Langmuir 2005, 21, 1599-1602
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Patterned Polyelectrolyte Multilayer: Surface Modification for Enhancing Selective Adsorption Feng Shi, Zhiqiang Wang,* Nan Zhao, and Xi Zhang* Key Lab of Organic Optoelectronics and Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 100084, People’s Republic of China Received August 30, 2004. In Final Form: November 16, 2004 This paper describes the use of surface chemical modification to enhance the difference of the surface charge on a patterned polyelectrolyte multilayer, which can be used for selectively adsorbing functional materials. We fabricated a patterned multilayer by combining the layer-by-layer self-assembly technique and photolithography and taking advantage of the different solubility of polyelectrolyte multilayers of diazo resins (DAR)/poly(acrylic acid) before and after UV irradiation. This patterned surface can be used as a matrix for selective adsorption of small molecular dyes, such as Methylene Blue. However the difference in surface charge on the patterned surface was not enough when we used it to selectively adsorb polystyrene (PS) nanoparticles using electrostatic force as the driving force. Therefore, we modified the patterned surface by interfacial chemistry. After modification, the patterned polyelectrolyte multilayer can be used as a good matrix for selective adsorption of PS nanoparticles with both positive and negative charges.
Introduction Polyelectrolyte multilayers are fabricated by the layerby-layer (LbL) self-assembly technique, as first introduced by Decher.1 It has been proved to be a versatile method to assemble layer nanostructures with tailored composition and architecture2 and shown to have promising applications as chemical sensors or biosensors,3 enzyme immobilization,4 hollow capsules,5 separation membranes,6 microporous films,7 light-emitting diodes,8 and erasable films.9 This inexpensive and versatile technique to fabricate patterned multilayer films and microstructures has been proved to be a feasible and novel method. Hammond et al.10 obtained a patterned multilayer by combining LbL self-assembly and the microprinting * Authors to whom correspondence may be addressed. E-mail:
[email protected];
[email protected]. (1) (a) Decher, G.; Hong, J. Makromol. Chem. Macromol. Symp. 1991, 46, 321. (b) Multilayer Thin Films-Sequential Assembly of Nanocomposite Materials; Decher, G., Schlenoff, J. B., Eds.; Wiley-VCH: New York, 2002. (2) (a) Decher, G. Science 1997, 277, 1232. (b) Zhang, X.; Shen, J. C. Adv. Mater. 1999, 11, 1139. (3) (a) Wang, B. Q.; Rusling, J. F. Anal. Chem. 2003, 75, 4229. (b) Sun, J. Q.; Sun, Y. P.; Zou, S.; Zhang, X.; Sun, C. Q.; Wang, Y.; Shen, J. C. Macromol. Chem. Phys. 1999, 200, 840. (c) Sun, C. Q.; Sun, Y. P.; Zhang, X.; Xu, H. D.; Shen, J. C. Anal. Chim. Acta 1995, 312, 207. (4) (a) Protein Architectures: Interfacing Molecular Assemblies and Immobilization Biotechnology; Lvov, Y., Moehwald, H., Eds.; Marcel Dekker: New York, 2000. (b) Sun, J. Q.; Sun, Y. P.; Wang, Z. Q.; Sun, C. Q.; Wang, Y.; Zhang, X.; Shen, J. C. Macromol. Chem. Phys. 2001, 202, 111. (c) Kong, W.; Zhang, X.; Gao, M. L.; Zhou, H.; Li, W.; Shen, J. C. Macromol. Rapid Commun. 1994, 15, 405. (5) (a) Lvov, Y.; Antipov, A. A.; Mamedov, A.; Moehwald, H.; Sukhorukov, G. B.; Nano Lett. 2001, 3, 125. (b) Antipov, A. A.; Sukhorukov, G. B.; Donath, E.; Moehwald, H. J. Phys. Chem. B 2001, 105, 2281. (c) Pastoriza, I.; Schoeler, E.; Caruso, F. Adv. Funct. Mater. 2001, 11, 122. (6) (a) Balachandra, A. M.; Dai, J. H.; Bruening, M. L. Macromolecules 2002, 35, 3171. (b) Harris, J. J.; DeRose, P. M.; Bruening, M. L. J. Am. Chem. Soc. 1999, 121, 1978. (7) (a) Fery, A.; Scholer, B.; Casssagneau, T.; Caruso, F. Langmuir 2001, 17, 377. (b) Mendelsohn, J. D.; Barrett, C. J.; Chan, V. V.; Pal, A. J.; Mayes, A. M.; Rubner, M. F. Langmuir 2000, 16, 5017. (c) Zhang; H. Y.; Fu, Y.; Wang D.; Wang, L. Y.; Wang, Z. Q.; Zhang, X. Langmuir 2003, 19, 8497. (8) (a) Rogach, A. L.; Koktysh, D. S.; Harrison, M.; Kotov, N. A. Chem. Mater. 2000, 12, 1526. (b) Wang, Y.; Tang, Z. Y.; Correa-Duarte, M. A.; Liz-Marza´n, L. M.; Kotov, N. A. J. Am. Chem. Soc. 2003, 125, 2830. (9) (a) Xie, A. F.; Granick, S. J. Am. Chem. Soc. 2001, 123, 3175. (b) Zhang, X. Y.; Zhu, Y. X.; Granick S. J. Am. Chem. Soc. 2001, 123, 6736.
technique. Rubner et al.11,12 fabricated micropatterned thin films using both ink-jet printing and photolithographic techniques by taking advantage of the different solubility of the hydrogen-bonding multilayer when the pH value was adjusted. Our group fabricated robust multilayer patterns by photolithographic techniques and taking advantage of the different solubility of polyelectrolyte multilayers of diazo resins (DAR)/poly(acrylic acid) (PAA) before and after UV irradiation.13 Recently, the self-assembly of various particles into twodimensional (2D) or three-dimensional (3D) patterned surface has generated wide interest for its potential application in many fields, such as photonic crystal devices, functional templates, catalysts for chemical and biological processes, and data storage.14-17 For example, Hammond’s group first demonstrated the use of patterned multilayers as a selective template for the deposition of particles by controlling and enhancing the driving forces that direct selective adsorption on patterned surfaces.18,19 Herein, we demonstrate how to modify the patterned surface by interfacial chemistry, and after modification, the patterned polyelectrolyte multilayer can be used as a good matrix for selective adsorption of polystyrene (PS) nanoparticles. An important advantage of this method is that the underlying polyelectrolyte multilayer platform opens up the possibility of introducing functionality into the multialyers, which can potentially be used as an active part of a device. (10) (a) Clark, S. L.; Montague, M.; Hammond, P. T. Supramol. Sci. 1997, 4, 141. (b) Clark, S. L.; Montague, M.; Hammond, P. T. Macromolecules 1997, 30, 7237. (11) (a) Kumar, A.; Whitesides, G. M. Appl. Phys. Lett. 1993, 63, 2002. (b) Xia, Y.; Mrksich, M.; Kim, E.; Whitesides, G. M. J. Am. Chem. Soc. 1995, 117, 9576. (12) (a) Yang, S. Y.; Rubner, M. F. J. Am. Chem. Soc. 2002, 124, 2100. (b) Yang, S. Y.; Mendelsohn, J. D.; Rubner, M. F. Biomacromolecules 2003, 4, 987. (13) Shi, F.; Dong, B.; Qiu, D. L.; Sun, J. Q.; Wu, T.; Zhang, X. Adv. Mater. 2002, 14, 805. (14) Velev, O. D.; Jede, T. A.; Lobo, R. F.; Lenhoff, A. M. Nature 1997, 389, 447. (15) Holland, B. T.; Blanford, C. F.; Stein, A. Science 1998, 281, 538. (16) Braun, P. V.; Wiltzius, P. Nature 1999, 402, 603. (17) Johnson, S. A.; Ollivier, P. J.; Mallouk, T. E. Science 1999, 283, 963. (18) Clark, S. L.; Hammond, P. T. Langmuir 2000, 16, 10206. (19) Clark, S. L.; Montague, M. F.; Hammond, P. T. Macromolecules 1997, 30, 7237.
10.1021/la0478393 CCC: $30.25 © 2005 American Chemical Society Published on Web 12/31/2004
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Experimental Section In the experiments, the following chemicals were used as supplied: (3-mercaptopropyl)-trimethoxysilane (MPTS), octyltrimethoxysilane (OTS), (3-aminopropyl)dimethylmethoxysilane (APMS), and PAA (Mn ∼ 8.2 × 105) were supplied by ACROS Organics. The Mn of Diazo resin was about 2640. The modification of the substrates was described as follows: A freshly cleaned silicon wafer was immersed in 1 × 10-5 mol/mL MPTS toluene solution for 12 h to form a self-assembled monolayer terminated with hydrosulfide groups. The hydrosulfide groups were then oxidized by 30% H2O2/HAc (v/v ) 1:5) into sulfonic acid groups. The multilayer patterned structure was formed by exposing the newly fabricated DAR/PAA film to UV irradiation to get crosslinked/non-cross-linked areas with the aid of a mask, followed by dissolution of the non-cross-linked areas from the substrate in the sodium dodecyl sulfate (SDS) aqueous solution. The UV irradiation was accomplished with a 1000 W mercury lamp. The distance between the lamp and the sample was about 45 cm. Twenty seconds of irradiation and 10 min of ultrasonication in SDS aqueous solution were used to produce robust multilayer patterns of DAR/PAA. The chemical modification process was as follows: A patterned multilayer surface was immersed in NH4F/H2O2/H2O (1:1:10 in molar) mixture solution for 30 s to remove sulfonic acid groups in the channels by dissolving the underlying silicon dioxide layer. Then, the substrate was rinsed with water and dried with nitrogen gas after being drawn out of the mixture solution. After the substrate was removed from the mixture solution and exposed to the atmosphere, a new silicon dioxide layer is formed, which is terminated with hydroxyl groups by oxidizing the outmost silicon layer. After the hydroxyl groups formed, the substrate could be chemically modified into the desired functional groups by immersing the substrate in a corresponding organosilane toluene solution, such as methyl groups (1 × 10-5 mol/L OTS solution, 12 h), hydrosulfide groups (1 × 10-5 mol/L MPTS solution, 12 h), or amino groups (1 × 10-5 mol/L APMS solution, 12 h). The selective adsorption was carried out by directly immersing the substrate into the aqueous solution of PS nanoparticles for about 30 min and then rinsing with water and drying with nitrogen gas. Two kinds of PS nanoparticles with different diameters were used. One was negatively charged with sulfonic acid groups on the surface, with a diameter about 100 nm, and the other was positively charged with a quaternary ammonium compound as the outmost group, with a diameter about 300 nm.20 Atomic force microscope (AFM) observation of the patterned surface of DAR/PAA and selective adsorption on the patterned surface was carried out ex situ with commercial instruments (Digital Instruments, Dimension 3100) using silicon cantilevers at tapping mode.
Results and Discussion Figure 1 shows the procedure of the pattern formation, the surface modification, and selective adsorption. The multilayer of DAR/PAA was fabricated using the LbL selfassembly technique.21,22 The deposition process was as follows: The substrate of a silicon wafer covered with sulfonic acid groups was alternately immersed in aqueous solutions of DAR (1 mg/mL) and PAA (1 mg/mL) for 20 min and rinsed with water and dried with nitrogen between immersions. The above steps were repeated in a cyclical fashion to form multilayer films. After ultraviolet photolithography through a striped mask, the film was developed by an ultrasonic technique in sodium dodecyl (20) Chen, X.; Cui, Z. C.; Chen, Z. M.; Zhang, K.; Lu, G.; Zhang, G.; Yang, B. Polymer 2002, 43, 4147. (21) (a) Sun, J. Q.; Wu, T.; Sun, Y. P.; Wang, Z. Q.; Zhang, X.; Shen, J. C.; Cao, W. X. Chem. Commun. 1998, 1853. (b) Sun, J. Q.; Wang, Z. Q.; Wu, L. X.; Zhang, X.; Shen, J. C.; Gao, S.; Chi, L. F.; Fuchs, H. Macromol. Chem. Phys. 2001, 202, 967. (22) (a) Sun, J. Q.; Wang, Z. Q.; Sun, Y. P.; Zhang, X.; Shen, J. C. Chem. Commun. 1999, 693. (b) Sun, J. Q.; Wu, T.; Zou, B.; Zhang, X.; Shen, J. C. Langmuir 2001, 17, 4035. (c) Sun, J. Q.; Wu, T.; Liu, F.; Wang, Z. Q.; Zhang, X.; Shen, J. C. Langmuir 2000, 16, 4620.
Figure 1. Procedural scheme of the experiment: the formation of patterned polyelectrolyte multilayers, the surface modification, and the selective adsorption of positively charged PS nanoparticles.
Figure 2. Schematic representation of reaction between DAR and PAA in multilayer fabricated DAR/PAA with UV irradiation.
sulfate (SDS) aqueous solution (0.25 mol/mL) for 10 min to obtain the patterned multilayer.13 Then the silicon surface modified with sulfonic acid groups in the channels was exposed, and the stripes of cross-linked polyelectrolyte multilayer were retained. The reaction between DAR and PAA in the multilayer is shown in Figure 2. The sulfonic acid groups on the surface could be etched completely by immersing the substrate into NH4F/H2O2/H2O (1:1:10 in molar) mixture solution for 30 s and a bare silicon wafer was exposed as the outmost layer in the channels. The polyelectrolyte multilayer stripes acted as the resist layers in this procedure. After the sample was removed from the etching solution, the surface of the bare silicon wafer could be immediately oxidized into silicon dioxide and a hydroxyl group covered surface was achieved. To obtain channels terminated with various functional groups, the substrate was immersed into a toluene solution of organosilane for about 12 h. With this method, we can change the functional groups in the channels from sulfonic acid groups to various functional groups, such as methyl, hydrosulfide, amino, and even fluorinated methyl. We hoped that the patterned method by combining the LbL technique and surface chemistry would help us to harness intrinsic interactions between surfaces and enhance the driving force for selective adsorption of functional materials.
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Table 1. X-ray Photoelectron Spectrometer (XPS) Data of Nitrogen and Sulfur Elements on the Surface and the Surface Contact Angle XPS peak (eV) substrate
N
S
silicon wafer with -OH 0 0 silicon wafer with -SH 0 164.1 silicon wafer with -SO3H 0 167.4 silicon wafer with DAR/PAA 399.2 a multilayer after irradiation silicon wafer after removing DAR/PAA 0 167.8 multilayer in SDS solution after ultrasonic, silicon wafer 0 0 etched in NH4F/H2O2/H2O solution after etching, modified by -SH 0 163.9 after etching, modified by -CH3 0 0 after etching, modified by -NH2 399.2 0 a
contact angle (deg) 29.3 56.4 29.4 62.3 28.1 29.9 58.0 80.5 70.1
Complex.
We studied the surface modification on silicon wafers with X-ray photoelectron spectrometry (XPS) and a contact angle instrument, since it is hard to locally detect the surface change on patterned multilayer films. The results are shown in Table 1. The bare silicon wafer was covered with the hydroxyl groups after being treated in H2O2 solution for 24 h. The contact angle measurement shows that the substrate is hydrophilic (29.3°) and there is no signal of sulfur and nitrogen elements. After the hydroxyl groups on the substrate were modified into the hydrosulfide groups, the surface contact angle changes from 29.3° to 56.4°, because the hydrosulfide groups are more hydrophobic than the hydroxyl groups. XPS data show that the signal of S2s at 164.1 eV in hydrosulfide groups appeared, which indicates that the substrate was successfully modified with the hydrosulfide groups. After the hydrosulfide groups on the substrate are oxidized into the sulfonic acid groups, the contact angle decreases from 56.4° to 29.4°. In this case, the XPS characterization clearly indicates a signal of S2p in sulfonic acid group at 167.4 eV. When the multilayer of DAR/PAA was deposited on the silicon wafer, which was modified with the sulfonic acid groups, and cross-linked under UV irradiation, the signal of N2s at 399.2 eV from DAR chain appeared. The signal of sulfur became complicated because of the various oxidation states of sulfur in the multilayer film. Moreover, the contact angle shows that the surface property becomes more hydrophobic (62.3°) after the formation of crosslinked multilayer, even though the PAA layer is the outmost layer. The reason for the increase of the contact angle could be explained as follows: After ultraviolet
irradiation, the side groups of DAR change from the hydrophilic diazonium groups to the hydrophobic ester groups. Although the top layer of the multilayer is PAA, the underlying cross-linked hydrophobic ester layers still affect the surface property due to the interpenetrated structure. After the substrate covered with un-cross-linked multilayer films was dissolved in the SDS solution, the N2s signal at 399.2 eV disappears and the S2p signal of the sulfonic acid group remains and the contact angle is 28.1°, which is similar to that of the substrate terminated with the sulfonic acid groups before multilayer formation. All these facts indicate that the polyelectrolyte multilayer films before UV irradiation can be removed completely by ultrasonic washing in SDS aqueous solution, which is also supported by UV-vis spectroscopy.13 After removal of the polyelectrolyte multilayer, the substrate terminated with the sulfonic acid groups was etched by a NH4F/H2O2/H2O mixture solution in order to remove the silicon dioxide layer and the sulfonic acid groups, which can be monitored by disappearance of the XPS signal of sulfur in the sulfonic acid group. In this case the contact angle is 29.9°, which is similar to that of an H2O2-treated silicon wafer. After NH4F etching, the surface groups have changed from the sulfonic acid groups to hydroxyl groups. This kind of active surface can be further chemically modified with desired functional groups by immersing the substrate into appropriate solutions of organosilane. After modification of hydrosulfide groups, the signal of S2s in hydrosulfide groups at 163.9 eV appears and the contact angle is 58.0°, which is similar to the substrate covered with the hydrosulfide groups before etching. After the substrate is modified with the amino groups, the signal of N2s at 399.2 eV can be found in XPS data, and the surface becomes more hydrophobic (70.1°) because the amino groups cannot fully be ionized in neutral water. The substrate terminated with the methyl groups cannot be detected by XPS, but it can be characterized by the contact angle. All of the above results indicate that we can obtain desired groups in the channels by surface chemistry. Atomic force microscopy (AFM) was employed to observe the topographic change during the etching process. Figure 3a is the AFM height image of the patterned multilayer before NH4F etching. The multilayer film is made of 10 bilayers of DAR/PAA, and the average thickness is about 25 nm from section analysis. Figure 3b is an image of etched patterned multilayers. After etching, the vertical distance between the stripes and the channels increase by about 5 nm, which corresponds to the thickness of the etched silicon dioxide layer. It also suggests that the
Figure 3. The AFM height images of robust multilayer stripes: (a) DAR/PAA multilayer before NH4F etching (data scale 50 µm × 50 µm); (b) DAR/PAA multilayer after NH4F etching (data scale 50 µm × 50 µm); (c) free-standing films of DAR/PAA multilayer under the condition of over etching (data scale 80 µm × 80 µm).
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Figure 4. The ex situ AFM height images of the patterned DAR/PAA multilayer surface, data scale 20 µm × 20 µm: (a) positively charged PS nanoparticles adsorbed on a -COOH/ -SO3H surface; (b) positively charged PS nanoparticles adsorbed on a -COOH/-OH surface; (c) positively charged PS nanoparticles adsorbed on a -COOH/-NH2 surface; (d) negatively charged PS nanoparticles adsorbed on a -COOH/-NH2 surface.
sulfonic acid groups on the silicon dioxide layer have been removed completely. In the etching process, we also observed an interesting phenomenon. In some areas the stripe of the polyelectrolyte multilayer film can be peeled from the substrate and exist as a free-standing film (Figure 3c), which is caused by overetching the silicon dioxide under the stripe. The stripe of the film appears just like yellow scarves blowing in the wind, as indicated by the AFM image. It can be twisted freely, so it appeared to be quite soft. Under the optical microscope, the stripe DAR/PAA multilayer is a translucent and sandy beige film. It is the first time that we are able to show our readers what a multilayer ultrathin film of DAR/PAA looks like. To detect the change of interfacial properties before and after the surface chemical modification process, polystyrene (PS) nanoparticles with positive charges are employed to observe the selectivity of adsorbates on the patterned surface. Figure 4a shows the AFM image of the patterned surface before the chemical modification. There are the carboxyl groups on the stripes, since the top layer is PAA, and sulfonic acid groups in the channels, as mentioned before. The adsorption of the positively charged PS nanoparticles shows no difference between the stripes terminated with the carboxyl groups and the channels terminated with the sulfonic acid groups. The positively charged PS nanoparticles adsorb densely everywhere with high coverage due to strong electrostatic attraction. In the neutral aqueous solution of PS nanoparticles, the carboxyl groups and sulfonic acid groups on the surfaces can ionize and lead to a high-density negative charge; therefore, this patterned surface has no selectivity for the adsorption of positively charged PS nanoparticles. After the substrate is immersed in the NH4F solution, the
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terminated groups in the channels are changed from the sulfonic acid groups to the hydroxyl groups. From Figure 4b, we can observe that the coverage of PS nanoparticles in the channels terminated with the hydroxyl groups is much lower than that on the stripes terminated with the carboxyl groups, which suggests that PS nanoparticles can preferentially adsorb onto the surface terminated with the carboxyl groups. This is because it still has many negative charges on the surface terminated with hydroxyl groups; however the charge density of hydroxyl groups is lower than that of the carboxyl groups in the neutral solution. This phenomenon also exists when we compare the surface terminated with carboxyl groups to that with methyl groups or hydrosulfide groups for the similar reason. In the latter case, the hydrophobic interaction between the PS nanoparticles and the terminate groups plays a role in surface adsorption. To further enhance the interfacial difference between the channels and the stripes, we changed the groups in the channels from hydroxyl groups to amino groups. We observed that there are almost no PS nanoparticles adsorbed on the amino group terminated areas, but there is high coverage on the stripes, as shown in Figure 4c. This result indicates that the surface terminated with amino groups is positively charged because of the protonation, and the strong electrostatic repulsion prevents the positively charged PS nanoparticles from adsorbing in the channels. We may draw a preliminary conclusion that tailoring surface chemistry can be used to enhance the selectivity of the patterned surface. When we observed Figure 4c very carefully, we find that there are still some PS nanoparticles adsorbed on the boundary of the amino groups terminated area. We speculate that the capillary forces in the channels may be responsible for this phenomenon. To confirm the above conclusion, we used negatively charged PS nanoparticles as an indicator to verify the different selectivity of the same patterned surface as in the case of Figure 4c. We can see clearly from Figure 4d that all the negatively charged PS nanoparticles are selectively adsorbed into the channels. This suggests that we can select the adsorption area of the PS nanoparticles on the patterned surfaces by changing the surface charges. Conclusions We have demonstrated that the combination of the LbL method and interfacial chemistry can be successfully used to fabricate multilayer patterns with tailored surface properties. Advantages of this method for enhancing selective adsorption will be manifold. For example, it can be used to obtain desired functional groups in the channels of the patterned surface fabricated by the LbL technique, which allows selective adsorption of various functional materials onto the patterned surface by electrostatic interactions, capillary forces, and hydrogen bonding. The patterned multilayer formed in this way could act as a matrix for confined self-assembly in the channels. Acknowledgment. We thank Professor Weixiao Cao (Peking University) for providing DAR. We thank Professor Bai Yang and Mr. Xin Chen (Jilin University) for providing PS nanoparticles. The research was funded by the Major State Basic Research Development Program (G2000078102), Key Project of the Ministry of Education, and National Natural Science Foundation of China (20334010, 20204003). LA0478393
