Research Article
www.acsami.org
Electrochemical Rectification of Redox Mediators Using PorphyrinBased Molecular Multilayered Films on ITO Electrodes
Marissa R. Civic and Peter H. Dinolfo*
Department of Chemistry and Chemical Biology, Rensselaer Polytechnic Institute, 125 Cogswell, 110 Eighth Street, Troy, New York
12180, United States
S Supporting Information
*
ABSTRACT: Electrochemical charge transfer through multilayer thin films of zinc and nickel 5,10,15,20-tetra(4ethynylphenyl) porphyrin constructed via copper(I)-catalyzed
azide−alkyne cycloaddition (CuAAC) “click” chemistry was
examined. Current rectification toward various outer-sphere
redox probes is revealed with increasing numbers of layers, as
these films possess insulating properties over the neutral
potential range of the porphyrin, then become conductive upon
reaching its oxidation potential. Interfacial electron transfer
rates of mediator−dye interactions toward [Co(bpy)3]2+,
[Co(dmb)3]2+, [Co(NO2-phen)3]2+, [Fe(bpy)3]2+, and ferrocene (Fc), all outer-sphere redox species, were measured by
hydrodynamic methods. The ability to modify electroactive films’ interfacial electron transfer rates, as well as current rectification
toward redox species, has broad applicability in a number of devices, particularly photovoltaics and photogalvanics.
KEYWORDS: electrochemical rectification, layer-by-layer assembly, wall-jet electrochemistry, interfacial electron transfer rates,
multilayer electrochemistry
■
INTRODUCTION
Tailoring electrode surfaces to control charge transfer reactions
is a critical aspect in the design of many types of electronic
devices. Macromolecular electronics,1 biosensors,2−4 catalytic
devices,5 and solar energy devices,6 among others, all utilize
current rectification to successfully function. Researching
current rectification of electrodes is therefore of great interest
due to its broad application.2,3,7−12
Electrochemical current rectification is the process by which
charge transfer between a modified electrode surface and
solution-phase redox probe is promoted in one direction, while
limited in the opposite. The direction of charge transfer is
determined by the relative thermodynamic potentials of the
surface-bound species and redox probe. Figure 1 shows a
schematic representation of anodic current rectification of the
faradaic response of redox probe M at a modified electrode
surface. The direct charge transfer (both cathodic and anodic,
right side of Figure 1) between M and the electrode is blocked
by the insulating properties of the surface-bound film. Anodic
charge transfer is possible when the electrode reaches the
oxidation potential of the redox-active film, which then
mediates oxidation of M (left side of Figure 1).1 The cathodic
reaction, however, is blocked due to unfavorable thermodynamics of the film potential versus that required to reduce M.
Electrochemical current rectification was first reported by
Murray and co-workers between discrete layers of redox-active
polymers comprised of Fe, Ru, and Os polypyridyl complexes
on electrode surfaces.13−15 Since then, numerous other
examples have been described in the literature. Mallouk and
© 2016 American Chemical Society
Figure 1. Schematic representation of anodic current rectification of a
surface-confined redox-active film toward a solution-based redox
probe, M.
co-workers described the rectifying properties between cationic
and anionic redox probes at electrode surfaces modified with
zeolites, pillared clay particles, and zirconium−phosphonatebased multilayers.16 Wrighton and co-workers showed
rectification of electrodes coated with quinone- and viologenReceived: May 11, 2016
Accepted: July 13, 2016
Published: July 13, 2016
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and “sep” is sepulchrate. We proposed based on results that
thicker multilayer structures block electron recombination
reactions between the reduced mediator and underlying
electrode surface.
One of the major contributing factors to DSC inefficiencies is
photovoltage loss due to the relatively high potential of
commonly used I−/I3−.38 Efforts to replace I−/I3− with cobalt
complexes have increased significantly6 since an early report in
2001.39 One advantage of utilizing cobalt and other transition
metal coordination complexes is the ability to tune the redox
potential of the mediator to match that of the sensitizing
chromophore.40,41 A drawback of these outer-sphere electrontransfer reagents is the relatively fast recombination kinetics
between a photoinjected electron in the semiconductor and the
oxidized mediator as compared to I−/I3−. To reduce the
recombination rates, several approaches have been taken
including deposition of ultrathin layers of alumina on the
semiconductor surface,42−44 as well as increasing the steric bulk
of the mediators45 or dyes.46−48 In fact, the DSC with the
current record efficiency utilizes a [Co(bpy)3]2+/3+ redox
couple in conjunction with a bulky Zn porphyrin push−pull
sensitizer.46
As the future of these devices lies with these types of redox
mediators, the rectification property of our molecular multilayers is of current interest. In our earlier work on the
development of this multilayer fabrication technique, we were
able to observe electrochemical current rectification of the
faradaic response of [Co(bpy)3]2+/3+ with a ZnTPEP-multilayer
modified ITO electrode such as the one shown in Figure 2. The
multilayer film showed insulating properties in its neutral form,
which blocked the direct oxidation and reduction of [Co(bpy)3]2+/3+, and then became conductive upon reaching the
porphyrin oxidation potential, mediating the oxidation of
[Co(bpy)3]2+.31 This blocking layer experiment was initially
performed to test the quality of the film and look for defects,
but the aqueous electrolyte employed did not allow us to
directly examine the electrochemical response of the porphyrin
multilayer.
Herein we report molecular multilayers of ZnTPEP
assembled via CuAAC induce unidirectional current flow
toward various outer-sphere redox species, effectively acting
as a passivation layer on ITO working electrodes. We examine
this rectification in films with one through four layers of
ZnTPEP, as well as use wall-jet electrochemistry to determine
interfacial electron transfer rates between four layers of
ZnTPEP and outer-sphere redox species [Co(bpy)3]2+, [Co(dmb)3]2+, [Co(NO2-phen)3]2+, [Fe(bpy)3]2+, and Fc0/+.
based polymers could be controlled by the pH of the
electrolyte.10,17,18 Kubiak and co-workers were able to show
current rectification toward the methyl ester of cobaltocenium
carboxylate and methyl viologen via nickel clusters assembled
on Au electrodes.11 Several other examples are based on Au
electrodes modified with n-alkanethiols. Uosaki et al. showed
that ferrocene-terminated undecanethiol on Au could mediate
the reduction of FeIII in solution.19 Creager et al. was able to
show rectification with Au electrodes modified with ferrocenecarboximide-terminated alkanethiols of various lengths toward
ferrocyanide ions in solution.7 Crooks et al. have shown
ferrocyanide ions can be effectively rectified at a gold electrode
via redox-active dendrimers and n-alkanethiols.9 Recently, the
Haga20−22 and van der Boom23,24 groups, among others,25 have
examined the electrochemical properties of mixed multilayer
films comprised of Ru, Os, and Fe polypyridyl complexes
assembled on indium tin oxide (ITO) electrodes. Through
judicious selection of the linking groups and molecular
precursors, they have been able to control the direction and
rates of electron transport through the multilayer films.
Our group has developed a methodology utilizing copper(I)catalyzed azide−alkyne [3 + 2] cycloaddition chemistry
(CuAAC)26,27 to construct molecular multilayer assemblies
on electrode surfaces using a range of building blocks including
porphyrins28−31 and perylene diimides.32 This method employs
a layer-by-layer (LbL) approach that links alternating molecular
components through self-limiting reactions at an interface. LbL
methods, a bottom-up approach to nanoscale fabrication, are
facile, broadly applicable, and easily implemented into surface
functionalization techniques.33−36 Figure 2 shows a schematic
■
RESULTS & DISCUSSION
Selection of Redox Probes. Figure 3 contains each redox
probe examined with its respective potential. All potentials
listed herein are referenced to the ferrocene/ferrocinium redox
(Fc0/+) couple. Polypyridyl cobalt complexes were chosen due
their outer-sphere redox mechanism and their recent
incorporation into DSCs.6,49 These species’ redox potentials
are easily tunable and were chosen to approach that of
ZnTPEP. An iron polypyridyl species [Fe(bpy)3](PF6)2 was
selected with a midpoint potential past the first oxidation of
ZnTPEP.
Current Rectification Studies. The top panel of Figure 4
contains representative cyclic voltammograms (CVs) for one
through four layers of ZnTPEP assembled on an ITO electrode
and are consistent with previous reports.30 All electrochemistry
Figure 2. Schematic representation of porphyrin-based molecular
multilayers fabricated using CuAAC in a LbL approach on ITO
electrodes.
of Zn(II) 5,10,15,20-tetra(4-ethynylphenyl)porphyrin
(ZnTPEP) and 1,3,5-tris(azidomethyl)benzene (N3-Mest)
multilayers used in this study assembled on ITO electrodes.
Our previous studies demonstrated an increase in performance of p-type dye-sensitized solar cells (DSC) incorporating
ZnTPEP multilayer functionalized photocathodes.37 Our
results indicated photovoltage was improved with increasing
layers when the commonly used iodide/triiodide redox couple
was replaced with cobalt-based outer-sphere redox shuttles
[Co(bpy)3]3+ and [Co(sep)]3+, where “bpy” is 2,2′-bipyridine
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Figure 3. Midpoint potentials for redox probes examined.
herein was collected using anhydrous acetonitrile with 0.1 M
tetrabutylammonium hexafluorophosphate (TBAPF6) electrolyte under a nitrogen atmosphere. The peak currents for the
ZnTPEP0/+ at +0.50 V and ZnTPEP+/2+ at +0.81 V show a
linear response with scan rate, consistent with a surface-bound
species (Figure S1).50 The integrated charge for both waves
also increases with additional layers of ZnTPEP added to the
film, closely matching the increase in UV−visible absorbance
(Figure S2).30
The lower panel of Figure 4 shows CVs for [Co(bpy)3]2+
over varying layers of ZnTPEP on ITO. A clean working
electrode produces a typical faradaic response for diffusioncontrolled species centered at −0.27 V. Addition of one to four
layers of ZnTPEP to an ITO working electrode via the
CuAAC-based LbL methodology changes the current response
for [Co(bpy)3]2+/3+. The oxidation wave shifts anodically with
increasing layers until it reaches the onset of ZnTPEP film
oxidation at ca. +0.30 V and remains consistent after two layers.
The oxidation wave sharpens and maintains approximately the
same current density at ca. +0.25 V from two to four layers.
The ZnTPEP film has a driving force of ∼0.77 V toward
[Co(bpy)3]2+ oxidation, thus allowing an alternate pathway for
mediator oxidation via the multilayer film. On the one hand,
the lack of any observed current at the standard potential of the
[Co(bpy)3]2+ implies the film is densely packed with minimal
pinhole defects. On the other hand, when examining the
reductive return wave for [Co(bpy)3]3+/2+, adding successive
layers of ZnTPEP results in a cathodic shift and a diminished
current density. This effect is due to the insulating properties of
the neutral ZnTPEP film, which would require a tunneling
mechanism for charge transfer from the underlying ITO
electrode to [Co(bpy)3]3+. The combined effects of mediated
oxidation of [Co(bpy)3]2+ by ZnTPEP+ and diminished
cathodic response due to the insulating nature of the neutral
film result in a large current rectification response for
[Co(bpy)3]2+/3+ that increases with thickness of the ZnTPEP
multilayer film.
Figure 5 shows CVs of all mediators at an ITO electrode
modified with four layers of ZnTPEP. The rectification effect
toward the mediators is greatest for the thickest films examined.
CVs of each mediator at one through four layers of ZnTPEP
are available in the Supporting Information. A significant peak
splitting is seen for [Co(dmb)3]2/3+ (Figure S3), [Co(bpy)3]2/3+ (Figure 4), and [Co(NO2-phen)3]2/3+ (Figure S4)
Figure 4. (top) CVs of one through four layers of ZnTPEP on ITO.
(bottom) CVs of a 1 mM solution [Co(bpy)3]2+ at a clean ITO
electrode (solid red line), SAM-functionalized ITO (long dashed
orange), and one (dashed yellow), two (short dashed green), three
(dash-dot cyan), and four (purple dash-dot-dot) layers of ZnTPEP on
ITO. The vertical dashed black lines are located at the ZnTPEP0/+ and
ZnTPEP+/2+ potentials.
after addition of only one layer of ZnTPEP due to the
insulating nature of the multilayer film. CVs of these mediators
at two or more layers shifts the oxidation wave anodically to the
onset of the film oxidation, consistent with mediated charge
transfer via ZnTPEP0/+ as outlined in Figure 1. The reductive
wave continues to decrease in peak current and shift
cathodically due to the increasing thickness of the neutral
film, consistent with a tunneling process. This results in an
increasing rectification effect that increases with additional
ZnTPEP layers. The hypothesis for ZnTPEP0/+ mediated
charge transfer from the solution mediators is supported by
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To confirm the observed oxidative shift of redox probes’
oxidation is due to electrochemical mediation via ZnTPEP+, an
analogous study was done over a film of four layers of NiTPEP.
NiTPEP has an oxidation potential of 0.74 V, which is 240 mV
more positive than that of ZnTPEP (0.50 V). Because of
NiTPEP’s higher potential, the anodic current response for the
redox probes should shift a corresponding amount if the
mediated process dominates the anodic current response for
thicker films. Figure 7 compares CVs of [Co(dmb)3]2+ over
Figure 7. CVs of 1 mM [Co(dmb)3]2+ over four layers of ZnTPEP
(red line) and NiTPEP (black line) on ITO. The oxidation of the
NiTPEP4 films appears as a reversible wave at +0.7 V.
Figure 5. CVs for [Co(bpy)3]2+ (purple line), [Co(dmb)3]2+ (dashdotted cyan), [Co(NO2-phen)3]2+ (dashed blue), Fc (medium dashed
green), and [Fe(bpy)3]2+ (short dashed red) over four layers of
ZnTPEP on ITO. The vertical dashed black lines are located at the
ZnTPEP0/+ and ZnTPEP+/2+ oxidation potentials, while the smaller
solid lines are located at each mediator’s thermodynamic potential.
four layers of ZnTPEP and NiTPEP. The oxidative peak for
[Co(dmb)3]2+ at a NiTPEP4 modified electrode is shifted +680
mV versus its standard potential, as compared to a +490 mV
shift for ZnTPEP4. The additional +190 mV shift is similar to
the difference in oxidation potential of the two porpohyrins and
is consistent with the mediated oxidation of [Co(dmb)3]2+ by
the films. The CVs of [Co(bpy)3]2+ and [Co(NO2-phen)3]2+ at
a NiTPEP4 modified electrode show similar anodic shifts
(Figure S5).
The electrochemistry of Fc at ZnTPEP and NiTPEP
modified ITO electrodes does not result in an anodic shift of
the oxidation wave as is seen for the larger charged cobalt
complexes. The smaller, neutral Fc may be able to penetrate the
multilayer films, resulting in oxidation at the thermodynamic
potential. There is a slight decrease in the cathodic current at
higher layers of ZnTPEP modified electrodes. This apparent
rectification effect is likely the result of the ferrocinium ions
being expelled from the nonpolar neutral film. However, Fc
oxidation is clearly not mediated by the ZnTPEP, as the peak
current does not shift toward the ZnTPEP0/+ wave; rather, it
remains centered at the thermodynamic potential (Figure
S6).51−54 A similar response is seen for NiTPEP films (Figure
S5).
The electrochemical response of Fe(bpy)32+ over a ZnTPEP
film presents a different situation. The midpoint potential of
Fe(bpy)32/3+ (+0.60 V) is +0.10 V more positive than
ZnTPEP0/+ and +0.21 V less than ZnTPEP+/2+. As a result,
electrochemically generated ZnTPEP+ cannot oxidize the
[Fe(bpy)3]2+. However, ZnTPEP2+ does have the driving
force required to oxidize [Fe(bpy)3]2+. Thus, there is a slight
anodic shift of the oxidation of Fe(bpy)32+ to the onset of the
ZnTPEP2/+ wave with increasing layers (Figure S7). A
rectifying effect is expected here, similar to the Co mediators,
CVs taken over a range of mediator concentrations. Figure 6
shows CVs of 0.1, 0.3, and 1.0 mM [Co(dmb)3]2/3+ at an ITO
electrode modified with four layers of ZnTPEP. As
concentration of the redox probe is increased, the anodic
current increases as expected with the onset potential
remaining the same.
Figure 6. CVs of [Co(dmb)3]2+ over a film of four layers of ZnTPEP
on ITO. Black solid line, blue long dash, and red medium dash are
1000, 300, and 100 μM respectively. The dashed black line at 0.5 V
corresponds to the midpoint potential of ZnTPEP0/+.
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although Fe(bpy)33+ has the thermodynamic driving force
required to oxidize ZnTPEP. Thus, the reduction peak of
Fe(bpy)33+ is shifted cathodically to the ZnTPEP0/+ wave.
However, the maximum peak splitting of Fe(bpy)33+ and
rectifying effect is limited by the close spacing at the
ZnTPEP0/+ and ZnTPEP2/+ potentials. The response of
[Fe(bpy)3]2+ at a NiTPEP4 film further supports this (Figure
S5D). The oxidation potential of NiTPEP is almost identical to
that of [Fe(bpy)3]2+. At four layers of NiTPEP there is no
observable peak splitting as compared to a bare electrode,
suggesting the NiTPEP film can also mediate both the
oxidation and reduction process.
Interfacial Electron Transfer Rates. For these multilayer
films to be applicable in devices utilizing electrochemical
rectification, the interfacial electron transfer rates need to be
determined and optimized. We employed a forced-convection
hydrodynamic flow method to provide a quantitative analysis of
the interaction between mediators and the oxidized films.
Rotating disk electrochemistry (RDE) is the typical hydrodynamic method used to evaluate interfacial electron transfer
rates; however, it is difficult to design RDE electrodes
containing conductive ITO glass.55 Wall-jet electrochemistry
offers an alternative forced-convection hydrodynamic method
that allows for the use of a wider range of planar electrode
materials, including ITO.56−60
To conduct wall-jet electrochemistry experiments, we
constructed our own apparatus based upon various groups’
designs (Figure S11).55,57 This apparatus, in brief, is composed
of a planar electrode with a defined working area and a jet that
delivers a centered stream of electrolyte to continuously
replenish the redox species under investigation. The stream is
normal to the electrode and applies a hydrodynamic flow
profile, creating a limiting current characteristic of a diffusioncontrolled electrochemical system. Provided that the electrode
diameter is sufficiently larger than that of the jet, the current
density across the electrode interface can be treated similarly to
RDE.57,59 Equation 1 is the representative Levich equation for a
wall-jet hydrodynamic system, where ilim is the limiting current
(A), n is the number of electrons, F is Faraday’s constant (C/
mol e−), C is the concentration of the redox species (M), DS is
the diffusion coefficient of the species, ν is the kinematic
viscosity (cm2/s), a is the diameter of the jet determined by the
gauge of the needle (cm), A is the working electrode area
(cm2), and υ is the flow rate (cm3/s).57,59
ilim = 0.898nFCDS2/3ν−5/12a−1/2A3/8υ3/4
Equation 4 introduces keff, which represents the rate constant
without factoring in the coverage of the surface-bound
electroactive species.
keff = ΓfilmkET
Figure 8 contains two examples of linear sweep voltammograms (LSVs) of redox mediators (Fc0 and [Co(dmb)3]2+) over
Figure 8. LSVs of 1 mM Fc (top) and 1 mM [Co(dmb)3]2+ (bottom)
at varying flow rates. Vertical line is at the midpoint potential of the
ZnTPEP0/+ film.
four layers of ZnTPEP on ITO taken using our wall-jet forced
convection instrument. Corresponding LSVs for remaining
mediators are included in the Supporting Information (Figures
S12−S14).
A Levich analysis (Figure S15) performed on each redox
mediator over four layers of ZnTPEP on ITO reveals a linear
limiting current with respect to ν0.75 (flow rate), confirming an
electrochemical system that is mass-transfer limited.50,57 A
Koutecky−Levich analysis for each mediator is shown in Figure
9. If the inverse of the limiting current is linear with respect to
ν−0.75, the intercept reveals the kinetic limiting current, as
shown previously in eq 2.
Application of the rate law (eqs 3 and 4) reveals the
interfacial electron transfer rate constants between the redox
mediators and the ZnTPEP film on the working electrode.
These values are included in Table 1. Limiting currents were
taken at the potential of ZnTPEP, except in the case of
[Fe(bpy)3]2+. A completely limiting current of [Fe(bpy)3]2+
was difficult to achieve, as scanning beyond 1 V irreversibly
damages the films. In this case, the ilim was taken at 0.99 V, the
section of the LSV that exhibited the most ilim character. The
(1)
Taking advantage of this flow profile allows determination of
interfacial rate constants through the Koutecky−Levich relation
(eq 2),50,55,57 where ik is the kinetically limited current.
1
ilim
=
1
1
+
2/3 −5/12 −1/2 3/8 3/4
ik
0.898nFCDS ν
a
A υ
(2)
The following rate law (eq 3) can then be applied, where
Γfilm is the film coverage (mol/cm2), and kET is the interfacial
electron transfer rate constant. Film coverage was determined
by integrating surface CVs for four layers of ZnTPEP and
dividing that obtained coverage value by four to approximate
the coverage at the surface of the film in contact with the
mediator.
iK = nFA ΓfilmCkET
(4)
(3)
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calculated values in Table 1 will be a product of these rates. We
are currently exploring the thickness and molecular linker
dependence on the through-film charge transfer rates of these
porphyrin multilayers systems.
■
CONCLUSIONS
We have examined the surface passivation properties of films
assembled via CuAAC click chemistry. We have shown our
films possess a rectifying property that we have demonstrated
toward five different outer-sphere redox mediators (Fc0/+,
[Co(bpy) 3 ] 2+ , [Co(dmb) 3 ] 2+ , [Co(NO 2 -phen) 3 ]2+, [Fe(bpy)3]2+). In these systems, current is mediated by the
electroactive film when in contact with a redox probe. This
process was examined by cyclic voltammetry. Using hydrodynamic forced convection voltammetry, we calculated
interfacial electron transfer rates between the mediators and
films of ZnTPEP. The inherent ability for films assembled via
this LbL method to encourage unidirectional current flow
toward outer-sphere redox species has broad application in
many types of devices.1,2,13
Figure 9. Koutecky−Levich analysis of each mediator: Fc (orange ▼);
[Co(bpy)3]2+ (red ●); [Co(dmb)3]2+ (yellow ■); [Co(NO2phen)3]2+ (green ◆); and [Fe(bpy)3]2+ (teal ▲). Limiting current
was taken at 0.50 V (ZnTPEP0/+) for all mediators except
[Fe(bpy)3]2+, which was taken at 0.99 V.
■
Table 1. Interfacial Electron Transfer Rates and Driving
Force for Electron Transfer for Oxidation of Various Redox
Mediators (1 mM) at a Working Electrode with Four Layers
of ZnTPEP
mediator
Fc
[Co(bpy)3]2+
[Co(dmb)3]2+
[Co(NO2phen)3]2+
[Fe(bpy)3]2+
a
kET [(cm3)(mol−1)(s−1)]
9.4
5.4
6.5
3.4
×
×
×
×
8
10
108
108
108
3.3 × 108
0/+
keff [(cm)(s−1)]
1.5
4.4
5.3
2.8
×
×
×
×
Materials. All solvents, ACS grade or higher, were purchased from
Sigma-Aldrich or Fisher Scientific and used as received. Anhydrous
acetonitrile used for all electrochemical experiments was purged with
nitrogen and recirculated for 48 h through a solid-state column
purification system (Vacuum Atmospheres Company, Hawthorne,
CA) prior to use.61 Toluene was purged with nitrogen and stored
under nitrogen over 4 Å molecular sieves. Sodium ascorbate (Aldrich)
was used as received. Zinc(II) 5,10,15,20-tetra(4-ethynylphenyl)
porphyrin (ZnTPEP),62 nickel(II) 5,10,15,20-tetra(4-ethynylphenyl)
porphyrin (NiTPEP),29 tris(benzyltriazolylmethyl)amine (TBTA),63
1,3,5-tris(azidomethyl)benzene , 6 4 and (3-azidopropyl)trimethoxysilane65 were synthesized according to literature methods.
Indium tin oxide (ITO) coated glass was purchased from Delta
Technologies (polished float glass slides, one-side coated, Rs of 4−8
Ω). TBAPF6 (Aldrich) was recrystallized twice from hot ethanol and
dried under vacuum overnight. Ferrocene (Aldrich) was purified via
sublimation.
Cobalt and Iron Complexes. All polypyridyl cobalt and iron
complexes [Co(bpy)3](PF6)2, [Co(dmb)3](PF6)2, and [Co(NO2phen)3](PF6)2 were synthesized following literature procedure47
with slight modifications. In minimal methanol, 1 equiv of CoCl2·
6H2O and 3.3 equiv of the corresponding ligand were stirred until
dissolved. The resulting solution was yellow to brown. The solution
was stirred under reflux for ∼4 h. To form the complex with the
hexafluorophosphate counterion, a saturated, aqueous solution of
potassium hexafluorophosphate was added dropwise, until no further
precipitation occurred. The resulting solid was filtered, washed well
with methanol and then diethyl ether, and dried under high vacuum
overnight. The product was further purified via anion exchange from
the PF6− salt to the Cl− salt and back to PF6− to ensure high purity.
Exchange to the Cl− salt was accomplished by dissolving the PF6− salt
in acetone and adding saturated TBACl, until no further precipitate
formed. The resulting solid was filtered and washed with acetone,
methanol, and diethyl ether. After each step involving solvation, the
dissolved complex was filtered through a fine frit.
[Fe(bpy)3](PF6)2 was synthesized following literature procedure
with slight modifications.66 Briefly, FeCl2·6H2O was dissolved in a
minimal amount of water. The 2,2′-dipyridyl (3 equiv) was added
slowly with stirring to form a dark red solution. A saturated aqueous
solution of sodium chloride was added to assist with recrystallization of
dark red solid, [Fe(bpy)3]Cl2. The chloride salt was anion exchanged
to the hexafluorophosphate salt as described vide supra for cobalt
complexes. Purity of all complexes was confirmed with both NMR and
cyclic voltammetry. Cyclic voltammograms for each redox probe are
included in Figure 4 and Figure S7.
ΔGET (V)
10−1
10−2
10−2
10−2
−0.50a
−0.70a
−0.77a
−0.12a
2.7 × 10−2
EXPERIMENTAL SECTION
−0.22b
b
Calculated from the ZnTPEP potential at 0.50 V. Calculated
from the ZnTPEP+/2+ potential at 0.81 V.
Levich analysis for [Fe(bpy)3]2+, therefore, is less linear than
the other mediators.
Examining the rates for each redox probe, values vary; Fc0
has the highest, while [Fe(bpy)3]2+ has the lowest, as evidenced
by its incomplete rectification seen in Figure S6. As Fc0 is the
smallest of the five, it is likely that it penetrates the films,
increasing the current and hence the kET. Outer-sphere cobalt
redox mediators are considerably larger species, and their kET
values are an order of magnitude smaller than that of Fc0.
Cobalt species also undergo a change in spin upon oxidation/
reduction, hence lengthening the time of electron transfer
processes.
Comparing ΔGET for the oxidation of cobalt redox probes by
ZnTPEP+ (ZnTPEP2+ for [Fe(bpy)3]2+) to the trend in kET for
this process, an increase in rate is seen for species with the
greatest driving force for electron transfer ([Co(dmb)3]2+ and
[Co(bpy)3]2+). Driving force for [Co(NO2-phen)3]2+ is 0.65 V
less than the bipyridyl-based species, and hence a slower kET is
observed. With a driving force of −0.22 V for oxidation of
[Fe(bpy)3]2+ by ZnTPEP2+, this redox probe results in an
electron transfer rate comparable to cobalt-based species.
However, the similarity in kET for these species (less than a
factor of 2) for a wide range of ΔGET (0.65 V) suggests that the
interfacial electron transfer process is not the limiting factor.
Rather, electron transfer, or hopping, through the porphyrin
multilayer film may be limiting the overall interfacial rate, as the
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ACS Applied Materials & Interfaces
■
Azido Self-Assembled Monolayer Formation. Prior to selfassembled monolayer (SAM) formation, ITO-coated glass slides were
sonicated in dilute Alconox for 10 min then rinsed well with deionized
(DI) water. Slides were rinsed successively with acetone, dichloromethane, methanol, and water and dried with a stream of nitrogen, and
submersed in 1:1:5 NH3/H2O2/H2O for 30 min. Upon removal, the
slides were rinsed well with DI water, dried under a stream of nitrogen,
and placed to further dry in a Schlenk flask at 0.1 mTorr for 1 h. The
appropriate volume of neat (3-azidopropyl)trimethoxysilane to make a
1 mM solution was added to the Schlenk tube, which was then
evacuated and purged with nitrogen three times. Dry toluene was
transferred via cannula to the Schlenk tube, ensuring slides were
submerged. The reaction vessel was then heated to 65−70 °C
overnight. When it cooled to room temperature, the ITO was removed
and sonicated in toluene for 5 min then rinsed with acetone,
dichloromethane, methanol, and water and dried under a stream of
nitrogen. Slides were annealed at 75 °C for 3 h under vacuum.
Multilayer Formation. Multilayers were assembled using our
previously reported method.31 In brief, for ethynyl porphyrin layers,
the SAM-functionalized ITO surface is placed for 10 min in contact
with a dimethyl sulfoxide (DMSO) solution containing less than 2%
water, 1.3 mM ZnTPEP, 0.33 mM CuSO4, 0.36 mM TBTA, and 0.48
mM sodium ascorbate. After the 10 min treatment, the slide was rinsed
with acetone, dichloromethane, methanol, 5 mM ethylenediaminetetraacetic acid in 50/50 water/ethanol, and water and then dried under
a stream of nitrogen. The azido-linker reaction consists of a DMSO
solution containing less than 8% water, 2.2 mM 1,3,5-tris(azidomethyl)benzene, 4.4 mM CuSO4, 4.8 mM TBTA, and 8.9
mM sodium ascorbate. Treatment for the linker layer was also 10 min,
and the same rinsing scheme was used. Multilayer growth was
confirmed using UV−vis spectroscopy. Electronic absorbance spectra
were collected using a PerkinElmer Lambda 950 spectrophotometer
with a solid sample holder that orients the sample normal to incident
light. Background spectra of the SAM-functionalized ITO surface were
subtracted from each spectrum.
Electrochemistry. All cyclic voltammetry experiments were
performed using a CHI440A (CH Instruments, Austin, TX)
potentiostat. Electrolyte solution for all experiments was 0.1 M
TBAPF6 in anhydrous acetonitrile and was both prepared and
deoxygenated with nitrogen immediately before use. All CVs were
referenced against a ferrocene internal standard.
Solution CVs were collected using a 1 mM solution of the
corresponding mediator in 0.1 M TBAPF6 in anhydrous acetonitrile.
The three electrode system used consisted of a Pt counter electrode, a
Pt disk working electrode, and a Ag/AgCl wire pseudoreference
electrode. The headspace of the cell was purged with a very low stream
of nitrogen during scans.
Electrochemical rectification CVs were collected using a threeelectrode setup containing a Pt wire counter electrode and a Ag/AgCl
wire pseudoreference electrode. The working electrode was a
functionalized ITO electrode defined by the cylindrically bored area
of a Teflon cone pressed against the slide and filled with either 0.1 M
TBAPF6 electrolyte or 1 mM solution of the corresponding mediator
in 0.1 M TBAPF6.67 Scan rates for all surface scans were 1000 mV/s
and 100 mV/s for solutions. All scans were directed anodically.
Wall-jet experiments were conducted using an instrument consisting
of a Harvard Apparatus Model 11 Plus single syringe pump (70−
2208) containing a 10 mL gas-tight syringe with a needle that was cut
and filed flat. The needle was lowered into a cell so that it was 4 mm
above the surface of the working electrode. The cell design is described
elsewhere.31,67 In brief, a hole bored in the bottom of a cylindrical
Teflon cone defined the working area of the functionalized ITO
electrodes (0.15 cm2 area). The Teflon cone was pressed firmly against
the ITO electrode and filled with electrolyte and mediator (0.1 M
TBAPF6 and 1 mM mediator in SPS acetonitrile). A Teflon plug was
placed in the top of the cone with holes bored to fit snugly a nitrogen
line, the wall-jet syringe, a Pt wire counter electrode, and a Ag/AgCl
wire pseudoreference electrode. A Levich analysis of the redox
mediators at a bare ITO electrode are included in the Supporting
Information (Figure S16).
Research Article
ASSOCIATED CONTENT
S Supporting Information
*
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acsami.6b05643.
Additional electrochemical scans of the multilayer films
with redox mediators. (PDF)
■
AUTHOR INFORMATION
Corresponding Author
*E-mail: dinolp@rpi.edu.
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
M.R.C. acknowledges a Slezak Memorial Fellowship. This
material is based upon work supported by the National Science
Foundation under Contract No. CHE-1255100.
■
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