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Cross-Linked and Chemically Functionalized Polymer Supports by Reactive Reversal Nanoimprint Lithography Wei Zhao,† Hong Yee Low,*,† and P. S. Suresh*,‡ Institute of Materials Research and Engineering, 3, Research Link, Singapore 117602, and Institute of Chemical and Engineering Sciences, 1, Pesek Road, Jurong Island, Singapore 627833 ReceiVed September 15, 2005. In Final Form: December 15, 2005 A new format of polymer support having cross-linked polymeric micro- and nanoarrays has been fabricated via reactive reversal nanoimprint lithography. Reactive reversal nanoimprint lithography is a relatively simple method to imprint highly cross-linked and chemically tunable polymers. An array of chloromethyl-functionalized cross-linked polystyrene has been imprinted on hard (silicon) and soft (polymer) substrates, and a model esterification reaction is demonstrated. The imprints have been found to be relatively stable under both static and dynamic stability tests carried out in various organic solvents. The chemical functionality is evenly distributed over the imprinted array. This method of fabricating polymer supports offers a high degree of freedom in terms of the choice of chemical functionality, the types of polymer matrix, and the size of the polymer support. The functional polymer support has potential applications for chemical and biological assays.
1. Introduction Nanoimprint lithography (NIL) has become a potential alternative to conventional lithographic technologies because of its ability to pattern materials into smaller scale more efficiently and economically.1 NIL is essentially similar to the conventional hot embossing technique, except that, in NIL, the focus is on the fabrication of nanometer-scale patterns. The nanometer-scale feature is reproduced from a mold, which can be made by e-beam lithography, meaning that the pattern resolution in NIL is not limited by the optical diffraction, as in the conventional photolithography technique. Thus, feature size as small as 10 nm has been demonstrated on polymers.2 Although e-beam lithography can be used to fabricate nanometer scale features, it is a slow and expensive process. However, once the mold is fabricated, NIL can be used for the routine fabrication of nanopatterns. NIL also offers the advantages of flexibility in terms of choice of materials that can be patterned and a flexible processing window to enable the patterning of various nanostructures. Its applications range widely from integrated circuits3 and nanofluidic channels4 to tissue engineering.5 However, in most of these applications, NIL serves only as a lithographic tool without imparting functionality on the imprinted structures. In this paper, we described the application of NIL to fabricate a functionalized polymer support. The nanoimprinted polymer support offers a number of advantages for potential applications in chemical and biological assays. As an example, resin bead solid supports are widely used in solid-phase organic synthesis,6-8 combinatorial synthesis,9,10 and * Corresponding author. E-mail:
[email protected] (H.Y.L.);
[email protected] (P.S.S.). † Institute of Materials Research and Engineering. ‡ Institute of Chemical and Engineering Sciences. (1) Chou, S. Y.; Krauss, P. R.; Renstrom, P. J. Appl. Phys. Lett. 1995, 67, 3114. (2) Cao, H.; Yu, Z.; Wang, J.; Tegenfeldt, J. O.; Austin, R. H.; Chen, E.; Wu, W.; Chou, S. Y. Appl. Phys. Lett. 2002, 81, 174. (3) Resnick, D. J.; Dauksher, W. J.; Mancini, D.; Nordquist, K. J.; Bailey, T. C.; Johnson, S.; Stacey, N.; Ekerdt, J. G.; Wilson, C. G.; Sreenivasan, S. V.; Schumaker, N. J. Vac. Sci. Technol. B 2003, 21, 2624. (4) Guo, J.; Cheng, X.; Chou, C. Nano Lett. 2004, 4, 69. (5) Yim, E.; Reano, R.; Pang, S.; Yee, A.; Chen, C.; Leong, K. Biomaterials 2005, 26, 5405. (6) Vaino, A. R.; Janda, K. D. J. Comb. Chem. 2002, 2, 579. (7) Hudson, D. J. Comb. Chem. 1999, 1, 403. (8) Blaney, P.; Grigg, R.; Sridharan, V. Chem. ReV. 2002, 102, 2607. (9) Bunin, B. A.; Plunkett, M. J.; Ellman, J. A. Proc. Natl. Acad. Sci. U.S.A. 1994, 91, 4708.
as supports for immunological assay screening studies. Although this is a fairly well-established technology, the use of resin beads as solid supports still faces some challenges.11,12 The effectiveness of resin beads as a solid support is governed by the accessibility of the functional groups; specifically, factors such as diffusion rate and the reaction kinetics are greatly affected by the size of the beads and the degree of resin swelling. Generally, the diffusion rate and reaction kinetics are higher when the resin has a higher degree of swelling13 and the bead size is smaller.14 However, there is a balance between the degree of cross-linking and the swelling property. A higher degree of cross-linking provides beads with higher mechanical strength and better chemical resistance at the cost of limited swelling, which leads to lower efficiency of the reaction due to inaccessible functional sites. Although decreasing the bead size increases the available surface area and thus allows for a higher population of the functional groups on the surface, there is a limit to how much the size of the beads can be reduced because of the difficulty in separating the beads during work up. This drawback may be overcome by anchoring the nanometer-scale solid support in an array format on a substrate. Using the nanoimprinting technique, a functionalized polymer resin array with a high and tunable surface/volume ratio can be obtained. Recently, NIL was used to fabricate nonfunctional poly(methyl methacrylate) nanopillars for immunoassay application.15 In this paper, we demonstrate the use of reactive reversal NIL to fabricate a cross-linked and chemically functionalized polymer support and a model chemical exchange reaction on the imprinted polymer support. 2. Experimental Section Treatment of Molds and Substrates. Si molds were supplied by the Institute of Microelectronics, Singapore. The substrates used for imprinting were prime Si wafers with a thickness of 0.5 mm and poly(ethylene 2,6-naphthalate) sheets (PEN, Goodfellow) with a thickness of 0.125 mm. (10) Arya, P.; Quevillon, S.; Joseph, R.; Wei, C. Q.; Gan, Z.; Parisien, M.; Sesmilo, E.; Reddy, P. T.; Chen, Z. X.; Durieux, P.; Laforce, D.; Campeau, L. C.; Khadem, S.; Couve-Bonnaire, S.; Kumar, R.; Sharma, U.; Leek, D. M.; Daroszewska, M.; Barnes, M. L. Pure Appl. Chem. 2005, 77, 163. (11) Walsh, D.; Wu, D.; Chang, Y. T. Curr. Opin. Chem. Biol. 2003, 7, 353. (12) Yu, Z.; Bradley, M. Curr. Opin. Chem. Biol. 2002, 6, 347. (13) Walsh, D. P.; Pang, C.; Parikh, P. B.; Kim, Y. S.; Chang, Y. T. J. Comb. Chem. 2002, 4, 204. (14) Groth, T.; Grotli, M.; Meldal, M. J. Comb. Chem. 2001, 3, 461.
10.1021/la052523w CCC: $33.50 © 2006 American Chemical Society Published on Web 05/09/2006
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Figure 1. Schematic diagram of an array of a polymer support. To facilitate mold release, an antisticking layer was deposited on the Si mold by immersing the mold into a heptane solution, which contained 10 mM 1H,1H,2H,2H-perfluorodecyl-trichlorosilane (FDTS, 96%, Lancaster). To enhance the adhesion between the cross-linked polymers and the substrates, surface modifications were conducted for both the Si and PEN substrates. For the Si substrate, a coupling agent, [3-(methacryloyloxy)propyl]trimethoxysilane (γ-MPS, 98%, Aldrich), was hydrolyzed in methanol/water (80:20 by volume) under a pH value of 4. A Si wafer was then immersed in the solution for 10 min. After treatment, the Si wafer was rinsed in water, 2-propanol, and acetone sequentially before use. For the PEN sheet, an oxygen plasma exposure (250 mT, 80 w, 5 s) was performed prior to the imprinting process to increase its surface energy, again for enhanced adhesion between the imprinted polystyrene (PS) and the PEN substrate. Precursor Preparation. A thermally cross-linkable recipe, containing styrene (99%, Fluka) as the monomer, divinylbenzene (80%, mixture of isomers, Aldrich) as the cross-linker, vinylbenzyl chloride (97%, Aldrich) as the functional monomer, and benzyl peroxide (Aldrich) as the thermal initiator, in the molar ratio of 60:13:25:2, was mixed and stirred over 6 h. Filtration through a 0.2-µm filter was performed before conducting imprinting. To tune the concentration of the functionalities, the molar ratio of the crosslinker and the initiator was fixed while tuning the concentration of the functional monomers. Reactive Reversal Nanoimprinting. The precursor was dispersed on a Si mold. The dispersion was made by drop-casting the precursor with the aid of a 0.5-µL micropipet to control the volume of the precursor for imprinting. Either silicon or polymer substrate was then placed on top of the mold to form a sandwich structure. Imprinting was carried out on a 4-in. imprinter (Obducat, Inc.) at a temperature of 110 °C for a period of 10 min under a pressure of 4 MPa. After releasing the mold, the substrate containing crosslinked resin was subjected to a reactive ion etching (RIE) process to remove the residual layer and then post-cured under vacuum at 110 °C for a period of 1 h. It is worth noting that the imprinting conditions, viz., temperature, time, and pressure, can be tuned according to the polymerization exotherm of the resin system. The polymerization exotherm was easily obtained through differential scanning calorimetry (DSC). Characterization. Scanning electron microscopy (SEM) images were taken on a JEOL JSM6700F field emission scanning electron microscope at 5 kV. All samples were sputtered with a thin gold layer before sample loading. Atomic force microscope (AFM) scanning was conducted on a Multimode Digital Instruments AFM in tapping mode. Fluorescence microscopy images were taken on a Leica DM-IRE2 confocal scanning microscope.
3. Results and Discussion 3.1. Nanoimprinting of Functionalized Cross-Linked PS Arrays. A schematic showing the concept of an imprinted polymer support is shown in Figure 1 in which an array of polymer supports with tunable size and chemical functionality is imprinted onto a chemically inert substrate. The polymer supports could (15) Kuwabara, K.; Ogino, M.; Motowaki, S.; Miyauchi, A. Microelectron. Eng. 2004, 73-74, 752.
Figure 2. Schematic diagram of the reactive reversal imprinting process.
be of various geometries and shapes; in this work, pillar structure ranging from 2 µm to 250 nm were fabricated. These micro- and nanopillars would be chemically and mechanically stable for the various potential applications and are thus preferably made up of a cross-linked polymer. The imprinting process is depicted in Figure 2. Briefly, a mixture of reactive resin is drop-cast on the mold, and a substrate is placed on top of the mold. The sandwich structure is then placed in the imprinting machine for the imprinting process. We refer to this process as reactive reversal imprinting, which is based on an earlier report on the reversal imprinting method.16 In contrast to conventional NIL, in which the polymer for imprinting is spin-coated on the substrate, reversal imprint is carried out by coating the polymer on the mold. In reactive reversal imprinting, instead of imprinting a polymer, we start with a resin mixture, which is composed of monomers, a cross-linker, and a functional comonomer. The resin mixture is drop-cast on the mold and cross-linked during the imprinting process; thus, we named it reactive reversal imprinting. The reactive reversal imprinting method is also contrasted with the step and flash technique, where a photocurable resin is dispensed on a substrate and imprinting is carried out with in situ photocuring.17 There are a number of advantages with the reactive reversal imprinting approach: first, by starting with a low viscosity resin, a complete filling of the precursor into the patterns (particularly the trenches) on the mold is achieved during the dispersion of the resin droplet, thus avoiding the incomplete filling phenomenon that is often seen in NIL processes;18 second, because of the extremely low viscosity (compared to polymer melt), the excess resin is easily squeezed out during the imprinting process, resulting in a very thin residual layer. A thin residual layer is important for this fabrication because the residual layer is removed by exposing (16) Huang, X. D.; Bao, L. R.; Cheng, X.; Guo, L. J.; Pang, S. W.; Yee, A. F. J. Vac. Sci. Technol. B 2002, 20, 2872. (17) Resnick, D. J.; Mancini, D.; Dauksher, W. J.; Nordquist, K.; Bailey, T. C.; Johnson, S.; Sreenivasan, S. V.; Ekerrdt, J. G.; Wilson, C. G. Microelectron. Eng. 2003, 69, 412. (18) Heyderman, L. J.; Schift, H.; David, C.; Gobrecht, J.; Schweizer, T. Microelectron. Eng. 2000, 54, 229.
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the imprinted polymer to RIE, such as oxygen plasma etching. A long RIE exposure time will not only roughen the surface of the imprinted polymer, but will also etch or degrade the chemical functional groups. It is obvious that minimum etching or degradation of the functional group is desirable. The imprinting temperature was set at 110 °C, at which the precursor containing styrene (monomer), vinylbenzyl chloride (functional comonomer), and divinylbenzene (cross-linker) started to polymerize, and resulted in a highly cross-linked PS. The imprinting temperature was selected according to the polymerization exotherm of the resin obtained from DSC. When another resin mixture is used, the imprinting temperature will be selected according to its reaction exotherm. The substrates used in this study are Si wafer and PEN. This serves to demonstrate the versatility of the technique: a hard substrate may be more suitable in some applications, while a polymer substrate offers the potential advantages of a rolled up polymer support because of its flexibility. Prior to imprinting, the Si substrate was treated with a functional silane, γ-MPS. The reactive methacrylate moiety in γ-MPS reacts with the styrene monomer during the reactive reversal imprinting, forming chemical bonds between the PS and the substrate, thus resulting in a strong adhesion. On the other hand, the PEN substrate was treated with oxygen plasma to increase its surface energy to enhance the adhesion to the cross-linked polymer. The SEM images of the polymer support on the Si substrate and on the PEN substrate, with pillar diameters ranging from 2 µm to 250 nm, are shown in Figure 3. From microscopy images (not shown here) and the SEM images, well-defined pillars were obtained over a large area; no defects (such as partial pillar) due to an incomplete filling phenomenon was observed for all the samples. The size, shape, and density of the pillars are determined by the features on the mold, meaning that high aspect ratio pillars or pillars less than 100 nm in diameter are also feasible using the reactive reversal NIL. The overall efficiency of the imprinted polymer support can be evaluated by the ratio of total functionalized surface area to the area of the entire substrate, which is proportional to h/r (r and h are the radius and height of the pillar, respectively). [For arrays with round pillars distributed in a square format, as depicted in Figure 1, in the case where the spacing between two adjacent pillars is 2 times the radius of the pillar, the functionalized surface area (Sfunction), the volume of each individual pillar (Vpillar), and the area of the substrate (Ssubstrate) are expressed in the following: Sfunction ) πr2 + 2πrh; Vpillar ) πr2h; Ssubstrate ) 4r2; thus, Sfunction/Ssubstrate ) π(1/4 + h/2r) and Sfunction/Vpillar ) 2/r + 1/h]. On the other hand, for each individual pillar, the efficiency is valued by the ratio of outer surface area to its volume, which can be express as (2/r + 1/h). By reducing the diameter of the pillar, both the overall efficiency and the efficiency of an individual pillar are enhanced, whereas increasing the height of the pillar contributes to the overall efficiency only. Therefore, pillars with higher aspect ratios and smaller diameters are expected to have higher overall efficiency. Another advantage of reversal imprinting is that it results in a thin residual layer. To measure the thickness of the residual layer, a scratch was made on the 2 µm, imprinted cross-linked PS on Si substrate. An AFM was employed to measure the height of the trench, which is equal to the thickness of the residual layer. Measurement was performed at different locations of the scratches. Figure 4 shows two of the scans results. A residual layer of about 5 nm was detected. The variation in the thickness of the residual layer was found to be within 1 nm along the scratch, indicating good uniformity of the imprinting. Comparing it with the height of the pillars (500 nm), the height/residual ratio for the 2 µm pillar was found to be around 100. A high height/
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Figure 3. SEM images of imprinted PS pillars: (A) 2 µm diameter PS pillar on silicon (aspect ratio: 1:2); (B) 250 nm diameter PS pillars on Si substrate (aspect ratio: 1:1); and (C) 250 nm diameter PS pillars on PEN substrate (aspect ratio: 1:1).
Figure 4. Measurement of residual layer thickness by AFM.
residual ratio ensures the fidelity of the features after oxygen plasma exposure. Stability of the pillar in different solvents is crucial for its applications. Both static and dynamic stability tests were conducted in water, tetrahydrofuran, ethanol, toluene, and dichloromethane. In static stability tests, samples were immersed into solvents for over 1 day at room temperature without stirring. For all the samples, no obvious changes in any dimension of the features were detected, as characterized using AFM and SEM. The stability in the features is due to the high degree of crosslinking in the resin. While a high cross-linking degree is a
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Figure 5. SEM images of an imprinted cross-linked PS array after exposure to solvents: (A) dichloromethane; (B) toluene.
Figure 6. Fluorescence microscope image of the imprinted polymer support after the solid-phase reaction.
drawback for the resin bead support because of its hindrance to swelling during chemical reactions, it is an advantage for the imprinted polymer support because it imparts high mechanical and chemical stabilities. In the dynamic stability test, magnetic stirring and shaking were applied to simulate more realistic reaction conditions. Representative SEM images of the samples that went through such tests are shown in Figure 5. Defects such as missing, dislodged, and merged pillars were seen occasionally in pillars on the Si substrate, particularly when using highly polar or polar aprotic solvents. This is partially due to the brittleness and rigidity of the Si substrate. When these substrates were bumping with the stirrer or with the wall of the reactor or with each other, the substrates could not absorb or dissipate the energy because of a lack of resilience, leading to the delamination of the imprinted pillar from the substrate. On the other hand, the imprinted polymer support on the PEN substrate remained mostly intact after the solvent stability tests. 3.2. Model Reaction on the Surface of the Imprinted Polymer Support. A model reaction on the imprinted polymer support was carried out in dichloromethane. The reaction is shown in Scheme 1:
A fluorophore, 9-carboxyacridine, was introduced covalently on a chloromethyl-substituted polymer support through an exchange reaction in the presence of triethylamine at room temperature for 5 h. After the reaction, the imprinted polymer support was sequentially rinsed in dichloromethane, acetone, deionized water, and ethanol and dried in air. Note that an excess amount of the reagents were used to ensure the completion of the reaction. The surface of the imprinted polymer support was examined using a fluorescence microscope. Because of the limitation of resolution of the fluorescence microscope, only the 2 µm polymer support with 10 µm spacing could be characterized with reliable results. A microscopic image of the fluorescence observed is shown in Figure 6. The line scan on a row of the PS pillars shows good uniformity of the fluorescence intensity on each individual pillar, which indicates uniform distribution of the chlorofunctional groups over the array of the PS pillars. No fluorescence was seen from the spacing between the PS pillars, showing that the residual layer was totally removed. A shadow was seen beside each shining pillar in the image. This optical phenomenon was attributed to the reflective surface of the Si substrate, which was confirmed by the absence of shadow in the polymer pillar on the PEN substrate. The intensity of the fluorescence emitted from each pillar can be compared by their signal amplitudes. Figure 7 shows a profile of the signal from one pillar. By monitoring the amplitude and thus the intensity of the fluorescence, the effect of the functional monomer concentration in the polymer support system is estimated. As shown in Figure 8, the fluorescence intensity increases with increasing comonomer (vinylbenzyl chloride) concentration. A best fitting was obtained for a fractional power law relationship between the comonomer concentration and the fluorescence intensity. This result is expected because the comonomer is assumed to be uniformly distributed in the volume of the entire pillar. However, the pillars do not swell in the reaction medium, meaning only the surface functional groups
Scheme 1. Imprinted Crosslinked PS Support Used to Conduct a Reaction between the Chloromethyl and 9-Carboxyacridinea
a
Note that the schematic is not drawn to scale.
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information only. (We are currently working on spectroscopy techniques to more accurately quantify the concentration of surface functional groups.) Theoretically, if the available functionalities at one concentration could be quantified, the available functionalities at any given concentration can be determined. This property of imprinted polymer supports made it possible to study chemical and biological processes quantitatively in nanoscale. In contrast, it is harder to quantify the concentration of the available functional groups in the resin bead support because a good proportion of the functional groups is buried within the beads, and their availability for a reaction depends on the swelling of the polymer matrix.
4. Conclusions Figure 7. A typical profile of a scan-line across the 2 µm pillars during signal amplitude measurement.
Figure 8. Fluorescence intensity of an imprinted pillar as a function of the comonomer concentration. The power law fitting is extrapolated to higher concentration. The average in each data point was taken from an average of three pillars in each sample.
are available for the reaction. Since the pillar dimensions are kept constant for this set of the experiment, the surface area of the pillars is the limiting factor. As mentioned earlier, if the surface area of the pillars is increased, for example, by increasing the aspect ratio, the result will be an increase in the available functional groups for the reaction. Furthermore, the accessible surface groups are on both the top surface and the sidewall of the pillar, while fluorescence imaging measures the top surface
In this work, we have demonstrated that NIL offers a unique approach to fabricating polymer supports. Such polymer supports could potentially be useful for applications such as chemical and biological assays and solid-phase reactions. The unique feature of nanoimprinted polymer support is summarized below: (a) The efficiency can be greatly enhanced without compromising the mechanical and chemical properties. The high surface/ volume ratio of a nanoscale array, as obtained from NIL, renders most of the functional sites accessible by the reagents without the need for resin swelling. (b) It is possible to achieve any feature size, feature shape, and feature density by replicating the desired mold. The number of the functional sites on the individual polymer support can be tuned by changing the concentration of the functional monomers and the size of the pillars. By proper choice of the comonomers, polymer supports having varying functionalities could be made easily. Since there is little swelling involved when used in reactions, the system is truly a solid-phase reaction that takes place at the solid/liquid interface. Such a simplified situation allows better control of chemical processes. (c) Imprinted polymer supports offer easy handling during reaction and postreaction processes, particularly the one made out of PEN film, as it can be conveniently rolled up. Through this work, we also demonstrated the feasibility of fabricating chemically functionalizable, stable nanoarrays by NIL on inert substrates. Acknowledgment. We thank Wang Yubo for his assistance in fluorescence microscopy characterization. LA052523W
