Article pubs.acs.org/Langmuir

Electron Transport through a Diazonium-Based Initiator Layer to Covalently Attached Polymer Brushes of Ferrocenylmethyl Methacrylate Mie Lillethorup, Kristian Torbensen, Marcel Ceccato, Steen Uttrup Pedersen, and Kim Daasbjerg* Department of Chemistry, Aarhus University, Langelandsgade 140, DK-8000 Aarhus C, Denmark Interdisciplinary Nanoscience Center (iNANO), Aarhus University, Gustav Wieds Vej 14, DK-8000 Aarhus C, Denmark S Supporting Information *

ABSTRACT: A versatile method based on electrografting of aryldiazonium salts was used to introduce covalently attached initiators for atom transfer radical polymerization (ATRP) on glassy carbon surfaces. Polymer brushes of ferrocenylmethyl methacrylate were prepared from the surface-attached initiators, and these films were thoroughly analyzed using various techniques, including X-ray photoelectron spectroscopy (XPS), infrared reflection−absorption spectroscopy (IRRAS), ellipsometry, and electrochemistry. Of particular interest was the electrochemical characterization of the electron transfer through the diazonium-based initiator layer to the redox centers in the polymer brush films. It was found that the apparent rate constant of electron transfer decreases exponentially with the dry-state thickness of this layer. To investigate the electron transfer in the brushes themselves, scanning electrochemical microscopy (SECM) was applied, thereby allowing the effect from the initiator layer to be excluded. The unusual transition feature of the approach curves recorded suggests that an initial fast charge transfer to the outermost-situated ferrocenyl groups is followed by a slower electron transport involving the neighboring redox units.

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solvent,12 counterion,13 temperature,9 and density of redox centers.10 Furthermore, simulation of cyclic voltammetric responses has served as an important tool to understand these systems.10 In general, studies of charge transport phenomena have shown the immense importance of the structure of the organic film.14,15 In this respect, the above-described deposition techniques are associated with some disadvantages because the polymer films often are disordered and/or lack stability.16 One way to overcome such issues is to use surface-initiated polymerizations in a “grafting-from” procedure to create polymer brushes which because of their high density are forced to stretch out to give highly ordered films.17 Living radical polymerizations, including atom transfer radical polymerization (ATRP), are popular methods to exert control over the polymerization and to obtain polymer brushes tethered to substrates. Previously, ATRP has been applied to synthesize Fccontaining polymer films, either by polymerization of a Fcfunctionalized monomer18−20 or by using a one-pot preparation, where the ATRP is combined with “click” chemistry to introduce the Fc units.21 Unfortunately, in these cases the

INTRODUCTION Study of charge transfer processes in electroactive polymermodified electrodes has constituted a subject of considerable and growing interest since the mid-1970s with many implications in the areas of catalysis,1 molecular recognition,2 switches,3 sensors,4 and electrochromic devices.5 Electronconducting polymers are usually categorized according to the mechanism of electron transport, that is, in conducting polymers electrons move through a delocalized conjugated system, while in redox polymers the electron transport occurs in terms of either electron hopping or electron exchange reactions between redox centers.6 Redox-active polymers prepared with ferrocene (Fc) have served as one of the most interesting and convenient model systems for the investigation of charge transfer due to the wellcharacterized reversible one-electron behavior of the ferrocenium/ferrocene (Fc+/Fc) redox unit.7 Without comparison, poly(vinylferrocene) (PVF) is the most widely studied system.8−10 Specifically, PVF has been grafted by plasma polymerization,9 resulting in a highly disordered and crosslinked polymer film. Alternatively, deposition of electrochemically oxidized PVF10 from a solution containing PVF8 or through dip coating11 has provided physically adsorbed polymer films. In investigations of PVF-modified electrodes, numerous effects have been examined such as the influence of © 2013 American Chemical Society

Received: July 23, 2013 Revised: September 20, 2013 Published: October 21, 2013 13595

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Scheme 1. Formation of Covalently Attached Initiator Film through Electrografting of 4-(2-Hydroxyethyl)benzenediazonium Tetrafluoroborate (1) To Obtain Grafted Substrates, GC-OHY (Y = e, 1, 5, or 10), Followed by Reaction with 2Bromoisobutyryl Bromide To Form Initiator-Based Films, GC-IY; in the Third Step FMMA Is Used as Monomer in ATRP To Produce Surface-Attached Fc-Containing Polymer Brushes, GC-IY-PFMMA

electrochemical properties of the films were not subjected to further investigation. To ensure a high stability of the polymer brushes, the covalent anchoring between the surface and the polymer chains is of particular interest and can be accomplished by electrografting an aryldiazonium salt as precursor for the ATRP initiator.22−25 In this work we combine electrografting of an aryldiazonium salt with the surface-initiated ATRP (SI-ATRP) technique to produce covalently attached polymer brushes of ferrocenylmethyl methacrylate (FMMA) on glassy carbon (GC). We report the characterization of the surface films using various surface-sensitive techniques such as cyclic voltammetry, ellipsometry, infrared reflection−absorption spectroscopy (IRRAS), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and scanning electrochemical microscopy (SECM). Particular emphasis is on the extent by which the blocking behavior of the grafted initiator films affects the electron transport between the substrate and the redox polymer film. Hence, initiator layers with different thickness and resistivity were synthesized using either electrolysis or cyclic voltammetry at three different sweep rates as the electrografting procedure. Moreover, the electron transfer mechanism between Fc moieties within the film was investigated by SECM in the feedback mode.

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by carrying out potentiostatic electrolysis at a potential 200 mV negative to that of Ep for 300 s. Alternatively, electrografting on blank substrates was performed by employing one cyclic voltammetric sweep between 0.4 and Ep − 0.2 V at a sweep rate = 1, 5, or 10 V s−1 to obtain GC-OH1, GC-OH5, or GC-OH10, respectively (Scheme 1). Afterward, the substrates were rinsed with MeCN followed by sonication in acetone for 10 min. Dry-state film thickness was measured by ellipsometry on GC plates. Preparation of Initiator-Modified Substrates. Hydroxylterminated substrates (GC-OHe, GC-OH1, GC-OH5, and GC-OH10) were immersed in a solution of 0.5 M 2-bromoisobutyryl bromide and 0.05 M triethylamine (TEA) in dichloromethane (DCM) at room temperature for 3 h followed by rinsing in DCM and sonication in DCM and acetone (10 min in each solvent) to obtain GC-Ie, GC-I1, GC-I5, and GC-I10. Dry-state thickness of the initiator film was measured by ellipsometry, while the chemical composition was analyzed by XPS. SI-ATRP from Initiator-Modified Substrates. Ferrocenylmethyl methacrylate (FMMA) (1.7 g, 6.0 mmol), CuIBr (1.0 mg, 0.0070 mmol), and CuIIBr2 (0.7 mg, 0.0031 mmol) were added to a dried Schlenk flask which was sealed by a rubber septum. The flask was deoxygenated three times using a procedure consisting of purging with Ar for 1 min followed by applying vacuum for 1 min. Meanwhile, 1.2 mL of acetone and N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA) (2.4 μL, 0.011 mmol) were added to a second Schlenk flask which was degassed by three freeze−pump−thaw cycles and backfilled with argon. This solution was transferred by a cannula to the first Schlenk flask, heated to 50 °C, and prestirred for 30 min before immersion of the GC-IY (Y = e, 1, 5, or 10) substrates. In general, identical initiator-modified GC disk electrodes and plates were derivatized simultaneously to enable direct comparison of electrochemical data and thickness measurements. One batch typically consisted of four plates with different initiator films. It was noted that variations in the small amounts of CuBr and CuBr2 weighed had a relatively large effect on the polymerization rate in the SI-ATRP. This meant that measured brush lengths could vary appreciably from one batch to another. Importantly, this had no effect on the reliability of the results obtained as evidenced from IRRAS analysis showing high consistency of data pertaining to different series of batches (vide inf ra). The substrates were exerted for 2 h of polymerization before being rinsed with acetone and sonicated in acetone and DCM for 10 min in each. Unreacted FMMA could be extracted for reuse from the quenched polymerization mixture by passing it through a column of Al2O3, eluting with acetone, and evaporating the solvent. The FMMAmodified substrates denoted GC-Ie-PFMMA, GC-I1-PFMMA, GC-I5PFMMA, and GC-I10-PFMMA (see Scheme 1), with the superscript referring to the grafting conditions of 1, were analyzed by electrochemistry, ellipsometry, IRRAS, AFM, and XPS.

EXPERIMENTAL SECTION

Electrochemical Setup. A standard three-electrode electrochemical setup (CH Instruments 660B or 601C) consisting of GC as working electrode, a platinum wire as auxiliary electrode, and a Ag/ AgI pseudoreference electrode [i.e., a silver wire immersed in an acetonitrile (MeCN) solution of 0.1 M Bu4NBF4 and 0.01 M Bu4NI] or a standard calomel electrode (SCE) as reference electrode was used in all electrochemical measurements, the exception being the SECM experiment, where a CH instrument 900B bipotentiostat was employed and a silver wire served as quasi-reference electrode. At the end of each experiment the standard potential of the Fc+/Fc couple, E0Fc+, was measured, and all potentials were referenced against SCE using a previous determination of E0Fc+ = 0.41 V vs SCE in MeCN.26 Electrografting of 4-(2-Hydroxyethyl)benzenediazonium Tetrafluoroborate (1). Glassy carbon (GC) plates or disk electrodes were immersed in a solution of 0.1 M Bu4NBF4/MeCN containing 2 mM of 1 (see Supporting Information for synthetic procedures). First, one cyclic voltammogram was recorded at a sweep rate = 0.1 V s−1 to measure the peak potential (Ep) of the reduction wave pertaining to 1. Subsequently, the substrate was further modified to produce GC-OHe 13596

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RESULTS AND DISCUSSION Initiator Platform. Scheme 1 outlines the two-step procedure used for the immobilization of the initiator film for ATRP on GC as previously described.23,25 In this work the thickness of the grafted film is varied through the sweep rate employed (ν = 1, 5, or 10 V s−1) in a cyclic voltammetric grafting experiment to obtain GC-OHY (Y = 1, 5, or 10) or by employing potentiostatic electrolysis for 5 min to obtain GCOH e . The second step comprises a nucleophilic acyl substitution of 2-bromoisobutyryl bromide with the surfaceanchored hydroxyl groups to extend the substrate film with 2bromo-2-methylpropanoyl initiator units. These initiatormodified substrates are henceforth named GC-IY. In the third and final step FMMA is used as monomer in ATRP to produce surface-attached Fc-containing polymer brushes, i.e., GC-IYPFMMA. Electrochemical Impedance Spectroscopy (EIS) and Thickness Measurements of GC-OHY. Figure 1 shows the

Information). Interestingly, RCT is within uncertainty the same (∼1 kΩ) for GC-OH10 and GC-OH5 but increases to 3 and 500 kΩ for GC-OH1 and GC-OHe, respectively. The latter value is consistent with an electrode that is totally blocked toward electron transfer. The overall results are also in agreement with those obtained in previous experiments on electrografted diazonium-based surface films of varying grafting densities and thicknesses.27 Furthermore, the trend observed from EIS for the GC-OHY films correlates well with results obtained from cyclic voltammetric redox probe experiments as described in the Supporting Information (Figure S3). From the dry-state film thickness, d, obtained by ellipsometry (see Table 1), it may be estimated that there is close to a monolayer film on GC-OH10 (d = 0.5 ± 0.1 nm) and GC-OH5 (d = 0.7 ± 0.1 nm), while 2−3 and 5−6 layers are formed on GC-OH1 (d = 2.0 ± 0.1 nm) and GC-OHe (d = 4.4 ± 0.4 nm), respectively.23,25 Although these values pertain to the dry rather than swollen state, the data are still in line with the EIS experiments (Table 1), in that RCT is similar for GC-OH10 and GC-OH5 but increases with the film thickness as one goes to GC-OH1 and, in particular, to the thickest GC-OHe film. XPS Data for GC-IY. Table 2 summarizes the atomic content of the relevant elements in the GC-IY films obtained from acylation of the GC-OHY electrodes using 2-bromoisobutyryl bromide. In accordance with expectation, the bromine content with 2.0% is highest for the GC-Ie film grafted by electrolysis and decreases as the grafting time is shortened in the sweeping technique going from 1.1% for GC-I1 to 0.4% for GC-I5 and further to 0.3% for GC-I10. Unfortunately, the absolute concentration of bromine cannot easily be used to estimate the percentage of acylation because XPS (penetration depth ∼10 nm) also detects contributions from the GC substrate to an extent that is dependent on the thickness of the grafted film. In principle, this issue could be solved by using metal substrates, but on the other hand, this would give weaker bonds between the substrate and the grafted film. Finally, it should be emphasized that no decrease in the bromine signal was experienced due to degradation induced by the X-ray beam itself. GC-IY-PFMMA (Y = e, 1, 5, or 10) Films. First, it may be noted that the content of iron from the Fc units detected in XPS is the same for all PFMMA brushes (Table 2) as expected if the penetration depth of the X-ray (∼10 nm) is smaller than the thickness of the brushes (≥25 nm). Also, the distribution ratio of Fe:O:C = 1:2.1:15.8 is close to the theoretically calculated 1:2:15 in all cases. In addition, d measured by ellipsometry and AFM scratch tests (Figures S4 and S5, Supporting Information) is 37, 28, 27, and 25 nm for Y = e, 1, 5, and 10, respectively (Table 2). The expectation that the thickest initiator layer, which according to XPS and EIS may be correlated with the highest surface density, leads to the formation of the thickest PFMMA film is thereby confirmed.23 Figure 2 shows spectra of three PFMMA films measured by IRRAS. Stretches pertaining to the ester group appear at 1727, 1260−1240, and 1170 cm−1, while the two bands at 1407 and 1107 cm−1 are characteristic for the Fc group.28 The bands in the IRRAS spectra of PFMMA films are the same for the various initiator platforms used. This is due to the fact that the initiator film in all cases is thin compared to the polymer film and that the initiator layer, like the brush itself, contains an ester functionality as its most profound IR-active moiety. Another important point to notice is the existence of a linear correlation between IRRAS absorbance and the ellipsometric

Figure 1. Nyquist plots for GC-OH1 (×), GC-OH5 (○), and GCOH10 (●) recorded on 2 mM Fc in 0.1 M Bu4NBF4/MeCN. Inset shows the corresponding Nyquist plot for GC-OHe (◆).

Nyquist plots for GC-OH1, GC-OH5, and GC-OH10 obtained from EIS measurements on Fc in 0.1 M Bu4NBF4/MeCN. As seen, they all display a semicircle characteristic of an interfacial charge transfer mechanism along with a low-frequency Warburg line at an angle of 45° ascribed to semi-infinite diffusion.27 In comparison, the Nyquist plot for GC-OHe consists of a depressed semicircle without displaying semi-infinite diffusion at low frequency (see inset in Figure 1). Table 1 shows the charge transfer resistance, RCT, determined from these measurements on the basis of the appropriate equivalent circuits (Figures S1 and S2, Supporting Table 1. Charge Transfer Resistance, RCT, Measured by Electrochemical Impedance Spectroscopy and Dry-State Film Thickness, d, Measured by Ellipsometry for GC-OHY (Y = e, 1, 5, or 10) Films electrode

RCTa (kΩ)

GC-OHe GC-OH1 GC-OH5 GC-OH10

500 3 1.0 0.8

± ± ± ±

100 1 0.1 0.2

db (nm) 4.4 2.0 0.7 0.5

± ± ± ±

0.4 0.1 0.1 0.1

a

Uncertainties are given as standard deviations determined from seven individual measurements. bUncertainties are given as standard deviations determined from three individual measurements. 13597

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Table 2. Dry-State Film Thickness, d, Measured by Ellipsometry and Atomic Percentage Obtained by XPS for GC-IY and GC-IYPFMMA (Y = e, 1, 5, or 10) Filmsa electrode GC GC-Ie GC-I1 GC-I5 GC-I10 GC-Ie-PFMMA GC-I1-PFMMA GC-I5-PFMMA GC-I10-PFMMA

d (nm) 5.0 2.5 1.1 0.7 37 28 27 25

± ± ± ± ± ± ± ±

0.1 0.5 0.3 0.1 1b 1b 2b 1b

O 1s (atom %) 2.1 8.9 7.9 3.9 4.3 11.5 11.1 10.9 10.9

± ± ± ± ± ± ± ± ±

0.2 0.3 0.1 0.1 0.5 0.3 0.1 0.2 0.1

C 1s (atom %) 97.9 83.0 86.9 94.0 93.3 83.3 83.6 83.8 83.8

± ± ± ± ± ± ± ± ±

0.2 0.5 0.3 0.2 0.7 0.2 0.1 0.1 0.2

N 1s (atom %) 5.9 3.6 1.2 1.1

± ± ± ±

0.2 0.2 0.1 0.1

Br 3p (atom %) 2.0 1.1 0.4 0.3

± ± ± ±

Fe 2p (atom %)

0.1 0.1 0.1 0.1 5.2 5.3 5.3 5.3

± ± ± ±

0.1 0.1 0.1 0.1

a

In addition to the listed elements, minor contributions from Na, Si, and P are also measured for some of the samples in XPS; uncertainties are given as standard deviations determined from three and four individual measurements in ellipsometry and XPS, respectively. bMeasured by both ellipsometry and AFM scratch tests; corrected for initiator layer thickness.

also provides a measure of the total amount of PFMMA present on the surface with the compactness of the different GC-IYPFMMA films being similar (see also Supporting Information). Surface Topography Imaging Using AFM. AFM was used to visualize the surface topography of GC-OHY and GCIY-PFMMA (Y = e, 1, 5, or 10). The AFM images of GC-OHY (Figure 3A) reveal a surface film consisting of nanosized domains. The homogeneity of the films improves going from GC-OH10 over GC-OH5 and GC-OH1 to GC-OHe. This suggests that the domains merge as the grafting time, and hence, the initiator density increases. For the GC-IY-PFMMA series a similar trend is observed (Figure 3B). The AFM image of GC-I10-PFMMA shows an inhomogeneous film morphology consisting of separated individual domains. As the initiator density increases these domains start to merge to form a more homogeneous film as seen for GC-Ie-PFMMA. Specifically, the root-mean-square surface roughness decreases in the order GC-I10-PFMMA, GCI5-PFMMA, GC-I1-PFMMA, and GC-Ie-PFMMA with the exact values being 2.0, 1.8, 1.1, and 0.9 nm, respectively. Without doubt conformational changes of the polymer films for the varying initiator densities contribute to this development.29 Cyclic Voltammetry of GC-IY-PFMMA. Figure 4 displays representative cyclic voltammograms of the four different GCIY-PFMMA films (i.e., disk electrodes) recorded at various sweep rates. In all cases the characteristic electrochemical

Figure 2. IRRAS spectra recorded for GC-IY-PFMMA films having various dry-state film thicknesses, d, = 36 (), 27 (···), and 18 nm (- - -). Inset shows a plot of the IRRAS absorbance pertaining to the CO stretch band at 1727 cm−1 as a function of d; samples originate from two different batches with Y = 1 and e (●; left to right) and Y = 10, 5, 1, and e (○; left to right), respectively, to show the high consistency in the data obtained.

dry-state thickness, d, as most easily evidenced from the changes in the integrated area of the carbonyl stretch band at 1727 cm−1 as a function of d for six films originating from two different batches (see inset in Figure 2). This emphasizes that d

Figure 3. AFM topographic images of (A) GC-OHY and (B) GC-IY-PFMMA (Y = e, 1, 5, or 10). 13598

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Figure 4. Cyclic voltammograms recorded of GC-Ie-PFMMA (-··), GC-I1-PFMMA (), GC-I5-PFMMA (···), and GC-I10-PFMMA (- - -) at ν = (A) 0.01, (B) 0.1, (C) 1, and (D) 10 V s−1 in 0.1 M Bu4NBF4/MeCN.

in the literature,33,34 if electrons are transferred across a thin nonconducting film. Hence, these results show that the constitution of the anchoring initiator film exerts a great effect on the electrochemical response of the PFMMA brushes. Determination of Heterogeneous Rate Constants. Without exception ΔEp increases as ν is enhanced with the effect becoming more and more pronounced as the thickness of the initiator film increases in the order GC-I10-PFMMA, GCI5 −PFMMA, GC-I1-PFMMA, and GC-Ie-PFMMA. This behavior may further be translated into a corresponding decrease in the apparent heterogeneous rate constant, ks.35 In other words, our results demonstrate that the charge transfer resistance arising from an inner layer electrografted from aryldiazonium salts (vide supra) significantly lowers the rate of electron transfer to the Fc groups in the brushes, thereby causing the observed increase in ΔEp. Determination of ks for a specific film can be obtained using the procedure suggested by Laviron.35 First, the transfer coefficient is calculated from the expression 1 − α = 62 mV/ ΔEp,1/2 for each recorded cyclic voltammogram fulfilling ΔEp > 200 mV; for the remaining near-reversible cases α is set to be 0.5. The α value exhibits a clear potential dependency as predicted by the Marcus density-of-state model36 with an increase from α = 0.5 to 0.9, if high overpotentials are required to oxidize the Fc groups. Using these α values, ks may then be calculated for each ν applied to a specific film.35 Table 3 displays the average ks values determined in this manner to be 0.05, 1.4, 2.6, and 3.4 s−1 for GC-Ie-PFMMA, GC-I1-PFMMA, GC-I5-PFMMA, and GC-I10-PFMMA, respectively. Clearly, an increase in ks is observed as the thickness and thereby charge transfer resistance of the initiator layer decrease.

response from the surface-confined Fc+/Fc redox couple of PFMMA is evident. For the GC-I1-PFMMA, GC-I5-PFMMA, and GC-I10-PFMMA films the peak separation, ΔEp, ≈ 0 mV at ν = 0.01 V s−1 (Figure 4A). This is as expected for a surfaceconfined reversible electron transfer between redox centers and an electrode.30 In contrast, the full width at half peak height, ΔEp,1/2, is ≤22 mV for the oxidative waves pertaining to GC-I1PFMMA, GC-I5-PFMMA, and GC-I10-PFMMA and ≤45 mV for the reductive wave under these conditions. This is substantially lower than the 90.6 mV theoretically predicted for a one-electron process.30 In fact, at this low sweep rate it is only fulfilled in the case of GC-Ie-PFMMA but then with ΔEp = 80 mV, pointing to a quasi-reversible nature of the electron transfer process. In general, low values of ΔEp,1/2 are indicative of the existence of attractive interactions between the redox centers according to the Frumkin isotherm.30,31 That the reductive wave is less sharp compared to the oxidative wave suggests that these interactions are significantly different for the redox centers in the reduced and oxidized states. Previously, Peerce and Bard have observed similar responses in the case of electrodeposited PVF.10 The presence of dissymmetric cathodic and anodic peaks has also been addressed by Laviron and Roullier, who by considering the interactions between electroactive sites and the electron transfer rate at the surface was able to describe the shape of such voltammograms.32 Figure 4B−D shows that the oxidation and reduction waves become more symmetrical as ν is enhanced and at the same time ΔEp and ΔEp,1/2 increase substantially. For example, for the GC-I1-PFMMA film ΔEp = 992 mV and ΔEp,1/2 = 580 mV at ν = 10 V s−1. Similarly large ΔEp values have been reported 13599

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obtained in a plot of ln ks vs d (Figure S6, Supporting Information). Such a low β excludes the possibility of having a true tunneling process, and the fact that d is not determined for the solvated but rather the dry state (by ellipsometry) will not alter this conclusion. A likely interpretation of the overall behavior is therefore that the initiator filmbeing formed in a kinetically controlled grafting processcontains numerous pinholes and, possibly, that the brushes possess a sufficiently high mobility to let the Fc units move close to the surface. Alternatively, Fc units may already have become deposited inside the initiator film during the SI-ATRP. A previous study on the kinetics of polymer brush formation provides support for such a scenario, in which growth takes place inside the initiator layer as well.25 Determination of Surface Coverages and Peak Currents. Another interesting parameter to consider is the surface coverage, Γ, which may be extracted from the cyclic voltammograms for the four PFMMA films by integrating the area under the oxidative and reductive waves. As the measurements were carried out on extended series of experiments with a decay of the electrochemical signals occurring over time due to the sweeping, Γ as well as the peak current, ip, had to be corrected. In spite of being slow the time-dependent decays of Γ and ip, were noted to be essentially exponential. From this observation the initial surface coverage, Γinit, the initial peak current, ip,init, and the pertinent decay rate constants, kdecay,ox and kdecay,red, could easily be determined (Figures S7 and S8, Supporting Information). In general, kdecay,ox and kdecay,red = (3−6) × 10−4

Table 3. Apparent Heterogeneous Rate Constant, ks, Determined Using Cyclic Voltammetry, Initial Surface Coverage of Fc Groups, Γinit, and Initial Surface Coverage of a Monolayer of Fc Units, Γinit,monolayer electrode GC-Ie-PFMMA GC-I1-PFMMA GC-I5-PFMMA GC-I10-PFMMA

ksa (s−1) 0.05 1.4 2.6 3.4

± ± ± ±

0.02 0.1 0.2 0.1

Γinitb (nmol cm−2) 6.5 11.6 12.9 15.7

± ± ± ±

0.5 0.1 0.1 1.0

Γinit,monolayerc (nmol cm−2) 0.25 0.23 0.28 0.44

± ± ± ±

0.03 0.01 0.01 0.04

a Average of the cathodic and anodic ks values derived as described in the text;35 uncertainties are standard deviations. bObtained from the plots in Figure S7 (see Supporting Information); uncertainties are given as standard deviations. cDetermined as Γinit,monolayer = 0.79 6 Γinit/(3d) (in nm), where d = 17, 33, 30, and 23 for GC-IePFMMA, GC-I1-PFMMA, GC-I5-PFMMA, and GC-I10-PFMMA, respectively (see Supporting Information for derivation of the expression for Γinit,monolayer). Note that these samples are not the same as those listed in Table 2.

Qualitatively, such effects were also recently observed for diazonium-grafted layers functionalized with Fc in simple structures34 or more complex dendrimeric architectures.37 For self-assembled monolayers on gold of alkanethiols terminated with Fc38 or other redox units39,40 ks usually exhibits an exponential dependency on d with the characteristic tunneling constant, β, being close to 1 Å−1. A similar relationship is also observed for the current data but with β = 0.1 Å−1 determined from the slope of the straight line

Figure 5. Plot of the initial peak currents, ip,init, for the anodic (▲) and cathodic (△) waves vs log ν for (A) GC-Ie-PFMMA, (B) GC-I1-PFMMA, (C) GC-I5-PFMMA, and (D) GC-I10-PFMMA. Insets show plots of the initial surface coverages, Γinit, for the anodic (●) and cathodic (○) waves vs log ν. 13600

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s−1, demonstrating that Fc+ possesses a relatively high stability in the films. Figure 5 displays plots of log ip,init vs log ν for all four GC-IYPFMMA films. In the case of Y = 1, 5, and 10, characterized by having the initial layer electrografted by sweeping, the plots clearly exhibit curvature. Interestingly, the slope is one at low sweep rates, as normally would be expected for a surfaceconfined reversible redox film, while at higher sweep rates the slope is close to be 0.5, as normally would be expected for a diffusional process. In contrast, in the case of GC-Ie-PFMMA a single slope = 0.65 is seen for the whole range of ν values. One possible interpretation of such phenomena would be that the limiting process at the very highest sweep rates employed is the transport of counterions in and out of the film. However, as already noted, the charge transfer processes themselves, all having ks ≤ 3.4 s−1, induce at least for the three thinnest films a transition from Nernstian to quasi-reversible and further to irreversible behavior upon increasing ν. Concomitantly, this would result in a broadening of the voltammetric waves and, hence, lowering of the peak current in line with the experimental observations. For GC-Ie-PFMMA the charge transfer process is always non-Nernstian as deduced from the broad and highly shifted voltammetric waves obtained at all sweep rates and further translated into the low ks = 0.05 s−1. To investigate to which extent the curvature in the plots of Figure 5 can be attributed to kinetic limitations of the relatively slow electron transfer step at the surface, we resorted to digital simulations. Figure S9 in the Supporting Information indeed shows that even for a simple redox system the results are fully explainable, if the increase in α as a function of ν is fully taken into account. Insets in Figure 5 show plots of Γinit vs v to gain further information on this issue. The general tendency is that Γinit decreases by 10−20% as ν is increased, the exception being GC-I10-PFMMA, in the case of which some larger fluctuations are found for ν < 0.05 V s−1. Overall, this suggests that the total number of Fc groups being electrochemically activated during the sweep is relatively constant, independent of the time scale of the experiment. In other words, the contribution from rearrangement of Fc groups, solvation of the brush structure, and diffusion of counterions into and out of the film induces no substantial limitations to the electron transfer process. Table 3 includes average values of Γinit = 6.5, 11.6, 12.9, and 15.7 nmol cm−2 for GC-Ie-PFMMA, GC-I1-PFMMA, GC-I5PFMMA, and GC-I10-PFMMA, respectively. Compared to other diazonium-based redox films, these coverages are 1−2 orders of magnitude higher,34,41−43 which is also expected, taking the polymeric nature of our films into account. On the assumption that the geometry of the monomeric units of PFMMA may be approximated by hexagonal close-packed (hcp) spheres (diameter = 0.79 nm), the initial surface coverage of a monolayer of Fc units, i.e., Γinit,monolayer, may be calculated from Γinit (see Supporting Information). In average, Γinit,monolayer = 0.30 ± 0.11 nmol cm−2 (Table 3, last column), which is the same as that obtained from a simple theoretical calculation of an optimized hcp structure (Supporting Information). The particularly high value of Γinit,monolayer found for GC-I10-PFMMA (= 0.44 nmol cm−2) may reflect the fact that with a very thin initiator layer film and consequently more pinholes present in this case, detection of more Fc units in the brush layer becomes possible. However, this trend is not

completely consistent, considering that Γinit,monolayer is more or less the same for GC-Ie-PFMMA and GC-I1-PFMMA. Working Model for the Electron Transport. Scheme 2 shows a plausible working model for the electron transport Scheme 2. Model Showing Electron Transport through FcContaining Polymer Brushes Generated from an Inner Initiator Film

process through the initiator and brush layers. Across the inner layer the transport may occur via diffusion of polymer-bound Fc groups through pinholes or defects in the film. In particular, for GC-I10-PFMMA and GC-I5-PFMMA with the thinnest inner layers, this effect could be large because of the high number of pinholes present. Along these lines the increase in charge transfer resistance with film thickness for GC-OHY (Y = e, 1, 5, or 10) noted in Table 1 (vide supra) may be attributed to a decrease in the number of defects/pinholes accompanied by an increase of the resistivity for a growing inner organic film. Another scenario to consider is that the SI-ATRP process as noted above may have left Fc units inside the initiator layer in the initial phases of the brush formation, in the case of which the electron transfer distance becomes smaller than anticipated. Finally, for the outer PFMMA brush part the electron transfer is expected to proceed either via electron exchange or hopping mechanisms.7,44 In general, for the GC-Ie-PFMMA film cyclic voltammetry revealed that the electron transport through the inner layer exerts full kinetic control, independent of the sweep rate employed. For GC-I1-PFMMA, GC-I5-PFMMA, and GC-I10PFMMA this process is comparatively faster and thermodynamic control is attainable at the lowest sweep rates applied. Hence, kinetic information on the electron transport through the PFMMA brush layer cannot be extracted for any of the films by means of the cyclic voltammetric technique. SECM on GC-IY-PFMMA (Y = e, 1, 5, or 10). For the purpose of obtaining kinetic information on the electron transfer through brushes SECM is an applicable technique.45−49 Already, this methodology has been used to investigate surfaceconfined Fc-containing dendrimers37,43,50 and Fc-terminated SAMs,51,52 where an oxidized form of an appropriately chosen solution-based redox mediator is generated under the SECM tip and subsequently reduced by the Fc moieties in the films. The result is the generation of a positive feedback current. If this current further is recorded as the tip is moved toward the surface film, a so-called approach curve is obtained, from which kinetic information subsequently may be extracted. In this work similar proof-of-concept experiments were carried out for the PFMMA brushes to show the prospect in using SECM for investigating the redox characteristics of the 13601

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films. For these experiments to be successful it is essential that the underlying GC substrate exerts no electrochemical effect whatsoever on the electron transfer processes. Hence, it should be completely insulated from the solution based redox probe not to mention the PFMMA film itself. In our case this requirement is fulfilled only for GC-Ie-PFMMA due to its thick initiator layer. Finally, a GC-Ie-PMMA film with PMMA denoting poly(methyl methacrylate) and thus containing no Fc moieties was synthesized23 to serve as a suitable reference system for the measurements. Figure 6 shows approach curves obtained under steady state conditions using tri-p-tolylamine (TTA) as solution-based

The distance for a given experiment, at which the transition from a positive to a negative feedback situation occurs, then reflects the point at which these inner exchange processes become unable to transport sufficiently large amounts of charge to the outer part to keep up the reduction of TTA+. Previously, a similar behavior was displayed in a SECM experiment on a thin film of Fc moieties immobilized on diazonium-grafted layers,55 although in that study the kinetic limitation was explained by a slow charge compensation step. These issues along with a quantification of inter and intra electron transfers between Fc moieties in the polymer films are the subject of ongoing investigations in our group.

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CONCLUSION

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ASSOCIATED CONTENT

In this work, electrochemically responsive polymer brushes of PFMMA have been synthesized by SI-ATRP from diazoniumbased initiator-modified glassy carbon substrates. One of the advantages of this technique is that highly robust and stable polymer brushes may be attached to the covalently bound initiator films. The diazonium-based initiator film was assembled in various electrochemical ways (sweeping or electrolysis) to obtain four different initiator platforms having their own specific property. Both the initiator films and the PFMMA brushes were characterized by spectroscopic and electrochemical experiments. As long as the electron transfer is activated through the initiator layer, the kinetics is determined by the properties of this layer solely as evidenced by e.g., the decrease in the overall electron transfer rate constant with the measured charge transfer resistance of the inner layer. It is suggested that the actual electron transport process through the inner layer occurs via diffusion of polymer-bound Fc groups through pinholes or defects in the film. Another scenario to consider is that the SI-ATRP process may have left Fc units inside the initiator layer in the initial phases of the brush formation, in the case of which the electron transfer distance becomes smaller than anticipated. Finally, SECM experiments allowed the redox properties of the film to be characterized from the solution side, thereby avoiding any effects from the initiator film. The unusual transition feature of the approach curves recorded under these conditions suggests that an initial fast electron transfer to the outermost-situated Fc groups is followed by a slower electron transport comprising the neighboring redox active units to sustain the reducing character of the outer part.

Figure 6. Approach curves in terms of normalized tip current, iT/iT,inf, vs normalized distance, L (= d/a), obtained in SECM at 70 nm thick GC-Ie-PFMMA film (□), 33 nm thick GC-Ie-PMMA film (○), and GC-OHe (×) using 1 mM tri-p-tolylamine as redox mediator in 0.1 M Bu4NBF4/MeCN; iT,inf is the steady-state current measured far from the substrate, a is the radius of the Pt tip electrode (= 12.5 μm), and d is the distance between the substrate and the tip electrode. The substrate is unbiased while the potential of the tip electrode, ET, = 1.05 V vs SCE; approach rate = 1 μm s−1. Theoretical curves for conducting (- - -) and insulating () substrates are included.

redox probe (E0TTA = 0.76 V vs SCE) for the GC-OHe, GCIe -PMMA, and GC-Ie -PFMMA films (on plates). Not surprisingly, the charge transfer properties of GC-OHe and GC-I e-PMMA are as those expected for an insulating substrate53 with a feedback current that decreases as the tip is approaching the film. In comparison, the approach curve obtained at GC-Ie-PFMMA displays a quite different behavior. Initially, a positive feedback with an increasing current is observed as predicted for a conducting substrate,54 but as the tip approaches the film and gets within a distance of 15 μm, the current decreases and eventually levels off, now being consistent with a negative feedback situation. A plausible interpretation of this peculiar transition is that it is due to limitations occurring in the electron transport within the PFMMA brush film. Although similar phenomena also are observed at substrates exhibiting simple intermediary heterogeneous charge transfer kinetics,51,52 the overall shapes of such approach curves differ too much for this to be the case here. Rather, the initial part of the approach curve, showing the characteristics of a positive feedback situation, indicates that the oxidized solution probe TTA+ becomes reduced by the outermost-situated Fc moieties in the film. In turn, these are reduced by neighboring redox active units in electron exchange or hopping processes (see Scheme 2) to sustain in this manner the reducing character of the outer part for some time at least.

* Supporting Information S

Materials and experimental procedures; Figures S1−S9. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel +45 8942 5965; Fax +45 8619 6199 (K.D.). Notes

The authors declare no competing financial interest. 13602

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ACKNOWLEDGMENTS The Danish Agency for Science, Technology and Innovation, Grundfos A/S, and SP Group A/S are gratefully acknowledged for financial support.

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