Preparation, Characterization, and Electrocatalytic Properties of mvRuO/RuCN and RP Hybrid Film-Modified Electrodes

Article abstract of DOI:10.1149/1.1666189

Ruthenium oxide/ruthenocyanide (ruthenium oxide/hexacyanoruthenate, or mvRuO/RuCN) and iron(III) ruthenocyanide (ruthenium purple, or RP) hybrid films have been prepared using consecutive cyclic voltammetry, and the deposition process and the films' elect

Full text of DOI:10.1149/1.1666189

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Journal of The Electrochemical Society, 151 ͑4͒ E168-E178 ͑2004͒
0013-4651/2004/151͑4͒/E168/11/$7.00 © The Electrochemical Society, Inc.
Preparation, Characterization, and Electrocatalytic Properties
of mvRuOÕRuCN and RP Hybrid Film–Modified
Electrodes
Shen-Ming Chenz and Sheh-Hung Hsueh
Department of Chemical Engineering, National Taipei University of Technology, Taipei, Taiwan 106, China
Ruthenium oxide/ruthenocyanide ͑ruthenium oxide/hexacyanoruthenate, or mvRuO/RuCN͒ and iron͑III͒ ruthenocyanide ͑ruthe-
nium purple, or RP͒ hybrid films have been prepared using consecutive cyclic voltammetry, and the deposition process and the
films’ electrocatalytic properties in electrolytes containing Rbϩ and Csϩ have been investigated. The hybrid ruthenium oxide/
ruthenocyanide and RP films showed four obvious and separated redox couples with formal potentials between Ϫ0.2 and 1.0 V in
monovalent cation aqueous solutions of RbNO3 or CsNO3 . The cyclic voltammograms recorded the deposition of the hybrid films
directly from the mixing of Ru3ϩ, Fe3ϩ, and Ru͑CN)64Ϫ ions in aqueous Rbϩ or Csϩ cation solutions. An electrochemical quartz
crystal microbalance and cyclic voltammetry were used to study the growth mechanism of the hybrid films. The electrochemical
properties of the films indicate that the redox process was confined to the immobilized and mixed ruthenium oxide/ruthenocyanide
and RP films. The electrocatalytic reduction properties of SO25Ϫ and S2O82Ϫ by the hybrid films were also determined, as well as
the electrocatalytic properties of dopamine and epinephrine. The electrocatalytic reactions of the hybrid films were investigated
using the rotating ring-disk electrode method.
© 2004 The Electrochemical Society. ͓DOI: 10.1149/1.1666189͔ All rights reserved.
Manuscript submitted July 7, 2003; revised manuscript received October 23, 2003. Available electronically March 3, 2004.
Iron ruthenocyanide ͑ruthenium purple, or RP͒ and ruthenium
the electrocatalytic activity measurements are applicable to analyti-
cal applications and to the electrochemical oxidation of low electro-
active compounds.
oxide/ruthenocyanide ͑ruthenium oxide/hexacyanoruthenate or
mvRuO/RuCN͒ are the ruthenium analogs of iron hexacyanoferrate.
Metal hexacyanoferrates and metal ruthenocyanides show interest-
ing redox chemistry that is accompanied by changes in their elec-
trochromic, ion exchange, and electrocatalytic properties. Metal
hexacyanoferrate1-6 chemically modified film electrodes show inter-
esting electrochemical properties. They are used in both chemistry
and in material science, in such areas as electroanalysis, chemical
sensing, and electrocatalysis,7-15 in studies on interfacial charges,
electrochromicity, in the study of ion-exchange and electron-transfer
processes,16-18 and in research on the chemical composition of sur-
face films.19
An RP film can be deposited onto a working electrode using a
two-step procedure,20,21 and an mvRuO/RuCN film can be deposited
onto a working electrode by cycling the potential between 0.5 and
1.0 V using Ru͑CN)26Ϫ in an aqueous 0.5 M KCl solution at pH
2.22-27 To realize practical applications of RP and ruthenium oxide/
ruthenocyanide film–modified electrodes for ion sensor applica-
tions, the deposition processes of these films must be fully charac-
terized.
An important and interesting possibility is a modified hybrid
electrode containing RP and mvRuO/RuCN films that have different
properties, which can be used to convert reactants to selected prod-
ucts. Hybrid films are highly relevant to contemporary material sci-
ence, and a hybrid RP and mvRuO/RuCN film is a member of this
interesting class of materials.
This paper discusses the successful preparation of hybrid films of
iron ruthenocyanide and ruthenium oxide/ruthenocyanide in aqueous
Rbϩ and Csϩ electrolytic solutions. These show good stability, as
well as well-defined redox couples and electrocatalytic properties.
No previous discussion has occurred in the literature on the prepa-
ration of RP and mvRuO/RuCN hybrid films, although a few reports
concerning their electrocatalytic properties28 and electrochemical
quartz crystal microbalance ͑EQCM͒ measurements29 have been
cited.
This paper discusses the first successful preparation of a hybrid
film of iron ruthenocyanide and ruthenium oxide/ruthenocyanide in
aqueous Rbϩ and Csϩ electrolytic solutions and their resulting elec-
trocatalytic properties. The paper also discusses the electrocatalytic
properties and EQCM measurements made during the deposition
and ion-exchange processes of the RP and mvRuO/RuCN hybrid
film. Simultaneous CV and microgravimetry using an EQCM can
continuously detect in situ the surface mass change on an electrode
in an experimental solution. This technique is useful for elucidating
the growth mechanism and electrochemical properties of RP and
mvRuO/RuCN films, as well as their hybrid films. The electrocata-
lytic reduction of SO25Ϫ and S2O28Ϫ by an RP and mvRuO/RuCN
hybrid film was observed, as well as the reversible electrocatalysis
of dopamine and epinephrine. The analytical methods used are es-
tablished techniques and are important for the determination of these
analytes. Dopamine and epinephrine are important neurotransmitters
in the mammalian central nervous system, and therefore the devel-
opment of electrochemical methods that can be used to monitor
these compounds is important. The electrocatalytic reduction of per-
oxosulfate and persulfate is of interest for environmental applica-
tions, and the electrocatalytic oxidation of dopamine and epineph-
rine is of interest for biomedical applications. The electrocatalytic
oxidation of dopamine was active, and the products of the oxidation
of dopamine could be electrocatalytically reduced. The electrocata-
lytic reactions of dopamine and epinephrine by an RP and mvRuO/
RuCN hybrid film were investigated using the rotating ring-disk
electrode ͑RRDE͒ method.
Experimental
Electrochemistry was performed using a Bioanalytical Systems
model CV-50W and CH Instruments CHI-400 and CHI-750 poten-
tiostats. CV was conducted using a three-electrode cell in which a
BAS glassy carbon ͑GC͒ electrode, a platinum electrode, and a tin
dioxide electrode were used as the working electrodes. The GC elec-
trode was polished with 50 nm alumina on Buehler felt pads and
then ultrasonically cleaned for 1 min. The auxiliary compartment
contained a platinum wire, which was separated from the rest of the
compartment by a medium-sized glass frit. All cell potentials were
measured using either a Ag͉AgCl͉KCl ͑saturated solution͒ reference
The fabrication of a chemically modified film electrode is easily
controlled using consecutive cyclic voltammetry ͑CV͒, because in
this synthetic method, an increase in the peak current of the film
induces the appropriate redox couple of the modified film. Results of
electrode or a Hg͉Hg2Cl2͉KCl ͑saturated solution͒ reference elec-
trode.
z E-mail: smchen78@ms15.hinet.net

Journal of The Electrochemical Society, 151 ͑4͒ E168-E178 ͑2004͒
The working electrode for the EQCM measurements was an 8
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MHz AT-cut quartz crystal with gold electrodes. The diameter of the
quartz crystal was 13.7 mm and the gold electrode diameter was 5
mm. The spectroelectrochemical cell consisted of a 1 mm cuvette, a
100-mesh platinum gauze used as a working electrode, a platinum
wire used as an auxiliary electrode, and an Ag͉AgCl reference elec-
trode.
RRDE experiments were performed using a Pine Instruments Co.
electrode in conjunction with a CH Instruments CHI-750 poten-
tiostat connected to a model AFMSRX analytical rotator. The rotat-
ing disk electrode ͑RDE͒, purchased from the Pine Instruments Co.,
consisted of a GC disk electrode and a GC ͑or platinum͒ ring elec-
trode.
UV-visible spectra were measured using a Hitachi model U-3300
spectrophotometer.
The ion chromatograph used in the experiments was a Dionex
Instruments ion chromatograph model DX-100, which consisted of a
pump, a conductivity detector, an electrochemical detector, and a
syringe loading system with a 25 ␮L sample loop. The columns used
throughout were an IonPac AG4A guard column, an IonPac AS4A
analytical column, and an anion self-regenerating suppressor col-
umn. All measurements were conducted at room temperature.
All chemicals used were of analytical grade. The aqueous solu-
tions were prepared using doubly distilled deionized water, and the
solutions were deoxygenated by purging with prepurified nitrogen
gas.
The electrochemical formation of the hybrid iron ruthenocyanide
and ruthenium oxide/ruthenocyanide films was performed by con-
tinuous cycling of the potential of the working electrode in a defined
potential range in a suitable aqueous solution containing Ru3ϩ
,
Fe3ϩ, and Ru͑CN)46Ϫ ions. Typical electrochemical formation of hy-
brid RP and mvRuO/RuCN films was performed by repetitive CV
on cycling the potential of the working electrode in a defined poten-
tial range in an aqueous Rbϩ or Csϩ supporting electrolyte solution
containing K4Ru͑CN)6 with Ru3ϩ ͑with the balancing anion being
chloride or nitrate͒.
Results and Discussion
Hybrid RP and mvRuO/RuCN film deposition in aqueous RbNO3
and CsNO3 solutions.—The electrochemical formation of hybrid
iron ruthenocyanide and ruthenium oxide/ruthenocyanide films on a
GC electrode was carried out using consecutive CV in aqueous
RbNO3 and CsNO3 solutions.
The hybrid RP and mvRuO/RuCN film showed four redox
couples with formal potentials at Ϫ0.05, 0.26, 0.73, and 0.94 V,
respectively, in aqueous monovalent cation RbNO3 solutions ͑Fig.
1A͒. Figure 1B shows the consecutive CVs of the hybrid RP and
mvRuO/RuCN film showing the four redox couples with formal
potentials at 0.03, 0.28, 0.64, and 0.90 V in aqueous CsNO3 solu-
tions. The reason for choosing cesium and rubidium as the electro-
lyte solution salts is that the films exhibited four clear redox couples
with cesium or rubidium electrolytes, but with lithium, potassium, or
sodium salt electrolytes the four redox couples were not well sepa-
rated or did not exhibit a well-defined shape.
Figure 1. Repetitive CVs of a GC electrode modified with a hybrid ruthe-
nium oxide/ruthenocyanide and iron͑III͒ ruthenocyanide film synthesized
from 2 ϫ 10Ϫ3 M Ru͑CN)46Ϫ added to aqueous 1 ϫ 10Ϫ3 M Ru3ϩ and 1
ϫ 10Ϫ3 M Fe3ϩ mixed in a pH 2.0 aqueous solution of: ͑A͒ 0.2 M RbNO3
and ͑B͒ 0.2 M CsNO3 . Scan rate 0.1 V/s.
close linear dependence of Ipa on the scan rate, with the ratio of the
anodic to cathodic peak current, Ipa /Ipc , equal to unity.
The results show the successful formation of a hybrid RP and
mvRuO/RuCN film and demonstrate that the voltammetric proper-
ties of the hybrid RP and mvRuO/RuCN film depend on the
monovalent cation (Rbϩ or Csϩ) of the electrolyte. The experimen-
tal results also show that the hybrid RP and mvRuO/RuCN films can
be formed directly from the mixing of Fe3ϩ, Ru3ϩ, and Ru͑CN)46Ϫ
ions by consecutive CV using Rbϩ and Csϩ cations as the electro-
lyte solution and give the four separate redox couples ͑see Fig. 1 and
Table I͒.
Figure 2A shows that a hybrid RP and mvRuO/RuCN film was
obtained on a GC electrode from the 0.2 M aqueous RbNO3 solution
at pH 2.0 and shows four chemically reversible redox couples for
various scan rates. The inset of Fig. 2A shows a plot of the anodic
peak current, Ipa , vs. scan rate at a potential of 0.15 V, illustrating a
Table I. The electrochemical properties of RP, mvRuOÕRuCN,
and RP and mvRuOÕRuCN hybrid films.
Film/electrolyte
pH
RbNO3 ͑V͒
CsNO3 ͑V͒
Ironruthenocyanide
Ruthenium oxide/ruthenocyanide
0.30
0.03
0.68
0.92
0.06
0.26
0.72
0.92
0.34
0.06
0.70
ϩ0.96
0.04
0.28
2.0
2.0
RP and mvRuO/RuCN
2.0
0.64
0.86

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Journal of The Electrochemical Society, 151 ͑4͒ E168-E178 ͑2004͒
where ⌫o ͑g/cm2͒, ␷, A, Ip, and M ͑g/mol͒ represent the surface
coverage concentration, the scan rate, the electrode area, the peak
current, and the effective molar mass, respectively. This result indi-
cates that the redox process was confined to the surface of the hybrid
RP and mvRuO/RuCN film on the GC electrode, confirming the
immobilized state of the hybrid RP and mvRuO/RuCN
film.30,31 The four redox couples are proposed to
arise
from
the
RuII-O͔/͓RuII͑CN) ]/ RuIII-O/͓RuII͑CN) ],
͓
͓
6
6
FeII-NC-RuIII͔/͓FeIII-NC-RuIII
,
͔
͓
͓
͓
RuIII-O͔/͓RuII͑CN) ]/ RuIII-O/͓RuIII͑CN) ],
and
͓
6
6
RuIII-O͔/͓RuIII͑CN) ]/ RuIV-O͔/͓RuIII͑CN) ] processes in passing
͓
6
6
from the negative to the positive potential regions.
Figure 3A͑a͒ shows RP formation CVs that exhibit one redox
couple with a formal potential at about Eo ϭ 0.30 V ͑vs. Ag͉AgCl͒
Ј
in an aqueous 0.2 M RbNO3 solution at pH 2.0. Figure 3A͑b͒ shows
the CVs of mvRuO/RuCN film formation, which exhibit three redox
couples with formal potentials at about Eo ϭ 0.03, 0.68, and 0.92
Ј
V ͑vs. Ag͉AgCl͒, respectively, in an aqueous 0.2 M RbNO3 solution
at pH 2.0. The CVs of RP and mvRuO/RuCN film formation exhibit
four redox couples with formal potentials at about Eo ϭ 0.06, 0.26,
Ј
0.72, and 0.92 V ͑vs. Ag͉AgCl͒, respectively, in an aqueous 0.2 M
RbNO3 solution at pH 2.0.
Figure 3B͑a͒ shows RP formation CVs that exhibit one redox
couple with a formal potential at about Eo ϭ 0.34 V ͑vs. Ag͉AgCl͒
Ј
in an aqueous 0.2 M CsNO3 solution at pH 2.0. Figure 3B͑b͒ shows
the CVs of mvRuO/RuCN film formation, which exhibit three redox
couples with formal potentials at about Eo ϭ 0.06, 0.70, and 0.96
Ј
V ͑vs. Ag͉AgCl͒, respectively, in an aqueous 0.2 M CsNO3 solution
at pH 2.0. The CVs of RP and mvRuO/RuCN film formation show
four redox couples with formal potentials at about Eo ϭ 0.04, 0.28,
Ј
0.64, and 0.86 V ͑vs. Ag͉AgCl͒, respectively, in an aqueous 0.2 M
CsNO3 solution at pH 2.0 ͑Table I͒.
RP, mvRuO/RuCN, and RP and mvRuO/RuCN hybrid film
deposition and in situ EQCM measurements.—We successfully
grew RP, mvRuO/RuCN, and RP and mvRuO/RuCN hybrid films on
gold electrodes from an aqueous 0.2 M RbNO3 solution at pH 2.0.
The CVs are shown in Fig. 4A, with Fig. 4B showing the growth of
the RP film on a gold electrode and the resulting change in EQCM
frequency. The RP film obtained from the aqueous 0.2 M RbNO3
solution at pH 2.0 showed a single redox couple occurring between
potentials of Ϫ0.2 and 0.6 V, with the formal potential being at
approximately 0.3 V ͑vs. Ag͉AgCl͒. Figure 4B shows the EQCM
frequency change recorded during the first ten cycles of the consecu-
tive CV. The voltammetric peak current in Fig. 4A and the frequency
decrease ͑or mass increase͒ in Fig. 4B are consistent with the growth
of an RP film on the gold electrode. The EQCM results show that
the deposition of the RP film occurred between the potential range
0.2 and Ϫ0.2 V ͑vs. Ag͉AgCl͒. This potential range is the region
where FeII-NC-RuII species form and deposit on the electrode
Figure 2. ͑A͒ CVs of a GC electrode modified with a hybrid RP and
mvRuO/RuCN film in an aqueous solution at pH 2.0 of ͑A͒ 0.2 M RbNO3
and ͑B͒ 0.2 M RbNO3 for scan rates of ͑a͒ 0.01, ͑b͒ 0.02, ͑c͒ 0.03, ͑d͒ 0.045,
͑e͒ 0.06, ͑f͒ 0.08, ͑g͒ 0.10, ͑h͒ 0.12, ͑i͒ 0.14, ͑j͒ 0.16, ͑k͒ 0.18, and ͑l͒ 0.20
V/s. Inset 2A: Plot of the anodic peak current ͑at a potential of 0.15 V͒ vs.
scan rate. Inset 2B: Plot of cathodic peak current at a potential of ͑a͒ 0.05 V
and ͑b͒ anodic peak current vs. scan rate at a potential of about 0.40 V.
͓
͔
surface.15,32-34
In the EQCM experiments, the change in mass at the quartz
crystal was calculated from the change in the measurement fre-
quency using the Sauerbrey equation35,36
Figure 2B shows that a hybrid RP and mvRuO/RuCN film was
obtained on a GC electrode from the 0.2 M aqueous CsNO3 solution
at pH 2.0 and shows three chemically reversible redox couples be-
tween potentials of Ϫ0.2 and 0.9 V for various scan rates. The inset
of Fig. 2B shows a plot of: ͑a͒ the cathodic peak current at a poten-
tial of 0.05 V and ͑b͒ the anodic peak current at about 0.40 V vs.
scan rate, illustrating a close linear dependence of Ip on the scan
rate, and that Ipa /Ipc was close to unity. The behavior shown in Fig.
2A and B is consistent with a surface-type behavior, a reversible
electron-transfer process at low scan rates. The peak current and
scan rate are related by the following equation30,31
Mass change ⌬m͒ ϭ Ϫ1/2͒ fϪ2͒ ⌬f ͒A k␳͒
͓2͔
1/2
͑
͑
͑
͑
͑
o
where A is the area of the gold disk coated onto the quartz crystal, ␳
is the density of the crystal, k is the shear modulus of the crystal, ⌬f
is the measured frequency change, and f0 is the oscillation fre-
quency of the crystal. A frequency change of 1 Hz is equivalent to a
change of 1.4 ng in mass. During the first CV scan, approximately
1890 ng/cm2 of RP was deposited on the gold electrode. Approxi-
mately 9450 ng/cm2 of RP was deposited on the gold electrode after
ten CV scans.
We also successfully grew mvRuO/RuCN on a gold electrode
from an aqueous 0.2 M RbNO3 solution at pH 2.0. The CVs are
shown in Fig. 5A, with Fig. 5B showing the growth of the mvRuO/
Ip ϭ n2F2␷A⌫o/4MRT
͓1͔

Journal of The Electrochemical Society, 151 ͑4͒ E168-E178 ͑2004͒
E171
Figure 4. ͑A͒ Consecutive CVs of a gold electrode modified with an iron
ruthenocyanide film synthesized from 1 ϫ 10Ϫ3 M Ru͑CN)64Ϫ and added
2 ϫ 10Ϫ4 M Fe3ϩ in an aqueous 0.2 M RbNO3 solution at pH 2.0. Scan rate
0.02 V/s. ͑B͒ Change in EQCM frequency recorded concurrently with the
consecutive CVs of Fig. 4A. Inset 4A: Plot of ͑a͒ cathodic peak current vs.
scan cycle and ͑b͒ cathodic peak current change during a single scan cycle
vs. scan cycle. Inset 4B: Plot of ͑a͒ total frequency change vs. scan cycle and
͑b͒ frequency change during a single scan vs. scan cycle.
Figure 3. Repetitive CVs of a GC electrode modified with ͑a͒ an iron͑III͒
ruthenocyanide film synthesized from 1 ϫ 10Ϫ3 M Fe3ϩ and 2 ϫ 10Ϫ3
M
Ru͑CN)46Ϫ , ͑b͒ a ruthenium oxide/ruthenocyanide film synthesized from 1
ϫ 10Ϫ3 M Fe3ϩ and 2 ϫ 10Ϫ3 M Ru͑CN)46Ϫ , and ͑c͒ a film synthesized
from 2 ϫ 10Ϫ3 M Ru͑CN)46Ϫ in a pH 2.0 aqueous solution of ͑A͒ 0.2 M
RbNO3 and ͑B͒ 0.2 M CsNO3 . Scan rate 0.1 V/s.
equation.35,36 During the first CV scan, approximately 725 ng/cm2 of
mvRuO/RuCN was deposited on the gold electrode. Approximately
3808 ng/cm2 of mvRuO/RuCN was deposited on the gold electrode
after seven CV scans ͑see Fig. 5͒.
Successful growth of the RP and mvRuO/RuCN hybrid film on a
gold electrode surface was achieved using an aqueous 0.2 M RbNO3
solution at pH 2.0. Figure 6A shows the CVs of the RP and mvRuO/
RuCN hybrid film deposition from an aqueous 0.2 M RbNO3 solu-
tion at pH 2.0.
Figure 6 shows the growth of the RP and mvRuO/RuCN hybrid
film and the resulting change in frequency in the EQCM measure-
ments on a gold electrode in an aqueous 0.2 M RbNO3 solution at
pH 2.0. The film obtained from the aqueous 0.2 M RbNO3 solution
at pH 2.0 showed four redox couples between potentials of Ϫ0.2
and 1.0 V, with the formal potentials being at about 0.06, 0.26, 0.72,
and 0.92 V ͑vs. Ag͉AgCl; see Fig. 6A͒. Figure 6B shows the EQCM
frequency change recorded during the first ten cycles of the consecu-
RuCN film on the gold electrode and the resulting change in the
EQCM frequency. The mvRuO/RuCN film obtained from the aque-
ous 0.2 M RbNO3 solution at pH 2.0 showed three redox couples
occurring between potentials of Ϫ0.2 and 1.0 V, with the formal
potentials being at approximately 0.03, 0.68, and 0.92 V ͑vs.
Ag͉AgCl͒. Figure 5B shows the EQCM frequency change recorded
during the first seven cycles of the consecutive CV. The EQCM
results show that the deposition of the film occurred between the
potential range of 0.85 and 1.0 V ͑vs. Ag͉AgCl͒. This potential range
is where RuIV-O͔/͓RuIII͑CN) ] species form and deposit on the
͓
6
electrode surface.
The change in mass at the quartz crystal was calculated from the
change in the measurement frequency using the Sauerbrey

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Journal of The Electrochemical Society, 151 ͑4͒ E168-E178 ͑2004͒
Figure 6. ͑A͒ Consecutive CVs of a gold electrode modified with a ruthe-
nium oxide/ruthenocyanide film synthesized from 1 ϫ 10Ϫ3 M Ru3ϩ, 1
ϫ 10Ϫ3 M Fe3ϩ, and 2 ϫ 10Ϫ3 M Ru͑CN)64Ϫ in an aqueous 0.1 M RbNO3
solution at pH 2.0. Scan rate 0.02 V/s. ͑B͒ Change in EQCM frequency
recorded concurrently with the consecutive CVs of Fig. 6A. Inset 6A: Plot of
͑a͒ cathodic peak current vs. scan cycle at a potential of 0.0 V and ͑b͒ change
in cathodic peak current over a single scan cycle vs. scan cycle. Inset 6B:
Plot of ͑a͒ total frequency change vs. scan cycle and ͑b͒ frequency change
over a single scan cycle vs. scan cycle.
Figure 5. ͑A͒ Consecutive CVs of a gold electrode modified with an RP film
synthesized from 1 ϫ 10Ϫ3 M Ru͑CN)64Ϫ added to 1 ϫ 10Ϫ3 M Ru3ϩ in an
aqueous 0.1 M RbNO3 solution at pH 2.0. Scan rate 0.02 V/s. ͑B͒ Change in
EQCM frequency recorded concurrently with the consecutive CVs of Fig.
5A. Inset 5A: Plot of ͑a͒ cathodic peak current vs. scan cycle at a potential of
Ϫ0.04 V and ͑b͒ change in cathodic peak current during a single scan cycle
vs. scan cycle. Inset 5B: Plot of ͑a͒ total frequency change vs. scan cycle and
͑b͒ frequency change over a single scan cycle vs. scan cycle.
hybrid films on gold electrodes, and these films all grow steadily vs.
time on a gold electrode. The EQCM measurements obviously show
that the deposition of the RP film occurred at a potential more nega-
tive than 0.2 V ͑see Fig. 4B͒ and that the deposition potential of the
mvRuO/RuCN film occurred at potentials more positive than 0.85 V
͑vs. Ag͉AgCl͒ ͑see Fig. 5B͒. The deposition potential of the RP and
mvRuO/RuCN hybrid film occurred over two potential ranges: one
more positive than 0.85 V ͑vs. Ag͉AgCl͒, and the other more nega-
tive than 0.2 V ͑vs. Ag͉AgCl͒. The results also show that the RP and
mvRuO/RuCN hybrid film, which exhibits four redox couples and
two deposition potential ranges, shows similar behavior to that of
combined RP and mvRuO/RuCN films.
tive CV. The voltammetric peak current in Fig. 6A and the frequency
decrease ͑or mass increase͒ in Fig. 6B are consistent with the growth
of the RP and mvRuO/RuCN hybrid film on the gold electrode. The
EQCM results show that the deposition of the film obviously oc-
curred between two potential ranges: one between 0.85 and 1.0 V
͑vs. Ag͉AgCl͒ and the second between 0.2 and Ϫ0.2 V ͑vs.
Ag͉AgCl͒. During the first CV scan, about 1300 ng/cm2 of RP and
mvRuO/RuCN hybrid film was deposited on the gold electrode, and
a total of about 9800 ng/cm2 of RP and mvRuO/RuCN hybrid film
was deposited on the gold electrode after ten CV scans. The EQCM
data in Fig. 4-6 were also used to estimate the RP and mvRuO/
RuCN deposition on the gold electrode for the RP and mvRuO/
RuCN hybrid film. The mass of RP and mvRuO/RuCN hybrid film
on the gold electrode surface was estimated to be about 5800 ng/cm2
of mvRuO/RuCN and about 4000 ng/cm2 of RP.
The single redox couple of RP is attributed to the ͓Fe͑III͒-NC-
Ru͑II͔͒/͓Fe͑II͒-NC-Ru͑II͔͒ redox reaction. There are three redox
couples of the mvRuO/RuCN film, which are proposed to
arise from the
͓
RuII-O͔/͓RuII͑CN) ]/ RuIII-O͔/͓RuII͑CN) ],
͓
͓
͓
EQCM measurements are a very good method for monitoring the
in situ growth of RP, mvRuO/RuCN, and RP and mvRuO/RuCN
6
6
RuIII-O͔/͓RuII͑CN) ]/ RuIII-O͔/͓RuIII͑CN) ],
and
6
6

Journal of The Electrochemical Society, 151 ͑4͒ E168-E178 ͑2004͒
E173
7A͑b͒ is consistent with an Rbϩ ion exchange of the redox couple
reaction ͑from potentials of Ϫ0.2 to 0.6 V͒.
The redox process of RP in Rbϩ and Csϩ electrolyte aqueous
solutions may be written as follows15,32-34
FeI4II͓RuII͑CN)6]3 ϩ 4eϪ ϩ 4Rbϩ ꢀ Rb4FeI4I͓RuII͑CN)6]3
͓3͔
FeI4II͓RuII͑CN)6]3 ϩ 4eϪ ϩ 4Csϩ ꢀ Cs4FeI4I͓RuII͑CN)6]3
͓4͔
Figure 7B shows the results of the CV and EQCM measurements of
an mvRuO/RuCN film in an aqueous 0.2 M RbNO3 solution at pH
2.0. EQCM and potential scanning measurements of an mvRuO/
RuCN film in an aqueous 0.2 M RbNO3 solution at pH 2.0 were also
performed. The EQCM frequency change was recorded over a
single cycle using CV between potentials of Ϫ0.2 and 1.0 V. The
CV frequency ͑or EQCM͒ responses recorded during a single cycle
using CV in an aqueous RbNO3 solution are shown in Fig. 7B͑a͒
and 7B͑b͒. The redox couple voltammetric current varied between
potentials of Ϫ0.2 and 1.0 V, and the frequency increased/decreased
͑or mass decreased/increased͒ with the cation exchange of the three
redox couple reactions.
The results showing the three redox couples with voltammetric
currents ͑and the frequency change͒ from potentials of Ϫ0.2 to 0.4
V ͑frequency increase/mass decrease͒, from potentials of 0.40 to
ϩ0.85 V ͑frequency decrease/mass increase͒, and from potentials of
0.85 to 1.0 V ͑no obvious change in frequency͒ are given in Fig.
7B͑b͒. The frequency increase ͑mass decrease͒ in Fig. 7B͑b͒ is con-
sistent with an Rbϩ ion exchange of the redox couple reaction from
potentials of Ϫ0.2 to 0.4 V. The frequency decrease/mass increase in
Fig. 7B͑b͒ is consistent with an Rbϩ ion exchange of the redox
couple reaction from potentials of 0.40–0.85 V, and with the fre-
quency showing no obvious change from potentials of 0.85–1.0 V.
Figure 8 shows the results of CV and EQCM measurements of an
RP and mvRuO/RuCN hybrid film in an aqueous 0.2 M RbNO3
solution at pH 2.0. EQCM and potential scanning measurements of
a hybrid RP and mvRuO/RuCN film in an aqueous RbNO3 solution
were also performed. The EQCM frequency change was recorded
over a single cycle using CV between potentials of Ϫ0.2 and 1.0 V.
The CV frequency ͑or EQCM͒ responses recorded for the hybrid RP
and mvRuO/RuCN film during a single cycle using CV in an aque-
ous RbNO3 solution are shown in Fig. 8A͑a͒ and 8A͑b͒. The redox
couple voltammetric current varied between potentials of Ϫ0.2 and
1.0 V, and the frequency increased/decreased ͑or mass decreased/
increased͒ with the cation exchange of the three redox couple reac-
tions.
Figure 7. ͑A,a͒ CVs of a gold electrode modified with an RP film in an
aqueous 0.1 M RbNO3 solution. ͑A,b͒ EQCM frequency change recorded for
this film in an aqueous 0.1 M RbNO3 solution. ͑B,a͒ CVs of a gold electrode
modified with a ruthenium oxide/ruthenocyanide film in an aqueous 0.1 M
RbNO3 solution. ͑B,b͒ EQCM frequency change recorded for this film in an
aqueous 0.1 M RbNO3 solution.
Results showing the four redox couples with voltammetric cur-
rents ͑and frequency change͒ in potential ranges of Ϫ0.2 to 0.2 V
͑frequency increase/mass decrease͒, 0.2–0.5 V ͑frequency increase/
mass decrease͒, 0.5-0.85 V ͑frequency decrease/mass increase͒, and
in the potential range 0.85-1.0 V ͑no obvious change in frequency͒
are given in Fig. 8A. The frequency increase ͑or mass decrease͒ in
Fig. 8A is consistent with an Rbϩ ion exchange of the two redox
couples ͑in the potential ranges of Ϫ0.2 to 0.2 and 0.2-0.5 V͒. The
frequency decrease ͑or mass increase͒ in Fig. 8A is consistent with
an Rbϩ ion exchange of the redox couple in the potential range
0.50-0.85 V, and with no obvious change in frequency in the poten-
tial range 0.85-1.0 V.
Kinetics of potential switching in the potential range of Ϫ0.2 to
0.2 V ͓Fig. 8B͑a͔͒, 0.2-0.5 V ͓Fig. 8B͑b͔͒, and 0.5-0.85 V ͓Fig.
8B͑c͔͒ were elucidated to assess the performance of a hybrid RP and
mvRuO/RuCN film in RbNO3 in these experiments. A square wave
potential was applied over a 2 s period during film cation exchange.
The results are shown in Fig. 8B. The reversibility of the hybrid RP
and mvRuO/RuCN film during the cycling and frequency change
was good, and cation exchange is obvious from the redox couples.
From the EQCM results, the kinetics of the cation exchange rate in
RuIII-O͔/͓RuIII͑CN) ]/ RuIV-O͔/͓RuIII͑CN) ] redox processes.
͓
͓
6
6
The electrochemical properties of RP and mvRuO/RuCN hybrid
films.—Figure 7A shows the CV and EQCM measurements on an
RP film in an aqueous 0.1 M RbNO3 solution. EQCM and potential
scanning measurements of an RP film in an aqueous RbNO3 solu-
tion at pH 2.0 were also performed. The EQCM frequency change
was recorded during a single cycle using CV between potentials of
Ϫ0.2 and 0.6 V. The CV frequency ͑and EQCM͒ responses recorded
during one cycle of the RP film using CV in an RbNO3 aqueous
solution are shown in Fig. 7A. The redox couple voltammetric cur-
rent varied between potentials of Ϫ0.2 and 1.0 V, and the frequency
increased/decreased ͑or mass decreased/increased͒ with the cation
exchange of the single redox couple reaction.
Results showing the single redox couple with voltammetric cur-
rent ͑and the frequency change͒ from potentials of Ϫ0.2 to 0.6 V
indicating the frequency increase/mass decrease are given in Fig.
7A͑a͒ and 7A͑b͒. The frequency increase ͑mass decrease͒ in Fig.

E174
Journal of The Electrochemical Society, 151 ͑4͒ E168-E178 ͑2004͒
molar mass is taken to be equivalent to 1 mol of positive charge
exchanged, i.e., 1 mol of Rbϩ).
The EQCM results in Fig. 7A show that an increase in frequency
͑or a decrease in mass͒ had occurred during the oxidation of the
mvRuO/RuCN film. During the CV, the frequency change was about
18.4 Hz ͑or 25.8 ng͒ for a 0.196 cm2 gold electrode. This frequency
change was smaller than that of the frequency exchange occurring at
the film ͑if the total Rbϩ ion exchange reaction in the film that had
a frequency change of about 50 Hz͒. These results show that there
was perhaps water ͑solvent͒ transfer in the switching of the RP film,
as water would exit an RP film upon cation insertion. The EQCM
results of ruthenium oxide/hexacyanoruthenate films also exhibit a
similar phenomenon.
Figure 6B and 8 show the EQCM results of a 9800 ng/cm2
(1.1 ϫ 10Ϫ9 mol/cm2͒ ruthenium oxide/hexacyanoruthenate and RP
hybrid film that was deposited on a gold electrode. For a one-
electron redox couple, n ϭ 1, the effective molar mass, M, was
estimated to be about 1840 g/mol at the peak potential of 0 V ͑as-
suming that one effective molar mass is taken to be equivalent to 1
mol positive charge, i.e., 1 mole of Rbϩ, exchanged͒.
The EQCM results in Fig. 8B show that an increase in frequency
͑or a decrease in mass͒ had occurred during the oxidation of the
ruthenium oxide/hexacyanoruthenate and RP hybrid film. During the
CV, the frequency change was about 67 Hz ͑or 93.8 ng͒ for a 0.196
cm2 gold electrode ͑assuming that the total Rbϩ ion-exchange reac-
tion had a frequency change of about 19 Hz͒. This frequency change
was larger than that of the frequency exchange occurring at the film.
These results also show that there was perhaps water ͑solvent, or
hydrated ion͒ transfer occurring during the switching of the film, as
water ͑or hydrated ions͒ would be exchanged in the hybrid film
upon cation insertion.
Electrocatalytic reduction of S2O28Ϫ and SO52Ϫ by hybrid
ruthenium(III) ruthenocyanide and RP films.—The electrocatalytic
reduction of S2O82Ϫ and SO25Ϫ by hybrid RP and mvRuO/RuCN
films in aqueous 0.2 M RbNO3 solutions at pH 2.0 was investigated.
Figure 9A shows the CVs of the hybrid RP and mvRuO/RuCN
film in an aqueous 0.2 M RbNO3 buffered solution at pH 2.0 in the
absence and presence of S2O82Ϫ . The cathodic peak current of the
hybrid RP and mvRuO/RuCN film for the redox couple at 0.05 V
͑vs. Ag͉AgCl͒ increased noticeably, while the anodic peak current
decreased as the concentration of S2O28Ϫ increased. The cathodic
peak current of the hybrid RP and mvRuO/RuCN film redox couple
at 0.30 V ͑vs. Ag͉AgCl͒ also increased, but a smaller electrocatalytic
current was observed. The experimental results also show the CVs
of a hybrid RP and mvRuO/RuCN film in an aqueous 0.2 M RbNO3
buffered solution at pH 2.0 in the presence of SO25Ϫ . An increase in
the cathodic peak current appears as the concentration of SO25Ϫ in-
creases in the aqueous 0.2 M RbNO3 solution at pH 2.0. Figure 9B
shows the CVs of a hybrid RP and mvRuO/RuCN film in an aque-
ous 0.2 M RbNO3 buffered solution at pH 2.0 in the absence and
presence of SO25Ϫ . The cathodic peak current of the two redox
couples of the hybrid RP and mvRuO/RuCN film at potentials of
about 0.05 and 0.30 V ͑vs. Ag͉AgCl͒ both increased noticeably, but
the other two redox couples of the hybrid RP and mvRuO/RuCN
film that occurred at potentials between 0.6 and 1.1 V ͑vs. Ag͉AgCl͒
stayed the same and showed no obvious electrocatalytic current. In
aqueous solutions of the cesium or rubidium electrolytes, the elec-
trocatalytic processes that developed from the redox couples were
easier to observe than in the corresponding potassium or sodium
aqueous electrolyte solutions.
Figure 8. ͑A,a͒ CVs of a gold electrode modified with an RP and mvRuO/
RuCN hybrid film in an aqueous 0.1 M RbNO3 solution. ͑A,b͒ EQCM fre-
quency change recorded for this film in an aqueous 0.1 M RbNO3 solution.
͑B͒ EQCM measurements on an RP and mvRuO/RuCN hybrid film in an
aqueous 0.1 M RbNO3 solution during potential switching from ͑a͒ Eappl
ϭ Ϫ0.2 to 0.2 V vs. Ag͉AgCl, ͑b͒ Eappl ϭ 0.2-0.45 V vs. Ag͉AgCl, and ͑c͒
E
appl ϭ 0.45-0.8 V vs. Ag͉AgCl with 2 s time pulses.
the potential ranges of Ϫ0.2 to 0.2 V ͓Fig. 8B͑a͔͒ and 0.2-0.5 V
͓Fig. 8B͑b͔͒ were both faster than the cation exchange rate in the
potential range of 0.5-0.85 V ͓Fig. 8B͑c͔͒.
The electrocatalytic activity was shown only through the two
negative redox couples of the hybrid RP and mvRuO/RuCN film.
For a one-electron redox couple, n ϭ 1, the effective molar
mass, M, was estimated to be about 2290 g/mol. Figure 7A shows
the EQCM results of a 9450 ng/cm2 ͑about 4.1 ϫ 10Ϫ9 mol/cm2͒
RP film that was deposited on a gold electrode ͑if one effective
S2O82Ϫ ϩ 2eϪ → 2SO24Ϫ
͓5͔
͓6͔
SO25Ϫ ϩ 2eϪ ϩ 2Hϩ → SO24Ϫ ϩ H2O

Journal of The Electrochemical Society, 151 ͑4͒ E168-E178 ͑2004͒
E175
Figure 10. ͑A͒ CVs of a hybrid ruthenium͑III͒ ruthenocyanide and iron͑III͒
ruthenocyanide film adhered to a GC electrode in a 0.1 M RbNO3 solution at
pH 5.0. ͑A͒ dopamine ϭ (a) 0.0, ͑b͒ 4 ϫ 10Ϫ3, ͑c͒ 7 ϫ 10Ϫ3, and ͑d͒
Figure 9. ͑A͒ CVs of a hybrid ruthenium͑III͒ ruthenocyanide and iron͑III͒
ruthenocyanide film adhered to a GC electrode in a 0.1 M RbNO3 solution at
pH 2.0. ͑A͒ SO2Ϫ ϭ (a) 0.0, ͑b͒ 5 ϫ 10Ϫ3, ͑c͒ 1.0 ϫ 10Ϫ2, and ͑d͒ 2.0
͓
͔
1 ϫ 10Ϫ2 M. ͑a ͒ Bare GC carbon electrode, dopamine ϭ 1 ϫ 10Ϫ2 M.
Ј
͓
͔
͑B͒ epinephrine ϭ (a) 0.0, ͑b͒ 4 ϫ 10Ϫ3
,
͑c͒ 7 ϫ 10Ϫ3
, and ͑d͒ 1
͓
͔
͓
͔
5
ϫ 10Ϫ2 M. ͑a ͒ Bare GC electrode, SO2Ϫ ϭ 2.0 ϫ 10Ϫ2 M. ͑B͒
ϫ 10Ϫ2 M. ͑a ͒ Bare GC electrode, epinephrine ϭ 1 ϫ 10Ϫ2 M.
Ј ͓ ͔
Ј
͓
͔
5
S O2Ϫ ϭ (a) 0.0, ͑b͒ 6 ϫ 10Ϫ3, ͑c͒ 1.2 ϫ 10Ϫ2, and ͑d͒ 2.4 ϫ 10Ϫ2 M.
͓
͔
2
8
͑a ͒ Bare GC electrode, S O2Ϫ ϭ 2.4 ϫ 10Ϫ2 M.
Ј
͓
͔
8
2
noticeably, and an increasing cathodic peak current ͑electrocatalytic
reduction͒ appeared as the concentration of epinephrine increased.
The other redox couple of the hybrid RP and mvRuO/RuCN film at
a potential of about 0.0 V ͑vs. Ag͉AgCl͒ showed no obvious elec-
trocatalytic current.
Both the anodic and cathodic peak currents increased as the con-
centration of dopamine ͑or epinephrine͒ increased. The electrocata-
lytic oxidation of dopamine ͑or epinephrine͒ and the reduction of its
oxidation products were both electrocatalytically active when using
a hybrid RP and mvRuO/RuCN film in an aqueous solution at pH
5.0 ͑see Table II͒.
Electrocatalysis of dopamine and epinephrine by hybrid RP and
mvRuO/RuCN films.—The electrocatalytic oxidation of dopamine
and epinephrine by a hybrid RP and mvRuO/RuCN film in an aque-
ous 0.2 M RbNO3 solution at pH 5.0 was investigated. Figure 10A
shows the CVs of a hybrid RP and mvRuO/RuCN film in an aque-
ous 0.2 M RbNO3 buffered solution at pH 5.0 in the absence and
presence of dopamine. The anodic peak current of the hybrid RP and
mvRuO/RuCN film of the redox couple at a potential of about 0.30
V ͑vs. Ag͉AgCl͒ increased noticeably, and the cathodic peak current
also increased, as the concentration of dopamine increased. The ca-
thodic peak current of the hybrid RP and mvRuO/RuCN film redox
couple at a potential of about 0.0 V ͑vs. Ag͉AgCl͒ showed no elec-
trocatalytic current. Figure 10B shows the CVs of a hybrid RP and
mvRuO/RuCN film in an aqueous 0.2 M RbNO3 buffered solution
at pH 5.0 in the absence and presence of epinephrine. The anodic
peak current of the redox couple of the hybrid RP and mvRuO/
RuCN film at a potential of about 0.30 V ͑vs. Ag͉AgCl͒ increased
Electrocatalytic oxidation of dopamine by a hybrid RP and
mvRuO/RuCN film in an aqueous solution at pH 5.0 is described by
the following reaction scheme37-39
͓7͔

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Journal of The Electrochemical Society, 151 ͑4͒ E168-E178 ͑2004͒
Table II. The electrocatalytic properties of dopamine with a hy-
brid RP and mvRuOÕRuCN film and a bare GC electrode in
aqueous solutions at pH 5.0.
Anodic peak Cathodic peak
potential
͑V͒
potential
͑V͒
Electrodea
⌬Ep ͑V͒
Reactantb
Dopamine
RP and
0.33
0.22
0.11
mvRuO/RuCN
GC
RP and
0.44
0.38
0.13
0.27
0.31
0.11
Dopamine
Epinephrine
mvRuO/RuCN
GC
0.56
0.21
0.35
Epinephrine
a GC is a bare glassy carbon electrode.
b The first cathodic peak potential.
The electrocatalytic oxidation of epinephrine by a hybrid ruthe-
nium͑III͒ ruthenocyanide and RP film in an aqueous solution at pH
5.0 is described by37-39
͓8͔
͓9͔
͓10͔
The CVs of epinephrine in an aqueous buffered solution at pH 5.0
shows one oxidation peak and two reduction peaks ͑Fig. 10B͒. The
irreversible oxidation peak occurred at a potential of about 0.57 V
͑vs. Ag͉AgCl͒ and stems from the oxidation reaction shown in Eq.
8-10. The two reduction peaks occurring at potentials of about 0.22
and Ϫ0.08 V ͑vs. Ag͉AgCl͒ arise from the reduction reactions
shown in Eq. 8 and 10.
Figure 11. ͑A͒ RRDE voltammogram of a hybrid ruthenium͑III͒ rutheno-
cyanide and iron͑III͒ ruthenocyanide film adsorbed on a GC disk electrode
with differing concentrations of dopamine in an aqueous 0.1 M RbNO3
solution at pH 5.0. dopamine ϭ (a) 0.0, ͑b͒ 2 ϫ 10Ϫ4, ͑c͒ 4 ϫ 10Ϫ4, ͑d͒
͓
͔
6 ϫ 10Ϫ4, and ͑e͒ 8 ϫ 10Ϫ4 M at a rotation rate of 2500 rpm. ER ϭ 0.15 V.
Inset 11A: Plot of ͑a͒ IR , ͑b͒ ID , and ͑c͒ Icat vs. ͓dopamine͔. ͑B͒ RRDE
voltammogram of a hybrid ruthenium͑III͒ ruthenocyanide and iron͑III͒ ru-
thenocyanide film adsorbed on a GC disk electrode in an aqueous 0.1 M
RbNO3 solution at pH 5.0 with different rotation rates: ͑a͒ 200, ͑b͒ 400, ͑c͒
Study of the electrocatalytic oxidation properties using the
RRDE method.—Figure 11 shows the electrocatalytic oxidation of
dopamine by a hybrid RP and mvRuO/RuC3N film using the RRDE
method. Figure 11A shows the dopamine present in an aqueous so-
lution at pH 5.0 with different concentrations of dopamine when the
hybrid RP and mvRuO/RuCN film modified ring GC electrode had
an applied potential of 0.1 V ͑vs. Ag͉AgCl͒ at 2500 rpm. Figure 11B
shows the dopamine present in an aqueous solution at pH 5.0 when
the hybrid RP and mvRuO/RuCN film modified ring GC electrode
600, ͑d͒ 900, ͑e͒ 1200, ͑f͒ 1600, and ͑g͒ 2500 rpm. dopamine ϭ 4
͓
.
͔
Ϫ1/2
ϫ 10Ϫ4 M. ER ϭ 0.15 V. Inset 11B: Plot of IϪ1 vs. ␻
In Fig. 11B, the value of ID is the oxidation current and the value
of IR is the reduction current shown in Eq. 7. The collection effi-
ciency for various rotating rates plotted as IR /ID vs. ␻1/2 was almost
constant, with an average value close to 0.25 ͑for ER ϭ 0.1 V͒.
These results are consistent with the current IR being the reduction
current shown in Eq. 7. The RRDE data were analyzed using the
Koutecky-Levich equation30,40,41 and plotted as IϪ1 vs. ␻Ϫ1/2, as
shown in the inset of Fig. 11B
had an applied potential of 0.1 V ͑vs. Ag͉AgCl͒ and dopamine
͓
͔
ϭ 4 ϫ 10Ϫ4 M for different rotation rates. The inset of Fig. 11A
shows a plot of ID and Icat (ID minus the disk current in the absence
of dopamine͒ and IR vs. ͓dopamine͔, illustrating a close linear de-
pendence of ID , Icat , and IR on the scan rate. The values of ID and
I
R are the oxidation and reduction currents at 0.37 ͑Fig. 11͒ and 0.42
1/I ϭ 1/Ik ϩ 1/Ilim,c
͓11͔
V ͑Fig. 12͒, respectively ͑where ID reaches a plateau͒. The ratio of
IR :ID was equal to 0.30, and the collection efficiency, N, is defined
as IR /ID .
where

Journal of The Electrochemical Society, 151 ͑4͒ E168-E178 ͑2004͒
E177
a potential of about Ϫ0.15 V for ID and IR . The best straight line
passing through the data points was then selected and the RRDE
data analyzed using the Koutecky-Levich equation and plotted as I1
vs. ␻Ϫ1/2, as shown in the insets of Fig. 11B and 12B.
The inset of Fig. 11B shows a plot of IϪ1 vs. ␻Ϫ1/2, for Ik
*
ϭ nFAk⌫C0 , where k is the rate constant of the chemical reaction
between the film and dopamine, and ⌫ is the coverage of the catalyst
on the electrode surface. ⌫ was estimated to be 7.8 ϫ 10Ϫ8 mol/cm2
for the RP film, and the rate constant was estimated to be 5.2
ϫ 102 MϪ1 sϪ1
.
Figure 12 shows the electrocatalytic oxidation of epinephrine by
a hybrid RP and mvRuO/RuCN film using the RRDE method. Fig-
ure 12A shows the epinephrine present in an aqueous solution at pH
5.0 when the hybrid RP and mvRuO/RuCN film modified ring GC
electrode had an applied potential of 0.1 V ͑vs. Ag͉AgCl͒ with dif-
ferent concentrations of epinephrine at 2500 rpm. The inset of Fig.
12A shows a plot of ID , Icat (Icat is ID,lim minus the disk current
when the reactant concentration is zero at the same potential in the
presence of catalytic film͒, and IR vs. ͓epinephrine͔, respectively,
illustrating a close linear dependence of ID , Icat , and IR on the scan
rate, and that IR :ID was equal to 0.30.
Figure 12B shows the oxidation of epinephrine present in a pH
5.0 buffered solution when a ring GC electrode potential was ap-
plied at Ϫ0.2 V ͑vs. Ag͉AgCl͒ at different rotation rates. The param-
eter ID is the oxidation current shown in Eq. 8 and of the further
oxidation shown in Eq. 10. In Fig. 12B, IR denotes the reduction
current of Eq. 8 and 10. The RRDE data were also analyzed using
the Koutecky-Levich equation40,41 (1/I ϭ 1/Ik ϩ 1/Ilim,c), where
Ilim,c is the observed limiting current of the disk and Ik ϭ nFA␬ ⌫
Ϫ1/2
͓epinephrine͔. Plots of IϪ1 vs. ␻
are shown in the inset of Fig.
12B. The surface coverage, ⌫, was estimated to be 8 ϫ 10Ϫ8
mol/cm2 for the RP in the hybrid film, as deduced from the chrono-
coulometry charge and the EQCM frequency. The rate constant of
the chemical reaction, ␬, was estimated to be an average value of
5.0 ϫ 102 MϪ1 sϪ1 from three epinephrine concentrations: 1
ϫ 10Ϫ4, 2 ϫ 10Ϫ4, and 4 ϫ 10Ϫ4 M.
The electrocatalytic properties depicted in Fig. 11 and 12 clearly
show the electrochemical oxidation and electrocatalytic reduction of
dopamine and epinephrine and their oxidation products by a hybrid
RP and mvRuO/RuCN film.
Conclusions
Figure 12. ͑A͒ RRDE voltammogram of a hybrid ruthenium͑III͒ rutheno-
cyanide and iron͑III͒ ruthenocyanide film adsorbed on a GC disk electrode
with differing concentrations of epinephrine in an aqueous 0.1 M RbNO3
Hybrid RP and mvRuO/RuCN films were successfully synthe-
sized using consecutive CV on various electrodes directly from
Ru3ϩ, Fe3ϩ, and Ru͑CN)46Ϫ ions in various electrolyte solutions
containing Rbϩ and Csϩ cations. The formal potentials and shapes
of the CVs show four obvious and separated redox couples which
depended on the electrolyte cation.
solution at pH 5.0. epinephrine ϭ (a) 0.0, ͑b͒ 1 ϫ 10Ϫ4, ͑c͒ 3 ϫ 10Ϫ4
,
͓
͔
and ͑d͒ 4 ϫ 10Ϫ4 M at a rotation rate of 2500 rpm. ER ϭ 0.15 V. Inset 12A:
Plot of ͑a͒ IR , ͑b͒ ID , and ͑c͒ Icat vs. ͓epinephrine͔ at 0.38 V. ͑B͒ RRDE
voltammogram of a hybrid ruthenium͑III͒ ruthenocyanide and iron͑III͒ ru-
thenocyanide film adsorbed on a GC disk electrode in an aqueous 0.1 M
RbNO3 solution at pH 4.0 with different rotation rates: ͑a͒ 200, ͑b͒ 400, ͑c͒
EQCM and CV were used to study the growth mechanisms of the
RP and mvRuO/RuCN films, and the hybrid RP and mvRuO/RuCN
films. EQCM and CV were used to study the in situ growth of the
RP, mvRuO/RuCN, and hybrid RP and mvRuO/RuCN films. The
results indicate that the redox processes were confined to the sur-
face, confirming the immobilized state of the RP and mvRuO/
RuCN, and the hybrid RP and mvRuO/RuCN films. The ionic ex-
change processes of the RP, mvRuO/RuCN, and the hybrid RP and
mvRuO/RuCN films were also discussed using EQCM and CV.
The hybrid RP and mvRuO/RuCN films can electrocatalytically
reduce SO25Ϫ and S2O28Ϫ . Hybrid RP and mvRuO/RuCN films are
electrocatalytically active in the oxidation of dopamine and epineph-
rine. The electrocatalytic reactions of dopamine and epinephrine
with hybrid RP and mvRuO/RuCN films were investigated using the
RRDE method and the rate constant of the chemical reaction was
estimated.
600, ͑d͒ 900, ͑e͒ 1200, ͑f͒ 1600, and ͑g͒ 2500 rpm. epinephrine ϭ 4
͓
͔
Ϫ1/2
ϫ 10Ϫ4 M. ER ϭ Ϫ0.2 V. Inset 12B: Plot of IϪ1 vs. ␻
.
Ilim,c ϭ 0.62nFAD02/3
␻
␷
͓12͔
1/2 Ϫ1/6
*
C0
The parameter Ilim,c is the measured limiting current of the disk, ␻ is
*
the rotation rate, D0 and C0 are the diffusion coefficient and the
bulk concentration of dopamine, respectively, and ␷ is the kinematic
viscosity of water in the experimental rotating rates. In Fig. 11 and
12, the zero current is observed at a potential of about Ϫ0.15 V, and
the values of ID and IR are the oxidation and reduction currents at
potentials of 0.37 ͑Fig. 11͒ and 0.42 V ͑Fig. 12͒, respectively ͑where
the value of ID reaches a plateau͒. The baseline zero current was at

E178
Journal of The Electrochemical Society, 151 ͑4͒ E168-E178 ͑2004͒
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