Journal of Electroanalytical Chemistry 699 (2013) 33–39

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Journal of Electroanalytical Chemistry

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Renewable silver-amalgam film electrode for voltammetric monitoringof solar photodegradation of imidacloprid in the presence of Fe/TiO2

and TiO2 catalysts

1572-6657/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.jelechem.2013.04.003

⇑ Corresponding author. Tel./fax: +381 21 454 065.E-mail address: valeria.guzsvany@dh.uns.ns.rs (V. Guzsvány).

Valéria Guzsvány a,⇑, Jelena Petrovic a, Jugoslav Krstic b, Zsigmond Papp a, Maria Putek c, Luka Bjelica a,Andrzej Bobrowski c, Biljana Abramovic a

a Department of Chemistry, Biochemistry and Environmental Protection, Faculty of Sciences, University of Novi Sad, Trg D. Obradovica 3, 21000 Novi Sad, Serbiab Institute of Chemistry, Technology and Metallurgy, Department of Catalysis and Chemical Engineering, University of Belgrade, Njegoševa 12, Belgrade, Serbiac Department of Building Materials Technology, Faculty of Materials Science and Ceramics, AGH-University of Science and Technology, Mickiewicza 30, 30-059 Krakow, Poland

a r t i c l e i n f o

Article history:Received 27 July 2012Received in revised form 28 December 2012Accepted 1 April 2013Available online 10 April 2013

Keywords:Silver-amalgam film electrodeImidaclopridVoltammetric monitoringSolar photodegradationFe/TiO2 catalyst

a b s t r a c t

A rapid and sensitive voltammetric method has been developed for the characterization and determina-tion of the imidacloprid insecticide by means of square-wave voltammetry (SWV) with a silver-amalgamfilm electrode (Hg(Ag)FE) in the aqueous Britton–Robinson buffer solution as the supporting electrolyte.In the investigated pH range between 2.0 and 9.0 two reduction peaks of imidacloprid were observed, andthey were both obtained in a fairly negative potential range, (approximately from �0.8 to �1.5 V). Theirshape strongly depended on the pH of the supporting electrolyte. The analytical measurements, based onthe first peak, were performed at the pH of 7.0, and imidacloprid was determined in the concentrationrange of 0.91�47.48 lg cm�3. The reproducibility of the analytical signal was characterized by a relativestandard deviation smaller than 1.0%, and the calculated values of detection and quantitation limits were0.27 lg cm�3 and 0.91 lg cm�3, respectively. The applicability of the elaborated SWV method was testedby monitoring imidacloprid concentration during its solar photolytic and photocatalytic degradation inthe presence of two heterogeneous catalysts: TiO2 and TiO2 modified with 1.9% w/w Fe (1.9% Fe/TiO2).Under photolytic conditions, imidacloprid degraded very slowly, and the presence of TiO2 as a catalystaccelerated the process, while 1.9% Fe/TiO2 showed a remarkable efficiency in the removal of imidaclo-prid under natural insolation. In all investigated cases the results obtained using the Hg(Ag)FE-SWVmethod were in agreement with those obtained by means of the comparative HPLC–DAD method, whichconfirmed that the developed voltammetric method can be used to monitor the degradation ofimidacloprid.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

Imidacloprid (((EZ)-1-(6-chloro-3-pyridylmethyl)-N-nitroimi-dazolidin-2-ylideneamine), Fig. 1A, inset) is the most frequentlyused insecticide from the group of neonicotinoids. There arenumerous literature reports on the photolytic [1–4], and photocata-lytic degradation [5–12] of this insecticide and its formulatedproducts. Our recent studies have shown that the O2/TiO2, UVAlight system (400–320 nm) can be successfully applied for its pho-tocatalytic degradation [10]. Furthermore, it is well-known thatdifferent iron-based catalysts, and especially the photo-Fentonprocess, based on the combination of the Fenton reagent (Fe2+/H2O2) and UV or vis light, are effective for the advanced oxidativeremoval of imidacloprid from water [8–12]. Different types of

heterogeneous catalysts like Fe/TiO2/pH via TiO2/SiO2, Fe0/Fe3O4

and Fe/TiO2 showed high efficiency in the removal of both imida-cloprid and some other harmful organic compounds [13–16].

High-performance liquid chromatography (HPLC) with diodearray (DA) or mass spectrometric (MS) detection is the analyticaltechnique that is commonly used to monitor photodegradation ofimidacloprid [2,3,5,7–13,17–19]. HPLC coupled with powerfuldetectors allows the monitoring of parent compound degradationkinetics, the detection and identification of degradation intermedi-ates, and their monitoring. The determination of imidacloprid andits major metabolite in soils by means of liquid chromatographywith pulsed reductive amperometric detection has also beenelaborated [20]. However, although the HPLC technique isundoubtedly the most popular one, it requires advanced equip-ment, it is costly, and rather time-consuming, as well. The alterna-tive methods that have been used for the monitoring of photolytic/photocatalytic degradation of imidacloprid are spectroscopy

Fig. 1. SWV curves obtained with Hg(Ag)FE for imidacloprid solution (25.41 lg cm�3, (2)) and the corresponding blanks (1) in Britton–Robinson buffer pH 2.0 (A), 3.0 (B), 6.0(C) and 7.0 (D). The inset (A): molecular formula of imidacloprid. The measurement parameters: pulse amplitude – 25 mV; polarization rate – 100 mV s�1.

34 V. Guzsvány et al. / Journal of Electroanalytical Chemistry 699 (2013) 33–39

[21,22] and voltammetry [23,24]. The recently developed voltam-metric methods based on the bismuth-film electrode [23] or car-bon paste electrode [24] allow the monitoring of the degradationof the electroactive fragment of imidacloprid, which is responsiblefor its physiological activity.

Generally, modern voltammetric methods are fast, sensitive,and inexpensive, and thus suitable for large-scale monitoring ofelectrochemically-active environmental pollutants [25,26], ofteneasily adaptable for on-site work. Mercury electrodes are obviouslythe best sensors for voltammetric determination of electrochemi-cally reducible compounds [27,28], especially those that are elec-troactive in fairly negative potential ranges; imidacloprid is oneof these compounds due to its reducible nitro group [29–31]. How-ever, because of fears of mercury toxicity, there is a tendency tosubstitute mercury with other non-toxic or less toxic electrodematerials [25].

Probably the most promising alternatives to mercury electrodesare bismuth-based electrodes [32] and various solid amalgam elec-trodes [33–41]. A wide potential window, low noise, a surface thatis easily renewable by electrochemical means, and mechanicalrobustness are very desirable features with regard to measure-ments in ‘‘flow systems’’, and they, along with easy preparationand regeneration, make them a very promising electroanalyticaltool compatible with the concepts of green analytical chemistry.The renewable silver-amalgam film electrode (Hg(Ag)FE) repre-sents a promising type of the amalgam-based electrode familywhich is applicable in the analysis of trace metals [35,39–44],and biologically active organic compounds [33,36–39,45,46].

Other alternatives include carbon-based composite electrodessuch as tricresyl-phospate based carbon paste electrode (TCP-CPE) in its simple unmodified form [47], or TCP-CPE bulk-modifiedwith bismuth particles [48], the Nafion�/nanoTiO2 Nafion� com-posite surface modified glassy carbon electrode [49] and the cup-per(II) phtalocyanine modified carbon ceramic electrode [50] for

differential-pulse voltammetric determination of imidacloprid inselected real samples, like spiked water, commercial formulationsor tomato samples. A basic characteristic of such sensor is thattheir negative potential measurement range is narrower than thatof Hg-based electrodes. In consequence, in the case of imidaclopridinstead of two separate peaks only the one that represents thereduction of the nitro group to hydroxylamine appears, whereasthe second step of the reduction of hydroxylamine to amine cannotbe observed because of the overlapping with hydrogen evolution.Still, the first peak, a well-defined and more intensive four-electronexchange signal, is in the majority of the procedures successfullyused for the determination of imidacloprid [29,31,47–49].

The objective of this work was a detailed investigation of theelectrochemical reduction of imidacloprid at the Hg(Ag)FE, optimi-zation of the highly sensitive SWV analytical method for the deter-mination of imidacloprid, and the application of the developedmethod to monitor its concentration in three different types ofphotodegradation samples. Namely, the applicability of the meth-od was tested by monitoring the solar photolytic and photocata-lytic degradation of imidacloprid in the presence of twoheterogenous catalysts: TiO2 in combination with H2O2, and the re-cently synthesized Fe-modified (1.9%, w/w) TiO2 (1.9% Fe/TiO2)nanocomposite [16] and H2O2. Comparative HPLC–DAD measure-ments were performed to validate the developed Hg(Ag)FE-basedSWV method.

2. Experimental

2.1. Reagents and solutions

All chemicals used were of analytical reagent grade. Imidaclo-prid (purity 99.9%) was purchased from Riedel-de Haen (Seelze,Germany). Primary stock solution was prepared by dissolving the

V. Guzsvány et al. / Journal of Electroanalytical Chemistry 699 (2013) 33–39 35

insecticide substance in doubly distilled water to obtain a standardsolution of 100 mg dm�3 concentration.

Britton–Robinson buffer solutions were prepared from a stocksolution containing 0.04 M phosphoric (Merck, Darmstadt,Germany), boric (Merck) and acetic (Merck) acids, respectively,by adding 0.2 M sodium hydroxide (Merck) to obtain the requiredpH values, covering the pH range 2.0–9.0.

The catalysts used in the photocatalytic degradation of imida-cloprid were TiO2 (Degussa P-25; 75% anatase and 25% rutile, spe-cific area 50 ± 15 m2 g�1, grain size about 20 nm, Degussa,Frankfurt, Germany) and 1.9% Fe/TiO2 nanocomposite catalyst[16], both applied at a load of 2.0 mg cm�3.

The concentration of the H2O2 stock solution (p.a., Roth, Kar-lsruhe, Germany) was 30%, and it was diluted to the required con-centration (234 lg cm�3). H2SO4 (p.a., Zorka, Šabac, Serbia) wasapplied for the pH adjustment in the case of photocatalyticsystems.

2.2. Apparatus

Voltammetric experiments were performed on an AUTOLABPGSTAT12 electrochemical analyzer operated via GPES 4.9 soft-ware (Ecochemie, The Netherlands). The cell stand included athree-electrode system with a renewable Hg(Ag)FE [35,39] asworking, a saturated calomel electrode (SCE) (Amel, Italy) as refer-ence, and a platinum (Amel) auxiliary electrode. All potentials arequoted vs. SCE reference electrode.

Comparative HPLC measurements were performed on an Agi-lent 1100 liquid chromatograph (Agilent Technologies Inc., USA),Zorbax Eclipse XDB-C18 (250 mm � 4.6 mm, 3.5 lm) column,and DA-detector.

2.3. Procedures

2.3.1. Preparation of solar photodegradation samplesThree solutions of the same imidacloprid concentration

(102.26 lg cm�3) were prepared, of which the one for photolyticexperiment contained only imidacloprid, while those for photocat-alytic experiments contained also H2O2 (234 lg cm�3) and TiO2 or1.9% Fe/TiO2 (2.0 mg cm�3). For the photocatalytic experiments,the pH was adjusted to 2.8 by adding H2SO4 [10,16]. All experi-ments were carried out in triplicate. Before starting the degrada-tion experiment all glassware was sterilized with dry heatsterilizer. The aqueous solutions of imidacloprid, with and withoutcatalyst, were exposed to natural sunlight at room temperature(25 �C) during four weeks of September 2010, and were manuallyshaked twice a day. Sampling was carried out daily and the sam-ples were kept frozen until SWV and HPLC–DAD measurements.

2.3.2. VoltammetryBefore starting measurement, the working electrode was acti-

vated in the chosen supporting electrolyte by cycling its potentialin the span from �0.20 to �1.60 V at a scan rate of 100 mV s�1.The SWV studies of the imidacloprid behavior were carried outin the Britton–Robinson buffers pH 2.0–9.0. In the case of themodel system, the stock solution of imidacloprid was dilutedwith doubly distilled water (5.00 cm3) and a correspondingBritton–Robinson buffer solution (5.00 cm3). To monitor thephotolytic and photocatalytic degradation, the samples werediluted with the Britton–Robinson buffer pH 7.0 in a ratio 1:3.Samples were analyzed without removal of the catalyst particles.

2.3.3. ChromatographyThe mobile phase was prepared by mixing 0.2% phosphoric acid

and acetonitrile in a ratio 7:3 (pH 2.6) [51]. The working tempera-ture of the column was 25 �C, flow rate 1.0 cm3 min�1, and the loop

volume 5.00 lL. The measurements were carried out at 270 nm,the imidacloprid retention time being 1.18 min. Before eachmeasurement, the samples were filtered through a Millex-GV(Millipore, SAD) membrane filter of pore size of 0.22 lm.

2.3.4. pH measurementsThe pH measurements were made using a combined glass

electrode (Jenway, UK), on a previously calibrated pH-meter(Radiometer, The Netherlands).

3. Results and discussion

As it is evident from the above, the experiments encompassedthe SWV characterization of imidacloprid in model solutions ofthe pH 2.0–9.0 using Hg(Ag)FE, optimization of the correspondinganalytical procedure for the determination of imidacloprid by SWVand its application for the monitoring of imidacloprid concentra-tion under the conditions of its solar photolytic and photocatalyticdegradation.

3.1. Voltammetric characterization of imidacloprid on Hg(Ag)FE

In view of the fact that the imidacloprid molecule contains anitroguanidine functional group which is electroactive on Hg elec-trode [29–31], it can be expected that its reduction at Hg(Ag)FEwill also result in a similar voltammetric pattern.

Since the voltammetric behavior of imidacloprid is significantlyinfluenced by the pH of the medium a detailed SWV study was car-ried out in the Britton–Robinson buffer (pH 2.0–9.0) at Hg(Ag)FE inthe potential span from �0.20 to �1.60 V. As can be seen fromFig. 1 (curves 2), the curves have two reduction peaks whoseshapes at the pH 2.0 and pH 6.0 differ significantly, which can bea consequence of the different degree of protonation of the imida-cloprid molecule. At the higher pH range from 6.0 to 9.0 (shown forthe pH 7.0) the two peaks are well separated, and the first of themis more intensive and of a symmetric shape. However, the thirdreduction peak which is on the Hg electrode observed at �2.2 V(not shown), is missing. Generally, both peaks obtained atHg(Ag)FE shift cathodically with the increase in the investigatedrange of pH, the first from �0.8 to �1.0 V, and the second from�1.1 to �1.5 V. All this indicates a complex electrode process inwhich electron exchange is significantly influenced by protons[29–31].

It should be pointed out that an unexpected peak [52] was reg-istered on the baselines (curves 1) for the supporting electrolytesof the pH > 3.0, which was most intensive at the pH 5.0. This peakwas also observed for the acetate buffer pH 4.5 (0.1 M) and phos-phate buffer pH 7.5 (0.1 M). Since a well defined baseline is a pre-requisite for a good analytical method, the best choice could be torecord the voltammograms for the solution of pH 7.0, as the analyt-ical signal is well shaped and separated most from the undesirablebaseline signal, which is at�1.20 V characterized by a reproducibleshape and intensity (Fig. 1D, curve 1). Although the baseline re-corded for the solution pH 2.0 does not contain the unexpectedpeak, the peaks of reduction of imidacloprid are not sensitive en-ough to be used for the determination. On the other hand, thepeaks for the solution pH 9.0 are well separated but imidaclopridhydrolyzes [22], and Hg(Ag)FE is not suitable in this medium.Hence, these pH values were not suitable for developing an analyt-ical method.

Before each set of measurements, Hg(Ag)FE was activated elec-trochemically by cycling its potential in the range from �0.20 to�1.60 V in the appropriate supporting electrolyte. In the case ofneed (which is recognized as the loss of the electrode sensitivity),chemical activation in 0.1 M HNO3 was applied to clean the sensor

Fig. 4. SWV curves recorded at Hg(Ag)FE for different imidacloprid concentrationsin Britton–Robinson buffer solution pH 7.0. Inset: dependence of Ip1 and Ip2 onimidacloprid concentration. Other experimental parameters are the same as inFig. 1.

Table 1Analytical parameters for Hg(Ag)FE-SWV and HPLC–DAD determination ofimidacloprid, r: linear regression coefficient; LOD: limit of detection; LOQ: limitof quantitation, n: number of measurements, RSD: relative standard deviation.

Parameter Hg(Ag)FE-SWV HPLC–DAD [47]

Concentration interval (lg cm�3) 0.91�47.48 0.075–5.0Intercept 0.0219 lA �0.57 mAU

3 �1 3 �1

36 V. Guzsvány et al. / Journal of Electroanalytical Chemistry 699 (2013) 33–39

surface. The cleaned electrode was again covered with amalgam bydipping into the attached mercury pool [35,39] and subjected toelectrochemical conditioning.

3.2. Determination of imidacloprid by SWV on Hg(Ag)FE

As can be seen from Fig. 2, the maximum current is registeredfor the solutions in the pH 7.0–9.0 for the first peak, and pH 2.0–4.0 for the second peak. Since the Ip values for the first peak aremuch higher, this peak and the pH 7.0 were chosen for the analyt-ical determinations.

The repeatability of the analytical signal was tested by record-ing successively 15 SWV curves for the imidacloprid concentrationof 3.26 lg cm�3 (curve 2) and 9.41 lg cm�3 (curve 3), which is,along with the baseline (curve 1), shown in Fig. 3. The relative stan-dard deviation (RSD) was 0.92% and 0.88% for the first and secondpeak, respectively. It should be noted that the RSD of the baselinepeak was 0.89%, which enabled analytical determinations in thepresence of this undesired signal.

The determination of imidacloprid was based on the lineardependence of the height of the first reduction peak on concentra-tion, as shown in Fig. 4. The calibration graph (inset) obtained forthe model solution under optimal experimental conditions showedlinearity in the range of 0.91–47.48 lg cm�3 (Table 1). As for thesecond reduction peak, there are two linear ranges of the Ip2–cdependence: in the range from 0.91 to 7.5 lg cm�3 and from 7.5to 25 lg cm�3, which can be described by the following relations:Ip = 0.0168 (lA cm3 lg�1 + 0.226 c (lg cm�3), r = 0.996, andIp = 0.902 lA cm3 lg�1 + 0.104 c (lg cm�3), r = 0.998, respectively.

Fig. 2. Dependence of Ip on pH: first peak (1), second peak (2).

Fig. 3. Repeatibility of the SWV signals (15 successive scans) at Hg(Ag)FE obtainedfor imidacloprid solution of 3.26 lg cm�3 (2) and 9.41 lg cm�3 (3) in Britton–Robinson buffer pH 7.0, and the corresponding baseline (1). Other experimentalparameters are the same as in Fig. 1.

Slope 0.196 lA cm lg 0.108 mAU cm ngr (%) 0.999 0.999LOD (lg cm�3) 0.27 0.023LOQ (lg cm�3) 0.91 0.075RSD (%) (n = 15) 0.92 1.10

The comparative HPLC–DAD method was applicable in a widerconcentration range 0.075–5.0 lg cm�3, but the developed SWVmethod has some other advantages. Namely, it is suitable for infield analysis, and the sample preparation is not so complex as itis usual in chromatography. Besides, the Hg(Ag)FE-based SWVmethod is less costly and is suitable for rapid measurements.

3.3. Application of the developed Hg(Ag)FE-SWV method formonitoring solar degradation of imidacloprid

The voltammetric method developed for the model solutionwas applied for the monitoring of the process of photolytic andphotocatalytic degradation of imidacloprid under solar irradiation.Since the photodegradation of neonicotinoids is accompanied bysignificant changes in the pH [23,24], and having in mind the highpH-dependence of the analytical signals, it was necessary to adjustthe pH of the investigated samples to the optimal value (pH 7.0).Further on, the unfavorable adsorption of TiO2 and 1.9% Fe/TiO2

catalysts onto the electrode surface had to be considered. Theexperiments showed that the addition of the Britton–Robinsonbuffer pH 7.0 at a relatively high concentration caused sedimenta-tion of the catalysts, and thus hindered their adsorption onto theelectrode surface. Hence, all measurements could be performedwithout sample filtering.

3.3.1. Photolytic degradationAs can be seen from Fig. 5B, only 20% of imidacloprid decom-

posed during the four-week time. The degradation reaction isapproximately of the first order, with the rate constantsk = 0.007 days�1, which can be described by the equation given in

Table 2The rate of phodegradation of imidacloprid in the presence of different catalystsmeasured by Hg(Ag)FE-based SWV and by comparative HPLC–DAD method, t (days).

Catalyst Kinetic equation

Hg(Ag)FE-SWV HPLC–DAD

– ln(ct/c0) = �0.007 t + 0.034(r = �0.967)

ln(ct/c0) = �0.007 t + 0.019(r = �0.988)

TiO2 + H2O2 ln(ct/c0) = �0.890 t + 0.142(r = �0.998)

ln(ct/c0) = �0.9147 t + 0.248(r = �0.998)

1.9% Fe/TiO2 + H2O2 ln(ct/c0) = �2.190 t + 0.697(r = �0.972)

ln(ct/c0) = �2.275 t + 0.625(r = �0.962)

Fig. 5. SWV curves (A, C, E), degradation curves (B, D, F insets, curves 1) of photolytic (B) and photocatalytic (D and F) degradation of imidacloprid in the presence of TiO2 (D)and Fe–TiO2 (F) catalysts and kinetic curves (insets) obtained by SWV (curves 1) and HPLC–DAD (curves 2).

V. Guzsvány et al. / Journal of Electroanalytical Chemistry 699 (2013) 33–39 37

Table 2. An identical kinetic curve obtained by the comparativeHPLC–DAD method confirms the validity of the developed voltam-metric method.

The moderate photosensitivity could be explained by the char-acteristics of the absorption spectrum of imidacloprid, which ischaracterized by two discrete absorption bands with maxima at212 and 270 nm, of which the latter is stronger and is probablydue to the electronic spectrum of the pyridine part of the com-pound [10]. Thanks to its chromophore, the molecule absorbs radi-ation with the absorbance maximum in the UVC (280–200 nm)range, but the absorption extends far to the UVB (320–280 nm).Since the solar radiation contains also UVB frequencies, there is apossibility of absorbing a certain amount of the energy that candrive photolysis. However, solar energy alone was not efficient en-ough to remove imidacloprid from the aqueous medium, andhence it was necessary to apply a catalyst.

3.3.2. Photocatalytic degradationThe process of solar photocatalytic degradation of imidacloprid

in the presence 1.9% Fe/TiO2 (Fig. 5E and F) and unmodified TiO2

(Fig. 5C and D) was monitored by both the Hg(Ag)FE–SWV andHPLC–DAD methods. To improve the efficiency of the catalyst[10,16], H2O2 was added to the initial solution. It should be notedthat the experiments were also carried out in the presence of H2O2

at pH 2.8 without the catalyst, and the achieved efficiency wasabout 20% and 80% during 5 and 28 days, respectively.

Natural solar irradiation in the presence of TiO2 and H2O2 accel-erated tremendously (by about 100 times) the imidacloprid

38 V. Guzsvány et al. / Journal of Electroanalytical Chemistry 699 (2013) 33–39

degradation compared to the photolysis. Under the given condi-tions, imidacloprid decomposed completely within 5 days, whichis evident from Fig. 5D. The degradation reaction is of pseudo-firstorder, and the determination results agree well with those of thecomparative HPLC–DAD method (Table 2). An important conclu-sion is that TiO2, which is a very effective catalyst in the presenceof UV light [10,16], accelerates also imidacloprid degradation un-der solar irradiation. The obtained high values of kinetic parame-ters can be attributed to the presence of the rutile form of TiO2

[10], and partly to the presence of H2O2 in the system.The imidacloprid degradation in the presence of 1.9% Fe/TiO2

and H2O2 was significantly faster than in the presence of unmodi-fied TiO2 and H2O2, which can be seen from Fig. 5F and Table 2. Asfor the role of the catalyst, it has been reported that the TiO2 sub-strate in the 1.9% Fe/TiO2 catalyst exists in rutile form, and the Fenanoparticles are dominantly located on the outer side of TiO2

particles [16]. Such a system behaves also as a heterogeneousphoto-Fenton system, and the supported �OH generation causesfast degradation of imidacloprid. Hence it can be supposed thatthe degradation reaction rate observed is a result of the simulta-neous combination of heterogeneous and homogeneous catalyticreactions related to TiO2 and photo-Fenton mechanisms. It is wellknown that at an acidic pH, the influence of the semiconductor-based pathway is lower than that of the degradation by thephoto-Fenton reaction [15]. However, above a critical pH, a syner-gistic effect between iron and TiO2 degradation mechanisms is ob-served, increasing thus the overall reaction rate. This could be thereason why the reaction rate in the presence of 1.9% Fe/TiO2 is eventhree times higher than that measured in the presence of unmod-ified TiO2.

Generally, the results of the developed Hg(Ag)FE-SWV methodagree well with those obtained by HPLC–DAD method. The existingsmall deviations (below 3%) can be ascribed to the operationalsteps or to the formation of small amounts of degradation productswith a nitro group. It is known from the literature [21] that suchintermediates are physiologically active to insects, so that the mea-surement of the signal of the reduction of nitro group can be usedto monitor these physiologically important and electroanalyticallyvery similar compounds.

The advantageous results obtained using the Fe-modified TiO2

support our previous opinion [10] that it would be advisable tolook for different iron-based catalysts because of the reasonableexpectation that they could be effective in the visible region, whichmay speak in favor of halogen lamp or natural insolation, com-pared to the expensive mercury lamp, which is also harmful to liv-ing organisms.

4. Conclusions

In summary, the study described above has demonstrated theapplicability of the developed SWV method with the Hg(Ag)FEfor the determination of the neonicotinoid imidacloprid in the Brit-ton–Robinson buffer, pH 7.0. The sensitivity of the procedure wasfound to depend considerably upon the pH of the supporting elec-trolyte and the Hg(Ag)FE condition. A linear response was obtainedin the concentration range of 0.91–47.48, with the LOQ and LOD of0.91 and 0.27 lg cm�3, respectively.

Moreover, the developed voltammetric procedure was alsofound to be effective for the monitoring of the concentrationchanges of imidacloprid during its solar photolytic and TiO2- or1.9% Fe/TiO2-assisted photocatalytic degradation. To the authors’knowledge, the method described herein is the first applicationof the Hg(Ag)FE for the electrochemical monitoring of the photo-degradation process of commercially available and widely used or-ganic pollutants.

Although commonly used HPLC analyses undoubtedly providemore information about the examined system, the voltammetricalternative utilizing the Hg(Ag)FE for detection offers a rapid, sim-ple, and inexpensive tool that may be used for basic screening andcapable of quickly obtaining the actual concentration profile(s) ofthe insecticide(s) in the course of the photodegradation process.

Acknowledgements

This document has been produced with the financial assistanceof the European Union (Project HU-SRB/0901/121/116 OCE-EFPTRWR Optimization of Cost Effective and EnvironmentallyFriendly Procedures for Treatment of Regional Water Resources).The contents of this document are the sole responsibility of theUniversity of Novi Sad Faculty of Sciences and can under no cir-cumstances be regarded as reflecting the position of the EuropeanUnion and or the Managing Authority. Additionally, the partialfinancial support of the Secretariat for Science and TechnologicalDevelopment of AP Vojvodina, Republic of Serbia (Grants No.114-451-02011/2007-02), CEEPUS II (CII-CZ-0212-04-1011) net-work and CEEPUS III network (CIII-CZ-0212-05-1112) areacknowledged.

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