Mixed Monolayer Organic Films via Sequential Electrografting from

Feb 11, 2013 - Eng. ACS Symposium Series, ACS Synth. .... Two general strategies can then be applied to “fill in” the sparse monolayer with a seco...
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Mixed Monolayer Organic Films via Sequential Electrografting from Aryldiazonium Ion and Arylhydrazine Solutions Lita Lee,† Paula A. Brooksby,† Yann R. Leroux,‡ Philippe Hapiot,‡ and Alison J. Downard*,† †

MacDiarmid Institute for Advanced Materials and Nanotechnology, Department of Chemistry, University of Canterbury, Private Bag 4800, Christchurch 8140, New Zealand. ‡ Institut des Sciences Chimiques (Equipe MaCSE), CNRS, UMR 6226, Université de Rennes 1, Campus de Beaulieu, Bat 10C, 35042 Rennes Cedex, France ABSTRACT: Sequential electrografting at glassy carbon from aryldiazonium salt solutions, or an aryldiazonium salt followed by an arylhydrazine, leads to the formation of covalently attached monolayer films incorporating two modifiers. In the first step, a 4((triisopropylsilyl)ethynyl)phenyl film is electrografted to the surface, followed by removal of the triisopropylsilyl group to give a submonolayer of phenylethynylene groups. Two general strategies can then be applied to “fill in” the sparse monolayer with a second modifier. In the first route, nitrophenyl groups are grafted to the phenylethynylene-modified surface by the oxidation of 4-nitrophenylhydrazine. Ferrocene can be coupled to the terminal alkyne groups on the surface via a click reaction with azidomethylferrocene; an electrochemical measurement of the amount of immobilized ferrocene demonstrates that the phenylethynylene layer retains close to full reactivity after the second grafting step. In the alternative strategy, ferrocene is coupled to the phenylethynylene layer prior to grafting nitrophenyl groups by the reduction of the 4-nitrobenzenediazonium ion or by the oxidation of 4-nitrophenylhydrazine. For all approaches, the optimization of the grafting conditions gives surface concentrations of ferrocene and nitrophenyl groups that are consistent with those of a mixed monolayer. The stepwise generation of mixed monolayers is also monitored by film thickness measurements by depth profiling using the atomic force microscope. Thickness values are consistent with the proposed film structure in each preparation step.

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INTRODUCTION The application of radical-based grafting methods for the preparation of covalently anchored, robust, nanoscale organic films on conducting, semiconducting, and nonconducting substrates is now well established.1−3 Grafting from aryldiazonium salt solutions is the most studied example of this general strategy;2,4,5 other modifiers include amines,6−8 iodonium salts,9,10 and hydrazines.11,12 Key features of radical-based surface modification are the generally strong attachment between the modifier and the surface and the tendency of the formation of loosely packed disordered multilayer films via radical attack on both the substrate surface and the grafted layer. Although strong surface anchoring is clearly an advantage for many practical applications, the formation of multilayers may be a disadvantage. In particular, tight control of interfacial properties is difficult or impossible with disordered multilayers, and a thick, poorly conducting layer may pose a significant barrier to electron transfer between film- or solution-based redox centers and the underlying substrate. Both of these factors are disadvantages for applications such as electrochemical sensors. A consideration of factors such as those described above highlights the need for reliable methods for the formation of monolayer films by diazonium-based grafting. In a monolayer, © 2013 American Chemical Society

every modifier is anchored to the surface, and hence the potential for disorder is reduced compared to that of multilayer structures. Additionally, faster electron transfer will occur across an insulating monolayer than the corresponding multilayer film because of the shorter distance for electron transfer. Several ingenious approaches based on the use of diazonium derivatives with a cleavable substituent have been investigated for the preparation of monolayers. After film formation, the substituent can be cleaved from the aryl ring, thus removing upper layers of the film and exposing a reactive functionality for further coupling reactions. On the basis of film thickness measurements, monolayers or near monolayers of thiophenolates have been prepared by grafting from the diazonium salt of a diaryl disulfide and subsequent reductive cleavage of the disulfide bond.13 In more recent work, the concept was further developed by the use of a sterically bulky cleavable group that restricts film grafting to close to a monolayer. In the first example of this approach, grafting a diazonium derivative with a bulky cationic alkylhydrazone group gave, after acid hydrolysis, a layer of benzaldehyde groups.14 More recently, this strategy was demonstrated by grafting a 4-((triisopropylsilyl)ethynyl)Received: January 22, 2013 Published: February 11, 2013 3133

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Scheme 1. Strategies for Film Preparationa

a

(a) A single-component monolayer film and (b) a mixed film.

Scheme 2. Strategies for the Preparation of Mixed Monolayer Films from a Deprotected TIPS-Eth Layer

solution containing two diazonium ions gives a mixed film, but when the reduction potentials of the modifiers differ (as would often be the case), the composition of the film does not reflect the composition of the solution and is difficult if not impossible to predict.17 A further limitation is that multilayer films are deposited by these approaches. We, and others, have recently reported an alternative strategy for the preparation of mixed films that relies on the sequential grafting of two diazonium derivatives. In both cases, the first modifier has a bulky protecting group that creates free space on the electrode surface. In the approach of Lacroix and co-workers, the grafting of the bithiophenebenzenediazonium ion in the presence of

phenyl (TIPS-Eth) film from the corresponding diazonium ion. Deprotection with tetrabutylammonium fluoride (TBAF) gave a monolayer of phenylethynylene groups (H-Eth) to which azidomethylferrocene (FcCH2N3) could be clicked (Scheme 1, step a).15,16 A second area of recent interest for radical-based grafting is the preparation of mixed layers (i.e., films incorporating more than one modifier). Mixed layers have many potential applications including, for example, biosensors where the recognition species is diluted in an antifouling film so that interactions with target analytes are not sterically hindered and nonspecific adsorption is minimized. Electrografting from a 3134

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Electrochemistry and Surface Modification. All electrochemical measurements were performed using an Eco Chemie Autolab PGSTAT302 potentiostat/galvanostat. GC working electrodes (3 mm diameter) were polished with 1 μm alumina, followed by ultrasonication in water and drying with a stream of N2(g). PPF working electrodes were mounted horizontally between an insulated metal base plate and a glass solution cell. A Viton or Kalrez O-ring and four springs from the base plate to the glass cell sealed the solution above the PPF while maintaining the copper electrical contact to the PPF external to the solution.20 The auxiliary electrode was a Pt mesh, and the reference electrode was a saturated calomel electrode (SCE) for aqueous solutions or a calomel electrode with 1 M LiCl for nonaqueous solutions. TIPS-Eth groups were electrografted to GC and PPF from a solution of 5 mM TIPS-Eth-ArN2+ and 0.1 M TBABF4 in ACN using five cycles between 0.60 and −0.75 V at a scan rate of 50 mV s−1. The electrode was rinsed with acetone, sonicated in ACN for 2 min, and dried with a stream of N2(g). TIPS was cleaved from the layer by immersing the modified electrode in a stirred solution of 0.05 M TBAF in THF for 20 min. NP groups were electrografted from solutions of NBD and NPH in 0.1 M TBABF4-ACN and 0.1 M KH2PO4 in water (pH 4.45), respectively. The concentration of the modifier, potential limits, and number of cyclic scans were varied as described in the Results and Discussion section; the scan rate was 100 mV s−1. After modification in ACN solution, electrodes were rinsed with acetone, sonicated in ACN for 2 min, and dried with a stream of N2(g). After modification in aqueous solution, electrodes were rinsed with water, sonicated in ACN for 2 min, and dried with a stream of N2(g). Ferrocene groups were coupled to H-Eth layers by click chemistry. Modified electrodes were immersed in a stirred 5 mL solution of FcCH2N3 (1 mg) in THF, and CuSO4 (2.5 mL, 0.01 M) was added. After 15 min, L-ascorbic acid (0.01 M in 2.5 mL water containing 15 mg of NaHCO3) was added dropwise to the solution, and the mixture was gently agitated for 3 h. A N2(g) atmosphere was maintained throughout the reaction. The resulting Fc-GC and Fc-PPF electrodes were rinsed with EDTA-Na2 (0.01 M) for 10 min and THF for 20 min and dried with a stream of N2(g). Cyclic voltammograms of immobilized Fc and NP groups were obtained in 0.1 M LiClO4−ethanol (EtOH) and 0.1 M H2SO4, respectively. For mixed films, the voltammetry of Fc groups was recorded prior to reduction of the NP groups. The surface concentration of immobilized Fc was determined by averaging the areas under the anodic and cathodic peaks from the second voltammetric cycle obtained at a scan rate of 200 mV s−1. The surface concentration of immobilized NP groups was estimated from the first cycle obtained at a scan rate of 100 mV s−1 using the NP reduction and the hydroxyaminophenyl oxidation peak areas and the number of electrons involved in each redox process.24 Voltammetric peak area analysis for NP was performed by curve fitting the data using Linkfit software.25 The geometric working electrode area was assumed for all surface concentration calculations. Reported surface concentrations are the means based on the number of samples indicated in the text. When two surface concentrations were averaged, the uncertainties indicate the range of values. When three surface concentrations were averaged, the reported uncertainties are standard deviations of the means. AFM Measurements. Film thickness measurements were made on PPF working electrodes by depth profiling with an AFM tip.20 This method involves removing a small section of film by scratching with an AFM tip and then recording average line profiles across the film and scratch. Two scratches were made on each sample, and at least eight transverse cross sections were chosen from the corresponding images, yielding at least eight average line profiles. Each profile gave two film thicknesses: one from the step on the right side of the section and the other from the left. Thus, the datum reported for each sample is the mean of at least 16 values, and the uncertainty is the standard deviation of the mean. Film thicknesses were ‘blank corrected’ after measuring the scratch depth (approximately 0.2 nm) at bare PPF. One sample was analyzed for each type of film, with the exception of Fc-

cyclodextrin gives a layer of bithiophene phenyl and cyclodextrin groups.18 The removal of cyclodextrin leaves pinholes in the film for the grafting of nitrophenyl (NP) groups. In our work, a mixed film was prepared by grafting a layer of TIPS-Eth to glassy carbon (GC), followed by NP groups (Scheme 1, step b), using nitrobenzenediazonium salt (NBD) as the second modifier.15 To demonstrate the retention of functionality of the ethynyl groups in the mixed film, the TIPS protecting group was removed and FcCH2N3 was coupled to the H-Eth layer. Surface concentration measurements showed that the yield of coupled Fc groups was close to that of a single-component deprotected H-Eth-GC surface but that a multilayer of NP groups was attached to the surface. Interestingly, when NBD was used in the alternative sequence of preparation steps shown in Scheme 2, step a, the grafting of NP groups to the deprotected H-Eth-GC surface eliminated the reactivity of the ethynylphenyl layer toward click chemistry. To date, there are no reports of well-characterized covalently attached mixed monolayers formed by radical-based grafting methods. Such films should have both the stability advantages of films grafted by radical-based methods and the advantages of monolayer films in terms of better controlled composition and morphology and fast electron-transfer rates across the film. In this work, we explore the preparation of mixed monolayer films using the TIPS-protected ethynylbenzenediazonium salt. We have compared the formation of mixed NP and H-Eth films through the three routes shown in Schemes 1, step b, and 2, steps a and b. NP was chosen as the second film component because its redox activity is well understood and allows an estimation of its surface concentration. NP groups have been grafted using both the corresponding diazonium ion and the hydrazine, with the goal of preparing a film with high reactivity for click chemistry (as evidenced by the reaction with FcCH2N3) and monolayer coverage of NP groups. Arylhydrazines have been shown to have a low susceptibility to multilayer formation, instead yielding monolayers (or near monolayers) of covalently attached films when grafted from aqueous solution at pH ≤7.11,12 Hence arylhydrazines are an attractive alternative to aryldiazonium salts for this work. Detailed electrochemical experiments have been undertaken at GC electrodes, and the thicknesses of the layers grafted onto smooth (rms roughness ≤0.6 nm) GC-like thin films (pyrolyzed photoresist film, PPF19) were measured by depth profiling using atomic force microscopy (AFM).

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EXPERIMENTAL SECTION

Materials and Reagents. 4-Nitrophenylhydrazine (NPH, BDH), tetrahydrofuran (THF, Merck), and tetrabutylammonium fluoride (TBAF, Acros Organics) were used as received. Acetonitrile (ACN) was dried in a solvent drying system. Tetrabutylammonium tetrafluoroborate (TBABF4),20 4-((triisopropylsilyl)ethynyl)aniline (TIPS-Eth-ArNH2),21 azidomethylferrocene (FcCH2N3),22 4-nitrobenzenediazonium tetrafluoroborate (NBD),23 and PPF20 were prepared by literature methods. Millipore Milli-Q water (resistivity >18 MΩ cm) was used for all aqueous solutions. 4-((Triisopropylsilyl)ethynyl)benzenediazonium tetrafluoroborate (TIPS-Eth-ArN2+) was synthesized by the adaptation of a previously described procedure.15 Briefly, TIPS-Eth-ArNH2 (1 equiv) was dissolved in acetone, and a 25 wt % aqueous solution of tetrafluoroboric acid (HBF4, 15 mL) was added. After the mixture was cooled in an ice bath, NaNO2 (3 equiv) was added to the solution and the reaction mixture was stirred overnight and filtered under vacuum. The diazonium salt was reprecipitated from an ACN and water mixture. 3135

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PPF, for which the reported film thickness is the mean of that obtained from two samples.

In experiments described by table entries I−VI, we investigated route 2a for the preparation of mixed films. After the deprotection of the TIPS-Eth-GC layer, NP groups were grafted from solutions of the corresponding hydrazine (NPH) or diazonium salt (NBD). Using a 1 mM NPH solution for grafting at the H-Eth-GC surface (experiment I), we found that the first scan shows a well-defined irreversible oxidation peak at Epa ≈ 0.35 V; the peak diminishes and moves to more positive potentials on repeat scans (Figure 1). This behavior is similar to

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RESULTS AND DISCUSSION Table 1 lists the electrochemical conditions used to graft NP groups for the mixed films and the corresponding surface Table 1. Preparation Conditions for Grafting NP Groups and Surface Concentrations of Films Prepared by the Routes Shown in Schemes 1, Step b, and 2 a route

expt

2a 2a

I II

2a

III

2a

IV

2a

V

2a

VI

2b

VII

2b

VIII

2b

IX

2b

X

1bd

XI

1bd

XII

NP grafting conditions

b

1 mM NPH, 3 × −0.2 to 0.8 V 0.5 mM NPH, 1 × −0.2 to 0.5 V blank: conditions the same as for II but in the absence of NPH 0.01 mM NBD, 3 × 0.6 to 0.2 V blank: conditions the same as for IV but in the absence of NBD bare GC; 0.01 mM NBD, 3 × 0.6 to 0.2 V 0.5 mM NPH, 1 × −0.2 - 0.5 V blank: conditions the same as for VII but in the absence of NPH 0.01 mM NBD, 3 × 0.6 to 0.2 V blank: conditions the same as for IX but in the absence of NBD 0.5 mM NPH, 1 × −0.2 to 0.5 V 0.01 mM NBD, 3 × 0.6 to 0.2 V

ΓFc × 10−10 mol cm−2c

ΓNP × 10−10 mol cm−2c

0.6 1.5 ± 0.2 (n = 2) 2.1 ± 0.4 (n = 2)

4.0 1.7 ± 0.1 (n = 2) N/A

1.6 ± 0.1 (n = 2) 2.0 ± 0.2 (n = 2)

0.6 ± 0.1 (n = 2) N/A

N/A

1.8 ± 0.4 (n = 5) 1.8 ± 0.6 (n = 3) N/A

1.6 ± 0.2 (n = 3) 1.9 ± 0.01 (n = 2) 1.5 ± 0.1 (n = 3) 1.3 ± 0.1 (n = 2)

1.6 ± 0.3 (n = 3) N/A

N/A

0.6 ± 0.1 (n = 3) 0.4 ± 0.2 (n = 3)

N/A

Figure 1. Three consecutive scans at a H-Eth-GC electrode in a solution of 1 mM NPH in 0.1 M KH2PO4(aq), pH 4.45. Scan rate = 100 mV s−1; initial potential = −0.2 V.

that on polished GC and is attributed to a three-electron oxidation of the arylhydrazine that generates an aryl radical that attacks the GC surface with the formation of a covalent C−C bond.12 The modified electrode was rinsed and cleaned by sonication, and Fc was “clicked” to the surface. Figure 2a shows that a well-defined, chemically reversible response is obtained for the Fc redox couple in the mixed film. The NP groups exhibit the expected irreversible reduction in aqueous acid solution27 (Figure 2b) arising from a mixed six-electron reduction to aminophenyl groups and a four-electron reduction to hydroxyaminophenyl groups. The latter are oxidized to nitrosophenyl groups at ∼0.35 V, and the chemically reversible hydroxyaminophenyl/nitrosophenyl couple can be seen on subsequent scans. From the areas under the cyclic voltammetric peaks, the surface concentrations of Fc and NP groups were determined, giving 0.6 × 10−10 and 4.0 × 10−10 mol cm−2 for Fc and NP, respectively. The surface concentration of Fc groups is significantly lower than that previously reported for Fc clicked to a single-component deprotected TIPS-Eth-GC surface,15 and hence it is clear that the reactivity of the H-Eth-GC surface has been significantly decreased by grafting the NP film. The reaction of ethynyl groups with radicals produced during the oxidation of NPH and/or the inaccessibility of ethynyl groups after grafting the NP film presumably accounts for this finding. Recognizing that the surface concentration of NP groups on this surface is approximately twice that expected for a monolayer, in the following experiments milder conditions were used for grafting from NPH solution. Table entry II shows that when the concentration of NPH was decreased to 0.5 mM and the upper potential limit of the single grafting scan was decreased from 0.8 to 0.5 V the surface concentration of NP groups ((1.7 ± 0.1) × 10−10 mol cm−2) was in the region expected for a monolayer film. The concentration of electroactive Fc groups clicked to this surface ((1.5 ± 0.2) × 10−10 mol cm−2) was much greater than in the presence of the thicker NP film, consistent with the greater retention of reactivity of

a

Unless noted, NP groups were grafted to H-Eth-GC or Fc-GC surfaces (Scheme 2). bConcentration of modifier and number of cyclic scans between the potentials indicated, starting at the first potential listed. cn is the number of samples analyzed. dNP groups were grafted to TIPS-Eth-GC surfaces.

concentrations of electroactive Fc and NP groups on GC. The route indicated corresponds to that shown in Schemes 1 and 2. In all experiments, the same conditions were used for grafting the TIPS-Eth layer, deprotecting the ethynyl group, and reacting with FcCH2N3. The surface concentrations of coupled Fc groups and grafted NP groups were determined from cyclic voltammograms obtained in 0.1 M LiClO4−EtOH and 0.1 M H2SO4, respectively. Also listed in Table 1 are the results of “blank” experiments in which the electrode was subjected to the same sequence of treatments as for grafting but in the absence of NBD and NPH. A close-packed layer of NP groups on an ideally flat surface has a calculated surface concentration of ∼12 × 10−10 mol cm−2.26 However, our previous experimental work on multilayer films grafted to PPF from NBD solution demonstrated that the films are loosely packed and a monolayer equivalent of NP groups attached to PPF has a surface concentration of 2.5 ± 0.5 × 10−10 mol cm−2.20 Thus to avoid the formation of multilayer films, grafting conditions were optimized to give surface concentrations of NP groups that did not exceed this value. 3136

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Figure 2. Repeat cyclic voltammograms of a GC electrode modified by route 2a using the conditions shown in Table 1, experiment I. Scans were performed in (a) 0.1 M LiClO4−EtOH at a scan rate of 200 mV s−1 and (b) 0.1 M H2SO4 at a scan rate of 100 mV s−1.

Figure 3. Repeat cyclic voltammograms of a GC electrode modified by route 2a using the conditions shown in Table 1, experiment II. Scans were performed in (a) 0.1 M LiClO4−EtOH at a scan rate of 200 mV s−1 and (b) 0.1 M H2SO4 at a scan rate of 100 mV s−1.

((1.8 ± 0.4) × 10−10 mol cm−2). To investigate further the relationship between the surface concentration of NP groups (grafted from NBD solution) and the surface concentration of electroactive Fc subsequently clicked to the surface, higher concentrations of NP groups were grafted following the route 2a approach. In these experiments, NP was grafted from 0.02 and 0.1 mM solutions of NBD. Figure 4 shows that as the surface concentration of NP groups increases, the surface concentration of Fc groups decreases. Hence using the diazonium salt to graft NP to the surface after the deprotection of the TIPS-Eth layer does not allow the preparation of a

the H-Eth-GC layer under these grafting conditions. As shown in Figure 3, the mixed film exhibits well-defined voltammetry for both redox centers. When a TIPS-Eth-GC film was treated to the same series of steps as in the experiment above but in the absence of NPH, the surface concentration of Fc groups was 2.1 ± 0.4 × 10−10 mol cm−2 (experiment III). Hence, within the experimental uncertainty the mixed film has the same surface concentration of Fc groups as the single-component film. However, when we consider the large experimental uncertainty for the blank experiment, it is most likely that a slightly smaller number of Fc groups can be clicked to the H-Eth-GC layer after grafting the NP layer. Table entries IV and V describe experiments similar to those above but based on grafting from NBD solution. In these experiments, much milder grafting conditions were used than in our previous work where we observed the complete loss of HEth-GC activity after the second grafting step.15 Here, a small concentration of NBD (0.01 mM) and a relatively positive lower potential limit (0.2 V) were used for grafting to prevent the formation of a thick multilayer NP film. Under these conditions, the concentration of Fc groups that were clicked to the surface in the mixed film ((1.6 ± 0.1) × 10−10 mol cm−2, table entry IV) was the same as that obtained by the NPH grafting described above and was close to that for a singlecomponent Fc film subjected to the same experimental conditions ((2.0 ± 0.2) × 10−10 mol cm−2, table entry V). However, the surface concentration of NP groups in the mixed film ((0.6 ± 0.1) × 10−10 mol cm−2) was significantly smaller than expected for a monolayer and significantly smaller than that grafted to a bare GC surface under the same conditions

Figure 4. Plot of surface concentration of NP groups vs Fc groups for mixed films grafted via route 2a and using NBD as the second modifier. 3137

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Table 2. Thicknesses, d, of Films on PPF film structure (Scheme 2) d (nm) a

A

Ba

Ca

D

E

F

0.4 ± 0.3

0.7 ± 0.3

0.8 ± 0.4

0.9 ± 0.3

0.8 ± 0.2

0.7 ± 0.2

NP groups grafted from NPH.

three preparation pathways (Scheme 2) that result in mixed films with monolayer surface concentrations. By taking account of the large relative uncertainties in the thicknesses of these thin films, it has been found that there are no significant differences between the films and all are consistent with the proposed monolayer structure.16,20 Most importantly, the second grafting step does not result in multilayer formation, suggesting that under these conditions, when NP groups are grafted to H-EthPPF and Fc-Eth-PPF surfaces, grafting occurs within the layers (most likely to the PPF surface) rather than on top of the preexisting layers. Although our results demonstrate that both the diazonium ion and hydrazine derivatives of NP can be used to prepare mixed monolayer films, grafting from the hydrazine has advantages over the diazonium route. First, multilayer formation appears to be inherently less favored for the hydrazine. Pedersen and Daasbjerg have suggested that this may be due to the deposition of a physisorbed film on top of the grafted layer.12 The physisorbed material limits the growth of the grafted layer and is removed in postgrafting cleaning steps. A second advantage of the use of a hydrazine in the second grafting step is that there is no evidence for spontaneous grafting from arylhydrazine solutions,12 whereas spontaneous grafting at graphitic carbon from aryldiazonium salt solutions is a facile process.29 The practical consequence of these different behaviors is that monolayers can be grafted from the hydrazine with considerably less stringent control of the grafting conditions than is required when grafting from the diazonium ion.

surface with high reactivity for click chemistry and a significant concentration of NP groups. An alternative strategy for the preparation of mixed monolayers is shown in Scheme 2, step b. The reaction of HEth-GC with FcCH2N3 prior to grafting the second modifier removes any possibility of radical attack on ethynyl groups during the second grafting step. Table entries VII−X describe the results of preparing mixed layers by this route, using NPH and NBD under the same grafting conditions as for route 2a. Table entry VII shows that grafting from NPH solution gives remarkably similar surface concentrations of electroactive Fc ((1.6 ± 0.2) × 10−10 mol cm−2) and NP ((1.8 ± 0.6) × 10−10 mol cm−2) groups to those found using the route 2a strategy (table entry II). Similar surface concentrations of Fc ((1.5 ± 0.1) × 10−10 mol cm−2) and NP ((1.6 ± 0.3) × 10−10 mol cm−2) are also obtained when NP groups are grafted from a small concentration of NBD under mild conditions (experiment IX). Thus route 2b gives films with surface concentrations of Fc and NP groups consistent with a mixed monolayer. In a final set of grafting experiments, the conditions used in routes 2a and 2b were applied to grafting NP groups to TIPSEth-GC surfaces prior to deprotection (Scheme 1, step b). Table entries XI and XII show that small concentrations of NP groups were grafted under these conditions; this is attributed to the shielding of the electrode surface by the bulky TIPS groups. The most striking result from these electrochemical measurements is that experiments II, VII, and IX all generate mixed films with the same composition and with total surface concentrations consistent with monolayer films. The reproducibility of ΓFc between the films prepared by different routes is attributed to reproducible grafting and the subsequent deprotection of TIPS-Eth-N2+ and the reproducible reaction of H-Eth groups with FcCH2N3. As shown by the data in Table 1, any degradation of H-Eth or Fc moieties in the subsequent grafting steps is relatively minor. Reproducible surface concentrations of NP groups in films prepared by different routes is also expected if the final film structures match those shown in cartoon form in Scheme 2. The concentration of NP groups that can be accommodated in a monolayer structure depends on the “spacing” of groups grafted in the first step, and this will be constant for the reproducible grafting of TIPS-EthN2+. However, although our observation of reproducible surface concentrations of NP groups in experiments II, VII, and IX is consistent with the proposed monolayer structure, these results do not eliminate the possibility that NP groups have coupled to the first grafted layer (rather than the surface) or form oligomeric strands, anchored to the surface or to the first grafted layer. AFM investigations were undertaken to address these points. The thickness of surface films at various stages of preparation by routes 2a and 2b were measured by AFM depth profiling. This technique involves completely removing a small section of film using an AFM tip and then measuring the depth of the shaved area.28 PPF, rather than GC, is used as the substrate because of its lower surface roughness. Table 2 shows the average thicknesses of the surface films at each stage of the

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CONCLUSIONS

Electrochemical investigations and film thickness measurements confirm that monolayer films incorporating two different modifiers can be prepared by electrografting. The key steps are (i) to prepare a sparse monolayer of H-Eth groups by grafting from a solution of the TIPS-Eth diazonium derivative, followed by the removal of the TIPS group, and (ii) to fill in the sparse layer by grafting from a solution of the second diazonium ion, or the arylhydrazine. Optimization of the grafting conditions for the second step ensures that multilayers do not form; arylhydrazines are particularly well suited to this application because they have a low tendency to form multilayers. The reactivity of the H-Eth groups to click chemistry is maintained in the mixed monolayer, allowing facile postgrafting coupling reactions. Alternatively, H-Eth groups can be reacted prior to the second grafting step. The use of TIPS as the protecting group in TIPS-Eth-ArN2+ results in a layer with approximately equal concentrations of the two modifiers. Future work will investigate how mixed monolayers with different proportions of two modifiers might be prepared using other ethynylbenzenediazonium ions with differently sized protecting groups. Applications of mixed monolayers, in which the advantages of stable covalent attachment and controlled monolayer composition can be exploited, are also of interest. 3138

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AUTHOR INFORMATION

Corresponding Author

*Tel: +64 3 364 2501. Fax: +64 3 364 2110. E-mail: alison. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the MacDiarmid Institute for Advanced Materials and Nanotechnology and in part by the Agence Nationale de la Recherche (contract ANR10-BLAN714, Cavity-zyme(Cu) project). L.L. thanks the MacDiarmid Institute for a doctoral scholarship. We thank Dr. John Loring for the use of Linkfit curve-fitting software.

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dx.doi.org/10.1021/la400303x | Langmuir 2013, 29, 3133−3139