Cobalt nanoflowers: Synthesis, characterization and derivatization to cobalt hexacyanoferrate - Electrocatalytic oxidation and determination of sulfite and nitrite

Article abstract of DOI:10.1016/j.electacta.2012.06.014

Cobalt hexacyanoferrate (CoHCF) nanostructure was synthesized by anodic oxidation of metallic cobalt nanoflowers in a solution of K₃Fe(CN) ₆. The synthesized CoHCF sample was then employed to prepare a modified carbon paste electrode. The modified electrode was characterized electrochemically in a phosphate buffer solution at physiological pH. Two redox transitions were appeared in the voltammograms which were related to the redox processes of Co^II/Co^III and Fe^II/Fe ^III in the solid state of CoHCF. The modified electrode was successfully applied to the electrooxidation of nitrite and sulfite and these substrates were oxidized electrocatalytically on the modified electrode surface via the active Fe^III species. The catalytic rate constants, the electron transfer coefficients and diffusion coefficients involved in the electrocatalytic oxidation of the compounds were reported. The modified electrode was applied to the amperometric determination of nitrite and sulfite.

Full text of DOI:10.1016/j.electacta.2012.06.014

Electrochimica Acta 77 (2012) 294–301
Contents lists available at SciVerse ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Cobalt nanoflowers: Synthesis, characterization and derivatization to cobalt
hexacyanoferrate—Electrocatalytic oxidation and determination
of sulfite and nitrite
H. Helia,∗, I. Eskandarib, N. Sattarahmadyc,d, A.A. Moosavi-Movahedie
a Laboratory of Analytical and Physical Electrochemistry, Department of Chemistry, Science and Research Branch, Islamic Azad University, Fars, 7348113111, Iran
b Department of Chemistry, Islamic Azad University, Firouzabad Branch, Firouzabad, Iran
c Department of Medical Physics, School of Medicine, Shiraz University of Medical Sciences, Shiraz, Iran
d Pharmaceutical Science Research Center, Shiraz University of Medical Sciences, Shiraz, Iran
e Institute of Biochemistry and Biophysics, University of Tehran, Tehran, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Cobalt hexacyanoferrate (CoHCF) nanostructure was synthesized by anodic oxidation of metallic cobalt
nanoflowers in a solution of K3Fe(CN)6. The synthesized CoHCF sample was then employed to prepare a
modified carbon paste electrode. The modified electrode was characterized electrochemically in a phos-
phate buffer solution at physiological pH. Two redox transitions were appeared in the voltammograms
which were related to the redox processes of CoII/CoIII and FeII/FeIII in the solid state of CoHCF. The
modified electrode was successfully applied to the electrooxidation of nitrite and sulfite and these sub-
strates were oxidized electrocatalytically on the modified electrode surface via the active FeIII species.
The catalytic rate constants, the electron transfer coefficients and diffusion coefficients involved in the
electrocatalytic oxidation of the compounds were reported. The modified electrode was applied to the
amperometric determination of nitrite and sulfite.
Received 25 February 2012
Received in revised form 30 May 2012
Accepted 5 June 2012
Available online 15 June 2012
Keywords:
Cobalt hexacyanoferrate
Cobalt nanoflowers
Electrocatalysis
Modified electrode
Nanoflowers
© 2012 Elsevier Ltd. All rights reserved.
Nitrite
Sulfite
1. Introduction
of its unique chemical and electrochemical properties [11,14–16].
In CoHCF, both cobalt and iron have two oxidation states and
The recent advancement of nanoscience and nanotechnology
has been accompanied by the synthesis of various sized and
shaped nanostructured materials with novel characteristics [1].
In recent years, synthesis, characterization and applications of
metal and metal oxide nanostructures have attracted the atten-
tion of researchers. In this regard, several synthesis methods have
been employed for the preparation of nanoparticles, nanorods,
nanowires, nanorings, nanotubes, nanosheets, nanoflakes and
nanoflowers [2–8]. In comparison with bulk materials, size and
shape of nanostructured materials affect their properties and
potential applications [9–13].
Metal hexacyanoferrates (MHCFs) with a general formula of
Ma[Fe(CN)6]b·nH2O (M: a metal ion), as an important class of
mixed-valence compounds, have a wide range of applications
including sensors, optical control, ion exchanging, electrocatalysis
and electrochromism [11,14]. Among the MHCFs, cobalt hexa-
cyanoferrate (CoHCF) is very attractive for researchers, because
redox couples of (II) and (III), leading to a multitude of compound
stoichiometries, redox states and unique electrocatalytic and
electrochromic properties, reversible photoinduced magnetization
and thermochromism behavior in a wide range of temperatures
[11,14–18].
Chemically modified electrodes have attracted much attention
in the study of the kinetics of electrocatalytic oxidation and reduc-
tion of many important redox systems [19]. In such an electrode,
an electrocatalytic system is generated and undergoes a rapid and
reversible electrode reaction. On the other hand, modified elec-
trodes can reduce the overpotential required for either oxidation
or reduction of a substrate. Modified electrodes have a wide range of
potential applications in electrochemical technology, energy con-
version and storage systems, information storage, electrochromic,
and display devices and in electroanalysis [10,15,16,20,21]. In this
regard, immobilization of surface-active materials that can, in prin-
ciple, alternate between various valence states under the effect
of external electric fields is of particular interest. These surface-
active materials are then capable of mediating fast electron transfer
between a substrate in the bulk of the solution and the elec-
trode surface. Among the chemically modified electrodes, carbon
∗
Corresponding author. Tel.: +98 728 46 92 114; fax: +98 728 46 92 110.
E-mail address: hheli7@yahoo.com (H. Heli).
0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.electacta.2012.06.014

H. Heli et al. / Electrochimica Acta 77 (2012) 294–301
295
paste electrodes modified by MHCFs have been vastly studied
[22].
agate mortar for 20 min. The resulting paste was kept at room tem-
perature in a desiccator before use. The paste was packed firmly
into a cavity (4.05 mm diameter, geometric surface are of 0.128 cm2
and 0.5 mm depth) at the end of a Teflon tube. Electrical contact was
established via a copper wire connected to the paste in the inner
hole of the tube. The electrode surface was gently smoothed by rub-
bing on a piece of weighing paper just prior to use. This procedure
was also used to regenerate the surface of the electrode.
Cobalt nanoflowers-modified carbon paste electrode (CoCPE)
was prepared by hand-mixing carbon microparticles, mineral oil
and cobalt nanoflowers with a 75/20/5% (w/w) ratio. Derivatization
of cobalt nanoflowers to cobalt hexacyanoferrate and prepara-
tion of cobalt hexacyanoferrate-modified carbon paste electrode
(MCPE) was performed by CoCPE anodizing. CoCPE was transferred
to a 10 mL solution of 0.5 M KCl containing 0.5 M K3[Fe(CN)6]. Then,
a positive potential of 1200 mV for 250 s was applied. During this
step, the cobalt nanoflowers immobilized on the electrode surface,
were oxidized under application of this highly positive potential
to the cobalt ions. In the presence of hexacyanoferrate ions in the
solution, these cobalt ions derivated to CoHCF and precipitated in
situ on the nanoflowers surface.
In the present study, we report a novel and template-free
approach to synthesize nanoflowers of cobalt. The nanoflowers
were then derivatized to cobalt hexacyanoferrate and applied as
a chemically modified electrode. The electrode represented fast
faradaic reactions and showed a high efficiency toward the elec-
trocatalytic oxidation of nitrite and sulfite as model compounds.
Based on the results, an amperometric procedure was developed
for the analysis of sulfite and nitrite with a high sensitivity and low
detection limit.
2. Experimental
2.1. Materials
All chemicals used in this study were of analytical grade
from Merck (otherwise those stated) and used without further
purification. Carbon microparticles (graphite fine powder) with a
particle size of less than 50 ␮m were also obtained from Merck.
Polyvinylpyrrolidone (PVP, molecular weight 40,000) was pur-
chased from Loba Chemie. All solutions were prepared with
redistilled water.
3. Results and discussion
2.2. Apparatus
3.1. XRD
Electrochemical measurements were carried out in a conven-
tional three-electrode cell containing 100 mM Na-phosphate buffer
solution, pH 7.4 (PBS) powered by an Autolab 302N potentio-
stat/galvanostat (The Netherlands). An Ag/AgCl, 3 M KCl and a
platinum disk were used as the reference and counter electrodes,
respectively. The system ran on a PC through GPES 4.9 software.
In order to obtain information about the morphology and size
of the synthesized cobalt particles, scanning electron microscopy
(SEM) was performed using a HITACHI Modele S-4160.
Fig. 1 represents an XRD pattern of cobalt nanoflowers. In
the pattern, slightly broadened peaks appeared. It is due to the
nanometer-size effect of the samples [23]. The crystal structure
of the nanoflowers is hexagonal close-packed (hcp) structure. The
main diffraction peaks at 42, 44.5, 48 and 76◦ are indexed to (1 0 0),
(0 0 2), (1 0 1) and (1 1 0) reflections, respectively, of a hexagonal
unit cell of a hcp cobalt structure (Joint Committee on Powder
Diffraction Standards, JCPDS card No. 05-0727).
Powder X-ray diffraction (XRD) patterns were measured by a
Philips X’Pert (The Netherlands) using CuK␣ radiation at 40 kV and
30 mA in the 2ꢀ degree ranging from 35◦ to 80◦.
3.2. Electron microscopy
2.3. Synthesis of cobalt nanoflowers
In order to investigate the surface morphology, structure and
particle size, cobalt particles were examined by SEM. Fig. 2 shows
SEM images of the synthesized cobalt particles with two different
magnifications. It clearly reveals uniform flower-like microspheres
with an average diameter of 5 ␮m. Higher magnification SEM indi-
cates that the flowery microsphere is built of intercrossed 2D
nanoflakes with a thickness of about 50 nm. All nanoflakes in a
single flowery microsphere seem to grow from a center. They are
aligned perpendicular to the surface of the spherical particles and
the flakes are connected with each other via edge sharing to form
nanoflakes unit. These results indicate that the flower-like cobalt
nanostructure can be synthesized successfully by the simple reduc-
tion method from a polyol medium.
It has been reported that CoCl2 forms a complex with PVP with
a formula of [(C6H9NO)2CoCl2]x, where x is the polymerization rate
of PVP [24]. This complex is formed via oxygen atoms in carboxides
and its nitrogen atoms in pyrrolidone rings due to the high polarity
of the lactam ring conferred by resonance stabilization facilitated
by near-planer ring geometry [25,26]. The resultant complex is sol-
uble in organic alcohols [26]. Therefore, the complex of PVP-CoCl2
in such a reaction system is formed. In consequence, a more stable
Co[(N2H4)3]2+ complex is formed. The reactions are as follows:
The cobalt nanoflowers were synthesized by the chemical
reduction in the presence of PVP from a polyol medium the boiling
point of the mixture. Hydrazine was employed as both reducing and
complexing agents. PVP also acts as both complexing and structure-
directing agents. In the synthesis procedure, 0.86 g CoCl2·6H2O and
4.0 g PVP were firstly dissolved in 180 mL ethylene glycol (EG) by
intensive stirring. The stirring was continued for 12 h and after
this time, a purple solution was obtained. Then, 20 mL hydrazine
monohydrate (80% V/V) was added to the solution and continued to
stirring for further 1 h. The solution color after this step was purple-
pink. The mixture was subsequently heated to the boiling point of
the resulting mixture (about 197 ◦C) by refluxing for 6 h. During this
step, the solution color initially turned to dark, and finally, a dark
precipitate was achieved. The system was then slowly cooled down
on the heating mantel to room temperature. The final product was
collected by centrifugation and washed with absolute ethanol for
six times to remove all remnants. The product was dried under a
nitrogen atmosphere and stored in degassed water. The final prod-
uct was a loose dark powder.
2.4. Preparation of the working electrodes
CoCl2 + PVP → CoCl2–PVP(complex)
(1)
(2)
Unmodified carbon paste electrode (UCPE) was prepared by
hand-mixing carbon microparticles and mineral oil with an 80/20%
(w/w) ratio. The paste was carefully mixed and homogenized in an
CoCl2–PVP + 3N2H4 → Co[(N2H4)3]2+ + PVP + 2Cl−

296
H. Heli et al. / Electrochimica Acta 77 (2012) 294–301
Fig. 1. An XRD pattern of cobalt nanoflowers.
The later complex can be reduced to cobalt particles accordingly.
The formation of cobalt can be formulated as follows [27–29]:
its spatial conformation. The cobalt–hydrazine complex provides
a special molecular environment favorable for the nucleation and
growth of cobalt particles with anisotropic morphologies. The
strong crystalline anisotropy of hexagonal cobalt also influences
the structure and the final shape in the complex synthesis solu-
tion. Obviously, both the structure of cobalt complex precursor and
the cobalt crystalline nature direct the formation of nanoflakes of
cobalt. Special hindrance, magnetic attraction of excessive crystal-
lites, and other kinetic factors caused the cobalt nanoflakes to curl
and aggregate in a flower-like structure.
Co[(N2H4)3]2+ → Co + N2 + 2N2H4 + H+
Co[(N2H4)3]2+ + N2H4 → Co + 4NH3 + 2N2 + H2 + 2H+
(3a)
(3b)
The growth of cobalt nanoflowers is based on the above reac-
tions. During the chemical reduction of metal ions, different
morphologies for the metal nanocrystalls may be attained inten-
sively influenced by the precursor of ligand–metal complex along
Fig. 2. SEM images of the as synthesized cobalt particles with two different magnifications.

H. Heli et al. / Electrochimica Acta 77 (2012) 294–301
297
100
80
for n-MCCE at low potential sweep rates. From the slope of these
linear plots and using the following equation [39]:
ꢀ
ꢁ
n2F2
Ip
=
ꢁAꢂ ∗
(6)
60
4RT
40
where Ip is the peak current, A is the geometric surface area of the
electrode, and ꢂ * is the surface coverage of the redox species, the
mean values for surface concentrations of redox species of CoIII/CoII
II
20
and FeIII/FeII are obtained as 2.62 × 10−8 and 4.42 × 10−8 mol cm−2
,
I
0
respectively. It should be noted that the values of ꢂ * obtained
here for cobalt and iron species are the “surface” concentrations
of these species when they are assumed as adsorbed redox species
at the surface of the composite electrode. When the potential was
swept with the values of >60 mV s−1 (high potential sweep rates),
the anodic and cathodic peak currents for both couples I and II
increased linearly with the square root of the potential sweep rate
(Fig. 4D and E). These linear dependencies suggest that diffusion-
controlled steps dominate in the redox couples I and II in long terms,
which can be witnessed from the peak currents in the time scale
of cyclic voltammetry. This is related to the insertion of counter
cations into CoHCF [40]. From the slopes of the lines represented in
Fig. 4D and E, and using the following equation for the semi-infinite
planar diffusion condition for a quasi-reversible kinetic [39]:
-20
-40
-60
-80
-150
50
250
450
650
850
1050
E vs (Ag/AgCl)/mV
Fig. 3. Typical cyclic voltammograms of MCPE in PBS with a potential sweep rate of
10 mV s−1
.
3.3. Study of the charge transfer kinetics across the
MCPE/solution interface
Ip = 2.99 × 105˛s1/2n3/2ADNa
+
1/2Cꢁ0.5
(7)
where A is the active surface area, DNa+ is the diffusion coef-
ficient of sodium cation, C is total concentration of redox sites
and ˛s is the electron-transfer coefficient (vide infra), the average
value of diffusion coefficients for Na+ was obtained for couple I as
1.13 × 10−10 cm2 s−1 and for couple II as 1.73 × 10−10 cm2 s−1. The
differences between the values of the diffusion coefficients can be
related to the differences in the composition and stoichiometry of
the compounds represented in Eqs. (4) and (5).
Laviron derived general expressions for the linear potential
sweep voltammetric response of surface-confined electro-reactive
species at small concentrations [41]. The expressions for the peak-
to-peak potential separation (ꢃEp) are as follows:
Fig. 3 shows typical cyclic voltammograms of MCPE in PBS. The
potential sweep rate was 10 mV s−1. The voltammogram exhibits
two well-defined quasi-reversible redox couples named I and II
at low and high potentials, respectively. The formal potentials (as
mid-peak in the voltammograms recorded at low potential sweep
rates) of the couples I and II are 365 and 830 mV, respectively.
The voltammogram shown is similar to those previously reported
[11,30] and couples I and II are related to the redox transitions of
CoIII/CoII and FeIII/FeII species immobilized at the electrode surface
via the following reactions [31–34]:couple I:
NaCoIII[FeII(CN)6] + Na+ + e− ꢀ Na2CoIIFeII(CN)6
(4a)
(4b)
Na0.4CoIII [FeII(CN)6] + Na+ + e− ꢀ Na1.4CoII [FeII(CN)6]
ꢂ
ꢃ
ꢄ
ꢅ
RT
˛
s
1.3
1.3
Epa = E0
+
+
ln 1 −
(8)
(9)
(1 − ˛s)nF
Y
couple II:
ꢄ
ꢅ
ꢄ
ꢅ
RT
˛
s
Epc = E0
ln
CoI1I.5[FeIII(CN)6] + Na+ + e− ꢀ NaCoII [FeII(CN)6]
CoIII[FeIII(CN)6] + Na+ + e- ꢀ NaCoIII[FeII(CN)6]
(5a)
(5b)
1.5
˛snF
Y
where
ꢄ
ꢅꢄ
ꢅ
RT
ks
Fig. 4A shows the cyclic voltammograms of MCPE in PBS
recorded at different potential sweep rates in range of
5–150 mV s−1
The peak-to-peak potential separation in the
Y =
F
(10)
nꢁ
a
.
and
voltammograms deviated from the theoretical value of zero and
increased at higher potential sweep rates. This result indicates a
limitation in the charge-transfer kinetics, which is due to: (a) chem-
ical interactions between the electrolyte ions and the cobalt species,
(b) dominance of electrostatic factors, (c) the lateral interactions of
the redox couples present on the surface and/or (d) non-equivalent
sites present in the cobalt species. In addition, the average values
of full width at half height for anodic couples I and II are about
228 and 197 mV. Both values are higher than the theoretical value
for non-interacting and one-electron surface sites of 90 mV [35].
This was attributed to repulsive site–site interactions [36,37] and
this repulsive intermolecular interaction is higher for redox cou-
ple I with respect to redox couple II. On the other hand, Prussian
Blue surfaces were reported to have a 50 mV peak width, indicating
attractive interactions [38].
ꢄ
ꢅ
˛s(1 − ˛s)nFꢃEp
RT
ln ks = ˛s ln(1 − ˛s) + (1 − ˛s) ln ˛s − ln
−
nFꢁ
RT
(11)
In these equations, Epa and Epc are the anodic and cathodic
peak potentials, respectively, and ks and ꢁ are the apparent charge
transfer rate constant and potential sweep rate, respectively. From
these expressions, ˛s can be determined by measuring the vari-
ation of the peak potential with respect to the potential sweep
rate, and ks for electron transfer across the MCPE/solution inter-
face can be determined by measuring the Ep values. Fig. 4F and G
show the plots of anodic and cathodic peak potentials with respect
to the logarithm of potential sweep rate for couples I and II, respec-
tively. It can be observed that for potential sweep rates higher than
70 mV s−1, the values of Ep are linearly depend on the logarithm
of the potential sweep rate indicated by Laviron. Using the plots
and Eqs. (8)–(11), the average values of ks for couples I and II were
For both couples I and II, plotting the anodic and cathodic peak
currents with respect to the potential sweep rate at low values
(lower than 45 mV s−1, Fig. 4B and C) shows linear dependencies.
These results indicate a surface-confined electrochemical behavior

298
H. Heli et al. / Electrochimica Acta 77 (2012) 294–301
Fig. 4. A: Cyclic voltammograms of MCPE in PBS recorded at different potential sweep rates of 5, 7, 9, 12, 15, 18, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 120, 130,
140 and 150 mV s−1. Dependency of the anodic (B) and cathodic (C) peak currents to the potential sweep rate at low values of the potential sweep rates. Dependency of
the anodic (D) and cathodic (E) peak currents to the square root of the potential sweep rate at high values of the potential sweep rates. Plots of anodic and cathodic peak
potentials with respect to the logarithm of potential sweep rate for couples I (F) and II (G).
obtained as 1.49 and 2.47 s−1. In addition, the value of ˛s,a (the
anodic electron-transfer coefficient) was obtained for couple I as
0.7 and 0.6 for couple II. The value of ˛s,c (the cathodic electron-
transfer coefficient) was also obtained for couple I as 0.3 and 0.4
for couple II. The discrepancies between ˛s,a and ˛s,c suggest that
the rate-limiting steps for the reduction and oxidation processes
might not be the same [42]. It should also be noted that ks indicates
the rate constant for the electron transfer process occurring at the
“surface” when electron jumps across the electrode/solution inter-
face. The apparent charge transfer rate constants obtained here are
both higher than those reported for the Fe2O3 core–CoHCF shell
nanoparticles [11], CoHCF-modified screen-printed electrodes [43]
and CoHCF–graphite wax composite electrode [44]. This can be
related to the special size and structure of the precursor (the
nanoflowers) of the synthesized CoHCF sample.
electrocatalyst of FeIII species which is generated at more positive
potentials. In the presence of 498 ␮M sulfite and nitrite (Fig. 5A
and B, insets), the anodic charge associated with couple II is quan-
titatively 121 and 143% that of the corresponding cathodic peak,
while, in the absence of the compounds, the anodic and cathodic
charge has a little difference. Iron species are present on the elec-
trode surface, and the species with the higher valence oxidizes
the compounds via chemical reactions followed by generation of
low-valence species. Along this line, the high-valence species is
regenerated through the external electrical circuit. Accordingly,
the compounds are twice oxidized via two EC’ processes. More-
over, the significant currents in the reverse sweep indicate that
the reactions of the compounds with the high-valence species are
the rate-determining steps of the oxidation processes. Comparison
of the voltammograms recorded using MCPE and UCPE revealed
that sulfite and nitrite are oxidized on the MCPE surface with a
higher rate at the same potentials or with the same rates at lower
potentials.
3.4. Electrocatalytic oxidation of sulfite and nitrite
Based on the results presented here, the electrocatalytic oxi-
dation of sulfite and nitrite can be expressed as follows. The
electrooxidation of FeII in the potential window,
Fig. 5 shows cyclic voltammograms of MCPE in PBS recorded in
the absence (curve a) and presence (curve b) of 4.76 mM sulfite (A)
or nitrite (B). The potential sweep rate was 10 mV s−1. In order to
compare, cyclic voltammograms of similar solutions were recorded
using UCPE, as presented in Fig. 5C and D. Upon addition of sulfite
or nitrite to PBS, the anodic current of couple II increased and the
corresponding cathodic current decreased while, the peak currents
related to couple I almost remained unchanged. These results can
be interpreted as an electrocatalytic oxidation mechanism of the
compounds. The results indicate that these compounds are oxi-
dized by the active moiety of FeIII. It seems that the active species
of CoIII which is generated at lower positive potentials cannot oxi-
dize the compounds and these are oxidized only by the stronger
FeIII + e ꢀ FeII
(12)
(13)
(14)
is followed by the oxidation of sulfite,
k
FeIII + SO23− → SO42− + FeII
cat,sul
or the oxidation of nitrite:
k
FeIII + NO−2 → NO2 + FeII
cat,nit

H. Heli et al. / Electrochimica Acta 77 (2012) 294–301
299
90
75
60
45
30
15
0
90
A
B
50
35
20
5
40
30
20
10
0
a
a
70
50
b
b
-
10
-
10
400 600 800 1000 1200
a
30
450
650
850
1050
E vs (Ag/AgCl)/mV
a
b
E vs (Ag/AgCl)/mV
b
10
-15
-10
0
200
400
600
800
1000
0
200
400
600
800
1000
E vs (Ag/AgCl)/mV
E vs (Ag/AgCl)/mV
7
6
7
C
D
6
5
4
3
2
1
0
a
5
a
b
4
b
3
2
1
0
-1
450
-1
400
550
650
750
850
950
1050
600
800
1000
1200
E vs (Ag/AgCl)/mV
E vs (Ag/AgCl)/mV
Fig. 5. A: Cyclic voltammograms of MCPE in PBS recorded in the absence (curve a) and presence (curve b) of 4.76 (main panel) or 498 (inset) mM sulfite. The potential sweep
rate was 10 mV s−1. B: Cyclic voltammograms of MCPE in PBS recorded in the absence (curve a) and presence (curve b) of 4.76 (main panel) or 498 (inset) mM nitrite. The
potential sweep rate was 10 mV s−1. Cyclic voltammograms of UCPE in PBS recorded in the absence (curve a) and presence (curve b) of 4.76 mM sulfite (C) and nitrite (D).
The potential sweep rate was 10 mV s−1
.
It has been reported that nitrite electrooxidation occurs as [45]:
100
90
80
70
60
50
40
30
20
10
0
NO2− → NO2 + e−
2NO2 + H2O ꢀ 2H+ + NO3− + NO2−
(15)
(16)
overall:
NO2− + H2O → 2H+ + NO3− + 2e−
(17)
Cyclic voltammograms of 498 ␮M sulfite and nitrite in PBS
using MCPE recorded at different potential sweep rates were
recorded (Supplementary materials S1 and S2). The anodic peak
currents increased linearly with the square root of the potential
sweep rate (Supplementary materials S1 and S2). This indicates
that the electrocatalytic oxidation processes of the compounds are
diffusion-controlled and occur in the bulk of solution. In addition,
the electron transfer coefficients for the electrooxidation reactions
can be obtained from the following equation [46]:
0
10
20
30
40
50
t/s
120
100
80
60
40
20
0
ꢄ
ꢅ
RT
Ep
=
ln ꢁ + constant
(18)
2˛F
which is valid for a totally irreversible diffusion-controlled process.
Using the dependency of anodic peak potential on the natural loga-
rithm of the potential sweep rate (Supplementary materials S1 and
S2), the values of electron transfer coefficients for the electrocat-
alytic oxidation of sulfite and nitrite were obtained as 0.4. Another
point in the voltammograms recorded at different potential sweep
rates in the presence of sulfite and nitrite is that upon increasing
potential sweep rate, the anodic peak shifts positively. This indi-
cates the existence of a kinetic limitation in the reactions between
CoHCF and the compounds. Another point in these voltammo-
grams is that plotting the current functions (peak current divided
by the square root of the potential sweep rate) against the square
root of the potential sweep rate (Supplementary materials S1 and
S2) reveal negative slopes. It further confirms the electrocatalytic
nature of the electrooxidation processes.
0
10
20
30
40
50
t/s
Fig. 6. A: Chronoamperograms for MCPE in the absence (curve a) and presence
(curves b–h) of 0.5, 1.0, 1.5, 2.0, 2.5, 3.0 and 3.5 ␮M sulfite. The potential step was
920 mV. Inset A1: plotting the net current with respect to the minus square roots of
time. Inset A2: plotting the Icat/Id values with respect to the square roots of time. B:
Chronoamperograms for MCPE in the absence (curve a) and presence (curves b–h)
of 1.7, 3.5, 5.1, 6.8, 8.5, 10.0 and 12.0 ␮M nitrite. The potential step was 920 mV.
Inset B1: plotting the net current with respect to the minus square roots of time.
Inset B2: plotting the Icat/Id values with respect to the square roots of time.
Chronoamperometry as well as other voltammetric techniques
was used in order to study the mass transfer kinetics and also to
obtain heterogeneous catalytic rate constant in EC’ mechanisms.
This is attained by setting the working electrode potentials to the
desired values [39]. Fig. 6 shows chronoamperograms for MCPE in

300
H. Heli et al. / Electrochimica Acta 77 (2012) 294–301
Table 1
Chronoamperometry, can also be used to evaluate the catalytic
rate constant according to [39]:
The determined parameters for the calibration curves of sulfite and nitrite using
MCPE.
ꢆ
Icat
−ꢅ
Sulfite
Nitrite
= ꢅ1/2
ꢄ
1/2erf(ꢅ1/2) + exp
(20)
ꢅ1/2
Id
Linear range (mM)
1.0–7.83
4.61
1.63
2.87
9.57
0.1–2.15
11.11
20.4
1.19
3.97
Slope (mA M−1
Intercept (␮A)
LOD (␮M)
)
where Icat and Id are the currents in the presence and absence of
the compounds, ␥ = kcatCt is the argument of the error function, kcat
is the catalytic rate constant and t is the elapsed time. In the cases
where ␥ > 1.5, erf(␥1/2) is almost equal to unity, the above equation
can be reduced to:
LOQ (␮M)
RSD (%)
3.17
3.27
Icat
= ꢅ1/2ꢄ1/2 = ꢄ1/2(kcatCt)1/2
(21)
the absence (curve a) and presence (curves b–h) of sulfite (A) and
nitrite (B) recorded using a potential step of 920 mV. Plotting the net
currents with respect to the minus square roots of time presented
linear dependencies (Fig. 6, insets A1 and B1). Therefore, diffusion-
controlled processes in the bulk of solution were dominated for
both oxidation steps. It was also confirmed by cyclic voltammetry
(Supplementary materials S1 and S2). By using the slope of these
lines, the diffusion coefficient of the electroreactive species can be
obtained according to Cottrell’s equation [39]:
Id
From the slopes of the Icat/Id versus t1/2 plots, presented in Fig. 6,
insets A2 and B2, the mean values of catalytic rate constants for the
electrooxidation of sulfite and nitrite were obtained as 584 and
935 cm3 mol−1 s−1, respectively.
Aiming at developing a simple procedure for the analysis of
sulfite and nitrite, we employed amperometry technique. Typi-
cal amperometric signals obtained during successive increments of
sulfite (A) and nitrite (B) to PBS are displayed in Fig. 7. Gentle stirring
for a few seconds was needed to promote solution homogenization
after each injection. The electrode response was quite rapid and
proportional to the concentration. The limits of detections (LOD)
and quantitations (LOQ) of the procedure were calculated accord-
ing to the 3SD/m and 10SD/m criteria, respectively, where SD is the
standard deviation of the intercept and m is the slope of the cali-
bration curves [47]. The determined parameters for the calibration
curves of sulfite and nitrite, accuracy and precision, LODs and LOQs
and the slopes of calibration curves are shown in Table 1. A com-
parison between the sensing utility of some modified electrodes for
sulfite and nitrite was also made (Supplementary materials S3 and
S4).
I = nFAD1/2Cꢄ−1/2t−1/2
(19)
where D is the diffusion coefficient and C is the bulk concentration.
The mean values of the diffusion coefficient for sulfite and nitrite
were found to be 2.05 × 10−5 and 2.01 × 10−5 cm2 s−1, respectively.
4. Conclusion
Nanoflowers of cobalt were synthesized in a polyol medium
in the presence of PVP as a complexing and structure directing
agent. Upon anodizing in a ferrycyanide solution, the nanoflowers
were derivatized to cobalt hexacyanoferrate. Cobalt hexacyanofer-
rate was employed to prepare a modified carbon paste electrode.
The electrode showed a fast redox system and electrocatalytic effi-
ciency toward the oxidation of sulfite and nitrite. The electrode,
as an amperometric sensor, was employed to analyze sulfite and
nitrite. The simplicity, sensitivity, selectivity, and short time of
analysis are the main advantages of the proposed amperometric
procedure.
Acknowledgements
We would like to thank the Research Councils of Islamic Azad
University and Shiraz University of Medical Sciences (91-01-36-
4469), the Iran National Science Foundation (INSF) and National
Institute of Elites (BMN) for supporting this research.
Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.electacta.
2012.06.014.
References
Fig. 7. A: Typical amperometric signals obtained during successive increments of
sulfite. Inset: a calibration curve for the amperometric measurements. B: Typical
amperometric signals obtained during successive increments of nitrite. Inset: a
calibration curve for the amperometric measurements.
[1] H.S. Nalwa, Encyclopedia of Nanoscience and Nanotechnology, American Sci-
entific Publishers, 2001.

H. Heli et al. / Electrochimica Acta 77 (2012) 294–301
301
[2] H. Heli, F. Faramarzi, N. Sattarahmady, Journal of Solid State Electrochemistry
14 (2010) 2275.
[26] S. Lahiri, S. Sarkar, Applied Radiation and Isotopes 65 (2007) 387.
[27] H.G. Zheng, J.H. Liang, J.H. Zeng, Y.T. Qian, Materials Research Bulletin 36 (2001)
47.
[28] F. Guo, H.G. Zheng, Z.P. Yang, Y.T. Qian, Materials Letters 56 (2002) 906.
[29] H.G. Zheng, J.H. Zeng, J.H. Liang, Acta Metallurgica Sinica 35 (1999) 837.
[30] D.R. Shankaran, S.S. Narayanan, Sensors and Actuators B 86 (2002) 180.
[31] R.O. Lezna, R. Romagnoli, N.R. de Tacconi, K. Rajeshwar, Journal of Physical
Chemistry B 106 (2002) 3612.
[3] G. Cao, D. Liu, Advances in Colloid and Interface Science 136 (2008) 45.
[4] B.I. Kharisov, Recent Patents Nanotechnology 2 (2008) 190.
[5] H. Heli, H. Yadegari, Electrochimica Acta 55 (2010) 2139.
[6] B. Cui, T. Veres, Microelectronic Engineering 84 (2007) 1544.
[7] H. Heli, H. Yadegari, A. Jabbari, Journal of Physical Chemistry C 115 (2011)
10889.
[8] H. Heli, H. Yadegari, A. Jabbari, Journal of Applied Electrochemistry 42 (2012)
279.
[32] N.R. de Tacconi, K. Rajeshwar, R.O. Lezna, Journal of Electroanalytical Chemistry
587 (2006) 42.
[9] H. Heli, M. Hajjizadeh, A. Jabbari, A.A. Moosavi-Movahedi, Analytical Biochem-
istry 388 (2009) 81.
[33] S.J.R. Prabakar, S.S. Narayanan, Analytical and Bioanalytical Chemistry 386
(2006) 2107.
[10] N. Sattarahmady, H. Heli, A.A. Moosavi-Movahedi, Biosensors & Bioelectronics
25 (2010) 2329.
[34] Z. Xun, C. Cai, W. Xing, T. Lu, Journal of Electroanalytical Chemistry 545 (2003)
19.
[11] H. Heli, S. Majdi, N. Sattarahmady, Materials Research Bulletin 45 (2010) 850.
[12] A.D. Yoffe, Advances in Physics 51 (2002) 799.
[35] E. Laviron, Journal of Electroanalytical Chemistry 122 (1981) 37.
[36] P. Daum, J.R. Lenhard, D. Rolison, R.W. Murray, Journal of the American Chem-
ical Society 102 (1980) 4649.
[13] M.K. Roy, B. Haldar, H.C. Verma, Nanotechnology 17 (2006) 232.
[14] N.R. de Tacconi, K. Rajeshwar, Chemistry of Materials 15 (2003) 3046.
[15] N. Sattarahmady, H. Heli, Analytical Biochemistry 409 (2011) 74.
[16] H. Heli, S. Majdi, N. Sattarahmady, Sensors and Actuators B 145 (2010) 185.
[17] S. Zamponi, M. Giorgetti, M. Berrettoni, P.J. Kulesza, J.A. Cox, A.M. Kijak, Elec-
trochimica Acta 51 (2005) 118.
[18] N. Sattarahmady, H. Heli, Sensor Letters, http://dx.doi.org/10.1166/sl.2012.
2338, in press.
[19] M. Fujihira, in: A.J. Bard, M. Stratmann (Eds.), Encyclopedia of Electrochemistry,
vol. 10, Wiley, 2007.
[37] J.M. Bolts, A.B. Bocarsly, M.C. Palazzotto, E.G. Walton, N.S. Lewis, M.S. Wrighton,
Journal of the American Chemical Society 101 (1979) 1378.
[38] D. Ellis, M. Eckhoff, V.D. Neff, Journal of Physical Chemistry 85 (1981) 1225.
[39] A.J. Bard, L.R. Faulkner, Electrochemical Methods, Wiley, New York, 2001.
[40] M.H. Pournaghi-Azar, H. Dastangoo, Electrochimica Acta 48 (2003) 1797.
[41] E. Laviron, Journal of Electroanalytical Chemistry 101 (1979) 19.
[42] H. Luo, Z. Shi, N. Li, Z. Gu, Q. Zhuang, Analytical Chemistry 73 (2001) 915.
[43] F. Arduini, A. Cassisi, A. Amine, F. Ricci, D. Moscone, G. Palleschi, Journal of
Electroanalytical Chemistry 626 (2009) 66.
[20] J.G. Redepenning, Trends in Analytical Chemistry 6 (1987) 18.
[21] R.J. Mortimer, Chemical Society Reviews 26 (1997) 147.
[22] A.A. Karyakin, Electroanalysis 13 (2001) 813.
[23] B.D. Cullity, Elements of X-ray Diffraction, Addison-Wesley Reading, 1978.
[24] S.S. Rashidova, V.A. Li, A.A. Kim USSR Patent No. 821445 (1981).
[25] Y.G. Sun, Y.N. Xia, Science 298 (2002) 2176.
[44] S.J.R. Prabakar, S.S. Narayanan, Talanta 72 (2007) 1818.
[45] B. Piela, P.K. Wrona, Journal of the Electrochemical Society 149 (2002) E55.
[46] J.A. Harrison, Z.A. Khan, Journal of the Electrochemical Society 28 (1970)
131.
[47] J.C. Miller, J.N. Miller, Statistics for Analytical Chemistry, 4nd ed., Ellis-Howood,
New York, 1994.