Electrochemical characterization of charge injection at electrodeposited platinum electrodes in phosphate buffered saline

Article abstract of DOI:10.1149/1.2354457

Platinum exhibits biocompatibility and chemical stability over a wide potential range and hence is a candidate material for implantable electrodes. Electrodeposition is of interest for the fabrication of implantable electrodes since it is compatible with a wide range of patterning techniques, including conventional microfabrication, soft-lithography, and electrochemical template synthesis. We show that surface roughness of platinum electrodes deposited from ammonium hexachloroplatinate solution is dependent on the deposition potential. We demonstrate that 600 μm diameter electrodeposited platinum microelectrodes can inject 200 nC of charge in a 2 ms biphasic pulse with a potential window as small as 200 mV. The Electrochemical Society

Full text of DOI:10.1149/1.2354457

C834
Journal of The Electrochemical Society, 153 ͑12͒ C834-C839 ͑2006͒
0
013-4651/2006/153͑12͒/C834/6/$20.00 © The Electrochemical Society
Electrochemical Characterization of Charge Injection
at Electrodeposited Platinum Electrodes in Phosphate
Buffered Saline
a
a
a
b, ,z
John J. Whalen III, Jeffrey Young, James D. Weiland, and Peter C. Searson *
a
Doheny Vision Research Center, University of Southern California, Los Angeles, California 90033,
USA
b
Department of Materials Science and Engineering, Johns Hopkins University, Baltimore, Maryland 21218,
USA
Platinum exhibits biocompatibility and chemical stability over a wide potential range and hence is a candidate material for
implantable electrodes. Electrodeposition is of interest for the fabrication of implantable electrodes since it is compatible with a
wide range of patterning techniques, including conventional microfabrication, soft-lithography, and electrochemical template
synthesis. We show that surface roughness of platinum electrodes deposited from ammonium hexachloroplatinate solution is
dependent on the deposition potential. We demonstrate that 600 ␮m diameter electrodeposited platinum microelectrodes can inject
2
©
00 nC of charge in a 2 ms biphasic pulse with a potential window as small as 200 mV.
2006 The Electrochemical Society. ͓DOI: 10.1149/1.2354457͔ All rights reserved.
Manuscript submitted March 17, 2006; revised manuscript received July 5, 2006. Available electronically October 11, 2006.
Stimulatory electrodes are used in a wide range of applications
with properties that can be tailored for specific applications. In this
paper we report on the electrochemical behavior of electrodeposited
platinum films and microelectrodes in phosphate buffered saline. We
show that the charge capacity is strongly dependent on the morphol-
ogy and surface chemistry of the films.
including deep brain and cochlear implants, retinal and cortical pros-
theses, stimulation of the bladder and spinal cord, and cardiac
1
,2
pacing. The electrical stimulation of cells requires the injection of
charge. In this type of biomedical application, a balanced-charge
biphasic waveform is generally used to prevent tissue damage in-
duced by irreversible electrochemical reactions occurring at the
Experimental
3
electrode/tissue interface. The performance of noble metal stimu-
Platinum was deposited from solution containing 17 mM
lating electrodes is determined in large part by the charge that can be
delivered within the potential limits associated with hydrogen and
͑
NH ͒ PtCl ͑Alfa Aesar͒ and 250 mM Na HPO ͑Alfa Aesar, ACS
4 2 6 2 4
8
1,3
grade͒ at pH 7.8. The details have been reported elsewhere. Plati-
num films were deposited on polycrystalline gold films or platinum
microelectrodes. Gold films ͑Ͼ200 nm thick͒ were thermally evapo-
rated onto Si͑111͒ wafers with a 20 nm chromium adhesion layer.
Prior to each experiment, films were immersed in acetone and rinsed
sequentially in isopropanol, ethanol, and water. Finally, the films
were immersed in 5 M nitric acid and rinsed in water. Platinum foils
oxygen evolution. Within these limits, charge injection can occur
through double-layer charging or by reversible faradaic processes
such as oxide formation and reduction, and hydrogen adsorption/
desorption. Irreversible faradaic processes that result in the forma-
tion of chemical species in bulk solution can adversely affect the
1
-3
tissue. Materials that have been studied for stimulating electrodes
4
-7
include platinum, iridium oxide, and titanium nitride.
͑
2
Alfa Aesar, Premion 99.99%͒ were annealed in air at 700°C for
h. After annealing the films were etched in nitric acid solution and
Platinum is widely used in implantable electrodes for both stimu-
lation and recording since it exhibits good biocompatibility and is
stable over a wide potential range. Since the double-layer capaci-
tance is typically 20 ␮F cm−2 and the potential window is about
rinsed in distilled, demonized water.
Platinum films were deposited at constant potential for 2 h cor-
responding to deposition film thicknesses of 30–195 ␮m. Films
were characterized by cyclic voltammetry and impedance spectros-
copy in phosphate buffered saline ͑Gibco Brand, Invitrogen͒: 0.1 M
K HPO , 0.1 M Na PO , and 0.15 M NaCl ͑pH 7.4͒.
1
V, the baseline capacitance available for cell stimulation by
−
2
double-layer charging alone is about 20 ␮C cm . Additional charge
can be obtained from the adsorption and desorption of hydrogen
+
−
2
4
3
4
͓
͓
͑
Pt + H ͑aq͒ + e ↔ Pt–H ͔, from oxide formation and reduction
ads
+
Charge injection experiments were performed on platinum disk
−
Pt + H O ↔ PtO͑s͒ + 2H + 2e ͔, and from anion adsorption
2
microelectrode arrays. The arrays were comprised of 600 ␮m diam
−
−
e.g., Pt + Cl ↔ Pt–Cl + e ͒. These processes are reversible
2
ads
͑A = 0.00283 cm ͒ disk electrodes embedded in a silicone elas-
since the corresponding back-reaction occurs on applying a potential
pulse of opposite polarity. Reactions involving the formation of
gases and soluble dissolution products are irreversible, since these
reaction products diffuse away from the electrode surface and can-
not be completely converted back to the corresponding reactants.
Although platinum can be dissolved in chloride solutions with the
tomer. Prior to electrodeposition of platinum, the platinum micro-
electrodes were cycled at least 40 times in 250 mM H SO ͑pH 1.8͒
2
4
over the potential range from −0.2 to 1.25 V ͑Ag/AgCl͒ at a scan
rate of 250 mV s− to ensure reproducible surfaces. Electrodes were
then rinsed in deionized water prior to electrodeposition of platinum
at potentials from −0.4 to − 0.8 V ͑Ag/AgCl͒ for 1 h.
1
formation of soluble species ͑e.g., PtCl2 and PtCl ͒, the rate of
−
2−
4
6
Platinum microelectrodes were characterized in 250 mM H SO
pH 1.8͒ and in phosphate buffered saline ͑PBS͒. Charge injection
2
4
reaction is sufficiently slow that the concentration released into the
surrounding tissue can be neglected, even after long periods of
͑
3
experiments were performed on the microelectrodes in a two-
electrode configuration using a biphasic current pulse with a dura-
stimulation.
8
In previous work we have shown that platinum can be deposited
9
tion of 1 ms. The experiments were performed in PBS at room
from ammonium hexachloroplatinate solution at neutral pH over a
potential range where hydrogen evolution can be avoided. Further-
more, we have shown that the morphology and grain size of the
deposited films is dependent on the deposition potential. These fea-
tures provide the opportunity to fabricate implantable electrodes
temperature with a platinum mesh counter electrode.
Results and Discussion
Platinum films.— Figure 1 shows voltammograms for a gold
electrode in supporting electrolyte and in ammonium hexachloro-
8
platinate solution. The details have been reported elsewhere.
Briefly, the onset of platinum ion reduction occurs at 0.2 V
͑Ag/AgCl͒; however, the current in the potential range from
*
Electrochemical Society Active Member.
E-mail: searson@jhu.edu
z
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Journal of The Electrochemical Society, 153 ͑12͒ C834-C839 ͑2006͒
C835
Figure 2. Current–voltage curves for annealed polycrystalline platinum foil
and electrodeposited platinum films in PBS solution ͑pH 7.4͒. The potential
range was −1.0 to 1.0 V and the scan rate was 50 mV s−
1
.
in diameter. At high magnification these features are porous, con-
sisting of needle-like crystals up to a few hundred nanometers in
length. The films deposited at −0.6 V exhibit similar characteristics,
although the features are somewhat larger. The nodular features seen
at low magnification are about 10 ␮m in diameter. At high magni-
fication these features are seen to be composed of faceted, needle-
like crystals up to 1 ␮m in length. At −0.7 and −0.8 V the films
appear smooth, bright, and dense, with grain sizes on the order of
1
00 nm. Although the evolution of the morphology with deposition
potential is unusual, the results were highly reproducible.
Thin films formed at −0.5 and −0.6 V are of particular interest
due to the high surface area. For stimulation applications, high-
surface-area electrodes exhibit increased charge capacity so that
lower potential limits can be used to achieve the capacities needed
for depolarization with minimal contribution from faradaic reac-
tions. High-surface-area platinum electrodes can be formed by sev-
2
6
−
eral methods. Platinum black is usually deposited from acidic PtCl
solution at high current density with the addition of Pb͑NO ͒ or
3
2
11,12
Pb͑CH COOH͒ .
undesirable due to its poor mechanical stability and the incorpora-
tion of lead into the deposited film. High-surface-area electrodes can
For implant applications, platinum black is
3
2
Figure 1. ͑a͒ Current–voltage curves for gold in supporting electrolyte
dashed line͒ and in ammonium hexachloroplatinate solution ͑solid line͒. The
scan rate was 50 mV s . ͑b͒ Plan-view SEM images of electrodeposited
͑
−
1
also be formed by cycling platinum in H SO at high rate for several
platinum films.
2
4
6
,13
hours.
During cycling, sequential oxide formation and reduction
results in the formation of a porous structure with roughness factors
as high as 70. The films deposited in this work can be deposited
quickly, have high surface area, are relatively robust, and do not
contain toxic impurities.
0
.2 to − 0.3 V corresponds to the reduction of Pt͑IV͒ to Pt͑II͒. The
onset of bulk deposition at −0.3 V is followed by a current peak at
0.5 V characteristic of nucleation and diffusion-controlled growth.
The equilibrium potential for hydrogen evolution at this pH is
0.71 V ͑Ag/AgCl͒, illustrating that platinum can be deposited
−
In previous work we have shown that with increasing deposition
8
overpotential, the films exhibit increasing ͑111͒ texture. At more
−
positive potentials ͑200͒, ͑220͒, and ͑311͒ peaks are also observed.
from this solution in a potential range positive to the onset of hy-
drogen evolution, thereby avoiding effects associated with the co-
deposition of hydrogen, such as internal stresses. This is not possible
in acidic solutions where the equilibrium potential for hydrogen
evolution is shifted positive by about 450 mV, close to the onset of
X-ray photoelectron spectroscopy has shown that the films deposited
at −0.5 to − 0.8 V exhibit a thin, compact PtO layer at the surface.
2
The film deposited at −0.4 V, however, has a much thicker mixed
oxide of PtO and PtO2.
In vitro characterization of implantable electrodes is usually per-
formed in phosphate-buffered saline. Figure 2 shows voltammo-
grams for the electrodeposited films and an annealed platinum foil in
PBS. In contrast to acidic solutions, the effective potential range for
charging is significantly larger at this pH due to the negative shift in
the onset of hydrogen evolution and the large overpotential for oxy-
gen evolution. The voltammogram for the annealed polycrystalline
platinum foil shows the same general features as in sulfuric acid
10
platinum deposition.
The morphology of platinum films deposited from this solution is
strongly dependent on the deposition potential, as shown in the plan-
view scanning electron microscope images in Fig. 1. Films depos-
ited at −0.4 V are smooth and bright. At high magnification, the
films appear dense with spherical grains approximately 10 nm in
diameter. At −0.5 V, the films exhibit nodular features about 1 ␮m
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C836
Journal of The Electrochemical Society, 153 ͑12͒ C834-C839 ͑2006͒
Figure 4. Parallel capacitance vs potential for an annealed polycrystalline
platinum foil in PBS. The inset shows the dependence of the exponent n on
potential.
Figure 4 shows the total capacitance C and exponent n vs poten-
tial for the annealed platinum film. The capacitance decreases from
−
2
about 120 ␮F cm at −0.6 V, in the hydrogen regime, to about
0 ␮F cm− at 0.8 V, in the oxide regime. The exponent n was
2
3
greater than 0.9 over most of the potential range. Similar curves
have been reported for the total capacitance of platinum in acidic
1
9,20
sulfate and acidic chloride solutions.
As described above, the
capacitance is somewhat larger than typical double-layer capaci-
tances due to the additional contribution from reversible ion adsorp-
tion ͑H , Cl , and OH ͒ over the whole potential range.
+
−
−
Figure 3. Complex plane plots for an annealed polycrystalline platinum foil
in PBS. The potentials are indicated in the plots.
Figure 5 shows complex plane plots of the impedance response
of the electrodeposited platinum films in PBS over the potential
range from −0.6 to 0.8 V. The complex plane plots for the film
deposited at −0.4 V are similar to those for the platinum foil. At
potentials from −0.6 to 0 V, a single semicircular loop is seen. At
0.2 and 0.4 V the loop is much larger, indicating a large increase in
the parallel resistance, and also exhibits some dispersion at lower
frequencies. At more positive potentials, at 0.6 and 0.8 V, the par-
allel resistance decreases resulting in a slightly smaller loop. The
complex plane plots for films deposited at −0.5 and −0.6 V are
much more capacitive, as expected from the high surface area. The
complex plane plots for the film deposited at −0.7 V are similar to
those for the annealed platinum foil.
The total capacitance and exponent of the electrodeposited films
are plotted vs potential in Fig. 6a. As for the case of the annealed
foil, the capacitance of the electrodeposited films decreases with
increasing potential. However, the magnitude of the capacitance
spans more than two orders of magnitude. The exponents for the
films deposited at −0.6 and −0.7 V are close to 0.9 over most of the
potential range, as seen in Fig. 6b, consistent with the results on the
annealed platinum foil. The lower exponents associated with the
films deposited at −0.4 V may be related to the surface chemistry.
As described above, the films deposited at −0.4 V exhibit mixed
with hydrogen adsorption and desorption peaks, a double-layer
charging region, an oxide formation plateau, and an oxide reduction
1
4
peak. Similar results have been reported for polished polycrystal-
4
,15,16
line platinum in PBS under comparable conditions.
The film
deposited at −0.4 V exhibits relatively broad features and large ca-
pacitive charging, presumably related to the relatively thick mixed
oxide at the surface. The high-surface-area film deposited at −0.5 V
exhibits large currents and broad, slightly sheared features charac-
teristic of a non-negligible film resistance. Similar features have
been reported for electrodes fabricated by electrodeposition of plati-
num onto high-surface-area gold with a roughness factor of about
1
7
1
0. The high-surface-area films deposited at −0.6 V exhibit
slightly smaller currents but more distinct features. The voltammo-
grams for the films deposited at −0.7 and −0.8 V are very similar to
the annealed platinum foil.
The electrochemical response of the electrodeposited platinum
films was also characterized using electrochemical impedance spec-
troscopy. Figure 3 shows complex plane plots for the annealed plati-
num foil in PBS in the potential range from −0.6 to 0.8 V. The
impedance response in the hydrogen and double-layer regime, from
Pt͑III͒/Pt͑IV͒ oxide rather than the thin PtO film seen at more nega-
−
0.6 to 0 V, shows a characteristic semicircular loop. At 0.2 and
2
tive deposition potentials. Figure 6c shows the capacitance obtained
from the impedance spectra at 0.4 V plotted vs the surface rough-
ness. The surface roughness was determined from the hydrogen de-
0
.4 V, corresponding to the onset of oxide formation, the complex
plane plots exhibit some dispersion at low frequencies; however, at
more positive potentials, 0.6 and 0.8 V, the complex plane plots are
slightly depressed semicircular loops.
8
sorption charge in voltammograms in H SO . The linear depen-
2
4
dence with a slope of one shows that the electrode capacitance is
directly related to the surface roughness. These results also demon-
strate that the differences in capacitance associated with the different
films are related only to the differences in surface area and not due
to differences in surface chemistry.
The semicircular loop is associated with charge transfer at the
interface and was modeled using an equivalent circuit with a resis-
tance, R , in series with a resistance, R , in parallel with a constant
s
p
phase element, CPE. The total impedance of the equivalent circuit is
n
R + R + R R Q͑i␻͒
s
p
s p
Z͑␻͒ =
͓1͔
Platinum microelectrodes.— Since stimulation and recording ap-
plications require microelectrodes, we have characterized the perfor-
mance of electrodeposited platinum microelectrodes. Figure 7 shows
voltammograms for platinum microelectrodes in 250 mM H SO .
n
1
+ R Q͑i␻͒
p
where the CPE is represented by an equivalent capacitance, Q, and
18
an exponent, n. In most cases, fitting produced n Ͼ 0.9, and we
take Q = C.
2
4
The voltammogram for the platinum microelectrode with no elec-
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Journal of The Electrochemical Society, 153 ͑12͒ C834-C839 ͑2006͒
C837
Figure 6. ͑a͒ The logarithm of the parallel capacitance vs potential for an
annealed polycrystalline platinum foil and electrodeposited platinum thin
films in PBS. ͑b͒ Exponent n vs potential. ͑c͒ Surface roughness vs the
capacitance measured at 0.4 V. The surface roughness was determined from
the hydrogen desorption charge in voltammograms in 50 mM H SO based
2
4
−
2
on a monolayer desorption charge of 210 ␮C cm . ͑ڣ͒ Udep = −0.5 V, ͑᭝͒
Udep = −0.6 V, ͑ᮀ͒ Udep = −0.7 V, ͑ϫ͒ Udep = −0.4 V, ͑᭺͒ foil.
The voltammograms for the electrodeposited microelectrodes,
shown in Fig. 7b, have the same general features, although the cur-
rents are larger, especially for the films deposited at −0.5 and
−
0.6 V.
Figure 8 shows the surface roughness of the electrodeposited
microelectrodes obtained from the hydrogen desorption charge. The
roughness factor for the microelectrode deposited at −0.4 V is about
Figure 5. Complex plane plots for electrodeposited platinum films in PBS.
The deposition potentials are indicated in each figure.
4
. In contrast, the roughness factor for the high-surface-area elec-
trodes deposited at −0.5 and −0.6 V are more than an order of
magnitude larger. The films deposited at more negative potentials
have roughness factors in the range 10–15.
trodeposited platinum ͑Fig. 7a͒ shows the same general features as
annealed polycrystalline platinum films with two well-defined hy-
drogen adsorption and desorption peaks, a double-layer charging
region, an oxide formation plateau, and an oxide reduction peak.
Charge injection experiments were performed on the electrode-
posited platinum microelectrodes using a biphasic current pulse, as
illustrated in Fig. 9. The charge needed to depolarize a cell is typi-
cally 5–500 nC, depending on the cell type, proximity to the cell,
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C838
Journal of The Electrochemical Society, 153 ͑12͒ C834-C839 ͑2006͒
Figure 7. Current–voltage curves for ͑a͒ polycrystalline platinum microelec-
trode and ͑b͒ electrodeposited microelectrodes in 250 mM H SO , pH 1.8.
2
4
Figure 9. ͑a͒ Schematic illustration of the biphasic waveform. ͑b͒ Potential–
time transients recorded for electrodeposited platinum microelectrodes.
The potential range was −0.2 to − 1.25 V, and the scan rate was
−
1
2
50 mV s . The deposition potentials are indicated in the figure.
9,21
and pulse duration.
The charge injected during each half-cycle in
1
00 mV. The films deposited at −0.7 and −0.8 V ͑Ag/AgCl͒ exhibit
these experiments was 200 nC ͑corresponding to a geometric charge
density of about 7.1 ϫ 10−5 C cm2͒.
intermediate behavior. These results are consistent with the surface
roughness values for the microelectrodes shown in Fig. 8. On
switching the polarity of the current to +200 ␮A the potential of the
microelectrode deposited at −0.4 V ͑Ag/AgCl͒ increases to about
Figure 9 shows the potential response ͑measured between the
working and counter electrodes͒ to ±200 ␮A current pulses 1 ms in
duration. The responses of all electrodes exhibit an initial, rapid
change in potential followed by a longer period of more gradual
potential displacement. On applying −200 ␮A, the initial potential
decrease of about 10 mV corresponds to the IR drop in the system
0
.25 V. The other electrodes show much smaller potential excur-
sions, increasing to about +0.1 V.
In considering the performance of implantable electrodes, the
two most important reactions to consider are hydrogen and oxygen
evolution. The equilibrium potential for the hydrogen couple at this
pH is Ueq = −0.644 V ͑Ag/AgCl͒, and the current onset in the vol-
tammogram at −0.84 V ͑Ag/AgCl͒ corresponds to an overpotential
of about 200 mV. The equilibrium potential for oxygen at this pH is
Ueq = 0.579 V ͑Ag/AgCl͒ and the current onset in the voltammo-
grams at 0.78 V ͑Ag/AgCl͒ corresponds to an overpotential of
about 200 mV. We should also consider the dissolution reaction for
͑
about 100 ⍀͒. The microelectrode deposited at −0.4 V exhibits a
large potential excursion, decreasing to about −0.4 V after 1 ms. In
contrast, the high-surface-area films deposited at −0.5 and −0.6 V
͑
Ag/AgCl͒ show relatively small potential excursions of less than
o
eq
2−
4
which the relevant equilibrium potential U ͑Pt/PtCl ͒ = 0.558 V
͑
Ag/AgCl͒.
The open circuit potentials ͑OCPs͒ of the electrodeposited plati-
num microelectrodes during the current pulse experiments were in
the range 0.329–0.352 V ͑Ag/AgCl͒. Thus, the most negative po-
tential for the electrodeposited microelectrodes in response to the
−
−
200 ␮A current pulse of about −0.4 V corresponds to about
0.07 V ͑Ag/AgCl͒, well above the equilibrium potential for hydro-
gen evolution. Similarly, the most positive potential in response to
the 200 ␮A current is about 0.11 or 0.44 V ͑Ag/AgCl͒ for the plati-
num electrodes except those deposited at −0.4 V ͑Ag/AgCl͒. The
electrodes deposited at −0.4 V exhibit a potential of 0.26 or 0.59 V
͑
Ag/AgCl͒, above the equilibrium potentials for oxygen evolution
and dissolution.
Figure 8. Surface roughness for electrodeposited platinum microelectrodes
obtained from the hydrogen desorption charge in the potential range from
Thus, the microelectrodes deposited at potentials between −0.5
and −0.8 V ͑Ag/AgCl͒ can deliver 200 nC of charge without evo-
−
0.2 to 0.1 V ͑assuming 210 ␮C cm−2 for monolayer desorption͒.
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Journal of The Electrochemical Society, 153 ͑12͒ C834-C839 ͑2006͒
C839
lution of hydrogen or oxygen. As described above, the injected
charge of 200 nC is much larger than typically needed to depolarize
a cell, especially for electrodes in close proximity to the cell. Indeed,
the high-surface-area films deposited at −0.5 and −0.6 V
Johns Hopkins University assisted in meeting the publication costs of this
article.
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