sensors

Article Enhanced Electrochemical Response of Diclofenac at a – Nanofiber Paste Electrode

Sorina Motoc 1, Florica Manea 2,*, Corina Orha 3 and Aniela Pop 2 1 “Coriolan Dragulescu” Institute of Chemistry, Romanian Academy, Mihai Viteazul 24, Timisoara 300223, Romania; [email protected] 2 Department of Applied Chemistry and Engineering of Inorganic Compounds and Environment, Politehnica University of Timisoara, P-ta Victoriei no.2, Timisoara 300006, Romania; [email protected] 3 National Condensed Matter Department, Institute for Research and Development in Electrochemistry and Condensed Matter, Timisoara, 1 P. Andronescu Street, Timisoara 300254, Romania; [email protected] * Correspondence: fl[email protected]; Tel.: +40-256403070

 Received: 8 February 2019; Accepted: 13 March 2019; Published: 17 March 2019 

Abstract: The requirements of the Water Framework Directive to monitor diclofenac (DCF) concentration in surface water impose the need to find advanced fast and simple analysis methods. Direct voltammetric/amperometric methods could represent efficient and practical solutions. Fullerene–carbon nanofibers in paraffin oil as a paste electrode (F–CNF) was easily obtained by simple mixing and tested for DCF detection using voltammetric and amperometric techniques. The lowest limit of detection of 0.9 nM was achieved by applying square-wave voltammetry operated under step potential (SP) of 2 mV, modulation amplitude (MA) of 10 mV, and frequency of 25 Hz, and the best sensitivity was achieved by four-level multiple pulsed amperometry (MPA) that allowed in situ reactivation of the F–CNF electrode. The selection of the method must take into account the environmental quality standard (EQS), imposed through the “watchlist” of the Water Framework Directive as 0.1 µg·L−1 DCF. A good improvement of the electroanalytical parameters for DCF detection on the F–CNF electrode was achieved by applying the preconcentration step for 30 min before the detection step, which assured about 30 times better sensitivity, recommending its application for the monitoring of trace levels of DCF. The electrochemical behavior of F–CNF as a pseudomicroelectrode array makes it suitable for practical application in the in situ and real-time monitoring of DCF concentrations in water.

Keywords: sodium diclofenac; voltammetric/amperometric detection; fullerene–carbon nanofiber paste electrode; real-time water monitoring

1. Introduction The environmental presence of diclofenac, a nonsteroidal anti-inflammatory drug (NSAID), has been found to exhibit toxicological effects on wildlife [1,2], although no direct toxicological effects in human beings have been reported. The environmental quality standard (EQS) for diclofenac that belongs to the “watchlist” of the Water Framework Directive (WFD) was set to 0.1 µg·L−1 in surface waters, and its concentrations will be widely monitored through Europe [3]. The monitoring of DCF concentration in aquatic environments necessitates analytical methods, from which several variants of chromatography are often used [4,5]. Also, for the quantitative determination of DCF, electrochemical methods and sensors have been reported [6–13], as electrochemical methods exhibit great potential for environmental monitoring because of their advantages of saving time, fast response, simplicity, low cost, and the avoidance of sample preparation. The electrode composition confers the electroanalytical performance of any electrochemical detection method. In general, the direct detection of DCF using a bare electrode is appropriate

Sensors 2019, 19, 1332; doi:10.3390/s19061332 www.mdpi.com/journal/sensors Sensors 2019, 19, 1332 2 of 14 only for relatively high concentrations of DCF, because the electrochemical response is poor owing to the low electron transfer during electrochemical oxidation on the electrode surface [6,7]. It is well-known that in order to enhance electrochemical performance, the effective strategy is to design a composite consisting of a highly electrocatalytic material and a substrate with good conductivity [12]. The exceptional electrical, chemical, and mechanical properties of carbon mean they have great potential in sensors applications, especially in composite form. The type of nanostructured carbon depends on the synthesis method that is dictated, as well as its price. Examples of nanostructured carbon materials include carbon nanofibers, nanowires, nanotubes, , nanoclusters, and , etc. Also, fullerene (C60) belongs to the nanostructured carbon class, and its electrocatalytic properties have been reported for various applications, including electrochemical sensors and detection methods [14–16]. The challenge in using nanostructured carbon electrode material for electroanalysis is to obtain inexpensive electrode material characterized by high electrocatalytic performance. The integration of fullerene within a composite electrode should improve the electrocatalytic activity based on its electrochemical behavior as a redox system, due to its remarkable feature of electron-accepting ability [16]. It has been reported that C60 can enhance the electron transfer reaction, provides reproducible catalytic responses, and exhibits chemical stability, which makes it an attractive candidate for electroanalytical applications [17,18]. Carbon nanofibers (CNF) are considered as one class of the appropriate supportive carbon materials due to the large surface-to-volume ratio and excellent electrical conductivity [19]. Also, they are cheaper in comparison with carbon nanotubes due to the synthesis method [20]. The effect of C60 on improving electroanalytical performance in the detection of vinclozolin [21], dopamine [22], and hemoglobin [23] has been reported using -based supportive materials. In our work, the effect of fullerene (C60) on DCF detection using a CNF support is studied. A simple method based on component mixing to obtain an electrode consisting of a paste of fullerene (F) and carbon nanofibers (F–CNF) and investigation of its electrochemical behavior in the presence of sodium diclofenac (DCF) for its electrochemical determination at trace levels in water are described. To our knowledge, no study has been published to date concerning the electroanalytical application of a fullerene—carbon nanofiber paste electrode. Voltammetric and amperometric techniques, i.e., cyclic voltammetry (CV), differential pulsed voltammetry (DPV), square-wave voltammetry (SWV), chronoamperometry (CA), and multiple pulsed amperometry (MPA), were used to develop enhanced and fast electrochemical methods for DCF determination in aqueous solutions.

2. Materials and Methods The composition of the fullerene–carbon nanofiber paste electrode (F–CNF) was obtained by mixing certain amounts of carbon nanofibers, paraffin oil, and fullerene to reach the ratio of 50 wt. % carbon nanofibers, 25 wt. % fullerene, and 25 wt. % paraffin oil. For comparison, a carbon nanofiber paste electrode (CNF) was similarly obtained with the composition of 75 wt. % carbon nanofibers and 25 wt. % paraffin oil. The mass ratio of fullerene, carbon nanofibers, and paraffin oil of 1:2:1 was chosen to assure the sufficient contribution of fullerene and electrode stability. For comparison, the ratio of 3:1 carbon nanofibers to paraffin oil as the carbon nanofiber paste was used. The carbon nanofibers (>98% purity), paraffin oil, and fullerene (C60, 98% purity) were of analytical standard, provided by Sigma Aldrich (Germany). Fourier transform infrared spectroscopy (FTIR) measurements of F–CNF and CNF in paraffin oil paste were obtained on a Vertex 70 spectrometer from Bruker at room temperature in the wavenumber range of 4000–400 cm−1 using transmission technique. The morphological surface characterization of F–CNF in comparison with the simple carbon nanofiber paste electrode (CNF) was studied by a scanning electronic microscope (SEM, Inspect S PANalytical model) coupled with an energy dispersive X-ray analysis detector (EDX). All the electrochemical measurements were performed using an Autolab potentiostat/galvanostat PGSTAT 302 (Eco Chemie, The Netherlands) controlled with GPES 4.9 software using a three-electrode Sensors 2019, 19, x FOR PEER REVIEW 3 of 15

All the electrochemical measurements were performed using an Autolab Sensors 2019, 19, 1332 3 of 14 potentiostat/galvanostat PGSTAT 302 (Eco Chemie, The Netherlands) controlled with GPES 4.9 software using a three-electrode cell, consisting of a F–CNF paste working electrode, a platinum cell,counter consisting electrode, of a F–CNFand a saturated paste working calomel electrode, reference a platinum(SCE) electrode. counter The electrode, F–CNF and paste a saturated electrode calomelwith disc reference geometry (SCE) was electrode. obtained The by F–CNFfilling a paste Teflon electrode mold, withresulting disc geometryin an active was surface obtained with by a fillingdiameter a Teflon of 3 mm. mold, As resultingthe supporting in an activeelectrolyte, surface 0.1 withM sodium a diameter sulfate of at 3pH mm. 5 was As used. the supportingPrior to use, electrolyte,the electrode 0.1 M sodiumwas electrochemically sulfate at pH 5 was stabilized used. Prior through to use, the10 electrodecontinuous was electrochemicallyrepetitive cyclic stabilizedvoltammograms through within 10 continuous the potential repetitive ranging cyclic between voltammograms −0.5 and +1.5 within V/SCE. the potentialNa2SO4 used ranging was betweenanalytical-grade−0.5 and reagent +1.5 V/SCE. from NaMerck,2SO4 andused DCF was was analytical-grade used as received reagent from from Amoli Merck, Organics and DCF Ltd. was All usedsolutions as received were prepared from Amoli with Organics doubly distilled Ltd. All and solutions deionised were water. prepared with doubly distilled and deionisedThe water.electrochemical techniques applied for electrochemical characterization and analytical applicationsThe electrochemical were cyclic voltammetry, techniques applied differential for electrochemical pulsed voltammetry, characterization square-wave and voltammetry, analytical applicationschronoamperometry, were cyclic and voltammetry, multiple pulsed differential amperometry. pulsed voltammetry, square-wave voltammetry, chronoamperometry, and multiple pulsed amperometry. 3. Results and Discussion 3. Results and Discussion 3.1. Structural and Morphological Characterization 3.1. Structural and Morphological Characterization The molecular structure of the fullerene–CNF paraffin oil paste was characterized by FTIR spectroscopyThe molecular (Figure structure 1). In accordance of the fullerene–CNF with the literature paraffin [24], oil the paste peaks was recorded characterized at 1427 cm by−1 FTIR, 1180 −1 spectroscopycm−1, 576 cm (Figure−1, and 5271). Incm accordance−1 corresponded with to the the literature presence [ of24 ],C60 the. The peaks vibrations recorded seen at at 1427 2925 cm cm−,1, −1 −1 −1 11802853 cm cm−1,, 1457 576 cm cm−1,, 1427cm and 527−1, cm and 1428corresponded cm−1 are associated to the presence with differ of Cent60. aliphatic The vibrations CH groups seen (CH at −1 −1 −1 −1 −1 2925and cmCH2 ,bonds), 2853 cm as reported, 1457 cm previously, 1427cm for, carbon and 1428 nanofibers cm are [25]. associated The broad with peak different at 3430 aliphatic cm−1 is CHcharacteristic groups (CH of and O–H CH stretching2 bonds), as from reported inter- previously and intramolecular for carbon hydrogen nanofibers bonds, [25]. The and broad the peakpeaks at at −1 34301652 cm cm−1 isand characteristic 1457 cm−1 are of O–Hcharacteristic stretching of from phenolic inter- resins and intramolecular [26]. hydrogen bonds, and the peaks at 1652 cm−1 and 1457 cm−1 are characteristic of phenolic resins [26].

1.05 a 1.00 0.95 b 0.90 3430 0.85 1176 1378 0.80 576

0.75 1427 b 0.70 1457 527 0.65

0.60 a 0.55 0.50 Transmittance, a.u. 0.45 0.40 2853 0.35 0.30 0.25

0.20 2925 500 1000 1500 2000 2500 3000 3500 4000 Wavenumber cm-1

Figure 1. FTIR spectra of carbon nanofiber (CNF)–paraffin oil paste (a, dotted line) and C60/fullerene

(F)–CNF–paraffinFigure 1. FTIR spectra oil paste of carbon (b, solid nanofiber line). (CNF)–paraffin oil paste (a, dotted line) and C60/fullerene (F)–CNF–paraffin oil paste (b, solid line). The electrode paste composition morphology was studied through SEM and the results are presentedThe inelectrode Figure2 .paste A good composition distribution morphology of both carbon was nanofibers studied through and fullerene SEM inand oil the paraffin results was are assured,presented and in a Figure randomized 2. A good arrangement distribution of both of both carbon carbon nanofibers nanofibers and fullerene and fullerene resulted. in oil paraffin was assured, and a randomized arrangement of both carbon nanofibers and fullerene resulted. 3.2. Cyclic Voltammetry Cyclic voltammetry using the classical potassium ferri/ferrocyanide redox system was used for the determination of the electroactive area of the fullerene–CNF–paraffin oil paste electrode. Cyclic voltammetry (CV) of the supporting electrolyte consisting of 4 mM K3[Fe(CN)6] in 1 M KNO3 was recorded at different scan rates (results not shown here), and the diffusion coefficient was determined as 10.86 × 10−6 cm2·s−1 according to the Randles–Sevcik Equation (1):

5 1/2 3/2 1/2 Ip = 2.69 × 10 AD n v C (1)

Sensors 2019, 19, 1332 4 of 14 where A represents the area of the electrode (cm2), n is the number of electrons participating in the reaction (and is equal to 1), D is the diffusion coefficient of the in solution, C is the concentration of the probe molecule in the solution and is 4 mM, and v is the scan rate (V·s−1); the linear dependence between peak current and the square root of the scan rate was achieved. Taking into account the theoretical diffusion coefficient value of 6.7 × 10−6 cm2·s−1 found in the literature data [27], the value of the electroactive electrode area was determined to be 0.249 cm2 versus the value 2 ofSensors the electrode2019, 19, x FOR geometric PEER REVIEW areaof 0.196 cm . 4 of 15

(b) (a)

Figure 2. SEM images of (a) CNF–paraffin oil paste and (b) F–CNF–paraffin oil paste. Figure 2. SEM images of (a) CNF–paraffin oil paste and (b) F–CNF–paraffin oil paste. The electrochemical behavior of DCF on both paste electrodes was investigated by cyclic 3.2. Cyclic Voltammetry voltammetry (CV) in a supporting electrolyte of 0.1 M Na2SO4. No peak corresponding to the DCF oxidationCyclic appeared voltammetry for the carbon using the nanofiber classical paste potassium electrode ferri/ferrocyanide without fullerene redox content system (results was used not shownfor the here).determination This should of the be explainedelectroactive by thearea absence of the offullerene–CNF–paraffin an electrocatalytic effect oil ofpaste CNF electrode. towards DCFCyclic electrooxidation, voltammetry (CV) or of by the a large support backgrounding electrolyte current consisting recorded of on4 mM the K simple3[Fe(CN) carbon6] in 1 nanofiber M KNO3 pastewas recorded electrode at increasing different atscan each rates usage (results and no thust shown overlapping here), and the electrochemicalthe diffusion coefficient response was for DCFdetermined electrooxidation. as 10.86 × 10 The−6 cm latter2·s−1 according would also to give the Randles–Sevcik information about Equation the instability (1): of the electrode composition. CV series recorded on the F–CNF paste electrode at various DCF concentrations are =×5 1/2 3/2 1/2 IADnvCp 2.69 10 (1) presented in Figure3, and an anodic peak corresponding to DCF oxidation is evidenced at the potential valuewhere of Aabout represents +0.75 the V/SCE area (peakof the II).electrode It must (cm be2 noticed), n is the that number the CV of shape electrons showed participating the presence in the of anodicreaction and (and corresponding is equal to cathodic1), D is peaksthe diffusion (Ia and Ib)coefficient characteristics of the inmolecule the carbon in redoxsolution, system C is[ 28the]. Theconcentration first stage inof thethe overallprobe molecule oxidation in of the DCF solution is given and by is the 4 DCFmM, sorptionand v is ontothe scan the carbonrate (V·s surface,−1); the evidencedlinear dependence by the diminution between peak of the current anodic and peak the of squa carbonre root oxidation, of the scan followed rate was by achieved. the second Taking stage ofinto the account DCF oxidation the theoretical process. diffusion It has beencoefficient reported value that of the6.7 electrochemical× 10−6 cm2·s−1 found oxidation in the literature of diclofenac data involves[27], the value a one-electron of the electroactive electrochemical-chemical electrode area was (EC) determined mechanism to followed be 0.249 cm by2 a versus chemical the reactionvalue of inthe which electrode 2,6 dichloroanilinegeometric area of and 0.196 2-(2-hydroxyprop-2-phenyl) cm2. acid acetic are formed [10]. A linear dependenceThe electrochemical between the useful behavior anodic of peakDCF currenton both and paste DCF electrodes concentration was is investigated noticeable in by the cyclic inset ofvoltammetry Figure3. (CV) in a supporting electrolyte of 0.1 M Na2SO4. No peak corresponding to the DCF oxidationSome appeared mechanistic for aspects the carbon related nanofiber to the electrooxidation paste electrode of without DCF on thefulleren F–CNFe content paste electrode (results cannot −1 beshown discussed here).through This should the study be explained of the scan by ratethe abse influence.nce of CVsan electrocatalytic recorded in the effect presence of CNF of 5 towards mg·L −1 DCFDCF andelectrooxidation, 0.1 M Na2SO 4orsupporting by a large electrolytebackground at current the scan recorded rates ranging on the from simple 0.01 carbon to 0.2 Vnanofiber·s are presentedpaste electrode in Figure increasing4. From at theseeach usage CVs, itand can thus be noticedoverlapping that thethe dependenceelectrochemical between response the for anodic DCF peakelectrooxidation. current and theThe square latter rootwould of thealso scan give rate info is notrmation linear about (see insetthe ofinstability Figure4 ),of which the electrode denotes acomposition. nonlinear diffusion-controlled CV series recorded oxidationon the F–CNF process. paste This electrode should at be various explained DCF by concentrations the fact that theare F–CNFpresented paste in electrodeFigure 3, can and work an anodic as a pseudomicroelectrode peak corresponding array, to DCF which oxidation is characterized is evidenced by spherical at the potential value of about +0.75 V/SCE (peak II). It must be noticed that the CV shape showed the presence of anodic and corresponding cathodic peaks (Ia and Ib) characteristics in the carbon redox system [28]. The first stage in the overall oxidation of DCF is given by the DCF sorption onto the carbon surface, evidenced by the diminution of the anodic peak of carbon oxidation, followed by the second stage of the DCF oxidation process. It has been reported that the electrochemical oxidation of diclofenac involves a one-electron electrochemical-chemical (EC) mechanism followed by a chemical reaction in which 2,6 dichloroaniline and 2-(2-hydroxyprop-2-phenyl) acid acetic are formed [10]. A linear dependence between the useful anodic peak current and DCF concentration is noticeable in the inset of Figure 3.

Sensors 2019, 19, 1332 5 of 14 and nonlinear diffusion patterns that are specific to macroelectrodes. Also, surface-controlled or complex processes should influence the linearity of the dependence between the anodic peak current and the square root of the scan rate. A complex process involving fullerene’s availability to act as a multiple electron acceptor [16] in DCF oxidation should be considered to explain nonlinear diffusion. The irreversible characteristic of the overall oxidation process is evidenced by the lack of the cathodic peak correspondingSensors 2019, 19, x to FOR the PEER anodic REVIEW DCF oxidation, and also through the dependence of the oxidation5 of 15 potential value and the logarithm of the scan rate.

1.5x10-4

1.0x10-4

5.0x10-5 8 Ia II 1 0.0

I/ A Ib 14 y= -0.3095 + 0.6425x; 2 -5 12 R =0.9528

-5.0x10 10

8 A

μ 6 I/

-4 Δ -1.0x10 4

2

0

-4 -2 0 5 10 15 20 25 -1.5x10 Conc / μM DCF

-0.5 0.0 0.5 1.0 1.5 Potential applied (V/SCE)

FigureSensors 3. Cyclic 2019, 19 voltammograms, x FOR PEER REVIEW recorded at the F–CNF paste electrode in 0.1 M Na2SO4 supporting6 of 15 −1 electrolyteFigure (curve 3. Cyclic 1) and voltammograms in the presence recorded of various at the DCF F–CNF concentrations: paste electrode curves in 0.1 2–8:M Na 1–72SO mg4 supporting·L DCF. Inset: Calibrationelectrolyte plots(curve of 1) the and currents in the presence versus of DCF various concentrations DCF concentrat at potentialions: curves value 2–8: of1–7 +0.75 mg·L V/SCE.−1 DCF. Inset: Calibration plots of the currents versus DCF concentrations at potential value of +0.75 V/SCE. 3.5x10-4 16 -4 Some mechanistic3.0x10 aspects14 related to the electrooxidation of DCF on the F–CNF paste electrode 12 2.5x10-4 can be discussed through the10 study of the scan rate influence. CVs recorded in the presence of 5

-4 A −1 2.0x10 2 μ 8 4 −1 mg·L DCF and 0.1 M Na SOI / supporting electrolyte at the scan rates ranging from 0.01 to 0.2 V·s Δ 6 are presented in1.5x10 Figure-4 4. From these CVs, it can be noticed that the dependence8 between the anodic 4 -4 peak current and1.0x10 the square 2root of the scan rate is not linear (see inset of Figure 4), which denotes a -5 0.10 0.15 0.20 0.25 0.30 0.35 nonlinear diffusion-controlled5.0x10 oxidationv 1/2 (Vs -1process.)1/2 This should be explained1 by the fact that the F– CNF paste electrode0.0 can work as a pseudomicroelectrode array, which is characterized by spherical and nonlinearI/ A -5.0x10diffusion-5 patterns that are specific to macroelectrodes. Also, surface-controlled or

0.86 complex processes-1.0x10 should-4 influence the linearity of the dependence between the anodic peak current 0.84 and the square -1.5x10root of-4 the scan rate. A complex process involving fullerene’s availability to act as a 0.82 -4 multiple electron-2.0x10 acceptor [16] in DCF oxidation should0.80 be considered to explain nonlinear

-4 / V E diffusion. The irreversible-2.5x10 characteristic of the overall oxidation0.78 process is evidenced by the lack of the cathodic peak-3.0x10 corresponding-4 to the anodic DCF oxidation,0.76 and also through the dependence of -4 0.74 the oxidation potential-3.5x10 value and the logarithm of the scan-2.0 rate. -1.8 -1.6 -1.4 -1.2 -1.0 log (v / Vs-1) -4 -4.0x10 -0.5 0.0 0.5 1.0 1.5 Potential applied (V/SCE)

Figure 4. Cyclic voltammograms recorded at the F–CNF paste electrode in 5 mg·L−1 DCF and 0.1 M

Na2SO4 supporting electrolyte at various scan rates: (1) 10, (2) 20, (3) 30, (4) 40, (5) 50, (6) 75, (7) 100, Figure 4. Cyclic voltammograms recorded at the F–CNF paste electrode in 5 mg·L−1 DCF and 0.1 M and (8) 200 m·Vs−1. Insets: upper: dependence of anodic peak current vs. square root of the scan rate; Na2SO4 supporting electrolyte at various scan rates: (1) 10, (2) 20, (3) 30, (4) 40, (5) 50, (6) 75, (7) 100, lower: dependence of peak potential vs. logarithm of the scan rate. and (8) 200 m·Vs−1. Insets: upper: dependence of anodic peak current vs. square root of the scan rate; lower: dependence of peak potential vs. logarithm of the scan rate.

3.3. Analytical Applications In order to develop the electroanalytical methods for the determination of DCF, two approaches were considered. The first one considered differential pulsed and square-wave voltammetries (DPV

and SWV) for enhancing the sensitivity and the lowest limit of detection (LOD) of DCF. The second considered the chronoamperometry and multiple pulsed amperometry (CA and MPA), being the simplest and fastest electrochemical methods for DCF determination.

3.3.1. DPV and SWV Both voltammetric techniques were applied under optimized operating conditions applied for the electrochemical determination of DCF on a boron-doped (BDD) electrode as reported in our previous work [7]. DPV technique was applied at an SP of 25 mV, an MA of 100 mV, and at the scan rate of 0.05 V·s−1 and differential pulsed voltammograms are shown in Figure 5. A good linearity between the anodic peak current recorded at +0.75 V/SCE and DCF concentration was reached (see inset of Figure 5). It must be mentioned that more than tenfold higher sensitivity was achieved using the F–CNF paste electrode in comparison with the BDD electrode operated under the same conditions [7]. A slight enhancement in sensitivity was achieved using DPV under these operating conditions. However, about six times lower LOD and respective limit of quantification (LOQ) were obtained, which proved the DPV to be superior in comparison with CV. Also, the SWV technique was tested under similarly optimized operating conditions reported by our group for the BDD electrode [7], and the results are presented in Figure 6. A larger DCF concentration range was detected using SWV operated under SP of 2 mV, MA of 10 mV, and frequency of 25 Hz, and a good linearity was reached, as can be seen in the inset of Figure 6. About two times better electroanalytical parameters were reached in comparison with the results of DPV.

Sensors 2019, 19, 1332 6 of 14

3.3. Analytical Applications In order to develop the electroanalytical methods for the determination of DCF, two approaches were considered. The first one considered differential pulsed and square-wave voltammetries (DPV and SWV) for enhancing the sensitivity and the lowest limit of detection (LOD) of DCF. The second considered the chronoamperometry and multiple pulsed amperometry (CA and MPA), being the simplest and fastest electrochemical methods for DCF determination.

3.3.1. DPV and SWV Both voltammetric techniques were applied under optimized operating conditions applied for the electrochemical determination of DCF on a boron-doped diamond (BDD) electrode as reported in our previous work [7]. DPV technique was applied at an SP of 25 mV, an MA of 100 mV, and at the scan rate of 0.05 V·s−1 and differential pulsed voltammograms are shown in Figure5. A good linearity between the anodic peak current recorded at +0.75 V/SCE and DCF concentration was reached (see inset of Figure5). It must be mentioned that more than tenfold higher sensitivity was achieved using the F–CNF paste electrode in comparison with the BDD electrode operated under the same conditions [7]. A slight enhancement in sensitivity was achieved using DPV under these operating conditions. However, about six times lower LOD and respective limit of quantification (LOQ) were obtained, which proved the DPV to be superior in comparison with CV. Also, the SWV technique was tested under similarly optimized operating conditions reported by our group for the BDD electrode [7], and the results are presented in Figure6. A larger DCF concentrationSensors 2019 range, 19, x FOR was PEER detected REVIEW using SWV operated under SP of 2 mV, MA of 10 mV, and frequency7 of 15 of 25 Hz, and a good linearity was reached, as can be seen in the inset of Figure6. About two times better electroanalytical parameters were reached in comparison with the results of DPV.

-6 3.5x10 2.5 y=0.1986 + 0.6891x; 10 2 2.0 R =0.992 3.0x10-6

1.5

-6 A

2.5x10 μ I/ 1.0 Δ

-6 2.0x10 0.5

-6 0.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

I/ A 1.5x10 DCF concentration / μM 1.0x10-6

5.0x10-7 1

0.0

-5.0x10-7

0.0 0.2 0.4 0.6 0.8 1.0 1.2 Potential applied (V/SCE)

Figure 5. Differential pulsed voltammograms recorded at the F–CNF paste electrode in 0.1 M Na2SO4

supportingFigure electrolyte5. Differential (curve pulsed 1) andvoltammograms in the presence recorded of various at the DCFF–CNF concentrations: paste electrode curves in 0.1 2–10:M Na2SO4 −1 0.1–0.9supporting mg·L DCF electrolyte; step potential(curve 1) and (SP) in 25 the mV; presen modulationce of various amplitude DCF concentrations: (MA) 100 mV; curves potential 2–10: 0.1– range:0.9 0 mg·L to +1.2−1 DCF; V/SCE. step Inset: potential Calibration (SP) 25 plotsmV; modulation of the currents amplitude recorded (MA) at E 100 = +0.75 mV; potential V/SCE versus range: 0 to DCF concentrations.+1.2 V/SCE. Inset: Calibration plots of the currents recorded at E = +0.75 V/SCE versus DCF concentrations.

3.0x10-6 1.8 y= 0.0543 + 1.0752x; 11 1.6 R2=0.955 1.4

1.2

1.0 A

μ 0.8

-6 I/ 2.0x10 Δ 0.6 0.4 0.2 1 0.0

-0.2 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 DCF concentration / μM I/A 1.0x10-6

0.0

0.00.20.40.60.81.01.2 Potential applied (V/SCE)

Figure 6. Square-wave voltammograms recorded at the F–CNF paste electrode in 0.1 M Na2SO4 supporting electrolyte (curve 1) and in the presence of various DCF concentrations: curves 2–6: 0.02– 0.1 mg·L−1 DCF and curves 7–11: 0.2–0.6 mg·L−1 DCF; step potential (SP) 2 mV; modulation amplitude

Sensors 2019, 19, x FOR PEER REVIEW 7 of 15

-6 3.5x10 2.5 y=0.1986 + 0.6891x; 10 2 2.0 R =0.992 3.0x10-6

1.5

-6 A

2.5x10 μ I/ 1.0 Δ

-6 2.0x10 0.5

-6 0.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

I/ A 1.5x10 DCF concentration / μM 1.0x10-6

5.0x10-7 1

0.0

-5.0x10-7

0.0 0.2 0.4 0.6 0.8 1.0 1.2 Potential applied (V/SCE)

Figure 5. Differential pulsed voltammograms recorded at the F–CNF paste electrode in 0.1 M Na2SO4 supporting electrolyte (curve 1) and in the presence of various DCF concentrations: curves 2–10: 0.1– 0.9 mg·L−1 DCF; step potential (SP) 25 mV; modulation amplitude (MA) 100 mV; potential range: 0 to +1.2 V/SCE. Inset: Calibration plots of the currents recorded at E = +0.75 V/SCE versus DCF concentrations. Sensors 2019, 19, 1332 7 of 14

3.0x10-6 1.8 y= 0.0543 + 1.0752x; 11 1.6 R2=0.955 1.4

1.2

1.0 A

μ 0.8

-6 I/ 2.0x10 Δ 0.6 0.4 0.2 1 0.0

-0.2 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 DCF concentration / μM I/A 1.0x10-6

0.0

0.00.20.40.60.81.01.2 Potential applied (V/SCE)

Figure 6. Square-wave voltammograms recorded at the F–CNF paste electrode in 0.1 M Na2SO4 Sensors 2019, 19, x FOR PEER REVIEW 8 of 15 supportingFigure 6. electrolyte Square-wave (curve voltammograms 1) and in the presencerecorded ofat variousthe F–CNF DCF paste concentrations: electrode in curves 0.1 M 2–6:Na2SO4 0.02–0.1supporting mg·L−1 electrolyteDCF and curves(curve 7–11:1) and0.2–0.6 in the presen mg·L−ce1 DCF;of various step DCF potential concentrations: (SP) 2 mV; curves modulation 2–6: 0.02– (MA) 10 mV; potential range: 0 to +1.2 V/SCE. Inset: Calibration plots of the currents recorded at E = amplitude (MA)−1 10 mV; potential range: 0 to +1.2−1 V/SCE. Inset: Calibration plots of the currents +0.750.1 V/SCEmg·L versusDCF and DCF curves concentrations. 7–11: 0.2–0.6 mg·L DCF; step potential (SP) 2 mV; modulation amplitude recorded at E = +0.75 V/SCE versus DCF concentrations. Regarding the sorption properties of fullerene for DCF, this inconvenience should be exploited Regarding the sorption properties of fullerene for DCF, this inconvenience should be exploited in in a positive way to improve the electroanalytical parameters of both sensitivity and, especially, a positive way to improve the electroanalytical parameters of both sensitivity and, especially, LOD by LOD by inclusion of the preconcentration step before the detection step applied for low DCF inclusionconcentrations. of the preconcentration In the preconcentration step before step, thethe detectionF–CNF paste step electrode applied is for immersed low DCF in concentrations. supporting Inelectrolyte the preconcentration containing low step, DCF the concentrations F–CNF paste at electrode the open iscircuit immersed potential in supportingfor a certain electrolytetime to containingassure its low sorption DCF concentrationsonto the electrode at the surface. open A circuit maximum potential preconcentration for a certain time factor to of assure about its 28 sorption was ontofound the at electrode 30 min (see surface. Figure A 7). maximum A longerpreconcentration sorption time led factorto diminution of about of 28 the was preconcentration found at 30 min (seefactor, Figure probably7). A longer due to sorption the fouling time effect led to starting diminution to manifest. of the preconcentration It is clear that this factor, preconcentration– probably due to thedetection fouling effectscheme starting necessities to manifest. a longer It time is clear for DCF that thisdetermination, preconcentration–detection but this is nevertheless scheme useful necessities for a longertrace concentration time for DCF levels determination, of DCF. but this is nevertheless useful for trace concentration levels of DCF.

1.0 30 Preconcentration factor Preconcentration 0.8 25

0.6 20 A

μ 15

I/ 0.4 Δ

10

0.2 5

0.0 0

0 1020304050 Sorption time/ min

Figure 7. Useful signals reached by square-wave voltammetry (SWV) recorded in the presence of −1 0.005Figure mg ·7.L UsefulDCF signals containing reached 0.1 Mby Nasquare-wave2SO4 supporting voltammetry electrolyte (SWV) at recorded F–CNF paste in the electrodes, presence of as a function0.005 mg·L of the−1 DCF sorption containing time in 0.1 the M preconcentration Na2SO4 supporting step electrolyte prior to detection. at F–CNF paste electrodes, as a function of the sorption time in the preconcentration step prior to detection.

3.3.2. CA and MPA Considering the simple, easy, and fast attributes of CA and MPA, various amperometric schemes were tested to obtain very good electroanalytical parameters. Conventional CA tested at a single level of the potential value of +1 V/SCE, which is higher than a value recommended by the CV through the DCF oxidation peak (+0.75 V/SCE), allowed us to reach CAs recorded at various DCF concentrations presented in Figure 8. Lower sensitivity was reached by CA, probably due to the fouling effect of the electrode surface.

Sensors 2019, 19, 1332 8 of 14

3.3.2. CA and MPA Considering the simple, easy, and fast attributes of CA and MPA, various amperometric schemes were tested to obtain very good electroanalytical parameters. Conventional CA tested at a single level of the potential value of +1 V/SCE, which is higher than a value recommended by the CV through the DCFSensors oxidation 2019, 19 peak, x FOR (+0.75 PEER V/SCE), REVIEW allowed us to reach CAs recorded at various DCF concentrations9 of 15 presented in Figure8. Lower sensitivity was reached by CA, probably due to the fouling effect of the electrode surface.

3.0x10-5

1.8 -5 2.5x10 1.6 y=0.05636 + 0.07733x; 2 1.4 R =0.990

1.2 -5 2.0x10 1.0 A

μ 0.8 I /

Δ 0.6 -5 I/ A I/ 1.5x10 0.4

0.2

0.0 -5 1.0x10 -0.2 0 5 10 15 20 DCF concentration/ μM

5.0x10-6 7

0.0 1 0 1020304050 Time / s

Figure 8. Chronoamperograms (CAs) recorded for a single level of the detection potential of +1 V/SCE at theFigure F–CNF 8. pasteChronoamperograms electrode in 0.1 M (CAs) Na2SO recorded4 supporting for a electrolytesingle level (curve of the 1) detection and in the potential presence of +1 of various DCF concentrations: curves 2–7: 1–6 mg·L−1 DCF. Inset: Calibration plots of the currents V/SCE at the F–CNF paste electrode in 0.1 M Na2SO4 supporting electrolyte (curve 1) and in the versuspresence DCF concentrations. of various DCF concentrations: curves 2–7: 1–6 mg·L−1 DCF. Inset: Calibration plots of the currents versus DCF concentrations. To assure the cathodic activation of the F–CNF paste electrode, CAs operating at the two potentials of +1 V/SCE and −0.3 V/SCE were applied, and the results are shown in Figure9. The cathodic To assure the cathodic activation of the F–CNF paste electrode, CAs operating at the two potential belongs to the hydrogen evolution potential range, being selected in accord with CV. It can be potentials of +1 V/SCE and −0.3 V/SCE were applied, and the results are shown in Figure 9. The noticed that a decreasing cathodic current occurs with DCF concentration increasing, which suggested cathodic potential belongs to the hydrogen evolution potential range, being selected in accord with that this simple procedure did not allow the activation of the F–CNF paste electrode at this medium CV. It can be noticed that a decreasing cathodic current occurs with DCF concentration increasing, cathodic potential value and no enhanced response was reached under these operating conditions which suggested that this simple procedure did not allow the activation of the F–CNF paste (see inset of Figure9). electrode at this medium cathodic potential value and no enhanced response was reached under Under these circumstances of involving sorption processes in the anodic oxidation and detection these operating conditions (see inset of Figure 9). of DCF on the F–CNF paste electrode, which was reflected also in the low sensitivity when using chronoamperometry with one and two levels of potential, multiple pulsed amperometry (MPA) was tested under several strategies3x10-5 in order to improve the electronalaytical parameters for DCF detection.

2 It is well-known that pulsed amperometric detection involves y1= 0.03875 in + situ 0.0809x;R cleaning=0.9888 and reactivation of the 2.0 2 y2= 0.06783 + 0.02137x; R =0.975 electrode surface during the-5 electrodetection process [29]. The responses of MPA corresponded to 2x10 1.5 E1 each potential pulse applied for a short duration, combining the anodic and cathodic polarization and

M 1.0 μ I/ avoiding the fouling effect on the electrode surface. TheΔ first variant of MPA applied consisted of the -5 1x10 0.5 E2 application of similar potentials with two levels of CA.8 By continuously applying the same potential values for the short duration of 0.05 s per pulse, the0.0 amperograms recorded at the F–CNF paste 0 1 0 5 10 15 20 25 electrode at −0.3 V/SCEI / A and +1 V/SCE are shown in Figure 10DCF. Enhanced concentration/ μM sensitivities were reached E1 for MPA in comparison with CA for both anodic and cathodic potential pulses (see inset of Figure 10). -5 The short durations of-1x10 pulse application impeded the sorption and fouling effect8 manifestation, which

-5 -2x10 1 E2 -3x10-5

0 20406080100 Time / s

Sensors 2019, 19, x FOR PEER REVIEW 9 of 15

3.0x10-5

1.8 -5 2.5x10 1.6 y=0.05636 + 0.07733x; 2 1.4 R =0.990

1.2 -5 2.0x10 1.0 A

μ 0.8 I /

Δ 0.6 -5 I/ A I/ 1.5x10 0.4

0.2

0.0 -5 1.0x10 -0.2 0 5 10 15 20 DCF concentration/ μM

5.0x10-6 7

0.0 1 0 1020304050 Time / s

Figure 8. Chronoamperograms (CAs) recorded for a single level of the detection potential of +1 V/SCE at the F–CNF paste electrode in 0.1 M Na2SO4 supporting electrolyte (curve 1) and in the presence of various DCF concentrations: curves 2–7: 1–6 mg·L−1 DCF. Inset: Calibration plots of the currents versus DCF concentrations.

To assure the cathodic activation of the F–CNF paste electrode, CAs operating at the two Sensors 2019, 19, 1332 9 of 14 potentialsSensors 2019 , of19, +1x FOR V/SCE PEER REVIEWand −0.3 V/SCE were applied, and the results are shown in Figure 109. ofThe 15 cathodic potential belongs to the hydrogen evolution potential range, being selected in accord with led toCV. the ItFigure bettercan be 9. efficiency noticedChronoamperograms that of DCFa decreasing detection. (CAs) cathodic recorded Ten times current for highertwo occurs levels sensitivity withof detection DCF at theconcentration potential, potential namely value increasing, +1 of +1 V/SCEwhichV/SCE wassuggested and achieved; activation that also,this potential thesimple cathodic of procedure−0.3 currentV/SCE (E2),di increasedd notat the allow F–CNF linearly the paste activation with electrode DCF, of increasing in the0.1 MF–CNF Na at2SO the paste4 potentialelectrodesupporting value at of this− electrolyte0.3 medium V/SCE, (curvecathodic and 1) a commendableand potential in the pres valueence sensitivity and of various no wasenhanced DCF reached, concentrations: response beingmarkedly was curves reached 2− higher8: 1 −under7 mg L−1 DCF. Inset: Calibration plots of the currents versus DCF concentrations at both potential thanthese that recordedoperating at conditions the anodic (see part. inset The of cathodic Figure 9). part assured the very good in situ reactivation of values (E1 and E2). the electrode surface.

Under these3x10 circumstances-5 of involving sorption processes in the anodic oxidation and

2 detection of DCF on the F–CNF paste electrode, which y1=was 0.03875 reflected+ 0.0809x;R =0.9888 also in the low sensitivity when 2.0 2 y2= 0.06783 + 0.02137x; R =0.975 using chronoamperometry-5 with one and two levels of potential, multiple pulsed amperometry 2x10 1.5 E1 (MPA) was tested under several strategies in order to improve the electronalaytical parameters for

M 1.0 μ I/

DCF detection. It is well-known that pulsed amperometricΔ detection involves in situ cleaning and -5 1x10 0.5 E2 reactivation of the electrode surface during the el8 ectrodetection process [29]. The responses of MPA corresponded to each potential pulse applied for0.0 a short duration, combining the anodic and 0 1 0 5 10 15 20 25 cathodic polarizationI / A and avoiding the fouling effect on DCFthe concentration/ electrode μM surface. The first variant of E1 MPA applied consisted of the application of similar potentials with two levels of CA. By -5 continuously applying-1x10 the same potential values for the short duration8 of 0.05 s per pulse, the amperograms recorded at the F–CNF paste electrode at −0.3 V/SCE and +1 V/SCE are shown in Figure 10. Enhanced sensitivities-5 were reached for MPA in comparison with CA for both anodic and -2x10 1 cathodic potential pulses (see inset of Figure 10). The short durationsE2 of pulse application impeded the sorption and fouling-5 effect manifestation, which led to the better efficiency of DCF detection. Ten -3x10 times higher sensitivity at the potential value of +1 V/SCE was achieved; also, the cathodic current increased linearly with0 DCF, increasing 20406080100 at the potential value of –0.3 V/SCE, and a commendable sensitivity was reached, being markedly higherTime than / sthat recorded at the anodic part. The cathodic part assured the very good in situ reactivation of the electrode surface.

Figure 9. Chronoamperograms (CAs) recorded for two levels of detection potential, namely +1 V/SCE

and activation potential of −0.3 V/SCE (E2), at the F–CNF paste electrode in 0.1 M Na2SO4 supporting electrolyte (curve 1) and in the presence of various DCF concentrations: curves 2−8: 1−7 mg L−1 DCF. Inset: Calibration plots of the currents versus DCF concentrations at both potential values (E1 and E2).

E2

4.0x10-4

200 y= 22.9333 + 6.4509x; R2=0.977 150

100 E1 I / A A μ I/

Δ 50 -4.5x10-4 E2 y= 5.08889 + 0.91667x; 0 2 -4 R =0.964 -5.0x10 0 5 10 15 20 25 30 E1 DCF concentration/ μM -5.5x10-4

-6.0x10-4

-6.5x10-4 0 100 200 300 400 500 time / s

Figure 10. Multiple pulsed amperograms (MPAs) recorded for two levels of the potential pulses − of +1Figure V/SCE 10.for Multiple 0.05 s (E1)pulsed and amperograms activation potential (MPAs) recorded of 0.3 V/SCE for two forlevels 0.05 of sthe (E2) potential at the F–CNFpulses of +1 pasteV/SCE electrode for in0.05 0.1 s M (E1) Na 2andSO4 activationsupporting potential electrolyte of (curve–0.3 V/SCE 1) and for in the0.05 presence s (E2) at of the various F–CNF DCF paste concentrations: 1–7 mg·L−1 DCF. Inset: Calibration plots of the currents versus DCF concentrations at electrode in 0.1 M Na2SO4 supporting electrolyte (curve 1) and in the presence of various DCF bothconcentrations: potential pulse 1–7 values mg·L (E1−1 andDCF. E2). Inset: Calibration plots of the currents versus DCF concentrations at both potential pulse values (E1 and E2).

Sensors 2019, 19, x FOR PEER REVIEW 11 of 15 Sensors 2019, 19, 1332 10 of 14 Another strategy was considered for MPA, applying it in according with the CV shape as the Anotherreference, strategy involving was the considered redox system for MPA, of fulleren applyinge that it in should according act withas an the electrocatalyst CV shape as thein DCF reference,oxidation involving and detection, the redox systemand the of amperograms fullerene that are should shown act in as Figure an electrocatalyst 11. in DCF oxidation and detection,The pulses and were the applied amperograms continuously are shown using in the Figure following 11. scheme: The pulses were applied continuously using the following scheme: 1. +0.3 V/SCE for a duration of 100 ms, where fullerene is in the reduced form; 1. 2.+0.3 +0.5 V/SCE V/SCE for afor duration a duration of 100 of 100 ms, ms, where where fullerene reduced is in fullerene the reduced is oxidized; form; 2. 3.+0.5 -0.3 V/SCE V/SCE for afor duration a duration of 100 of ms, 100 where ms, reducedwhere H fullerene2 evolution is oxidized; occurs alongside other reduction processes; 3. −0.3 V/SCE for a duration of 100 ms, where H2 evolution occurs alongside other 4.reduction +1 V/SCE processes; for a duration of 50 ms, considering the detection potential that corresponded to DCF 4. +1 V/SCEoxidation. for a duration of 50 ms, considering the detection potential that corresponded to DCF oxidation.

0.0008 E4

0.0006

120 y=0.93333 + 4.31863x; 2 100 R =0.997

0.0004 80 E3

A 60 μ

I / E4 Δ 40 I / A

20 0.0002 y= -12.06667 + 4x; 2 0 R =0.983

0 5 10 15 20 25 30 E2 DCF concentration/ μM 0.0000 E1

-0.0002 E3

0 100 200 300 400 500 time/ s Figure 11. Multiple pulsed amperograms (MPAs) recorded for four levels of the potential pulses of +0.3 V/SCE for 0.1 s (E1), +0.5 V/SCE for 0.1 s (E2), −0.3 V/SCE for 0.1 s (E3), and +1 V/SCE for Figure 11. Multiple pulsed amperograms (MPAs) recorded for four levels of the potential pulses of 0.05 s (E4) at the F–CNF paste electrode in 0.1 M Na2SO4 supporting electrolyte and in the presence +0.3 V/SCE for 0.1 s (E1), +0.5 V/SCE for 0.1 s (E2), −0.3 V/SCE for 0.1 s (E3), and +1 V/SCE for 0.05 s of various DCF concentrations: 1–7 mg·L–1 DCF. Inset: Calibration plots of the currents versus DCF (E4) at the F–CNF paste electrode in 0.1 M Na2SO4 supporting electrolyte and in the presence of concentrations at both E3 and E4 potential pulses. various DCF concentrations: 1–7 mg·L–1 DCF. Inset: Calibration plots of the currents versus DCF It isconcentrations obvious that at the both better E3 and electroanalytical E4 potential pulses. parameters were achieved using MPA under the operating conditions presented above. The currents recorded at E1 and E2 potential values did not varyIt is linearlyobvious with that DCFthe better concentration electroanalytical due tofullerene-related parameters were surface achieved processes using MPA occurring, under the operating conditions presented above. The currents recorded at E1 and E2 potential values did not which significantly influenced the DCF oxidation process and, implicitly, the detection sensitivity due vary linearly with DCF concentration due to fullerene-related surface processes occurring, which to the fact that the reduced fullerene can act as an efficient electron mediator for DCF oxidation, leading significantly influenced the DCF oxidation process and, implicitly, the detection sensitivity due to to the considerable enhancement of the analytical sensitivity [14]. About four times better sensitivity the fact that the reduced fullerene can act as an efficient electron mediator for DCF oxidation, at the detection potential of +1 V/SCE was achieved by integration of both E1 and E2 potential pulses leading to the considerable enhancement of the analytical sensitivity [14]. About four times better within the MPA-based detection strategy in comparison with two-level MPA-based detection strategy. sensitivity at the detection potential of +1 V/SCE was achieved by integration of both E1 and E2 All electroanalytical parameters determined for each electrochemical technique and detection scheme potential pulses within the MPA-based detection strategy in comparison with two-level MPA-based are summarized in Table1. detection strategy. All electroanalytical parameters determined for each electrochemical technique and detection scheme are summarized in Table 1. Table 1. Electroanalytical parameters for DCF detection on the F–CNF paste electrode.

Sensors 2019, 19, 1332 11 of 14

Table 1. Electroanalytical parameters for DCF detection on the F–CNF paste electrode.

Conditions, Sensitivity Correlation LOD a LQ b RSD c Technique E/V vs. SCE µA·µM−1 Coefficient (R2) (µM) (µM) (%) CV +0.75 0.642 0.952 0.0568 0.1893 0.1531 DPV +0.77 0.689 0.992 0.0102 0.0341 1.0097 SWV +0.75 1.076 0.955 0.0009 0.0029 0.1028 CA +1 V 0.077 0.990 0.0905 0.3019 0.3733 +1 V 0.080 0.988 1.2788 4.2628 1.8678 CA −0.3 V 0.021 0.975 4.4203 14.7345 1.7280 −0.3 V−0.05 s 6.450 0.977 6.1520 20.5068 2.8758 MPA +1 V−0.05 s 0.916 0.964 14.9974 49.9915 1.1871 +0.3 V−0.1 s - d ---- +0.5 V−0.1 s - - - - - MPA −0.3 V−0.1 s 4.318 0.997 3.1324 10.4413 2.6473 +1 V−0.05 s 4.000 0.983 1.8874 6.2915 0.4043 a,b The lowest limit of detection and the lowest limit of quantification, respectively, determined in accordance with the literature [30]; c For three replicates; d - means not determined.

It can be noticed that the lowest limits of detection and quantification were reached when applying SWV, while the best sensitivities were achieved by the MPA technique. In comparison with other electrodes reported in the literature for voltammetric/amperometric detection of DCF [6–8,12], the F–CNF paste electrode exhibited enhanced electroanalytical performance regarding both sensitivity and the lowest limit of detection.

3.3.3. Analysis of DCF in Spiked Tap Water

The C60/fullerene-modified carbon nanofiber paste (F–CNF) electrode was directly used to determine the presence of DCF in tap water, envisaging its detection in a real water matrix without deliberately adding any supporting electrolyte. Not every type of electrode is able to detect an analyte without supporting electrolyte; only one that can act as an array/ensemble of microelectrodes. From the effect of the scan rate study presented above, a pseudomicroelectrode array behavior was concluded to occur, which justified the further testing of the electrode to detect DCF in tap water. A different calibration plot was determined for SWV in application with tap water for DCF concentrations ranging from 1 to 5 mg·L−1. A smaller sensitivity of 0.38 µA·µM−1 was achieved in tap water (Figure 12) −1 in comparison with the sensitivity of 1 µA·µM reached in 0.1 M Na2SO4 supporting electrolyte (Figure6), which concluded that calibration is required in a real water matrix. Based on this calibration plot, the practical analytical application of the proposed SWV method was further established by determining DCF concentrations in tap water without any preliminary treatment. A recovery test was performed by analyzing three parallel tap water samples, which were spiked with 1 and 5 mg·L−1 DCF. The recovery test was run directly in tap water without supporting electrolyte. The recovery values higher than 95% and the relative standard deviation (RSD) values less than 5% for both concentrations indicated good recovery and reproducibility of the results and the great potential of the F–CNF paste electrode to be used for in situ and real-time water quality monitoring. Repeatability of the sensor was evaluated by comparing the results of the determination of a solution containing 5 mg·L−1 DCF over three days. The relative standard deviation of less than 4% demonstrated an appropriate repeatability of the proposed sensor. The electrode was tested for a time period of two months, and a 99% electrochemical response was found at the end of this period for 5 mg·L−1 DCF, which indicated a good stability and life time. Sensors 2019, 19, x FOR PEER REVIEW 13 of 15 Sensors 2019, 19, 1332 12 of 14

0.000010

0.000005 6

1 1.4 y= -0.047 + 0.3802x; 1.2 R2=0.990

I/ A 0.000000 1.0

A 0.8 μ I/

Δ 0.6

0.4 -0.000005 0.2

0.0

-0.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 DCF concentration / μM -0.000010 0.0 0.2 0.4 0.6 0.8 1.0 1.2 Potential applied (V/SCE)

Figure 12. Square-wave voltammograms recorded on the F–CNF paste electrode in tap water without supportingFigure electrolyte12. Square-wave (curve 1)voltammograms and in the presence recorded of various on the DCFF–CNF concentrations: paste electrode curves in tap 2–6: water −1 0.1–0.5without mg·L supportingDCF; step electrolyte potential (SP) (curve 2 mV; 1) and modulation in the presence amplitude of various (MA) 10 DCF mV; concentrations: potential range: curves 0 to +1.22–6: V/SCE.0.1–0.5 mg·L Inset:−1 DCF; Calibration step potential plots of (SP) the 2 currents mV; modulation recorded atamplitude E = +0.75 (MA) V/SCE 10 mV; versus potential DCF concentrations.range: 0 to +1.2 V/SCE. Inset: Calibration plots of the currents recorded at E = +0.75 V/SCE versus DCF concentrations. 4. Conclusions

C60/fullerene–carbonBased on this calibration nanofibers plot, in the paraffin practical oil inanal aytical weight application ratio of 1:2:1, of the comprising proposed aSWV F–CNF method pastewas electrode, further exhibitedestablished good by determining dispersion ofDCF both concentrations carbon fillers in and tap chemicalwater without stability any inpreliminary both Na2SOtreatment.4 supporting A recovery electrolyte test andwas tapperformed water. Its by electrochemical analyzing three behavior parallel tap in the water presence samples, of sodium which were diclofenacspiked (DCF) with 1 studied and 5 mg·L by cyclic−1 DCF. voltammetry The recovery made test itwas appropriate run directly to in be tap tested water in without some variants supporting of voltammetric/amperometric-basedelectrolyte. The recovery values higher analytical than applications. 95% and the Inrelative comparison standard with deviation the performance (RSD) values of a boron-dopedless than 5% diamondfor both concentrations electrode for DCF indicated detection good [7 recovery], the F–CNF and electrodereproducibility showed of enhancedthe results and voltammetric/amperometricthe great potential of the response, F–CNF paste due electrode to the electrocatalytic to be used for effect in situ of and the fullerenereal-time water towards quality the anodicmonitoring. oxidation of DCF. The lowest limit of detection of 0.9 nM was achieved by applying square-waveRepeatability voltammetry of the operated sensor under was evaluated SP of 2 mV, by MA comparing of 10 mV, the and results frequency of the of determination 25 Hz, which of a is appropriatesolution containing for detecting 5 mg·L DCF− concentrations,1 DCF over three according days. The to relative environmental standard quality deviation standard of less (EQS) than 4% imposeddemonstrated through the an “watchlist” appropriate of repeatability the Water Framework of the proposed Directive. sensor. Also, The a goodelectrode improvement was tested of for a the electroanalyticaltime period of two parameters months, for and DCF a 99% detection electrochemica on the F–CNFl response electrode was wasfound achieved at the end by applyingof this period a preconcentrationfor 5 mg·L−1 DCF, step which for 30 indicated min before a good the detectionstability and step, life which time. assured about 30 times better sensitivity. Simple, fast, and good electrochemical response for DCF detection was achieved by four-level4. Conclusions multiple pulsed amperometry (MPA) that allowed in situ reactivation of the F–CNF paste electrode. However, this method can be selected for DCF concentrations that exceed the EQS in water C60/fullerene–carbon nanofibers in paraffin oil in a weight ratio of 1:2:1, comprising a F–CNF samples,paste or electrode, for application exhibited with pharmaceuticalgood dispersion formulations. of both carbon The electrochemicalfillers and chemical peculiarities stability of thein both F–CNF electrode, as a pseudomicroelectrode array, make it appropriate for in situ and online analytical Na2SO4 supporting electrolyte and tap water. Its electrochemical behavior in the presence of sodium applicationsdiclofenac for (DCF) DCF monitoring studied by incyclic real surfacevoltammetry water. made it appropriate to be tested in some variants of Authorvoltammetric/amperometric-based Contributions: Conceptualization, F.M.;analytical Investigation, applications. S.M., In C.O. comparison and A.P.; Writing—originalwith the performance draft of a preparation,boron-doped S.M. and diamond F.M.; Writing—review electrode for and DCF editing, detectio F.M. andn [7], A.P. the F–CNF electrode showed enhanced Funding:voltammetric/amperometricThis research received no external response, funding. due to the electrocatalytic effect of the fullerene towards the anodic oxidation of DCF. The lowest limit of detection of 0.9 nM was achieved by applying Acknowledgments: This work was supported by a grant of the Romanian Ministry of Research and Innovation, projectsquare-wave number PN-III-P1-1.2-PCCDI-2017-0245/26 voltammetry operated under PCCDI/2018 SP of 2 mV, (SUSTENVPRO), MA of 10 mV, withinand frequency PNCDI III. of 25 Hz, which is appropriate for detecting DCF concentrations, according to environmental quality standard (EQS) Conflicts of Interest: The authors declare no conflict of interest. imposed through the “watchlist” of the Water Framework Directive. Also, a good improvement of the electroanalytical parameters for DCF detection on the F–CNF electrode was achieved by

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