Mechanism of potentiostatic deposition of MnO2 and electrochemical characteristics of the deposit in relation to carbohydrate oxidation

Article abstract of DOI:10.1016/j.electacta.2008.07.082

Cyclic voltammetric (CV) and chronoamperometric (CA) studies on potentiostatic deposition of MnO₂ on Pt from Mn(II) solution in very weakly alkaline media show the process to be controlled by a one-electron transfer step, which means that the deposition proceeds through the generation of Mn(III). The electrocatalytic activity of the deposited electrode towards carbohydrate oxidation is found to be maximum at an optimum amount of deposition. Chronopotentiometric (CP) and CV measurements show that the oxidation of carbohydrates on the deposited electrodes follows a catalytic EC (electrochemical-chemical) mechanism via electrolytic formation of Mn(V) and its subsequent consumption either by disproportionation or by chemical reaction in the presence of carbohydrates. The rate constants of the reaction of Mn(V) with dextrose and fructose have been obtained from CA results. The relative order of the oxidation currents for dextrose and fructose as well as their dependence on carbohydrate concentration has been discussed. Replacement of Pt by carbon as the electrode support material does not affect the electrocatalytic activity of the MnO₂ deposit. The observed linear variation of the steady state oxidation currents with carbohydrate concentration can be exploited for analytical application.

Full text of DOI:10.1016/j.electacta.2008.07.082

Electrochimica Acta 54 (2008) 289–295
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Mechanism of potentiostatic deposition of MnO2
and electrochemical characteristics of the deposit
in relation to carbohydrate oxidation
∗
Debasmita Das, Pratik Kumar Sen, Kaushik Das
Department of Chemistry, Jadavpur University, Kolkata 700032, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 30 June 2008
Received in revised form 30 July 2008
Accepted 31 July 2008
Available online 8 August 2008
Cyclic voltammetric (CV) and chronoamperometric (CA) studies on potentiostatic deposition of MnO2 on
Pt from Mn(II) solution in very weakly alkaline media show the process to be controlled by a one-electron
transfer step, which means that the deposition proceeds through the generation of Mn(III). The electro-
catalytic activity of the deposited electrode towards carbohydrate oxidation is found to be maximum at
an optimum amount of deposition. Chronopotentiometric (CP) and CV measurements show that the oxi-
dation of carbohydrates on the deposited electrodes follows a catalytic EC (electrochemical–chemical)
mechanism via electrolytic formation of Mn(V) and its subsequent consumption either by dispropor-
tionation or by chemical reaction in the presence of carbohydrates. The rate constants of the reaction of
Mn(V) with dextrose and fructose have been obtained from CA results. The relative order of the oxidation
currents for dextrose and fructose as well as their dependence on carbohydrate concentration has been
discussed. Replacement of Pt by carbon as the electrode support material does not affect the electrocat-
alytic activity of the MnO2 deposit. The observed linear variation of the steady state oxidation currents
with carbohydrate concentration can be exploited for analytical application.
Keywords:
MnO2 electrocatalyst
Potentiostatic deposition
Carbohydrate electro-oxidation
Catalytic EC mechanism
Carbohydrate estimation
©
2008 Elsevier Ltd. All rights reserved.
1. Introduction
The overall reaction in the electrochemical deposition of MnO2
from an electrolyte containing Mn2+ ions is [13]:
The electrochemical oxidation of Mn(II) is of considerable
importance from both practical and fundamental viewpoints. Its
oxidation in acidic solution produces various higher-valent Mn
species such as Mn(III), Mn(IV) and Mn(VII) [1]. The strong oxidis-
ing properties of Mn(III) and Mn(VII) have been utilised in many
analytical techniques [2] and also for decomposition of organic
compounds [3,4]. The Mn(IV) state is usually available in MnO2.
In addition to the naturally occurring form, known as pyrolusite,
MnO2 can be prepared by both chemical (CMD) and electrochem-
ical (EMD) means. The anodic deposition method is widely used
for preparing hydrated and amorphous MnO2 having high energy
Mn2 + 2H O → MnO + 4H + 2e−
+
+
2
2
However, it is unlikely that the reaction takes place in a single
step. The mechanism of anodic deposition of MnO2 from Mn(II)
solutions has been investigated extensively [6,16–22]. The general
3+
conclusion is that the first step is the generation of Mn as a pre-
cursor, which subsequently disproportionates to Mn2 and Mn at
low acid concentration or hydrolyses to form an intermediate prod-
uct of insulating MnOOH at high acid concentration. Formation of
EMD has also been studied in different carboxylic, sulphonic and
phosphoric acid media [23–26].
+
4+
and power density. The EMD variety is usually ␥-MnO . Much
2
effort have been spent to characterise the electrodeposited material
and to study the reduction of ␥-MnO2 under various conditions in
view of its widespread application in batteries and supercapacitors
Recently we have reported the use of MnO2 prepared electro-
chemically (by galvanostatic technique) and chemically from both
aqueous and reverse micellar media as an anode catalyst for oxida-
tion of carbohydrates and poly-ols [27–30]. The electro-oxidation
was found to proceed via a catalytic EC mechanism in which Mn(V)
is generated electrochemically and consumed chemically in succes-
sion. Electrochemical oxidation of these carbohydrates has several
possible important applications in diverse fields such as medical
(estimation of blood sugar), environmental (treatment of wastewa-
ter containing carbohydrates), industrial (analysis of food products
[
5–13]. The usefulness of EMD as an oxygen reduction electrode has
also been reported [14,15].
∗ Corresponding author. Tel.: +91 33 2414 6666; fax: +91 33 2414 6584.
E-mail address: kdas@chemistry.jdvu.ac.in (K. Das).
0
013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2008.07.082

2
90
D. Das et al. / Electrochimica Acta 54 (2008) 289–295
Fig. 1. SEM photographs of the MnO2 deposit on carbon: (a) 2.5 min and (b) 10 min deposition; (c) EDAX spectrum and surface composition (inset).
and synthesis of materials) and energy conversion (development
of carbohydrate fuel cell) [31–36].
2. Experimental
In this paper, we have used anodic deposition by potentiostatic
technique, which is expected to give more control on the process by
choosing a suitable potential and thereby restricting the side reac-
tions to a minimum. The mechanism of potentiostatic deposition
2.1. Electrode preparation
The electro-deposition of MnO2 was carried out potentiostat-
ically at 0.9 V vs. saturated calomel electrode (SCE) on platinum
(Pt) and carbon (C) electrodes from a solution composed of 0.01 M
of MnO , the electrochemistry of the deposit and its electrocat-
2
alytic activity towards the oxidation of dextrose and fructose have
been studied by using cyclic voltammetry, chronoamperometry,
chronopotentiometry and steady state polarization measurements.
MnSO4 (AR, Merck) and 0.5 M Na SO4 (GR, Merck). The deposition
time was varied from 0.5 min to 30 min. After deposition, the elec-
2
Fig. 3. Plots of (a) anodic peak current densities (jp) vs. v1/2 and (b) peak potential
(E ) vs. potential scan rate (v) for 0.01 M MnSO in 0.5 M Na SO .
p 4 2 4
Fig. 2. Cyclic voltammograms for different concentrations of MnSO4 in 0.5 M Na2SO4
−1
at a scan rate of 0.005 V s
.

D. Das et al. / Electrochimica Acta 54 (2008) 289–295
291
obtained from the EDAX spectrum showed that the deposit con-
tained S, possibly by incorporation of SO4= within the MnO lattice,
2
as observed earlier [28].
3.2. Electrochemistry during deposition of MnO2
2
+
Cyclic voltammograms for different concentrations of Mn
−
1
solutions on Pt at a scan rate of 0.005 V s (Fig. 2) exhibited an
anodic peak around 0.9 V which increased in height and shifted
slightly towards more positive potentials with increasing Mn2+ con-
centration. At higher concentration, another anodic peak appeared
at a more positive potential of ∼1.4 V. However, the cathodic peak at
∼
0.75 V was found to be roughly 10 times smaller than the anodic
one.
The anodic peak obtained in the cyclic voltammogram of Mn2+
solution must be related to the formation of the MnO deposit
2
+
2
and, therefore, may be ascribed to the oxidation of Mn to some
higher oxidation state, Mn3+ or Mn . In the former case, Mn
4+
4+
3+
is formed subsequently by disproportionation of Mn . The peak
current increased with Mn concentration showing a small neg-
Fig. 4. Logarithmic chronoamperograms for different concentrations of Mn2+ solu-
2+
tions in 0.5 M Na2SO4 at 0.95 V vs. SCE.
ative deviation from linearity while the peak potential shifted to
more positive values. These observations jointly suggest that the
thickness of the electro-generated MnO2 layer increases with Mn2+
concentration and the peak characteristics are controlled by the dif-
fusion of Mn2+ ions through this layer. The anodic peak observed
at considerably more positive potential (∼1.4 V) for higher con-
centration of Mn2+ ions indicates the generation of still higher
oxidation state of Mn, possibly Mn(VII). On the other hand, much
smaller cathodic peak, compared to the anodic one, obtained dur-
ing the reverse sweep and its practically unchanged position with
trode was thoroughly washed in double distilled water and dried
in air.
2.2. Electrochemical measurements
Electrochemical characteristics of the MnO deposition process,
2
the MnO2 deposited electrodes and electro-oxidation of dextrose
GR, Merck) and fructose (Extrapure, Merck) on these were studied
(
2+
by cyclic voltammetry (CV), chronoamperometry (CA), chronopo-
tentiometry (CP) and steady state polarization measurements. A
conventional three-electrode cell was used with a Pt foil as counter
and a SCE as reference electrodes. All potentials given in the text are
Mn concentration suggests that the MnO2 layer formed is quite
stable.
a
The observed linear variation of anodic peak current (i ) with
p
square root of potential scan rate (v) at fixed Mn2+ concentration
with respect to SCE. The supporting electrolyte was 0.5 M Na SO4
(Fig. 3a) indicates that the rate-determining step is the diffusion of
2
2+
(GR, Merck). The electrochemical equipments used were a com-
Mn ions. The peak potential was found to be practically indepen-
a
puter aided potentiostat/galvanostat (AEW2-10, Ministat, Sycopel
Scientific Ltd., UK) and an Autolab PG STAT 12 (Eco Chemie, The
Netherlands). The experimental temperature was 30 ± 2 C.
dent of v, implying reversibility (Fig. 3b). Therefore, ip is related to
v by the following expression [37]:
◦
a
= (2.69 × 10 )n3/2AD C v
5
1/2
0
∗
1/2
= BCVv1/2
i
p
0
2.3. Surface studies
where BCV is the slope of ia vs. v1/2 plot. The numerical constant
p
Surface morphology of the electrochemically prepared deposit
a
corresponds to expressing i in ampere, electrode surface area (A)
p
was examined with a scanning electron microscope, SEM (JEOL,
360). The surface composition was obtained from energy disper-
sive X-ray analysis, EDAX (INCA X-Stream Oxford Instruments, UK).
2
2+
2
−1
in cm , diffusion coefficient of Mn ions (D ) in cm s , its con-
centration (C ) in mol cm and v in V s while n is the number of
0
6
−3
−1
0
electrons transferred.
The deposition of MnO2 was also studied using chronoamper-
ometry at different Mn concentrations. The chronoamperograms
3. Results and discussion
2+
were found to follow Cottrell equation [37] and the slopes of cur-
3.1. Surface characteristics of the deposited layer
−1/2
rent vs. (time)
plots reached limiting values at potentials more
positive than ∼0.9 V. Chronoamperograms recorded for different
Fig. 1 shows the SEM pictures and the EDAX spectrum of the
MnO2 deposits obtained on carbon under potentiostatic control at
.9 V for deposition times of 2.5 min and 10 min. At lower deposition
time the material was needle-shaped, while at higher deposition
time the surface of the deposit became smoother. The composition
2+
concentrations of Mn solutions at 0.95 V are presented in Fig. 4.
The current at time t, denoted by i(t), follows the Cottrell equation
0
[
37], showing diffusion control:
1/2
∗
nFAD0
C
BCA
0
i(t) =
=
t1/2
␲1/2
t1/2
Table 1
BCV, BCA and n values [see text] at various Mn2 concentrations in 0.5 M Na2SO4
+
−1/2
where BCA is the slope of i vs. t
plot.
Mn2+ concentration (M)
BCV (A V
−1/2
s1/2
)
BCA (A s1/2
)
n
By combining the expressions for BCV and BCA, the following
equation can be obtained:
0
0
0
0
.002
.005
.01
3.44
7.05
11.4
0.69
1.60
2.26
3.55
0.99
0.78
1.02
0.85
ꢀ
ꢁ
2
1
4.4
BCV
n = 2
BCA
.02
16.34

2
92
D. Das et al. / Electrochimica Acta 54 (2008) 289–295
Table 2a
Anodic and cathodic peak current densities (jp) and potentials (Ep) vs. SCE for the
deposited electrode (charge density = 120 mC cm ) at different potential scan rates
in 0.5 M Na2SO4
Table 2b
Chronopotentiometric transition times (ꢀ) and quarter-wave potentials (Eꢀ/4) vs.
SCE for the deposited electrode (charge density = 120 mC cm ) at different current
densities (j) in 0.5 M Na2SO4
−2
−2
Scan rate (V s 1
−
)
Anodic
Cathodic
j (mA cm
−2
)
0
0
0
0
0
0
0
.002
0.4
0.5
56
0.7
1.0
16
1.5
jp (mA cm 2
−
)
)
)
)
)
)
)
0.38
0.915
−0.18
ꢀ
(s)
69.5
0.86
29.5
0.88
8
0.82
Ep (V)
0.43
Eꢀ/4 (V)
0.87
0.85
.005
jp (mA cm 2
−
0.58
0.95
−0.20
−0.40
Ep (V)
0.80
0.44
two small cathodic peaks at ∼0.8 V and ∼0.4 V, for which the cur-
rents varied linearly with potential scan rate.
The deposited electrode showed a single chronopotentiometric
CP) transition to oxygen evolution region. The transition times (ꢀ)
and the quarter-wave potentials (Eꢀ/4) at varying current densities
(j) are given in Table 2b. The increase in the transition time with
decreasing current density was more rapid than that demanded by
Sand equation [37].
The CV and the CP responses of the deposited electrode can be
analysed by considering the primary electrochemical process to be
.01
jp (mA cm 2
−
0.94
0.965
−0.50
−0.80
−0.43
Ep (V)
0.80
(
.015
jp (mA cm 2
−
1.16
0.98
−0.60
−0.88
Ep (V)
0.775
0.42
.02
jp (mA cm 2
−
1.44
0.99
−1.04
−1.30
Ep (V)
0.74
0.45
.03
jp (mA cm 2
−
1.76
0.99
−1.20
−1.68
Mn(IV) − ne− → Mn(∗)
Ep (V)
0.76
0.40
.05
To establish the identity of Mn (*) the model proposed earlier
28,29] is used, by which it is postulated that Mn (*) is continuously
jp (mA cm 2
−
2.98
1.00
−2.34
−3.30
[
Ep (V)
0.75
0.42
generated electrochemically and consumed chemically (e.g., by dis-
proportionation). Two parameters denoted by SCV and SCP, obtained
from CV and CP data, respectively, are expressed as follows [28,29]:
The value of n, thus evaluated, is found to be fairly close to unity.
Table 1 summarises the values of BCV, BCA and n at different Mn2+
concentrations.
ꢂ
ꢀ
ꢁꢃ
1
4
nF
RT
SCV
=
nFN0
Based on the above discussion it is concluded that the primary
2
+
electrochemical step during anodic deposition of MnO2 from Mn
solution is a one-electron transfer process, by which Mn3 is gen-
erated. Thereafter, the latter rapidly disproportionates to produce
MnO2 as the deposit on the electrode surface according to the fol-
lowing scheme.
+
SCP = [nFN0]
N0 being the surface density of active Mn(IV) sites in the deposit
based on the geometrical surface area.
Therefore,
Mn2 → Mn3+ + e−
+
ꢀ
ꢁꢀ
ꢁ
4
RT
F
SCV
SCP
n =
+
2
Mn3 → Mn2+ + Mn4+
Using the observed values of S (90.3 mC V 1 cm ) and SCP
−
−2
CV
−
2
(
13.8 mC cm ), n is calculated to be 0.68. As n should be an inte-
Mn4 + 2H O → MnO + 4H
+
+
2
2
ger, the observed value may be taken to be close to unity. Hence,
the intermediate Mn (*) species is likely to be Mn(V) as was con-
cluded for both chemically and galvanostatically deposited MnO2
There is some doubt in the literature regarding the actual state of
the Mn3+ species generated as an intermediate at the electrode
surface. Some authors consider it to remain in the adsorbed state
[
28,29]. The deviation of n from unity can also arise from the differ-
ence in the effective N0 values between CV and CP measurements.
Such difference may arise due to the relatively longer experimen-
tal times required for CP compared to CV, whereby some active
Mn(IV) sites in the bulk of the deposit, in addition to the surface
ones, may become involved in the former technique. Therefore,
[
6,16,17] while others speculate the formation of MnOOH [18–21].
The real situation may be a mixture of both of these [22], although,
under the present experimental condition of very slightly alkaline
environment, the latter proposition seems to be more likely.
SCV is expected to be more appropriate for calculating N tak-
0
−
2
3
.3. Electrochemistry of the deposited electrode
ing n = 1. N0 at a deposition charge of 120 mC cm
is found to
−
8
−2
be 9.75 × 10 mol cm . Total amount of deposition can be esti-
mated by using the deposition charge and assuming the deposition
to be an overall two-electron transfer process with perfect current
MnO2 was deposited on Pt potentiostatically from 0.01 M Mn2+
solution in 0.5 M Na SO at 0.9 V (vs. SCE). The potential was chosen
2
4
−
7
−2
on the basis of the results obtained from cyclic voltammetric and
chronoamperometric studies reported earlier. The charge passed
during deposition varied linearly with the deposition time, indi-
cating a fairly constant rate of growth (33.74 mC cm min ) of
the deposit.
efficiency. This is calculated to be 6.2 × 10 mol cm . A compar-
ison with the value of N0 shows that approximately ∼16% of the
total number of Mn(IV) sites remains active at the surface of the
deposited mass.
−
2
−1
The cathodic peaks obtained during the negative potential scan
are not very prominent, indicating that total reduction of the
deposited film does not occur. The observed linear dependence
of the peak currents with potential scan rate suggests that the
cathodic process involves surface-bound redox sites. The two peaks
Cyclic voltammogram (CV) of the deposited electrode in the
supporting electrolyte of 0.5 M Na SO4 solution showed an anodic
2
peak at ∼1.0 V. The peak current increased non-linearly with v and
appeared to approach a limiting value (Table 2a). There were also

D. Das et al. / Electrochimica Acta 54 (2008) 289–295
293
−
1
Fig. 5. Cyclic voltammograms of MnO2/Pt electrodes in different solutions at
Fig. 6. Variation of peak current densities (jp) at 0.01 V s with carbohydrate con-
centrations (C).
−1
0
.01 V s . Blank denotes the supporting electrolyte (0.5 M Na2SO4) present in all
the solutions.
the potentiostatically deposited material indicates that it is more
porous than the chemically deposited ones by which a greater con-
tact area with the solution becomes possible in the former case.
Fig. 7 shows the semi-logarithmic plots of steady state current
vs. potential, obtained potentiodynamically under sufficiently slow
may be assigned to successive reductions of Mn(IV) to Mn(III) and
then Mn(III) to Mn(II) according to the following scheme:
MnO + H + e− → MnOOH
+
2
MnOOH + H+ + e− → Mn(OH)2
−1
potential scan rate (0.5 mV s ) on MnO2 deposited on Pt and car-
bon bases. Substantially higher currents were observed for fructose
than dextrose in both the cases. Significantly, the electrocatalytic
activity of MnO2 deposit towards oxidation of the carbohydrates
remains practically unchanged on replacement of Pt by the much
cheaper material C as the substrate. The Tafel slopes calculated from
the linear portions of the potential vs. log current density profiles
are around ∼0.2 V for both dextrose and fructose.
Mn(III) is somewhat stabilised by the alkaline environment [38]
generated within the deposited layer during reduction.
3.4. Electro-oxidation of the carbohydrates
The catalytic activity of the deposited electrodes towards oxi-
dation of carbohydrates was assessed from slow sweep rate
To evaluate the kinetic parameters of oxidation of the carbohy-
−1
(
0.5 mV s ) potentiodynamic scans. The current densities obtained
drates, chronoamperograms of MnO /Pt electrodes at 1.0 V were
2
at a fixed potential of 0.9 V in the presence of carbohydrates
in the electrolyte passed through a maximum with deposition
recorded in different concentrations of dextrose and fructose in
the solution. For an electrochemical process following catalytic EC
mechanism, chronoamperometric technique can be used to deter-
mine the rate constant (k) of the catalyst regeneration step [37], i.e.,
the reaction between Mn(V) and carbohydrate in the present case.
−2
charge density of 120 mC cm . Therefore, the deposition time cor-
responding to this charge density was used for further studies
on oxidation of the carbohydrates. Possibly, at higher deposition
charge densities the porosity of the deposit decreases, thereby
impeding the access of the carbohydrate molecules within its bulk.
The electrocatalytic activity of potentiostatically deposited
MnO2 towards carbohydrate oxidation is demonstrated by the
enhancement of the Mn(IV) → Mn(V) oxidation peak current with
addition of dextrose and fructose in the electrolyte (Fig. 5). This
behaviour indicates that a catalytic EC mechanism is operative, as
found earlier for both chemically and galvanostatically prepared
MnO /Pt electrodes [28–30].
2
Mn(IV) → Mn(V) + e−
Mn(V) + carbohydrate → Mn(IV) + lactone
The increase in the peak current densities continued up to ∼0.4 M
for both dextrose and fructose before approaching limiting values
(
Fig. 6). Such limiting behaviour must be due to the requirement of
adsorption of the carbohydrate molecules at the deposit-solution
interface prior to electron transfer, which is likely to reach satura-
tion values at higher carbohydrate concentrations. Although MnO2
deposits prepared chemically from either aqueous or reverse micel-
lar media show qualitatively the same behaviour, the saturation
points were found to be at much lower carbohydrate concentra-
tions (<0.1 M). The observed higher saturating concentration for
Fig. 7. Plots of steady state potential (vs. SCE) vs. log current density for oxidation
of 0.2 M dextrose and fructose on MnO2/Pt and MnO2/C electrodes.

2
94
D. Das et al. / Electrochimica Acta 54 (2008) 289–295
Table 3
Steady state current densities (after ∼1 min) at 1.0 V vs. SCE in solutions with increas-
ing concentrations of dextrose and fructose
Current density (␮A cm 2
−
)
Concentration (mM)
Dextrose
Fructose
0
2
4
6
8
24
54
82
114
146
182
24
120
240
400
560
720
10
The relevant relationship is
ꢄ␲1/2
erf(ꢁ1/2) + exp(−ꢁ)
ꢅ
i
=
ꢁ1/2
(1)
ꢁ1
/2
id
where i and id are the oxidation currents in the presence and
absence of carbohydrates, respectively, at time t, while ꢁ = kc t (c0
0
being the bulk concentration of carbohydrate) and erf denotes the
error function given by the following equation[37]:
ꢆ
ꢇ
ꢂ
ꢃ
2
␲1/2
ꢁ1/2
1
1
10
1
42
erf(ꢁ1/2) =
1 − 3ꢁ +
ꢁ −
2
ꢁ + · · ·
3
(2)
(3)
under the condition 0 ≤ ꢁ ≤ 2.
Again,
ꢂ
ꢃ
1
1
6
1
2
3
4
exp(−ꢁ) = 1 − ꢁ + 2ꢁ − ꢁ +
ꢁ − · · ·
24
Now, using Eqs. (2) and (3) in Eq. (1), one obtains
ꢂ
ꢃ
i
1
1
30
1
168
2
3
4
=
1 + ꢁ − 6ꢁ +
ꢁ −
ꢁ + · · ·
id
which may be approximated as
i
1
6
2
2
=
1 + kc t − (kc ) t
(4)
0
0
id
3
−1 −1 −3
, mol dm and s,
Here k, c0 and t are expressed in dm mol
s
Fig. 8. Polynomial fits of i/id vs. t [see text] for (a) fructose and (b) dextrose.
respectively. From the polynomial fit of i/i vs. t (Fig. 8), the values
d
3
−1 −1
of k are evaluated to be 78 and 134 dm mol
s for dextrose and
fructose, respectively. Higher value of the rate constant for fructose
than dextrose can be accounted for by considering the presence of
4. Conclusions
two OH groups attached to the carbon atoms adjacent to the C
O
The electrocatalytic activity of potentiostatic MnO2 deposits
towards carbohydrate oxidation has been shown to be due to
the repeated transitions between Mn(IV) and Mn(V) states. Both
MnO /Pt and MnO /C electrodes are found to be equally effective
in this regard. The oxidation currents increase linearly with carbo-
hydrate concentrations, which lead to the possibility of using these
electrodes as carbohydrate sensor. The large difference between the
oxidation currents for dextrose and fructose can be rationalised by
considering jointly the structural features of the molecules and the
rate constants of the catalyst regeneration step.
group in fructose, compared to only one in dextrose. Adsorption of
the carbohydrate molecules on the electrode surface via hydrogen
bonding with such suitably oriented OH groups is a pre-requisite
for efficient electro-oxidation of the C O group [28]. Moreover,
the intermediate oxidation product in case of fructose is likely to
be a ␥-lactone, which is more stable [39] than the corresponding
intermediate ␦-lactone expected from dextrose oxidation.
2
2
3.5. Electroanalytical application in carbohydrate estimation
The steady state currents (after ∼1 min) in excess of that
Acknowledgements
obtained in the supporting electrolyte alone (blank) at a fixed
potential of 1 V were found to increase fairly linearly with dex-
trose and fructose concentrations (Table 3). The slopes of these lines
are related to the sensitivities of carbohydrate estimation by this
Financial assistance from Jadavpur University is gratefully
acknowledged. One of the authors (DD) thanks Jadavpur University
for providing a research fellowship.
−
2
−1
technique. The calculated values are 15.7 and 70.9 ␮A cm mM
for dextrose and fructose, respectively. The sensitivity value for
dextrose is found to be higher than that reported for its estima-
tion (9.6 ␮A cm mM ) using meso-porous Pt [40]. Moreover, the
same electrode could be used for a series of measurements with-
out any significant loss in activity, which indicated its stability in
performance.
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