In the Laboratory
Exploration of Solid-Supported Reactions with Gold Nanoparticles Grigoriy Sereda* and Vikul Rajpara Department of Chemistry, University of South Dakota, Vermillion, South Dakota 57069 *
[email protected] Because of their practical importance in organic synthesis, polymer-supported reactions have been used increasingly in the laboratory curriculum (1-3). One of the latest experiments in this Journal employs color reactions to monitor the attachment of 4-hydroxybenzoic acid to an aminomethylated polystyrene resin (4). We present an experiment that demonstrates four important steps in the utilization of polymer supports for manipulation of nanoparticles (attachment of a linker to the solid support, chemical modification of the linker, binding of nanoparticles to the linker, and cleavage of the nanoconjugate back in solution). The experiment takes one four-hour laboratory period. Methodology used in this approach is similar to the handling of gold nanoparticles in research applications (5, 6). Because of the intense color of gold nanoparticles, students can easily observe their distribution between the polymer beads and solution without additional reagents or equipment. Experimental Section In the first experiment, students learn the basic techniques of polymer-supported synthesis (working with a fritted peptide synthesis reactor, handling of the polymer beads, and detection of functional groups on their surface) via the acylation of aminomethylated polystyrene with maleic anhydride, 3 (Scheme 1). The presence of the primary amino group in the starting polymer material is confirmed by fluorescamine, 1, and the carboxy groups in the modified polymer are detected by the staining with malachite green, 2. IR spectroscopy can also be used as an alternative or a supplemental method for characterization of the functionalized polymer. The experiment is performed in parallel with a similar experiment that utilizes 4-hydroxybenzoic acid instead of maleic anhydride, 3 (4). Both experiments normally take 2.5-3 h. A 2.5% solution of malachite green, instead of a 25% solution as suggested by Hailstone and co-workers (4), is used, which is more consistent with the known analytical protocol (7). We discuss the difference between fluorescamine, 1, employed for detection of amino groups and trinitrobenzenesulfonic acid used by Hailstone and co-workers (4). For the second experiment, hexanethiol-coated gold nanoparticles (1.5-2 nm in diameter) and 6-acetylthiohexanoic acid, 4, were prepared by the instructor prior to the experiment, according to known procedures (8, 9). The experiment starts from the attachment of the linker 4 to commercially available 2-chlorotrityl chloride resin (step 1, 30 min, Scheme 2), followed by deprotection of the thiol group (step 2, 45 min). The beads prepared on step 2 are reacted with 5,50 -dithiobis(2-nitrobenzoic acid), 5; the resulting orange-colored beads indicate the presence of thiol groups on the surface of the beads that impart affinity to gold nanoparticles (GNP). 978
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Figure 1. TEM image of carboxy-functionalized gold nanoparticles, coupled with 1,2-diaminoethane.
Next, the thiol-functionalized polymer beads are added to a purple solution of gold nanoparticles, and the mixture is stirred for 2 h. The minimum required time for this step is 1 h. After 4 h, the solution becomes completely colorless because of the complete consumption of nanoparticles by the polymer beads. It is also acceptable to stir the mixture overnight, so the time of the experiment can be easily tailored to different laboratory schedules. The dark color of the polymer beads clearly indicates the successful attachment of gold nanoparticles. In the last step (15 min), the gold nanoparticles are cleaved from the polymer beads by 2% trifluoroacetic acid in dichloromethane, which is apparent because of the dark purple color of the solution and weakening color of the beads. The purple solution color indicates that the gold particles remain in the nanometer size range. If a transmission electron microscope (TEM) is available, the students can see the gold nanoparicles. The TEM image in Figure 1 shows carboxy-functionalized 1.5-2 nm gold nanoparticles in the presence of 1,2-diaminoethane. Because of the electrostatic interaction between carboxylate and ammonium groups, some of nanoparticles are associated in dimers (circled on the picture), which is typical for monocarboxylated gold nanoparticles (6). The interparticle distance is about 2-3 nm, which is consistent with the expected value for the stretched conformations of the ligand and the linkage. Attachment of one or more carboxy groups to nanoparticles makes them soluble in a more polar solvent (10% methanol in dichloromethane), which does not dissolve the initial hexanethiol-coated nanoparticles. Therefore, the
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Vol. 87 No. 9 September 2010 pubs.acs.org/jchemeduc r 2010 American Chemical Society and Division of Chemical Education, Inc. 10.1021/ed100379u Published on Web 07/14/2010
In the Laboratory
Scheme 1. Attachment of the Maleate Group to the Aminomethylated Polymer (PS)
Scheme 2. Polymer-Supported Functionalization of Gold Nanoparticles (GNP)
percentage of carboxylated nanoparticles in the dissolved fraction is believed to be close to 100%. Hazards Methanol, ethanol, triethylamine, piperidine, and diisopropylethylamine are flammable and irritating to eyes and skin. Trifluoroacetic acid is corrosive, harmful by inhalation or on contact with eyes or skin. Tetrahydrofuran is volatile, highly flammable, and may contain peroxides. Peroxides are explosive. They are easily formed from tetrahydrofuran and air in ambient light and may accumulate up to dangerous concentration when tetrahydrofuran dries out. Dichloromethane has been found to cause cancer in animals. Dimethylformamide is harmful by inhalation or on contact with skin or eyes and causes harm to an unborn child. Low permeability protective gloves (nonlatex type), safety glasses, and a fume hood should be used. The normal precautions should be taken for working at reduced pressure with suction filtration.
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Summary Gold nanoparticles are among the most available building blocks for modern technology, imparting catalytic, optical, and electrical properties to nanodevices. Functionalization of nanoparticles with appropriate organic molecules is essential to utilize their unique properties. The solid-phase technique explored in this experiment is gaining increasing attention of material chemists as a tool of manipulation with nanoparticles. Second-year undergraduate students participating in the workshop unanimously reported that their exposure to the chemistry of polymers, including techniques of solid-supported synthesis, positively affected their appreciation of science and motivation toward gaining further research experience. Acknowledgment We acknowledge NSF NPURC Grant 0532242 and the Director, Office of Science, Office of Biological & Environmental
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Research, Biological Systems Science Division, of the U.S. Department of Energy under Contract No. DE-FG02-08ER64624 for financial support of this work. We also thank the student participants of the polymer workshop: K. Boone, A. Jones, K. McInerney, D. Parrott, S. Parsons, B. Studelska, and Y. Zou. Literature Cited € C-engel, O. € J. Chem. 1. Taralp, A.; Turkseven, C. H.; C-akmak, A. O.; Educ. 2002, 79, 87. 2. Vilaseca, L.; Bardaji, E. J. Chem. Educ. 1995, 72, A99. 3. Buglass, A.; Waterhouse, J. J. Chem. Educ. 1987, 64, 371. 4. Hailstone, E.; Huther, N.; Parsons, A. J. Chem. Educ. 2003, 80, 1444. 5. Worden, J.; Dai, Q.; Shaffer, A.; Huo, Q. Chem. Mater. 2004, 16, 3746.
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6. Sung, K.; Mosley, D.; Peelle, B.; Zhang, Sh.; Jacobson, J. J. Am. Chem. Soc. 2004, 126, 5064. 7. Attardi, M.; Porcu, G.; Taddei, M. Tetrahedron Lett. 2000, 41, 7391. 8. Hostetler, M.; Wingate, J.; Zhong, Ch.; Harris, J.; Vachet, R.; Clark, M.; J. Londono, D.; Green, S.; Stokes, J.; Wignall, G.; Glish, G.; Porter, M.; Evans, N.; Murray, R. Langmuir 1998, 14, 17. 9. Svedhem, S.; Hollander, C.; Shi, J.; Konradsson, P.; Liedberg, B.; Svensson, S. J. Org. Chem. 2001, 66, 4494.
Supporting Information Available Handout for instructors. This material is available via the Internet at http://pubs.acs.org.
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