In the Laboratory
Preparation of Dppe-Stabilized Gold Nanoparticles
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Keenan E. Dungey,* David P. Muller, and Tammy Gunter Chemistry Program, University of Illinois at Springfield, Springfield, IL 62703-5407; *
[email protected] Nanotechnology is the manipulation of matter at the nanometer (10᎑9 m) scale and promises to make great changes in our culture (1). The importance of nanotechnology has been recognized by the federal government, which has funded the National Nanotechnology Initiative since 2001. Nanotechnology includes science and engineering concepts and tools, and chemists—with their long experience at manipulating matter—are ideally suited to play key roles in its development. While there are many graduate programs and professional journals devoted to nanotechnology research, there are few examples of nanotechnology in undergraduate education. This gap between undergraduate and graduate education is decreasing rapidly, notably with the modules for introductory chemistry produced by the University of Wisconsin– Madison Materials Research Science and Engineering Center (2), including a laboratory on the stoichiometry of a ferrofluid (3). Other examples of nanotechnology-related laboratory experiences include the preparation and characterization of porous silicon by materials science and engineering students (4). A capstone course in nanotechnology for two-year college students is available at the Pennsylvania State University Nanofabrication Facility (5). At the junior level, chemistry students can study the kinetics of the self-assembly of gold colloid monolayers (6). For senior chemistry majors, nanocrystalline Y2O3:Eu3+ phosphor can be prepared by solid-state synthesis (7) and CdSe nanocrystals by organometallic synthesis (8). The laboratory described here is an additional aid in bringing nanotechnology to the undergraduate, specifically at the upper-division level. The advantages of this laboratory are that the reactions are performed at room temperature in the presence of air, it can be readily inserted into the inorganic synthesis curriculum in place of a coordination compound synthesis, and it does not require expensive equipment. Learning Objectives The purpose of this experiment is to introduce students to nanotechnology through the preparation of nanoparticles and (optionally) their visualization using transmission electron microscopy (TEM). Students will also become familiar with nonaqueous solvents, biphasic reactions, phase-transfer agents, ligands to stabilize growing nanoparticles, and bidentate ligands. Students will practice the following laboratory skills: low-temperature condensation, extraction, recrystallization, various spectroscopies (NMR, IR, Vis), using the rotary evaporator, consulting the professional literature, and using the TEM. Overview of the Experiment The first two weeks of the four-week project are used to prepare the ligand 1,2-bis(diphenylphosphino)ethane (dppe) (9). Optionally, the project can be shortened to only two weeks by purchasing the dppe (the ligand is available from www.JCE.DivCHED.org
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several chemical suppliers at a cost of approximately $150 for 50 g). Nucleophilic substitution of dichloroethane by sodium diphenylphosphide in liquid ammonia results in dppe, which is purified by mixed solvent (dichloromethane兾ethanol) recrystallization. By the third week, the students will have read the procedure in the professional literature for preparing triphenylphosphine-stabilized gold nanoparticles (10) and adapted it for using their dppe. The biphasic (water兾toluene) gold solution is prepared with the phase-transfer agent, tetraoctylammonium bromide. The prepared dppe is added to the mixture, and then an aqueous solution of NaBH4 is added to rapidly reduce the HAuCl4 to gold. The nanoparticles are recovered by extracting the organic layer, drying with sodium sulfate, and evaporating the toluene. This solid is further purified by washing at least five times with heptane兾methanol and saturated aqueous sodium nitrite. Finally, the nanoparticles are precipitated from chloroform upon addition of pentane. Materials and Equipment The chemicals needed for this procedure are readily available from commercial sources. Liquid ammonia is obtained from an ammonia gas cylinder equipped with an “educator tube.” Other than the glass-covered magnetic stir bars, dry ice condensers, and oil bubblers, only standard glassware and instrumentation is needed. Access to a transmission electron microscope is optional for use with advanced students. Hazards The solvents used in this experiment are flammable. Dichloromethane is considered a human carcinogen and the vapors should be avoided. Sodium metal reacts violently with water. Sodium metal and sodium borohydride are strong reducing agents. Ammonia is a toxic and irritating gas and should be handled by the instructor in a hood using rubber gloves. Dry ice and dry ice兾ethanol baths are extremely cold. HAuCl4 is a solid acid and should be handled with gloves and plastic spatula. Results Students were able to prepare dppe in decent yields (40– 50%) and to match the literature spectra (Figure 1B) and melting point. Upon binding to the nanoparticle, the ligand symmetry decreased as indicated by doublet bands in the IR spectrum collapsing into single bands (e.g., bands at 727 and 692 cm᎑1 in Figure 1B collapse to 692 cm᎑1 in Figure 1A and bands at 505 and 474 cm᎑1 in Figure 1B collapse to 692 cm᎑1 in Figure 1A ). Upon reduction, the cloudy-white gold-dppe mixture rapidly changed to a dark-black mixture. The students characterized the nanoparticles by visible absorbance spectroscopy (Figure IN1 in the Supplemental Material W ). With a
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In the Laboratory
ments from students included, “The finished product did not look like what I expected. I thought I would see gold, but I saw black gold.” and [the four-week experiment was] “tedious but rewarding.” Conclusion This advanced laboratory in inorganic synthesis introduces students to the nanoscale: the region between molecules and microscale. It can easily be implemented in place of a coordination compound synthesis. The laboratory is similar to a research experience, involving multiple steps and taking multiple weeks during which students consult professional literature. Acknowledgments Figure 1. Infrared spectra of (A) dppe-stabilized gold nanoparticles and (B) dppe (KBr disk method).
We thank two years of CHE 423 students for patience with a new teacher and a new experiment, Gary Trammell at UIS for assisting with the NMR spectra, Nada Chang of the Biology Program at UIS for training in the use of the TEM, Jan Dungey for assistance in preparing the manuscript, and the anonymous reviewers for helpful suggestions in revising the manuscript. We gratefully acknowledge funding from the UIS Science Division, the Camille and Henry Dreyfus Foundation, and the Council on Undergraduate Research Undergraduate Summer Research Fellowship sponsored by NSF (for DPM). W
Supplemental Material
The student handout and instructor notes, including comments on the experiment, additional results, grading criteria, equipment and reagent lists, and hazard alerts, are available in this issue of JCE Online. Literature Cited Figure 2. Transmission electron micrograph of gold nanoparticles.
technician’s support, groups of students used the TEM to visualize their nanoparticles (Figure 2). The gold particles had a diameter of approximately 7 nm and tended to aggregate into clusters of three or more (note the triangle and pentagon shapes due to clustering). Though this is a challenging project, student response was favorable overall. The students struggled to follow the published gold nanoparticle procedure since the professional literature does not give the step-by-step instructions that they were used to having in laboratory courses. Typical errors were miscalculating the quantities of materials needed and choosing the wrong glassware, which were easily remedied by guidance from the instructor. The nanoparticle formation reaction has a low yield and, at the scale we worked at, some students lost their product during purification. On a positive note, students recognized the importance of nanotechnology in the introduction to their reports and correctly analyzed the visible absorbance spectrum in their discussion section. Com-
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1. Atkinson, W. I. Nanocosm; Amacom: New York, 2003. 2. The University of Wisconsin–Madison Materials Research Science and Engineering Center Interdisciplinary Educations Group. http://mrsec.wisc.edu/edetc/index.html (accessed Feb 2005). 3. Berger, P.; Adelman, N. B.; Beckman, K. J.; Campbell, D. J.; Ellis, A. B.; Lisensky, G. C. J. Chem. Educ. 1999, 76, 943– 948. 4. Parkhutik, V. P.; Canham, L. T. Phys. Status Solidi A-Appl. Res. 2000, 182, 591–598. 5. Fonash, S. J. J. Nanopart. Res. 2001, 3, 79–82. 6. Keating, C. D.; Musick, M. D.; Keefe, M. H.; Natan, M. J. J. Chem. Educ. 1999, 76, 949–955. 7. Bolstad, D. B.; Diaz, A. L. J. Chem. Educ. 2002, 79, 1101– 1104. 8. Kippeny, T.; Swafford, L. A.; Rosenthal, S. J. J. Chem. Educ. 2002, 79, 1094–1100. 9. Girolami, G. S.; Rauchfuss, T. B.; Angelici, R. J. Synthesis and Technique in Inorganic Chemistry, 3rd ed.; University Science Books: Sausalito, CA, 1999. 10. Weare, W. W.; Reed, S. M.; Warner, M. G.; Hutchison, J. E. J. Am. Chem. Soc. 2000, 122, 12890–12891.
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