On Electrogenerated Acid-Facilitated Electrografting of Aryltriazenes to

Apr 15, 2013 - Page 1 ... to Create Well-Defined Aryl-Tethered Films. Jesper Vinther,. †,‡,§ .... generated aryldiazonium ions to create films wi...
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On Electrogenerated Acid-Facilitated Electrografting of Aryltriazenes to Create Well-Defined Aryl-Tethered Films Jesper Vinther,†,‡,§ Joseph Iruthayaraj,† Kurt Gothelf,†,‡,§ Steen U. Pedersen,*,†,‡ and Kim Daasbjerg*,†,‡ †

Department of Chemistry, Aarhus University, Langelandsgade 140, DK-8000 Aarhus C, Denmark Interdisciplinary Nanoscience Center (iNANO), Aarhus University, DK-8000 Aarhus C, Denmark § Center for DNA Nanotechnology (CDNA), Danish National Research Foundation, Aarhus University, DK-8000 Aarhus C, Denmark ‡

S Supporting Information *

ABSTRACT: The mechanism of electrogenerated acidfacilitated electrografting (EGAFE) of the aryltriazene, 4(3,3-dimethyltriaz-1-enyl)benzyl-1-ferrocene carboxylate, was studied in detail using electrochemical quartz crystal microbalance (EQCM) and cyclic voltammetry. The measurements support the previously suggested mechanism that electrochemical oxidation of the EGA agent (i.e., N,N′−diphenylhydrazine) occurs on the forward oxidative sweep to generate protons, which in turn protonate the aryltriazene to form the corresponding aryldiazonium salt close to the electrode surface. On the reverse sweep, the electrochemical reduction of the aryldiazonium salt takes place, resulting in the electrografting of aryl groups. The EGAFE-generated film consists of a densely packed layer of ferrocenyl groups with nearly ideal electrochemical properties. The uncharged grafted film contains no solvent and electrolyte, but counterions and solvent can easily enter and be accommodated in the film upon charging. It is shown that all ferrocene moieties present in the multilayered film are electrochemically active, suggesting that the carbon skeleton possesses a sufficiently high flexibility to allow the occurrence of fast electron transfers between the randomly located redox stations. In comparison, EQCM measurements on aryldiazonium-grafted films reveal that they have a substantially smaller electrolyte uptake during charging and that they contain only 50% electroactive ferrocenyl groups relative to weight. Hence, half of these films consist of entrapped supporting electrolyte/solvent and/or simply electrochemically inactive material due to solvent inaccessibility.

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transfer between a charging Fc-containing film and an aquasolvated electrolyte. At low coverage, there is sufficient space for fully hydrated counteranions to associate with the Fc groups during charging because the Fc groups are separated sparsely. As the coverage increases, the average free space between Fc groups becomes smaller, and the counteranions are forced to reduce their hydration shell to move into the narrow gap space. Moreover, the amount of water molecules entering the film decreases with increased size of the counteranion. The most promising approach to produce redox active layers of high chemical and electrochemical stability is to exploit the ability of aryl radicals as grafting agents because they form strong covalent bonds between the electrode surface and the organic film.23 Most conveniently, the aryl radicals are generated from easily reducible aryldiazonium23,24 or diaryliodonium25,26 salts, but also aryltriazenes,1,4,5,27 arylhydrazines,28,29 triarylsulfonium salts,30 and Grignard reagents31,32 may serve as suitable precursors. The main disadvantage of using these highly efficient grafting agents is that the film

INTRODUCTION Redox-active ferrocene (Fc) containing films have been extensively used for modifying electrodes, owing to their capability to store and release charges fast and reversibly at modest anodic potentials. Hence, they have served multifarious applications such as hybrid molecular/semiconductor memory devices,1,2 reference electrodes in nonaqueous systems,3 reporter labels for grafting efficiency4,5 and in aptamer-based sensors,5,6 electrocatalysis in biosensors,7 surface-driven switching of liquid crystals,8 and charge-transfer studies of alkanes9 and nucleic acids.10 Often, the electrochemical response of the surface-confined Fc groups deviates from the ideal Nernstian behavior expected for an immobilized reversible redox couple.11,12 The surface density and layered structure of the immobilized Fc units along with restricted solvation13 often affect the heterogeneous charge-transfer kinetics,14 redox potential,15,16 and total charge capacity.17 For self-assembled monolayers18 and electroprecipitated films containing Fc moieties, the effect of electrolytes and film densities on the redox behavior has been thoroughly investigated.13,16,19−22 In aqueous solution, a free volume constraint determines the amount of water molecules that © XXXX American Chemical Society

Received: December 21, 2012 Revised: March 14, 2013

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Scheme 1. Mechanism Proposed for Covalent Immobilization of Fc-Containing Aryl Groups (G) on Pt Surfaces through the EGAFE of an Aryltriazene (T) Involving the Intermediate Formation of the Corresponding Aryldiazonium Ion (D).4

prospects in using aryltriazenes as precursors to aryldiazonium ions with avoidance of spontaneous grafting as one of the particular features. Furthermore, the EQCM-D technique provides information on the electroactivity and solvent accessibility for the surface-confined Fc moieties and, through this, important insight into the morphology of the grafted film is obtained. Among other things, it is revealed that the EGAFE procedure provides a higher charging capacity of the deposited film compared to that obtained in a direct aryldiazonium approach, which is also investigated herein.

formation occurs through uncontrollable and irreversible radical reactions that most often result in a multilayered and disorganized film.24 However, in a recent work we showed that it was possible to deprotect aryltriazenes using an electrogenerated acid (EGA)33−35 to facilitate the grafting process of the thus generated aryldiazonium ions to create films with a good controllability of the layer thickness.4 Specifically, this strategy denoted EGA-facilitated electrografting (i.e., EGAFE) could be utilized in a repetitive double-potential step technique to control the thickness of a Fc-containing film from submonolayer to 4−5 well-defined multilayers. Scheme 1 shows the overall concept of the EGAFE process. The first step involves formation of the EGA at an anodic potential in close proximity to the electrode surface through oxidation of N,N′-diphenylhydrazine (DPH). The protons thus generated will convert the aryltriazene [4-(3,3-dimethyltriaz-1enyl)benzyl-1-ferrocene carboxylate (T)] to the corresponding aryldiazonium functionality. Upon stepping to a cathodic potential, the aryldiazonium ions (D) are reduced in the true grafting step to form the aryl radicals that are responsible for the covalent immobilization of the Fc containing aryl groups (G) onto the surface.4 The electrochemical signal from the Fc moieties immobilized in this manner was found to exhibit Nernstian characteristics with a full width at half-height, ΔEfwhm, equal to the ideal 90.6 mV and with a small peak potential separation, ΔEp, of only 9 ± 4 mV. Moreover, a plot of log ip versus log ν, where ip is the peak oxidation current and ν the sweep rate, resulted in a linear relationship with a slope equal to unity, as expected for a surface-confined reversible redox system. The EGAFE of DPH/T is, to the best of our knowledge, the first procedure to give via the diazonium route covalently attached Fc films that exhibit truly Nernstian behavior. In this paper the mechanistic details of the procedure are investigated by means of an electrochemical quartz crystal microbalance and dissipation (EQCM-D) setup. This study not only confirms the mechanism suggested in our previous study4 but also details the

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EXPERIMENTAL SECTION

Chemicals. Acetonitrile (MeCN) (anhydrous, 99.9%, Lab-Scan), N,N′-diphenylhydrazine (DPH), and ferrocene (Fc) (Sigma-Aldrich) were used without further purification. Tetrabutylammonium tetrafluoroborate (Bu4NBF4) was synthesized using standard procedure. 4(3,3-Dimethyltriaz-1-enyl)benzyl-1-ferrocene carboxylate (T) was synthesized as described elsewhere.4 EQCM Electrodes. Platinum-coated quartz crystals (0.3 μm thickness according to the manufacturer specification) with a diameter = 1 cm and geometrical surface area = 0.785 cm2 were obtained from Q-sense. One might question if the geometric surface area is a sufficiently good approximation of the electrochemically active surface area, considering that the former does not take into account surface roughness. However, Galina et al. found that this approximation is valid for a gold-coated sensor from Q-sense.36 Prior to EQCM measurements, the platinum-coated area of the crystals was exposed to a piranha solution, which comprises 1:3 (v:v) 30% H2O2:H2SO4 (caution: piranha solution is a very strong oxidant and should be handled with extreme care), at room temperature for 30 min and afterward rinsed thoroughly with Milli-Q water and ethanol. Electrochemical Quartz Crystal Microbalance (EQCM). All electrochemical measurements were performed using a Q-sense E4 instrument (Q-sense AB, Sweden) connected with an electrochemical module, QEM 401, having an available volume of ca. 100 μL above the sensor. The potentiostat was a CH Instruments Model 601C. Cho et al. have given a thorough and detailed protocol for running EQCM-D experiments.37 The electrochemical setup consisted of a Pt-covered EQCM crystal as a working electrode, a Pt plate as a counter electrode, and Ag/AgCl (PEEK, from Cypress systems) as a reference electrode. B

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Figure 1. (A) First (black line) and second (red line) cyclic voltammogram recorded on 2 mM T and 2 mM DPH at a Pt-coated EQCM crystal, using ν = 0.1 V s−1 in 0.1 M Bu4NBF4/MeCN and with a >300 s rest time spent at the open-circuit potential between the two cycles. (B) Corresponding normalized changes in frequency recorded during the first (black line) and second (red line) voltammetric cycle. The measurements pertain to the second of two PtT films investigated (vide inf ra). Charging. After electrografting producing either PtT or PtD films, the electrochemical cell was thoroughly rinsed using 0.1 M Bu4NBF4/ MeCN at a flow rate = 0.1 mL min−1. Three potential cycles (from −0.08 to 1.03 V vs SCE) were then carried out at ν = 0.1 V s−1, under static conditions with the pump switched of, while recording Δf and ΔD simultaneously along with the current profile. Between each cycle the electrochemical cell was disconnected and the background electrolyte was exchanged by pumping in fresh solution for >250 s. The net mass change, Δmc(E), of the grafted electrodes during the recording of the cyclic voltammograms and thus charging of the Fc units in the film was calculated from eq 1. The linearized part of the (Δf vs time) curve recorded prior to or between the individual cycles was used to compensate for a slight time-dependent drift in Δf. In no cases was the effect found to be higher than 5 × 10−4 Hz s−1, meaning that the total drift during any of the recordings would never exceed 0.01 Hz within the experimental time frame. For background-corrected recordings, Δf and Δmc(E) will always be close to zero at the beginning of each new cycle. While Δmc(E) mainly provides information on the amount of electrolyte drawn into and out of the film during the charging/ discharging process, the cyclic voltammograms themselves contain essential information on the electroactivity of the film. This is due to the fact that the continuous consumption of moles of electrons per area unit, Δnc(E), during a sweep may easily be calculated from eq 3.12

The reference electrode was calibrated3 using the redox potential E0 = 0.70 V versus SCE for the immobilized Fc-containing aryl groups.4 The EQCM equipment was used to continuously record changes in resonance frequency (Δf) and dissipation (ΔD) at different overtones during one complete experiment, including electrografting and several charging processes. The switch potential was used to postsynchronize the EQCM and cyclic voltammetric experiments in order to link development in Δf and the current (i) with the electrode potential (E). Grafting of aryltriazene (T). For the EGAFE of T at a Pt electrode, two cyclic voltammetric cycles were carried out on a solution of 2 mM T and 2 mM DPH in 0.1 M Bu4NBF4/MeCN by varying the potential in the range from −0.70 to 0.75 V versus SCE at a sweep rate (ν) = 0.1 V s−1. The sweeping occurs under static conditions with the pump switched off to avoid flow effects. Between the two cycles the electrochemical cell was disconnected for >300 s. This modified electrode is denoted PtT. Grafting of aryldiazonium ion (D). In these experiments T was first converted quantitatively to D by adding 2 equivalents of HBF4 to a solution containing 2 mM T in 0.1 M Bu4NBF4/MeCN. The sweeping procedure employed was, by and large, the same as that described for the EGAFE procedure (vide supra). The only difference was that direct electrografting had to be carried out immediately upon generation of D because of the ability of the Fc group itself to reduce the aryldiazonium functionality and that the sweeping profile 0.3 → 0.75 → −0.7 V versus SCE was employed to avoid an initial direct reduction of the diazonium salt. The uncertainty given on measured film properties is estimated to be ±10%. The film obtained in this manner is denoted PtD. Surface Coverages. In consideration of the fact that the deposited layer is rigid and that no viscoelastic changes occur at the electrode interface, the relationship between the mass change and the frequency change is given by the Sauerbrey equation (eq 1).38

Δm(E) = ΔfS

Δnc(E) =

∫0

t

i dt =

1 FAv

∫E

E(t )

i

i dE

(3)

In this equation, F is the Faraday constant, A is the geometric surface area of the platinum electrode, t is time, and Ei is the initial potential. Prior to integration, the recorded current was background corrected, using the procedure described above in the interval from 0.3 to 1.0 V versus SCE on the forward sweep and between 0.95 and 0.3 V vs SCE on the reverse sweep. The faradaic current was set to zero outside these intervals. Values of Δnc(E) were calculated, letting the potential E vary from Ei = 0.3 V versus SCE, located before the rise of the wave to Es = 1.03 versus SCE, located after the wave. Note that Δnc(Es) corresponds to the total surface coverage of electroactive Fc functionalities of the electrografted films, which henceforth is denoted ΓCV c , to emphasize that this value is obtained from the cyclic voltammetric experiment (eq 4).

(1)

In this equation Δm(E) denotes the mass change per unit area occurring at the potential E upon sweeping from the selected initial, Ei and switch, Es, potentials. Dependent on whether the mass changes are measured during grafting or the subsequent charging experiment involving the immobilized Fc groups, the notations Δmg and Δmc, respectively, will be used. The parameter S is a characteristic constant of the quartz crystal (= 17.7 ng cm−2 Hz−1 herein). , associated with the Grafting. The surface grafting density, ΓEQCM g formation of grafted films is obtainable through eq 2. Γ gEQCM = Δmg (total)/MG

1 FA

ΓCV c = Δnc(Es) =

1 FAv

∫E

Es

i

i dE

(4)

Ellipsometry. Dry film thicknesses of the electrografted film on the Pt-coated EQCM crystals were obtained using a rotating analyzer ellipsometer (Dre, Germany). All measurements were performed at 75° angle of incidence. The ellipsometric parameters of the bare (Δs, ψs) and grafted (Δg, ψg) substrates were measured in air at ambient temperature, where Δ is the phase shift and tan(ψ) is the amplitude

(2)

In this expression, Δmg(total) is the total mass change obtained after two voltammetric cycles and MG is the molar mass of the grafted agent G (= 319.2 g mol−1). C

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Figure 2. (A) First (black line), second (red line), and third (blue line) cyclic voltammogram recorded along with the normalized backgroundcorrected change in frequency, Δf, at a PtT crystal using ν = 0.1 V s−1 in 0.1 M Bu4NBF4/MeCN and with a >250 s rest time spent at the open-circuit potential between each cycle. (B) Mass gain, Δmc, plotted against the moles of electrons consumed, Δnc, during the first (black line), second (red line), and third (blue line) voltammetric cycle. The measurements pertain to the second of two PtT films investigated (vide inf ra). ratio upon reflection. The complex refractive index of the bare substrate was calculated from the measured Δs and ψs values. A threelayer optical model consisting of the Pt substrate with a complex refractive index, the grafted layer given by its refractive index and thickness, and the surrounding medium (air) was used to calculate the overall reflection coefficients for in-plane (Rp) and out-of-plane (Rs) polarized lights. The real and the imaginary parts of the refractive index of the bare Pt substrate were obtained by measuring the clean plates prior to modification. Ellipsometric measurements were performed on the same area of the plates before and after electrografting. Because the measurements are carried out on a dried and thus collapsed film, the refractive index of the layer is fixed at a constant value (real = 1.55; imaginary = 0), independent of the thickness. The average and the standard deviation values reported correspond to data points obtained from measuring three spots on each plate.

similar feature is observed, if the oxidation of DPH is carried out in absence of T (Figure S1 of the Supporting Information), this phenomenon can be ascribed to desorption of some physisorbed material, most likely initially adsorbed DPH. On the first reverse sweep, a significant decrease in Δf (∼14 Hz) coincides with the appearance of the broad reduction wave in the cyclic voltammogram (see Figure 1A). This supports our previously suggested mechanism of EGAFE, in that the aryldiazonium ions must be formed in close proximity to the electrode surface, thereby allowing them to be electrografted once the potential is lowered. Clearly, Δf has not reached its finish point at the end of the first potential cycle, meaning that the diffusion layer at this point still contains the in situgenerated aryldiazonium ions. More so, after disconnecting the cell for >300 s, the grafting process continues spontaneously as evidenced through the small frequency drop observed between the end of the first cycle and the beginning of the second cycle. The formation of a thin and partly insulating film during the first voltammetric cycle affects the second cycle, in that the oxidation wave pertaining to DPH becomes positively shifted by ∼0.2 V (Figure 1A). Interestingly, the size of the oxidation wave of DPH is not becoming smaller at this slightly more positive potential, which can be explained by the fact that the generation of small amounts of charged Fc+ moieties in the film makes the electrode conducting through mediated chargetransport processes. The broad reduction wave around 0.0 V on the reverse sweep is largely suppressed23,24 and all in all, a small decrease in Δf of ∼3 Hz39,40 is observed between the start and the end of the second cycle. Hence, less electrografting of the aryldiazonium salt is taking place during the second cycle in accordance with the development expected for a self-limiting grafting process. The total decrease in Δf for the grafted PtT film during the two cycles, including the contribution from the spontaneous grafting taking place in between them, is 20.8 ± 2.1 Hz. The fact that the change in dissipation, ΔD, is negligible during the electrografting cycles (Figure S2 of the Supporting Information) indicates that the PtT film is thin and rigid, therefore justifying the use of the simple Sauerbrey model in calculating the mass change, Δmg, associated with the film formation (eq 1). From this, it is found that the recorded total decrease in Δf (= 20.8 ± 2.1 Hz) corresponds to Δmg(total) =

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RESULTS EGAFE of Aryltriazene (T) Monitored by EQCM. Figure 1 shows the recordings of two sequential cyclic voltammograms and the concomitant normalized frequency change, Δf, during the electrografting of the aryltriazene T in the presence of DPH at a Pt-coated EQCM crystal to afford grafted films denoted PtT. First, it should be noted that compared with the corresponding voltammogram recorded on glassy carbon in our previous study,4 showing a single peak at 0.7 V versus SCE, the wave herein is significantly broadened out on the first sweep to the interval 0.3−0.75 V versus SCE (see Figure 1A). Nevertheless, it is evident that oxidation processes take place, and according to previous knowledge,4 these encompass the concomitant oxidations of DPH and the Fc functionality in T. These processes lead to the formation of azobenzene/2H+ and the Fc+ moiety, respectively. Furthermore, on the first reverse sweep, a broad reduction wave appears at ∼0 V versus SCE, which is attributed to a combined reduction of liberated protons from the oxidation of DPH and the reduction of the diazonium ion (D) formed in situ due to the EGA-facilitated protonation of T. Figure 1B shows the frequency changes, Δf, recorded in EQCM as a response to the voltammetric sweeping. On the first oxidative sweep, Δf remains reasonably constant initially, confirming the inability of the aryltriazene to undergo spontaneous grafting. However, at 0.2 V versus SCE, a small but distinct increase in Δf (∼4 Hz) takes place concomitantly with the appearance of the anodic current wave pertaining to the oxidation of DPH in the cyclic voltammogram. Since a D

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Table 1. Ellipsometrically Determined Dry Film Thickness, d, Sauerbrey Total Mass Gain per Unit Area Upon Grafting, Δmg(total), Surface Grafting Density Upon Grafting, ΓEQCM , Sauerbrey Total Mass Gain per Unit Area Upon Charging of Films, g Δmc(Es), Electrochemically Determined Surface Density of Fc Moieties, ΓCV c , and Slopes, ∂Δmc/∂Δnc, from Plots of Δmc Versus Δnc for PtT or PtD Films electrodesa T

Pt PtT PtD

d (nm)

Δmg(total) (ng cm−2)

ΓEQCM (nmol cm−2) g

Δmc(Es) (ng cm−2)

−2 ΓCV c (nmol cm )

∂Δmc/∂Δncb (g mol−1)

3.9 ± 0.2 3.4 ± 0.1 4.3 ± 0.1

409 ± 41 368 ± 37 653 ± 65

1.28 ± 0.1 1.15 ± 0.1 2.05 ± 0.2

165 ± 17 124 ± 12 49 ± 5

1.27 ± 0.1 1.09 ± 0.1 1.00 ± 0.1

124 114 44

a Obtained by carrying out two voltammetric cycles at ν = 0.1 V s−1 on the appropriate solutions of either T or D in 0.1 M Bu4NBF4/MeCN. bSlopes obtained from the plots of Δmc versus Δnc for the second cycle in Figure 2B and Figure S5B of the Supporting Information.

368 ± 37 ng cm−2 or a surface grafting density ΓEQCM = 1.15 ± g 0.1 nmol cm−2, according to eq 2. Grafting of Aryldiazonium Ion (D) Monitored by EQCM. To enable a comparison of the EGAFE approach with the much more common procedure that exploits a direct electrografting of the diazonium salt, experiments were carried out, in which T first was converted quantitatively to D by adding 2 equivalents of HBF4 to the solution. This direct electrografting to afford the electrodes denoted PtD had to be carried out immediately upon generation of D because of the ability of the Fc group itself to reduce the diazonium functionality. Figure S3 of the Supporting Information shows the normalized frequency changes, Δf, recorded in EQCM as a response to the voltammetric sweeping. A significant decrease in Δf (∼26.5 Hz) coincidences with the broad reduction wave of the diazonium salt during the first cycle and corresponds to a mass gain of 469 ng cm−2. During the second cycle, Δf decreases further by 10.4 Hz, corresponding to deposition of additional 184 ng cm−2 on the electrode. In total, this gives the = 2.05 PtD electrode Δmg(total) = 653 ± 65 ng cm−2 or ΓEQCM g ± 0.2 nmol cm−2, which are 1.7 times the corresponding values obtained for PtT. Electrochemical Charging of PtT. Electrochemical charging/discharging of Fc groups in the PtT film may be exploited to obtain quantitative information about the overall electrochemical activity and to study the coupled transport of counterions and solvent in and out of the films.12,41 Moreover, analyses of the current−voltage shapes may reveal details regarding interactions between Fc and generated Fc + functionalities.16 Figure 2A shows the response in Δf during the recording of three consecutive cyclic voltammetric cycles at a PtT electrode in a blank electrolyte solution consisting of 0.1 M Bu4NBF4/ MeCN. Simultaneous monitoring of the dissipation, ΔD, showed relatively small changes in this parameter, indicating that the viscoelastic properties of the layer were, by and large, not affected by the charging/discharging process (Figure S4 of the Supporting Information). The dry-state film thickness, d, was also measured by ellipsometry. In this particular case d = 3.4 nm corresponding to a 3−4 layer thick multilayered film, considering that the length of one grafting molecule is 1.1 nm (estimated using the PM3 facility in the Chem3D Ultra 11.0 program package, CambridgeSoft). The cyclic voltammograms of the Fc+/Fc couple recorded at ν = 0.1 V s−1 are characterized by the standard potential E0 = 0.70 V versus SCE, ΔEfwhm, = 88 ± 3 mV, and ΔEp = 26 ± 3 mV. The latter value is slightly higher than the previously reported one (ΔEp = 9 ± 4 mV) for the same system using glassy carbon as a working electrode.4 In comparison, an ideal Nernstian electron transfer process would have ΔEfwhm = 90.6

mV and ΔEp = 0 V as its characteristic values, indicating that there may exist relatively weak lateral interactions between the redox units in the PtT film, whereas ion transport limitations can be considered to be minimal. In addition, cyclic voltammetric experiments using different sweep rates in the interval from 50−200 mV s−1 show little variation in these parameters. The overall changes in i, Δf, and ΔD are essentially reversed upon reversal of the potential sweep, indicating that the reconstruction abilities of the film are high. Another noteworthy feature is that the changes in i, Δf, and ΔD occur synchronously, demonstrating that the charging of the Fc moieties to generate Fc+ is associated with a mass gain coupled to the transport of BF−4 into the film to retain charge balance. This feature becomes even more evident if the changes in Δf are converted to mass changes per area unit, Δmc, using eq 1 and plotted against the consumption of moles of electrons per area unit, Δnc, obtained by integration of the voltammetric waves, according to eq 3. Figure 2B shows such plots of Δmc versus Δnc for the three voltammetric cycles recorded. Interestingly, the plots of the experimental data do not only confirm the high degree of reversibility in the charging/discharging processes but also exhibit linear relationships. The slopes are 107 and 114 g mol−1 for the first and the second/third cycle, respectively. Presumably, the slightly lower value for the first cycle can be attributed to the occurrence of desorption of noncovalently attached material during sweeping, as also seen in other studies on aryldiazonium salts.42 Noteworthy, the slopes obtained for the last two cycles are very close to that expected (i.e., 128 g mol−1, if one molecule of BF4− should diffuse along with one solvent molecule of MeCN into the film for each positive charge generated on Fc during sweeping). This may be interpreted as if the electrolyte experiences no problems in accessing the entire film, no matter if Fc is located in the outer or more compact inner parts. Note that Δmc(Es) = 124 ± 12 ng cm−2 (for the second cycle) is at the end point, where the maximum amount of electrolyte solution has been able to enter the film. The corresponding −2 denotes the value of ΓCV c ≡ Δnc(Es) = 1.09 ± 0.1 nmol cm total moles of electrons consumed during the electrochemical charging process and therefore constitutes an electrochemical determination of the surface coverage (see eq 4). Electrochemical Charging of PtD. Experimental data and graphs for the PtD electrode are presented in Figure S5 of the Supporting Information. The overall behavior pertaining to the cyclic voltammetric and EQCM experiments is the same as for PtT, although, a closer look at the extracted values of Δmc(Es) −2 (= 49 ± 5 ng cm−2) and ΓCV c (= 1.00 ± 0.1 nmol cm ) reveals that they differ appreciably, as discussed in greater detail below. Only the absolute numbers of ΔEfwhm = 108 ± 3 mV and ΔEp = E

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18 ± 3 mV are comparable to those of PtT (i.e., E0 = 0.70 V vs SCE, ΔEfwhm = 88 ± 3 mV, and ΔEp = 26 ± 3 mV) indicating that, at least, the electron transfer characteristics for the two films cannot be too different.

Figure 3 provides a pictorial comparison of the different electrochemical responses of the PtD and PtT films as expressed through the linear Δmc versus Δnc plots, extracted from Figure 2B and Figure S5B of the Supporting Information, respectively. In general, the uptake of BF−4 and MeCN during charging occurs to maintain charge neutrality, the osmotic balance, and to stabilize, through solvation, the charged Fc+ film.41 For the two PtT films, we find that the slope, ∂Δmc/∂Δnc = 124 and 114 g mol−1, respectively, corresponds by and large to the molar mass of 128 g mol−1 for the uptake of one BF−4 ion along with one solvent molecule per unit charging. It is also noteworthy that the uptake of BF−4 and MeCN during charging only induces small changes in the dissipation (Figure S3 of the Supporting Information).

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DISCUSSION Mass Changes and Surface Coverages. In Table 1, all relevant data obtained for the PtT and PtD films from the cyclic voltammetric and EQCM experiments are collected. In the first rows, specific results for two PtT films having slightly different film thicknesses are shown. As seen, the reproducibility in (= 1.28 ± 0.1 and 1.15 ± 0.1 nmol cm−2) is high, if the ΓEQCM g 13% smaller dry film thickness, d, measured for the second PtT film is taken into account. The values of ΓcCV were determined for the charging associated with the second voltammetric cycle (see Figure 2), while the first cycle was omitted because of the more irreproducible results to which the initial restructuring of the film is always giving rise.4 For both PtT films, the close (= 1.27 ± 0.1 and 1.09 ± 0.1 nmol similarity between ΓCV c (= cm−2, respectively) and the corresponding values of ΓEQCM g 1.28 ± 0.1 and 1.15 ± 0.1 nmol cm−2, respectively) implies that essentially all Fc moieties present in the films are electrochemically accessible, and that there can only be a minor . contribution from electrolyte/solvent to ΓEQCM g Hence, the thickest film is also the one containing the most electroactive Fc units. The normalization of the values of ΓCV c with respect to what would correspond to a monolayer (d ≈ 1.1 −2 for both PtT nm) leads to ΓCV c (monolayer) = 0.35 nmol cm films. This value is in agreement with the 0.4 nmol cm−2 we found for the same chemical system, using glassy carbon electrodes.4 Also, it is in reasonable agreement with a previous calculation by Chidsey et al.,43 in which the coverage of Fc units on the basis of a hexagonal closest-packing was estimated to be 0.45 nmol cm−2. In addition, Leroux et al.44 obtained 0.44 nmol cm−2, as the experimentally determined value for Fc units attached to a monolayer of phenylacetylene using click chemistry. Without a doubt, the slightly smaller value of ΓCV c (monolayer) we found is due to the fact that the Fccontaining molecule studied herein will occupy a larger volume than a single Fc molecule would itself. In the last row, the results for PtD prepared in a two-cycle procedure through a direct electrografting of the diazonium salt (produced in the bulk solution from T/HBF4) gave Δmg(total) = 2.05 ± 0.2 nmol cm−2. This is = 653 ± 65 ng cm−2 or ΓEQCM g 1.6 times the amount deposited for the first of the two PtT films investigated, while the corresponding increase in the thickness with d = 4.3 ± 0.1 nm only is 0.4 nm (i.e., 10% higher). It is indeed expected that the direct electrografting should be more efficient than the EGAFE procedure, considering that the diazonium ion formation in the latter case only takes place in the diffusion layer. In this context, it becomes particularly interesting to examine the charging process of PtD. As seen from Table 1, ΓCV c = 1.0 ± (= 2.05 ± 0.1 nmol cm−2, which constitutes only 50% of ΓEQCM g 0.2 nmol cm−2) and is even lower than the value of ΓCV c (= 1.27 ± 0.1 nmol cm−2) for PtT. Hence, a substantial fraction of the mass deposited in the direct electrografting of the diazonium ion consists of electrochemically silent Fc units and/or entrapped electrolyte/solvent molecules. For Fc groups to be inactive, it would require that at least certain parts of the PtD film are inaccessible for the electrolyte/solvent.

Figure 3. Mass gain, Δmc, plotted against the moles of electrons consumed, Δnc, for PtT (o) and PtD (■). Data are taken from Figure 2B and S5B of the Supporting Information; straight lines are obtained from linear regression. Slopes corresponding to 86.8 g mol−1 (BF4−) and 128 g mol−1 (MeCN + BF4−) are shown as dashed lines for comparative reasons.

In comparison, ∂Δmc/∂Δnc has as low a value as 44 g mol−1 for the PtD film, which is less than half of that obtained for PtT and even lower than MBF4− = 86.8 g mol−1. This suggests that the supporting electrolyte or solvent may be entrapped in the PtD film to enable the required charge compensation of the generated Fc+. This would also imply that Bu4N+ in contrast to BF4− should be able to leave (and upon discharging re-enter) the film, which would be in line with the small extent of swelling occurring but hard to explain from ion size considerations alone. In any case, the low swelling effect is EQCM expected from the small value of ΓCV = 0.4 (vide supra). c /Γg Electrochemical Stability of the PtT Film. The electrochemical stability of the Fc-containing layer may be evaluated from the development in the sequential cyclic voltammograms recorded during charging/discharging (see Figure 2). Interestingly, the data extracted show that ΓCV c decreases 5.5% going from the first to the second cycle but only 2.9% from the second to the third one. The question which remains is then to which extent is this decrease in ΓcCV due to a loss of electrochemical activity of the Fc moieties or if desorption of material from the surface has taken place. This question may be answered from an analysis of the raw data pertaining to Δf for the PtT film (Figure S6 of the Supporting Information).42 First of all, it may be noted that the first cycle results in a small increase in Δf for the neutral Fc film, going from 0 to 0.6 Hz after the first voltammogram with a gradual 0.1 Hz decrease toward 0.5 Hz as the layer is allowed to F

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corresponding aryldiazonium ion. Second, a subsequent reductive potential is required to electrograft the aryldiazonium ion. The previously suggested mechanism4 for EGAFE is supported by the EQCM measurements. In agreement with previous findings, the EGA method provides good controllability of the grafting. Furthermore, the surface coverage of the grafted film calculated from the Sauerbrey equation is equal to the electrochemically determined coverage of the Fc moieties, indicating that all grafted Fc groups are electroactive. The reversible mass change observed during the charging/discharging cycle of the Fc layer indicates the dynamic nature of the grafted film, wherein the counterions move in and out of the film, depending on the applied potential. The film exhibits close to ideal electrochemical behavior for a surface immobilized reversible redox couple in cyclic voltammetry. In comparison, EQCM measurements on direct electrografting of the aryldiazonium ion D generated in a bulk solution of HBF4/T reveal that both thicker and more compact films are produced. In this case, a substantial fraction of the mass deposited (∼50%) consists of electrochemically silent Fc units and/or entrapped electrolyte/solvent molecules. For Fc groups to be inactive, it would require that at least certain parts of the PtD film are inaccessible for the electrolyte/solvent. It should also be noted that the consumption of material effectively will be much higher in this case, since the stability of aryldiazonium salts in most grafting solvents is limited in contrast to that of aryltriazenes. Indeed, the triazene group is often used as a protection group for the diazonium group in synthesis. In the particular case studied herein, the stability issue is even more pertinent because of the presence of the reducing ferrocenyl group, which will induce a relatively fast dediazoniation process, unless the diazonium group is protected.

relax for the 325 s spent at the open-circuit potential. On the second and third cycle, Δf for the neutral Fc film both starts and ends at 0.5 Hz, although the film relaxation after the voltammetric cycles is relatively slow. Hence, it may be concluded that desorption of material only takes place during the first cycle and not to any appreciable extent. Presumably, the relatively long time required after each cycle for attaining the original state can be attributed to a slow exchange of solvent and ions between the film and the medium. In comparison, the plateau value of Δf for the charged Fc+ film experiences a relatively larger change going from −7.2 over −6.6 to −6.1 Hz for the first, second, and third cycle, respectively (Figure S6 of the Supporting Information). Hence, the loss of electrochemical activity of Fc moieties is the main reason for the decrease in ΓCV c . Further, this can be attributed to the effect known as charge trapping,13 where potential sweeping causes some Fc+ moieties to become isolated and therefore unable to participate in additional redox activity. It should also be mentioned that these small changes in Δf are accompanied with concomitant small changes in ΔD as expected, if the solvent-induced film swelling effect becomes increasingly smaller for each cycle (Figure S4 of the Supporting Information). Structures of PtT and PtD Films. On the basis of the above results gathered for the PtT film, the following structural information about the neutral and positively charged state may be deduced: (i) Essentially all Fc moieties present in the films are electrochemically active, (ii) the uncharged grafted film has a low solvent and electrolyte content, (iii) the easy accessibility of counterions upon charging suggests that a potential-induced film swelling takes place, (iv) the high charging efficiency of the multilayered film suggests the occurrence of a fast electron transfer between Fc moieties, and (v) a well-defined nearNernstian behavior in cyclic voltammetry indicates that the neighboring interaction between Fc moieties upon charging is small.45 Such fast charge transport in the film implies a high degree of freedom in the structural rearrangement of the grafted multilayered film with minimal cross coupling present to hinder the access of solvent and counterions. In comparison, the PtD films are thicker containing regions, in which solvent and counterions have restricted access and therefore unable to assist in the oxidation of Fc groups by counter-balancing the positive charge generated. This is manifested in the observation that only half of the deposited material is electroactive (with respect to weight), although regions seem to exist, where entrapped BF−4 can provide the appropriate conditions for the oxidation process to take place. In spite of fewer active redox stations, the charge transport at these locations is only affected relatively little as evidenced from the still near-Nernstian cyclic voltammetric behavior of the electrochemically active Fc groups.

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ASSOCIATED CONTENT

S Supporting Information *

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. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The Danish Agency for Science, Technology and Innovation, the Danish National Research Foundation, Grundfos, and the SP Group are gratefully acknowledged for financial support.

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CONCLUSION The electrografting of the aryltriazene T on Pt-coated EQCM crystal assisted by either the electrogenerated acid (EGA) approach or through addition of two equivalents of HBF4 was studied using EQCM. In the former case, where the corresponding aryldiazonium ion D is generated in the diffusion layer at the electrode surface only, it is demonstrated that the surface grafting may be precisely controlled by a double-potential excursion. First, an oxidative potential is required to produce the EGA, which protonates the aryltriazene and through this induces a deprotection step to form the

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