Factors influencing voltammetric reduction of 5-nitroquinoline at boron-doped diamond electrodes

Article abstract of DOI:10.1007/s00706-015-1621-6

The voltammetric signal of 5-nitroquinoline with reducible nitro and quinoline moieties largely depends on the pH of the indifferent electrolyte, electrode pretreatment, activation between individual scans, and boron concentration of the BDD film electrod

Full text of DOI:10.1007/s00706-015-1621-6

Monatsh Chem (2016) 147:21–29
DOI 10.1007/s00706-015-1621-6
ORIGINAL PAPER
Factors influencing voltammetric reduction of 5-nitroquinoline
at boron-doped diamond electrodes
1
1
2,3
1
Jana Vos a´ hlov a´
•
Jaroslava Zav a´ zalov a´
•
V a´ clav Petr a´ k
•
Karolina Schwarzov a´ -Peckov a´
Received: 17 September 2015 / Accepted: 24 November 2015 / Published online: 14 December 2015
Ó Springer-Verlag Wien 2015
Abstract The voltammetric signal of 5-nitroquinoline
with reducible nitro and quinoline moieties largely depends
on the pH of the indifferent electrolyte, electrode pre-
treatment, activation between individual scans, and boron
concentration of the BDD film electrode. Anodic pre-
Graphical abstract
-
3
treatment at ?2.4 V for 5 min in 0.5 mol dm H SO and
2
4
2
0 s stirring between individual scans assured repeat-
able signals of nitro group in the whole pH range 2.0–12.0;
-
7
in acetate buffer pH 5.0 limit of detection is 2 9 10
-
3
mol dm for differential pulse voltammetry. The reduc-
tion of quinoline skeleton is visible in the pH range of
6
.0–11.0. Presence of oxygen in the measured solutions led
Keywords Voltammetry Á
to slight increase of peak heights and acceptable increase of
its relative standard deviation. BDD films with metallic
type of conductivity deposited at B/C ratio
Boron-doped diamond electrode Á Boron concentration Á
Reduction Á Electrochemistry
2
000–8000 ppm exhibit faster electron transfer at lower
potential for nitro group reduction than semiconductive
films 500 and 1000 ppm.
Introduction
Boron doped diamond introduced for electroanalysis in
1992 [1] gained a deserved popularity especially for elec-
trooxidation of organic compounds of biological,
pharmaceutical, and environmental significance [2–5]. Its
mechanical and electrochemical properties are among
others significantly influenced by morphology of the BDD
films, boron concentration, and electrode pretreatment,
when high positive/negative current densities or potentials
&
Karolina Schwarzov a´ -Peckov a´
kpeckova@natur.cuni.cz
1
Charles University in Prague, Faculty of Science, Department
of Analytical Chemistry, UNESCO Laboratory of
Environmental Electrochemistry, Albertov 6, CZ-12843
Prague 2, Czech Republic
(Correctly C± 2.0 V) in the region of water decomposition
reactions are applied for few seconds to minutes. As results
of this anodic/cathodic pretreatment, oxygen-terminated
(O-BDD) or hydrogen-terminated (H-BDD) surfaces are
produced, very often with different capabilities of pre-
vention of surface passivation, enhancement of the
voltammetric signals, and ensuring of repeatable and
reproducible response of particular analytes [6–9].
2
3
Department of Functional Materials, Institute of Physics
ASCR, v.v.i., Na Slovance 2, 18221 Prague 8,
Czech Republic
Faculty of Biomedical Engineering, Czech Technical
University in Prague, S ´ı tn a´ 3105, 272 01 Kladno,
Czech Republic
123

2
2
J. Vos a´ hlov a´ et al.
Boron-doping level plays crucial role in basic electro-
to a split of the original four-electron reduction described
in Eq. (1) and two reductive signals corresponding to
Eqs. (3) and (4), also leading to hydroxylamine as the final
product can be observed [36, 37, 40].
chemical characteristics, e.g., electrical conductivity of the
BDD film and kinetics of electron transfer [1, 7, 10]. The
few studies concerned with influence of boron concentra-
tion on electroanalytical parameters for organic compounds
À
ÁÀ
2
ArNO þ e $ ArNO
ð3Þ
ð4Þ
2
[
11, 12] including our reports on oxidation of benzophe-
ÁÀ
À
þ
none-3 [13] and 2-aminonaphthalene [14] report that
semiconductive films exhibit more sluggish kinetics for
ArNO2 þ 3e þ 4H ! ArNHOH þ H2O
Compared to the reduction of the nitro group, the
reduction of the quinoline skeleton proceeds at more
negative potentials, close to the onset of supporting
electrolyte [38, 39]. Quinoline (Q) itself is
polarographically reducible in two steps in alkaline
media according to Eq. (5) and (6) yielding
3
-/4-
surface-sensitive redox marker [Fe(CN)6]
as well as
decreased sensitivity towards mentioned analytes than
metallic films. The predicted threshold for the semicon-
2
0
ductive/metallic transition is at *2 9 10 boron atoms
3
per cm [15] (theoretical value), i.e. *1000–2000 ppm
(
experimental values) [14, 16, 17], which is the B/C ratio in
dihydroquinoline (QH ) in the first step [Eq. (5)] and
2
the gaseous phase during the chemical vapour deposition of
BDD films.
tetrahydroquinoline (QH ) in the second one [Eq. (6)]
4
[
38, 41]:
It can be traced in reviews [2–4] and monographs [5, 9]
devoted to electroanalysis of organic compounds by means
of BDD-based electrodes that lower attention has been paid
to their utilization for electrochemical reductions despite
the favorable characteristic for such applications: relatively
wide potential window in the cathodic region and low
sensitivity towards oxygen evolution [18, 19]. Among
organic reducible compounds, substances containing nitro
group at the aromatic skeleton represent an extensive
group, where pharmaceuticals, agrochemicals, and envi-
ronmental pollutants are present. Most of them are toxic,
probably due to a reactive nitro-radical in their metabolic
pathway [20, 21].
À
þ
Q þ 2e þ 2H ! QH2
ð5Þ
ð6Þ
À
þ
QH þ 2e þ 2H ! QH
2
4
Also electrooxidation of the quinoline skeleton is
relatively hardly achievable [42, 43] and thus there are
not many studies devoted to utilization of these processes
in voltammetry. Modification of electrode surface [42, 43]
or presence of surfactant [39] was tested to afford results
utilizable in electroanalysis.
The aim of this study is to extend the knowledge on the
electro reduction of aromatic nitro group and quinoline
skeleton at BDD electrodes. For this purpose, an environ-
mental pollutant, formed as product of incomplete
combustion of fossil fuels, 5-nitroquinoline was selected as
model compound [38, 45]. It was previously studied in our
laboratory at mercury, amalgam-based and carbon film
electrodes, as obvious from the overview in Table 1 sum-
marizing voltammetric methods used for determination of
5-nitroquinoline [38, 44, 45]. In this study, special attention
has been paid to the influence of BDD electrode pretreat-
ment, boron-doping level, and oxygen presence on
voltammetric signal of 5-nitroquinoline to present the
variety of specific factors influencing voltammetric analy-
sis at BDD electrodes.
The few determinations based on nitro group reduction
at BDD-based materials were suggested for some nitro-
phenols [22–26] and aminonitrophenols [27], and nitro-
group containing pesticides (methylparathion [28]), drugs
(
chloramphenicol [29], nitrofurazone [30, 31], selected
benzazepines [32]), and derivatives of polycyclic aromatic
hydrocarbons (1-nitropyrene [33], 3-nitrofluoranthene
[
35]).The reduction of nitro group has been investigated
among the first electrochemical processes of organic
compounds at dropping mercury and other mercury elec-
trodes [20, 35, 36], later on it was extended on solid
electrodes including carbon and amalgam based electrodes
[
20, 37]. In aqueous acidic and neutral media, indepen-
dently on the electrode material, the first step of the
reduction relies on the four-electron reduction of nitro
group to the hydroxylamino group [Eq. (1)]. In acidic
media, further two-electron reduction to amine may occur
Results and discussion
Mechanism of reduction of 5-nitroquinoline
(
Eq. 2) [35, 36, 40].
The mechanism of reduction of 5-nitroquinoline was
studied using pH dependence in BR buffer of pH 2.0–12.0
using DC and DP voltammetry and further by cyclic
voltammetric experiments. As relatively extended literature
exists on the mechanism of reduction of nitro group at
aromatic skeleton at liquid mercury and solid electrodes
À
þ
ArNO þ 4e þ 4H ! ArNHOH þ H O
ð1Þ
ð2Þ
2
2
À
þ
ArNHOH þ 2e þ 2H ! ArNH2 þ H2O
At mercury-based electrodes and solid electrodes, in the
alkaline or non-aqueous media the lack of protons may lead
1
23

Factors influencing voltammetric reduction of 5-nitroquinoline at boron-doped diamond…
Table 1 Overview of voltammetric methods for the determination of 5-nitroquinoline
23
-
3
-3
Electrode
m-AgSAE
LOQ/lmol dm
LDR/lmol dm
Method
pH, medium
References
[46]
-
pH 9.0; 0.05 mol dm borate buffer
3
0.3
0.2–100
0.4–100
DPV
DCV
DPV
DCTP
DPP
0
.5
a
–
Carbon film
DME
0.5
0.9
pH 11.0, BR buffer
pH 3.0, BR buffer
pH 3.0, BR buffer
[44]
[45]
a
–
a
–
0
.09
.01
a
–
-3
0.2 mol dm NaOH
0
DPP
a
–
-3
0.2 mol dm NaOH
HMDE
0.02
DPV
DCTP direct current tast polarography, DCV direct current voltammetry, DPV/P differential pulse voltammetry/polarography, DME dropping
mercury electrode, HMDE hanging mercury drop electrode, m-AgSAE mercury meniscus-modified silver solid amalgam electrode, LDR linear
dynamic range, LOQ limit of quantification
a
Not given
[
20, 35–37], analogies and differences could be found in
A
B
the behavior of the studied compound.
DP and DC voltammetry–influence of pH
Figure 1a, b represents pH-dependence of DC and DP
voltammograms of 5-nitroquinoline. Relatively well-
shaped main reduction signal can be traced in the whole pH
range tested (2.0–12.0), with the slope of the peak potential
Ep vs. pH dependence of -83.52 mV between pH 2.0–5.0.
It corresponds to the nitro group reduction to hydroxy-
lamine according to Eq. (1). This signal is accompanied by
indistinctive signals at more negative potential at pH values
Fig. 2 Dependence of the a peak currents (I
peak in the presence (closed square) and absence (open square) of
oxygen and dependence of the b peak potentials (E ) on pH.
Measured for 5-nitroquinoline (c = 1910 mol dm ) using DP
voltammetry. The error bars are constructed as standard deviations
p
) of the first cathodic
p
-
4
-3
6
.0–12.0, as obvious from peak potential Ep vs. pH
dependence at Fig. 2a. These are of different origins:
(
n = 5)
1
.
For the most alkaline media 11.0 and 12.0 the splitting
of the nitro-group reduction into two steps according to
Eqs. (3) and (4) is foreseen, the reduction peaks of
both processes lay within a narrow potential region of
250 mV. Thus, the first, pH-independent step corre-
sponds to a fast one electron reduction of the nitro
group to a nitro radical [Eq. (3)] and the second step
Fig. 1 Selected a DC, b DP
voltammograms of
-
4
5
-nitroquinoline (c = 1 9 10
-
3
mol dm ) at BDD electrode
B/C = 4000 ppm) in BR buffer
pH 2.0–12.0; the pH values are
(
noted by the curves. The inset
c) in (b) shows DP
(
voltammograms in the presence
of oxygen. Scan rate for DPV is
-
1
2
5
0 mV s and for DCV is
0 mV s
-1
123

2
4
J. Vos a´ hlov a´ et al.
corresponds to the three electron reduction of the nitro
radical to the hydroxylamine [Eq. (4)]. This type of
splitting of the main reduction peak was described for
example for reduction of 5-nitroquinoline and 6-nitro-
quinoline [38] and nitronaphtalenes [37] at amalgam
electrodes. Generally it occurs when the transfer of the
second electron in the overall four-electron reduction
(5): The reduction of the quinoline skeleton [Eq. (5)] is
preceded by reduction of the hydroxylamino derivative
to 5-aminoquinoline [Eq. (2)]. The latter reaction is
enabled by stabilization of the product of dehydration of
the hydroxylamine intermediate through resonance
structures involving heterocyclic nitrogen. This path-
way was found for several heterocyclic nitro derivatives
[36, 40], nevertheless is not common for nitro deriva-
tives of polycyclic aromatic hydrocarbons, which
undergo reduction to the amino derivative only in acidic
media [36]. The peak corresponding to reduction of the
[
Eq. (1)] is inhibited, as e.g., in non-aqueous media or
surfactant containing media at mercury electrodes [20,
6] or at solid electrodes in alkaline media [37],
3
including BDD electrode [30, 31], where the rate of
electron transfer is diminished by the solid character of
the electrode surface and simultaneously the lack of
protons influences the reaction pathway. Clearly the
surface of BDD electrodes has the same inhibiting
effect on nitro group reduction of 5-nitroquinoline in
alkaline media as other solid electrode materials.
Nevertheless, this reduction splitting of aromatic nitro
group cannot be assessed as general rule at BDD
electrodes, because in previous reports on reduction of
quinoline skeleton is characterized by dE /dpH value of
p
52.2 mV in pH range 6.0–10.0, which is close to the
theoretical value of 59 mV for an electrochemical
reaction with equal number of protons and elec-
trons, in accordance with Eq. (5). At these pH values
the quinoline skeleton is not protonized (pKA of
5-nitroquinoline is 2.73) [46]. The reduction of quino-
line skeleton does not appear in the range of pH 2.0–5.0
of BR buffer at BDD electrode probably due to shorter
potential window in acidic media for BDD electrode.
•
-
3
-nitrofluoranthene [34], formation of ArNO2 was not
reported and its stabilization is obviously connected to
boron-doping level and other factors influencing
electrochemical properties of BDD surface, and further
content of organic cosolvent, and structure of the
aromatic compound itself [30, 31].
These two reduction peaks at the end of potential win-
dow appeared also in voltammograms of 5-nitroquinoline
at meniscus modified silver solid amalgam electrode in pH
range 7.0–12.0 [38].
Furthermore, the influence of oxygen presence on the
peak height and repeatability of 5-nitroquinoline reduction
was investigated in solutions open to air in all pH range
tested. Obviously oxygen reduction is inhibited at BDD
electrode and the potential of the first cathodic reduction
peak is in the range from ca -450 mV to -750 mV, i.e. in
the region of reduction of 5-nitroquinoline. As expected,
the DP voltammograms (Fig. 1b) of oxygen-free solution
exhibit of about 10–30 % lower current response in the
presence of 5-nitroquinoline than these when oxygen is
present (DP voltammograms at Fig. 1c, evaluation of peak
heights at Fig. 2a). This effect is mostly pronounced in
alkaline media, but importantly, the peak height repeata-
bility is not affected and remains acceptable for all
investigated media, mostly in the range from 0.7 to 3.3 %
for DPV with oxygen, and from 0.8 to 4.5 % for DPV
without oxygen. For DCV higher values with maximum of
2
.
In BR buffer pH 5.0–10.0 two pH-dependent signals at
far negative potentials of ca -1000 to -1250 mV were
observed. To confirm whether these signals can be
assigned to the reduction of the quinoline skeleton, the
reduction of quinoline has been investigated under the
same conditions. An example of DP voltammogram of
quinoline in BR buffer pH 9.0 is given in Fig. 3. It shows
one reduction peak at the potential of -1183 mV while
the curve of 5-nitroquinoline shows two steps reduction
at the potentials of -1117 and -1255 mV, presumably
corresponding to processes described in Eqs. (2) and
7.1 % with oxygen and 6.5 % without oxygen were
achieved (relative standard deviation evaluated from five
measurements). These results are promising for determi-
nation of electrochemically reducible organic compounds
by means of BDD-based sensors in the presence of oxygen.
This could be very advantageous from the analytical point
of view because oxygen removal from solutions might be
problematic and its traces cause problems e.g. when using
electrochemical detection at mercury-based electrochemi-
cal sensors in liquid flow techniques [38, 47, 48].
-
1
Fig. 3 DP voltammograms (scan rate 20 mV s ) of 5-nitroquinoline
(
medium pH 9.0 (c)
-4
-3
a) and quinoline (b) (for both c = 1 9 10 mol dm ) in BR buffer
1
23

Factors influencing voltammetric reduction of 5-nitroquinoline at boron-doped diamond…
Fig. 4 Cyclic voltammograms
25
of 5-nitroquinoline
(
0
A
B
-
c = 1 9 10 mol dm ) in
4
-3
-
3
.1 mol dm acetate buffer pH
5.0: a ten consecutive cycles
from ?1.0 V to -1.3 V with
the first cathodic scan starting at
?
rate 100 mV s , and b the first
0.36 V (dashed line), scan
-1
-
1
cycle for scan rates (mV s ): 5
a), 10 (b), 20 (c), 40 (d), 80 (e),
60 (f), 320 (g), 640 (h), 1280
ch), 2560 (i), 5120 (j)
(
1
(
-
3
Further measurements were performed in 0.1 mol dm
acetate buffer pH 5.0; in this media only the main reduction
signal corresponding to Eq. (1) is present.
Cyclic voltammetry
-
3
Cyclic voltammograms in 0.1 mol dm acetate buffer pH
.0 exhibited features typical for electrochemical reduction
5
of aromatic nitro group (Fig. 4a). It is the main irreversible
cathodic peak p , corresponding to nitro group reduction
k1
to hydroxylamine [Eq. (1)], followed by the pair of peaks
pa1 and pk2 in the reversed anodic/second cathodic scan at
the potentials of ca ?300 and 0 mV, corresponding to
quasireversible oxidation/reduction of the pair hydroxy-
lamino/nitroso derivative [Eq. (7); pa1 and pk2 in Fig. 4a].
This suggestion is confirmed by the fact that the cathodic
peak pk2 is absent in the first cathodic scan, similarly, the
anodic peak pa1 is absent when starting the scan at 0 V in
positive direction. The behavior is in agreement with lit-
erature [40].
Fig. 5 Eight differential pulse voltammograms of 5-nitroquinoline
-4
-3
-3
c = 1 9 10 mol dm ) in 0.1 mol dm pH 5.0 acetate buffer
(
using different pretreatment of the electrode in 0.5 mol dm sulfuric
acid with positive or negative potential: a anodic pretreatment (5 min,
-3
?2.4 V), b cathodic pretreatment (10 min, -2.4 V). The scan rate
-1
was 20 mV s
(B/C 4000 ppm). Anodic pretreatment at the potential of
À
ArNHOH $ ArNO þ 2e þ 2Hþ
?2.4 V for 5 min and cathodic pretreatment at the poten-
ð7Þ
Further cycling leads to decrease of pk1 and increase of
-
3
tial of -2.4 V for 10 min in 0.5 mol dm sulfuric acid
and three types of activation between individual scans
directly in the measured solution were tested: anodic
activation, cathodic activation, and stirring without appli-
cation of potential. It was necessary because without any
activation the signal height of 5-nitroquinoline is decreas-
ing as obvious from Fig. 5: For eight consecutive scans,
anodic pretreatment exhibits faster stabilization of elec-
trode response and ca 100 mV more negative peak
potential than the cathodic pretreatment with a fast decline
in peak height for the first three scans. This decline is
caused by instability of the H-terminated surface resulting
from cathodic pretreatment; that surface is known to be
relatively unstable not only in solutions, but also in air [49].
Thus, the activation between individual scans was also
necessary. For all activation modes relative stability of
electrode response was achieved, but as the application of
cathodic or anodic potential had no explicitly positive
pa1 and pk2 as result of surface passivation (p ) and for-
k1
mation of reaction products (pa1 and p ). The main
k2
reaction–reduction of nitro group to hydroxylamine
[
Eq. (1)] is controlled by diffusion as proved by linear
1
course of the peak current I vs. scan rate v dependence
/2
p
-
in the range from 10 to 80 mV s characterized by the
1
1
/2
-1
regression
R = 0.996); corresponding voltammograms are depicted
at Fig. 4b.
line:
Ip/nA = -257.3v /(mV s ) ? 2.8
(
Pretreatment of BDD electrode and calibration
dependences of 5-nitroquinoline
Optimal combination of electrode pretreatment and acti-
vation between individual scans was tested in
-
.1 mol dm acetate buffer pH 5.0 with BDD electrode
3
0
123

2
6
J. Vos a´ hlov a´ et al.
Table 2 Parameters of the calibration straight lines and limits of
detection and quantification for the reductive determination of
(E = 1 2.4 V, t = 5 min) or cathodic (E = 22.4 V, t = 10 min)
pretreated BDD electrode (B/C 4000 ppm)
5
-nitroquinoline using DC and DP voltammetry with anodic
3
23
21
a
23
23
BDD pretreatment LDR/lmol dm
R
Slope/nA dm lmol
Intercept/nA RSD (%)
LOQ/lmol dm
LOD/lmol dm
DPV
Anodic
0.5–100
0.5–75
0.988 -6.02 ± 0.06
0.996 -4.59 ± 0.05
-4.1 ± 0.2
6.5
0.66
1.68
0.20
0.50
Cathodic
-10.4 ± 0.7 12
DCV
Anodic
10–100
7.5–75
0.997 -9.73 ± 0.12
0.998 -7.61 ± 0.14
24.1 ± 6.4 10
8.9
2.7
4.7
Cathodic
-31.1 ± 1.1 13
15.7
-3
Supporting electrolyte was 0.1 mol dm acetate buffer pH 5.0, 20 s stirring between individual scans applied
LOQ limit of quantification, LOD limit of detection, R correlation coefficient
a
Relative standard deviation (RSD) of ten times repeated measurement at the lowest concentration of LDR (linear dynamic range)
-
3
Table 3 Peak potentials E
standard deviations (RSD) evaluated from DC and DP voltammo-
p
, peak heights I
p
and their relative
0.1 mol dm acetate buffer pH 5.0. Measured by BDD electrodes
with B/C ratio in the range of 500–8000 ppm
-
4
-3
grams of 5-nitroquinoline (c = 1 9 10
B/C ratio/ppm DPV
mol dm
)
in
DCV
E
p
/mV
I
p
/nA
RSD/%
E
p
/mV
p
I /nA
RSD/%
5
1
2
4
8
00
-561
-498
-388
-438
-442
-579 ± 15
-255 ± 13
2.7
5.2
1.8
2.5
1.2
-797
-536
-443
-510
-519
-2502 ± 84
-691 ± 53
-3616 ± 57
-3720 ± 52
-3581 ± 83
3.4
7.2
1.6
1.4
2.3
000
000
000
000
-2065 ± 38
-1630 ± 40
-1589 ± 18
-
4
-3
effect and RSD values of peak height were comparable
with these when using stirring between individual scans,
only stirring for 20 s was used to assure repeatable signals.
For DC and DP voltammetry the RSD values of peak
height were 6.5 and 0.5 % for cathodic pretreatment and
of 1 9 10
mol dm
solution of 5-nitroquinoline in
-
3
0.1 mol dm acetate buffer pH 5.0. Obviously, the peak
height for electrodes with metallic type of conductivity
(2000, 4000, and 8000 ppm) is comparable and signifi-
cantly higher than for semiconductive BDD electrodes
(500 ppm and 1000 ppm). Simultaneously, their peak
potential is shifted to more positive values confirming the
easier reduction of nitro group using 2000–8000 ppm
electrodes. Further, favorable repeatability of peak height
with RSD values B2.5 % for this set of electrodes was
achieved. Among them, the 2000 ppm electrode exhibits
additional favorable characteristics: low background cur-
rent and negative shift in onset of supporting electrolyte
caused by hydrogen evolution, which enables visualization
of the reduction of the quinoline skeleton at the potential of
about -910 mV; at the 4000 and 8000 ppm electrodes this
signal is only insinuated.
-
4
2
.1 and 4.6 % for anodic pretreatment (c = 1 9 10
-
3
mol dm , n = 10), respectively. For shorter times insta-
bility of electrode response was observed for all activation
modes.
Parameters of the calibration straight lines for both types
of pretreatment are given in Table 2. Better limits of
-
7
-6
-3
detection in the 10 and 10 mol dm concentration
range for DPV and DCV was obtained using anodic pre-
treatment, which is given by lower values of peak height
repeatability for the lowest measurable concentration and
higher value of the slope, i.e., parameters used for calcu-
lation of detection limit. Anodic pretreatment compared to
cathodic one should be also preferred with respect to the
extent of the linear dynamic range.
Obviously, the electrodes with metallic type of con-
ductivity perform similarly as in oxidation of
benzophenone-3 [13], where the 2000 ppm electrode
exhibited the highest slope of the linear calibration
dependence. This electrode with the boron-doping level
just above the semiconductive/metallic conductivity
threshold seems favorable in terms of electroanalytical
performance.
Boron-doping level of BDD
Boron-doping level significantly affects the height of the
peak and its potential, as summarized in Table 3 and shown
in Fig. 6, where are depicted DC and DP voltammograms
1
23

Factors influencing voltammetric reduction of 5-nitroquinoline at boron-doped diamond…
Fig. 6 A DC and B DP
27
A
B
voltammograms of
-
4
5-nitroquinoline (c = 1 9 10
-3
mol dm ) in 0.1 mol dm
-
3
acetate buffer pH 5.0 measured
with BDD electrodes with
different boron concentration
(
(
(
B/C ratio): (a) 500 ppm,
b) 1000 ppm, (c) 2000 ppm,
d) 4000 ppm, (e) 8000 ppm.
-
1
Scan rate for DPV is 20 mV s
-
and for DCV is 50 mV s
1
Conclusion
used for the development HPLC-ED method enabling
separation and detection of nitro group containing aromatic
compounds.
The voltammetric reduction of 5-nitroquinoline was elu-
cidated at boron doped diamond (BDD) electrode in
aqueous media of pH 2.0–12.0. The signal of the nitro
group largely depends on the pH of the indifferent elec-
trolyte, electrode pretreatment, activation between
individual scans, and boron concentration of the BDD film
electrode. In alkaline media, the four-electron reduction of
the nitro to the hydroxylamino group occurs in two sepa-
rated steps with the first one being a one electron reduction
of the nitro group to the nitro-radical anion—a mechanism
pathway previously recognized for 5-nitroquinoline at
amalgam [38] electrodes or other nitro group containing
aromatics at BDD electrodes [30]. Presence of oxygen in
the measured solutions led to slight increase of peak
heights and acceptable increase of its relative standard
deviation. The detection limit for DPV achieved using
optimized protocol, i.e. anodic pretreatment at ?2.4 V for
Experimental
Stock solution of 5-nitroquinoline (99 %, Sigma-Aldrich,
Czech Republic) was prepared by dissolving exact quantity
-
3
in deionized water for final concentration of 1 9 10
-
mol dm . The experiments were carried out in Britton-
3
-
3
Robinson (BR) buffer or 0.1 mol dm acetate buffer (pH
5.0) at laboratory temperature. BR buffers were prepared
by mixing a solution of phosphoric, acetic and boric acid
-
3
(concentration of each 0.04 mol dm , all p.a., Lach-Ner,
Czech Republic) with an appropriate amount of
-
3
0.2 mol dm
sodium hydroxide solution. All solutions
were prepared in deionized water (Millipore, Billerica,
MA, USA). Other used chemicals were: acetic acid (Lach-
Ner, Neratovice, Czech Republic), quinoline (Merck,
Czech Republic). All measurements were performed using
computer controlled Eco-Tribo Polarograph with PolarPro
software (version 5.1, Eco-Trend Plus, Prague, Czech
Republic) in a three electrode arrangement involving
platinum wire auxiliary electrode and silver–silver chloride
-
3
5
min in 0.5 mol dm H SO and 20 s stirring between
2 4
-
7
individual scans assured limit of detection in the 10
-
3
mol dm concentration range, which is comparable with
detection limits obtained at other solid electrode materials
(
compare in Table 1) including amalgam [38] and carbon
film [44] electrodes. The nitro group reduction results in
well-developed, observable signals at BDD films with
metallic type of conductivity deposited at B/C ratio
-
3
reference electrode (Ag|AgCl, 3 mol dm
KCl) (both
Elektrochemick e´ detektory, Turnov, Czech Republic). As
working electrodes served boron doped diamond electrodes
with boron-doping level 500, 1000, 2000, 4000, and
8000 ppm (B/C ratio during microwave-plasma assisted
chemical vapor procedure described in [13]).Obtained
BDD films at Si wafers were placed in Teflon electrode
body constructed in our laboratory [32] with geometric
2
000–8000 ppm, but not using semiconductive films 500
and 1000 ppm. On the other hand, the reduction of the
quinoline skeleton close to the onset of supporting elec-
trolyte is well observable only at the 2000 ppm electrode.
2
This might be connected with the increasing content of sp
impurities, existence of boron clusters [17, 50] and other
factors influencing electron transfer and processes limiting
the potential window when increasing boron-doping level.
To conclude, BDD electrodes seem to be good analyti-
cal alternative to determinations based on reduction of
aromatic nitro group. For this purpose, highly doped BDD
films are recommendable. The experimental data might be
2
surface area of 5.72 mm (disk diameter 2.7 mm). If not
-
3
otherwise stated, the 4000 ppm films and 0.1 mol dm
acetate buffer pH 5.0 as supporting electrolyte was used.
Differential pulse voltammetry was performed using the
-
1
scan rate of 20 mV s with pulse amplitude -50 mV for
-
80 ms. Scan rate 50 mV s was used for DC voltammetry
1
123

2
8
J. Vos a´ hlov a´ et al.
-
and 100 mV s for cyclic voltammetry, if not otherwise
1
ˇ
Applications (an Overview). In: Kalcher K, Metelka R, Svancara
I, Vyt rˇ as K (eds), Sensing in Electroanalysis. University Press
Centre, University of Pardubice, Pardubice, p 21
stated.
-
3
The BDD electrode was pretreated in 0.5 mol dm
1
0. Zivcova ZV, Frank O, Petrak V, Tarabkova H, Vacik J, Nesladek
M, Kavan L (2013) Electrochim Acta 87:518
sulfuric acid for 5 min with potential ? 2.4 V before the
measurement every day. During DP and DC voltammetry
was BDD electrode activated for 20 s between each scan.
The influence of boron content was tested at
11. Trouillon R, O’Hare D, Einaga Y (2011) Phys Chem Chem Phys
3:5422
1
1
2. Guinea E, Garrido JA, Rodriguez RM, Cabot PL, Arias C, Cen-
tellas F, Brillas E (2010) Electrochim Acta 55:2101
5
00–8000 ppm electrodes after anodic pretreatment or
-
13. Zavazalova J, Prochazkova K, Schwarzova-Peckova K (2016)
Anal Lett 49:80
3
5
min in 0.5 mol dm sulfuric acid at the potential of
2.4 V, 20 s stirring between individual voltammetric
1
4. Vosahlova J, Zavazalova J, Schwarzova-Peckova K (2014) Chem
Listy 108:s270
?
scans was applied. All measurements were carried out at
laboratory temperature. The pH measurements were carried
out by digital pH Meter 3510 (Jenway, UK) with combined
glass electrode.
1
5. Williams AW, Lightowl EC, Collins AT (1970) J Phys C: Solid
State Phys 3:1727
16. Harfield JC, Toghill KE, Batchelor-McAuley C, Downing C,
Compton RG (2011) Electroanalysis 23:931
1
7. Bernard M, Baron C, Deneuville A (2004) Diam Relat Mat
3:896
The solutions for measurements were prepared in
3
0 cm volumetric flasks by measuring of proper volume
1
1
18. Yano T, Tryk DA, Hashimoto K, Fujishima A (1998) J Elec-
trochem Soc 145:1870
19. Ernst S, Aldous L, Compton RG (2011) J Electroanal Chem
of the 5-nitroquinoline stock solution and filling by BR
-
buffer of the required pH or 0.1 mol dm acetate buffer
3
6
0. Vyskocil V, Barek J (2011) Curr Org Chem 15:3059
63:108
pH 5.0 up to the mark. For DPV, the peak heights (I ) were
p
2
measured from the straight line connecting minima on both
sides of the peak. In DCV they were measured from the
line prolonging the voltammetric curve before the onset of
the voltammetric signal of 5-nitroquinoline.
21. World Health Organization (2003) Selected Nitro- and Nitro-
Oxy-Polycyclic Aromatic Hydrocarbons. WHO, Geneva
2
2
2. Musilova J, Barek J, Peckova K (2011) Electroanalysis 23:1236
3. Garbellini GS, Salazar-Banda GR, Avaca LA (2007) J Braz
Chem Soc 18:1095
All calibration curves were measured in triplicate. The
calibration dependences were processed using linear
regression method. For voltammetric measurements, limits
of detection (LOD) were calculated as the concentration of
the analyte, which gave the signal equal to three times the
standard deviation of peak heights estimated from ten
consecutive measurements of the lowest measurable
concentration.
24. Pedrosa VA, Codognoto L, Machado SAS, Avaca LA (2004) J
Electroanal Chem 573:11
2
5. Pedrosa VA, Suffredini HB, Codognoto L, Tanimoto ST,
Machado SAS, Avaca LA (2005) Anal Lett 38:1115
6. Karaova J, Barek J, Schwarzova-Peckova K (2016) Anal Lett
49:66
2
27. Dejmkova H, Barek J, Zima J (2011) Int J Electrochem Sci
:3550
8. Garbellini GS, Salazar-Banda GR, Avaca LA (2009) Food Chem
16:1029
6
2
2
1
9. Chuanuwatanakul S, Chailapakul O, Motomizu S (2008) Anal Sci
24:493
Acknowledgments The research was financially supported by the
Grant Agency of the Charles University in Prague (Project GAUK
30. Juliao MSD, Almeida EC, La Scalea MA, Ferreira NG, Compton
RG, Serrano SHP (2005) Electroanalysis 17:269
6
84213) and Charles University in Prague (Project SVV).
3
1. Juliao MSD, Ferreira EI, Ferreira NG, Serrano SHP (2006)
Electrochim Acta 51:5080
3
2. Martins I, Canaes LD, Doretto KM, Rath S (2010) Electroanal-
ysis 22:455
References
3
3
3. Yosypchuk O, Barek J, Vyskocil V (2012) Anal Lett 45:449
4. Cizek K, Barek J, Fischer J, Peckova K, Zima J (2007) Electro-
analysis 19:1295
1
2
. Fujishima A, Einaga Y, Rao TN, Tryk DA (2005) Diamond
Electrochemistry. Elsevier, Amsterdam
. Peckova K, Musilova J, Barek J (2009) Crit Rev Anal Chem
3
3
3
3
3
4
5. Laviron E, Vallat A, Meunier-Prest R (1994) J Electroanal Chem
3
3
9:148
79:427
6. Zuman P, Fijalek Z, Dumanovic D, Suznjevic D (1992) Elec-
troanalysis 4:783
3
4
5
. Musilova J, Barek J, Peckova K (2009) Chem Listy 103:469
. Peckova K, Barek J (2011) Curr Org Chem 15:3014
. Peckova-Schwarzova K, Zima J, Barek J (2015) Determination of
Aromatic Hydrocarbons and Their Derivatives. In: Moretto LM,
Kalcher K (eds) Electrochemical Analysis by Electrochemical
Sensors and Biosensors. Springer, New York, p 931
7. Peckova K, Barek J, Navratil T, Yosypchuk B, Zima J (2009)
Anal Lett 42:2339
8. Jiranek I, Peckova K, Kralova Z, Moreira JC, Barek J (2009)
Electrochim Acta 54:1939
9. Dar RA, Brahman PK, Tiwari S, Pitre KS (2012) Colloid Surf
B-Biointerfaces 98:72
0. Gal M, Hromadova M, Pospisil L, Hives J, Sokolova R, Koli-
voska V, Bulickova J (2010) Bioelectrochemistry 78:118
1. Pech J (1934) Collect Czech Chem Commun 6:126
2. Zhan XM, Liu LH, Gao ZN (2011) J Solid State Electrochem
6
7
8
9
. Holt KB, Bard AJ, Show Y, Swain GM (2004) J Phys Chem B
1
08:15117
. Hutton LA, Iacobini JG, Bitziou E, Channon RB, Newton ME,
Macpherson JV (2013) Anal Chem 85:7230
. Suffredini HB, Pedrosa VA, Codognoto L, Machado SAS,
Rocha-Filho RC, Avaca LA (2004) Electrochim Acta 49:4021
. Zavazalova J, Barek J, Peckova K (2013/2014) Boron Doped
Diamond Electrodes in Voltammetry: New Designs and
4
4
1
5:1185
1
23

Factors influencing voltammetric reduction of 5-nitroquinoline at boron-doped diamond…
29
4
4
3. Geto A, Amare M, Tessema M, Admassie S (2012) Anal Bioanal
Chem 404:525
4. Jiranek I, Rumlova T, Barek J (2010) Voltammetric Determina-
tion of 5-nitroquinoline at a Carbon Film Electrode. In: Navr a´ til
T, Barek J (eds) XXX Modern Electrochemical Methods. BEST
Servis, Usti nad Labem, Jet ˇr ichovice, p 93
5. Vyskocil V, Jiranek I, Danhel A, Zima J, Barek J, Wang J,
Peckova K (2011) Collect Czech Chem Commun 76:1991
6. Armarego WL (1962) J Chem Soc:4094
47. Peckova K, Vrzalova L, Bencko V, Barek J (2009) Collect Czech
Chem Commun 74:1697
48. Yosypchuk O, Karasek J, Vyskocil V, Barek J, Peckova K (2012)
Sci World J, ID 231986
49. Salazar-Banda GR, Andrade LS, Nascente PAP, Pizani PS,
Rocha RC, Avaca LA (2006) Electrochim Acta 51:4612
50. Azevedo AF, Baldan MR, Ferreira NG (2013) J Phys Chem
Solids 74:599
4
4
123