Surface-Attached Poly(glycidyl methacrylate) as a Versatile Platform

Jul 16, 2014 - epoxide ring-openings of the side chains. These polymer brushes represent an attractive chemical platform to deliberately introduce...
0 downloads 0 Views 886KB Size
Download PDF
Article pubs.acs.org/Macromolecules

Surface-Attached Poly(glycidyl methacrylate) as a Versatile Platform for Creating Dual-Functional Polymer Brushes Mie Lillethorup,† Kyoko Shimizu,† Nicolas Plumeré,*,‡ Steen U. Pedersen,*,†,§ and Kim Daasbjerg*,†,§ †

Department of Chemistry, Aarhus University, Langelandsgade 140, DK-8000 Aarhus C, Denmark Faculty of Chemistry and Biochemistry, Ruhr University Bochum, Universitätsstr. 150, D-44780 Bochum, Germany § Interdisciplinary Nanoscience Center (iNANO), Aarhus University, Gustav Wieds Vej 14, DK-8000 Aarhus C, Denmark ‡

S Supporting Information *

ABSTRACT: Novel types of dual-functional surface-attached polymer brushes were developed by post-polymerization modification of poly(glycidyl methacrylate) brushes on glassy carbon substrates. Azide and alcohol groups were initially introduced by epoxide ring-openings of the side chains. These polymer brushes represent an attractive chemical platform to deliberately introduce other molecular units at specific sites. In this work, ferrocene and nitrobenzene redox units were immobilized through the two groups to create redox polymers. In-depth analysis by infrared reflection− absorption spectroscopy and X-ray photoelectron spectroscopy revealed an almost quantitative conversion of the modification reactions. The electrochemical activity of the ferrocenyl part of this diode-like system was fully expressed with an electron transfer rate constant = 1.2 s−1 and surface density = 0.19 nmol cm−2 per nm section of the film, independent of its thickness. In contrast, for the nitrobenzene moieties diffusion of counterions (i.e., tetraalkylammonium) easily becomes the rate-controlling step, thereby leaving a substantial fraction of them electrochemically inactive.

■

RAFT)15,16 polymerization from appropriately immobilized initiator molecules have been used to grow polymer brushes on substrates. In principle, the most straightforward way to synthesize polymer brushes functionalized repetitively would be to employ a monomer containing the functional group of interest. Unfortunately, the conditions used for the polymerization of one kind of functional monomer to another are usually not the same, meaning that for each new monomer type all polymerization parameters would have to be optimized anew to have the right conditions for a controlled polymerization. In addition, elaborate and expensive synthesis of specialized monomers in the large quantities usually required for controlled polymerization reactions may be required. An alternative approach would be to grow a polymer brush containing a readily modifiable group once for all and, subsequently, postmodify this brush to obtain the desired functionality (or functionalities).17 In this context poly(glycidyl methacrylate) (PGMA) represents one of the most versatile platforms, considering that the epoxide group readily reacts with numerous types of nucleophiles, including carboxylates, alcohols, amines, thiols, and azides.17,18 Recently, it was demonstrated for polymers in solution that the hydroxyl group formed after the epoxide ring-opening reaction also

INTRODUCTION Functionalization of various substrate surfaces is of interest in research fields, including life science (i.e., sensors and biochips),1−3 molecular electronics,4 and chromatography.5,6 Moreover, the importance of a broader fundamental understanding of advanced materials has intensified the research interest. The most straightforward strategy to decorate surfaces goes through functional self-assembled monolayers (SAMs), where numerous sophisticated structures ranging from ferrocene-functionalized alkylthiols on gold7 to highly complex molecular assemblies of porphyrin−fullerene conjugates on indium−tin oxide electrodes8 have been presented. Such assemblies, also referred to as molecular nanotechnology,9 have the advantage that the modification is approaching control at molecular level. The main disadvantage of this, otherwise easily adaptable, approach is the limited number of substrates susceptible to SAM formation along with the low chemical, electrochemical, and mechanical stability of SAMs. In general, high density of functional groups can be created on a limited surface area, if the so-called skyscraper approach is used to extend the structure in the third dimension. This is, for example, widely applied in enzyme electrodes for sensing10 or energy conversion, 11,12 where an electron mediator is incorporated in a polymer matrix. Construction of polymer brushes represents a convenient way of obtaining the desired polymeric layer in a controlled fashion.13 In particular, surfaceinitiated atom transfer radical polymerization (SI-ATRP)14 and reversible addition−fragmentation chain-transfer (SI© XXXX American Chemical Society

Received: April 27, 2014 Revised: July 2, 2014

A

dx.doi.org/10.1021/ma500872b | Macromolecules XXXX, XXX, XXX−XXX

Macromolecules

Article

containing 0.1 M Bu4NBF4 or a saturated solution of ∼0.01 M Me4NBF4. At the end of each experiment, ferrocene was added to the electrolyte solution, and the formal redox potential of the Fc+/Fc couple was measured. All potentials were referenced against SCE using a previous determination of E0Fc+ = 0.41 V vs SCE in MeCN.27 Electrografting of 4-(2-Hydroxyethyl)benzenediazonium Tetrafluoroborate. GC plates were immersed in an oxygen-free solution of 0.1 M Bu4NBF4/MeCN at room temperature containing 2 mM 4-(2-hydroxyethyl)benzenediazonium tetrafluoroborate. First, one cyclic voltammogram was recorded on a test substrate at a sweep rate = 5 V s−1 to measure the peak potential (Ep) of the diazonium reduction wave. Subsequently, the substrates were modified by employing one voltammetric sweep between 0.5 V and Ep − 0.2 V at the same sweep rate to obtain hydroxyl-terminated substrates (denoted GC-OH). After modification the substrates were rinsed with MeCN followed by sonication in acetone for 10 min. Preparation of Initiator-Modified Substrates. GC-OH plates were immersed in a solution of 0.5 M 2-bromoisobutyryl bromide and 0.05 M TEA in DCM at room temperature for 3 h followed by rinsing in DCM and sonication in acetone for 10 min to obtain the initiatormodified substrates denoted GC-I. Preparation of Surface-Attached PGMA Brushes Using SIATRP. A solution of GMA (2.67 mL, 2.0 M) and Milli-Q water/ propan-2-ol (1:1; 7.33 mL) in a Schlenk flask was purged with argon for 30 min to remove oxygen. CuICl (29.7 mg, 0.030 M), CuIICl2· 2H2O (22.2 mg, 0.013 M), and BiPy (169 mg, 0.11 M) were added while the argon purging continued another 30 min. GC-I substrates were immersed in the solution, the flask was sealed with a rubber septum, and the polymerizations were allowed to run for 3 h under stirring at 25 °C. Subsequently, the plates were rinsed with acetone and water and sonicated in acetone for 10 min. The PGMA-modified substrates, denoted GC-PGMA, were analyzed by ellipsometry, infrared reflection−absorption spectroscopy (IRRAS), and X-ray photoelectron spectroscopy (XPS). Epoxide Ring-Opening of GC-PGMA Using NaN3. GC-PGMA plates were immersed in a solution of NaN3 (228 mg, 3.5 mmol) and NH4Cl (188 mg, 3.5 mmol) in 7.5 mL of DMF at 50 °C for 7 h under stirring. These substrates, denoted GC-PGMA-N3, were rinsed by water and acetone and sonicated in acetone. Analysis was performed by ellipsometry, IRRAS, and XPS. Functionalization of GC-PGMA-N3 with Propargylic Ferrocene Carboxylate Using Click Chemistry. 10 mL of Milli-Q water/ t-BuOH (3:1) was added to a Schlenk flask containing propargylic ferrocene carboxylate (4.6 mg, 0.017 mmol), CuSO4·5H2O (1.5 mg 0.006 mmol), and NaAsc (12 mg, 0.06 mmol), and the solution was purged with argon for 10 min. The GC-PGMA-N3 plates were immersed, letting the reaction proceed under stirring overnight at 40 °C. The resulting ferrocenyl-modified substrates (denoted GCPGMA-Fc) were rinsed in water followed by sonication in water and acetone. Ellipsometry, IRRAS, and XPS were used for the characterization part. Functionalization of GC-PGMA-Fc with 4-Nitrophenyl Isocyanate. TEA (2.5 μL, 0.02 mmol) was added to a solution of 4nitrophenyl isocyanate (32.8 mg, 0.20 mmol) in 10 mL of DCM. The solution was sonicated to obtain a suspension and heated to 40 °C, before the samples (GC-PGMA-Fc) were immersed overnight under stirring. The dual-functional surface-attached polymer brushes thereby obtained (denoted GC-PGMA-Fc/NO2) were rinsed and sonicated in acetone. Analysis was performed by ellipsometry, IRRAS, XPS, and cyclic voltammetry. Starting from GC-PGMA-N3, GC-PGMA-NO2 could be obtained under the same reaction conditions.

could be utilized in a second post-polymerization modification step, to create dual-functional polymers.19,20 The fact that two functionalities in this manner can be chemically linked to each repeating unit makes PGMA one of the most promising candidates for attaching unique properties to polymer brushes that may be exploited as they are or tuned to fulfill specific purposes. Additionally, poly(N-isopropylacrylamide)21 and polymeric activated esters, namely poly(pentafluorophenyl 4vinylbenzenesulfonate),22 have also been utilized for such double modifications. However, a question remains as to how easily these procedures may be applied to surface-attached polymer brushes. The task of introducing two new functionalities is not easy, considering the much more severe sterical constraints for these systems. Klok and co-workers have demonstrated how the interplay between graft density of the polymer brushes and size and polarity of molecules immobilized during single postpolymerization modifications influenced not only the conversion factors but also the spatial distribution of functionalities.23,24 Finally, it is worth mentioning a study by Rahane et al., on the synthesis of multifunctional polymer brushes utilizing postpolymerization modifications by orthogonal thiol-click reactions.25 In this work we demonstrate that PGMA brushes grown from an SI-ATRP process on glassy carbon (GC) can be used as a versatile chemical platform in the construction of doublefunctionalized films. A ring-opening reaction of the pendant epoxide groups with NaN3 gives repeating azide and alcohol groups, which are used as a chemical platform to create for the first time dual-functional polymer brushes consisting of two redox units, i.e., ferrocene and nitrobenzene. These postpolymerization modifications prove to be virtually quantitative with a high electrochemical activity of the film as the result. Without doubt, the possibility of introducing two functionalities separated by a defined distance in a polymer brush structure will be valuable in the future design of novel sensing and catalysis systems.

■

EXPERIMENTAL SECTION

Materials. HPLC grade dichloromethane (DCM), acetonitrile (MeCN), and hexane were all from Sigma-Aldrich. DCM and MeCN were dried according to standard procedures. N,N-Dimethylformamide (DMF), acetone, and propan-2-ol were all HPLC grade and purchased from VWR. Triethylamine (TEA), tert-butanol, oxalyl chloride, 2-bromoisobutyryl bromide, 2,2′-bipyridine (BiPy), CuICl, CuIICl2·2H2O, glycidyl methacrylate (GMA), sodium azide (NaN3), CuSO4·5H2O, sodium ascorbate (NaAsc), and 4-nitrophenyl isocyanate were used as received from Sigma-Aldrich. Ammonium chloride was purchased from Fluka. 4-(2-Hydroxyethyl)benzenediazonium tetrafluoroborate was synthesized as previously reported.26 The supporting electrolyte, tetrabutylammonium tetrafluoroborate, was prepared using a standard procedure, and tetramethylammonium tetrafluoroborate was purchased from Fluka. The inhibitor was removed from GMA by passing it through a column containing Al2O3. Propargylic ferrocene carboxylate was synthesized as described in the Supporting Information. Electrochemical Setup. A standard three-electrode electrochemical setup (CH Instruments 660B or 601C) consisting of a glassy carbon (GC) plate as working electrode, a platinum wire as auxiliary electrode, and a Ag/AgI pseudoreference electrode (i.e., a silver wire immersed in a MeCN solution containing 0.01 M Bu4NI and either 0.1 M Bu4NBF4 or saturated Me4NBF4) as reference electrode was used in all electrochemical measurements. The electrochemical analysis of the modified GC plates was conducted at room temperature in an oxygen-free and anhydrous MeCN solution

■

RESULTS AND DISCUSSION Preparation of PGMA Brushes. The immobilization of ATRP initiators on GC substrates was carried out following a two-step procedure (see Experimental Section), in which a surface-attached benzylic alcohol electrografted from 4-(2hydroxyethyl)benzenediazonium tetrafluoroborate was converted in an acylation reaction to an ester containing a tertiary B

dx.doi.org/10.1021/ma500872b | Macromolecules XXXX, XXX, XXX−XXX

Macromolecules

Article

Scheme 1. Poly(glycidyl methacrylate) Brushes Grown by Atom Transfer Radical Polymerization from Initiator-Modified Glassy Carbon Substrates Followed by Postpolymerization Modifications To Create Dual-Functional Polymer Brushes

in agreement with a previous assignment (Figure S1, Supporting Information).29 The C−Cl contribution at 287.0 eV was omitted from this peak fitting. Interestingly, as revealed from XPS depth profiling of a 40 nm PGMA film, the atomic concentration of Cl is almost constant (0.19 ± 0.03%) starting from the top layer and throughout the PGMA film, which suggests that the endcapped PGMA chains are uniformly distributed in the dry-state film (Figure S2, Supporting Information). At the same time it was noted that the Cl content obtained from survey spectra, in general, decreased with increasing thickness of the PGMA films, indicating that end-group analysis should be applicable in a calculation of the average number of repeating units in each polymer chain. On that assumption we find for this specific 40 nm film the number to be 53 ± 7 (see Supporting Information), corresponding to an average molecular weight, M = (7.6 ± 1.3) × 103 g mol−1. Finally, from the density of PGMA = 0.805 g cm−3 and the thickness of the PGMA film, a grafting density = 2.6 ± 0.4 chains per nm2 can be obtained. This suggests that the PGMA chains are in the brush regime.30 With a content of Br in the initiator film of 0.4% this is indeed expected to be so according to previous studies.26,28 In principle, the number of PGMA chains calculated would be overestimated, if traces of the catalyst, CuCl/CuCl2, should be left in the PGMA brush, but with the Cu content being below the detection limit this does not seem to be the case. In fact, an underestimation of the calculated grafting density is more likely, considering that termination reactions always will occur with a loss in Cl as consequence. Figure 2 shows the analysis of GC-PGMA by infrared reflection−absorption spectroscopy (IRRAS). The bands at 3000 and 2945 cm−1 are assigned to C−H sp3 bond stretching, 1736 cm−1 to the CO bond stretch of the unconjugated ester group, 1490−1440 cm−1 to C−H deformations, and 1300− 1050 cm−1 to the C−O stretch. The presence of the epoxide group is confirmed by the band appearing at 909 cm−1.31 A linear relationship equivalent to that for the film thickness is found for the IR absorbance, if plotted against the polymerization time (not shown). Ring-Opening Reaction of GC-PGMA with NaN3. The initial step in postmodifying GC-PGMA consists of nucleophilic ring-opening of the central epoxide unit. For PGMA polymer brushes and PGMA in solution the kind of nucleophiles used for that purpose has mainly involved amines,24,29,32 NaN3,33 and, most recently, thiols, in the case of which both linear19 and branched34 polymer systems were studied. In this work on surface-attached PGMA brushes, NaN3

bromoalkyl group, i.e., the radical initiator. A full description of this well-known procedure along with a characterization of the initiator-modified GC substrates is available elsewhere.28 Noteworthy, the electrochemical grafting of the diazonium salt was carried out by a single voltammetric sweep to ensure the formation of a thin and electrochemically nonblocking film. PGMA brushes were grown from the initiator-modified GC substrates by SI-ATRP at room temperature in water/propan-2ol using CuCl/CuCl2 and BiPy as the catalyst system (Scheme 1). The polymerization is controlled as evidenced from the linear relationship observed between the polymer brush thickness, d, and polymerization time (Figure 1). From 6 independent measurements d was determined to be 25 ± 2 nm after 3 h. The PGMA-modified GC substrates are henceforth denoted GC-PGMA.

Figure 1. Dry film thickness, d, of PGMA brushes measured by ellipsometry versus the polymerization time.

A survey spectrum obtained by X-ray photoelectron spectroscopy (XPS) of the GC-PGMA film shows as expected the C and O components of PGMA, (Table 1) with a C/O ratio = 2.7 that is only slightly higher than the expected value of 2.3. We attribute the excess in carbon content to contributions from the underlying GC substrate accessed through the PGMA brush. A component in the survey spectrum of GC-PGMA at 200.0 eV is attributed to Cl 2p from the end-capped PGMA chains. Deconvolution of the C 1s high-resolution spectrum is C

dx.doi.org/10.1021/ma500872b | Macromolecules XXXX, XXX, XXX−XXX

Macromolecules

Article

Table 1. Atomic Concentrations Obtained by Means of XPS for GC-PGMA, GC-PGMA-N3, GC-PGMA-Fc, and GC-PGMA-Fc/ NO2 C 1s (atom %) a

GC-PGMA GC-PGMA-N3b GC-PGMA-Fcc,d GC-PGMA-Fc/NO2d,e

72.7 58.5 71.8 68.8

± ± ± ±

0.3 0.4 0.5 0.4

O 1s (atom %)

Cl 2p (atom %)

± ± ± ±

0.3 ± 0.1 0.4 ± 0.1 0.1 ± 0.1

27.0 22.2 16.4 17.9

0.3 0.2 0.4 0.2

N 1s (atom %)

Fe 2p (atom %)

18.9 ± 0.5 8.6 ± 0.2 11.2 ± 0.2

2.9 ± 0.1 2.0 ± 0.1

Brush length = 25 ± 2 nm. bBrush length = 25 ± 3 nm. cBrush length = 65 ± 3 nm. dTraces of Na are also detected, presumably originating from the NaAsc used as a reducing agent during the “click” reaction. eBrush length = 87 ± 3 nm.

a

Figure 2. IRRAS recorded on GC-PGMA prior and after post-polymerization modifications: PGMA (black), GC-PGMA-N3 (red), GC-PGMA-Fc (green), and GC-PGMA-Fc/NO2 (blue). For comparison, a spectrum of GC-PGMA-NO2 is shown in the upper panel (light blue).

band at 909 cm−1 combined with the appearance of the broad band at 3580 cm−1, pertaining to O−H stretching, completes the picture of a successful ring-opening reaction. Noteworthy, the CO band for GC-PGMA-N3 is broadened compared to that of GC-PGMA which, most likely, is due to partial hydrogen bonding between hydroxyl groups from the ringopened epoxide and CO ester groups. “Click” Functionalization of GC-PGMA-N3. The CuAAC is included in the class of “click” reactions, which, in general, fulfill requirements such as being wide in scope, selective, high yielding, and with minimal purification needs.38 Specifically, the utility of the “click” reaction could be illustrated herein by reacting GC-PGMA-N3 with propargylic ferrocene carboxylate to obtain surface-attached redox polymers, GC-PGMA-Fc (Scheme 1). Because of their reversible one-electron redox response, ferrocene (Fc) groups do not only give straightforward access to electrochemical quantification of the reaction efficiency but also add responsive features to brushes.39 Concerning the characterization of the GC-PGMA-Fc film, its thickness is determined to be 65 ± 3 nm, which is 2.5 times larger than the 25 ± 3 nm determined for GC-PGMA-N3. The XPS analysis confirms a close to 100% conversion, in that the elemental composition of GC-PGMA-Fc gives N/Fe = 2.95, close to the expected 3 (Table 1). Most notable in the highresolution C 1s spectrum is the appearance of a component at 284.7 eV (Figure S1), assigned to the C’s in the two cyclopentadienyl rings of the ferrocene moiety and CC of the triazole. The high-resolution N 1s spectrum (Figure S3) shows a broad peak, which can be resolved into two components centered at 400.4 and 401.8 eV of 2:1 ratio.

was the preferred nucleophile, not only because of a high nucleophilicity and conversion efficiency but also because of its usefulness as a chemical platform for introducing countless other substituents via the versatile copper-catalyzed azide− alkyne cyclization (CuAAC) “click” reaction. The reaction with NaN3 in DMF at 50 °C for 7 h was conducted in the presence of NH4Cl to protonate the alkoxides formed during the ringopening, thereby preventing side reactions.35 The transformation of GC-PGMA to GC-PGMA-N3 had no implications for the ellipsometrically determined film thickness which may be attributed to the small size of the azide group. However, from the appearance of N in the XPS survey spectrum and the extracted atomic concentration = 18.9 ± 0.5% (see Table 1) the reaction conversion may be calculated to be 85%. Most reliably, this is done from the O/N ratio (= 1.17) considering that the C/N ratio (= 3.10) will be affected by the contribution from carbon at the GC surface. Deconvolution of the high-resolution C 1s spectrum gives five peaks located at binding energies of 285.0, 285.8, 286.1, 286.8, and 289.0 eV (Figure S1), corresponding to C−C/C−H, C*−(CO)−O, C−N, C−O, and O−C*O functional groups, respectively.36 Deconvolution of the N 1s peaks at 404.3 and 400.7 eV gives a 1:2 ratio (Figure S3, Supporting Information) as expected for an azide group having a much electron-poorer central N compared with the other two nitrogen atoms.37 Also, in IRRAS the presence of the azide group in GCPGMA-N3 was confirmed, in that the band observed at 2112 cm−1 is assigned to the asymmetric stretch of an organic azide (Figure 2).31 In addition, the disappearance of the epoxide D

dx.doi.org/10.1021/ma500872b | Macromolecules XXXX, XXX, XXX−XXX

Macromolecules

Article

from amide II, at 1510 and 1345 cm−1 from asymmetric and symmetric stretches of NO2, respectively, and at 1218 cm−1 from amide V (stretch of the N−aryl bond in Narylcarbamates)31 all confirm the successful formation of dual-functional polymer brushes. Electrochemical Analysis. Analysis by ellipsometry, XPS, and IRRAS has been able to reveal close to full conversion of epoxide ring-opening of GC-PGMA with NaN3 followed by two essentially quantitative postpolymerization steps, i.e., the copper-catalyzed azide−alkyne cyclization of azide groups with propargylic ferrocene carboxylate and the carbamate formation from the nucleophilic addition of the hydroxyl groups to 4nitrophenyl isocyanate. The introduction of the redox-active Fc and nitrobenzene groups further allows us to perform an electrochemical analysis to provide more detailed insight into the molecular environment of these groups. A series consisting of 6 GC-PGMA-Fc/NO2 samples with film thickness ranging from 6 to 90 nm were therefore subjected to cyclic voltammetric studies. Before looking into the overall trends for this series, the general electrochemical features under both reductive and oxidative conditions will be described using the 35 nm GC-PGMA-Fc/NO2 sample as a representative example. Figure 3 shows the three first cyclic voltammograms recorded for this sample on the reduction of the nitrobenzene

These bonding energies agree with previous reports on the triazole functionality,40,41 the formation of which in GCPGMA-Fc is the immediate chemical result of the “click” reaction between the azide group and alkyne. At the same time the high-energy peak at 404.3 eV, assigned to the electrondeficient central N of the azide group in GC-PGMA-N3, is completely gone, thus underlining the 100% efficiency of the “click” reaction. The IRRAS analysis fully supports this conclusion, in that the asymmetric stretch of the organic azide observed at 2112 cm−1 for GC-PGMA-N3 is absent after the “click” reaction (Figure 2). The extra component at the CO band observed at lower wavenumbers (1700 cm−1) originates from the CO stretch mode of a conjugated ester (here conjugated to Fc), which also gives rise to more intensive C−O bands. Furthermore, the C− C stretch at 1458 cm−1 and sp2-hybridized C−H bands above 3000 cm−1 are consistent with the cyclopentadienyl rings of GC-PGMA-Fc. Dual-Functional PMGA Brushes. The free hydroxyl groups present in GC-PGMA-Fc can be utilized to introduce a second functionality to each repeating unit of the polymer brushes. In this work nitrobenzene moieties were immobilized by reacting GC-PGMA-Fc with 4-nitrophenyl isocyanate to obtain GC-PGMA-Fc/NO2 through formation of carbamate bonds (Scheme 1). Nitrobenzene groups are, like Fc groups, electrochemically active and readily undergo one-electron reductions (rather than oxidations). The thickness of the film on GC-PGMA-Fc/NO2 measured by ellipsometry is 87 ± 3 nm or 34% larger than that of GCPGMA-Fc. The XPS analysis given in Table 1 confirms that this increase in thickness can be attributed to the introduction of nitrobenzene groups, noticing that the O/N ratio = 1.59 is very close to the predicted 1.6. Surprisingly, the N/Fe ratio (= 5.6) is higher than the expected 5, which would suggest that some Fe is lost during the last postmodification step or that Fc units to a higher extent than nitrobenzene groups are prone to be buried deeper into the film. With a 10 nm analysis depth this would let them more easily escape detection. In the highresolution C 1s spectrum of GC-PGMA-Fc/NO2 a broadening of the O−CO component at 289.2 eV is seen compared to that of GC-PGMA-Fc (Figure S1). This is attributed to the contribution from the carbamate, N−(CO)−O, that appears at slightly higher binding energy than the ester O−CO. The high-resolution N 1s spectrum (Figure S3) reveals a 3:1:1 ratio for three nitrogen components at 400.5, 401.9, and 406.1 eV. Clearly, the new peak observed at 406.1 eV pertains to the electron-deficient N in the nitro group,42 while the other two peaks again are attributed to the three nitrogen atoms in the triazole unit. According to a previous report, the binding energy of the carbamate N appears at 400.3 eV,43 thus overlapping with the first triazole component. Hence, the experimentally observed 3:1:1 ratio is in line with that predicted for a postmodification step that is quantitative. Figure 2 depicting the IRRAS spectrum of GC-PGMA-Fc/ NO2 is in agreement with carbamate formation, in that the broad O−H band at high wavenumber for GC-PGMA-Fc is seen to have been replaced by a narrower and slightly weaker band at 3330 cm−1, which is attributed to the N−H stretch of carbamate. The CO band is additionally broadened compared to that of GC-PGMA-Fc since it now includes three CO stretch modes, where the one at 1743 cm−1 corresponds to the CO stretch in carbamates. Furthermore, bands at 1600 cm−1 from CC (i.e., aromatic), at 1550 cm−1

Figure 3. First (black), second (red), third (green), fourth (blue), and fifth (magenta) cyclic voltammograms of GC-PGMA-Fc/NO 2 recorded at a sweep rate = 0.02 V s−1 in 0.01 M Me4NBF4/MeCN. The first three cycles are restricted to the reductive region of the nitrobenzene group (−0.65 V → −1.65 V → −0.65 V), while the last two cycles include the positive potential region of the Fc group (−0.25 V → −1.65 V → 1.05 V → −0.25 V).

group in the negative potential direction followed by reoxidation of the nitrobenzene radical anion on the reverse sweeps. The standard potential, E°, = −1.32 V vs SCE obtained as the midpoint of the two peak potentials is in close agreement with previous determinations for this redox group.44 At the second cathodic scan, the surface coverage of nitro groups, Γ(NO2), which may be extracted by integrating the area under the redox wave,45 reaches a maximum of 4.7 × 10−9 mol cm−2. Upon further cycling a decrease in Γ(NO2) is seen, most probably caused by protonation of the radical anion by the residual amounts of water always present in an acetonitrile solution.46 In any case this number is lower than expected, if all E

dx.doi.org/10.1021/ma500872b | Macromolecules XXXX, XXX, XXX−XXX

Macromolecules

Article

Figure 4. (A) Plot of surface coverages, Γ(NO2) (black circle) and Γ(Fc) (red circle), obtained by integration of the reduction wave of the second cathodic and the first anodic cyclic voltammogram, respectively, recorded in 0.01 M Me4NBF4/MeCN (sweep rate = 0.02 V s−1), against the dry film thickness, d, of GC-PGMA-Fc/NO2, the exception being for the 90 nm thick GC-PMGA-Fc/NO2 film, where the maximum Γ(NO2) value was reached at the ninth cycle. (B) Plot of the normalized absorbance of the IRRAS bands pertaining to the sum of the asymmetric and symmetric NO2 stretches at 1510 and 1345 cm−1 (green triangle) and the C−C stretch from Fc at 1458 cm−1 (blue square) against d.

almost disappeared, whereas the Fc/Fc+ redox wave is essentially unchanged. This indicates that the irreversible oxidation wave can be attributed to a cleavage of the carbamate bond. It is noted that this irreversible oxidation wave is not seen on the second anodic cycle or even the first one, if the potential range from the beginning was restricted to the anodic region of Fc only. In other words, the generation of the radical anion of the nitrobenzene groups is a prerequisite for bringing this phenomenon forward. We hypothesize that the role of the nitrobenzene radical anion is to act as a base and deprotonate the carbamate group, thereby lowering its oxidation potential and facilitating the oxidative cleavage. This interpretation finds support in a preliminary investigation of freely diffusing isopropyl (4-nitrophenyl)carbamate in 0.1 M Bu4NBF4/ MeCN, where the pertinent oxidation wave only appears in cyclic voltammetry, if the compound first has been reduced on a reductive sweep. In any case, the coverage determined of Fc is independent of whether the nitrobenzene group was cleaved off or not, strongly suggesting that all Fc groups are electrochemically addressable with a uniform distribution in the film. In contrast, the observation that successive sweeping is required to reach the nitrobenzene groups in the film suggests that the nitrobenzene groups have an ability to assemble in polar clusters, thereby protecting themselves from the electrolyte solution. To address this issue specifically, the electrochemical signature of NO2 and Fc was analyzed using cyclic voltammetry for all six GC-PGMAFc/NO2 samples with film thickness ranging from 6 to 90 nm. Figure 4A shows plots of Γ(NO2) and Γ(Fc) against d for the series of GC-PGMA-Fc/NO2 films. First, the linear relationship of Γ(Fc) with d may be noted, and it, furthermore, correlates with the increase in IRRAS absorbance of the Fc band at 1458 cm−1 (see Figure 4B and Figure S6 in the Supporting Information), which suggests a uniform distribution of Fc groups throughout the polymer brushes. The slope of the former gives a surface density = (0.19 ± 0.08) nmol cm−2 (per nm section of the film). A more than twice as high value (= 0.43 nmol cm−2) was obtained for polymer brushes constructed directly from ferrocenylmethyl methacrylate,28 which would

nitrobenzene groups in the brush had been accessible to the electrochemical process (vide inf ra). The shape of the voltammograms is indicative of a diffusioncontrolled process, despite the fact that the nitrobenzene groups are surface attached.45 The only true diffusing species in this process is the tetramethylammonium ion from the supporting electrolyte since it needs to be transported to the reduction sites to compensate for the developing negative charge at the nitrobenzene groups upon reduction. If the tetramethylammonium ion in the electrolyte solution is replaced with the more bulky tetrabutylammonium ion, at least 10 cyclic voltammograms are required before a maximum in Γ(NO2) is reached (Figure S4, Supporting Information). This signifies that the postmodified polymer brushes have a sufficiently large compactness to impede the diffusion of the counterions inside the film. Figure 3 also displays the result of extending two cyclic voltammograms to cover the reversible oxidation process of the Fc group with E° = 0.69 V vs SCE. The waves are bell-shaped with a peak separation, ΔEp, of only 30 mV, which is in line with a surface-confined redox process. In this case the transport of counterions during oxidation is taken care of by the small BF4− that exerts no kinetic limitations on the electron transfer process. The apparent rate constant of electron transfer, ks, was determined to be 1.2 ± 0.2 s−1 based on Laviron’s formalism (Figure S5, Supporting Information).47 In comparison, for poly(ferrocenylmethyl methacrylate) brushes a slightly higher value of 2.6 ± 0.2 s−1 was found.28 The surface coverage of the electrochemically accessible Fc groups, Γ(Fc), estimated from the integration of the background-subtracted Fc+ reduction wave, is 8.2 × 10−9 mol cm−2, which is almost twice as high as that determined for Γ(NO2). In general, such prominent features are very important for the development of efficient electrocatalysis systems, where integration of the molecular catalyst with associated electron mediators in polymer brushes would be important for ensuring that the overall electron transfer process is fast. A large irreversible oxidation peak is seen at 0.45 V vs SCE on the first anodic sweep. On the subsequent cycle this irreversible oxidation along with the nitrobenzene signal has F

dx.doi.org/10.1021/ma500872b | Macromolecules XXXX, XXX, XXX−XXX

Macromolecules

Article

immobilized secondary amines can undergo inter- and intrachain cross-linking reactions with neighboring epoxide moieties, which decreases the conversion efficiency in itself and, furthermore, decreases the mobility/flexibility of the polymer chains and, hence, constrains the diffusion of species inside the brush layer. In our study a comparatively stronger nucleophile, N3−, is used which is expected to increase the rate of postmodification considerably in accordance with observations. The same positive effect arises from the use of the aprotic N,Ndimethylformamide as solvent rather than water providing a favorable swelling of the brushes. Noteworthy, XPS depth profiling measurements demonstrated that the GC-PGMA brushes were in the brush regime (vide supra). Even then, both chemical postmodifications steps proceeded with excellent yields, showing that quantitative postmodifications may be achieved for high-density brushes, if the conditions are optimal. Noteworthy, the steric conditions of these resulting dualfunctionalized brushes hinder severely the diffusion of tetraalkylammonium ions inside the film but not tetrafluoroborate ions as evidenced from electrochemical experiments.

also be expected from the relative size difference of the Fccontaining units. In contrast, Γ(NO2) reaches a maximum for a 60 nm film, noticing that almost the same value as for Γ(Fc) is obtained for the thinnest films, while for the thickest films it seems to approach zero. As already concluded from the spectroscopic analysis (vide supra), the small electrochemical signal of the NO2 groups is not due to them not being physically present in the film. In fact, even for the thickest film no notable O−H band appears as would have been the case, if these had not reacted fully with the 4-nitrobenzene isocyanate in the last postmodification step (Figure S7, Supporting Information). This is further substantiated by a linear plot obtained of the absorbance of the nitrobenzene and the Fc groups in IRRAS as a function of d (see Figure 4B), which shows that the distribution of both groups is uniform and the same throughout the film. Hence, the electrochemical reduction of NO2 groups is limited by the diffusion of counterions and their ability to enter the polymer brush. For the thickest 90 nm film this effect is, in particular, pronounced, in that nine cycles were required before an appreciable signal was attainable with the tetramethylammonium as cation. A study on hyperbranched ferrocenyl polymer films has observed similar limitations in diffusion of counterions for thick polymer films.48 Brooksby and Downard encountered also deviations from the linear correlation between the electrochemically determined coverage and film thickness in their investigations of 4nitroazobenzene films.49 It was suggested that the electrochemical reduction of nitro groups in the film in aqueous acidic medium started from the solution−film interface, then becoming limited by the diffusion of counterions (i.e., protons) within the layer. Furthermore, when grafting a nonelectrochemically active polyphenylene layer on a 4-nitrobenzene film, a decrease in the rate of electron transfer caused by slow counterion permeation has been observed.44 In this context two studies by the Klok group should be mentioned, where the distribution of functional groups in postmodified polymer brush layers was addressed.23,24 They demonstrated how the interplay between graft density of the polymer brushes and size and polarity of molecules immobilized during the post-polymerization modifications influenced the conversion factors as well as the spatial distribution of functionalities. In the first of these studies side-chain hydroxyl groups of poly(2-hydroxyethyl methacrylate) brushes were reacted with p-nitrophenyl chloroformate and then further modified with deuterated leucine and serine.23 The postmodification with leucine only exhibited high conversion throughout the polymer brush at low grafting density, whereas the conversions obtained by reaction with serine was independent of the density. This study signifies that the nature of the nucleophilic compound (e.g., differences in size and polarity) influences both its reactivity and ability to penetrate the polymer brush. In their second study dealing with post-polymerization modification of a 100 nm thick PGMA brush with the primary amine, propylamine, and with bovin serum albumin (BSA) in aqueous solutions, propylamine became incorporated homogeneously throughout the brush whereas the much larger BSA were only immobilized in the top part.24 Despite the homogeneous distribution in the former case, the conversion was only 40−60% after 48 h reaction time, which is considerable lower than the conversions reported herein. After the epoxide ring-opening with primary amines, the

■

CONCLUSION This study demonstrated that polymer brushes of poly(glycidyl methacrylate) (PGMA) and, in particular, the azide-reacted and ring-opened PGMA brushes may serve as a versatile chemical platform for synthesizing easily tunable dual-functional polymer brushes through post-polymerization modifications. As a proofof-concept ferrocene and nitrobenzene moieties were incorporated through reactions at the azide and hydroxyl groups at the side chains of ring-opened PGMA. Analysis by infrared reflection−absorption spectroscopy, ellipsometry, and X-ray photoelectron spectroscopy provided detailed information on all steps and demonstrated full conversion in all cases. The distributions of both groups were shown to be uniform and the same throughout the film. Subjected to an electrochemical analysis, the electrochemical responsive polymer brushes comprising ferrocene and nitrobenzene groups displayed reversible redox responses. All ferrocene groups were electrochemically active (requiring diffusion of the tetrafluoroborate counterion into the film) as revealed from a linear correlation between the coverage and the film thickness which, furthermore, provided a surface density of 0.19 nmol cm−2 (per nm section of the film), independent of the film thickness. In contrast, the coverage of electrochemically accessible NO2 reached a maximum at a polymer brush thickness of 60 nm, thereafter decreasing to approach zero for the thickest film (∼90 nm). This is explained by the increasingly larger sterical constraints of the high-density postmodified polymer brushes that limit the diffusion into the film of the larger tetramethylammonium counterions required for the reduction process. In future studies, it would be interesting to apply different post-polymerization modifications of PGMA brushes to co-immobilize electron relays and active molecules for sensing or electrocatalytic applications.

■

ASSOCIATED CONTENT

S Supporting Information *

Materials and experimental procedures; Figures S1−S7. This material is available free of charge via the Internet at http:// pubs.acs.org. G

dx.doi.org/10.1021/ma500872b | Macromolecules XXXX, XXX, XXX−XXX

Macromolecules

■

Article

(26) Iruthayaraj, J.; Chernyy, S.; Lillethorup, M.; Ceccato, M.; Røn, T.; Hinge, M.; Kingshott, P.; Besenbacher, F.; Pedersen, S. U.; Daasbjerg, K. Langmuir 2011, 27, 1070−1078. (27) Daasbjerg, K.; Pedersen, S. U.; Lund, H. In General Aspects of the Chemistry of Radicals; Alfassi, Z. B., Ed.; Wiley: Chichester, UK, 1999; pp 385−427. (28) Lillethorup, M.; Torbensen, K.; Ceccato, M.; Pedersen, S. U.; Daasbjerg, K. Langmuir 2013, 29, 13595−13604. (29) Barbey, R.; Klok, H.-A. Langmuir 2010, 26, 18219−18230. (30) Brittain, W. J.; Minko, S. J. Polym. Sci., Part A: Polym. Chem. 2007, 45, 3505−3512. (31) Sokrates, G. In Infrared and Raman Characteristic Group Frequencies: Tables and Charts, 3rd ed.; Wiley: New York, 2004. (32) Xu, F. J.; Cai, Q. J.; Li, Y. L.; Kang, E. T.; Neoh, K. G. Biomacromolecules 2005, 6, 1012−1020. (33) He, F.; Luo, B.; Yuan, S.; Liang, B.; Choong, C.; Pehkonen, S. O. RSC Adv. 2014, 4, 105−117. (34) Gadwal, I.; Khan, A. Polym. Chem. 2013, 4, 2440−2444. (35) Tsarevsky, N. V.; Bencherif, S. A.; Matyjaszewski, K. Macromolecules 2007, 40, 4439−4445. (36) Beamson, G.; Briggs, D. In High Resolution XPS of Organic Polymers, The Scienta ESCA300 Database; Wiley: Chichester, 1992. (37) Wollman, E. W.; Kang, D.; Frisbie, C. D.; Lorkovic, I. M.; Wrighton, M. S. J. Am. Chem. Soc. 1994, 116, 4395−4404. (38) Kolb, H. C.; Finn, M. G.; Sharpless, K. B. Angew. Chem., Int. Ed. 2001, 40, 2004−2021. (39) Lallana, E.; Tirelli, N. Macromol. Chem. Phys. 2013, 214, 143− 158. (40) Liu, X.; Zheng, H.-N.; Ma, Y.-Z.; Yan, Q.; Xiao, S.-J. J. Colloid Interface Sci. 2011, 358, 116−122. (41) Ciampi, S.; Böcking, T.; Kilian, K. A.; James, M.; Harper, J. B.; Gooding, J. J. Langmuir 2007, 23, 9320−9329. (42) Ortiz, B.; Saby, C.; Champagne, G. Y.; Bélanger, D. J. Electroanal. Chem. 1998, 455, 75−81. (43) Davies, P. R.; Keel, J. M. Surf. Sci. 2000, 469, 204−213. (44) Ceccato, M.; Nielsen, L. T.; Iruthayaraj, J.; Hinge, M.; Pedersen, S. U.; Daasbjerg, K. Langmuir 2010, 26, 10812−10821. (45) Bard, A. J.; Faulkner, L. R. In Electrochemical Methods. Fundamentals and Applications, 2nd ed.; John Wiley & Sons: New York, 2001. (46) Nielsen, L. T.; Ceccato, M.; Holm, A. H.; Kristensen, M. V.; Pedersen, S. U.; Daasbjerg, K. Langmuir 2009, 25, 12160−12168. (47) Laviron, E. J. Electroanal. Chem. 1979, 101, 19−28. (48) Huo, J.; Wang, L.; Yu, H.; Deng, L.; Ding, J.; Tan, Q.; Liu, Q.; Xiao, A.; Ren, G. J. Phys. Chem. B 2008, 112, 11490−11497. (49) Brooksby, P. A.; Downard, A. J. J. Phys. Chem. B 2005, 109, 8791−8798.

AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected], Tel +49 49234 3229434, Fax +49 234 3214420 (N.P.). *E-mail [email protected], Tel +45 8942 3908, Fax +45 8619 6199 (S.U.P.). *E-mail [email protected], Tel +45 8942 5965, Fax +45 8619 6199 (K.D.). Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The Danish National Research Foundation, Center for Oxygen Microscopy and Imaging as well as the Cluster of Excellence RESOLV (EXC 1069) funded by the Deutsche Forschungsgemeinschaft (DFG) are gratefully acknowledged for financial support.

■

REFERENCES

(1) Adhikari, B.; Majumdar, S. Prog. Polym. Sci. 2004, 29, 699−766. (2) Lange, U.; Roznyatovskaya, N. V.; Mirsky, V. M. Anal. Chim. Acta 2008, 614, 1−26. (3) Jonkheijm, P.; Weinrich, D.; Schröder, H.; Niemeyer, C. M.; Waldmann, H. Angew. Chem., Int. Ed. 2008, 47, 9618−9647. (4) McCreery, R. L. Chem. Rec. 2012, 12, 149−163. (5) Plumeré, N.; Speiser, B.; Dietrich, B.; Albert, K.; Pesek, J. J.; Matyska, M. T. Langmuir 2009, 25, 13481−13487. (6) Block, H.; Maertens, B.; Spriestersbach, A.; Brinker, N.; Kubicek, J.; Fabis, R.; Labahn, J.; Schäfer, F. In Methods in Enzymology; Richard, R. B., Murray, P. D., Eds.; Academic Press: New York, 2009; Vol. 463, Chapter 27, pp 439−473. (7) Lee, L. Y. S.; Sutherland, T. C.; Rucareanu, S.; Lennox, R. B. Langmuir 2006, 22, 4438−4444. (8) Yamada, H.; Imahori, H.; Nishimura, Y.; Yamazaki, I.; Ahn, T. K.; Kim, S. K.; Kim, D.; Fukuzumi, S. J. Am. Chem. Soc. 2003, 125, 9129− 9139. (9) Gooding, J. J.; Ciampi, S. Chem. Soc. Rev. 2011, 40, 2704−2718. (10) Ohara, T. J.; Rajagopalan, R.; Heller, A. Anal. Chem. 1993, 65, 3512−3517. (11) Kothe, T.; Plumeré, N.; Badura, A.; Nowaczyk, M. M.; Guschin, D. A.; Rögner, M.; Schuhmann, W. Angew. Chem., Int. Ed. 2013, 52, 14233−14236. (12) Kothe, T.; Pöller, S.; Zhao, F.; Fortgang, P.; Rögner, M.; Schuhmann, W.; Plumeré, N. Chem. Eur. J. 2014, DOI: 10.1002/ chem.201402585. (13) Milner, S. T. Science 1991, 251, 905−914. (14) Hui, C. M.; Pietrasik, J.; Schmitt, M.; Mahoney, C.; Choi, J.; Bockstaller, M. R.; Matyjaszewski, K. Chem. Mater. 2013, 26, 745−762. (15) Hong, C.-Y.; You, Y.-Z.; Pan, C.-Y. Chem. Mater. 2005, 17, 2247−2254. (16) Li, Y.; Benicewicz, B. C. Macromolecules 2008, 41, 7986−7992. (17) Gauthier, M. A.; Gibson, M. I.; Klok, H.-A. Angew. Chem., Int. Ed. 2009, 48, 48−58. (18) Günay, K. A.; Theato, P.; Klok, H.-A. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 1−28. (19) De, S.; Khan, A. Chem. Commun. 2012, 48, 3130−3132. (20) De, S.; Stelzer, C.; Khan, A. Polym. Chem. 2012, 3, 2342−2345. (21) Reinicke, S.; Espeel, P.; Stamenović, M. M.; Du Prez, F. E. ACS Macro Lett. 2013, 2, 539−543. (22) Kakuchi, R.; Theato, P. Polym. Chem. 2014, 5, 2320−2325. (23) Schüwer, N.; Geue, T.; Hinestrosa, J. P.; Klok, H.-A. Macromolecules 2011, 44, 6868−6874. (24) Barbey, R.; Laporte, V.; Alnabulsi, S.; Klok, H.-A. Macromolecules 2013, 46, 6151−6158. (25) Rahane, S. B.; Hensarling, R. M.; Sparks, B. J.; Stafford, C. M.; Patton, D. L. J. Mater. Chem. 2012, 22, 932−943. H

dx.doi.org/10.1021/ma500872b | Macromolecules XXXX, XXX, XXX−XXX