pubs.acs.org/Langmuir © 2010 American Chemical Society
Selective Surface Chemistry Using Alumina Nanoparticles Generated from Block Copolymers Randall M. Stoltenberg,† Chong Liu,‡ and Zhenan Bao*,§ †
Department of Chemistry, Stanford University, Stanford, California 94305, United States, Department of Materials Science, Stanford University, Stanford, California 94305, United States, and § Department of Chemical Engineering, Stanford University, Stanford, California 94305, United States
‡
Received October 11, 2010. Revised Manuscript Received November 18, 2010 Developing orthogonal surface chemistry techniques that perform at the nanoscale is key to achieving precise control over molecular patterning on surfaces. We report the formation and selective functionalization of alumina nanoparticle arrays generated from block copolymer templates. This new material provides an alternative to gold for orthogonal surface chemistry at the nanometer scale. Atomic force microscopy and X-ray photoelectron spectroscopy confirm these particles show excellent selectivity over silica for phosphonic and carboxylic acid adsorption. As this is the first reported synthesis of alumina nanoparticles from block copolymer templates, characterizations via Fourier transform infrared spectroscopy, Auger electron spectroscopy, and transmission electron microscopy are presented. Reproducible formation of alumina nanoparticles was dependent on a counterintuitive synthetic step wherein a small amount of water is added to an anhydrous toluene solution of block copolymer and aluminum chloride. The oxidation environment of the aluminum in these particles, as measured by Auger electron spectroscopy, is similar to that of native aluminum oxide and alumina grown by atomic layer deposition. This discovery expands the library of available surface chemistries for nanoscale molecular patterning.
Introduction Creating chemically patterned surfaces is a central theme in surface science and is the basis for modern biotechnology and semiconductor processing technology. A main focus in this field is the development of systems displaying orthogonal functionality; surfaces on which defined areas will interact preferentially with certain classes of chemicals. Lithographic tools such as photo-,1-3 e-beam,4 and dip-pen5 lithography have been regularly employed to create such orthogonal interfaces. As pattern dimensions push toward the nanometer scale, however, these techniques are confronted with challenges in resolution or throughput, especially when attempting to approach the molecular level. Generating patterns for orthogonal chemistry at the nanometer scale has farreaching scientific and technological implications as it strikes at one of the main aims of nanotechnology: precise control over molecular and atomic placement. Recently, a number of reports have arisen in which block copolymer (BCP)-templated gold nanoparticle (AuNP) arrays formed on silica surfaces serve as selective functionalization sites for thiol-labeled biologicals on the nanometer scale.6-8 The main advantage to using this scheme, as opposed to the lithographic techniques mentioned, is rapid, parallel formation of nanoscale *Corresponding author. E-mail:
[email protected]. (1) Laibinis, P. E.; Hickman, J. J.; Wrighton, M. S.; Whitesides, G. M. Science 1989, 245, 845–847. (2) Orski, S. V.; Poloukhtine, A. A.; Arumugam, S.; Mao, L.; Popik, V. V.; Locklin, J. J. Am. Chem. Soc. 2010, 132, 11024–11026. (3) Xu, H.; Hong, R.; Lu, T.; Uzun, O.; Rotello, V. M. J. Am. Chem. Soc. 2006, 128, 3162–3163. (4) Harold, E. G.; Wolfgang, F. Nanotechnology 2007, 18, 135101. (5) Demers, L. M.; Ginger, D. S.; Park, S.-J.; Li, Z.; Chung, S.-W.; Mirkin, C. A. Science 2002, 296, 1836–1838. (6) Arnold, M.; Cavalcanti-Adam, E. A.; Glass, R.; Bl€ummel, J.; Eck, W.; Kantlehner, M.; Kessler, H.; Spatz, J. P. ChemPhysChem 2004, 5, 383–388. (7) Cavalcanti-Adam, E. A.; Volberg, T.; Micoulet, A.; Kessler, H.; Geiger, B.; Spatz, J. P. Biophys. J. 2007, 92, 2964–2974. (8) Walter, N.; Selhuber, C.; Kessler, H.; Spatz, J. P. Nano Lett. 2006, 6, 398– 402.
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features by simple processing such as spin- or dip-coating. The nanoparticles formed can be on the order of 5-10 nm,9-11 a scale not easily accessed by any conventional lithography technique. With particles of this size, applications include localizing single proteins12 and studying cell adhesion.7 However, the chemistry involved, while highly selective, is relatively unstable as the strength of the Au-thiol interaction is weaker than other surface modifications such as silanes on silica or phosphonic acids on alumina.13 Additionally, the presence of gold can preclude the integration of visible light spectroscopy; AuNPs can interefere constructive or destructively with light absorption and emission depending on their geometry.14-16 Hence, there is a need to expand the available library of orthogonal surface chemistry at the nanometer scale beyond the Au-thiol interaction. As a remedy, we report arrays of BCP-templated alumina nanoparticles that can be selectively modified with biologically relevant functional groups. These particles show excellent selectivity over silica for the adsorption of phosphonic and carboxylic acids as shown by XPS and AFM. Additionally, alumina is optically transparent and will not couple electromagnetically with pendant fluorophores, as may be the case with BCP-templated AuNPs. Also, the availability of two interacting functional groups and the stronger interaction of phosphonic and carboxylic acids with metal oxides in general make these alumina nanoparticles a (9) Aizawa, M.; Buriak, J. M. Chem. Mater. 2007, 19, 5090–5101. (10) Yun, S.-H.; Yoo, S. I.; Jung, J. C.; Zin, W.-C.; Sohn, B.-H. Chem. Mater. 2006, 18, 5646–5648. (11) F€orster, S.; Antonietti, M. Adv. Mater. 1998, 10, 195–217. (12) Wolfram, T.; Belz, F.; Schoen, T.; Spatz, J. P. Biointerphases 2007, 2, 44–48. (13) Dubey, M.; Weidner, T.; Gamble, L. J.; Castner, D. G. Langmuir 2010, 26, 14747–14754. (14) Dulkeith, E.; Ringler, M.; Klar, T. A.; Feldmann, J.; Mu~noz Javier, A.; Parak, W. J. Nano Lett. 2005, 5, 585–589. (15) H€artling, T.; Reichenbach, P.; Eng, L. M. Opt. Express 2007, 15, 12806– 12817. (16) Vukovic, S.; Corni, S.; Mennucci, B. J. Phys. Chem. C 2009, 113, 121–133. (17) Mani, G.; Johnson, D. M.; Marton, D.; Dougherty, V. L.; Feldman, M. D.; Patel, D.; Ayon, A. A.; Agrawal, C. M. Langmuir 2008, 24, 6774–6784.
Published on Web 12/06/2010
DOI: 10.1021/la104094h
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more robust system for molecular patterning.17-19 Here, it is appropriate to note that phosphonic acids have been reported to form stable monolayers on silica surfaces.20,21 In those cases, stable monolayers are formed through a combination of concentrated deposition and applied heat over many hours. In this report, the surfaces are exposed to rather dilute solutions of phosphonic acids for relatively short periods of time, and the surfaces are not heated. Carboxylic acids, on the other hand, have no reported tendency to form stable layers on silica, though they have been reported to bond to hydrogen-terminated silicon surfaces.22 Interestingly, given the vast array of materials templated by BCPs, this is the first report of alumina nanoparticles formed in this fashion. While the synthesis presented herein follows the general procedure for making these materials, we discovered an additional and counterintuitive processing step required to make reproducible nanoparticles. While the preparation of the BCP solution is generally performed under anhydrous conditions, we found that a small amount of water must be added at a certain point in the synthesis to form alumina nanoparticle arrays in a reliable manner. As this is the first preparation of this material, characterizations via Fourier transform infrared spectroscopy (FTIR), Auger electron spectroscopy (AES), and transmission electron microscopy (TEM) are also presented.
Scheme 1. Chemical Structures of Molecules Used in This Study
Experimental Section AlCl3-BCP Solution Preparation. Poly(styrene-b-4-vinylpyridine) (PS-P4VP, Mn 557K-75K, Polymersource, Inc., Montreal, Canada) was dissolved in anhydrous toluene (EMD, 99.8%) in a N2 glovebox environment at a concentration of 4 mg mL-1 and stirred for 1 h. AlCl3 (Sigma-Aldrich, Milwaukee, WI; anhydrous, 99.999%) was added in a molar ratio (moles AlCl3: moles 4VP) of 1. This solution was stirred for 24 h and then filtered through a 0.45 μm PTFE syringe filter (Whatman). For water-treated solutions, a 2 mL aliquot of the stock solution was dispensed into a separate vial, and 20 μL of deionized water was added. The solution was vigorously stirred for 2 min, after which the solution was refiltered. Fourier Transform Infrared Spectroscopy (FTIR). Aliquots of 4 mg mL-1 BCP solutions were drop-cast on a heated glass slide (70 C), and the solvent was allowed to evaporate completely. Films were released from the glass slides with a razor blade, and the free-standing polymer films were analyzed by transmission FTIR (Thermo Scientific Nicolet 6700 FT-IR) using a KBr beamsplitter and a deuterated triglycine sulfate (DTGS) detector. The range of analysis was 4000-400 cm-1. 64 spectra were averaged for the background and for each sample. AlOx Nanoparticle Arrays. Glass and Si wafer pieces were cleaned by immersion in a 6:1:1 H2O:NH4OH:H2O2 solution at 60 C for 20 min. After rinsing in DI water and drying under a stream of compressed air, substrates were exposed to UV-ozone for 5 min. Films of AlCl3-loaded BCP were formed by spin-coating the parent solution at 5000 rpm for 40 s on cleaned substrates. NP arrays were generated by subjecting the AlCl3-BCP films to oxygen plasma (PlasmaPrep-5, GaLa Instrumente) for 5-30 min (0.4 mbar of O2, 50 W). The surfaces were then annealed at 250 C for 10 min and exposed to UV-ozone for 5 min. NPs were characterized by atomic force microscopy (AFM, DI Nanoscope IIIa controller), (18) Flynn, N. T.; Tran, T. N. T.; Cima, M. J.; Langer, R. Langmuir 2003, 19, 10909–10915. (19) Willey, T. M.; Vance, A. L.; van Buuren, T.; Bostedt, C.; Terminello, L. J.; Fadley, C. S. Surf. Sci. 2005, 576, 188–196. (20) Hanson, E. L.; Schwartz, J.; Nickel, B.; Koch, N.; Danisman, M. F. J. Am. Chem. Soc. 2003, 125, 16074–16080. (21) Gouzman, I.; Dubey, M.; Carolus, M. D.; Schwartz, J.; Bernasek, S. L. Surf. Sci. 2006, 600, 773–781. (22) Sieval, A. B.; Demirel, A. L.; Nissink, J. W. M.; Linford, M. R.; van der Maas, J. H.; de Jeu, W. H.; Zuilhof, H.; Sudh€olter, E. J. R. Langmuir 1998, 14, 1759–1768.
446 DOI: 10.1021/la104094h
Auger electron spectroscopy (AES, PHI 700), and TEM (FEI Tecnai G2 F20 X-TWIN). AES spectra were processed by running a 9-point Savitsky-Golay smooth to the raw data and subsequently differentiating the smoothed data. TEM samples were prepared by spin-coating the AlCl3-BCP solution on Pdcoated Au TEM grids (Ted Pella, Inc.) followed by O2 plasma treatment. Particle diameter was measured by SEM (FEI XL30 Sirion). Nanoparticle Funcitonalization. The following chemicals were used as received from their suppliers (see Scheme 1): 3-aminopropylphosphonic acid (2, Sigma), 12-pentafluorophenoxydode cylphosphonic acid (3, SiKEMIA, Montpellier, France), and 12 mercaptododecylphosphonic acid (4, SiKEMIA). Solutions of 2 were prepared in deionized water. Solutions of 3 and 4 were prepared in 200 proof EtOH (Sigma-Aldrich, Milwaukee, WI). Solutions of a custom synthesized carboxylic acid dendrimer (1, Frontier Scientific, Logan, UT) were prepared (see Supporting Information) in deionized water. NP arrays were immersed in these solutions for varying amounts of time and then characterized by AFM and XPS (PHI 5000 Versaprobe, Al KR). All XPS spectra were referenced to a C 1s binding energy of 284.8 eV. Atomic Layer Deposition of Alumina. A Æ100æ Si wafer (Silicon Quest International, Santa Clara, CA) was cleaned in a piranha solution (70:30% v/v concentrated H2SO4:30% H2O2; use extreme caution) for 10 min, rinsed thoroughly in deionized water, and dried under a stream of compressed N2. A thin layer of alumina (10 nm) was deposited by atomic layer deposition (Cambridge NanoTech, Savannah system) using the following temperature settings: 60 C (source), 120 C (substrate), 100 C (lid), 160 C (exhaust).
Results and Discussion Formation and Characterization of Alumina Nanoparticle Arrays. The formation of alumina NPs from BCP templates has not been reported despite the large body of literature devoted (23) Aizawa, M.; Buriak, J. M. J. Am. Chem. Soc. 2005, 127, 8932–8933. (24) Abes, J. I.; Cohen, R. E.; Ross, C. A. Chem. Mater. 2003, 15, 1125–1131. (25) Abes, J. I.; Cohen, R. E.; Ross, C. A. Mater. Sci. Eng. C 2003, 23, 641–650.
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Scheme 2. BCP Micellization in a Selective Solvent and Subsequent Isolation of Metal Precursors within the Core of the Micelle
to BCP-templated materials and their applications.9,10,23-31 This may be attributed to the fact that their preparation, as described in this report, requires that the presence of water in the BCP solution be carefully controlled. The typical formation of BCP templated materials begins with the dissolution of the BCP in a solvent that selectively dissolves one of the blocks. This induces micelle formation, and the insoluble block is isolated from the solvent. A common BCP used to generate nanoparticle arrays is polystyrene-b-polyvinylpyridine (PS-PVP) with the nitrogen in pyridine ring in either the 2- or 4-position. Dissolved in toluene, THF, or a mixture of these solvents, the PVP blocks segregate from the solvent, inducing micellization, while the PS blocks maintain the solubility of the micelles as illustrated in Scheme 2. For this type of BCP, isolation of metal precursors within the micelles is due to association of the metal species with the vinylpyridine units either electrostatically or by coordination.11 In this study, AlCl3, a Lewis acid, was added to a toluene solution of PS-P4VP with the expectation that 4VP, a Lewis base, would sequester the AlCl3 within the micelles. These solutions were prepared in a N2 glovebox to avoid premature hydration of AlCl3. When solution preparation was attempted in a normal atmosphere, the added AlCl3 would not dissove and remained suspended in solution as a white precipitate. PS-P4VP was observed to accommodate AlCl3 in anhydrous toluene solution in mole ratios up to 2 (mol AlCl3:mol 4VP). Beyond a ratio of 2, the solution turns from clear to yellow to deep orange over a matter of hours. If the solution is allowed to stir for more than 24 h, it eventually turns black with a brown precipitate. When filtered, these blackened solutions did not yield micelle arrays. Thus, for this system, the stability of the micelle solution is sensitive to the loading ratio, which should not exceed 2. It is presumed excess AlCl3 degrades the PS-P4VP as has been described for AlCl3 and polyethylene,32 though neither the mechanism of degradation nor the degradation products were explored in this work. All of the NP arrays used in this study were formed by using AlCl3-BCP solutions with an AlCl3:4VP ratio of 1. The nature of the interaction between AlCl3 and the BCP and the effect of water on this interaction (described later) were examined by transmission FTIR; the spectra are displayed in Figure 1. The band near 1000 cm-1 is (26) K€astle, G.; Boyen, H.-G.; Weigl, F.; Lengl, G.; Herzog, T.; Ziemann, P.; Riethm€uller, S.; Mayer, O.; Hartmann, C.; Spatz, J. P.; M€oller, M.; Ozawa, M.; Banhart, F.; Garnier, M. G.; Oelhafen, P. Adv. Funct. Mater. 2003, 13, 853–861. (27) Li, X.; G€oring, P.; Pippel, E.; Steinhart, M.; Kim, D. H.; Knoll, W. Macromol. Rapid Commun. 2005, 26, 1173–1178. (28) Li, X.; Lau, K. H. A.; Kim, D. H.; Knoll, W. Langmuir 2005, 21, 5212–5217. (29) Spatz, J. P.; M€ossmer, S.; Hartmann, C.; M€oller, M.; Herzog, T.; Krieger, M.; Boyen, H.-G.; Ziemann, P.; Kabius, B. Langmuir 1999, 16, 407–415. (30) Yoo, S. I.; Sohn, B.-H.; Zin, W.-C.; An, S.-J.; Yi, G.-C. Chem. Commun. 2004, 2850–2851. (31) Yun, S.-H.; Sohn, B.-H.; Jung, J. C.; Zin, W.-C.; Lee, J.-K.; Song, O. Langmuir 2005, 21, 6548–6552. (32) Ivanova, S. R.; Gumerova, E. F.; Berlin, A. A.; Minsker, K. S.; Zaikov, G. E. Russ. Chem. Rev. 1991, 60, 225.
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Figure 1. FTIR spectra of films of PS-P4VP BCP with no modification (gray), loaded with AlCl3 (black dashed), and loaded with AlCl3 and treated with water (black solid). Spectra are normalized within the region shown. The band shift from 993 to 1004 cm-1 indicates coordination of AlCl3 to the vinylpyridine units of the BCP.
associated with pyridine ring deformation and is shifted to higher wavenumber in the AlCl3-BCP film compared to the unmodified BCP. This shift is indicative of association of the N in the pyridine ring to another entity33-35 and is interpreted here as evidence of coordination of 4VP to AlCl3. AFM images of AlCl3-loaded BCP films on silica surfaces are displayed in Figure 2. The density of the micelles can be controlled by the solution concentration and the spin speed; however, for this study, the spin speed was maintained at 5K rpm. Solution concentrations between 0.2 and 2.0 mg/mL yield semiordered, monomicellar films. At concetrations at or above 2.0 mg mL-1, multilayers are formed, and at concentrations below 0.2 mg mL-1, the resulting films are discontinuous, and the micelles no longer exhibit short-range order. BCP solutions of 1 mg mL-1 were used in the remainder of this study as they provide relatively dense single-layer micelle arrays. After film formation, the substrates were exposed to O2 plasma, which simultaneously removes the BCP template and oxidizes the inorganic material in the micelle core to yield surface bound nanoparticles. The chemical composition of the particles was probed by Auger electron spectroscopy (AES) and TEM (Figure S1). Figure 3 displays the results of AES analysis of the NPs formed from films of AlCl3-BCP solution. The Al KLL transition near 1390 eV was used to monitor the presence of Al on the surface. Al was not detectable by AES in the interstitial regions of the NP array, suggesting that before O2 plasma treatment Al is isolated within the micelle cores and does not exist in the PS matrix of the micelle film to any appreciable extent. However, a significant amount of carbon was observed within the particles, (33) Lee, J. Y.; Painter, P. C.; Coleman, M. M. Macromolecules 1988, 21, 954– 960. (34) Cesteros, L. C.; Meaurio, E.; Katime, I. Macromolecules 1993, 26, 2323– 2330. (35) Diab, M. A.; El-Sonbati, A. Z.; El-Sanabari, A. A.; Taha, F. I. Polym. Degrad. Stab. 1989, 24, 51–58.
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Figure 2. AFM height images of AlCl3-infused BCP micelle films on SiO2 spin-cast from toluene solutions of varying concentration: (a) 2, (b) 1, (c) 0.8, (d) 0.4, (e) 0.2, and (f) 0.1 mg mL-1. The height scale for each image is 20 nm (frame f). Each image is 5 5 μm.
Figure 4. AES spectra of nanoparticles from AlCl3-BCP solutions with (gray) and without (black) water treatment.
Figure 3. (a) SEM micrograph of an array of nanoparticles. (b) AES spectra taken at the points indicated in panel a. The scale bar in (a) is 200 nm.
and this carbon could not be removed by the annealing procedure used in this study. Additionally, particle heights as measured by AFM varied significantly from substrate to substrate and between batches of solution. In an effort to reduce the carbon content of the NPs and remedy the inconsistent NP size, a small amount of water (e1% v/v) was added to the micelle solution before spincoating. This water treatment eliminated the C peak in the AES spectrum as seen in Figure 4 and yielded more reproducible and uniform NPs. The average diameter of NPs from water-treated solutions was 26 ( 5 nm as measured by SEM. Also, NPs formed from water-treated BCP-AlCl3 solutions showed little variation in size over a range of plasma and annealing times. This addition of water was seemingly counterintuitive considering the effect water had on solutions prepared in a normal atmosphere. However, the AES data suggest that the Al in the micelles retains some of the associated polymer unless it is hydrolyzed prior to plasma treatment. We attempted this step based on a similar treatment reported for a BCP-templated NP array using TiCl4 as the precursor.28 In 448 DOI: 10.1021/la104094h
that system, a film of water-swollen PS-P2VP micelles was exposed to TiCl4 vapor in inert atmosphere, confining the hydrolysis of TiCl4 to the micelle cores. Here, reagents are added in reverse order: water is added to AlCl3-BCP solution to hydrolyze the inorganic material already sequestered in the micelle core. As the reaction between AlCl3 and water occurs in solution, stirring for a short amount of time (
