In the Laboratory edited by
Green Chemistry
Mary M. Kirchhoff Green Chemistry Institute Washington, DC 20036
Patterning Self-Assembled Monolayers on Gold
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Green Materials Chemistry in the Teaching Laboratory Lallie C. McKenzie, Lauren M. Huffman, Kathryn E. Parent, and James E. Hutchison* Department of Chemistry and Materials Science Institute, University of Oregon, Eugene, OR 97403-1253; *
[email protected] John E. Thompson Department of Chemistry, Lane Community College, Eugene, OR 97405
In developing new educational materials for a greener organic chemistry laboratory curriculum (1), we aimed to design new laboratory exercises that emphasized the applications of organic chemistry to complement recently introduced greener synthesis labs (1, 2). In particular, labs that focus on organic materials chemistry were a high priority. We describe a convenient experiment that demonstrates self-assembled monolayer (SAM) chemistry, organic thin-film patterning, and the use of molecular functionality to control macroscopic properties.1 We also show how organic chemistry can be applied to engineer surface properties and pattern modern microelectronic devices, such as “plastic” transistors (3, 4). Students explore the properties of alkanethiol monolayers assembled on easily prepared, readily available, and inexpensive gold films on vinyl. The hydrophobicity of different monolayers can be studied, and the hydrophobic or hydrophilic patterns can be generated on the surface by straightforward masking or printing methods. The laboratory activities highlight several important concepts in green chemistry (5), including dematerialization (6) and the use of monolayer films to prevent surface degradation or contamination.2 Organic materials such as polymers and thin films are of increasing technological value to society. The development of these new materials often requires an interdisciplinary approach that integrates product design, synthesis, and application into materials development. This type of approach has not typically been addressed in the undergraduate curriculum. The need for laboratory modules that introduce materials chemistry (7) and surface chemistry is growing as the dimensions of micro- and nanostructures decrease (8). Currently, only a few educational materials (7, 9–11) are available that emphasize the important concepts and techniques of organic materials, surface chemistry, and nanoscience. The laboratory exercise described provides an excellent opportu-
nity to demonstrate how material properties can be altered through the use of SAM chemistry and organic thin-film patterning. SAMs are single molecular layers that spontaneously assemble onto certain surfaces driven by specific interactions between the surface and the monolayer-forming molecules (12). In some cases, such as for the extensively studied alkanethiolates on gold surfaces (12), a highly ordered, twodimensional film results from the assembly process (Scheme I). When these single-molecule-thick layers are assembled on surfaces, the surface properties of the material are changed, while the bulk properties of the object remain the same. These changes in surface properties can be harnessed for a number of important applications, including corrosion protection, models of biological membranes, and as molecular resists for patterning microelectronic devices.3 In addition to illustrating materials chemistry concepts, this laboratory exercise is ideally suited to introduce several important green chemistry principles. The use of a surface treatment addresses an important goal of green chemistry— waste reduction. Organic thin films protect products from contamination, thus reducing the use of cleaning materials and protecting the product from environmental deterioration (e.g., corrosion or fouling). The monolayer experiment provides an excellent example of dematerialization, a green chemistry concept important in materials chemistry, because much less material is consumed (∼1000 times less) when a single molecular layer is used to coat the surface instead of a thicker polymer film. Finally, the self-assembly process used to prepare SAMs offers several green chemistry advantages. The process occurs at room temperature, is highly efficient, and produces less solvent waste than commonly used film-forming procedures that rely on spin coating. Green chemistry is directly reinforced in the laboratory because the solvents and reagents are greener, and only small quantities of relatively harmless waste are generated by the self-assembly process. The Laboratory Exercise
Scheme I. Highly ordered two-dimensional film of alkanethiolates on a gold surface.
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Given the demonstrated technological applicability of alkanethiol SAMs, this system seemed an ideal candidate as an organic laboratory experience; however, the gold substrates typically used are difficult to prepare4 and expensive to purchase. We have discovered that a new substrate, a gold film
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on vinyl available from signage companies, is functional, convenient, and inexpensive. Because the gold is supported on a vinyl film, it is also lightweight and flexible, common attributes of organic materials. In contrast to the traditional gold film on glass preparation, the convenience of the described procedure allows students to prepare their own gold substrates. In addition to typical lab supplies, a source of ozone5 for cleaning and patterning substrates is needed to perform the experiment. Using the gold on vinyl substrates, students prepare SAMs with functionalized alkanethiols on gold surfaces and investigate the influence of monolayers of alkanethiols with different terminal functionality upon the surface properties. SAMs are prepared by soaking the gold substrate in solutions of low (1 mM) alkanethiol concentration. The thiol head group (RSH) binds strongly to the gold surface, creating a dense monolayer with the hydrocarbon tail group pointing outwards from the surface (see Scheme I). By controlling the nature of the terminal group, one can engineer the properties of the surface. Students investigate how the nature of the terminating group (methyl versus carboxylic acid) affects the wettability of the surface (Figure 1). The wettability is measured by determination of the contact angles of water on the surface.6 The SAM formed from the methyl-terminated thiol is hydrophobic and has a high contact angle, θ > 110 (Figure 1A), compared to the hydrophilic acid-terminated SAM
that exhibits a low contact angle, θ < 10 (Figure 1B). Students will typically obtain contact angles of 100–110 for mercaptohexadecane on gold when a good source of ozone is used for cleaning the substrates prior to assembly.7 Students can also gain experience with monolayer patterning methods such as microcontact printing (8) that has been used in industry to produce flexible plastic electronic devices (3). This technique can be used to pattern gold electrodes (contacts) and interconnecting wires on the surfaces of devices such as electronic paper (13). Students can generate chemical patterns on the surface of their gold slides (using a thiol “inked” rubber stamp) with an alkanethiol possessing a hydrophobic tail group followed by soaking the substrate in a solution of an alkanethiol with a hydrophilic tail group. If students are careful in blotting excess thiol off of their stamp before applying it to the gold surface, they produce reproducible, well-defined patterns. Another convenient patterning method (Figure 2) starts with a monolayer coated slide. Some parts of the film are exposed to ozone and rinsed away, leaving bare regions on the film. Backfilling these voids with a second type of thiol generates the pattern on the gold (Figure 2B). Students typically generate well-defined patterns using ozone patterning when a good UV–ozone generator is used.5 The easiest, and most striking, method of visualizing the monolayer pattern involves observing how water interacts
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Figure 1. Schematics of water droplets on (A) hydrophobic and (B) hydrophilic surfaces. In these two cases, the interaction between the surface and water droplet is determined by the terminal functionality of the alkanethiol. Even though the bulk of the monolayer and the substrate are the same, water wets the terminal carboxylic acid in (B), while water does not wet the methyl tail groups in (A).
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In the Laboratory
with the surface—through steam condensation, water sheeting, water pooling, and so forth. The students can explore ways of visualizing the patterns and compare the properties of these surfaces to those of homogeneous films prepared at the beginning of the experiment. This laboratory procedure encourages students to use their own creativity during the learning process. Various options are available for patterning the surface and for visualization of the SAMs. Students can use their own ideas to manipulate the hydrophobic and hydrophilic regions of the surface. Additional challenges such as trying to make water droplets move uphill (14) or traverse across a surface can extend the complexity of the laboratory experience. In this way, students are made partners in the learning process, which promotes greater understanding of concepts, enhances problem-solving abilities, and increases student satisfaction (15). Student responses to this lab exercise have been positive. They cite the exposure to a materials application of organic chemistry and the use of chemistry to manipulate surface properties as highlights of the exercise. Although the experiment was designed as an experiment for the sophomore undergraduate organic chemistry curriculum, extensions into other areas are possible. General chemistry students could conduct the laboratory exercise with ease. Demonstrations of tunable surface properties are available through the use of such products as Rain-X and Rain-X AntiFog on glass slides. This experiment also could be used with
physical chemistry students with the incorporation of Young’s equation and its relation to contact angle (9). Hazards The procedures involve the use of volatile, flammable solvents (acetone and ethanol) and thiol-containing compounds (mercaptohexadecane and mercaptoundecanoic acid). Mercaptoundecanoic acid may be an irritant to eyes, respiratory system, and skin. Although the stench associated with volatile thiols typically requires the use of a fume hood, the two thiols employed here are nonvolatile enough to be used on the bench top, if desired. All solvents and rinse water should be collected in separate waste containers and disposed of properly. These organic substances pose little hazard if handled and disposed of properly. Summary Patterning with alkanethiol SAMs on gold surfaces is a developing area of research. All supplies are benign, inexpensive, and easily obtained. Benefits of teaching with this procedure include technological applicability, commonly available starting materials, and emphasis on practical and green lessons. This laboratory exercise allows students in all areas of chemistry a chance to explore surface chemistry while learning about current, practical research methods.
Figure 2. Patterning an alkanethiol monolayer via selective ozone exposure. The pattern on the surface (A) was generated by the ozone patterning procedure (B) and visualized with water vapor. The vapor condenses to form droplets on the hydrophobic region that forms the letters, while the droplets wet the hydrophilic area surrounding the letters. The patterning procedure (B) involves (1) covering part of a hydrophobic SAM coated slide with a plastic pattern (mask) and exposing the uncovered regions to ozone generated by UV irradiation in the presence of oxygen (such as in UV–ozone cleaner). The unprotected sulfur headgroups of the SAM react with ozone to form water-soluble sulfonates. Upon rinsing (2), the SAM is removed from the areas that were exposed and remains in the areas that were covered. The empty areas are then filled in (3) by soaking the slide in the acid-terminated thiol solution.
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Supplemental Material
Detailed introductory and background information, instructions for students, and notes for the instructors are available in this issue of JCE Online.
6. More details about the determination and interpretation of contact angles can be found in the Supplemental MaterialsW and in ref 9. 7. The contact angle is a good measure of the efficiency of ozone cleaning and self-assembly processes.
Acknowledgments
Literature Cited
This work was supported by the University of Oregon, the National Science Foundation (CHE-9702726 and DUE0088986), and the American Chemical Society. LCM acknowledges support from the NSF–REU program and Mentor Graphics, Inc. We thank Leif Brown, Gary Nolan, and the students enrolled in fall and winter term CH337G and CH338G (2000–2002 academic years) at the University of Oregon for their assistance in optimizing and testing these experiments. JEH is an Alfred P. Sloan Research Fellow and a Camille Dreyfus Teacher–Scholar. Notes 1. Although the initial laboratory activity was designed for use in the organic chemistry laboratory, it appears to be equally applicable across the curriculum. For examples, see the Supplemental Materials.W 2. Application of these films prevents waste by increasing material lifespan and reducing the use of solvents and cleaning agents. 3. See the Supplemental MaterialsW for more details about microcontact printing and patterning devices using this method. 4. Thin gold films are usually prepared by vapor deposition in a high vacuum evaporation chamber. This equipment is expensive and not widely available. In addition, preparation of the films is labor intensive, requiring extensive cleaning and careful handling of the materials. 5. A convenient method for producing ozone to clean gold substrates and pattern SAMs is to use a commercially available UV– ozone cleaner (we used a Boekel Model 135500 that we purchased for $1600. It accommodates about 25 samples at a time). A less expensive alternative for ozone cleaning and patterning is described in the Supplemental Materials.W
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