Hands-on Activity Thirsty for Gold

Learning Objectives

After this activity, students should be able to:

• Describe how nanoscale gold interacts with light differently than bulk gold.
• Explain how the color of gold nanoparticle solutions is dependent on particle or cluster size.
• Explain how nanoscale gold can be used for sensing chemical and biological agents.

Educational Standards

Each TeachEngineering lesson or activity is correlated to one or more K-12 science, technology, engineering or math (STEM) educational standards.

All 100,000+ K-12 STEM standards covered in TeachEngineering are collected, maintained and packaged by the Achievement Standards Network (ASN), a project of D2L (www.achievementstandards.org).

In the ASN, standards are hierarchically structured: first by source; e.g., by state; within source by type; e.g., science or mathematics; within type by subtype, then by grade, etc.

NGSS: Next Generation Science Standards - Science

International Technology and Engineering Educators Association - Technology

State Standards

Materials List

Each group needs:

• 20 ml gold colloid (have students synthesize these gold nanoparticles using supplies listed below and instructions in the Procedure section; alternatively: purchase the gold nanoparticles—see suggested source below, or ask for a small supply from your local university)
• Gatorade Ice^TM (colorless flavor), 2 drops
• pickle juice with little color, 2 drops
• Pedialyte^® (flavorless), 2 drops
• PowerAde^TM (any colorless flavor), 2 drops
• 4 1-dram glass vials (at https://www.sks-bottle.com/340c/fin11a.html or https://www.amazon.com/Glass-Vials-Dram-Pack-12/dp/B002JV6976)
• 4 disposable plastic pipets
• glass waste container with screw-top lid
• gloves and goggles for each student
• Thirsty for Gold Worksheet, one per student

Supplies for gold nanoparticle fabrication by teacher for the entire class:

• 20 ml of 1.0 mM HAuCl₄, gold (III) chloride hydrate; for example, part #244169-500 mg for $98 at www.sigmaaldrich.com. NOTE: To make 20 ml solution of 1.0 mM HAuCl₄, dissolve 7 mg of solid gold (III) chloride hydrate in distilled water.
• 2 ml of 38.8 mM Na₃C₆H₅O₇, sodium citrate anhydrous; for example, part #W302600-Sample K for $40 at www.sigmaaldrich.com. NOTE: to make 2 ml of 38.8 mM sodium citrate anhydrous, dissolve 14.86 mg of solid Na₃C₆H₅O₇ in 2 ml of distilled water.
• stirring hot plate
• magnetic stir bar
• distilled water, 50 ml (this is water for the above solution preparation and additions as solutions boil)
• (optional) refrigerator, for cooling solution, and fume hood

To purchase gold nanoparticles (or ask for a small supply from your local university biochemistry or chemistry programs):

• Get enough for 20 ml gold colloid per group
• Example source of gold colloid: part #752568 for $298 at www.sigmaaldrich.com. NOTE: It is much less expensive to source raw chemicals and make solutions, and raw materials last for multiple activities.

Worksheets and Attachments

Pre-Req Knowledge

Students must be able to perform simple algebra. It is also helpful if students have been introduced to basic physics involving wavelengths and frequencies.

Introduction/Motivation

Since ancient times, gold and silver nanoparticles have been used for staining glass with intense colors. In fact, it is recorded that ancient Romans used gold colloid for this very purpose. This is just one example in which nanomaterials have been around of for hundreds of years, but only recently have we been able to see and control this technology at its very small length scale. Why is gold so special?

Gold, in its bulk form is a very intense yellow appearance and similarly silver is very reflective or white color. This is explained as a phenomenon called surface plasmon resonance (SPR). Although it sounds complicated, it is easy to understand if you visualize electron interactions resulting from metallic bonding. Because metallic bonding does not require sharing of valence electrons and that sub-shell energy levels are filled, free electrons become present in the atomic structure. This is why metals are superior electrical conductors. Free electrons are present, in the form of a cloud, facilitating easy transport of such electrons when perturbed by a potential. This electron cloud also influences the optical properties of metallic metals. Imagine a cloud of moving electrons surrounding the atoms in a metallic material. According to quantum mechanics, these electrons exhibit two behaviors, depending on the experimental technique, particles or waves; this is known as particle-wave duality. If we imagine electrons as waves, they have some sort of associated energy level, which is inversely proportional to their wavelengths. What if this characteristic wavelength was equal to a particular incident light wavelength? Only that portion of light energy with equal wavelengths can be absorbed by the electron cloud. When this occurs, a resonance or electron cloud vibration commences dissipating energy. The remaining wavelengths of light are reflected off of the electron cloud. This process occurs only at the surface and illustrates how some light is reflected or absorbed. But this still does not explain why gold and silver are so special. How are nanoparticles affected by this phenomenon?

Gold has electron absorption cloud wavelengths in the infrared wavelength range (>750 nm) and therefore all visible light is reflected and a yellow shiny luster is seen. Silver appears white because the metal absorbs slightly smaller wavelengths than gold, reflecting only combinations of shorter wavelength colors to create a silvery appearance. But this only explains bulk metallic materials. When we consider nanoparticles, a very fundamental property change comes into play: surface area-to-volume ratio. Because nanoparticles are so small, the number of atoms is drastically decreased per crystal, however, the surface area remains large with respect to the number of atoms per volume. This large surface area enables an increased SPR effect. One effect of this is that gold nanoparticles exhibit SPR in certain portions of visible wavelengths, thus rendering the various reflected colors that gold can have at this size scale. Like quantum dots, particle or cluster size has an influence on the reflected color. Smaller particles absorb blue-green spectrums (~400-500 nm) and larger particles absorb the red light spectrum (>700 nm). If the light is absorbed, the opposite is reflected and thus defines the object color.

These unique physical and optical characteristics lend gold nanoparticles to a variety of biosensing applications, such as home pregnancy devices, DNA testing and (for the purpose of this activity) electrolyte detecting. These applications require knowledge of SPR, particle size, shape, ionic interactions between particles and biochemistry. How do color changes occur during a biosensing experiment?

We know that increasing gold nanoparticle size yields a blue-green color shift from yellow-red color. When synthesizing gold nanoparticles, a chemist or material scientist does not rely on physically growing larger-diameter gold spheres as with quantum dots. S/he relies on cluster forming or particle agglomeration during experimentation to increase the net nanoparticle size. Consider a 5 nm gold nanoparticle that has a corresponding negative charge on the surface (electron cloud) that repels neighboring gold nanoparticles. When an electrolyte (salt) is dissolved in water, cations (positive) and anions (negative) are formed and free floating. Chemistry and physics tell us that the positive cations will bind to the negatively charged gold nanoparticle surface because of charge attraction, creating a neutral cation-gold nanoparticle. As more and more cations bind to gold nanoparticle surfaces, the particles begin to agglomerate into clusters. These clusters are much larger than the original gold nanoparticle and are accompanied by a change in solution color. Thus, by forming clusters, the net size of gold nanoparticles have increased, changing their surface plasmon resonance susceptibility and changing the wavelength of absorption. What you end up seeing is a different color, consistent with the size-color effect.

Today, we want to understand this phenomenon first-hand by experimenting with gold nanoparticles and sports drinks of varying electrolyte content. By using our gold nanoparticle biosensor, we will determine which sport drink has more electrolytes. While doing this, we will explore the electromagnetic radiation spectrum, nanoparticle size-color effect and other questions that relate chemistry and physics to nanotechnology. When we are finished you will at least know what sports drink to consume after breaking a mental sweat.

Procedure

Before the Activity

• Decide to either purchase gold nanoparticles or have students synthesized their own as part of the experiment. Have students follow the steps below to fabricate the gold nanoparticles.
• Gather materials and make copies of the Thirsty for Gold Worksheet, one per student.
• For each group, use pipets to fill four vials with each electrolyte.
• Place all other supplies (pipets, glassware) at each lab station.

Gold Nanoparticle Fabrication (Student Instructions)

• Pour 20 ml of 1.0 mM HAuCl₄ into a 50 ml beaker. Add a magnetic stir bar. Heat the solution to boiling on a stir/hot plate while stirring with the magnetic stir bar.
• After the solution begins to boil, add 2 ml of 38.8 mM Na₃C₆H₅O₇. Continue to boil and stir the solution until it is a deep red color (about 10 minutes). As the solution boils, add distilled water as needed to keep the total solution volume near 22 ml. How does the solution visibly change? The sodium citrate reduces the Au ions to nanoparticles of Au metal. Excess citrate anions in solution stick to the Au metal surface, giving an overall negative charge to each Au nanoparticle.
• When the solution is a deep red color, turn off the hot plate and stirrer. Cool the solution to room temperature.
• Place vials in a fume hood or on table as a central access location.

With the Students—Overall Procedure

1. Divide the class into groups of three or four students each. Hand out the worksheets.
2. Have students review the materials list on their worksheets to make sure they have all items.
3. Have students answer the pre-activity questions on the worksheet.
4. Have students read the worksheet, follow its instructions and answer its questions.
5. Have students turn in their worksheets for grading.
6. Conclude with a class discussion to compare results and conclusions, as described in the Assessment section.

Student Lab Procedure (also on the worksheet)

1. Take four glass vials and add 3-4 drops of the gold nanoparticles to each.
2. To each vial, add 2 drops of a different electrolyte and observe what happens to the color of the nanoparticles. Carefully keep track of which liquid was added to which vial.
3. Record your observations in the worksheet data table.
4. After recording your observations, answer the remaining worksheet questions.
5. Clean up your lab station.

Vocabulary/Definitions

electromagnetic radiation: A means of energy travel through a vacuum. Has characteristic wavelengths and frequencies.

electron cloud: The region of negative charge surrounding an atomic nucleus that is associated with an atomic orbital.

plasmon: Oscillations of electron cloud against the fixed positive ions in a metal.

resonance: The selective response of an object or system that vibrates in step with an externally applied vibration.

surface plasmon resonance: The excitation of surface plasmons by light for planar or nanoparticle surfaces.

Assessment

Pre-Activity Assessment

Worksheet: Have students answer the pre-activity questions about sports drinks. Review and discuss their predictions at activity end to gauge their understanding of the concepts.

Activity Embedded Assessment

Worksheet: The attached Thirsty for Gold Worksheet is designed to guide students in recording observations, analyzing their observations and drawing conclusions based on observations. Through this worksheet, they explore more general physics and chemistry concepts on light energy, wavelength, frequency, electrolytes, ionic bonding and chemical bonding. Students need to read questions thoroughly to identify pertinent information to solve each problem. Expect students to finish the worksheet in class.

Post-Activity Assessment

Worksheet: Have students turn in their completed worksheets for grading. Review their answers to gauge their mastery of the concepts.

Closing Class Discussion: Lead a post-activity discussion to compare results and conclusions, including answers to the worksheet questions. Gauge students' understanding of how they used nanotechnology for electrolyte detection and the relationship of cluster/particle size to the electromagnetic radiation spectrum. How can gold nanoparticles be used to sense chemical agents for the benefit of humans? What sensing applications might they invent?

Team Poster Project: After students have completed the two lessons and four activities of the unit, assign student pairs to each create posters that summarize what was learned during the unit. This summary assessment is fully described in the NanoTech: Insights into a Nano-Sized World unit document, and includes a grading rubric for the teacher.

Safety Issues

• All supplies used in this activity are safe.
• Handle gold nanoparticle solution vials with care and abide by laboratory safety and handling procedures, as necessary.
• Have students wear gloves and goggles.
• Dispose of the solutions in a glass waste container with screw-top lid.

Troubleshooting Tips

To minimize waste, label the plastic pipets to denote which is used with each sports drink.

References

Copyright

Contributors

Supporting Program

Acknowledgements

This curriculum was created by the University of Houston's College of Engineering with the support of National Science Foundation GK-12 grant no. DGE 0840889. However, these contents do not necessarily represent the policies of the National Science Foundation, and you should not assume endorsement by the federal government.