pubs.acs.org/Langmuir © 2009 American Chemical Society
Submicrometer Chemical Patterning with High Throughput Using Magnetolithography Amos Bardea and Ron Naaman* Department of Chemical Physics, The Weizmann Institute, Rehovot 76110, Israel Received February 17, 2009. Revised Manuscript Received April 6, 2009 This letter demonstrates the ability to pattern surfaces chemically with submicrometer resolution by applying the simple and inexpensive magnetolithography (ML) method. This method allows fast patterning of large surfaces without having to face contamination problems or the need to remove the substrate from the solution. With ML it is possible to obtain pattern whose width is narrower than the width of the lines in the mask. By applying the green fluorescent protein (GFP), we were able to probe a 30 nm line of hydrophobic molecules patterned on a substrate coated with a hydrophilic monolayer.
Chemical patterning of surfaces is performed by methods varying from high-resolution, low-throughput techniques that are based on STM and/or AFM1-4 to printing-based methods5-10 and patterning that is based on photolithography.11,12 Recently, we introduced the magnetolithography (ML) method in which a paramagnetic mask is applied for the chemical patterning of surfaces with high-throughput and resolution.13 In contrast to other parallel lithography techniques such as nanocontact printing14 and nanoimprinting lithography,15 ML is a backside lithography technique that has the advantage of ease in producing multilayers with high accuracy of alignment and with the same efficiency for all layers. ML is performed by applying a magnetic field to the substrate by paramagnetic metal masks on its back side in the presence of a constant magnetic field. The mask defines the spatial distribution and shape of the magnetic field applied to the substrate. The *Corresponding author. E-mail:
[email protected]. (1) Martinez, R. V.; Losilla, N. S.; Martinez, J.; Tello, M.; Garcia, R. Nanotechnology 2007, 18, 084021. (2) Hong, S.; Mirkin, C. A. Science 2000, 288, 1808–1811. (3) Zhang, H.; Amro, N. A.; Disawal, S.; Elghanian, R.; Shile, R.; Fragala, J. Small 2007, 3, 81–85. (4) Salaita, K.; Wang, Y.; Fragala, J.; Vega, R. A.; Liu, C.; Mirkin, C. A. Angew. Chem. 2006, 45, 7220–7223. (5) Hoeppener, S.; Maoz, R.; Sagiv, J. Nano Lett. 2003, 3, 761–767. (6) (a) Geissler, M.; Bernard, A.; Bietsch, A.; Schmid, H.; Michel, B.; Delamarche, E. J. Am. Chem. Soc. 2000, 122, 6303–6304. (b) Michel, B.; Bernard, A.; Bietsch, A.; Delamarche, E.; Geissler, M.; Juncker, D.; Kind, H.; Renault, J. P.; Rothuizen, H.; Schmid, H.; Schmidt-Winkel, P.; Stutz, R.; Wolf, H. IBM J. Res. Dev. 2001, 45, 697–719. (7) Loo, Y. L.; Willett, R. L.; Baldwin, K. W.; Rogers, J. A. Appl. Phys. Lett. 2002, 81, 562–564. (8) Jacobs, H. O.; Whitesides, G. M. Science 2001, 291, 1763–1766. (9) Schmid, H.; Wolf, H.; Allenspach, R.; Riel, H; Karg, S.; Michel, B Adv. Funct. Mater. 2003, 13, 145–153. (10) (a) Quist, A. P.; Pavlovic, E.; Oscarsson, S. Anal. Bioanal. Chem. 2005, 381, 591–600. (b) Xu, C.; Taylor, P.; Ersoz, M.; Fletcher, P. D. I.; Paunov, V. N. J. Mater. Chem. 2003, 13, 3044–3048. (c) Lange, S. A.; Benes, V.; Kern, D. P.; Horber, J. K. H.; Bernard, A. Anal. Chem. 2004, 76, 1641–1647. (d) Schmalenberg, K. E.; Buettner, H. M.; Uhrich, K. E. Biomaterials 2004, 25, 1851–1857. (11) Buxboim, A.; Bar-Dagan, M.; Frydman, V.; Zbaida, D.; Morpurgo, M.; Bar-Ziv, R. Small 2007, 3, 500–510. (12) Stewart, M. E.; Motala, M. J.; Yao, J.; Thompson, L. B.; Nuzzo, R. G. J. Nanoeng. Nanosyst. 2007, 220, 81–138. (13) Bardea, A.; Naaman, R. Small 2009, 5, 316–319. (14) Li, H. W.; Muir, B. V. O; Fichet, G.; Huck, W. T. S. Langmuir 2003, 19, 1963–1965. (15) Hoff, J. D.; Cheng, L. J.; Meyhofer, E.; Guo, L. J.; Hunt, A. J. Nano Lett. 2004, 4, 853–857.
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second component in ML is ferromagnetic nanoparticles (NPs) that reside on the substrate according to the field induced by the mask. ML can be applied either in the positive or negative mode. In the positive mode, the magnetic NPs react chemically or interact via chemical recognition with the substrate. Hence, the magnetic NPs are immobilized at selected locations, where the mask induces a magnetic field, resulting in a patterned substrate. In the negative mode, the magnetic NPs do not interact chemically with the substrate. Hence, once they pattern the substrate, they block their site on the substrate. The exposed areas, not covered by the NPs, can in this stage be covered by molecules that chemically bind to the substrate. After the binding of these molecules, the NPs are removed, resulting in a “negatively” patterned substrate. Recently, we have demonstrated the negative ML approach, on a micrometer scale, by blocking the adsorption of a biotin monolayer on a substrate by magnetic NPs and then using NPs to block biotin-avidin biorecognition.13 Another unique feature of the ML method is that in a nonequilibrium state, with short times and low concentrations of magneto NPs, it is possible to obtain patterns whose width is narrower than the width of the lines in the mask. This is due to the gradient of the magnetic field within the line width defined by the mask. This unique feature is the focus of the current work. According to our results, by using ML it is possible to localize protein at a predefined spot on the surface with sub-100-nm resolution. This is done using negative ML for the self-assembly of a line of hydrophobic monolayers onto a flat gold surface covered with a hydrophilic monolayer. Figure 1 depicts the negative ML strategy for patterning the 200-nm-thick gold-coated silicon substrates by hydrophobic/ hydrophilic monolayers. First, inert magnetic Fe3O4 NPs (10 nm diameter, dissolved in toluene) are attracted to a silicon substrate (300 μm thick) that was coated with a 200 nm highquality gold layer by an e-beam evaporator. The mask induces a magnetic field of about 100 G on the gold surface. Next, a hydrophilic reagent with a hydroxyl headgroup, 11-mercapto1-undecanol (11MUD, HS-C11OH), is self-assembled onto the substrate at places not covered by the NPs. After the adsorption of 11MUD, the magnetic mask is removed, and the substrate is sonicated and washed in order to remove the magnetic
Published on Web 4/21/2009
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Figure 1. Stepwise negative magnetic lithography strategy for patterning gold-coated silicon by hydrophobic/hydrophilic monolayers. First, inert magnetic nanoparticles (NPs) are attracted to the gold substrate, where the mask induces a magnetic field (step 1). Next, hydrophilic reagent 11MUD is self-assembled onto the substrate at places not covered by the NPs (step 2). Then, the magnetic mask and the nanoparticles are removed (step 3), and subsequently the HDT is adsorbed on the substrate in the areas previously covered by the NPs (step 4). In the last stage (step 5), the surface was exposed to GFP that was adsorbed on the hydrophobic lines and was repelled from the hydrophilic background.
NPs. Subsequently, the hydrophobic reagent, hexadecanethiol (HDT, SH-C15CH3), is adsorbed on the substrate in those areas previously covered by the NPs. Both molecules, HDT and 11MUD, are dissolved (10 mM solution) in ethanol; the adsorption time is 3 h at room temperature. To verify the patterning by hydrophobic molecules, we used green fluorescent protein (GFP). First, we determined whether GFP interacts more strongly with hydrophobic surfaces. For this test, a series of gold substrates were coated with self-assembled monolayers composed of mixtures of hydrophobic/hydrophilic reagents. We used different molar fractions of HDT and 11MUD to change the hydrophobicity of the gold substrates gradually. Figure 2 compares the receding contact angles (CA) measured with water droplets and the fluorescence intensity at 510 nm, measured from GFP after the monolayer-covered substrates were immersed in a 10 nM GFP solution for 30 min. In the case of the CA, there is an abrupt jump in the contact angle at a molar fraction of 0.2 HDT. Following this change, the CA remains constant. This steplike change in the contact angle may indicate a phase separation in which each of the molecules forms a domain that includes only one type of molecule. The water droplets are averaged over a large area and therefore are appreciably affected by the hydrophobic domains; therefore, they have large contact angles in the case of monolayers made from the hydrophilic/ hydrophobic mixtures. When the fluorescence from the GFP is monitored, the amount of GFP on the surface is linearly correlated with the molar fraction of HDT. This finding is again consistent with the formation of separate domains for each molecule. The GFP is adsorbed better on the hydrophobic domains; therefore, as their concentration increases, more GFP is adsorbed and more fluorescence is observed. Hence, GFP can efficiently detect hydrophobic patterns on the nanometer scale. 5452
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Figure 2. Comparison between the receding contact angles (CA) measured with water droplets and the fluorescence intensity at 510 nm measured from GFP after the monolayer-covered substrates were immersed in GFP solution.
Using the negative ML approach, we patterned lines of hydrophobic molecules on a surface covered with hydrophilic molecules. Then the surface was exposed to GFP and washed (Figure 1). Figure 3 reveals the SEM images of the patterned surface after it was exposed to GFP. The GFP appears as a bright line, as already reported for SEM images of proteins.16 In Figure 3a, a low-resolution (more than 1-μm-wide) line was obtained when we used a high concentration (50 μg mL-1) of magnetic NPs that were exposed to the magnetic field for 2 min. A much narrower line, with a width of about 30 nm, was obtained when in the process we used a dilute (5 μg mL-1) solution of (16) See, for example, Thess, A.; Hutschenreiter, S.; Hofmann, M.; Tampe, R.; Baumeister, W.; Guckenberger, R. J. Biol. Chem. 2002, 277, 36321-36328. Bayramoglu, G.; Arıca, M. Y. React. Funct. Polym. 2009, 69, 189-196.
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Figure 3. SEM images of the patterned surface after it was exposed to GFP. (A) After adsorbing 50 μg mL-1 magnetic NPs for 2 min.
(B) After adsorbing 5 μg mL-1 magnetic NPs for 2 min. (C) SEM image of a uniform 30 nm line-width pattern of GFP. (D) Three-dimensional image of the line shown in image C.
Figure 4. Magnetic field distribution above the mask, as calculated using the COMSOL program. (A) Field distribution at distances of 1.5, 3.5, and 7.5 times the width of the mask. (B) Sites with the strongest intensity of magnetic field on top of a magnetic mask line width at distances corresponding to 1.5, 3.5, and 7.5 times the line width of the pattern on the mask.
magnetic NPs and exposed it again to the magnetic field for 2 min. The protein is adsorbed on the hydrophobic lines and is repelled from the hydrophilic background. It is important to realize that 30 nm molecular patterning was achieved despite the fact that the lines in the magnetic mask that Langmuir 2009, 25(10), 5451–5454
induced the magnetic field had a width of 50 μm. The higher resolution was obtained by either reducing the concentration of the NPs, as shown in Figure 3, or shortening the adsorption time so that the system does not reach equilibrium. Under these conditions, the NPs are first adsorbed in the high-field part, DOI: 10.1021/la900601w 5453
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namely, only at the center of the magnetic line. This is because the gradient of the magnetic field, within the line width defined by the mask, is stronger in the center than at the edges of the mask lines (Figure 4). Figure 4 shows the results of field simulation on the mask as a function of the distance from the mask. It clearly demonstrates an interesting property of ML: when the substrate is in close proximity to the mask one observes edge effect resulting from enhancement of the magnetic field at the edges of the lines in the mask, when the substrate is relatively far away from the mask, the magnetic field on the substrate weakens; however, it peaks in the center of the line on the mask and therefore can induce the adsorption of nanoparticles with patterns much narrower than the patterns on the mask. It is interesting that the line width obtained is quite uniform. Its uniformity depends on the size of the NPs because when NPs are smaller the line that can be
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obtained is much more uniform. However, smaller NPs have a smaller magnetic dipole and therefore require a higher permanent magnetic field. For the 10-nm-diameter particles and because three particles define the line width, the width of the 30 nm lines has fluctuatuons of (30%. Here we demonstrated the ability to achieve chemical patterning of surfaces with submicrometer resolution by applying the simple and inexpensive ML method. This method allows fast patterning of large surfaces without having to encounter contamination problems or the need to remove the substrate from the solution. In addition, the patterning of the substrate by backside ML is not affected by the topography or planarity of the surfaces. Acknowledgment. This work was partially supported by the Grand Center for Sensors and Security.
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