Langmuir 2004, 20, 9089-9094
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Influence of the Foundation Layer on the Layer-by-Layer Assembly of Poly-L-lysine and Poly(styrenesulfonate) and Its Usage in the Fabrication of 3D Microscale Features Dejian Zhou,†,‡ Andreas Bruckbauer,‡ Matthew Batchelor,‡ Dae-Joon Kang,*,† Chris Abell,‡ and David Klenerman*,‡ Nanoscience Centre, University of Cambridge, 11 J J Thomson Avenue, Cambridge CB3 0FF, U.K., and Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, U.K. Received June 4, 2004. In Final Form: July 23, 2004 The layer-by-layer (LBL) assembly of a polypeptide, poly-L-lysine (PLL), with poly(styrenesulfonate) sodium salt (PSS) on flat template-stripped gold (TSG) surfaces precoated with a self-assembled monolayer of alkanethiols terminated with positive (pyridinium), negative (carboxylic acid), and neutral [hexa(ethylene glycol)] groups is investigated. Both the topography and the rate of film thickness growth are found to be strongly dependent on the initial surface foundation layer. LBL assembly of PLL and PSS on patterned TSG surfaces produced by micro contact printing leads to structurally distinct microscale features, including pillars, ridges, and wells, whose height can be controlled with nanometer precision.
Introduction Layer-by-layer (LBL) assembly, which uses the alternative sequential deposition of a charged polymer with an oppositely charged polymer, has been demonstrated to be a simple but powerful, versatile, and economical approach for building up organized ultrathin organic films on surfaces.1 A wide range of charged materials, including charged polymers,1 organic dyes,2 biomolecules,3 and nanoparticles,4 have been successfully incorporated into the LBL systems, with a correspondingly wide range of potential applications, including in electroluminescent LEDs and conducting, photovoltaic, optical, and biological sensors.1,5 The most commonly exploited adhesion forces for adjacent layers in the LBL systems have been electrostatic interactions (ionic self-assembly) which require no directional orientation of the molecules.1 Other interactions, such as covalent bonding,4b,6 coordination bonding,7 hydrogen bonding,8 hydrophobic interactions,9 and biospecific interactions,3b have also been exploited, * To whom correspondence should be addressed. E-mail:
[email protected] (D.-J.K.);
[email protected] (D.K.). † Nanoscience Centre. ‡ Department of Chemistry. (1) (a) Decher, G. Science 1997, 277, 1232-1237. (b) Hammond, P. T. Curr. Opin. Colloid Interface Sci. 1999, 4, 430-442. (c) Faul, C. F. J.; Antonietti, M. Adv. Mater. 2003, 15, 673-683. (2) (a) Lang, J.; Lin, M. H. J. Phys. Chem. B 1999, 103, 1139311397. (b) Clark, S. L.; Handy, E. S.; Rubner, M. F.; Hammond, P. T. Adv. Mater. 1999, 11, 1031-1035. (3) (a) Jessel, N.; Atalar, F.; Lavalle, P.; Mutterer, J.; Decher, G.; Schaaf, P.; Voegel, J. C.; Ogier, J. Adv. Mater. 2003, 15, 692-695. (b) Zhou, D. J.; Bruckbauer, A.; Ying, L. M.; Abell, C.; Klenerman, D. Nano Lett. 2003, 3, 1517-1520. (c) Ai, H.; Fang, M.; Jones, S. A.; Lvov, Y. M. Biomacromolecules 2002, 3, 560-564. (d) Constantine, C. A.; Mello, S. V.; Dupont, A.; Cao, X. H.; Santos, D.; Oliveira, O. N.; Strixino, F. T.; Pereira, E. C.; Cheng, T. C.; Defrank, J. J.; Leblanc, R. M. J. Am. Chem. Soc. 2003, 125, 1805-1809. (4) (a) Kotov, N. A.; Dekany, I.; Fendler, J. H. J. Phys. Chem. 1995, 99, 13065-13069. (b) Chan, E. W. L.; Lee, D. C.; Ng, M. K.; Wu, G. H.; Lee, K. Y. C.; Yu, L. P. J. Am. Chem. Soc. 2002, 124, 12238-12243. (c) Pris, A. D.; Porter, M. D. Nano Lett. 2002, 2, 1087-1091. (5) (a) Tokuhisa, H.; Hammond, P. T. Adv. Funct. Mater. 2003, 13, 831-839. (b) Wang, T. C.; Cohen, R. E.; Rubner, M. F. Adv. Mater. 2002, 14, 1534-1537. (6) Shi, F.; Dong, B.; Qiu, D. L.; Sun, J. Q.; Wu, T.; Zhang, X. Adv. Mater. 2002, 14, 805-809.
albeit to a lesser extent. In most cases, a homogeneous film is fabricated on a flat substrate, which offers no positional control over the buildup of the LBL assembly on the substrate surface. However, to use these thin films in microelectronic devices, sensors, and optical memory devices, the ability of patterning and controlling the device architecture is required.2b,10 While there have been numerous studies focusing on different assembly systems, the influence of the surface properties of the substrate on the LBL assembly is less well studied.10a,12 One convenient approach to surface modification is to use self-assembled monolayers (SAMs), whose surface properties can be tailored through the modification of the thiol terminal group.11 SAMs terminated with oligo(ethylene glycol) (EGnOH) groups can resist nonspecific adsorption of biomolecules13 and strongly charged polymers.10b,c Microscale features have been prepared through the selective adsorption of polyelectrolytes on a carboxylic acid (COOH)-terminated template over an EG3OH background.10b,c The adsorption of the polyelectrolytes on surfaces is dependent on assembly (7) (a) Bharathi, S.; Nogami, M.; Ikeda, S. Langmuir 2001, 17, 74687471. (b) Krass, H.; Papastavrou, G.; Kurth, D. G. Chem. Mater. 2003, 15, 196-203. (8) (a) Zhang, H. Y.; Fu, Y.; Wang, D.; Wang, L. Y.; Wang, Z. Q.; Zhang, X. Langmuir 2003, 19, 8497-8502. (b) Hao, E. C.; Lian, T. Q. Langmuir 2000, 16, 7879-7881. (9) Serizawa, T.; Kamimura, S.; Kawanishi, N.; Akashi, M. Langmuir 2002, 18, 8381-8385. (10) (a) Clark, S. L.; Hammond, P. T. Langmuir 2000, 16, 1020610214. (b) Clark, S. L.; Montague, M. F.; Hammond, P. T. Macromolecules 1997, 30, 7237-7244. (c) Jiang, X. P.; Clark, S. L.; Hammond, P. T. Adv. Mater. 2001, 13, 1669-1673. (d) Jiang, X. P.; Zheng, H. P.; Gourdin, S.; Hammond, P. T. Langmuir 2002, 18, 2607-2615. (e) Kidambi, S.; Chan, C.; Lee, I. J. Am. Chem. Soc. 2004, 126, 4697-4703. (11) (a) Bain, C. D.; Evall, J.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7155-7164. (b) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7164-7175. (c) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559-3568. (12) (a) Tien, J.; Terfort, A.; Whitesides, G. M. Langmuir 1997, 13, 5349-5355. (b) Jiang, X. P.; Ortiz, C.; Hammond, P. T. Langmuir 2002, 18, 1131-1143. (13) (a) Prime, K. L.; Whitesides, G. M. Science 1991, 252, 11641167. (b) Zhou, D. J.; Wang, X. Z.; Birch, L.; Rayment, T.; Abell, C. Langmuir 2003, 19, 10557-10562.
10.1021/la048619s CCC: $27.50 © 2004 American Chemical Society Published on Web 09/09/2004
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Langmuir, Vol. 20, No. 21, 2004
Chart 1. Chemical Structures of the Two Polymers PLL and PSS and the Three Differently Charged Alkyl Thiols EG6OH, MHDA, and MHP‚Br Used in This Study
conditions, such as pH,10a,14 ionic strength,10b and solvent quality.15 In addition, different polymers are also found to have different surface morphologies when deposited on surfaces.10a To date, the most studied LBL systems are either “strong-strong”, where the polycation and polyanion are both strong polyelectrolytes whose charge density is independent of pH,1,10b or “weak-weak”,12a,14 where the charge density of both polyelectrolytes is strongly affected by pH. “Weak-strong” combinations of the polyelectrolytes are less well documented.15 In this study, we investigated a weak-strong combination, where the weak polycation is a polypeptide,3a poly-L-lysine (PLL), and the strong polyanion is poly(styrenesulfonate) (PSS) (Chart 1). PLL is selected as the weak polycation in favor of other polycations, such as poly(allylamine hydrochloride) (PAH), because of its biocompatibility.3a Its neutral polypeptide backbone can provide both inter- and intra-molecular hydrogen bonding, and its alkylamine side chains can provide electrostatic and hydrophobic interactions. We investigated their LBL assembly on flat template-stripped gold (TSG) surfaces precoated with SAMs of a positive, a negative, and a neutral hexa(ethylene glycol) thiol, whose molecular structures are shown in Chart 1.13 We found the initial SAM foundation layer strongly affects both the topography and the rate of the film thickness growth. Through the exploitation of the difference in LBL growth rate on different surfaces, structurally distinct features, including micro-pillars, -ridges, and -wells, were directly and controllably built up on TSG surfaces, patterned with microscale SAM templates by micro contact printing (µCP).16 The height of the features can be controlled with nanometer resolution, by simply controlling the number of assembly cycles. Experimental Section Materials. Poly-L-lysine aqueous solution (PLL; 0.1%), poly(styrenesulfonate) sodium salt (PSS; average MW 70000), 16mercaptohexadecanoic acid (MHDA; 95%), 2-butanol, and other chemicals and reagents were purchased from Sigma-Aldrich Co. (Dorset, U.K.) and used as received unless otherwise stated. The concentration for the polymers employed in the LBL assembly was 1 mg/mL for PLL and 2 mg/mL (containing 1 mM CuCl2) for PSS, prepared with ultrapure MilliQ water (resistance >18 MΩ‚ (14) (a) Shiratori, S. S.; Rubner, M. F. Macromolecules 2000, 33, 4213-4219. (b) Hiller, J.; Mendelsohn, J. D.; Rubner, M. F. Nat. Mater. 2002, 1, 59-63. (c) Lavalle, P.; Gergely, C.; Cuisinier, F. J. G.; Decher, G.; Schaaf, P.; Voegel, J. C.; Picart, C. Macromolecules 2002, 35, 44584465. (15) (a) Poptoshev, E.; Schoeler, B.; Caruso, F. Langmuir 2004, 20, 829-834. (b) Cho, J.; Quinn, J. F.; Caruso, F. J. Am. Chem. Soc. 2004, 126, 2270-2271. (c) Halthur, T. J.; Elofsson, U. M. Langmuir 2004, 20, 1739. (16) (a) Xia, Y. N.; Rogers, J. A.; Paul, K. E.; Whitesides, G. M. Chem. Rev. 1999, 99, 1823-1848. (b) Li, H. W.; Kang, D. J.; Blamire, M. G.; Huck, W. T. S. Nano Lett. 2002, 2, 347-349.
Zhou et al. cm). The pH of the PLL and PSS solutions was found to be ∼6.0. No pH and ionic strength adjustments were taken for the polymer solutions. Before usage, the solutions were filtered through a Whatman syringe filter (0.20 µm pore size, Whatman Plc.). MHDA was further purified by flash chromatography on a silica gel column using a mixed solvent of ethyl acetate/dichloromethane (1:9 v/v) as eluent and recrystallized from ethanol. (6-Mercaptohexyl)-N-pyridinium bromide (MHP‚Br) and (11-mercaptoundecyl)hexa(ethylene glycol) alcohol (EG6OH) were synthesized as described in previous publications.3b,13b Ultraflat TSG surfaces were prepared following a literature procedure using chemical stripping with THF.17 Once prepared, the TSG surfaces were thoroughly rinsed with ethanol and then MilliQ water and blown dry with a stream of N2. They were used immediately to minimize contamination.18 Homogeneous SAMs terminated with carboxylic acid (MHDA, negative), pyridinium (MHP, positive), and hexa(ethylene glycol) (EG6OH) groups were prepared by incubation of freshly prepared TSG surfaces with a 2 mM solution of the corresponding thiol in ethanol (for MHDA and EG6OH) or water (for MHP) for 12 h, thoroughly rinsed with ethanol and MilliQ water, and blown dry with N2. Micro Contact Printing. Two PDMS stamps, one with features of 2 µm stripes separated by 2 µm gaps and the other of 2 µm dot arrays, were prepared by replication of corresponding masters prepared by standard photolithography. A literature procedure was used to print the microscale SAM template.16 Because of the stability difference between SAMs with different chain lengths, the thiol with a longer alkyl chain (MHDA) was used to print while the thiol with a shorter chain (MHP) was used to back fill to minimize the possible thiol exchange during incubation. Briefly, the stamps were first inked by wiping the stamp surface with a 2 mM solution of MHDA in 2-butanol with a cotton swab. After being blown dry with N2, the stamp was brought into conformal contact with a freshly prepared TSG and kept in contact for 20-30 s before being lifted from the surface.3b For producing 2 µm dots or stripes of a MHDA SAM template, a single printing step using the 2 µm dot or stripe stamp was used. For producing MHP square arrays, the 2 µm stripe stamp was printed twice on the same surface with the stamp rotated by 90° for the second print. After printing, the unstamped gold regions were filled with a MHP SAM by incubation of the surface with a 2 mM aqueous solution of MHP‚Br for 2 h, rinsed with methanol containing 0.05 M HCl to remove noncovalently attached thiols and then with MilliQ water, and blown dry with N2. LBL Assembly of PLL and PSS. A standard LBL assembly procedure was used to assemble the PLL/PSS films.1 The assembly was carried out at the native pH (∼6.0 from our measurement) of the polymer solutions without additional salt. Under such conditions, the PLL side chain amines (pKa ≈ 10) would be extensively protonated and positively charged, while the polypeptide (polyamide) backbone remains neutral and uncharged. The substrates (SAM-coated TSG surfaces) were first soaked in the PLL solution (1 mg/mL) for 30 min, rinsed with MilliQ water (pH ≈ 6.0 from our measurement, no salt added), then dipped into the PSS solution (2 mg/mL) for another 30 min, and rinsed with MilliQ water (this constitutes an assembly cycle, abbreviated as PLL/PSS). The surfaces were not dried between the different assembly steps. Identical pHs for the polymer solutions and the rinsing water were used to minimize any disruption to the film quality during the washing step. The process was repeated until a desired number (n) of polymer bilayers ((PLL/ PSS)n) were deposited. The fabrication process was monitored by UV-vis absorption spectroscopy, atomic force microscopy (AFM), and ellipsometry. Before measurements were taken, the polymer-coated surfaces were rinsed with MilliQ water and dried under a stream of N2. Ellipsometry Measurement. The thicknesses of the SAMs and the PLL/PSS layers on TSG surfaces were measured on an EL X-02C ellipsometer (DRE, Germany) at an incident angle of (17) (a) Wagner, P.; Hegner, M.; Guntherodt, H. J.; Semenza, G. Langmuir 1995, 11, 3867-3875. (b) Zhou, D. J.; Sinniah, K.; Abell, C.; Rayment, T. Langmuir 2002, 18, 8278-8281. (18) (a) Zhou, D. J.; Sinniah, K.; Abell, C.; Rayment, T. Angew. Chem., Int. Ed. 2003, 42, 4934-4937. (b) Wang, X. Z.; Zhou, D. J.; Rayment, T.; Abell, C. Chem. Commun. 2003, 474-475.
LBL Assembly of PLL and PSS 70° using a He-Ne laser at 632.8 nm.11,18 A three-phase model (gold/organic/air), using a fixed refractive index of 1.50 (the imaginary part of the refractive index was assigned to be zero since the film is transparent at 632.8 nm) and assuming a homogeneous film for the organic layer, was used to evaluate the film thickness. After the required number of PLL and PSS assembly cycles, the surface was thoroughly rinsed with MilliQ water and dried under a stream of N2 before measurements were taken. At least five different spots on each sample were measured, and the averaged thickness values (extracted by the corresponding SAM thickness for each sample) for the polymer films were used. UV-Vis Spectra. The assembly of a PLL/PSS film was followed by measuring the absorption spectra of the films on a Cary 300 Bio UV-vis spectrophotometer (Varian Inc., California). Fused quartz slides were used as substrates. They were thoroughly cleaned by soaking with piranha solution (70:30 v/v ratio of concentrated H2SO4 and 30% H2O2; caution! piranha reacts violently with many organic compounds, and extreme care should be taken) for 1 h, then rinsed with MilliQ water, and dried under a stream of N2. An absorption spectrum of a cleaned blank quartz slide was first recorded and used as the reference. Multilayer films of PLL/PSS were then assembled onto this slide under the conditions described above. The absorption spectra were recorded at a scan rate of 600 nm/min with a resolution of 2 nm. Atomic Force Microscopy. AFM experiments were carried out on a Digital Instrument (Veeco, Santa Babara, CA) Dimension 3100 atomic force microscope with a Nanoscope IV controller.18 The height of the AFM scanner was calibrated against a TGZ01 calibration grating (25.0 ( 1.0 nm, MikroMasch Eesti OU, Estonia) and also using gold colloids (5 nm in diameter). All the images were collected at 22 ( 1 °C. For topographic imaging in air, tapping mode AFM with ultrasharp MikroMasch silicon cantilevers (NSC15 series, 125 µm long, tip radius
