Langmuir 2008, 24, 8959-8963
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Surface Chemical Functionalization of Cylindrical Nanopores Derived from a Polystyrene-Poly(methylmethacrylate) Diblock Copolymer via Amidation Yongxin Li and Takashi Ito* Department of Chemistry, Kansas State UniVersity, 111 Willard Hall, Manhattan, Kansas 66506 ReceiVed March 31, 2008. ReVised Manuscript ReceiVed May 19, 2008 This paper demonstrated covalent functionalization of surface -COOH groups on cylindrical nanopores derived from a polystyrene-poly(methylmethacrylate) diblock copolymer (PS-b-PMMA) via amidation mediated by 1-ethyl3-(3-dimethylaminopropyl)carbodiimide (EDC). The surface functionalization led to conversion of the surface charge of the nanopores and also to the shrinkage of effective pore radius, as verified using cyclic voltammetry for PS-bPMMA-derived nanoporous films immobilized on gold substrates. For native PS-b-PMMA-derived nanoporous films, the redox current of anionic Fe(CN)63- decreased with increasing solution pH due to the deprotonation of the surface -COOH groups, whereas those of cationic Ru(NH3)63+ and uncharged 1,1′-ferrocenedimethanol (Fc(CH2OH)2) were similar regardless of pH. In contrast, upon EDC-mediated amidation of the nanopore surface with ethylenediamine, the redox current of Ru(NH3)63+ decreased with decreasing pH and those of Fe(CN)63- and Fc(CH2OH)2 were independent of pH. The decrease in redox current of Ru(NH3)63+ at acidic pH was consistent with the presence of -NH2 groups on the nanopore surface as a result of the covalent immobilization of ethylenediamine. Furthermore, the redox current of Fc(CH2OH)2 decreased upon amidation of the nanopores with tetraethyleneglycol monoamine ((PEO)4NH2), reflecting the shrinkage of the effective pore radius. The control of the surface charge and effective radius of the nanopores via EDC-mediated amidation will provide a simple means for controlling the selectivity of molecular mass transport through PS-b-PMMA-derived nanopores.
Introduction This paper describes surface functionalization of vertically aligned cylindrical nanopores formed in a thin film of a polystyrene-poly(methylmethacrylate) diblock copolymer (PSb-PMMA) immobilized on a gold substrate. The chemical functionalization was performed via amidation mediated by 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) for the surface -COOH groups of the native PS-b-PMMA-derived nanopores.1 Atomic force microscopy (AFM) and cyclic voltammetry (CV) were employed to verify changes in the surface charge and effective radius of the PS-b-PMMA-derived nanopores upon chemical modification. Recently, nanoporous films obtained from cylinder-forming block copolymers (BCPs)2–5 have attracted much attention owing to their potential use as masks for lithography,6 templates for metal or silica nanowire synthesis,7,8 templates for nanoparticle arrays,9,10 and membranes for filtration of viruses.11 The nanopore diameter in a BCP-derived nanoporous film is uniform and can * To whom correspondence should be addressed. E-mail:
[email protected]. Telephone: 785-532-1451. Fax: 785-532-6666. (1) Li, Y.; Maire, H. C.; Ito, T. Langmuir 2007, 12771–12776. (2) Hillmyer, M. A. AdV. Polym. Sci. 2005, 190, 137–181. (3) Li, M.; Coenjarts, C. A.; Ober, C. K. AdV. Polym. Sci. 2005, 190, 183–226. (4) Fasolka, M. J.; Mayes, A. M. Annu. ReV. Mater. Res. 2001, 31, 323–355. (5) Olson, D. A.; Chen, L.; Hillmyer, M. A. Chem. Mater. 2008, 20, 869–890. (6) Park, M.; Harrison, C.; Chaikin, P. M.; Register, R. A.; Adamson, D. H. Science 1997, 276, 1401–1404. (7) Thurn-Albrecht, T.; Schotter, J.; Ka¨stle, G. A.; Emley, N.; Shibauchi, T.; Krusin-Elbaum, L.; Guarini, K.; Black, C. T.; Tuominen, M. T.; Russell, T. P. Science 2000, 290, 2126–2129. (8) Kim, H.-C.; Jia, X.; Stafford, C. M.; Kim, D. H.; McCarthy, T. J.; Tuominen, M.; Hawker, C. J.; Russell, T. P. AdV. Mater. 2001, 13, 795–797. (9) Zhang, Q.; Xu, T.; Butterfield, D.; Misner, M. J.; Ryu, D. Y.; Emrick, T.; Russell, T. P. Nano Lett. 2005, 5, 357–361. (10) Bandyopadhyay, K.; Tan, E.; Ho, L.; Bundick, S.; Baker, S. M.; Niemz, A. Langmuir 2006, 22, 4978–4984. (11) Yang, S. Y.; Ryu, I.; Kim, H. Y.; Kim, J. K.; Jang, S. K.; Russell, T. P. AdV. Mater. 2006, 18, 709–712.
be controlled in the range from 5 to 100 nm by varying the molecular weight of the BCP,2–5 making it possible to adjust the feature size in lithography, the diameter of synthesized nanowires, and the size-based selectivity in nanoparticle deposition and virus filtration. In addition, surface chemical modification of BCPderived cylindrical nanopores will permit us to control the selectivity and enhance the efficiency of the applications involving mass transport within the nanopores, as demonstrated with cylindrical nanopores based on track-etched polymer membranes and anodic alumina membranes.12–14 However, less attention has been given to surface functionalization of BCP-derived nanopores so far. Hillmyer and co-workers reported esterification between trifluoroacetic anhydride and residual -OH groups within nanopores prepared from polystyrene-polylactide diblock copolymers,15 and amidation of surface -COOH groups of nanoporous films prepared from polystyrene-polydimethylacrylamide-polylactide triblock copolymers.16 They also synthesized polystyrene-polyisoprene-polylactide triblock copolymers to obtain nanoporous films having derivatizable surface alkene groups.17 In these polymers, the nanoporous structures were formed as a result of the hydrolysis of cylindrical polylactide domains, and the surface functional groups on the nanopores were predictable. More recently, diblock copolymers whose two fragments were connected by a disulfide bond were synthesized.18 (12) Baker, L. A.; Jin, P.; Martin, C. R. Crit. ReV. Solid State Mater. Sci. 2005, 30, 183–205. (13) Martin, C. R.; Nishizawa, M.; Jirage, K.; Kang, M.; Lee, S. B. AdV. Mater. 2001, 13, 1351–1362. (14) Ito, Y.; Park, Y. S.; Imanishi, Y. J. Am. Chem. Soc. 1997, 119, 2739– 2740. (15) Zalusky, A. S.; Olayo-Valles, R.; Wolf, J. H.; Hillmyer, M. A. J. Am. Chem. Soc. 2002, 124, 12761–12773. (16) Rzayev, J.; Hillmyer, M. A. J. Am. Chem. Soc. 2005, 127, 13373–13379. (17) Bailey, T. S.; Rzayev, J.; Hillmyer, M. A. Macromolecules 2006, 39, 8772–8781. (18) Klaikherd, A.; Ghosh, S.; Thayumanavan, S. Macromolecules 2007, 40, 8518–8520.
10.1021/la800992f CCC: $40.75 2008 American Chemical Society Published on Web 07/01/2008
8960 Langmuir, Vol. 24, No. 16, 2008 Scheme 1
The disulfide moieties in these new types of BCPs can be cleaved using a reducing agent under mild conditions to obtain nanoporous structures. The sulfide groups formed on the resulting nanopores will allow for immobilizing chemical moieties on the nanopore surface. In contrast, surface chemical functionalization of cylindrical nanopores prepared from a PS-b-PMMA has not been demonstrated, despite the widespread use of this polymer in the applications mentioned above. This is probably because the surface functional groups on the native nanopores were not identified. For example, FTIR external reflection spectra did not give any peaks assigned to the surface functional groups.1 Recently, we have shown the presence of -COOH groups on the nanopore surface through systematic CV measurements on recessed nanopore-array electrodes (RNEs) based on PS-bPMMA-derived nanoporous films immobilized on gold substrates.1 The redox current of Fe(CN)63- decreased with increasing solution pH from 4.6 to 6.3 because of the electrostatic repulsion between the anionic redox species and the deprotonated -COOH groups on the nanopore surface.1 In this paper, we demonstrated covalent functionalization of -COOH groups on the PS-b-PMMA-derived nanopore surface via EDC-mediated amidation (Scheme 1). EDC has been widely used as a carboxyl activating agent for the coupling of a primary amine to immobilize biomolecules on a surface via an amide bond.19,20 The chemical functionalization of the nanopore surface through simple procedures led to conversion of the nanopore surface charge and shrinkage of the effective pore radius, and thus would widen the applicability of PS-b-PMMA-derived nanoporous materials.
Experimental Section Chemicals and Materials. All solutions were prepared with water having a resistivity of 18 MΩ cm or higher (Barnstead Nanopure Systems). Two types of PS-b-PMMA (57K PS-b-PMMA: Mn ) 39 800 g/mol for PS and 17 000 g/mol for PMMA, Mw/Mn ) 1.06; 71K PS-b-PMMA: Mn ) 50 000 g/mol for PS and 21 000 g/mol for PMMA, Mw/Mn ) 1.08) were purchased from Polymer Source and used as received. Potassium nitrate (Fisher Chemical), ethylenediamine (Fisher Chemical), potassium ferricyanide (Acros Organics), hexaammineruthenium (III) chloride (Acros Chemical), 1,1′-ferrocenedimethanol (Fc(CH2OH)2; Aldrich Chemical), 1-ethyl3-(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC; Chemimpex international), N-hydroxysuccinimide (N-HSS; Acros Organics), and tetraethyleneglycol monoamine ((PEO)4NH2; Molecular Biosciences, Inc.) were used as received. Gold-coated Si wafers, which were prepared by sputtering 10 nm of Ti followed by 20 nm (19) Muck, A.; Svatos, A. Talanta 2007, 74, 333–341. (20) Balamurugan, S.; Obubuafo, A.; Soper, S. A.; Spivak, D. A. Anal. Bioanal. Chem. 2008, 390, 1009–1021.
Li and Ito of Au onto Si(100) wafers, were purchased from LGA Thin Films (Foster City, CA). Preparation of PS-b-PMMA Films. Gold-coated Si wafers were cleaned in a Novascan PSD-UVT UV-ozone system for 15 min prior to use. A thin film of PS-b-PMMA was prepared on the gold substrate by spin-coating (2000 rpm) from its toluene solution (0.7%, w/v) and was then annealed at 170 °C in vacuum (ca. 0.3 Torr) for 60 h to form cylindrical PMMA domains in the film. The PMMA domains were degraded via UV irradiation using a Novascan PSDUVT UV-ozone system (ca. 20 mW/cm2) under an Ar atmosphere. This involves simultaneous cross-linking of the PS matrix and degradation of the PMMA domains.7,21,22 Subsequently, the degraded PMMA domains were removed by rinsing with glacial acetic acid (AcOH).7,21 The thickness of the PS-b-PMMA film was measured using a J. A. Woollam alpha-SE spectroscopic ellipsometer. The ellipsometric thickness of the annealed PS-b-PMMA films prior to the PMMA degradation was in the range of 28-35 nm (for 57K PS-b-PMMA) and 35 ∼ 43 nm (for 71K PS-b-PMMA), which offered cylindrical PMMA domains aligned perpendicular to the underlying gold substrate.1 Surface Modification of PS-b-PMMA-Derived Nanopores. PSb-PMMA-derived nanoporous films on gold were immersed in an aqueous solution containing 0.24 g of EDC and 0.03 g of N-HSS in a 1 mL phosphate buffer (0.1 M, pH 6) for 7 h with gentle shaking.16 Subsequently, the films were soaked in a phosphate buffer solution of ethylenediamine or (PEO)4NH2 for 24 h (39 µL/mL and 50 µL/ mL for ethylenediamine and (PEO)4NH2), respectively) with gentle shaking. The samples were rinsed with the phosphate buffer and then with water prior to AFM and electrochemistry measurements. As control experiments, the same procedures were performed without EDC and N-HSS to verify the effect of the amidation on electrochemical data. AFM Measurements. AFM images were obtained by tappingmode imaging in air, using a Digital Instruments Multimode AFM with Nanoscope IIIa electronics. Tapping-mode AFM probes from Applied Nanostructures (cantilever length, 125 µm; force constant, 40 N/m; resonant frequency, 300 kHz) were used. The reported average and standard deviation of the pore radius were obtained from 60-200 pores on at least three separate samples. Electrochemistry Measurements. CV measurements were performed in a three-electrode cell containing a Ag/AgCl (3 M KCl) reference electrode and a Pt counter electrode using a CH Instruments model 618B electrochemical analyzer. A gold substrate coated with a PS-b-PMMA-derived nanoporous film (serving as the working electrode) was immobilized at the bottom of the cell as reported previously.1 The diameter of the film area in contact with the solution, which was defined by a circular hole formed on the upper glass plate, was 0.65 cm. The pH of the solution was adjusted by adding a dilute KOH or HCl solution containing the supporting electrolyte.
Results and Discussion Mass transport of charged species through charged cylindrical nanoscale pores is strongly influenced by electrostatic interactions.12,13 Since a redox current measured in electrochemistry experiments reflects the mass transport of redox species to an electrode,23 electrochemical methods provide a simple and powerful means for characterizing the geometry and chemical properties of nanopores.1,24–28 In this study, we employed CV to characterize PS-b-PMMA-derived nanoporous films whose surface -COOH groups were modified with ethylenediamine or (PEO)4NH2 via EDC-mediated amidation (Scheme 1). Ethylenediamine will offer a surface -NH2 group after the other amino (21) Thurn-Albrecht, T.; Steiner, R.; DeRouchey, J.; Stafford, C. M.; Huang, E.; Bal, M.; Tuominen, M.; Hawker, C. J.; Russell, T. P. AdV. Mater. 2000, 12, 787–791. (22) Jeong, U.; Ryu, D. Y.; Kim, J. K.; Kim, D. H.; Russell, T. P.; Hawker, C. J. AdV. Mater. 2003, 15, 1247–1250. (23) Bard, A. J.; Faulkner, L. R. Electrochemical Methods, Fundamentals and Applications, 2nd ed.; Wiley: New York, 2001. (24) Ito, T.; Audi, A. A.; Dible, G. P. Anal. Chem. 2006, 78, 7048–7053.
Functionalization of PS-b-PMMA-Derived Nanopores
Figure 1. CVs (scan rate: 0.02 V/s) of (a) 3.0 mM K3Fe(CN)6, (b) 3.0 mM Fc(CH2OH)2, and (c) 3.0 mM Ru(NH3)6Cl3 in 0.1 M KNO3 at pH 2.8, 4.2, and 6.8 on a gold substrate coated with a native 57K PS-bPMMA-derived nanoporous film (28 nm thick).
group forms an amide bond with a -COOH group on the nanopore surface. The nanopores will be positively charged upon protonation of the NH2 groups, whereas the native nanopores are negatively charged at neutral to basic pH as a result of deprotonation of the surface -COOH groups.1 (PEO)4NH2 would be long enough to shrink the effective pore radius upon its covalent immobilization on the nanopore surface. In contrast, ethylenediamine would be too short to clearly observe a change in effective pore radius. EDC-Mediated Amidation of PS-b-PMMA-Derived Nanopores with Ethylenediamine. Figure 1 shows CVs of (a) Fe(CN)63-, (b) Fc(CH2OH)2, and (c) Ru(NH3)63+ on a RNE based on a planar gold substrate coated with a native 57K PSb-PMMA-derived nanoporous film. The redox current of Fe(CN)63- at the acidic pH was larger than that at the neutral pH (Figure 1a). In contrast, those of Fe(CH2OH)2 and Ru(NH3)63+ were very similar regardless of the pH (Figure 1b,c). The smaller peak current (ip) of Fe(CN)63- at the higher pH was consistent with the deprotonation of the surface -COOH groups, as reported previously.1 The very similar ip of Ru(NH3)63+ at the three pH conditions suggested negligible effect of electrostatic attraction on the redox current, perhaps because the CV measurements were performed in the presence of 0.1 M KNO3. In the absence of a supporting electrolyte, an increase in redox current due to the electrostatic attraction was previously observed on a single glass-nanopore electrode (30 nm pore radius).26 Chemical functionalization of the 57K PS-b-PMMA-derived nanopores with ethylenediamine changed the pH dependence of CVs of the redox species. As shown in Figure 2a, the CVs of Fe(CN)63- were similar regardless of the solution pH, in contrast to the data shown in Figure 1a. For Fc(CH2OH)2, the pHdependent changes in CV were negligible (Figure 2b) as with Figure 1b. Remarkably, the redox current of Ru(NH3)63+ decreased with decreasing the pH of the solution from 6.8 to 2.8 (Figure 2c). This is probably due to the electrostatic repulsion between the cationic Ru(NH3)63+ and the protonated -NH2 groups of ethylenediamine molecules immobilized on the nanopores (Scheme 1). The pH range that gave the ip change was lower than that expected from the pKa of -NH3+ groups,29 but was consistent with the apparent pKa reported for the surface amino groups of (25) Zhang, B.; Zhang, Y.; White, H. S. Anal. Chem. 2004, 76, 6229–6238. (26) Wang, G.; Zhang, B.; Wayment, J. R.; Harris, J. M.; White, H. S. J. Am. Chem. Soc. 2006, 128, 7679–7686. (27) Jeoung, E.; Galow, T. H.; Schotter, J.; Bal, M.; Ursache, A.; Tuominen, M. T.; Stafford, C. M.; Russell, T. P.; Rotello, V. M. Langmuir 2001, 17, 6396– 6398. (28) Laforgue, A.; Bazuin, C. G.; Prud’homme, R. E. Macromolecules 2006, 39, 6473–6482. (29) Speight, J. G. Lange’s Handbook of Chemistry, 14th ed.; McGraw-Hill: New York, 2005.
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Figure 2. CVs (scan rate: 0.02 V/s) of (a) 3.0 mM K3Fe(CN)6, (b) 3.0 mM Fc(CH2OH)2 and (c) 3.0 mM Ru(NH3)6Cl3 in 0.1 M KNO3 at pH 2.8, 4.2, and 6.8 on a gold substrate coated with a 57K PS-b-PMMAderived nanoporous film (28 nm thick) after EDC-mediated amidation with ethylenediamine. CVs of the electrode prior to the amidation are shown in Figure 1.
Figure 3. CVs (scan rate: 0.02 V/s) of (a) 3.0 mM K3Fe(CN)6, (b) 3.0 mM Fc(CH2OH)2 and (c) 3.0 mM Ru(NH3)6Cl3 in 0.1 M KNO3 at pH 2.8, 4.2, and 6.8 on a gold substrate coated with a 57K PS-b-PMMAderived nanoporous film (33 nm thick) after amidation procedures using ethylenediamine without EDC and N-HSS.
polymer films and self-assembled monolayers.30–33 Alternatively, the pH range may reflect the presence of residual -COOH groups that would reduce the net positive charge of the nanopore surface originating from the -NH3+ groups. On the other hand, the ip of Fe(CN)63- for the -NH2-terminated nanopores was similar at the different pH values (Figure 2a), suggesting negligible effect of the electrostatic attraction on the redox current as with Ru(NH3)63+ on the native nanopores (Vide supra). As control experiments, the amidation procedures were examined on an RNE based on a gold electrode coated with a thin 57K PS-b-PMMA-derived nanoporous film without EDC and N-HSS, which are known to be required for amidation in an aqueous solution.19,20 CVs of Fe(CN)63-, Fc(CH2OH)2, and Ru(NH3)63+ on such a RNE (Figure 3) were very similar to those on a RNE with a native PS-b-PMMA-derived nanoporous films (Figure 1). These control experiments support that the change in CVs of Fe(CN)63- (from Figure 1a to Figure 2a) and Ru(NH3)63+ (from Figure 1c to Figure 2c) were observed as a result of the covalent functionalization of the nanopore surface with ethylenediamine. Figure 4 summarizes the peak current values (ip) of the three redox species at different pH before and after the ethylenediamine modification. These values were determined from CV data (30) Holmes-Farley, S. R.; Bain, C. D.; Whitesides, G. M. Langmuir 1988, 4, 921–937. (31) Chatelier, R. C.; Hodges, A. M.; Drummond, C. J.; Chan, D. Y. C.; Griesser, H. J. Langmuir 1997, 13, 3043–3046. (32) Vezenov, D. V.; Noy, A.; Rozsnyai, L. F.; Lieber, C. M J. Am. Chem. Soc. 1997, 119, 2006–2015. (33) Ito, T.; Citterio, D.; Bu¨hlmann, P.; Umezawa, Y. Langmuir 1999, 15, 2788–2793.
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Li and Ito
Figure 5. Tapping-mode AFM images (∆z ) 30 nm) of the surfaces of a thin 71K PS-b-PMMA-derived nanoporous film (42 nm thick) on a gold substrate (a) before and (b) after the EDC-mediated amidation with (PEO)4NH2. Figure 4. pH dependence of the peak current (ip) of K3Fe(CN)6 (red), Fc(CH2OH)2 (blue), and mM Ru(NH3)6Cl3 (black) on gold substrates coated with 57K PS-b-PMMA-derived nanoporous films (a) before and (b) after EDC-mediated amidation with ethylenediamine. Table 1. Pore Radii Obtained from AFM Images (rAFM), Peak currents in CVs of 1,1′-Ferrocenedimethanol (ip), and Effective Pore Radii (reff) Calculated from the ip before and after EDC-Mediated Amidation with (PEO)4NH2 film type
rAFM (nm)a
ip (µA)b
reff (nm)c
57K-PS-b-PMMA before amidation 9.5 ( 0.9 after amidation 10.3 ( 1.2
34.4 ( 2.3 22.2 ( 1.9
11.1 ( 0.4 8.9 ( 0.4
71K-PS-b-PMMA before amidation 11.9( 1.4 after amidation 10.9 ( 1.4
32.6 ( 2.5 20.8 ( 2.8
12.2 (0.5 9.8 (0.7
a Average ( standard deviation obtained from 60-200 pores on AFM images of at least three separate films. b Average ( standard deviation measured from three separate PS-b-PMMA-derived RNEs. c Calculated from ip values and pore density obtained from AFM images (900 pores/µm2 for 57K PS-b-PMMA, 700 pores/µm2 for 71K PS-b-PMMA) using eq 1 in ref 1.
obtained using three separate RNEs. The relatively small standard deviations of the current data (relative standard deviation < 15%) indicate that the chemical functionalization of the PS-b-PMMAderived nanopores was quite reproducible. The effective pore radius, calculated from ip and pore density (900 pores/µm2; Table 1) using eq 1 in ref 1, changed from 10.2 ( 0.4 nm (pH 2.8) to 6.4 ( 0.5 nm (pH 6.8) for Fe(CN)63- on RNEs with native nanopores, and from 10.6 ( 0.6 nm (pH 6.8) to 8.0 ( 0.5 nm (pH 2.8) for Ru(NH3)63+ on RNEs modified with ethylenediamine. The change in effective pore radius was in the range between 2dD and 4dD, where dD is the Debye length (∼1 nm in 0.1 M KNO3). These results suggest that the decrease in effective pore radius reflected the electrostatic repulsion between the charged redox species and the electrical double layer extended from the charged nanopore surface.1,26 The reduction of the flux of ionic species due to electrostatic repulsion (permselective mass transport) was previously observed within chemically functionalized cylindrical nanopores based on track-etched membranes, anodic alumina membranes,12,13 as well as single glass-nanopore electrodes.26 The Shrinkage of PS-b-PMMA-Derived Nanopores via EDC-Mediated Amidation with Long Molecules. The ip of uncharged Fe(CH2OH)2 did not change upon amidation with ethylenediamine (Figure 4a,b; blue), probably because ethylenediamine molecules are too short to change the effective pore radius upon their immobilization. In contrast, (PEO)4NH2 is a relatively long molecule (ca. 2 nm in length in all-trans configuration according to the CPK model) that would result in
Figure 6. CVs (scan rate: 0.02 V/s) of 3.0 mM Fc(CH2OH)2 in 0.1 M KNO3 (pH 6.8) on a gold substrate coated with a PS-b-PMMA-derived nanoporous film before and after EDC-mediated amidation with (PEO)4NH2: (a) 57K PS-b-PMMA-derived nanoporous film (33 nm thick); (b) 71K PS-b-PMMA-derived nanoporous film (38 nm thick).
a significant change in the effective pore radius. Controlled shrinkage of the pore diameter via chemical functionalization with long molecules was previously demonstrated with other nanopore systems.14,34,35 Thus, we modified the nanopore surface with (PEO)4NH2 to reduce the effective pore radii of 57K and 71K PS-b-PMMA-derived nanopores. Figure 5 shows tapping-mode AFM images of a 71K PS-bPMMA-derived nanoporous film (42 nm thick) on a gold substrate (a) before and (b) after the EDC-mediated amidation with (PEO)4NH2. The AFM measurements were performed in air, and thus the images represent the surfaces of the dry films. Before and after the amidation with (PEO)4NH2, the density of the nanopores did not change (ca. 700 pores/µm2), and the change in the pore radius was not apparent in these AFM images (Table 1). This is probably because the yield of the surface amidation was not very high and the resulting, loosely packed PEO brush on the nanopore surface collapsed onto the nanopore surface under dry conditions.36,37 Similar results were also obtained for 57K PS-b-PMMA-derived nanoporous films (Table 1). Figure 6 shows CVs of Fc(CH2OH)2 before and after the amidation with (PEO)4NH2 on nanoporous films derived from (a) 57K PS-b-PMMA and (b) 71K PS-b-PMMA. For these experiments, uncharged Fc(CH2OH)2 was chosen to discuss the effective pore radius because of its negligible electrostatic effects. As shown in Figure 6, the ip of Fc(CH2OH)2 decreased upon amidation with (PEO)4NH2 for both PS-b-PMMA-derived nanoporous films. Table 1 summarizes the average and standard (34) Schepelina, O.; Zharov, I. Langmuir 2006, 22, 10523–10527. (35) Wanunu, M.; Meller, A. Nano Lett. 2007, 7, 1580–1585. (36) Ito, T.; Sun, L.; Bevan, M. A.; Crooks, R. M. Langmuir 2004, 20, 6940– 6945. (37) Hunter, R. J. Foundations of Colloid Science; Oxford University Press: New York, 2001.
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neutral to basic pH due to the presence of residual -COOH groups on the nanopore surface. Interestingly, the PEO layer was shown to reduce the nonspecific adsorption of protein molecules38 onto the nanopores. These studies, including an application of PEO-modified nanoporous films, will be reported elsewhere.
Conclusions
Figure 7. CVs (scan rate: 0.02 V/s) of (a) 3.0 mM K3Fe(CN)6 and (b) 3.0 mM Ru(NH3)6Cl3 in 0.1 M KNO3 at pH 2.8, 4.2, and 6.8 on a gold substrate coated with a 57K PS-b-PMMA-derived nanoporous film (33 nm thick) after EDC-mediated amidation with (PEO)4NH2.
deviation of ip before and after the amidation with (PEO)4NH2 obtained from three separate samples. The change in effective pore radius, calculated from the ip values (Table 1), was ca. 2 nm for both films, which coincided with the length of the extended PEO molecule (ca. 2 nm). This suggests that (PEO)4NH2 was immobilized on the nanopore surfaces, resulting in the shrinkage of the effective pore radius. However, the (PEO)4NH2 molecules did not form a densely packed layer on the nanopore surface, as suggested by the AFM image in Figure 5. This conclusion was also supported by the CV data of Fe(CN)63- and Ru(NH3)63+ (Figure 7). If all of the -COOH groups on the native nanopores reacted with (PEO)4NH2, the surface charge of the nanopores would be neutral. However, pH dependence of CVs of both the redox molecules on a PEOmodified 57K PS-b-PMMA-derived nanoporous film (Figure 7) was similar to that on a native nanoporous film (Figure 1). This indicates that the nanopore surface was negatively charged at
In this paper, we demonstrated the EDC-mediated amidation of surface -COOH groups of native PS-b-PMMA-derived nanopores with amine-terminated molecules. The surface charge and the effective radius of the nanopores could be tailored via covalent immobilization of ethylenediamine molecules and long PEO-containing molecules, respectively. CV measurements permitted us to assess the surface functionalization of PS-bPMMA-derived cylindrical nanopores. The EDC-mediated amidation of PS-b-PMMA-derived nanopores will provide a simple means for tailoring the surface properties and pore size of the nanopores, and thus permit us to control the selectivity and efficiency in chemical sensing and catalysis using PS-b-PMMAderived nanoporous films. Acknowledgment. The authors thank Shaida Ibrahim for her assistance with PS-b-PMMA film preparation and also for critically reading the manuscript. The authors also thank Helene Maire and Shinobu Nagasaka for their assistance with PS-bPMMA film preparation. The authors gratefully acknowledge the American Chemical Society Petroleum Research Fund and Kansas State University for financial support of this work. LA800992F (38) Senaratne, W.; Andruzzi, L.; Ober, C. K. Biomacromolecules 2005, 6, 2427–2448.
