Take-Home Nanochemistry: Fabrication of a Gold- or Silver

May 15, 2012 - ... window cling containing gold or silver nanoparticles. This lab covers small portions of three successive laboratory periods and is ...
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Laboratory Experiment pubs.acs.org/jchemeduc

Take-Home Nanochemistry: Fabrication of a Gold- or SilverContaining Window Cling Dean J. Campbell,* Richard B. Villarreal, and Tamara J. Fitzjarrald Department of Chemistry, Bradley University, Peoria, Illinois 61625-0208, United States S Supporting Information *

ABSTRACT: The purpose of this laboratory experiment is to introduce aspects of materials chemistry, such as polymers and nanoparticle synthesis and properties, to students by their fabrication of a take-home polydimethylsiloxane window cling containing gold or silver nanoparticles. This lab covers small portions of three successive laboratory periods and is designed to touch on a variety of topics relevant for general chemistry students, although the lab could also be adapted for students with more or less advanced backgrounds.

KEYWORDS: First-Year Undergraduate/General, General Public, Inorganic Chemistry, Laboratory Instruction, Hands-On Learning/Manipulatives, Colloids, Materials Science, Nanotechnology, Oxidation/Reduction, Polymerization

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activities.14−16 When the liquid kit components are mixed together (Scheme 1) silicon−hydrogen bonds attached to PDMS oligomers (species 2) oxidatively add to a platinum catalyst (possibly Karstedt’s catalyst formed by the reaction of chloroplatinic acid, H2PtCl6, and divinyltetramethyldisiloxane). Vinyl groups attached to PDMS oligomers (species 1) insert into the platinum−hydride bond and then the alkyl and silyl groups reductively eliminate to produce −Si−CH2−CH2−Si− cross-links within the PDMS.17−19 The ability of this form of PDMS to cross-link from liquid precursors to a rubbery solid on the time scale of hours allows it to be cast into a wide variety of shapes, dependent on the molds available. After the cross-linking is essentially complete, however, some of the leftover silicon−hydrogen bonds can still reduce some metal-containing ions, such as tetrachloroaurate(III), tetrachloropalladate(II), tetrachloroplatinate(II), and silver(I), that are carried into the elastomer by organic solvents.20−22 The metallic particles produced by this reduction reaction are embedded within the polymer matrix, which restricts particle aggregation. The PDMS is transparent to visible light wavelengths, so the colors of the nanoparticles can be examined both by the eye and by visible light spectroscopy.20−22 The experimental procedure is designed to cover portions of three periods of a general chemistry lab. The two overnight time intervals between the lab sessions are necessary to allow time for the PDMS to cross-link and to allow organic solvents to leave the PDMS. The actual time spent on the procedure is

esearch in materials at the nanoscale (1−100 nm) size regime continues to abound. At the same time, the field of chemistry education has also incorporated nanoscale structures and concepts (see, for example, the May 2011 issue of this Journal).1−10 The optical properties of gold and silver are popular illustrations of how the properties of nanoscale substances can be different from molecular or bulk substances. For example, nanoscale gold metal particles often have reddish colors due to their absorption of light by their plasmons (collective free-electron oscillations within the particles).11 The reddish colors associated with small gold particles have been known for centuries, for example, in aqueous suspensions used for medicinal purposes and in glass objects where the particles were used for pigmentation.1,11−13 Silver metal nanoparticles often have yellowish colors and have also been used to add color to glass.1,13 Many educational activities involving metal nanoparticles (and their optical properties) produce them in a liquid suspension by reduction of the appropriate metal species, and then these resulting particles are sometimes incorporated into other structures.1−10 In this laboratory experiment, students can grow and characterize gold or silver nanoparticles embedded within solid silicone matrices and then can take the tinted polymer samples home as decorative window clings, as these objects will adhere to glass by intermolecular forces without the need for glue or tape. The matrix material, polydimethylsiloxane (PDMS) can be produced as a colorless, transparent elastomer that is used for a variety of applications, such as encapsulation of electronic components to protect them from moisture and dirt. Sylgard 184 Elastomer is a kit produced by the Dow Corning Corporation that has been used for other chemistry education © XXXX American Chemical Society and Division of Chemical Education, Inc.

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Scheme 1. Hydrosilylation Cross-Linking Reaction of PDMS

Figure 1. General scheme for production of the PDMS window cling: (A) mapping out the personalized markings on the underside of the Petri dish; (B) placing the black dry-erase markings in the dish that will transfer to the PDMS; (C) pouring the PDMS precursor mixture; (D) removing the cross-linked PDMS window cling; (E) visible light absorbance spectra for gold and silver nanoparticles in PDMS; (F) examples of the final window clings.

the marker ink or the powder, the markings are transferred into the PDMS as lines of pigment. For written dish markings to be reversed initially so they are oriented correctly in the PDMS, a preliminary set of markings can be made on the underside, outside flat surface of a transparent Petri dish, Figure 1A. Then the dish can be flipped over and the same markings (in reverse) can be traced on the inside flat surface of the dish, Figure 1B. The PDMS components are then mixed in a weighing boat and poured into the mold, Figure 1C. For example, 3.3 g of PDMS will fill a 48 mm diameter Petri dish to a depth of a few millimeters. Placing the mold into a drying oven set to ∼60 °C at least overnight ensures that the polymer is adequately crosslinked. During the second lab period, the mold is removed from the oven and cooled to room temperature. Then, the cross-linked PDMS (with the incorporated ink marks) is carefully removed

not long, so this lab can be integrated with other laboratory experiments.

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EXPERIMENT

The steps involved in the production of the gold nanoparticlecontaining window cling are shown in Figure 1. The first lab period involves the fabrication of a disk-shaped window cling by cross-linking the PDMS within a mold such as a plastic Petri dish. Before casting, the window cling can be customized by adding markings to the bottom interior of the dish that can transfer to the cross-linked PDMS. One way to do this is to use a black dry-erase marker to write a message or picture onto the bottom interior of the dish. Another approach is to scratch the desired markings into the inside of the dish and then rub a powdered inert pigment (e.g., TiO2) into the scratches. When the mixed PDMS precursors are cross-linked in contact with B

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The simple procedures described in this laboratory experiment can be connected to a variety of chemical topics. The student laboratory procedure in the Supporting Information contains questions related to these topics (and the instructor notes contain the answers): • redox reactions (e.g., which species is the oxidizing agent) • polymer chemistry (e.g., viscosity, cross-linking, solventswelling) • polarity (e.g., only solvents of intermediate polarity can both dissolve the salts and penetrate the PDMS) • kinetics (e.g., cross-linking rate is temperature dependent) • unit conversions (e.g., how many gold atoms in a 40 nm diameter sphere) • color relationships (e.g., gold nanoparticles appear red because they absorb green light) • advantages (and disadvantages) of trapping nanoparticles within a solid matrix (e.g., environmental and health issues) • adhesion (e.g., in this case, short-range London forces and dipole−dipole attractions are involved) As written, the laboratory procedure is designed to recap multiple course concepts for first-year undergraduate students. This lab experiment was tested with about nine students in a summer session of general chemistry II lab over three successive lab periods late in the course when many of the course concepts have been introduced. The experiment was performed as an extra credit activity in addition to the regularly scheduled lab activities. All the students were able to construct the window cling (one tore his removing it from the mold; once torn, they are nearly impossible to repair). The students appeared to have mastery of most of the questions as well, although some common incorrect answers led to rewording of the questions for future laboratories. With appropriate supervision, high-school and middle-school students could also perform these relatively simple chemical manipulations and therefore experience polymer and nanoscale chemistry. For example, nonmetallized window clings of cross-linked PDMS incorporating glitter have been successfully produced with a group of seventh and eighth grade girls. Additionally, this lab activity could be performed as a classroom demonstration.

from that mold, Figure 1D, and soaked in a dilute solution of a gold- or silver-containing salt. More nonpolar solvents such as tetrahydrofuran or ethyl acetate penetrate PDMS more easily than more polar solvents such as isopropyl alcohol or water, but if the solvent is too nonpolar, the salts will not dissolve into the soaking solution. The intensity of color in the PDMS samples also varies with soaking time and concentration of soaking solution. Soaking the PDMS for about 1 h in 5 mM sodium tetrachloroaurate(III) or silver(I) tetrafluoroborate dissolved in ethyl acetate gives good results. The soaking solutions, which can be used for multiple sample soaks, should be used and stored in closed, dark containers to minimize evaporative losses and photoreduction of the metal species in solution. After the allotted time, the colored, solvent-swollen PDMS is removed from the soaking solution, rinsed in clean solvent, and placed between two sheets of aluminum foil to slowly dry in a fume hood at least overnight. The dry PDMS can be studied spectroscopically in the third lab period. The absorbance spectra in Figure 1E were taken by simply holding samples of metal nanoparticles in PDMS in the light beam of an Agilent 8453 UV−vis spectrophotometer (referenced to air). Students can take their samples home as window clings, which reversibly adhere to glass or other smooth, flat, hard surfaces, Figure 1F. The metal nanoparticles should continue to stay trapped within this rather unreactive elastomer indefinitely.

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HAZARDS The polymers used in these experiments are messy and can be mild irritants; gloves and goggles should be worn. The steps involving volatile solvents such as ethyl acetate or tetrahydrofuran should be performed in a fume hood and away from potential ignition sources. Tetrahydrofuran has been known to form potentially explosive peroxides. Solutions of silver compounds can leave dark spots on skin. The cross-linked PDMS is chemically unreactive in a dry, room-temperature-type environment, but will degrade when subjected to high temperatures or highly acidic, basic, or oxidizing conditions.

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DISCUSSION Scanning and transmission electron microscopy performed on various metal nanoparticles that were grown in PDMS in this fashion indicate that they are polydisperse: varied in both shapes and sizes (“nanogravel” might be a good descriptor).22 However, many of the particles have dimensions less than 100 nm, making them true nanostructures. A possible extension to this laboratory experiment is to expose the nanoparticles in PDMS to chlorine gas in a fume hood to illustrate the relative ease of oxidation and reduction of silver and gold. Small quantities of chlorine can be produced by combining a drop of household chlorine bleach and a drop of hydrochloric acid on a flat PDMS surface.23 The gas can diffuse into the PDMS and turn the gold and silver nanoparticles colorless (apparently by oxidizing them to the metal chloride salts). Placing the decolorized PDMS samples overnight in a drying oven at 70 °C brings the color at least partially back much more effectively for gold-containing PDMS than silvercontaining PDMS, apparently by thermally decomposing the more easily reduced gold chloride. A sample of gold nanoparticles in PDMS has been cycled between colorless and reddish at least 10 times by treatment with chlorine gas and heating, although the PDMS becomes brittle in the process.

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ASSOCIATED CONTENT

* Supporting Information S

Student laboratory procedures, instructor notes, illustrations of chlorine reaction experiments. This material is available via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful to Bradley University for funding through the Sherry and Special Emphasis Programs. Electron microscopy was conducted at the University of Washington NanoTech User Facility and the Nano Research Facility at Washington University in St. Louis, members of the NanoC

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technology Infrastructure Network (NNIN). We are also grateful for contributions from Rob Ihrig, Stacy Swanson, Ellen Freidinger, Kylee Korte, Josiah Miller, Brad Andersh, Edward Flint, Kristine Campbell, and the students in the Summer 2011, General Chemistry II Lab.

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REFERENCES

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