Journal of Sensor Technology, 2013, 3, 84-93
http://dx.doi.org/10.4236/jst.2013.33014 Published Online September 2013 (http://www.scirp.org/journal/jst)
An Electrochemical Nitrite Sensor Based on a Multilayer
Film of Polyoxometalate
Yosra Sahraoui1,2, Sana Chaliaa3, Abderrazak Maaref1, Amor Haddad3,
Nicole Jaffrezic-Renault2*
1Laboratory of Interfaces and Advanced Materials, Faculty of Sciences of Monastir,
Monastir, Tunisia
2University of Lyon, Institute of Analytical Chemistry, Villeurbanne, France
3 Laboratory of Materials and Crystallochemistry, Superior Institute of Applied Science and Technology,
Mahdia, Tunisia
Received July 8, 2013; revised August 8, 2013; accepted August 15, 2013
Copyright © 2013 Yosra Sahraoui et al. This is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
ABSTRACT
In this work, we have developed an electrochemical sensor for nitrite detection, based on a polyoxometalate (POM)
namely mono-lacunary keggin anion [SiW11O39]8− cited as (SiW11). Electrochemical characterization of SiW11 shows
two-step reduction processes, with formal potentials of −0.5 V (I) and −0.68 V (II). Oppositely charged polyelectrolyte
(poly (allylamine hydrochloride) (PAH)) and (SiW11) were assembled alternately to modify glassy carbon electrode.
The electrochemical behavior of the modified electrode was studied in detail using cyclic voltammetry (CV). The re-
sults showed that SiW11/PAH/GC electrode present good electrocatalytic activity for the reduction of nitrite. The sensor
showed a dynamic range from 100 µM to 3.6 mM of nitrite and no interference from other classical anions. Experimen-
tal factors that affect electron-transfer rate in these films, such as pH effect and layers number, were systematically
analyzed.
Keywords: Nitrite Sensor; Polyoxometalate; Mono-Lacunary Keggin Anion; Layer by Layer Assembly Process
1. Introduction
The presence of nitrite in groundwater and atmosphere is
an essential precursor in the formation of nitrosamines,
many of which have been proven to be powerful carcino-
gens [1-3]. The increase in nitrite concentration in the
blood causes a decrease in oxygen transport by the blood
causing methemoglobinemia, better known as the “blue
baby syndrome” reaction of nitrite with Fe (II) forming
hemoglobin to methemoglobin, HbFe (III) [4]. For these
reasons, determination of nitrite has received consider-
able attention. Many methods have been developed for
nitrite determination including spectrophotometry [5],
chromatography [6] and electrochemical methods [7-12],
which are advantageous over the other methods in terms
of cost and time.
Electrochemical determination of nitrite ion requires a
large over potential at almost all electrode surfaces
[13-15]. The better way to achieve nitrite detection at
low overpotentials is the use of mediators confined at the
electrode surface. Over the years, numbers of modifica-
tion procedures have been reported for the construction
of modified electrodes [17-19]. Many studies reported
the capability of hexacyanoferrate anion complex [20]
for the homogeneous electrocatalytic reduction of nitrite
and employed this catalytic reduction method for deter-
mination of it in real samples. They demonstrated also,
that the poly-ortho-toluidine can catalyze the nitrite re-
duction as heterogeneous catalysis [21].
Polyoxometalates (POMs), a large family of charged,
nanoscopic inorganic clusters, are attractive materials for
electrode modification because of their reversible redox
behavior, good chemical stability and electronic conduc-
tivity [22]. These compounds are also resistant to oxida-
tive degradation due to the fact that their fundamental
elements are in their higher oxidation state [23-26].
These properties make them very attractive in many
fields such as electrochemistry, catalysis and for bio-
medicinal tasks [23,27-30].
There is a different type of polyoxometalates [27], and
in our study Keggin-type POMs are specifically used. They
*Corresponding author.
C
opyright © 2013 SciRes. JST

Y. SAHRAOUI ET AL. 85
display a quite well-known structural motif that is com-
posed of a tetrahedrally coordinated central hetero atom
surrounded by four W3O13 groups (triads) that are con-
nected in a corner-sharing fashion via oxygen atoms [31,
32]. Lacunary Keggin structures [SiW11O39]8− (SiW11)
used in this study, are derived from the parent Keggin
type by the removal of specific WO6 moieties, followed
by an optional rotation of the remaining WO6 octahedra
[33,34]. As a consequence, (SiW11) is characterized by
the presence of more terminal oxygen atoms, which give
them more negative charge than other POMs structure.
Layer-by-layer (LBL) self-assembly has proved to be a
promising method for the fabrication of ultrathin films. It
is based on the alternate adsorption on the substrate sur-
face of oppositely charged species from dilute solutions
and film formation is attributed primarily to electrostatic
interaction and Van der Waals forces. It provides thick-
ness control at the nanometer level, which can be easily
adapted for automated fabrication. It is applicable to any
substrate shape and also permits co-assembly with dif-
ferent functional components [35]. Owing to these ad-
vantages, the layer-by-layer approach has been utilized to
fabricate POM-containing multilayer film consisting of
synthetized or natural polyelectrolytes. Several studies
used the parent Keggin silicotungstate [α-SiW12O40]4− as
the anion and, for example, polyaniline [36], chitosan [37]
or poly (diallyldimethylammonium chloride) [38] as the
counter cation source. There are only few examples of
the use of lacunary Keggin polyoxotungtate anions in
films prepared by the layer-by-layer self-assembly method
[39]. It has been reported that metal-substituted Keggin
silicotungstate could be used as mediator for the elec-
troreduction of nitrite [40]. In this paper, we show the
capability of SiW11 anion immobilized by electrodeposi-
tion and LBL methods, for the electrocatalytic reduction
of nitrite and this property is applied for the first time for
the design of a selective amperometric sensor for nitrite.
2. Experimental
2.1. Materials
Ultrapure milliQ water with a resistance of 18.2 MΏ cm
was used for all experiments. P-phenylenediamine (PPD),
poly (allylamine hydrochloride) (PAH) (MW 15,000),
sulfuric acid (H2SO4), sodium nitrite (NaNO2), the fol-
lowing product were purchased from commercial sources
and used without further purification from Sigma-Al-
drich. Gold nanoparticles 20 nm diameter, stabilized
suspension in 0.1 M PBS were commercial products
from Sigma-Aldrich. Solution of redox probe (1.0 mM)
was prepared dissolving the appropriate amounts of

34
6
Fe CN


in 1 M KCl.
K8 [-SiW11O39]·13H2O was synthesized as the litera-
ture described [41]. Briefly, a solution of 4 M HCl was
added over 10 min, to a mix of sodium meta-silicate so-
lution (2.23 g, 10 mM) (Solution A), and sodium tung-
state (36.29 g, 110 mM).
Solid Potassium Chloride (20 g) was then added to the
solution with gentle stirring, after 15 min the precipitate
was collected by filtration. Purification was achieved by
dissolving the product in water (200 mL) and insoluble
material being removed by filtration on a fine frit. The
salt was precipitated again by addition of solid potassium
chloride (10 g) before being collected by filtration,
washed with 2 M potassium chloride and air dried to give
K8[-SiW11O39]· 13H2O as a white powder (13.86 g,
33.6%).
2.2. Apparatus
Electrochemical measurements were carried out using a
potentiostat-galvanostat VOLTALAB 40 PGZ/301, 746
VA Trace Analyzer (Metrohm) equipped with a 747 VA
Stand. A three-electrode system was used, the side arms
contained Ag/AgCl (sat. KCl) reference electrode and a
platinum counter electrode with a surface area of ap-
proximately 1 cm2. The working electrode was a SiW11
modified glassy carbon electrode (3 mm diameter, sur-
face area: 7 mm2). Prior to coating, the GCE was condi-
tioned by a polishing/cleaning procedure. The GCE was
successively polished with 1.0, 0.3, and 0.05 µm α-Al2O3
paste and then rinsed with ultra-pure water to remove any
residual alumina and finally sonicated for 5 min in an
ultrasonic bath and dried under a stream of pure nitrogen.
SEM characterization of the films was performed us-
ing a Quanta TM 250 microscope (FEI) in degraded pres-
sure mode; no gold coating was required. The infrared
spectra were recorded as KBr pellets, in the 4000 - 400
cm−1 range on a Nicolet 470 FT-IR spectrophotometer.
Dynamic light scattering (Zetasizer Nano-ZS, Malvern
Instruments) was used for the determination of the zeta
potential.
2.3. Preparation of the Modified Electrodes
SiW11 modification of GC electrode was performed by
using electrodeposition and layer-by-layer methods for
the building of multilayers, by alternately dipping the
desired substrate in PAH and SiW11 solutions, in cyclic
mode [42]. Figure 1 displays a schematic representation
of the different layers of the SiW11/LBL film.
The GC electrode was first placed in 2 mM PPD solu-
tion and the potential was repeatedly scanned. PPD was
electropolymerized onto GCE surface by amine cation
radical formation. The oxidation peak gradually dimin-
ishes and is almost absent after the 6th cycle, this obser-
vation indicates the formation of a coating on the elec-
trode surface (cf. Figure 2).

Y. SAHRAOUI ET AL. 86
PPD
I
SiW
11
II
GC
P
AH
Re
p
eat
I, III
III
Figure 1. Schematic representation of 5-SiW11/PAH/PPD/
GCE multilayer films.
Figure 2. Cyclic voltammograms on a freshly polished GCE
in 2 mM PPD solution, for the (1) first, (2) second, (3) third,
(5) fifth and (6) sixth cycles, at a scan rate: 100 mV/s.
The polyPPD.GC electrode was then placed in 2 mM
(SiW11) solution, 0.1 M H2SO4 and at the same time a
cyclic potential sweep was conducted in the potential
range 0.8 V to −0.8 V at a scan rate of 100 mV/s−1, for 25
cycles. In this way, a SiW11 monolayer was deposited on
the surface of polyPPD/GCE [42]. Then, the resulting
electrode (SiW11/polyPPD/GC) was transferred to 2 mM
PAH (pH = 3) for 15 min, for resulting in one layer of
positive charge. This procedure (SiW11/PAH) was re-
peated until obtaining 5 layers of SiW11.
3. Results and Discussion
3.1. Characterization of SiW11 in Solution
The prepared compound was characterized by infrared
spectroscopy, and cyclic voltammetry in acidic aqueous
solution, because Keggin type and its derivative are un-
stable in neutral and basic solutions. The cyclic voltam-
mogram becomes ill-defined and peak current much
smaller, such an observation has been reported by Cheng
et al. [43] on Keggin-type polyoxometallates. The FT-IR
spectra of the lacunary keggin silicotungstate in KBr
pressed pellets are presented in (Figure 3).
The attributions of FT-IR spectra exhibits characteris-
tic bands at 3424.0, 1634 (OH); 988.4 as(Si-O); 945.4
cm−1 (W = O); 882.1, 733.8, 624.7 as (W-O-W); 532.2
(O-Si-O) cm−1. The silicotungstate vibrational spectrum
was in agreement with previously reported analysis [44].
The redox behavior of SiW11 in solution (conditions: 2
mM SiW11 in 0.1 M H2SO4 solution) was studied by cy-
clic voltammetry and polarography. The dissolved SiW11
shows that the CV curve exhibits two pairs of successive
redox waves with cathodic peaks located respectively
EpcI = −0.5 V (I) and EpcII = −0.68 V (II), and peak
separation potentials (∆Ep) of 65 and 44 mV respectively,
in the potential range from +0.8 V to −0.8 V (Figure
4(a)). This result is consistent with that in literature [45].
These redox waves correspond to the reduction of the
tungsten centers within the SiW11 (WVI → WV). The re-
duction of heteropolyanions is accompanied by protona-
tion, therefore, the pH of solution has a great effect on
the electrochemical behavior of heteropolyanions.
The pH effect on the electrochemical behavior of the
Figure 3. The FT-IR spectra of the K8[-SiW11O39]·13H2O.
Figure 4. Cyclic voltammogras of: (a) 2 mM SiW11 in 0.1 m
M H2SO4 solution and (b) multilayer film of 5-SiW11/PAH/
PPD/GCE at a scan rate: 100 mV/s.

Y. SAHRAOUI ET AL. 87
soluble SiW11 was studied by polarography. The peak

(step 2)

time they are
gr
potentials for both redox couples shift to the more nega-
tive direction with increasing pH. Plots of Ep of the two
successive redox waves (step I and step II) versus pH for
the (SiW11) (Figure 1S), show good linearity in the pH
range from 0.8 to 4.3. Slopes in this pH range are −69,
−85 mV pH step (I) and (II), respectively, which are
close to the theoretical value of −60 mV per pH for the
2e−/2H+ redox process [46]. The above results may indi-
cate the two overall redox process of (SiW11) in acidic
solution as follows:
(step 1)
VI 8VVI 8
11 3922939
SiWO2e 2HHSiWWO
 
 
VVI8 VVI8
229 39229 39
HW WO2e2HHSiW WO
 
 
3.2. Characterization of the Modified Electrode
3.2.1. Electrochemical Characterization
Define abbreviations and acronyms the first
used in the text, even after they have been defined in the
abstract. Abbreviations such as IEEE, SI, MKS, CGS, sc,
dc, and rms do not have to be defined. Do not use abbre-
viations in the title or heads unless they are unavoidable.
Through the attachment of polyPPD containing NH2
oup to the GCE, the modified electrode (poly) PPD/
GCE was positively charged at least up to pH 6.1 [47].
The PPD-modified GCE with an amido-terminated mo-
nolayer can be used as a charge-rich precursor to assem-
ble oppositely charged species by layer-by-layer electro-
static interaction [48]. The adsorption of a layer of SiW11
is evidenced through the behavior of the modified elec-
trode in presence of

34
Fe CN  redox probe (cf
6 
b). The presen
H with SiW11 was
ev
f
Si
Figure 5, curves a andce of the negative
layer of SiW11 increases ΔEp by more than 100 mV and
decreases, mainly the cathodic peak, which is due to the
electrostatic repulsion of the probe.
The electrostatic interaction of PA
idenced by using gold nanoparticles initially covered
with a citrate monolayer, a layer on PAH being adsorbed
on their surface, their zeta potential was found to be +44
mV. After adsorption of SiW11, the zeta potential became
equal to −1.1 mV, then showing the neutralization of the
positive charge by successful adsorption of SiW11.
Figure 5 shows that with increasing the number o
W11 layers from one to five, the peak maxima of

34
Fe CN

redox probe decrease, transducing a
6 
decrease of charge t11 ransfer through the SiW LBL film.
This phenomenon can be due to the increase of the elec-
trostatic repulsion between

34
Fe CN

and the
6 
BL film. more negatively charged SiW11 L





34
6
Fe CN
Figure 5. Cyclic voltammograms of (1 mM
th (PPD/PAH/
Figure 4 shows a comparison between the cyclic vol-
ta
mical stability of the SiW11/PAH/PPD/
G
3.2.2. SEM Characterization of SiW11/polyPPD Film
-
,
1 M KCl), at modified electrodes wiSiW11) n
for n = 1, 3 and 5.
mmogram of the soluble and immobilized SiW11 in
LBL films of 5-SiW11 layers. Immobilized in LBL film,
SiW11 displays a close similar electrochemical behavior
to that of soluble SiW11. These observations demonstrate
that the electrochemical behavior of the SiW11 anion is
maintained in the multilayer films [40]. Nevertheless,
EpcI was shifted to 0.454 V and EpcII to 0.722 V versus
Ag/AgCl. The redox peaks were broadered and ∆EpI was
found to be around 150 mV whereas ∆EpII was around
70 mV. The broadening can be related to the large cou-
lombic repulsion between the negative sites of highly-
charged polyanions in the same layer, as in [37]. The
decrease of reversibility could be attributed to the de-
crease of charge transfer rate through the poorly conduc-
tive PPD layer.
The electroche
C electrode was investigated in 0.1 M H2SO4 solution
at a scan rate of 100 mV s−1. After 300 cycles, the catho-
dic peak current (peak II) still remains about at 93% of
the initial value. These results indicate that the SiW11/
PAH/polyPPD/GC electrode present a good stability.
In order to observe the SiW11/polyPPD film in an accu
rate way, the film was electrodeposited onto gold elec-
trode, by the method described above. Near the border,
we can clearly see that the majority of the electrode sur-
face is covered by the PPD film and there is no appear-
ance of dendrite structure (Figure 6(a)), so we can say
that the morphologies seen at the polyPPD/gold electrode
surface are similar to those at the polyPPD/GC surface.
The successful immobilization of SiW11 in the electrode
surface is confirmed by the energy-dispersive X-ray (ED-
X) analyses (Figure 7), showing the presence of W and

Y. SAHRAOUI ET AL. 88
(a)
(b)
Figure 6. (a) PPD/SiW11 monolayer film (×400) (WD =
Working distance, HFW = High Field View (distance x, y
scanned), det LFD = type of used detector is ETD Everhart
Thornley (called secondary electron detector = topographic
contrast), mag = Magnification value to a Polaroid format,
HV = accelerating voltage); (b) PPD/SiW11 monolayer film
(×6000) (WD = Working distance, HFW = High Field View
(distance x, y scanned), det LFD = type of used detector is
ETD Everhart Thornley (called secondary electron detector
= topographic contrast), mag = Magnification value to a Po-
laroid format, HV = accelerating voltage).
Figure 7. Representative EDX analysis of multilayer films.
O
his study, amplification of the image obtained at
th
3.3. Reduction of Nitrite Using GCE/SiW/PAH
In throtonated,
2
(3)
Nitrite might be reduced to NO instantly [5
ac
. Several large crystallites and dendrites can be seen
(size around 200 µm), especially in the central zone of
the electrode, which showed a significantly rough sur-
face.
In t
e electrode center revealed the presence of highly po-
pulated crystallite regions (Figure 6(b)). Besides, the
electrode surface appeared as a tri-dimensional structure
with a high “apparent” rugosity. The image shows that
the product consists of nanometer-sized platelet struc-
tures (80 - 300 nm thick) and that some of the nanos-
tructures agglomerate together. The formation of dendrite
structure after deposition of SiW11 has already been ob-
served in [40].
11
Multilayer-Modified Electrode
is acidic solution, most nitrite ions are p
the question is which are the actual reactive species, be-
cause the pKa of HNO2 is 3.3 [49,50] Equation (3) sug-
gest that the active species could be HNO2 and/or NO.
As HNO2 disproportionates in fairly acidic solution, even
though the rate of reaction (3) is known to be low
[49,50].
23
3HNOHNO2NOHO
1] by re-
ting with 8
21139
HSiW O

and 8
41139
HSiW O as soon as
HNO2 arriveC/PPD1) films on
electrode surface. The resulting 8
21139
HSiW O is then re-
duced to 8
41139
HSiWO
s at the (G/PAH/SiW1

to facilitation effect
for the redrite. The electrocatalytic behavior
of a SiW11 modified electrode towards nitrite can be ex-
plained by the following mechanism:
HNOHNO H

 
ate the medi
uction of nit
2O 2
(4)
N
O1e NO


Figure 8 shows the behavior of SiW11 immobilized
LB
the reduction peak II of
Si
Ipc of the second couple are
0.
(5)
88
411392211392
HSiWO2HNO2NOHSiWO2HO


(6)
in
L film of 5-SiW11 layers in 0.1 M H2SO4 aqueous
solution at pH 1.2 containing nitrite in various concentra-
tions (from 0.1 mM to 3.6 mM).
The catalytic effect appears on
W11, it increases whereas the corresponding oxidation
current decreases. This typical for a reduction process
mediated by a reduction catalyst. Equation (6) presents
the possible overall process.
We find that the ratios Ipa/
45, 0.51 and 0.6 corresponding to nitrite concentrations
of 1, 2 and 2.8 mM respectively, this ratio is increased
with stepwise addition of nitrite. No response is observed
on the bare GC electrode, in the range of 0.8 to −0.8 V in

Y. SAHRAOUI ET AL. 89
Figure 8. Cyclic voltammograms showing the catalytic ac
.1 M HSO4 solutions containing 0.4 mM and 0.6 mM
3.4. Effect of the Number of Layers on the
In orect of the number of layers on
c
re
3.5. Calibration Curve and Reproducibility
line-
3.6. Life Time
e nitrite sensor was investigated over a
-
tivity of SiW11/LBL film (5-SiW11 layers) at different nitrite
concentrations: 0, 1, 2 and 2.8 mmol/L.
02
of 2
NO , as shown in (Figure 8(e)).
Nitrite Reduction
der to confirm the eff
the electrochemical properties, the electrochemical be-
haviors of SiW11-modified electrode were investigated.
With increasing number of SiW11 layers, the catalyti
duction of nitrite increases. For example, the electro-
catalytic current observed with 5- SiW11 layers is higher
than the current observed with 2- SiW11 layers in the
presence of 0.1 mM of nitrite. The response of the modi-
fied electrode toward nitrite is practically constant when
number of layers is high than 5 (Figure 9).
The catalytic peak maximum at potential −0.68 V is
arly dependent on the nitrite concentration, in the range
0.1 mM - 3.6 mM, with correlation coefficient of 0.9991,
as shown in Figure 10. The reproducibility of the nitrite
sensor was studied by using three different sensors, the
cathodic peak current corresponding to 2.8 mM of nitrite
still remains about 93%.
The stability of th
period of 75 days. The sensor was stored under air at
room temperature and the current response to 3 mM of
nitrite injection in 0.1 M H2SO4 pH 1.2 was checked at
regular intervals. No significant change (< 10%) was
observed within this period of time. The stability of this
sensor can be attributed to the large amount of SiW11
which is deposited on the surface of the electrode.
Figure 9. The relationship between peak currents (EpcII)
versus number of layers (2 to 10 layers).
Figure 10. Calibration curveLinear relationship between
the catalytic current and nitricentrations at −0.6 V vs.
wed that SiW11 modified electrode
perties towards nitrate molecular
monstrate that the second redox cou-
can act as a mediator for the catalytic
:
te con
Ag/AgCl. Electrolyte (0.1 M H2SO4), scan rate 100 mV/s.
3.7. Interferences
Our experiments sho
has no catalytic pro
(Figure 2.A.S). The response of SiW11 modified elec-
trode was also tested toward other ions such as phosphate
(Figure 2.B.S) and perchlorate (Figure 2.C.S), with con-
centrations between 2 mM and 6 mM. No significant in-
crease in the current was observed. Hence, this proves
the selective determination of nitrite because it eliminates
the major interferences such as nitrate.
4. Conclusions
In this study, we de
ple of SiW anion
11
reduction of nitrite in aqueous solution with H2SO4 con-
centration (0.1 M). The electrochemical behavior of the

Y. SAHRAOUI ET AL. 90
modified electrode was studied using cyclic voltammetry.
Catalytic reduction of nitrite can be employed as a new
method for determination of nitrite in real sample such as
weak liquor existing in the wood and paper industry.
This method is simple, low-cost and precise for routine
control and can be carried out directly without any sepa-
ration or pretreatment due to the selective electrocatalytic
reduction of nitrite.
The catalytic reduction peak current showed a linear
dependent on the nitrite concentration and a linear cali-
br
EGIDE through UTIQ
by PHC Maghreb program
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This work was supported by
program No. 09G 1128 and
UE
No 27960UG.
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Y. SAHRAOUI ET AL. 93
Appendix
Figure 1S. The relationship between cathodic potentials of
two redox waves (EpcI and EpcII) and pH.
Figure 2.A.S. Cyclic voltammograms of the SiW11/LBL/GC
electrode in 0.1 M H2SO4 solution containing (a) 0 mM, (b)
2 mM, (c) 4 mM and (d) 6 mMScan rate: 100 mV/s. 
3
NO .
Figure 2.B.S. Cyclic voltammograms of the SiW11/LBL/GC
electrode in 0.1 M H2SO4 solution containing (a) 0 mM, (b)
2 mM, (c) 4 mM and (d) 6 mMScan rate: 100 mV/s. 
4
HPO .
Figure 2.C.S. Cyclic voltammograms of the SiW11/LBL/GC
electrode in 0.1 M H2SO4 solution containing (a) 0 mM, (b)
2 mM, (c) 4 mM and (d) 6 mM4. Scan rate: 100 mV/s. ClO