubes in pre-treatment step followed by enhanced sensitivity in subsequent accumulation step. DPSV attained relatively low limits of detection and quantification which in comparison to concurent HPLC method leeds to conclusion that DPSV can be acknowledged as handy and fast alternative for CLM determination especially for on-site applications. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This study was carried out as part of the national scientific projects TR340 08, III 450 06, TR 31043, and III 43009 which are financed by the Ministry of Education, Science and Technological Development of the Republic of Serbia.
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´ M.Z. Momcˇ ilovic´ and J.S. Milicevi ´ c´ et al., Voltammetric sensor based on Pt nanoparticles Please cite this article as: M.S. Ran d¯elovic, suported MWCNT for determination of pesticide clomazone in water samples, Journal of the Taiwan Institute of Chemical Engineers, https://doi.org/10.1016/j.jtice.2019.10.013