Electrochemical Rectification of Redox Mediators Using Porphyrin-Based Molecular Multilayered Films on ITO ElectrodesMarissa R. Civic and Peter H. Dinolfo*

Department of Chemistry and Chemical Biology, Rensselaer Polytechnic Institute, 125 Cogswell, 110 Eighth Street, Troy, New York12180, United States

*S Supporting Information

ABSTRACT: Electrochemical charge transfer through multi-layer thin films of zinc and nickel 5,10,15,20-tetra(4-ethynylphenyl) porphyrin constructed via copper(I)-catalyzedazide−alkyne cycloaddition (CuAAC) “click” chemistry wasexamined. Current rectification toward various outer-sphereredox probes is revealed with increasing numbers of layers, asthese films possess insulating properties over the neutralpotential range of the porphyrin, then become conductive uponreaching its oxidation potential. Interfacial electron transferrates 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 byhydrodynamic methods. The ability to modify electroactive films’ interfacial electron transfer rates, as well as current rectificationtoward 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 reactionsis a critical aspect in the design of many types of electronicdevices. Macromolecular electronics,1 biosensors,2−4 catalyticdevices,5 and solar energy devices,6 among others, all utilizecurrent rectification to successfully function. Researchingcurrent rectification of electrodes is therefore of great interestdue to its broad application.2,3,7−12

Electrochemical current rectification is the process by whichcharge transfer between a modified electrode surface andsolution-phase redox probe is promoted in one direction, whilelimited in the opposite. The direction of charge transfer isdetermined by the relative thermodynamic potentials of thesurface-bound species and redox probe. Figure 1 shows aschematic representation of anodic current rectification of thefaradaic response of redox probe M at a modified electrodesurface. The direct charge transfer (both cathodic and anodic,right side of Figure 1) between M and the electrode is blockedby the insulating properties of the surface-bound film. Anodiccharge transfer is possible when the electrode reaches theoxidation potential of the redox-active film, which thenmediates oxidation of M (left side of Figure 1).1 The cathodicreaction, however, is blocked due to unfavorable thermody-namics 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-activepolymers comprised of Fe, Ru, and Os polypyridyl complexeson electrode surfaces.13−15 Since then, numerous otherexamples have been described in the literature. Mallouk and

co-workers described the rectifying properties between cationicand anionic redox probes at electrode surfaces modified withzeolites, pillared clay particles, and zirconium−phosphonate-based multilayers.16 Wrighton and co-workers showedrectification of electrodes coated with quinone- and viologen-

Received: May 11, 2016Accepted: July 13, 2016Published: July 13, 2016

Figure 1. Schematic representation of anodic current rectification of asurface-confined redox-active film toward a solution-based redoxprobe, M.

Research Article

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© 2016 American Chemical Society 20465 DOI: 10.1021/acsami.6b05643ACS Appl. Mater. Interfaces 2016, 8, 20465−20473

based polymers could be controlled by the pH of theelectrolyte.10,17,18 Kubiak and co-workers were able to showcurrent rectification toward the methyl ester of cobaltoceniumcarboxylate and methyl viologen via nickel clusters assembledon Au electrodes.11 Several other examples are based on Auelectrodes modified with n-alkanethiols. Uosaki et al. showedthat ferrocene-terminated undecanethiol on Au could mediatethe reduction of FeIII in solution.19 Creager et al. was able toshow rectification with Au electrodes modified with ferrocene-carboximide-terminated alkanethiols of various lengths towardferrocyanide ions in solution.7 Crooks et al. have shownferrocyanide ions can be effectively rectified at a gold electrodevia redox-active dendrimers and n-alkanethiols.9 Recently, theHaga20−22 and van der Boom23,24 groups, among others,25 haveexamined the electrochemical properties of mixed multilayerfilms comprised of Ru, Os, and Fe polypyridyl complexesassembled on indium tin oxide (ITO) electrodes. Throughjudicious selection of the linking groups and molecularprecursors, they have been able to control the direction andrates 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 assemblieson electrode surfaces using a range of building blocks includingporphyrins28−31 and perylene diimides.32 This method employsa layer-by-layer (LbL) approach that links alternating molecularcomponents through self-limiting reactions at an interface. LbLmethods, a bottom-up approach to nanoscale fabrication, arefacile, broadly applicable, and easily implemented into surfacefunctionalization techniques.33−36 Figure 2 shows a schematic

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 perform-

ance of p-type dye-sensitized solar cells (DSC) incorporatingZnTPEP multilayer functionalized photocathodes.37 Ourresults indicated photovoltage was improved with increasinglayers when the commonly used iodide/triiodide redox couplewas replaced with cobalt-based outer-sphere redox shuttles[Co(bpy)3]

3+ and [Co(sep)]3+, where “bpy” is 2,2′-bipyridine

and “sep” is sepulchrate. We proposed based on results thatthicker multilayer structures block electron recombinationreactions between the reduced mediator and underlyingelectrode surface.One of the major contributing factors to DSC inefficiencies is

photovoltage loss due to the relatively high potential ofcommonly used I−/I3

−.38 Efforts to replace I−/I3− with cobalt

complexes have increased significantly6 since an early report in2001.39 One advantage of utilizing cobalt and other transitionmetal coordination complexes is the ability to tune the redoxpotential of the mediator to match that of the sensitizingchromophore.40,41 A drawback of these outer-sphere electron-transfer reagents is the relatively fast recombination kineticsbetween a photoinjected electron in the semiconductor and theoxidized mediator as compared to I−/I3

−. To reduce therecombination rates, several approaches have been takenincluding deposition of ultrathin layers of alumina on thesemiconductor surface,42−44 as well as increasing the steric bulkof the mediators45 or dyes.46−48 In fact, the DSC with thecurrent record efficiency utilizes a [Co(bpy)3]

2+/3+ redoxcouple in conjunction with a bulky Zn porphyrin push−pullsensitizer.46

As the future of these devices lies with these types of redoxmediators, the rectification property of our molecular multi-layers is of current interest. In our earlier work on thedevelopment of this multilayer fabrication technique, we wereable to observe electrochemical current rectification of thefaradaic response of [Co(bpy)3]

2+/3+ with a ZnTPEP-multilayermodified ITO electrode such as the one shown in Figure 2. Themultilayer 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 theporphyrin oxidation potential, mediating the oxidation of[Co(bpy)3]

2+.31 This blocking layer experiment was initiallyperformed to test the quality of the film and look for defects,but the aqueous electrolyte employed did not allow us todirectly examine the electrochemical response of the porphyrinmultilayer.Herein we report molecular multilayers of ZnTPEP

assembled via CuAAC induce unidirectional current flowtoward various outer-sphere redox species, effectively actingas a passivation layer on ITO working electrodes. We examinethis rectification in films with one through four layers ofZnTPEP, as well as use wall-jet electrochemistry to determineinterfacial electron transfer rates between four layers ofZnTPEP and outer-sphere redox species [Co(bpy)3]

2+, [Co-(dmb)3]

2+, [Co(NO2-phen)3]2+, [Fe(bpy)3]

2+, and Fc0/+.

■ RESULTS & DISCUSSIONSelection of Redox Probes. Figure 3 contains each redox

probe examined with its respective potential. All potentialslisted herein are referenced to the ferrocene/ferrocinium redox(Fc0/+) couple. Polypyridyl cobalt complexes were chosen duetheir outer-sphere redox mechanism and their recentincorporation into DSCs.6,49 These species’ redox potentialsare easily tunable and were chosen to approach that ofZnTPEP. An iron polypyridyl species [Fe(bpy)3](PF6)2 wasselected with a midpoint potential past the first oxidation ofZnTPEP.

Current Rectification Studies. The top panel of Figure 4contains representative cyclic voltammograms (CVs) for onethrough four layers of ZnTPEP assembled on an ITO electrodeand are consistent with previous reports.30 All electrochemistry

Figure 2. Schematic representation of porphyrin-based molecularmultilayers fabricated using CuAAC in a LbL approach on ITOelectrodes.

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herein was collected using anhydrous acetonitrile with 0.1 Mtetrabutylammonium hexafluorophosphate (TBAPF6) electro-lyte under a nitrogen atmosphere. The peak currents for theZnTPEP0/+ at +0.50 V and ZnTPEP+/2+ at +0.81 V show alinear response with scan rate, consistent with a surface-boundspecies (Figure S1).50 The integrated charge for both wavesalso increases with additional layers of ZnTPEP added to thefilm, 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 workingelectrode produces a typical faradaic response for diffusion-controlled species centered at −0.27 V. Addition of one to fourlayers of ZnTPEP to an ITO working electrode via theCuAAC-based LbL methodology changes the current responsefor [Co(bpy)3]

2+/3+. The oxidation wave shifts anodically withincreasing layers until it reaches the onset of ZnTPEP filmoxidation at ca. +0.30 V and remains consistent after two layers.The oxidation wave sharpens and maintains approximately thesame 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 formediator 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 minimalpinhole defects. On the other hand, when examining thereductive return wave for [Co(bpy)3]

3+/2+, adding successivelayers of ZnTPEP results in a cathodic shift and a diminishedcurrent density. This effect is due to the insulating properties ofthe neutral ZnTPEP film, which would require a tunnelingmechanism for charge transfer from the underlying ITOelectrode to [Co(bpy)3]

3+. The combined effects of mediatedoxidation of [Co(bpy)3]

2+ by ZnTPEP+ and diminishedcathodic response due to the insulating nature of the neutralfilm result in a large current rectification response for[Co(bpy)3]

2+/3+ that increases with thickness of the ZnTPEPmultilayer film.Figure 5 shows CVs of all mediators at an ITO electrode

modified with four layers of ZnTPEP. The rectification effecttoward the mediators is greatest for the thickest films examined.CVs of each mediator at one through four layers of ZnTPEPare available in the Supporting Information. A significant peaksplitting 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)

after addition of only one layer of ZnTPEP due to theinsulating nature of the multilayer film. CVs of these mediatorsat two or more layers shifts the oxidation wave anodically to theonset of the film oxidation, consistent with mediated chargetransfer via ZnTPEP0/+ as outlined in Figure 1. The reductivewave continues to decrease in peak current and shiftcathodically due to the increasing thickness of the neutralfilm, consistent with a tunneling process. This results in anincreasing rectification effect that increases with additionalZnTPEP layers. The hypothesis for ZnTPEP0/+ mediatedcharge transfer from the solution mediators is supported by

Figure 3. Midpoint potentials for redox probes examined.

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 ITOelectrode (solid red line), SAM-functionalized ITO (long dashedorange), and one (dashed yellow), two (short dashed green), three(dash-dot cyan), and four (purple dash-dot-dot) layers of ZnTPEP onITO. The vertical dashed black lines are located at the ZnTPEP0/+ andZnTPEP+/2+ potentials.

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CVs taken over a range of mediator concentrations. Figure 6shows CVs of 0.1, 0.3, and 1.0 mM [Co(dmb)3]

2/3+ at an ITOelectrode modified with four layers of ZnTPEP. Asconcentration of the redox probe is increased, the anodiccurrent increases as expected with the onset potentialremaining the same.

To confirm the observed oxidative shift of redox probes’oxidation is due to electrochemical mediation via ZnTPEP+, ananalogous study was done over a film of four layers of NiTPEP.NiTPEP has an oxidation potential of 0.74 V, which is 240 mVmore positive than that of ZnTPEP (0.50 V). Because ofNiTPEP’s higher potential, the anodic current response for theredox probes should shift a corresponding amount if themediated process dominates the anodic current response forthicker films. Figure 7 compares CVs of [Co(dmb)3]

2+ over

four layers of ZnTPEP and NiTPEP. The oxidative peak for[Co(dmb)3]

2+ at a NiTPEP4 modified electrode is shifted +680mV versus its standard potential, as compared to a +490 mVshift for ZnTPEP4. The additional +190 mV shift is similar tothe difference in oxidation potential of the two porpohyrins andis consistent with the mediated oxidation of [Co(dmb)3]

2+ bythe 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 ofthe oxidation wave as is seen for the larger charged cobaltcomplexes. The smaller, neutral Fc may be able to penetrate themultilayer films, resulting in oxidation at the thermodynamicpotential. There is a slight decrease in the cathodic current athigher layers of ZnTPEP modified electrodes. This apparentrectification effect is likely the result of the ferrocinium ionsbeing expelled from the nonpolar neutral film. However, Fcoxidation is clearly not mediated by the ZnTPEP, as the peakcurrent does not shift toward the ZnTPEP0/+ wave; rather, itremains centered at the thermodynamic potential (FigureS6).51−54 A similar response is seen for NiTPEP films (FigureS5).The electrochemical response of Fe(bpy)3

2+ over a ZnTPEPfilm presents a different situation. The midpoint potential ofFe(bpy)3

2/3+ (+0.60 V) is +0.10 V more positive thanZnTPEP0/+ 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 drivingforce required to oxidize [Fe(bpy)3]

2+. Thus, there is a slightanodic shift of the oxidation of Fe(bpy)3

2+ to the onset of theZnTPEP2/+ wave with increasing layers (Figure S7). Arectifying effect is expected here, similar to the Co mediators,

Figure 5. CVs for [Co(bpy)3]2+ (purple line), [Co(dmb)3]

2+ (dash-dotted cyan), [Co(NO2-phen)3]

2+ (dashed blue), Fc (medium dashedgreen), and [Fe(bpy)3]

2+ (short dashed red) over four layers ofZnTPEP on ITO. The vertical dashed black lines are located at theZnTPEP0/+ and ZnTPEP+/2+ oxidation potentials, while the smallersolid lines are located at each mediator’s thermodynamic potential.

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 are1000, 300, and 100 μM respectively. The dashed black line at 0.5 Vcorresponds to the midpoint potential of ZnTPEP0/+.

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 theNiTPEP4 films appears as a reversible wave at +0.7 V.

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although Fe(bpy)33+ has the thermodynamic driving force

required to oxidize ZnTPEP. Thus, the reduction peak ofFe(bpy)3

3+ is shifted cathodically to the ZnTPEP0/+ wave.However, the maximum peak splitting of Fe(bpy)3

3+ andrectifying effect is limited by the close spacing at theZnTPEP0/+ and ZnTPEP2/+ potentials. The response of[Fe(bpy)3]

2+ at a NiTPEP4 film further supports this (FigureS5D). The oxidation potential of NiTPEP is almost identical tothat of [Fe(bpy)3]

2+. At four layers of NiTPEP there is noobservable peak splitting as compared to a bare electrode,suggesting the NiTPEP film can also mediate both theoxidation and reduction process.Interfacial Electron Transfer Rates. For these multilayer

films to be applicable in devices utilizing electrochemicalrectification, the interfacial electron transfer rates need to bedetermined and optimized. We employed a forced-convectionhydrodynamic flow method to provide a quantitative analysis ofthe interaction between mediators and the oxidized films.Rotating disk electrochemistry (RDE) is the typical hydro-dynamic method used to evaluate interfacial electron transferrates; however, it is difficult to design RDE electrodescontaining conductive ITO glass.55 Wall-jet electrochemistryoffers an alternative forced-convection hydrodynamic methodthat allows for the use of a wider range of planar electrodematerials, including ITO.56−60

To conduct wall-jet electrochemistry experiments, weconstructed our own apparatus based upon various groups’designs (Figure S11).55,57 This apparatus, in brief, is composedof a planar electrode with a defined working area and a jet thatdelivers a centered stream of electrolyte to continuouslyreplenish the redox species under investigation. The stream isnormal to the electrode and applies a hydrodynamic flowprofile, creating a limiting current characteristic of a diffusion-controlled electrochemical system. Provided that the electrodediameter is sufficiently larger than that of the jet, the currentdensity across the electrode interface can be treated similarly toRDE.57,59 Equation 1 is the representative Levich equation for awall-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 isthe diffusion coefficient of the species, ν is the kinematicviscosity (cm2/s), a is the diameter of the jet determined by thegauge of the needle (cm), A is the working electrode area(cm2), and υ is the flow rate (cm3/s).57,59

ν υ= − −i nFCD a A0.898lim S2/3 5/12 1/2 3/8 3/4

(1)

Taking advantage of this flow profile allows determination ofinterfacial rate constants through the Koutecky−Levich relation(eq 2),50,55,57 where ik is the kinetically limited current.

ν υ= + − −i i nFCD a A

1 1 10.898k Slim

2/3 5/12 1/2 3/8 3/4(2)

The following rate law (eq 3) can then be applied, whereΓfilm is the film coverage (mol/cm2), and kET is the interfacialelectron transfer rate constant. Film coverage was determinedby integrating surface CVs for four layers of ZnTPEP anddividing that obtained coverage value by four to approximatethe coverage at the surface of the film in contact with themediator.

= Γi nFA CkK film ET (3)

Equation 4 introduces keff, which represents the rate constantwithout factoring in the coverage of the surface-boundelectroactive species.

= Γk keff film ET (4)

Figure 8 contains two examples of linear sweep voltammo-grams (LSVs) of redox mediators (Fc0 and [Co(dmb)3]

2+) over

four layers of ZnTPEP on ITO taken using our wall-jet forcedconvection instrument. Corresponding LSVs for remainingmediators are included in the Supporting Information (FiguresS12−S14).A Levich analysis (Figure S15) performed on each redox

mediator over four layers of ZnTPEP on ITO reveals a linearlimiting current with respect to ν0.75 (flow rate), confirming anelectrochemical system that is mass-transfer limited.50,57 AKoutecky−Levich analysis for each mediator is shown in Figure9. If the inverse of the limiting current is linear with respect toν−0.75, the intercept reveals the kinetic limiting current, asshown previously in eq 2.Application of the rate law (eqs 3 and 4) reveals the

interfacial electron transfer rate constants between the redoxmediators and the ZnTPEP film on the working electrode.These values are included in Table 1. Limiting currents weretaken 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 irreversiblydamages the films. In this case, the ilim was taken at 0.99 V, thesection of the LSV that exhibited the most ilim character. The

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 theZnTPEP0/+

film.

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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 thesmallest of the five, it is likely that it penetrates the films,increasing the current and hence the kET. Outer-sphere cobaltredox mediators are considerably larger species, and their kETvalues 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 transferprocesses.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 thegreatest 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 isobserved. With a driving force of −0.22 V for oxidation of[Fe(bpy)3]

2+ by ZnTPEP2+, this redox probe results in anelectron transfer rate comparable to cobalt-based species.However, the similarity in kET for these species (less than afactor of 2) for a wide range of ΔGET (0.65 V) suggests that theinterfacial electron transfer process is not the limiting factor.Rather, electron transfer, or hopping, through the porphyrinmultilayer film may be limiting the overall interfacial rate, as the

calculated values in Table 1 will be a product of these rates. Weare currently exploring the thickness and molecular linkerdependence on the through-film charge transfer rates of theseporphyrin multilayers systems.

■ CONCLUSIONSWe have examined the surface passivation properties of filmsassembled via CuAAC click chemistry. We have shown ourfilms possess a rectifying property that we have demonstratedtoward five different outer-sphere redox mediators (Fc0/+,[Co(bpy)3]

2+, [Co(dmb)3]2+, [Co(NO2-phen)3]

2+, [Fe-(bpy)3]

2+). In these systems, current is mediated by theelectroactive film when in contact with a redox probe. Thisprocess was examined by cyclic voltammetry. Using hydro-dynamic forced convection voltammetry, we calculatedinterfacial electron transfer rates between the mediators andfilms of ZnTPEP. The inherent ability for films assembled viathis LbL method to encourage unidirectional current flowtoward outer-sphere redox species has broad application inmany types of devices.1,2,13

■ EXPERIMENTAL SECTIONMaterials. All solvents, ACS grade or higher, were purchased from

Sigma-Aldrich or Fisher Scientific and used as received. Anhydrousacetonitrile used for all electrochemical experiments was purged withnitrogen and recirculated for 48 h through a solid-state columnpurification system (Vacuum Atmospheres Company, Hawthorne,CA) prior to use.61 Toluene was purged with nitrogen and storedunder 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-t r is(az idomethyl)benzene ,64 and (3-az idopropyl)-trimethoxysilane65 were synthesized according to literature methods.Indium tin oxide (ITO) coated glass was purchased from DeltaTechnologies (polished float glass slides, one-side coated, Rs of 4−8Ω). TBAPF6 (Aldrich) was recrystallized twice from hot ethanol anddried under vacuum overnight. Ferrocene (Aldrich) was purified viasublimation.

Cobalt and Iron Complexes. All polypyridyl cobalt and ironcomplexes [Co(bpy)3](PF6)2, [Co(dmb)3](PF6)2, and [Co(NO2-phen)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 untildissolved. The resulting solution was yellow to brown. The solutionwas stirred under reflux for ∼4 h. To form the complex with thehexafluorophosphate counterion, a saturated, aqueous solution ofpotassium hexafluorophosphate was added dropwise, until no furtherprecipitation occurred. The resulting solid was filtered, washed wellwith methanol and then diethyl ether, and dried under high vacuumovernight. The product was further purified via anion exchange fromthe 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 precipitateformed. The resulting solid was filtered and washed with acetone,methanol, and diethyl ether. After each step involving solvation, thedissolved complex was filtered through a fine frit.

[Fe(bpy)3](PF6)2 was synthesized following literature procedurewith slight modifications.66 Briefly, FeCl2·6H2O was dissolved in aminimal amount of water. The 2,2′-dipyridyl (3 equiv) was addedslowly with stirring to form a dark red solution. A saturated aqueoussolution of sodium chloride was added to assist with recrystallization ofdark red solid, [Fe(bpy)3]Cl2. The chloride salt was anion exchangedto the hexafluorophosphate salt as described vide supra for cobaltcomplexes. Purity of all complexes was confirmed with both NMR andcyclic voltammetry. Cyclic voltammograms for each redox probe areincluded in Figure 4 and Figure S7.

Figure 9. Koutecky−Levich analysis of each mediator: Fc (orange ▼);[Co(bpy)3]

2+ (red ●); [Co(dmb)3]2+ (yellow ■); [Co(NO2-

phen)3]2+ (green ◆); and [Fe(bpy)3]

2+ (teal ▲). Limiting currentwas 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 DrivingForce for Electron Transfer for Oxidation of Various RedoxMediators (1 mM) at a Working Electrode with Four Layersof ZnTPEP

mediator kET [(cm3)(mol−1)(s−1)] keff [(cm)(s−1)] ΔGET (V)

Fc 9.4 × 108 1.5 × 10−1 −0.50a

[Co(bpy)3]2+ 5.4 × 108 4.4 × 10−2 −0.70a

[Co(dmb)3]2+ 6.5 × 108 5.3 × 10−2 −0.77a

[Co(NO2-phen)3]

2+3.4 × 108 2.8 × 10−2 −0.12a

[Fe(bpy)3]2+ 3.3 × 108 2.7 × 10−2 −0.22b

aCalculated from the ZnTPEP0/+ potential at 0.50 V. bCalculatedfrom the ZnTPEP+/2+ potential at 0.81 V.

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Azido Self-Assembled Monolayer Formation. Prior to self-assembled monolayer (SAM) formation, ITO-coated glass slides weresonicated in dilute Alconox for 10 min then rinsed well with deionized(DI) water. Slides were rinsed successively with acetone, dichloro-methane, methanol, and water and dried with a stream of nitrogen, andsubmersed in 1:1:5 NH3/H2O2/H2O for 30 min. Upon removal, theslides 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. Theappropriate volume of neat (3-azidopropyl)trimethoxysilane to make a1 mM solution was added to the Schlenk tube, which was thenevacuated and purged with nitrogen three times. Dry toluene wastransferred via cannula to the Schlenk tube, ensuring slides weresubmerged. The reaction vessel was then heated to 65−70 °Covernight. When it cooled to room temperature, the ITO was removedand sonicated in toluene for 5 min then rinsed with acetone,dichloromethane, methanol, and water and dried under a stream ofnitrogen. 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 contactwith a dimethyl sulfoxide (DMSO) solution containing less than 2%water, 1.3 mM ZnTPEP, 0.33 mM CuSO4, 0.36 mM TBTA, and 0.48mM sodium ascorbate. After the 10 min treatment, the slide was rinsedwith acetone, dichloromethane, methanol, 5 mM ethylenediaminete-traacetic acid in 50/50 water/ethanol, and water and then dried undera stream of nitrogen. The azido-linker reaction consists of a DMSOsolution containing less than 8% water, 2.2 mM 1,3,5-tris-(azidomethyl)benzene, 4.4 mM CuSO4, 4.8 mM TBTA, and 8.9mM sodium ascorbate. Treatment for the linker layer was also 10 min,and the same rinsing scheme was used. Multilayer growth wasconfirmed using UV−vis spectroscopy. Electronic absorbance spectrawere collected using a PerkinElmer Lambda 950 spectrophotometerwith a solid sample holder that orients the sample normal to incidentlight. Background spectra of the SAM-functionalized ITO surface weresubtracted 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 MTBAPF6 in anhydrous acetonitrile and was both prepared anddeoxygenated with nitrogen immediately before use. All CVs werereferenced 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, aPt disk working electrode, and a Ag/AgCl wire pseudoreferenceelectrode. The headspace of the cell was purged with a very low streamof nitrogen during scans.Electrochemical rectification CVs were collected using a three-

electrode setup containing a Pt wire counter electrode and a Ag/AgClwire pseudoreference electrode. The working electrode was afunctionalized ITO electrode defined by the cylindrically bored areaof a Teflon cone pressed against the slide and filled with either 0.1 MTBAPF6 electrolyte or 1 mM solution of the corresponding mediatorin 0.1 M TBAPF6.

67 Scan rates for all surface scans were 1000 mV/sand 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 cutand filed flat. The needle was lowered into a cell so that it was 4 mmabove the surface of the working electrode. The cell design is describedelsewhere.31,67 In brief, a hole bored in the bottom of a cylindricalTeflon cone defined the working area of the functionalized ITOelectrodes (0.15 cm2 area). The Teflon cone was pressed firmly againstthe ITO electrode and filled with electrolyte and mediator (0.1 MTBAPF6 and 1 mM mediator in SPS acetonitrile). A Teflon plug wasplaced in the top of the cone with holes bored to fit snugly a nitrogenline, the wall-jet syringe, a Pt wire counter electrode, and a Ag/AgClwire pseudoreference electrode. A Levich analysis of the redoxmediators at a bare ITO electrode are included in the SupportingInformation (Figure S16).

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsami.6b05643.

Additional electrochemical scans of the multilayer filmswith redox mediators. (PDF)

■ AUTHOR INFORMATIONCorresponding Author*E-mail: dinolp@rpi.edu.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSM.R.C. acknowledges a Slezak Memorial Fellowship. Thismaterial is based upon work supported by the National ScienceFoundation under Contract No. CHE-1255100.

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