Properties of materials change at the nanoscale. In bulk at the macroscale, the element of gold is gold colored, but at the nanoscale, the element of gold is red to purple in color. The formation of gold nanoparticles can be therefore observed by a change in color since small nanoparticles of gold are red. The layer of absorbed citrate anions on the surface of the nanoparticles keep the nanoparticles separated, and the presence of this colloidal suspension can be detected by the reflection of a laser beam from the particles. Switching to a smaller anion allows the particles to approach more closely and another color change is observed.
Safety:
- Wear eye protection
- Chemical Gloves Recommended
- Never look directly into a laser or shine a laser at another person
Inclusive Teaching Practices:
Inclusive teaching refers to methods that are designed to engage students in learning that is meaningful, relevant, and accessible to all. Equitable learning environments provide supports to address individual student needs and promote learning for all students. Creating an inclusive classroom is a semester long process that should begin before the term with development of your syllabus and lesson plans.
This nanogold particle synthesis lab contains specific inclusive teaching strategies that align with the learning objectives of the experiment. Creating Learning Objectives makes it clear for all students, regardless of their background, what knowledge or skills you expect them to learn by conducting the laboratory/experiment.
Learning Objectives – Students will:
- Synthesize gold nanoparticles and observe how the color of nanomaterials can be different than the color of bulk materials.
- Explore how changes in reducing agent concentration/strength and mixing speed effect the color of the nanoparticles.
- Use various methods to determine whether gold nanoparticles were formed (e.g. diffraction of a laser pointer beam or spectrophotometry).
- Measure the effect of the electrolytes in several different solutions on the nanoparticles.
- Interpret the collected data to accurately rank materials based upon their impact on the nanoparticles.
- Work in groups to practice safe and effective laboratory practices while synthesizing and testing the nanoparticles.
We find that it works best to have students work in groups of 3-4 for this laboratory. Forming functional groups is a challenging process that all instructors face in the classroom.
Some strategies to build functional groups :
- Deliberately assign students to small, heterogeneous groups that do not isolate underrepresented students. (e.g., when possible, avoid having one female student in a group with 3 male students.)
- Pair students who may be less proficient in English with culturally-sensitive classmates. (Wlodkowski and Ginsberg, 1995)
- Balance leadership roles across ethnic and gender groups and monitor the roles in group activities. (Van Note Chism and Pruitt, 1995; McKay, 2001). For example, assign students in each group roles (e.g., recorder, reporter, discussion facilitator). Make sure that students rotate roles regularly.
- Help students learn to work in groups by discussing group process issues in class and by addressing issues as they arise. (Saunders and Kardia, 2000) For example, have students explore Constructive and Destructive Group Behaviors to help them understand what positive and negative traits they bring to their group.
For more resources go to our Inclusive Teaching Practices page.
Materials:
Stock Solutions for 25 batches
- 1.0 mM hydrogen tetrachloroaurate: The solid is hygroscopic so purchase HAuCl4.3H2O (Aldrich 244597 or 520918) in 1.0 g quantities and use the entire bottle. Dissolve 1.0 g HAuCl4.3H2O in 250 mL distilled water to make a 10.0 mM stock solution of gold(III) ions that can be kept for years if stored in a brown bottle. Dilute 25 mL of stock to 250 mL to make the 1.0 mM concentration for this experiment.
- 1% trisodium citrate: Dissolve 0.5 g Na3C6H5O7.2H2O (sodium citrate) in 50 mL distilled water. NOTE: Sodium citrate does not keep well so it is best to make fresh solutions prior to each synthesis.
- NaCl solution: Dissolve at least 0.5 g of NaCl in 10 mL distilled water or use a saturated solution.
- Optional sports drinks: Gatorade Ice, Powerade, Flavorless Pedialyte, Pickle Juice. Solutions with little to no color work best.
Equipment:
- 50 mL Erlenmeyer flask or beaker
- 1″ or 1 cm stir bar
- Stirring hotplate
- Laser pointer, polarizing filter
- Droppers and test tubes or cuvettes
Protocol:
Step 1. Rinse all glassware with pure water before starting. Add 20 mL of 1.0 mM HAuCl4 to a 50 mL beaker or Erlenmeyer flask on a stirring hot plate. Add a magnetic stir bar and bring the solution to a rolling boil.
Step 2. To the rapidly-stirred boiling solution, quickly add 2 mL of a 1% solution of trisodium citrate dihydrate, Na3C6H5O7.2H2O. The gold sol gradually forms as the citrate reduces the gold(III). Remove from heat when the solution has turned deep red or 10 minutes has elapsed.
- (Gaps in the movie indicate equal gaps in time. The total elapsed time is approximately 10 times the movie length.)
- The presence of a colloidal suspension can be detected by the reflection of a laser beam from the particles.
- The light from a laser pointer may be polarized. When polarized light causes plasmon emission the beam may disappear at some angles. When the beam from the laser is visible, is it invisible in a view perpendicular to the first?
Step 3. Record the visible spectrum of the solution. If necessary, add additional water to the cuvette to get the absorbance on scale.
Step 4. Put a small amount (5-10 drops) of the gold nanoparticle solution into 5 test tubes or microcentrifuge tubes. Use one tube as a color reference and add an equal volume of NaCl solution, water, sports drink, or pickle juice to the other tubes. Does the color of the solution change as the addition of chloride (or other electrolyte) makes the nanoparticles clump closer together?
- Option 1: this part could be done in a cuvette with the visible spectrum recorded after each addition.
- Option 2: test electrolyte content in sports drinks by counting drops needed to change the color of 7 drops of gold nanoparticle solution.
- Option 3: you can ask students to retrieve a clear liquid of their choice that they would like to test in addition to some of the other solutions.
Conclusions
Before the addition of the reducing agent, the gold is in solution in the Au+3 form. When the reducing agent is added, gold atoms are formed in the solution, and their concentration rises rapidly until the solution exceeds saturation. Particles then form in a process called nucleation. The remaining dissolved gold atoms bind to the nucleation sites and growth occurs.
See “Producing gold colloids” at https://chem.beloit.edu/edetc/nanolab/gold/Manufacturing%20high-quality%20gold%20sol.pdf
- Why do more concentrated or stronger reducing agents tend to give smaller nanoparticles?
- Why does rapid mixing of the reducing agent give more monodispersed particles?
- What is the wavelength and fwhm (peak width at half the maximum height) of the visible absorption?
- Summarize the evidence that you made solid nanoparticles.
- If you tested several salt solutions, report your results and rank the solutions in order from the least to the most electrolytes.