In the Laboratory
From the Research Bench to the Teaching Laboratory: Gold Nanoparticle Layering
W
Maria Oliver-Hoyo* and Ralph W. Gerber Department of Chemistry, North Carolina State University, Raleigh, NC 27695; *
[email protected] Over the past decade, there has been a dramatic increase in the interest and the study of nanotechnology (1, 2). This trend strongly supports the introduction of nanoscience techniques and methodology into the undergraduate chemistry laboratory curriculum. Experiments published in this Journal dealing with nanoscience phenomena range from simple approaches (3) to advanced chemistry concepts and biological and medical applications (4–8). We present an experimental procedure to introduce chemistry students to synthetic, mechanistic, and measurable properties of gold nanoparticle layering techniques. Nanoscience experiments published in this Journal require the use of specialized equipment not normally available in undergraduate teaching laboratories and techniques too advanced for novice students, such as MALDI TOF, STM, AFM, and TEM among others (9– 13). Other published experiments use numerous hazardous chemicals and take too long to be suitable for an undergraduate chemistry laboratory environment (14, 15). This experiment is based on a well-documented research protocol that we modified to be used in undergraduate teaching laboratories (14, 15). The modifications are 1. Eliminating the use of highly corrosive or hazardous chemicals such as piranha or aqua regia. 2. Reducing the time needed for cleaning (time saved: 5 h), silanation (timed saved: 3 h), and deposition of the gold layer and cross-linker (time saved: 45 min for each gold layer). 3. Modifying storage restrictions on the materials and chemicals used in the experiments. The use of a vacuum food saver and a pack of silica gel desiccant extended the shelf-life of the chemicals, eliminating the need for refrigeration or desiccators.
The experimentation and testing, along with the appropriate documentation needed to make the aforementioned modifications are fully documented in the Supplemental Material.W The procedure developed was tested by a high school student and then used by physical chemistry laboratory students. Synthesis and measurements of gold nanoparticle suspensions are conducted in two, three-hour laboratory sessions. There were several specific educational objectives incorporated into the development of this experiment. In the laboratory, we wanted to show how the chemistry on the nanoscale (oxidation–reduction and gold nanoparticle layering) had a measurable visual effect in the macro scale. The experiment is designed in such a way that synthesis of colloidal gold nanoparticles, formation of self-assembled monolayers (SAMS), and conductivity on the nanoscale can be monitored visually and quantitatively measured using UV– vis spectrophotometric analysis and a conductivity testing apparatus. 1174
Journal of Chemical Education
•
Background The process of gold nanolayering involves producing gold colloidal suspensions to subsequently layer gold particles onto a glass surface with additional layers adsorbed to previous ones (14, 15). Sodium citrate and the hydrogen tetrachloroaurate (Figure 1) are the building blocks for this process where hydrogen tetrachloroaurate is the acid form of gold(III) chloride. The formation of gold colloidal suspensions involve the reduction of Au3+ to Au0 by sodium citrate resulting in the formation of a face-centered cubic lattice and aggregation of the lattices to form nanoparticles in a size dictated by the concentration of the sodium citrate. There is as yet no definitive mechanism for the reduction of Au3+ to Au0 by sodium citrate or for the growth of the nanoparticles through the aggregation of the gold face-centered cubic lattice. Glass slides are treated with 3-aminopropyltriethoxysilane to provide a linking mechanism for the first layer of gold nanoparticles onto the glass coverslips. The mechanism by which the 3-aminopropyltriethoxysilane is attached to the glass surface is depicted in Figure 2. New Si⫺O⫺Si bonds create bridges from the glass to the amine, which acts as the gold nanoparticle receptor. Subsequent layers are obtained by alternating between cysteamine (2-mercaptoethylamine) and the gold nanoparticles where the cysteamine acts as a cross-linker between the gold nanoparticles. There is not a definitive mechanism for the attachment of the thiol to the gold; however, the proposed adsorption reaction is believed to be (16) Au(s) + RS–H(solv)
RS–Au(s) + 1/2 H2(solv)
where R is H2NCH2CH2. Layers are built up until the glass has the appearance of a pure gold reflective surface by alter-
Figure 1. Structures of the compounds used in the nanolayering.
Vol. 84 No. 7 July 2007
•
www.JCE.DivCHED.org
In the Laboratory
nating between the 12 nm colloidal gold and the cysteamine cross-linker. Noble metal nanoparticles embedded in or attached to a dielectric exhibit a strong absorption peak owing to a collective motion of free electrons, that is, a surface plasmon resonance. For isolated spherical particles, the resonance peak generally occurs in the visible part of the spectrum. The particular frequency depends on the particle size and the dielectric constants of the metal and of the surrounding medium. For multiple particles and multiple layers, the electromagnetic coupling between neighboring particles shift the plasmon absorption bands (17). As the layers of gold are added, a noticeable shift in the plasmon band of the visible spectrum shows transmission of pink to blue–magenta (Figure 3) as a result of the loss of freedom for electron vibration from electromagnetic coupling of the gold nanoparticles within close proximity to each other. A scaled representation of the main components of the layering process and of the molecular attachments that produce self-assembled monolayers are shown in Figure 4. Understanding the Nanoscale It is challenging for students to visualize the sizes of the molecules they are working with. The following description of nanoparticles in relation to common sizes is an attempt to provide students with relevant analogies to the nanoscale. Using a single marble 12 mm in diameter (approximately 0.5 in.) we can make comparisons to the 12 nm gold colloidal
Table 1. Comparison of Particles to Marbles
12 nm Particles Dimension
Number
Coverage/ mm
12 mm Marbles Number
Width
750,000.
9.
750,000.
Length
4,166,666.
50.
4,166,666.
Thickness
14,833.
0.178
Coverage/ mi
14,833.
5.59 31.1 0.11
particle. The coverslips being used for layering in this experiment are 9 mm × 50 mm × 0.178 mm. If the 12 nm particles were placed end to end across the 9 mm width of the coverslip it would require 750,000 particles to cover this width while 750,000, 0.5 in. marbles placed end to end would span 5.59 mi. The 50 mm length of the cover slip would require 4,166,666 nanoparticles, which compares to 31.1 mi for the marbles. Even the thickness of 0.178 mm would require 14,833 nanoparticles or 0.11 mi (584 ft) of marbles. Table 1 shows these relationships in tabulated form.
Figure 3. Colors exhibited due to the shift in the plasmon bands.
Figure 2. Representation of the attachment of 3-aminopropyltriethoxysilane to a glass surface: R1 is the ethoxy group and R2 is the aminopropyl group.
www.JCE.DivCHED.org
•
Figure 4. Scaled representation of molecule attachments in self-assembled monolayers. Cysteamine, the cross-linker between gold particles, is bonded to the gold particle.
Vol. 84 No. 7 July 2007
•
Journal of Chemical Education
1175
In the Laboratory Table 2. UV–Vis Absorbance Data of Colloidal Suspensions and Coloring of Layers UV Abs, 12 nm Colloid/nm
UV Abs, Layer 1/nm
Color Observation
UV Abs, Layer 2/nm
Color Observation
UV Abs, Layer 3/nm
Color Observation
Authors' Tests
518.1 ± 0.7
533.7 ± 1.5
Pink
657.0 ± 0.7
Lt Blue
657.2 ± 0.6
Gray
HS Senior
518.6 ± 2.0
534.0 ± 0.8
Pink
655.7 ± 0.7
Lt Blue
657.0 ± 0.7
Gray
P-Chem Group 1
519.0 ± 1.4
532.9 ± 0.7
Pink
657.6 ± 0.7
Lt Blue
657.7 ± 0.7
Gray
P-Chem Group 2
519.1 ± 1.4
534.1 ± 1.0
Pink
656.3 ± 0.7
Lt Blue
658.1 ± 0.7
Gray
P-Chem Group 3
518.5 ± 0.5
533.2 ± 1.0
Pink
657.7 ± 0.9
Lt Blue
658.0 ± 0.8
Gray
Overall Average
518.7 ± 0.4
533.6 ± 1.0
Pink
656.9 ± 0.7
Lt Blue
657.6 ± 0.7
Gray
Test Participants
Experimental Data
Literature Cited
A summary of students’ data is presented in Table 2. These data include test results from the authors followed by results obtained by a high school senior and different groups of physical chemistry students. Each reported entry is the average of readings obtained from ten slides prepared by each participant or group.
1. Office of Science and Technology Policy. http://www.ostp.gov (accessed Jun 2006). 2. Roco, M. C. Government Nanotechnology Funding. http:// www.nano.gov/html/res/IntlFundingRoco.htm (accessed Jun 2006). 3. McFarland, Adam D.; Haynes, Christy L.; Mirkin, Chad A.; Van Duyne, Richard P.; Godwin, Hilary A. J. Chem. Educ. 2004, 81, 544A. 4. Bolstad, David B.; Diaz, Anthony L. J. Chem. Educ. 2002, 79, 1101–1104. 5. Watkins, John J.; Zhang, Bo; White, Henry S. J. Chem. Educ. 2005, 82, 712–719. 6. Hale, Penny S.; Maddox, Leone M.; Shapter, Joe G.; Voelcker, Nico H.; Ford, Michael J.; Waclawik, Eric R. J. Chem. Educ. 2005, 82, 775–778. 7. Smith, David K. J. Chem. Educ. 2005, 82, 393–400. 8. Lagorio, María Gabriela. J. Chem. Educ. 2004, 81, 1607–1611. 9. Powell, Cedric J. J. Chem. Educ. 2004, 81, 1734–1750. 10. Hipps, K. W.; Scudiero, L. J. Chem. Educ. 2005, 82, 704–711. 11. Bentley, Anne K.; Farhoud, Mohammed; Ellis, Arthur B.; Lisensky, George C.; Nickel, Anne-Marie L.; Crone, Wendy C. J. Chem. Educ. 2005, 82, 765–768. 12. Heinz, William F.; Hoh, Jan H. J. Chem. Educ. 2005, 82, 695– 703. 13. Williams, Geoffrey L.; Vohs, Jason K.; Grege, Jonathan J.; Fahlman, Bradley D. J. Chem. Educ. 2005, 82, 771–774. 14. Keating, Christine D.; Musick, Michael D.; Keefe, Melinda H.; Natan, Michael J. J. Chem. Educ. 1999, 76, 949–955. 15. Musick, Michael D.; Keating, Christine D.; Lyon, Andrew L.; S Botsko, Steven L.; Peña, David J.; Holliway, William D.; McEvoy, Todd M.; Richardson, John N.; Natan, Michael J. Chem. Mater. 2000, 12, 2869–2881. 16. Blanchard, G. J.; Karpovich, D. S. Langmuir 1994, 10, 3315– 3322. 17. Sweatlock, L. A.; Penninkhof, J. J.; Maier, S. A.; Polman, A.; Atwater, H. Mat. Res. Soc. Symp. Proc. 2004, 797, W4.6.1– W4.6.6.
Hazards The gold colloidal preparation should be performed in a hood. The 3-aminopropyltriethoxysilane is moisture sensitive and should be maintained under an argon or nitrogen environment. It is harmful by ingestion, inhalation, and if absorbed through the skin. It is corrosive and destructive of mucous membranes. Hydrogen tetrachloroaurate is corrosive and will stain any contacting surface. Absolute ethanol should be handled with care owing to its flammability. 2-Mercaptoethylamine is irritating to eyes, respiratory system, and skin. W
Supplemental Material
Experimental details, materials, equipment, and solution preparation instructions are available in this issue of JCE Online. Testing and experimentation used in the modification of the procedures for cleaning, silanation, layering, storage, and chemical usage are also included. Acknowledgments We thank Jaap Folmer at North Carolina State University and his physical chemistry students along with Ashley Beale, an intern from Southeast High School in Raleigh, North Carolina, for testing the new gold layering protocols. This work has been possible due to the generous funding provided by the National Science Foundation (CAREER Award No. REC-0346906 & NIRT 0403871).
1176
Journal of Chemical Education
•
Vol. 84 No. 7 July 2007
•
www.JCE.DivCHED.org
