Effect of metals on silver electrodeposition: Application to the detection of cisplatin

Article abstract of DOI:10.1016/j.electacta.2004.08.042

In the present work, the influence of several metals (Co, Ru, Pd, Os, Pt, Cu, Pb), deposited on a carbon paste electrode, towards silver electrodeposition was tested. First, adequate conditions for the electrodeposition of metals on the electrode were found. Then, the cyclic voltammograms registered (silver deposition curves and analytical signals) showed that Co, Cu, Pt and Pd were able to accelerate silver electrodeposition. Finally, a valid methodology for the detection of cisplatin was established. It is based on the deposition of silver on a Pt (from cisplatin) modified electrode and the analytical signal corresponds to the anodic stripping of the deposited silver. A limit of detection of 3.2 × 10^-9 mol dm^-3 (1 ng cm ^-3) cisplatin was obtained.

Full text of DOI:10.1016/j.electacta.2004.08.042

Electrochimica Acta 50 (2005) 1895–1902
Effect of metals on silver electrodeposition
Application to the detection of cisplatin
David Hern a´ ndez-Santos, Mar ´ı a Bego n˜ a Gonz a´ lez-Garc ´ı a,
∗
Agust ´ı n Costa-Garc ´ı a
Departamento de Qu ´ı mica F ´ı sica y Anal ´ı tica, Facultad de Qu ´ı mica,
Universidad de Oviedo, 33006 Oviedo, Asturias, Spain
Received 26 January 2004; received in revised form 3 May 2004; accepted 28 August 2004
Available online 30 November 2004
Abstract
In the present work, the influence of several metals (Co, Ru, Pd, Os, Pt, Cu, Pb), deposited on a carbon paste electrode, towards silver
electrodeposition was tested. First, adequate conditions for the electrodeposition of metals on the electrode were found. Then, the cyclic
voltammograms registered (silver deposition curves and analytical signals) showed that Co, Cu, Pt and Pd were able to accelerate silver
electrodeposition. Finally, a valid methodology for the detection of cisplatin was established. It is based on the deposition of silver on a Pt
(
from cisplatin) modified electrode and the analytical signal corresponds to the anodic stripping of the deposited silver. A limit of detection
−
9
−3
−3
of 3.2 × 10 mol dm (1 ng cm ) cisplatin was obtained.
2004 Elsevier Ltd. All rights reserved.
©
Keywords: Cisplatin; Silver electrodeposition; Carbon paste electrodes; Cyclic voltammetry
1
. Introduction
cyanines have been deposited on glassy carbon electrodes for
the electrooxidation of cysteine [4].
The modification of an electrode surface employing met-
Thebulkelectrodepositionofmetalsonthesurfaceofelec-
trodes can also form electrocatalytic layers. In some cases,
the electrocatalytic properties do not rely on the metal itself
but on derivatives such as metallic oxides. Wang et al. [5]
electrodeposited Pd and Pt on a glassy carbon electrode for
the electrocatalytic detection of formaldehyde, acetaldehyde
and propionaldehyde in a FIA system. Casella et al. [6] em-
ployed Pd modified glassy carbon electrodes as detectors in
chromatography for oxalic acid. Pt is also commonly em-
ployed for the electrocatalytic detection of different analytes
[7,8].
The underpotential deposition (upd) process, discovered
by Rogers et al. [9], is known as the phenomenon in which
up to a monolayer of metal deposits onto a foreign metal
substrate (usually the electrode) at potentials positive from
the reversible Nernst potential due to distinct interaction be-
tween them. The upd of a metal can form on the electrode
als or metallic complexes can give rise to electrodic surfaces
able to facilitate and/or accelerate electrochemical processes
and also to provoke electrocatalytic reactions. These surfaces
can usually be formed by means of chemical deposition of the
metal or metallic complex or by means of metal electrodepo-
sition.
Such layers can be formed by just placing a solution of the
metallic complex on the surface of the electrode for a time.
Cobalt metalophthalocyanines are commonly employed in
this way, as they are able to catalyse electrochemical reac-
tions. Electrodes so modified have been employed for the
detection of formaldehyde [1], trichloroacetic acid [2] or hy-
drazine [3]. Also, osmium, rhodium and ruthenium phthalo-
∗
Corresponding author. Tel.: +34 985 103 488; fax: +34 985 103 125.
E-mail address: costa@fq.uniovi.es (A. Costa-Garc ´ı a).
0
013-4686/$ – see front matter © 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2004.08.042

1
896
D. Hern a´ ndez-Santos et al. / Electrochimica Acta 50 (2005) 1895–1902
surface a metallic layer that shows high catalytic activity to-
wards several electrodic processes. Thus, cyclopentanol was
electrocatalytically oxidised employing Pt electrodes mod-
ified with underpotentially deposited Co, Ge and Pb [10].
Moreover, Pauling and J u¨ ttner [11] showed that the upd co-
electrodeposition of Tl and Ag modifies the catalytic proper-
ties of a gold electrode.
The employment of platinum complexes as electrochem-
ical labels in biological assays is based on the ability of
square planar Pt(II) complexes with a free coordination site
to bind to DNA (as previously described) or to proteins. This
is commonly employed to attach markers, i.e. enzymes or flu-
orochromes previously coordinated to the Pt complex, to the
target molecule via the proprietary Universal Linkage System
TM
In our research group, carbon paste electrodes have been
modified with colloidal gold by physical adsorption and has
been demonstrated that it accelerates silver electrodeposi-
tion [12–14]; however, in this case the modification of the
electrode was employed to detect colloidal gold and not the
reagent that suffers the electrochemical reaction (silver). Col-
loidal gold facilitates the electrodeposition of silver because
silver reduction process occurs at a less negative potential
than in the absence of gold on the surface of the electrode.
This fact generates a range of potentials in which silver reduc-
tion only occurs when the metal is adsorbed on the electrode;
(ULS ) labelling technology from Kreatech Biotechnology
[30].
2. Experimental
2.1. Reagents
Carbon paste was prepared by thorough mixing of
3
0.24 cm of Uvasol paraffin oil (Merck) with 1 g of Ultra
“F” purity graphite powder (Ultra Carbon). It was prepared
weekly.
−
3
therefore, if an anodic scan is performed in 1.0 mol dm
NH3 after silver electrodeposition at one of these potentials,
an oxidation peak of silver at +0.100 V is obtained. The
amount of electrodeposited silver is proportional to that of
the adsorbed metal, so silver stripping allows the detection
of colloidal gold.
Other strategies based on silver amplification for DNA
detection (using colloidal gold) [15,16] or for conducting
purposes [17] have been described.
In this work, the effect of several metals (electrodeposited
on a carbon paste electrode) towards silver electrodeposition
is studied. Then, cisplatin (cis-[Pt(NH3)2Cl2]) is selected as
a model metallic complex that could act as electrochemical
label and that contains one of the metals able to acceler-
ate silver electrodeposition. Thus, an adequate methodology,
based on the electrodeposition of silver, is developed in order
to quantify this complex.
Water was obtained from an ultrapure Millipore Milli-Q
water system.
Analyticalgrade(Merck)H2SO4, HNO3, HCl, NH3, NaCl
and AgNO3 were used.
The following reagents were employed: RuCl3·xH2O,
Pd(NO3)2·2H2O,
Cu(NO3)2·3H2O,
Co(NO3)2·6H2O,
Zn(NO3)2·6H2O, FeCl2·4H2O (all of them from Fluka),
OsCl3, (NH4)2[PtCl4] (both from Riedel-de Ha e¨ n),
Pb(NO3)2 (from Merck) and cis-[Pt(NH3)2Cl2] “cisplatin”
−3
(from Sigma). Solutions were prepared in 0.1 mol dm
HNO3, excepting the (NH4)2[PtCl4] solutions, which were
prepared in 0.1 mol dm HCl; solutions of cisplatin were
−
3
◦
prepared daily in NaCl 0.9% and stored at 4 C protected
from light with aluminium foil.
−
3
The 0.04 mol dm silver nitrate solutions were made
3
3
daily and 0.134 cm were added to a 25 cm cell contain-
ing 1.0 mol dm NH3 to obtain a final concentration of
2 × 10 mol dm Ag for each experiment.
−
3
Cisplatin has been employed as an anticancer drug since
−
4
−3
+
1
969, when Rosenberg et al. [18] discovered its chemothera-
peutic properties. When cisplatin enters into the cells it coor-
dinates to different molecules such as enzymes (affinity for
sulphur groups), RNA or DNA, being the interaction with
DNA the responsible of the therapeutic action. Cisplatin can
coordinate to N7 of two neighbouring guanine and/or ade-
nine bases, as this nitrogen does not form H bonds with
other bases, in the same or in opposite DNA strands. As a
consequence of the employment of cisplatin in cancer treat-
ment, it became necessary to detect the molecule in biologi-
cal samples [19–26]. The most employed technique has been
HPLC coupled to different kinds of detection (electrochemi-
cal, spectrophotometric) but the electrochemical detection of
cisplatin, by differential pulse polarography, without previ-
ous separation has also been described [26]. Moreover, Sadik
and co-workers [27–29] have described some silver based
methodologies for the electrochemical sensitive detection of
cisplatin on avidin covered, silver coated (electrochemically
deposited)goldelectrodesmodifiedwithbiotinylateddouble-
stranded DNA.
2.2. Apparatus
Voltammetric measurements were performed with an
ECO CHEMIE ␮Autolab Type II potentiostat/galvanostat
coupled to a PC and controlled by Autolab GPES software
version 4.6 for Windows.
The electrochemical arrangement used was a conventional
three-electrodecell. Ahomemadecarbonpasteelectrodewith
asurfacediameterof3 mmwasusedastheworkingelectrode.
It is made of a hollow Teflon tube in which the carbon paste
is packed under pressure into a 2 mm deep well; a stainless
steel rod acts as the electrical contact between the carbon
paste and the potentiostat; the paste is further compacted by
polishing manually on a sheet of paper. The reference elec-
trode was an Ag/AgCl electrode; the counter electrode was
a 1.5 mm × 1 mm platinum wire. Cells were protected from
light with aluminium foil when silver solutions or cisplatin
solutions were employed.

D. Hern a´ ndez-Santos et al. / Electrochimica Acta 50 (2005) 1895–1902
1897
Fig. 1. Analytical procedures developed to evaluate the effect of deposited metals on silver electrodeposition.
2
2
.3. Analytical procedures
2.3.2. Evaluation of the effect of metals on silver
electrodeposition
.3.1. Electrodeposition of metals
The following procedure is depicted in Fig. 1:
The following procedures were developed in order to test
if metals were electrodeposited on the surface of a carbon
paste electrode.
1. Electrode activation. The surface of the electrode was
−
3
activated using a 0.1 mol dm H SO stirred solution
2
4
Procedure 1: First, the surface of the electrode was acti-
for 2 min at +1.50 V (for cisplatin, a constant current of
+13 ␮A was employed).
2. Electrodeposition of the metal. The electrode was im-
mersed in a metal solution and the adequate electrode-
position potential (different for each metal) was held for
5 min with stirring.
−
3
vated using a 0.1 mol dm H2SO4 stirred solution for 2 min
at +1.50 V. Then, the electrode was immersed in a metal so-
lution and a cyclic voltammogram from +1.50 V to −0.90 V,
−
1
at a scan rate of 50 mV s , was recorded. Rinsing with water
was carried out between each step. Doing this, an adequate
electrodeposition potential for each metal could be selected.
Procedure 2: First, the surface of a new electrode was acti-
vated as in Procedure 1. Then, the electrode was immersed in
a metal solution and the adequate electrodeposition potential
3. Oxidation step. It was performed employing
a
−
3
0.1 mol dm
H SO stirred solution for 1 min at
2
4
+1.40 V (for cisplatin, a constant current of +0.5 ␮A was
employed).
(
2
different for each metal) selected in Procedure 1 was held for
min with stirring. Finally, the electrode was introduced in
4. Recording the cyclic voltammogram. As different kinds
of studies were developed, a silver deposition curve or an
analytical signal was recorded for an electrode.
- Silver deposition curves were obtained in the follow-
ing way: the electrode was immersed in a silver so-
the electrolyte solution (without metal) and a cyclic voltam-
mogram from the electrodeposition potential to +1.50 V was
recorded. Rinsing with water was carried out between each
step.
−
4
−3
lution (2 × 10 mol dm silver nitrate contained in

1
898
D. Hern a´ ndez-Santos et al. / Electrochimica Acta 50 (2005) 1895–1902
Table 1
Half-wave potentials of oxidation and reduction processes of metals on carbon paste electrodes
Metal
Concentration
Potential of reduction
process (E1/2) (V)
Potential of oxidation
process (E1/2) (V)
Electrodeposition
potential selected (V)
−
3
(mmol dm
)
Ru
Os
Cu
Co
Pt
1
1
1
1
0.1
0.01
1
+0.93
+0.24
−0.01
+1.20
+0.72
+0.10
+0.35
+0.91
+0.25
−0.40
+0.70
0.00
−0.20
−0.30
−0.40
−0.40
−0.60
−0.01
−0.21
−0.57
Pd
Pb
−
3
Thus, following the Procedure 1 described in Section
.3.1, the cyclic voltammograms obtained showed the reduc-
1
+
.0 mol dm NH3) and a cyclic voltammogram from
0.30 V to −1.20 V at a scan rate of 50 mV s−1 was
2
tion process for each metal (as well as the oxidation process
in the anodic scan). Taking it into account, an adequate re-
duction potential was chosen for each case. Table 1 lists the
half-wavepotentials(E1/2)oftheoxidationandreductionpro-
cesses, as well as the electrodeposition potential selected for
each metal. When cobalt solutions were used, no reduction
process appeared. However, if the cathodic scan in Procedure
recorded.
-
Analytical signals were obtained in the following
way: the electrode was immersed in a silver solu-
−
3
4
−3
tion (2 × 10 mol dm
silver nitrate contained in
−
1
.0 mol dm NH3) and silver electrodeposition, devel-
oped at a potential of −0.18 V for 45 s, followed by an
anodic stripping scan, from −0.18 V to +0.30 V with a
−
1
1
−
was recorded from +1.50 V to less negative potentials than
0.30 V the oxidation peak in the anodic scan was not ob-
scan rate of 50 mV s , were performed, giving rise to
the cyclic voltammogram corresponding to the analyti-
cal signal.
tained. Thus, −0.30 V was selected and demonstrated to be
an adequate electrodeposition potential for cobalt. In order
to prove that the metals remained deposited on the surface
of the electrode after their reduction, Procedure 2 of Sec-
tion 2.3.1 was carried out. In the cyclic voltammograms ob-
tained, an oxidation process for each metal appeared, as well
as a change in its intensity proportional to the concentration
of the solution employed. This confirmed that all the met-
als were electrodeposited on the surface of the carbon paste
electrode.
Rinsing with water was carried out between each step.
Background silver deposition curves and background analyt-
ical signals were recorded before each experiment following
the same procedure but employing only the electrolyte (with-
out metal) in Step 2.
A new electrode was employed for each measurement.
2
.3.3. Oxidation of metals in NH3
The following procedure was developed in order to test
which electrodeposited metals could interfere in the analyti-
cal signal obtained as described above.
3
.2. Effect of deposited metals on silver
electrodeposition
Firstly, the steps “Electrode activation” and “Electrode-
position of metals” were performed as Steps 1 and 2 de-
As previously mentioned, the ability of a metal for ac-
celerating silver electrodeposition would be indicated in the
shift, towards less negative potentials, of the silver reduction
process. The higher the shift, the higher the effect.
In order to test it, silver deposition curves, recorded fol-
lowing the analytical procedure described in Section 2.3.2,
similar to the one showed in Fig. 2 were obtained for each
metal. Concretely, this figure shows the voltammogram ob-
−
4
−3
scribed above, respectively, employing 1 × 10 mol dm
metal solutions in “Electrodeposition of metals” step. Then,
−3
theelectrodewasimmersedin1.0 mol dm NH3 andacyclic
voltammogram was recorded from −0.70 V to +1.20 V at a
−
1
scan rate of 50 mV s
.
−
5
−3
2+
tained when a 1 × 10 mol dm Cu solution was em-
ployed (voltammogram “b”, thick line) and the one obtained
in absence of metal (voltammogram “a”, thin line). Thus, this
metal accelerates silver electrodeposition because silver re-
duction process is shifted towards less negative potentials.
3
. Results and discussion
3
.1. Electrodeposition of metals on a carbon paste
electrode
ꢀE (0.086 V) indicates the difference between the half-wave
In order to evaluate which metals were able to catalyse the
potential of the silver reduction process on the electrode sur-
face without metal (E1/2[n], in this case −0.540 V) and the
half-wave potential of the silver reduction process when the
metal is electrodeposited on the electrode surface (E1/2[s], in
this case −0.454 V). Thus, the higher ꢀE, the higher the ef-
fect. Table 2 lists the ꢀE values for all the metals tested. Only
electrodeposition of silver, they must be previously deposited
on the surface of the carbon paste electrode. Therefore, the
first assays were focused on the selection of an adequate elec-
trodeposition potential, as well as on demonstrating that the
metals remained deposited on the electrode.

D. Hern a´ ndez-Santos et al. / Electrochimica Acta 50 (2005) 1895–1902
1899
adequate potential in this range, an anodic scan gives rise to a
silver oxidation process only if the metal was previously de-
posited on the surface of the electrode. This is shown in Fig. 3
for Cu and Os, where cyclic voltammograms correspond to
the analytical signals recorded following the procedure de-
scribed in Section 2.3.2. When there is no metal on the elec-
trode (Fig. 3A(b) and B(b)) silver is not electrodeposited at
−
0.18 V, so there is no oxidation process. Moreover, only the
electrodeposited Cu is able to provoke silver electrodeposi-
tion and an oxidation peak of silver is obtained (Fig. 3B(a));
the electrodeposited Os does not show this effect, so −0.18 V
is not enough to electrodeposit silver and register its oxidation
peak (Fig. 3A(a)).
3
.3. Electrochemical behaviour in NH3 of
Fig. 2. Cyclic voltammograms (silver deposition curves) recorded in
electrodeposited metals
−
3
−4
−3
+
1
−
.0 mol dm NH3 containing 2 × 10 mol dm Ag , from +0.30 V to
−1
1.20 V at a scan rate of 50 mV s , when (a) (thin line) Cu is not deposited
The aim of this work is to show the possibility of de-
tecting a metallic complex (cisplatin, as described below)
using the accelerated electrodeposition of silver provoked
by the metal. Then, a future research will deal with the
employment of the metallic complex as electrochemical la-
bel of biomolecules (proteins or DNA) that would be used
in immuno- or genosensing devices, where the labelled
molecules would be captured to give the analytical signal
based on the accelerated electrodeposition of silver. There-
fore, as the detection of the metallic label would be achieved
via the commonly used sequential heterogeneous assays on
immuno- or genosensors, there would be no possibility of
finding other metals on the sensor device that could interfere
in the detection of the metallic label.
on the electrode and (b) (thick line) Cu is deposited on the electrode employ-
−
5
−3
2+
2+
ing a 1 × 10 mol dm Cu solution. Cu electrodeposition conditions:
0.20 V, 5 min.
−
Table 2
Difference between the half-wave potential of the silver reduction process
on the carbon paste electrode in absence and in presence of electrodeposited
metals
Metal
ꢀE (V)
Os
Pb
Ru
Cu
Co
Pt
0.029
0.049
0.051
0.086
0.096
0.171
0.252
Pd
However, the accelerated deposition of silver could also be
used to detect a metallic complex (i.e. cisplatin) in biological
samples (which is out of the scope of this work). In this case,
metallic interferences present in the sample would need to
be considered. For this reason, the next study shows, for the
metals used in this work, the way in which they could be
evaluated.
Pd, Pt, Co and Cu facilitate silver electrodeposition; for Os,
Pb and Ru there are hardly differences in the silver reduction
processes. Moreover, as deduced from ꢀE values, the effect
is higher for Pd and Pt.
Fig. 2 also shows that there is a range of potentials in
which silver reduction only occurs when the metal is de-
posited on the electrode. So, if silver is electrodeposited at an
Of course, those metals that accelerate silver electrode-
position are potential interferences for the detection of other
−
3
−4
−3
+
Fig. 3. Cyclic voltammograms (analytical signals) recorded in 1.0 mol dm NH3 containing 2 × 10 mol dm Ag , from −0.18 V to +0.30 V at a scan
−1
−5
−3
rate of 50 mV s , when (A) Os and (B) Cu are employed (a) (thin line) metal is deposited on the electrode [from a 1 × 10 mol dm OsCl3 solution in (A)
−5
−3
3+
and from a 1 × 10 mol dm Cu(NO3)2·3H2O solution in (B)]; (b) (thick line) metal is not deposited on the electrode (background analytical signals). Os
2
+
+
electrodeposition conditions: 0.00 V, 5 min. Cu electrodeposition conditions: −0.20 V, 5 min. Ag electrodeposition conditions: −0.18 V, 45 s.

1
900
D. Hern a´ ndez-Santos et al. / Electrochimica Acta 50 (2005) 1895–1902
metal whose electrodeposition potential is more negative. For
example, Co would interfere in the detection of Pd or Pt.
Another interference could be the oxidation process of
−
3
metals at high concentrations in 1.0 mol dm NH3. As pre-
viously showed, the analytical signal obtained is due to the
reoxidation of silver, which occurs in NH3 at +0.100 V. Thus,
−
3
it is important that in 1.0 mol dm NH3 there is no other an-
odic peak at that potential, which could interfere with the
analytical signal. Therefore, it was necessary to study the
electrochemical behaviour of metals electrodeposited at high
concentrations on a carbon paste electrode when an anodic
−
3
scan is performed in 1.0 mol dm NH3. Following the ana-
lytical procedure described in Section 2.3.3, Pd, Pt, Cu and
Os did not show any anodic process. Co, Pb and Ru showed
anodic processes at +0.955 V, +0.897 V and +0.528 V, respec-
tively, but none of these processes interfere with the one of
silver at +0.100 V.
Other metals usually present in biological samples such
as Fe or Zn were also tested. However, results obtained fol-
lowing the procedures described in Section 2.3.1 (data not
shown) showed that none of them remained deposited to the
electrode surface after applying an adequate electroreduction
potential on the metal solution, so they could not affect silver
electrodeposition nor show an anodic process in NH3 and,
therefore, would not interfere.
Fig. 4. Reproducibility of the analytical signal of cisplatin (n = 4). Cyclic
voltammograms (analytical signals) recorded in 1.0 mol dm NH3 con-
−
3
−
4
−3
+
taining 2 × 10 mol dm Ag , from −0.18 V to +0.30 V at a scan rate of
−
1
−8
−3
5
0 mV s , when Pt (from a 3 × 10 mol dm cisplatin solution) is de-
posited on the electrode. Cisplatin electrodeposition conditions: −0.40 V,
+
5
min. Ag electrodeposition conditions: −0.18 V, 45 s.
50 ␮C (10-fold higher peak area than for Cu), due to the
higher activity of Pt towards silver electrodeposition.
In this analytical procedure, the “Oxidation step” and the
“
Electrode activation” step are carried out at constant current.
Doing this, the reproducibility in the analytical signal is better
than the one obtained if constant potentials were applied.
Thus, a relative standard deviation of 11.1% was observed
for five measurements with a mean peak area of 3.55 ␮C
Anyway, possible specific metallic interferences should
be evaluated (in a similar way than the one showed here) and
further research should be carried out in order to avoid them
for the detection of a metallic complex in a biological sample
using the accelerated silver electrodeposition.
−
8
−3
when a 3 × 10 mol dm cisplatin solution was employed
see Fig. 4).
(
3
.4. Detection of cisplatin
3.4.1. Influence of the “Oxidation step”
From the four metals able to catalyse silver electrode-
If the “Oxidation step” in H2SO4, performed after the
position, Pt was selected as the most adequate one to de-
velop an analytical methodology for the detection of a metal
that can act as electrochemical label of biological molecules.
The reason is the existence of cis-diamminedichloro plat-
inum(II) (cisplatin). Cisplatin and other similar Pt complexes
are molecules able to bind to DNA and other biomolecules.
Thus, detection of cisplatin would allow the detection of
biomolecules labelled with cisplatin (or other similar Pt com-
plex). So far, the employment of cisplatin (apart of being an
anti-cancerdrug)isjusttoactaslinkerbetweenabiomolecule
and a label (mainly enzymes or fluorochromes) [30] but it
has never been detected as a label itself. Thus, if cisplatin
“Electrodeposition” step, is not carried out, silver is elec-
trodeposited on the carbon paste electrode in absence of
metals. This effect was studied employing cisplatin. Fig. 5
shows the silver deposition curves recorded when the oxi-
dation step is not carried out (Fig. 5A; E1/2[n] = −0.451 V,
E1/2[s] = −0.381 V) and when it is carried out (Fig. 5B;
E1/2[n] = −0.545 V, E1/2[s] = −0.359 V). A comparison of
these curves and values of the half-wave potentials shows
that if the oxidation step is not included in the analytical pro-
cedure, the silver reduction process is shifted towards less
negative potentials in absence of Pt. Therefore, at −0.18 V
silver is electrodeposited and high peak areas are obtained
for background analytical signals.
(or a similar Pt complex) could be detected, the other la-
bels (enzymes, fluorochromes) used for the detection of the
biomolecule would not be necessary and electrochemical de-
tection of the Pt-labelled biomolecules could be performed,
which would allow employing cisplatin (or other similar Pt
complex) as a label itself and not only as linker.
Hydrogenevolutionduringtheelectrodepositionstep, per-
formed at negative potentials (−0.40 V for cisplatin), is prob-
ably the responsible of this behaviour. If the oxidation step
is not carried out, H2 is not removed from the surface of the
electrode and, even in absence of Pt, silver electrodeposits
at low cathodic potentials. If the oxidation step is carried
out, hydrogen is removed from the electrode and silver elec-
trodeposition only occurs in the presence of Pt. When the
influence of the oxidation current applied in the oxidation
The analytical signal for cisplatin is similar to the
2+
one showed for Cu in Fig. 3. However, a solution of
−
7
−3
1
× 10 mol dm cisplatin (100-fold lower concentration
than Cu) gives rise to an oxidation peak of silver of about

D. Hern a´ ndez-Santos et al. / Electrochimica Acta 50 (2005) 1895–1902
1901
Fig. 6. Calibration plot of cisplatin concentration.
These results agreed with the behaviour showed by silver
when its electrodeposition was accelerated by colloidal gold
[13].
Finally, the study of the effect of cisplatin electrodepo-
sition time showed that the signals increased with the elec-
trodeposition time, reaching a maximum at 5 min.
3
.4.3. Effect of cisplatin concentration on the analytical
signal
Fig. 6 shows the evolution of the analytical signal with the
cisplatin concentration. The calibration curve was linearly
−8
−3
fitted on a linear ln(y) − x form from 1 × 10 mol dm
−
7
−3
to 1 × 10 mol dm cisplatin, according to the following
equation (r = 0.9990):
Fig. 5. Cyclic voltammograms (silver deposition curves) recorded in
−
3
−4
−3
+
1
−
.0 mol dm NH3 containing 2 × 10 mol dm Ag , from +0.30 V to
7
−3
−1
1.20 V at a scan rate of 50 mV s , when cisplatin is not deposited on the
ln (peak area/␮C) = 0.13 + 3.83 × 10 [cisplatin]/mol dm
electrode surface (voltammograms a and c, thin lines) and after cisplatin
electrodeposition (voltammograms b and d, thick lines). (A) Oxidation step
is not carried out; (B) Oxidation step is carried out. Cisplatin concentration:
× 10 mol dm . Cisplatin electrodeposition conditions: −0.40 V, 5 min.
−
9
−3
−3
A limit of detection of 3.2 × 10 mol dm (1 ng cm
)
cisplatin was calculated considering the three times of the
standard deviation divided by the slope of the calibration
curve.
−
7
−3
1
step on the analytical signal was studied, it was observed
that a minimum oxidation (+0.05 ␮A) was enough to avoid
the electrodeposition of silver in absence of Pt.
4. Conclusions
For the first time, in this work the electrodeposition of
3
.4.2. Effect of silver electrodeposition conditions and
silver on carbon paste electrodes modified with several elec-
trodeposited metals (Co, Ru, Pd, Os, Pt, Cu, Pb) has been
studied. It was demonstrated that Os, Ru and Pb do not af-
fect silver electrodeposition but Pd, Pt, Co and Cu do, be-
ing Pd and Pt those who show a higher accelerating effect.
When these metals are electrodeposited on the surface of
the electrode, silver reduction process occurs at a less neg-
ative potential than on the bare electrode and the anodic
stripping of electrodeposited silver gives rise to an oxida-
tion peak at +0.100 V in NH3. Also, an adequate method-
ology based on the electrodeposition of silver for the de-
termination of cisplatin, a Pt containing complex, has been
described. The proposed procedure is sensitive but suscepti-
ble to interference of some metals and the precision is not so
high.
cisplatin electrodeposition time on the analytical signal
Silver electrodeposition potential, silver deposition time
and cisplatin electrodeposition time effects were studied em-
ploying a 1 × 10 mol dm solution of cisplatin; optimised
values are those applied in the analytical procedure described
in Section 2.3.2.
−
7
−3
First, it was observed that, at potentials less negative than
0.18 V, themorenegativethesilverelectrodepositionpoten-
−
tial, the higher the analytical signal and also constant back-
ground analytical signals were obtained. But when the depo-
sition potential was more negative than −0.18 V, background
analytical signals increased, and faster than the analytical
signals, so the most adequate potential was −0.18 V.
Then, the effect of silver electrodeposition time was stud-
ied; the voltammetric signals increased with the deposition
time, obtaining a good signal to background ratio at 45 s.
Work is in progress to show the feasibility of the employ-
ment of square planar Pt(II) complexes as electrochemical

1
902
D. Hern a´ ndez-Santos et al. / Electrochimica Acta 50 (2005) 1895–1902
labels in biological assays, for example in the detection of
Pt-labelled target oligonucleotides in genosensors.
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12] D. Hern a´ ndez-Santos, M.B. Gonz a´ lez-Garc ´ı a, A. Costa-Garc ´ı a, Elec-
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David Hern a´ ndez Santos is grateful to the ‘Consejer ´ı a de
Educaci o´ n y Cultura del Principado de Asturias’ for his re-
search grant. Support of this work was provided by Project
FC02-PC-REC01-05.
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