Bismuth film electrode for stripping voltammetric measurement of sildenafil citrate

Article abstract of DOI:10.1016/j.electacta.2011.11.049

A simple and fast procedure for measuring low concentration of sildenafil citrate (SC), also known as Viagra, at the ex situ prepared bismuth film electrode (BiFE) is demonstrated. The BiFE was prepared ex situ on the surface of a glassy carbon substrate electrode and employed in adsorptive cathodic stripping voltammetric mode in 0.1 M acetate buffer solution (pH 4.5) in the presence of dissolved oxygen. Electroanalytical properties of the BiFE were studied and compared with the ex situ prepared mercury film electrode, lead film electrode and with bare glassy carbon electrode. The BiFE revealed a LoD (3σ) of 1.8 × 10^-8 mol L^-1 using 120 s accumulation time and good linear response in the examined concentration range of 1.0 × 10^-7 to 1.0 × 10^-6 mol L^-1 SC with a correlation coefficient (R²) of 0.995 using short accumulation time of 30 s. The obtained very good repeatability of 1.5% (c = 1.0 × 10^-6 mol L^-1, n = 15) corroborated potential applicability of the BiFE for measuring SC, for example in pharmaceutical formulations. A successful preliminary study using constant current chronopotentiometric stripping mode has been also carried out.

Full text of DOI:10.1016/j.electacta.2011.11.049

Electrochimica Acta 60 (2012) 274–277
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Bismuth film electrode for stripping voltammetric measurement of sildenafil
citrate
Hanna Sophaa, Samo B. Hocevara,∗, Boris Pihlarb, Bozidar Ogorevca
a Analytical Chemistry Laboratory, National Institute of Chemistry, Hajdrihova 19, SI-1000 Ljubljana, Slovenia
b Faculty of Chemistry and Chemical Technology, University of Ljubljana, Asˇkercˇeva cesta 5, SI-1000 Ljubljana, Slovenia
a r t i c l e i n f o
a b s t r a c t
Article history:
A simple and fast procedure for measuring low concentration of sildenafil citrate (SC), also known as
Viagra®, at the ex situ prepared bismuth film electrode (BiFE) is demonstrated. The BiFE was prepared
ex situ on the surface of a glassy carbon substrate electrode and employed in adsorptive cathodic strip-
ping voltammetric mode in 0.1 M acetate buffer solution (pH 4.5) in the presence of dissolved oxygen.
Electroanalytical properties of the BiFE were studied and compared with the ex situ prepared mercury
film electrode, lead film electrode and with bare glassy carbon electrode. The BiFE revealed a LoD (3ꢀ)
of 1.8 × 10−8 mol L−1 using 120 s accumulation time and good linear response in the examined concen-
tration range of 1.0 × 10−7 to 1.0 × 10−6 mol L−1 SC with a correlation coefficient (R2) of 0.995 using short
accumulation time of 30 s. The obtained very good repeatability of 1.5% (c = 1.0 × 10−6 mol L−1, n = 15)
corroborated potential applicability of the BiFE for measuring SC, for example in pharmaceutical formu-
lations. A successful preliminary study using constant current chronopotentiometric stripping mode has
been also carried out.
Received 19 September 2011
Received in revised form 9 November 2011
Accepted 12 November 2011
Available online 23 November 2011
Keywords:
Sildenafil citrate
Viagra
Bismuth electrode
Stripping voltammetry
Stripping chronopotentiometry
© 2011 Elsevier Ltd. All rights reserved.
1. Introduction
and measured with electrochemical DNA biosensor [17]. Among
these electrode materials, lead and mercury are well-known as
toxic heavy metal pollutants being the latter associated also with
significant difficulties in its handling, disposal, and subjected to
very strict regulations.
Recently, sildenafil citrate (SC, also Viagra®),
name for 1-[[3-(6,7-dihydro-1-methyl-7-oxo-3-propyl-1-H-
pyrazolo[4,3-d]pyrimidin-5-yl)-4-ethoxy-phenyl]sulfonyl]-4-
methyl-piperazine citrate (Fig. 1) has attracted considerable
attention due to its application for oral therapy of erectile dys-
function. Initially it was investigated for its use in the case of
hypertension and angina pectoris [1].
The measurements of SC are usually carried out by means of
techniques such as liquid chromatography [2–8] or spectropho-
tometry [9,10] with appropriate sample preparation and time
consuming SC pre-concentration. On the other hand, electrochem-
ical techniques offer an attractive approach for direct, fast, simple
and inexpensive measurements of electroactive organic and inor-
ganic species, particularly in combination with stripping protocols
which enable unique pre-concentration of the analyte in/on the
surface of the selected working electrode. Hence, in the last few
years, several attempts were taken to measure SC electrochemically
using different types of electrodes and methods, e.g. SC was voltam-
metrically oxidized at boron-doped diamond electrode [11,12] and
at glassy carbon electrode [13], reduced at hanging mercury drop
electrode [14,15] and at the in situ prepared lead film electrode [16],
a
trivial
Approximately one decade ago, bismuth film electrode was
introduced as an alternative to its mercury counterpart and has
been already widely accepted in numerous electroanalytical lab-
oratories [18]. Namely, bismuth has been recognized as a “green”
element exhibiting comparable electroanalytical performance to
that of mercury. By now, bismuth electrodes have been used
in different configurations, e.g. bismuth film electrode prepared
on a glassy carbon [19], carbon paste [20], carbon fiber [21] or
boron-doped diamond substrate electrode [22], bismuth oxide and
bismuth powder bulk modified carbon paste electrodes [23,24],
screen printed electrode [25], bismuth sputtered electrode [26]
and bismuth bulk electrode [27], used for measuring inorganic
[28–30] and organic analytes [31–34]. However, there is only a
small number of works reporting on the application of the bis-
muth film electrode for measuring organic analytes in combination
with the electrochemical stripping analysis, i.e. including the pre-
concentration step [32].
In this work, we presented the successful application of the ex
situ prepared bismuth film electrode (BiFE) for measuring low con-
centration levels of SC in combination with adsorptive cathodic
stripping voltammetric mode in a non-deaerated 0.1 M acetate
buffer solution.
∗
Corresponding author. Tel.: +386 1 4760 230; fax: +386 1 4760 300.
E-mail address: samo.hocevar@ki.si (S.B. Hocevar).
0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2011.11.049

H. Sopha et al. / Electrochimica Acta 60 (2012) 274–277
275
Fig. 1. Structural formula of sildenafil citrate.
2. Experimental
potential of usually −0.6 V was applied to the working electrode
for a definite period of time. After the equilibration period of 10 s, a
stripping voltammogram was recorded using a square-wave poten-
tial scan with a frequency of 25 Hz, an amplitude of 50 mV, and a
step potential of 4 mV.
2.1. Apparatus
All measurements were performed using a modular elec-
trochemical system AUTOLAB PGSTAT12 (Eco Chemie, Utrecht,
Netherlands) in combination with GPES software (Eco Chemie).
The usual three-electrode configuration was employed with the
bismuth (BiFE), mercury (MFE) or lead film electrode (PbFE) all
prepared ex situ on a glassy carbon (GCE) disk (d = 2 mm) as the
working electrode, an Ag/AgCl/KCl (satd.) reference electrode and
a platinum wire as the counter electrode. A computer controlled
magnetic stirrer rotating at approximately 300 rpm was employed
during the accumulation step. All experiments were carried out at
room temperature of 23 1 ◦C in a 20 ml voltammetric cell.
3. Results and discussion
It has been proposed that the anodic voltammetric signals of
SC at the bare GCE, recorded at ca 1.0 V and 1.2 V vs. Ag/AgCl
in acetate buffer solution (pH 4.7) containing 30% of acetonitrile,
are attributed to the oxidation of the piperazine ring of the SC
molecule [13]. For the cathodic processes, observed at the mer-
cury electrode at approximately −1.05 V and −1.15 vs. Ag/AgCl in
0.01 M HClO4 (pH 2) [14], and also at the GCE at ca −1.4 V [13],
the reduction of the sulfonyl group was suggested by Rodriguez
et al. [14,15], but opposed by Zuman et al. [13], yet neither of these
claims were supported by a clear experimental evidence. Our ini-
tial experiments were focused on the comparative examination of
SC using cyclic voltammetry in combination with the bare glassy
carbon electrode and the ex situ prepared bismuth film electrode
with the potential scanning in the negative direction in the range
of −0.3 V to −1.5 V. As can be seen in Fig. 2, two cathodic peaks
were obtained at both electrodes at nearly identical peak poten-
tials, i.e. at −1.26 V and −1.40 V at the bare GCE (dashed line), and
at −1.25 V and −1.40 V at the BiFE (solid line) indicating the same
totally irreversible reduction electrode processes. In this case, the
BiFE exhibited a higher signal vs. GCE at −1.26 V, however, the
smaller signals obtained at more negative potential of −1.40 V were
associated with lower reproducibility. It is important to note, that
when switched to cathodic stripping voltammetric mode, the GCE
revealed also significantly lower reproducibility.
2.2. Reagents and solutions
All chemicals were of analytical grade purity. The standard solu-
tions of bismuth(III), mercury(II) and lead(II) were provided by
Merck and further diluted as required, whereas Sildenafil Citrate
(SC) was obtained from LGC Standards (Germany). A stock stan-
dard solution of 1 × 10−3 M SC was prepared in water and stored
at 4 ◦C. If not stated otherwise, a 0.1 M acetate buffer solution with
pH 4.5 was used as a supporting electrolyte. Water used through-
out the work was first deionized and then further purified using
Elix 10/Milli-Q Gradient unit (Millipore, Bedford, USA).
2.3. Preparation of the BiFE, MFE and PbFE
The bismuth film electrode was prepared ex situ on the surface
of a glassy carbon substrate electrode. Prior to plating, the glassy
carbon electrode was polished with a 0.05 ␮m alumina slurry on
a polishing pad. The electrode was then rinsed thoroughly with
water and placed in a plating solution consisting of a 0.1 M acetate
buffer (pH 4.5) and 5 mg L−1 Bi(III). A constant potential of −1.0 V
was applied to the working electrode for 60 s while the solution
was maintained under stirring conditions. After the modification
step, the electrode was rinsed with purified water and ready to use.
MFE and PbFE were prepared using a similar procedure, except that
the plating solutions consisted of 10 mg L−1 Hg(II) in 0.01 M HCl for
MFE and 20 mg L−1 Pb(II) in 0.1 M acetate buffer solution (pH 4.5)
for PbFE. The potential of −0.6 V was applied for 300 s in the case
of MFE and −1.2 V for 120 s in the case of PbFE.
2 A
μ
-1.6
-1.2
-0.8
-0.4
Potential / V
2.4. Measurement procedures
Fig. 2. Cyclic voltammograms of 5.0 × 10−6 mol L−1 SC at the ex situ prepared BiFE
If not stated otherwise, measurements were carried out using
the cathodic stripping voltammetric mode. An accumulation
(solid line) and at bare GCE (dashed line) in 0.1 M acetate buffer solution (pH 4.5)
purged with argon before measurement. Scan rate: 100 mV s−1
.

276
H. Sopha et al. / Electrochimica Acta 60 (2012) 274–277
1.2
A
1
5 µA
0.8
0.6
0.4
0.2
0
120
100
80
60
40
20
0
BiFE
MFE
GCE
PbFE
A
D
C
B
-0.4
-0.7
-1
-1.3
-0.9
-1.1
-1.3
Potential / V
-1.5
Accumulation potential / V
2
1.5
1
B
b
Fig. 3. Comparison of the adsorptive cathodic square-wave stripping voltammetric
signals of SC obtained at the ex situ prepared BiFE (A), MFE (C), PbFE (D), and at bare
GCE (B) in 0.1 M acetate buffer solution (pH 4.5). The relative comparison is shown
in the inset. Accumulation potential: −0.6 V, accumulation time: 90 s, square-wave
conditions: frequency of 25 Hz, amplitude of 50 mV, and step potential of 4 mV.
a
0.5
0
To confirm these observations, a potential scan rate dependence
study was performed with the BiFE in the linear sweep voltammet-
ric mode with potential scan rates in the range of 10–1000 mV s−1
(in the potential range of −0.6 V to −1.5 V) using two protocols:
(i) without holding the potential at −0.6 V (immediate start of the
scanning) and (ii) with holding the potential at −0.6 V for 60 s before
starting the potential scan (not shown). Surprisingly, in both cases
the dependence of the peak current (the peak at −1.26 V as in
Fig. 2—solid line) was nearly linear with the scan rate exhibiting
regression coefficients (R2) of 0.997 in the case of (i) and 0.993 in the
case of (ii). This indicates a relatively fast chemisorption of the SC to
the bismuth film surface. The same result was obtained also by con-
ducting the square-wave frequency dependence study performing
cathodic stripping voltammetric scans with accumulation of the SC
(concentration of 5 × 10−6 mol L−1) at −0.6 V for 60 s. In this case
the square-wave peak height was linear with the frequency in the
range of 8–250 Hz with R2 of 0.998.
Since the preliminary studies revealed favorable operation of
the bismuth electrode for stripping voltammetric measurement of
SC, the ex situ prepared BiFE was further compared with the ex situ
prepared MFE, the ex situ prepared PbFE and with the bare GCE
(Fig. 3). In all four cases the stripping peaks for SC can be observed
at approximately −1.2 V suggesting similar reduction processes at
all four electrodes with the lowest background currents recorded at
the BiFE and bare GCE. The peak current registered at the BiFE was
significantly higher than current responses at the other examined
ex situ prepared electrodes, i.e. 35% higher than at the bare GCE, 22%
higher than at the MFE and 62% higher than at the PbFE compared
to the normalized signal for BiFE, i.e. 100%. In addition, the second
signals could be also observed at more negative potentials, however
these signal were considerably lower and associated with poorer
reproducibility.
To set the optimal conditions for measuring SC at the ex
situ prepared BiFE the effect of pH, accumulation potential and
accumulation time were investigated. Studying the effect of dif-
ferent pH values of the acetate buffer solutions, i.e. pH 3.5, 4.5,
and 5.5, revealed that pH 4.5 allowed the most favorable opera-
tion of the bismuth film electrode, whereas in the solution with
pH 5.5 the reduction peak was significantly lower and shifted
towards more negative potential value. In the measurement solu-
tion adjusted to pH 3.5 the signal for SC was even lower, broader
and, as expected, appearing at slightly less negative potential. This
implies on the involvement of protonation/deprotonation in the
redox process of SC. Other important parameters with a strong
influence upon the stripping performance are the accumulation
potential and accumulation time. The dependence of the stripping
signal on the accumulation potential is depicted in Fig. 4A which
shows a considerable increase of the stripping signal when the
0
100
200
300
Accumulation time / s
Fig. 4. The effect of accumulation potential (A) and accumulation time (B) upon the
stripping voltammetric response of 5.0 × 10−7 mol L−1 (A) and 3.0 × 10−7 mol L−1 (B-
a) and 7.0 × 10−7 mol L−1 (B-b) SC at BiFE. Accumulation time: 60 s (A), accumulation
potential: −0.6 V (B). Other conditions are as in Fig. 3.
accumulation potential was changed from −0.3 V to −0.6 V. After
applying more negative accumulation potentials than −0.6 V, the
electrode response decreased gradually until the examined accu-
mulation potential of −1.1 V, thus an accumulation potential of
−0.6 V was selected as optimum. The effect of the accumulation
potential is obviously more complex. Since the whole process
relies on the adsorption of the SC on the surface of the bis-
muth electrode, we can speculate that in this case, the more
negative potential at the bismuth film working electrode atten-
uated the affinity of the analyte towards the electrode surface
due to several possible reasons: the beginning of the hydrogen
evolution might result in locally higher pH which, as already
mentioned before, caused the decreased stripping signal of SC.
Additionally, in the region of more negative accumulation poten-
tials, the initiation of the SC reduction might affect its deposition.
Fig. 4B demonstrates the effect of the accumulation time on the
stripping signal in combination with two investigated concen-
trations of SC, i.e. 3 × 10−7 mol L−1 and 7 × 10−7 mol L−1. With
prolonging accumulation times the signal first increased almost
linearly and started to level off at accumulation times higher than
120 s in the case of 3 × 10−7 mol L−1 SC, whereas for the higher
examined concentration of SC, i.e. 7 × 10−7 mol L−1, the surface sat-
uration effect could be observed at accumulation times higher than
60 s.
A possible effect of the citrate ligand was examined via adding
relatively high concentrations of MgCl2 (vs. concentration of SC)
in the measurement solution of 0.1 M acetate buffer containing
1 × 10−6 mol L−1 SC aimed at complexing the citrate with mag-
nesium ions. Upon the additions of MgCl2, the stripping signals
remained practically unchanged. Furthermore, the addition of
small amounts, i.e. 2 × 10−5 mol L−1 and 1 × 10−4 mol L−1 of citrate
into the measurement solution of 0.1 M acetate buffer containing
1 × 10−6 mol L−1 SC did not affect the electrode’s stripping voltam-
metric response towards SC (not shown).
Aimed at getting more insights into the analytical performance
of the ex situ prepared bismuth film electrode, its stripping voltam-
metric response was followed while successively increasing the
concentration of SC in steps of 0.5 × 10−7 mol L−1 (Fig. 5). Good lin-
ear concentration dependence was registered in the concentration

H. Sopha et al. / Electrochimica Acta 60 (2012) 274–277
277
4. Conclusions
1.2
0.9
0.6
0.3
0
The bismuth film electrode prepared ex situ at the glassy
carbon substrate electrode was proposed for the first time for
convenient adsorptive stripping voltammetric measurements of
sildenafil citrate at low concentration levels. A good linear response
in the examined concentration range of 1.0 × 10−7 mol L−1 to
1.0 × 10−6 mol L−1 and an excellent repeatability of 1.5% were
obtained. Apart from the bismuth non-toxic character, the com-
parison of the ex situ prepared BiFE with its mercury and lead ex
situ prepared counterparts as well as with the bare glassy carbon
electrode showed a superior performance of the BiFE, thus broaden-
ing the applicability of bismuth based sensors towards measuring
various organic analytes.
0
2
4
6
8
10
Concentration / 10-7
M
1µA
-1.1
-1.2
-1.3
-1.4
Potential / V
Fig. 5. Stripping voltammograms for successive additions of SC in 0.5 × 10−7 mol L−1
steps from 1.0 × 10−7 mol L−1 to 1.0 × 10−6 mol L−1 SC together with background
response recorded at BiFE (every second scan is shown). The inset shows the corre-
sponding calibration plot. Accumulation potential: −0.6 V, accumulation time: 30 s.
Other conditions are as in Fig. 3.
Acknowledgement
Financial support from the Slovenian Research Agency (P1-
0034) is gratefully acknowledged.
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