pubs.acs.org/Langmuir © 2010 American Chemical Society
Orthogonal Functionalization of Silicon Substrates Using Self-Assembled Monolayers Nicole Herzer,† Claudia Haensch,† Stephanie Hoeppener,*,‡ and Ulrich S. Schubert†,‡ †
Laboratory of Macromolecular Chemistry and Nanoscience, Center for NanoMaterials (cNM), Eindhoven University of Technology, Den Dolech 2, 5600 MB Eindhoven, The Netherlands, and ‡Laboratory of Organic and Macromolecular Chemistry, Friedrich-Schiller-Universit€ at Jena, Humboldtstrasse 10, 07743 Jena, Germany Received December 18, 2009. Revised Manuscript Received February 15, 2010 A fabrication process for multifunctional surfaces is designed leading to five different functional moieties (amine, thiol, carboxylic acid, fluoro, and methyl) being present on a single structured surface. The multifunctional surface is created by combining UV-ozone patterning, electro-oxidative lithography, the local deposition of self-assembled monolayers (SAMs), and surface modification schemes. Besides the characterization with conventional surface-sensitive techniques, the nature of the locally functionalized regions is demonstrated by self-assembly of three different probe nanomaterials (Si nanoparticles, Au nanoparticles, and hydroxyl functionalized micelles). A versatile fabrication approach for complex surfaces with addressable functionalities can be created, and it was possible to integrate five different functionalized areas on one substrate.
Introduction Multifunctional surface structures allow numerous potential applications in bioassays, sensors, and others. An elegant approach to introduce functionality to surfaces utilizes the spontaneous selfassembly of molecular species onto solid substrates. They provide stable, easy to prepare, transparent monolayer coatings with reliable quality and allow the efficient tailoring of specific surface properties, e.g., the wettability, chemical addressability, or bioaffinity. Several examples for the use of self-assembled monolayers (SAMs) with respect to the fabrication of micro- and nanostructured functional surfaces are reported in literature.1-5 Thereby, a large number of structuring techniques have been employed to structure monolayers, i.e., soft lithography, photolithography, dip-pen and electro-oxidative lithography, nanoimprint lithography,6-11 etc., to implement the fabrication of patterned surfaces. Surprisingly, these techniques mainly address the generation of bifunctional patterns, whereby frequently only one component is chemically active while the second molecular building block is used as a passivation layer.12-15 Only a few examples with respect *Corresponding author: Fax 0049(0)3641948202; e-mail s.hoeppener@ uni-jena.de. (1) Herzer, N.; Hoeppener, S.; Schubert, U. S.; Fuchs, H.; Fischer, U. C. Adv. Mater. 2008, 20, 346. (2) De la Rica, R.; Baldi, A.; Mendoza, E.; Paulo, A. S.; Llobera, A.; FernandezSanchez, C. Small 2008, 4, 1076. (3) Lin, Y.-C.; Yu, B.-Y.; Lin, W.-C.; Chen, Y.-Y.; Shyue, J.-J. Chem. Mater. 2008, 20, 6606. (4) Khatri, O. P.; Murase, K.; Sugimura, H. Jpn. J. Appl. Phys. 2008, 47, 5048. (5) Zeira, A.; Chowdhury, D.; Maoz, R.; Sagiv, J. ACS Nano 2008, 2, 2554. (6) Grigorescu, A. U.; Hagen, C. W. Nanotechnology 2009, 20, 292001/31pp. (7) Menard, E.; Meitl, M. A.; Sun, Y.; Park, J.-U.; Shir, D.J.-L.; Nam, Y.-S.; Jeon, S.; Rogers, J. A. Chem. Rev. 2007, 107, 1117. (8) Rosa, L. G.; Liang, J. J. Phys.: Condens. Matter 2009, 21, 483001/18pp. (9) Smith, R. K.; Lewis, P. A.; Weiss, P. S. Prog. Surf. Sci. 2004, 75, 1. (10) Wouters, D.; Hoeppener, S.; Schubert, U. S. Angew. Chem., Int. Ed. 2009, 48, 1732. (11) Onclin, S.; Ravoo, B. J.; Reinhoudt, D. N. Angew. Chem., Int. Ed. 2005, 44, 6282. (12) Li, Q.; Zheng, J.; Liu, Z. Langmuir 2003, 19, 166. (13) Hoeppener, S.; Schubert, U. S. Small 2005, 1, 628. (14) Lin, M.-H.; Chen, C.-F.; Shiu, H.-W.; Chen, C.-H.; Gwo, S. J. Am. Chem. Soc. 2009, 131, 10984. (15) Pang, I.; Kim, S.; Lee, J. Surf. Coat. Technol. 2007, 201, 9426.
8358 DOI: 10.1021/la9047837
to prototypes of monolayer-based multifunctional surfaces are reported in literature utilizing photochemical reactions by irradiation through a mask,16,17 the selective deposition of molecules on a defined spot by pipetting,18-20 or microcontact printing.21,22 Thereby, each of the approaches has inherent limitations. The use of photochemical reactions to form multifunctional surfaces requires specific synthetic routes as well as dedicated fabrication strategies. Microcontact printing requires the preparation of special stamps which allows the delivery of different molecules to certain spots. Pipetting allows the local coupling of molecules onto the surface, which requires mild reaction conditions as the reaction is directly performed on the spots. In this article a combination of easy and fast structuring techniques (photolithography and electro-oxidative lithography) and pipetting as well as surface chemical modification reactions is introduced and has been implemented into a fabrication process to combine areas of five different SAMs on a single substrate. Thereby, a class of self-assembling molecules, namely trichlorosilanebased precursor molecules, was chosen which provide a high stability and good quality of the SAMs. Moreover, these monolayers allow modifying technological relevant surfaces, i.e., glass and silicon. Because of the stability of these molecular systems, a variety of surface reactions can be used to generate a large number of different functional groups. We developed an approach that can be potentially combined with different structuring techniques by implementing compatible surface reactions that allow the sequential functionalization of surface areas, which preserve the chemical integrity of the (16) Ryan, D.; Parviz, B. A.; Linder, V.; Semetey, V.; Sia, S. K.; Su, J.; Mrksich, M.; Whitesides, G. M. Langmuir 2004, 20, 9080. (17) del Campo, A.; Boos, D.; Spiess, H. W.; Jonas, U. Angew. Chem., Int. Ed. 2005, 44, 4707. (18) Zammatteo, N.; Jeanmart, L.; Hamels, S.; Courtois, S.; Louette, P.; Hevesi, L.; Remacle, J. Anal. Biochem. 2000, 280, 143. (19) Beyer, M.; Felgenhauer, T.; Bischoff, F. R.; Breitling, F.; Stadler, V. Biomaterials 2006, 27, 3505. (20) Kim, D.-H.; Shin, D.-S.; Lee, Y.-S. J. Pept. Sci. 2007, 13, 625. (21) Renault, J. P.; Bernard, A.; Juncker, D.; Michel, B.; Bosshard, H. R.; Delamarche, E. Angew. Chem., Int. Ed. 2002, 41, 2320. (22) Geissler, M.; McLellan, J. M.; Chen, J.; Xia, Y. Angew. Chem., Int. Ed. 2005, 44, 3596.
Published on Web 03/05/2010
Langmuir 2010, 26(11), 8358–8365
Herzer et al.
Article
Figure 1. AFM and water contact angle characterization of the patterning. Scheme 1. Schematic Overview of the Patterning by an UV-Ozone Photoreactor
previously functionalized surface areas. In the present study we utilize a chemically inert n-octadecyltrichlorosilane (OTS) monolayer to perform the required patterning steps. Within this monolayer four differently functionalized areas have been integrated to demonstrate the possibility to create tailor-made multifunctional structured surfaces. Surface derivatization reactions have been characterized on the structured surfaces. The chemical compatibility of the implemented functional groups on the patterned surface has been additionally tested by the site-selective assembly of suitable probe materials, e.g., nanoparticles. Thereby, the main focus is placed on the compatibility of the functionalization scheme rather than on the obtainable feature sizes which potentially could be scaled down.
Results and Discussion Photopatterning of OTS. An OTS monolayer was selfassembled on a silicon wafer which serves as a chemically inert and robust passivation layer. OTS was chosen due to its high stability and chemically inert character.23,24 In a first step the patterning of the OTS monolayer was performed by irradiation in an UV-ozone photoreactor, whereby a photomask was placed on top of the OTS monolayer (Scheme 1) to locally protect the SAM. (23) Sagiv, J. J. Am. Chem. Soc. 1980, 102, 92. (24) Maoz, R.; Sagiv, J. J. Colloid Interface Sci. 1984, 100, 465. (25) Herzer, N.; Eckardt, R.; Hoeppener, S.; Schubert, U. S. Adv. Funct. Mater. 2009, 19, 2777.
Langmuir 2010, 26(11), 8358–8365
Figure 2. FT-IR spectra of the OTS monolayer before UV-ozone patterning and after UV-ozone patterning of the covered monolayer and of the exposed areas.
The removal of the OTS from the silicon wafers by the UV-ozone photoreactor was previously reported.25 The photomask provides square holes with a diameter of 5 mm. This mask was chosen as it permits the use of characterization techniques such as FT-IR spectroscopy (Figures 2 and 3) and water contact angle measurements (Figures 1 and 4), which require a minimum spot size to perform the measurements. Additionally the patterning process was analyzed by means of atomic force microscopy (AFM, Figure 1). The AFM image of the OTS monolayer before and after treatment as well as the treated area showed a smooth surface. The water contact angle measurement for the OTS before treatment revealed a water contact angle above 100° while the covered OTS after UV-ozone treatment showed only a small decrease of DOI: 10.1021/la9047837
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Figure 3. Modification steps and characterization by FT-IR spectroscopy: (a) bromine monolayer, (b) fluoro monolayer, (c) thiocyanate monolayer, (d) azide monolayer, (e) thiol monolayer, and (f) amine monolayer.
Figure 4. Water contact angle measurements of the different functionalized areas during the fabrication process.
the water contact angle of 5°. The noncovered areas revealed a strong decrease of the water contact angle down to
