Enhanced Electrochemical Response of Diclofenac at a Fullerene-Carbon Nanofiber Paste Electrode
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sensors
Article
Enhanced Electrochemical Response of Diclofenac at
a Fullerene–Carbon 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; sorinailies@acad-icht.tm.edu.ro
2 Department of Applied Chemistry and Engineering of Inorganic Compounds and Environment,
Politehnica University of Timisoara, P-ta Victoriei no.2, Timisoara 300006, Romania; aniela.pop@upt.ro
3 National Condensed Matter Department, Institute for Research and Development in Electrochemistry and
Condensed Matter, Timisoara, 1 P. Andronescu Street, Timisoara 300254, Romania; orha.corina@gmail.com
* Correspondence: florica.manea@upt.ro; 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/sensorsSensors 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 nanomaterials 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,
nanoparticles, nanoclusters, and graphene, 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 carbon nanotube-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-electrodeSensors 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
counter
cell, electrode,
consisting and a saturated
of a F–CNF paste workingcalomel reference
electrode, (SCE) electrode.
a platinum counter The F–CNFand
electrode, paste electrode
a saturated
with disc
calomel geometry
reference (SCE) was obtainedThe
electrode. by F–CNF
filling apaste
Teflon mold, with
electrode resulting in an active
disc geometry wassurface
obtainedwith by a
diameter of 3 mm. As the supporting electrolyte, 0.1 M sodium sulfate
filling a Teflon mold, resulting in an active surface with a diameter of 3 mm. As the supporting at pH 5 was used. Prior to use,
the electrode
electrolyte, was electrochemically
0.1 M sodium sulfate at pH 5 was stabilized
used. Priorthrough
to use, the 10electrode
continuous repetitive cyclic
was electrochemically
voltammograms
stabilized throughwithin the potential
10 continuous ranging
repetitive between
cyclic −0.5 and +1.5
voltammograms withinV/SCE. Na2SO4 used
the potential rangingwas
analytical-grade
between −0.5 and reagent
+1.5 V/SCE. from Na Merck,
2 SO4 and
usedDCF was was used as received
analytical-grade reagentfromfrom Amoli Organics
Merck, and DCF Ltd.wasAll
solutions
used were prepared
as received from Amoli withOrganics
doubly distilled
Ltd. Alland deionised
solutions werewater.
prepared with doubly distilled and
The water.
deionised electrochemical techniques applied for electrochemical characterization and analytical
applications were cyclic voltammetry,
The electrochemical techniques applied differential pulsed voltammetry,
for electrochemical square-wave
characterization andvoltammetry,
analytical
chronoamperometry,
applications were cyclic and multiple pulsed
voltammetry, amperometry.
differential 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
The molecular
spectroscopy (Figure structure of the fullerene–CNF
1). In accordance paraffin
with the literature [24],oilthe
paste
peakswas characterized
recorded by −1FTIR
at 1427 cm , 1180
spectroscopy −1 ,
cm−1, 576 cm(Figure
−1, and 527 1). cm
In −1accordance
corresponded withtothetheliterature
presence [24],
of C60the peaks
. The recorded
vibrations at at
seen 1427
2925cmcm −1,
1180 cm − 1 , 576 cm − 1 , and 527 cm − 1 corresponded to the presence of C . The vibrations seen at
2853 cm , 1457 cm , 1427cm , and 1428 cm are associated with different aliphatic CH groups (CH
−1 −1 −1 −1 60
2925 −1 2853 cm−1 , 1457 cm−1 , 1427cm−1 , and 1428 cm−1 are associated with different aliphatic
andcm CH2 ,bonds), as reported previously for carbon nanofibers [25]. The broad peak at 3430 cm−1 is
CH groups (CHof
characteristic and CHstretching
O–H 2 bonds), as reported
from inter-previously for carbon hydrogen
and intramolecular nanofibers bonds,
[25]. The broad
and the peak
peaksatat
3430 cm −1 is characteristic of O–H stretching from inter- and intramolecular hydrogen bonds, and the
1652 cm and 1457 cm are characteristic of phenolic resins [26].
−1 −1
peaks at 1652 cm−1 and 1457 cm−1 are characteristic of phenolic resins [26].
1.05
1.00
a
0.95 b
0.90
1176
0.85 3430
1378
576
Transmittance, a.u.
0.80
1427
0.75
b
1457
0.70
527
0.65
0.60
a
0.55
0.50
0.45
2853
0.40
0.35
0.30
0.25
2925
0.20
500 1000 1500 2000 2500 3000 3500 4000
-1
Wavenumber cm
Figure 1. FTIR spectra of carbon nanofiber (CNF)–paraffin oil paste (a, dotted line) and C60 /fullerene
(F)–CNF–paraffin oil paste
Figure 1. FTIR spectra (b, solidnanofiber
of carbon 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
presented
The in Figure 2.paste
electrode A good distribution
composition of both carbon
morphology wasnanofibers and fullerene
studied through in oilthe
SEM and paraffin was
results are
assured, and a randomized arrangement of both carbon nanofibers and fullerene resulted.
presented in Figure 2. A good distribution of both carbon nanofibers and fullerene 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):
I p = 2.69 × 105 AD1/2 n3/2 v1/2 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 molecule 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
of the electrode
Sensors geometric
2019, 19, x FOR area of 0.196 cm2 .
PEER REVIEW 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 Na2 SO4 . No peak corresponding to the DCF
oxidation appeared
Cyclic for the carbon
voltammetry using the nanofiber
classicalpaste electrode
potassium without fullerene
ferri/ferrocyanide redoxcontent
system (results
was used not
shown here). This should be explained by the absence of an electrocatalytic
for the determination of the electroactive area of the fullerene–CNF–paraffin oil paste electrode. effect of CNF towards
DCF
Cyclic electrooxidation,
voltammetry (CV) or of
bythe
a large background
supporting current
electrolyte recorded
consisting ofon
4 mMthe K simple
3[Fe(CN) carbon
6] in 1nanofiber
M KNO3
paste electrodeatincreasing
was recorded different atscan each usage
rates and not
(results thusshown
overlapping the electrochemical
here), and response was
the diffusion coefficient for
DCF
determined as 10.86 × 10
electrooxidation. Thecm
−6 latter
2 would alsotogive
·s according
−1 information about
the Randles–Sevcik the instability
Equation (1): of the electrode
composition. CV series recorded on the F–CNF paste electrode at various DCF concentrations are
I p = 2.69 × 105 AD1 / 2n3 / 2v 1 / 2C (1)
presented in Figure 3, and an anodic peak corresponding to DCF oxidation is evidenced at the potential
value
whereofAabout +0.75the
represents V/SCE
area (peak
of the II). It must(cm
electrode be 2noticed that
), n is the the CVofshape
number showed
electrons the presence
participating of
in the
anodic and corresponding cathodic peaks (Ia and Ib) characteristics in
reaction (and is equal to 1), D is the diffusion coefficient of the molecule in solution, C is thethe carbon redox system [28].
The first stage in
concentration of the
the overall oxidationinofthe
probe molecule DCF is given
solution by is
and the4 DCF
mM, sorption
and v is onto
the scanthe carbon
rate (V·s surface,
−1); the
evidenced by the diminution of the anodic peak of carbon oxidation,
linear dependence between peak current and the square root of the scan rate was achieved. Taking followed by the second stage
of the DCF oxidation process. It has been reported that the electrochemical
into account the theoretical diffusion coefficient value of 6.7 × 10 cm ·s found in the literature data
−6 2 −1 oxidation of diclofenac
involves a one-electron
[27], the value electrochemical-chemical
of the electroactive electrode area was (EC)determined
mechanismtofollowedbe 0.249 cm by 2aversus
chemical the reaction
value of
in which 2,6 dichloroaniline and
the electrode geometric area of 0.196 cm . 2-(2-hydroxyprop-2-phenyl)
2 acid acetic are formed [10]. A linear
dependence between the useful anodic peak current and DCF concentration
The electrochemical behavior of DCF on both paste electrodes was investigated by cyclic is noticeable in the inset
of Figure 3. (CV) in a supporting electrolyte of 0.1 M Na2SO4. No peak corresponding to the DCF
voltammetry
Some appeared
oxidation mechanistic foraspects related
the carbon to the electrooxidation
nanofiber paste electrode of without
DCF on the F–CNFcontent
fullerene paste electrode
(results can not
be discussed
shown here).through the study
This should of the scan
be explained by rate influence.
the absence ofCVs recorded in theeffect
an electrocatalytic presence
of CNF mg·L−1
of 5 towards
DCF
DCF and 0.1 M Na2 SO4orsupporting
electrooxidation, by a large electrolyte
background at current
the scanrecorded
rates ranging
on thefromsimple 0.01carbon
to 0.2 V ·s−1 are
nanofiber
presented in Figure
paste electrode 4. From
increasing theseusage
at each CVs, andit can be overlapping
thus noticed that the the electrochemical
dependence between response thefor
anodic
DCF
peak current and the
electrooxidation. Thesquare
latter root
would of the
alsoscan
give rateinformation
is not linearabout
(see inset
the of Figure 4),ofwhich
instability denotes
the electrode
acomposition.
nonlinear diffusion-controlled
CV series recordedoxidation on the F–CNF process.pasteThis shouldatbevarious
electrode explained DCFbyconcentrations
the fact that the are
F–CNF paste electrode can work as a pseudomicroelectrode array,
presented in Figure 3, and an anodic peak corresponding to DCF oxidation is evidenced which is characterized 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 ofSensors 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
Sensors 2019, 19, x FOR PEER REVIEW 5 of 15
peak corresponding to the anodic DCF oxidation, and also through the dependence of the oxidation
potential value and the logarithm of the scan rate.
-4
1.5x10
-4
1.0x10
-5
5.0x10 8
Ia II
1
I/ A
0.0
Ib 14
y= -0.3095 + 0.6425x;
-5 12 R2=0.9528
-5.0x10 10
8
ΔI/ μA
6
-4
-1.0x10 4
2
0
-4 -2
-1.5x10 0 5 10
Conc / μM DCF
15 20 25
-0.5 0.0 0.5 1.0 1.5
Potential applied (V/SCE)
Figure 3. Cyclic
Sensors voltammograms
2019, 19, x FOR PEER REVIEW recorded at the F–CNF paste electrode in 0.1 M Na2 SO4 supporting6 of 15
electrolyte (curve 1) and in the presence
Figure 3. Cyclic voltammograms recorded of various DCF
at the concentrations:
F–CNF curves
paste electrode in 0.1 2–8:
M Na1–7
2SOmg ·L−1 DCF.
4 supporting
Inset: Calibration
electrolyteplots
(curveof1)the
andcurrents versus of
in the presence DCF concentrations
various at potential
DCF concentrations: value
curves 2–8:of
1–7+0.75
mg·LV/SCE.
−1 DCF.
Inset: Calibration plots of the currents versus DCF concentrations at potential value of +0.75 V/SCE.
-4
3.5x10 16
-4
3.0x10 aspects related to the electrooxidation of DCF on the F–CNF paste electrode
Some mechanistic 14
-4 12
can be discussed2.5x10
through the study of the scan rate influence. CVs recorded in the presence of 5
10
-4
ΔI / μA
mg·L−1 DCF and2.0x10
0.1 M Na2SO4 supporting electrolyte at the scan rates ranging from 0.01 to 0.2 V·s−1
8
-4
8
6
are presented in1.5x10
Figure 4. From these CVs, it can be noticed that the dependence
4
between the anodic
peak current and the -4square root of the scan rate is not linear (see inset of Figure 4), which denotes a
1.0x10 2
0.10 0.15 0.20 0.25 0.30 0.35
-5
5.0x10
nonlinear diffusion-controlled oxidation
v 1/2 (Vs-1process.
)1/2 1
This should be explained by the fact that the F–
I/ A
CNF paste electrode0.0 can work as a pseudomicroelectrode array, which is characterized by spherical
-5
and nonlinear -5.0x10
diffusion patterns that are specific to macroelectrodes. Also, surface-controlled or
-4 0.86
complex processes
-1.0x10should influence the linearity of the dependence between the anodic peak current
0.84
root of-4 the scan rate. A complex process involving fullerene’s availability to act as a
and the square -1.5x10 0.82
-4
multiple electron
-2.0x10acceptor [16] in DCF oxidation should be considered to explain nonlinear
E/V
0.80
-4
diffusion. The irreversible
-2.5x10 characteristic of the overall oxidation process is evidenced by the lack of 0.78
the cathodic peak corresponding
-3.0x10
-4
to the anodic DCF oxidation, and also through the dependence of 0.76
the oxidation potential
-3.5x10 value and the logarithm of the scan rate.
-4 0.74
-2.0 -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
Na2 SO4 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 . Insets: upper: dependence of anodic peak current vs. square root of the scan rate;
−1
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 secondSensors 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 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
Sensors 2019, 19, x FOR PEER REVIEW 7 of 15
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.
-6
3.5x10 2.5
y=0.1986 + 0.6891x; 10
2.0
R2=0.992
-6
3.0x10
1.5
-6
ΔI/ μA
2.5x10
1.0
-6
2.0x10 0.5
I/ A
-6
1.5x10 0.0
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
DCF concentration / μM
-6
1.0x10
5.0x10
-7 1
0.0
-7
-5.0x10
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 Na2 SO4
supporting
Figure electrolyte (curve
5. Differential 1) and
pulsed in the presence
voltammograms of various
recorded at theDCF
F–CNFconcentrations: curves
paste electrode in 0.1 2–10:
M Na2SO4
0.1–0.9 mg · L −1 DCF; step potential (SP) 25 mV; modulation amplitude (MA) 100 mV; potential
supporting electrolyte (curve 1) and in the presence of various DCF concentrations: curves 2–10: 0.1–
range:
0.90 mg·L
to +1.2 V/SCE.
−1 DCF; stepInset: Calibration
potential (SP) 25 plots of the currents
mV; modulation recorded
amplitude at E100
(MA) = +0.75 V/SCE versus
mV; potential 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.
-6
3.0x10
1.8
y= 0.0543 + 1.0752x; 11
R2=0.955
1.6
1.4
1.2
1.0supporting 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
-6
3.0x10
1.8
y= 0.0543 + 1.0752x; 11
R2=0.955
1.6
1.4
1.2
1.0
ΔI/ μA
0.8
-6
2.0x10 0.6
0.4
0.2
0.0
1
-0.2
I/A
-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
-6
1.0x10
0.0
0.0 0.2 0.4 0.6 0.8 1.0 1.2
Potential applied (V/SCE)
Figure 6. Square-wave voltammograms recorded at the F–CNF paste electrode in 0.1 M Na2 SO4
Sensors 2019, 19, x FOR PEER REVIEW 8 of 15
supporting electrolyte
Figure 6. (curve
Square-wave 1) and in the presence
voltammograms recordedofat various
the F–CNFDCFpaste
concentrations:
electrode incurves
0.1 M 2–6:
Na2SO4
0.02–0.1 − 1
mg·L electrolyte
DCF and curves − 1
mg·L DCF;
supporting (curve 7–11:
1) and0.2–0.6
in the presence step DCF
of various potential (SP) 2 mV; curves
concentrations: 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
0.1 (MA)
mg·L 10 and
−1 DCF mV;curves
potential range:
7–11: 0 mg·L
0.2–0.6 to +1.2 V/SCE.
−1 DCF; stepInset: Calibration
potential (SP) 2 plots
mV; of the currents
modulation amplitude
+0.75 V/SCE versus DCF concentrations.
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
inclusion of the preconcentration
concentrations. step before
In the preconcentration step, the
the detection
F–CNF paste stepelectrode
applied isforimmersed
low DCFinconcentrations.
supporting
In electrolyte
the preconcentration
containing low step, theconcentrations
DCF F–CNF pasteatelectrode
the open iscircuit
immersed in supporting
potential for a certain electrolyte
time to
containing
assure itslow DCF concentrations
sorption at the
onto the electrode open A
surface. circuit potential
maximum for a certain time
preconcentration to of
factor assure its28sorption
about was
onto the at
found electrode surface.
30 min (see FigureA 7).
maximum
A longerpreconcentration
sorption time led factor of aboutof28the
to diminution was found at 30 min
preconcentration
(see Figure
factor, 7). A longer
probably due tosorption timeeffect
the fouling led tostarting
diminution of the preconcentration
to manifest. factor,
It is clear that this probably due to
preconcentration–
thedetection
fouling effect starting to manifest. It is clear that this preconcentration–detection scheme
scheme necessities a longer time for DCF determination, but this is nevertheless useful necessities
for
a longer time for DCF
trace concentration determination,
levels of DCF. but this is nevertheless useful for trace concentration levels
of DCF.
1.0
30
Preconcentration factor
0.8
25
0.6 20
ΔI/ μA
15
0.4
10
0.2
5
0.0
0
0 10 20 30 40 50
Sorption time/ min
Figure 7. Useful signals reached by square-wave voltammetry (SWV) recorded in the presence of
0.005 L−Useful
mg·7.
Figure 1 DCFsignals reached
containing by Na
0.1 M square-wave voltammetry
2 SO4 supporting (SWV)
electrolyte at recorded in theelectrodes,
F–CNF paste presence of
as a
0.005 mg·L
function −1 DCF containing 0.1 M Na2SO4 supporting electrolyte at F–CNF paste electrodes, as a
of the sorption time in the preconcentration step prior to detection.
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 aSensors 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 peak
2019, 19, (+0.75
x FOR PEERV/SCE),
REVIEW allowed us to reach CAs recorded at various DCF concentrations
9 of 15
presented in Figure 8. Lower sensitivity was reached by CA, probably due to the fouling effect of the
electrode surface.
-5
3.0x10
1.8
-5
2.5x10 1.6 y=0.05636 + 0.07733x;
1.4 R2=0.990
1.2
-5
2.0x10 1.0
ΔI / μA
0.8
I/ A
0.6
-5
1.5x10 0.4
0.2
0.0
-5
1.0x10 -0.2
0 5 10 15 20
DCF concentration/ μM
-6
5.0x10
7
1
0.0
0 10 20 30 40 50
Time / s
Figure 8. Chronoamperograms (CAs) recorded for a single level of the detection potential of +1 V/SCE
at theFigure
F–CNF8.paste electrode in 0.1 M(CAs)
Chronoamperograms Na2 SO 4 supporting
recorded for a electrolyte (curve
single level of the1) detection
and in thepotential
presenceof +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
versus DCF concentrations.
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 potentials
of +1 V/SCE and −0.3 V/SCE were applied, and the results are shown in Figure 9. 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 Figure 9).
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 strategies
3x10
-5 in order to improve the electronalaytical parameters for DCF detection.
It is well-known that pulsed amperometric detection involves in situ cleaning and reactivation of the
2.0
2
y1= 0.03875 + 0.0809x;R =0.9888
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
ΔI/ μM
1.0
avoiding the fouling effect-5on the electrode surface. The first variant of MPA applied consisted of the
1x10 0.5
E2
application of similar potentials with two levels of CA. By continuously applying the same potential
8
I/A
values for the short duration of 0.05 s per pulse, the amperograms recorded at the F–CNF paste
0.0
0 +1 V/SCE are shown in1Figure 10.DCFEnhanced
0 5 10 15 20 25
electrode at −0.3 V/SCE and 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 effect 8 manifestation, which
-5
-2x10
1
E2
-5
-3x10V/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
potentials
Sensors 2019, of
19, +1 V/SCE
x FOR and −0.3
PEER REVIEW V/SCE were applied, and the results are shown in Figure 10 9. of
The
15
cathodic potential belongs to the hydrogen evolution potential range, being selected in accord with
CV.
led to ItFigure
the can be9.efficiency
better Chronoamperograms
noticed that
of DCF (CAs)cathodic
a decreasing
detection. recorded for higher
twooccurs
current
Ten times levels of detection
with
sensitivityDCF potential,
concentration
at the potentialnamely +1
increasing,
value of
+1 V/SCE V/SCE
which was and
suggested activation
achieved; potential
thatalso,
this the
simple of −0.3 V/SCE
procedure
cathodic (E2),
currentdid at the
not allow
increased F–CNF paste electrode
the activation
linearly with DCF, of in 0.1 M Na
the F–CNF
increasing 2SO
at the paste
4
supporting
electrode electrolyte (curve 1) and in the presence of various DCF concentrations: curves 2−8: 1−7
potential valueatofthis
−0.3medium
V/SCE,cathodic potential value
and a commendable and no was
sensitivity enhanced
reached,response was reached
being markedly higher under
these mg L−1 DCF.conditions
operating Inset: Calibration
(see plots
inset of of the currents
Figure 9). versus DCF concentrations at both potential
than that recorded at the anodic part. The cathodic 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
detection of DCF on the F–CNF paste electrode, which was reflected also in the low sensitivity when
2.0
2
y1= 0.03875 + 0.0809x;R =0.9888
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
ΔI/ μM
1.0
DCF detection. It is well-known
-5
that pulsed amperometric detection involves in situ cleaning and
reactivation of the1x10
0.5
electrode surface during the electrodetection process [29]. The responses of MPA E2
8
I/A
corresponded to each potential pulse applied for a short duration, combining the anodic and
0.0
cathodic polarization0 and avoiding the fouling 1effect on DCF
0 5 10 15 20 25
the electrode
concentration/ 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 duration 8 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
-2x10sensitivities were reached for MPA in comparison with CA for both anodic and
-5
1
cathodic potential pulses (see inset of Figure 10). The short durations
E2 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
20 at40the potential
60 value
80 of –0.3100 V/SCE, and a commendable
sensitivity was reached, being markedly higher than that recorded at the anodic part. The cathodic
Time / s
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 Na2 SO4 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
4.0x10
200
y= 22.9333 + 6.4509x;
150
R2=0.977
I/A
100 E1
ΔI/ μA
50
-4
-4.5x10 E2
0
y= 5.08889 + 0.91667x;
-4 R2=0.964
-5.0x10 0 5 10 15 20 25 30
-4
E1 DCF concentration/ μM
-5.5x10
-4
-6.0x10
-4
-6.5x10
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 0.05 s (E1)
Multiple pulsedand activation potential
amperograms of −0.3 V/SCE
(MPAs) recorded for twofor 0.05ofsthe
levels (E2) at the F–CNF
potential pulses of +1
pasteV/SCE
electrode in 0.1 M Na 2 SO 4 supporting electrolyte (curve 1) and in the presence
for 0.05 s (E1) and activation potential of –0.3 V/SCE for 0.05 s (E2) at the of various DCFpaste
F–CNF
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
both concentrations:
potential pulse 1–7
values (E1
mg·L −1 and
DCF.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
reference,strategy
Another involving
wasthe redox system
considered of fullerene
for MPA, applyingthat
it in should act with
according as antheelectrocatalyst in DCF
CV shape as the
oxidation and detection, and the amperograms are shown in Figure 11.
reference, involving the redox system of fullerene that should act as an electrocatalyst in DCF oxidation
and detection,
The pulsesand theapplied
were amperograms are shown
continuously usinginthe
Figure 11. scheme:
following
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/SCEV/SCE
for afor a duration
duration of 100
of 100 ms, ms, where
where reduced
fullerene is infullerene is oxidized;
the reduced form;
2. 3.+0.5 -0.3
V/SCEV/SCE
for afor a duration
duration of 100ofms,
100 ms, reduced
where where Hfullerene
2 evolution occurs alongside other reduction
is oxidized;
3. processes;
−0.3 V/SCE for a duration of 100 ms, where H2 evolution occurs alongside other
4. +1 V/SCE
reduction for a duration of 50 ms, considering the detection potential that corresponded to DCF
processes;
4. oxidation.
+1 V/SCE 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;
100
R2=0.997
0.0004 80
E3
I/A
ΔI / μ A
60
E4
40
0.0002 20
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 Na2 SO4 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 is concentrations
obvious that attheboth E3 and
better E4 potential pulses.
electroanalytical parameters were achieved using MPA under
the operating conditions presented above. The currents recorded at E1 and E2 potential values
It is obvious that the better electroanalytical parameters were achieved using MPA under the
did not vary linearly with DCF concentration due to fullerene-related surface processes occurring,
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 Table 1.
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,bThe 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)
in comparison with the sensitivity of 1 µA·µM−1 reached in 0.1 M Na2 SO4 supporting electrolyte
(Figure 6), 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
I/ A y= -0.047 + 0.3802x;
R2=0.990
1.2
0.000000 1.0
0.8
Δ I/ μ A
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
supporting
Figureelectrolyte (curve 1)voltammograms
12. Square-wave and in the presence of various
recorded on the DCF
F–CNFconcentrations: curves
paste electrode 2–6:water
in tap
0.1–0.5 mg · L −1 DCF; step potential (SP) 2 mV; modulation amplitude (MA) 10 mV; potential range:
without supporting electrolyte (curve 1) and in the presence of various DCF concentrations: curves
0 to +1.2
2–6: V/SCE. Inset:
0.1–0.5 mg·L Calibration
−1 DCF; plots of(SP)
step potential the 2currents recorded at
mV; modulation E = +0.75
amplitude V/SCE
(MA) versus
10 mV; 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–carbon nanofibers
Based on this calibration plot,inthe
paraffin oil in
practical a weightapplication
analytical ratio of 1:2:1, comprising
of the proposedaSWV F–CNF method
pastewaselectrode,
further exhibited
established good dispersion of
by determining DCFboth carbon fillersinand
concentrations tap chemical stability
water without anyin both
preliminary
Na2 SO 4 supporting
treatment. electrolyte
A recovery testand
wastap water. Its
performed byelectrochemical
analyzing threebehavior
parallel tapin the presence
water of sodium
samples, which were
spiked(DCF)
diclofenac with 1studied
and 5 mg·L −1 DCF.
by cyclic The recovery
voltammetry test it
made was run directly
appropriate to in
betap water
tested in without supporting
some variants
electrolyte. The recovery values higher
of voltammetric/amperometric-based thanapplications.
analytical 95% and theIn relative
comparisonstandard withdeviation (RSD) values
the performance
less than 5% diamond
of a boron-doped for both concentrations
electrode for DCF indicated good[7],
detection recovery
the F–CNF and electrode
reproducibility
showed of enhanced
the results and
the great potential of theresponse,
voltammetric/amperometric F–CNF paste due electrode to be used foreffect
to the electrocatalytic in situof and real-time water
the fullerene towards quality
monitoring.
the anodic oxidation of DCF. The lowest limit of detection of 0.9 nM was achieved by applying
square-wave Repeatability
voltammetry of the sensorunder
operated was evaluated
SP of 2 mV,byMA comparing
of 10 mV,the andresults of the
frequency ofdetermination
25 Hz, which of a
solution containing
is appropriate for detecting 5 mg·L DCF over three
DCF−1concentrations, days. The
according to relative standard
environmental deviation
quality of less
standard than 4%
(EQS)
demonstrated
imposed through the an“watchlist”
appropriateofrepeatability of the proposed
the Water Framework sensor.
Directive. Also, The electrode
a good was tested
improvement of for a
time period of two
the electroanalytical months,for
parameters and a 99%
DCF electrochemical
detection on the F–CNF response was was
electrode found at the end
achieved of this period
by applying
for 5 mg·L−1 DCF,
a preconcentration stepwhich
for 30indicated
min before a good stability and
the detection lifewhich
step, time. assured about 30 times better
sensitivity. Simple, fast, and good electrochemical response for DCF detection was achieved by
4. Conclusions
four-level multiple pulsed amperometry (MPA) that allowed in situ reactivation of the F–CNF paste
electrode. C However, this method can be selected for DCF concentrations that exceed the EQS in water
60/fullerene–carbon nanofibers in paraffin oil in a weight ratio of 1:2:1, comprising a F–CNF
samples, or for application
paste electrode, exhibited with pharmaceutical
good dispersion formulations.
of both carbon The electrochemical
fillers and chemical peculiarities of the
stability in both
F–CNF electrode, as a pseudomicroelectrode array, make it appropriate for in situ and
Na2SO4 supporting electrolyte and tap water. Its electrochemical behavior in the presence of sodium online analytical
applications
diclofenac for (DCF)
DCF monitoring
studied byin real surface
cyclic water.made it appropriate to be tested in some variants of
voltammetry
voltammetric/amperometric-based
Author analytical
Contributions: Conceptualization, F.M.; applications.
Investigation, S.M., In comparison
C.O. with the performance
and A.P.; Writing—original draft of a
boron-doped
preparation, S.M. anddiamond electrode for
F.M.; Writing—review andDCF detection
editing, F.M. and [7],
A.P. the F–CNF electrode showed enhanced
voltammetric/amperometric
Funding: response,
This research received no external due to the electrocatalytic effect of the fullerene towards the
funding.
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 voltammetry operated under
number PN-III-P1-1.2-PCCDI-2017-0245/26 SP of 2 mV,
PCCDI/2018 MA of 10 mV, within
(SUSTENVPRO), and 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 bySensors 2019, 19, 1332 13 of 14
References
1. Oaks, J.L.; Gilbert, M.; Virani, M.Z.; Watson, R.T.; Meteyer, C.U.; Rideout, B.; Shivaprasad, H.L.; Ahmed, S.;
Jamshed Iqbal Chaudhry, M.; Arshad, M.; et al. Diclofenac residues as the cause of vulture population
decline in Pakistan. Nature 2004, 427, 630–633. [CrossRef] [PubMed]
2. Risebrough, R. Conservation biology: Fatal medicine for vultures. Nature 2004, 427, 596–598. [CrossRef]
[PubMed]
3. Directive 2013/39/EU of the European Parliament and of the Council of 12 August 2013 amending Directives
2000/60/EC and 2008/105/EC as regards priority substances in the field of water policy. Off. J. Eur. Union
2013, L226, 1–17.
4. Tavazzi, S.; Paracchini, B.; Suurkuusk, G.; Mariani, G.; Loos, R.; Ricci, M.; Gawlik, B.M. Analysis of diclofenac
in water, Water Framework Directive, Watch List Methods, JRC Technical Reports. Publ. Off. Eur. Union 2014.
[CrossRef]
5. Schmidt, S.; Hoffman, H.; Garbe, L.A. Liquid chromatography-tandem mass spectrometry detection of
diclofenac and related compounds in water samples. J. Cromatogr. A 2018, 1538, 112–116. [CrossRef]
[PubMed]
6. Manea, F.; Ihos, M.; Remes, A.; Burtica, G.; Schoonman, J. Electrochemical determination of diclofenac
sodium in aqueous solution on Cu-doped zeolite-expanded graphite-epoxy electrode. Electroanalysis 2010,
22, 2058–2063. [CrossRef]
7. Ihos, M.; Remes, A.; Manea, F. Electrochemical determination of diclofenac using boron-doped diamond
electrode. J. Environ. Prot. Ecol. 2012, 13, 2096–2103.
8. Motoc, S.; Manea, F.; Iacob, A.; Martinez-Joaristi, A.; Gascon, J.; Pop, A.; Schoonman, J. Electrochemical
selective and simultaneous detection of diclofenac and ibuprofen in aqueous solution using HKUST-1
metal-organic framework-carbon nanofiber composite electrode. Sensors 2016, 16, 1719. [CrossRef]
9. Afkhami, A.; Bahiraei, A.; Madrakian, T. Gold nanoparticle/multi-walled carbon nanotube modified glassy
carbon electrode as a sensitive voltammetric sensor for the determination of diclofenac sodium. Mater. Sci.
Eng. C 2016, 59, 168–176. [CrossRef]
10. Aguilar-Lira, G.Y.; Álvarez-Romero, G.A.; Zamora-Suárez, A.; Palomar-Pardavé, M.; Rojas-Hernández, A.;
Rodríguez-Ávila, J.A.; Páez-Hernández, M.E. New insights on diclofenac electrochemistry using graphite as
working electrode. J. Electroanal. Chem. 2017, 794, 182–188. [CrossRef]
11. Shalauddin, M.; Akhter, S.; Bagheri, S.; Karim, M.S.A.; Kadri, N.A.; Basirun, W.J. Immobilized copper ions
on MWCNTS-Chitosan thin film: Enhanced amperometric sensor for electrochemical determination of
diclofenac sodium in aqueous solution. Int. J. Hydrog. Energy 2017, 42, 19951–19960. [CrossRef]
12. Eteya, M.M.; Rounaghi, G.H.; Deiminiat, B. Fabrication of a new electrochemical sensor based on
Au-Pt bimetallic nanoparticles decorated multi-walled carbon nanotubes for determination of diclofenac.
Microchem. J. 2019, 144, 254–260. [CrossRef]
13. Mofidi, Z.; Norouzi, P.; Seidi, S.; Ganjali, M.R. Determination of diclofenac using electromembrane extraction
coupled with stripping FFT continuous cyclic voltammetry. Anal. Chim. Acta 2017, 972, 38–45. [CrossRef]
14. Sherigara, B.; Kutner, W.; D’Souza, F. Electrocatalytic properties and sensor applications of fullerene and
carbon nanotubes. Electroanalysis 2003, 15, 753–772. [CrossRef]
15. Rather, J.A.; de Wael, K. Fullerene-C60 sensor for ultra-high sensitive detection of bisphenol-A and its
treatment by green technology. Sens. Actuators B Chem. 2013, 176, 110–117. [CrossRef]
16. Zhang, X.; Jiao, K.; Iao, G.; Liu, S.; Li, S. Voltammetric study of fullerene C60 and fullerene C60 nanotubes
with sandwich method. Synth. Met. 2009, 159, 419–423. [CrossRef]
17. Pilehvar, S.; De Wael, K. Recent Advances in Electrochemical Biosensors Based on Fullerene-C60
Nano-Structured Platforms. Biosensors 2015, 5, 712–735. [CrossRef] [PubMed]
18. Baig, N.; Sajid, M.; Saleh, T.A. Recent trends in nanomaterial-modified electrodes for electroanalytical
applications. Trends Anal. Chem. 2019, 111, 47–61. [CrossRef]
19. Sukanya, R.; Sakthivel, M.; Chen, S.-M.; Chen, T.-W. A new type of terbium diselenide nanooctagon
integrated oxidized carbon nanofiber: An efficient electrode material for electrochemical detection of morin
in the food sample. Sens. Actuators B Chem. 2018, 269, 354–367. [CrossRef]You can also read
