Fabrication of Stimulus-Responsive Nanopatterned Polymer Brushes

NANO LETTERS

Fabrication of Stimulus-Responsive Nanopatterned Polymer Brushes by Scanning-Probe Lithography

2004 Vol. 4, No. 2 373-376

Marian Kaholek,†,§ Woo-Kyung Lee,†,§ Bruce LaMattina,‡,§ Kenneth C. Caster,§ and Stefan Zauscher*,†,§ Department of Mechanical Engineering and Materials Science, 144 Hudson Hall, Box 90300, Duke UniVersity, Durham, North Carolina 27708-09300, Army Research Office, P.O. Box 12211, Research Triangle Park, North Carolina 27709-2211, and Center for Biologically Inspired Materials and Material Systems, Duke UniVersity, Durham, North Carolina 27708-09300 Received November 19, 2003; Revised Manuscript Received December 21, 2003

ABSTRACT Stimulus-responsive, surface confined poly(N-isopropylacrylamide) (pNIPAAM) brush nanopatterns were prepared on gold-coated silicon substrates in a “grafting-from” approach that combines “nanoshaving”, a scanning probe lithography method, with surface-initiated polymerization using atom transfer radical polymerization (ATRP). The reversible, stimulus-responsive conformational height change of these nanopatterned polymer brushes was demonstrated by inverse transition cycling in water, and water−methanol mixtures (1:1, v:v). Our findings are consistent with the behavior of laterally confined and covalently attached polymer chains, where chain mobility is restricted largely to the out-of-plane direction. Our nanofabrication approach is generic and can likely be extended to a wide range of vinyl monomers.

The fabrication of nanopatterned polymer arrays with control over chemical functionality, shape, feature dimension, and interfeature spacing on the nanometer length scale has attracted considerable attention as these structures can be exploited in devices on the nano- and microscale, with applications for sensors,1 combinatorial arrays,2 protein affinity separations,3 and in micro- and nanofluidics.4 On the macro- and microscale, two approaches are used to fabricate surface attached polymer layers: a “bottom-up” or “grafting-from” approach in which covalently attached polymers are grown by surface-initiated polymerization from the substrate, yielding more densely packed polymer brush structures5 than those produced in a “grafting-to” approach that seeks to direct macromolecules to a surface and immobilize them there. Significant progress has been made in nanopatterning of biomacromolecules and polymers in a “grafting-to” approach using scanning probe lithography (SPL) techniques such as “nanoshaving” and “nanografting”,6-11 “dip-pen” nanolithography (DPN),12,13 and direct-write DPN.14-19 However, the nanofabrication of polymeric structures in a “grafting from” * Corresponding author. Phone: 1-919-660-5360, Fax: 1-919-660-8963, E-mail: [email protected] † Department of Mechanical Engineering and Materials Science, Duke University. ‡ Army Research Office. § Center for Biologically Inspired Materials and Material Systems, Duke University. 10.1021/nl035054w CCC: $27.50 Published on Web 01/16/2004

© 2004 American Chemical Society

approach using SPL remains largely unexplored.20-23 Herein we present a new, simple strategy to fabricate nanopatterned, surface-confined, stimulus-responsive poly(N-isopropylacrylamide) (pNIPAAM) brushes prepared in a “grafting-from” approach that combines SPL based on “nanoshaving” with surface-initiated polymerization using atom transfer radical polymerization (ATRP).24 ATRP has been the workhorse polymerization methodology used to prepare surface-attached polymer brushes of controlled structure, and has also been applied to polymerize NIPAAM.25,26 pNIPAAM is a stimulus-responsive polymer with a lower critical solution temperature (LCST) of about 32 °C in pure water.27 In addition to temperature, co-solvents can cause an inverse phase transition in pNIPAAM. For example, addition of 50% methanol by volume to aqueous pNIPAAM solutions leads to co-nonsolvency,27,28 effectively lowering the LCST of pNIPAAM to below 0 °C. Below the LCST, pNIPAAM is hydrated and the chains are in an extended conformational state. Above the LCST, pNIPAAM is in a hydrophobically collapsed conformational state. Patterns made from “smart” polymers with triggerable phase transition behavior, such as pNIPAAM, can be exploited in devices on the nano- and microscale, with potential applications for protein affinity separations3 and in micro- and nanofluidics.4 To pattern gold substrates, precoated with a self-assembled monolayer (SAM) of methyl-terminated 1-octadecanethiol (ODT), we used nanoshaving,6 where an AFM tip is

Figure 1. Contact mode AFM height images (20 µm × 20 µm) and corresponding typical height profiles of a pNIPAAM brush line nanopattern imaged at room temperature in (a) air, (b) MQ-grade water, and (c) a mixture of MeOH/water (1:1, v:v). The pNIPAAM line pattern was generated by first removing a thiol-resist through “nanoshaving” under large normal forces (∼50 nN) using an AFM and subsequent surface-initiated polymerization of NIPAAM for 60 min using a backfilled, covalently attached thiol initiator (1). The labels 1-5 associated with the set of parallel pNIPAAM brush lines indicate the “nanoshaving” time of a line in minutes.

employed as a nanomechanical tool to selectively remove the “thiol resist”. We applied large normal forces (∼50 nN) and high scan speeds (∼20 µm/s) to remove the resist and create a pattern of straight “trenches” on the substrate surface. Subsequently, the freshly exposed gold surface in these trenches was backfilled by self-assembly of the thiol initiator (1), ω-mercaptoundecyl bromoisobutyrate (BrC(CH3)2COO(CH2)11SH),29 to form an initiator pattern of parallel lines. Nanopatterned pNIPAAM brushes were prepared by ATRP, after exposing the initiator-patterned surfaces for 60 min at room temperature to a polymerizing solution of N-isopropylacrylamide in water at a low methanol concentration (2.6 vol %) (Scheme 1). The growing polymer brush Scheme 1. Preparation of Surface-Confined PNIPAAM Polymer Brush Nanopatterns by Combining (a) “Nanoshaving” and (b) Surface-Initiated ATRP Using a Surface-Tethered Thiol Initiator (1)

adopts an extended conformation27,28 under these reaction conditions, and brush growth is vigorous. In ATRP, a highly reactive and, in our case, surface-tethered, organic radical is generated along with a stable Cu(II) species that can be regarded as a persistent metalloradical, which is not able to initiate radical polymerization in the polymerizing solution.24 This means that polymerization is strictly confined to the surface-attached, growing polymer chains. 374

Figure 2. Average line-height plotted for the pNIPAAM brush lines labeled 1-5 in Figure 1 as a function of solvent condition. Legend: Line-height in air (patterned bar; brush collapsed), and line-heights after cyclic exposure (2 cycles) to first MQ-grade water (gray bars; brush swollen), and then to a 1:1 (v:v) MeOH/water mixture (white bar; brush collapsed).

Figure 1 shows a typical pNIPAAM brush line-pattern with line-widths ranging from 300 to 500 nm, imaged in (a) air, (b) water, and (c) a mixture of water-MeOH (1:1, v:v). The AFM topographic images were collected in contact mode while scanning laterally (scan angle ) 90°) over the lines labeled 1-5 (Set 1), where the line numbers correspond directly to the nanoshaving time in minutes. It is important to note that the equilibrium brush heights, in absence of any applied load, are likely larger than the measured apparent heights obtained from contact mode imaging.30 The polymer brush heights obtained from averaged height profiles of lines 1-5 in Figures 1a-c depend not only on the solvent (discussed below) but also on the nanoshaving conditions such as tip force and shaving time. In Figure 2 is plotted the average line-height for the patterned pNIPAAM brush lines labeled 1-5 in Figure 1 as a function of solvent conditions. The data in Figure 2 show that brush height increases with increasing shaving time (indicated in minutes directly by the line number) assuming that the shaving force has remained approximately constant while shaving the lines of Set 1. For example, the average heights of line 1 (1 min shaving), are 13, 34, and 11 nm in air, water, and a mixture of water-MeOH (1:1, v:v), respectively (Figure 2). In the case of line 5 (5 min shaving), the corresponding heights Nano Lett., Vol. 4, No. 2, 2004

Figure 3. Average line-width plotted for the pNIPAAM brush lines labeled 1-5 in Figure 1 as a function of solvent condition. Legend: Line-width in air (patterned bar; brush collapsed), and line-widths after cyclic exposure (2 cycles) to first MQ-grade water (gray bars; brush swollen), and then to a 1:1 (v:v) MeOH/water mixture (white bar; brush collapsed).

are 23, 60, and 35 nm. This dependence of brush height on shaving time is likely associated with the degree of resist removal, where at short shaving times the number of residual thiol resist molecules is larger than that at long shaving times. The degree of resist removal will directly affect the initiator surface density that can be achieved by backfilling. Brush height is a function of initiator surface density, where low initiator densities lead to low brush heights. This was recently shown by Huck et al.29 where SAMs containing 10% and 50% of thiol initiator (1) grew poly(methyl methacrylate) (PMMA) brushes to approximately 1/10 and 1/2 the thickness of PMMA brushes initiated from SAMs comprising 100% of the thiol initiator. Figure 3 shows that the average line-widths typically increase with increasing shaving time (lines 2-5). This dependence of lateral feature width on shaving time can likely be explained by drift of the open-loop XY-scanner employed in the AFM; the uncertainty in holding a selected line position (slow scan axis) increases with increasing scanning time. The unexpectedly large line width of line 1 is likely due to an unusually large drift of the slow scan axis. Use of a closed-loop XY-scanner should overcome this problem. The line pattern in Figure 1 and the data in Figure 2 also show that the conformation of nanopatterned pNIPAAM brushes is significantly affected by the solvent conditions. When exposed to water at room temperature, dry pNIPAAM brushes swell significantly and more than double their height. pNIPAAM brushes are also responsive to the solvent composition. For example, in pure water and at temperatures below the LCST, pNIPAAM brushes are in a good solvent and the brush adopts an extended conformation. After exposure to a water-MeOH (1:1, v:v) mixture (poor solvent), the brush adopts a hydrophobically collapsed conformation.27,28 We demonstrated the reversibility of this stimulusresponsive conformational change of nanopatterned brushes by cyclic exposure to water, and water-methanol mixtures (1:1, v:v) (inverse transition cycling). Figure 2 shows that the average heights of, for example, lines 1 and 5 decrease after addition of 50% (v) MeOH as a co-solvent to pure water, by 68% and 42%, respectively. In a second transition cycle the original brush line heights were not completely recovered (Figure 2). This may be due to an Nano Lett., Vol. 4, No. 2, 2004

incomplete rehydration of the brush in the experimental time frame. Figure 1 shows that a second set of lines (Set 2), oriented perpendicularly to lines 1-5 of Set 1, has smaller feature heights and appears not to be as responsive to solvent conditions as the lines of Set 1, although the shaving times for lines in Set 1 and in Set 2 were the same (we are grateful to the reviewer to have pointed out this apparent discrepancy). The perceived lack of responsiveness for lines in Set 2 is in part due to the chosen out-of-plane scaling and the orientation of the three-dimensional plots. To convince ourselves of the stimulus response of all lines on the pattern, we reexamined the same sample after it had been stored for four months in a humid environment. This analysis clearly showed that both sets of lines are equally stimulus-responsive and that storage affected the remeasured feature heights little (Supporting Information). We attribute the smaller feature heights of lines in Set 2 to overall smaller shaving forces, as the lines of Set 2 were shaved after all lines in Set 1 had been completed. It is likely that the applied nanoshaving forces at the time of shaving the lines from Set 2 were diminished due to drift in the cantilever deflection setpoint of the AFM. Small shaving forces led to less efficient removal of the thiol resist, ultimately again lowering the surface density of initiator (1). Figure 3 shows that the averaged widths of the pNIPAAM line patterns are affected only slightly by the solvent conditions. The line widths are slightly smaller in air and in the water-MeOH mixture (1:1, v:v) when compared to the widths in pure water. Our findings are thus consistent with the expected behavior of laterally confined and covalently attached stimulus-responsive polymer chains, where chain mobility is largely restricted to the out-of-plane direction.31 Here we have demonstrated the prototypical fabrication of nanopatterned, surface-confined, stimulus-responsive pNIPAAM brushes in a “grafting-from” approach using a simple strategy that combines SPL with surface-initiated polymerization using ATRP. We were able to fabricate polymer brush nanopatterns with an aspect ratio (height/ width) of about 1/10 in an extended state, and we expect that further improvements in the nanoshaving process such as the use of sharpened probe tips, closed-loop position control of the XY-scanner, and careful control of the shaving conditions, such as speed and applied force, will result in considerably reduced feature dimensions. Our nanofabrication approach is generic and can likely be extended to a wide range of vinyl monomers. The “living” nature of ATRP initiators24 allows the reinitiation of pNIPAAM nanopatterns, and we are currently using this property to synthesize stimulus-responsive block copolymers that have two different trigger mechanisms. Polymer nanopatterning, as demonstrated here, could be of significant importance for the fabrication of silicon-based devices because the patterned polymer brushes could serve as robust barriers to a range of wet chemical etchants.32,33 We are currently evaluating the effectiveness of nanopatterned stimulus-responsive polymer brushes for applications in biosensors, in which stimulus-responsive fusion proteins34 can be reversibly addressed to the patterned surface through 375

hydrophobic interactions. This application would potentially allow for protein separation from complex mixtures and require only minute amounts of sample. Acknowledgment. The authors thank the National Science Foundation for support through grants NSF EEC021059, NSF DMR-0239769 CAREER AWARD, and ARO DAADG55-98-D-0002. We thank Mr. Hongwei Ma (Department of Biomedical Engineering, Duke University) for the synthesis of the thiol initiator and Dr. Sang-Jung Ahn (Department of Computer Science, Duke University) for his contributions to scanning probe lithography. Supporting Information Available: Experimental details, and additional analysis of the line patterns. This material is available free of charge via the Internet at http:// pubs.acs.org. References (1) Bailey, R. C.; Hupp, J. T. Anal. Chem. 2003, 75, 2392-2398. (2) Stoykovich, M. P.; Cao, H. B.; Yoshimoto, K.; Ocola, L. E.; Nealey, P. F. AdV. Mater. 2003, 15, 1180-1184. (3) Nath, N.; Chilkoti, A. AdV. Mater. 2002, 14, 1243-1247. (4) Beebe, D. J.; Moore, J. S.; Yu, Q.; Liu, R. H.; Kraft, M. L.; Jo, B. H.; Devadoss, C. Proc. Natl. Acad. Sci. U.S.A. 2000, 97, 1348813493. (5) Prucker, O.; Ru¨he, J. Macromolecules 1998, 31, 592-601. (6) Liu, G. Y.; Xu, S.; Qian, Y. L. Acc. Chem. Res. 2000, 33, 457-466. (7) Wadu-Mesthrige, K.; Xu, S.; Amro, N. A.; Liu, G. Y. Langmuir 1999, 15, 8580-8583. (8) Liu, G. Y.; Amro, N. A. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 5165-5170. (9) Kenseth, J. R.; Harnisch, J. A.; Jones, V. W.; Porter, M. D. Langmuir 2001, 17, 4105-4112. (10) Case, M. A.; McLendon, G. L.; Hu, Y.; Vanderlick, T. K.; Scoles, G. Nano Lett. 2003, 3, 425-429. (11) Jang, C. H.; Stevens, B. D.; Phillips, R.; Calter, M. A.; Ducker, W. A. Nano Lett. 2003, 3, 691-694. (12) Lee, K. B.; Park, S. J.; Mirkin, C. A.; Smith, J. C.; Mrksich, M. Science 2002, 295, 1702-1705.

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Nano Lett., Vol. 4, No. 2, 2004