Materials Included In Kit

Additional Materials Required

Prelab Preparation

To prepare the 0.04 M silver nitrate solution, measure 10 mL of 0.1 M silver nitrate and dilute to 25 mL of DI water. This will be more than enough solution for 15 groups of students.

Safety Precautions

Silver nitrate is corrosive and highly toxic. Avoid contact with eyes and skin. It will stain skin and clothing. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Please review Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. The colloidal silver solution is stable and may be stored in a dark bottle to avoid exposure to light. The colloid can be broken by adding a small amount of 6 M hydrochloric acid, which precipitates the silver. Solid silver may be placed in the trash according to Flinn Suggested Disposal Method #26a. The remaining solution or filtrate is acidic and may be neutralized according to Flinn Suggested Disposal Method #24b.

Lab Hints

• This laboratory activity can reasonably be completed in one 50-minute class period. The prelaboratory assignment may be completed before coming to lab, and the data compilation and calculations may be completed the day after the lab.
• An ice bath may be used to bring the silver colloid solution to room temperature more rapidly.

Teacher Tips

• There are enough materials included in this kit to conduct this activity five times with a class of 30 students working in pairs.
• An extension to this lab would be to observe the absorbance of the silver nanoparticles with a spectrophotometer. Fill a cuvet two-thirds full with the colloidal silver solution and measure the absorbance every 10 nm from 400 nm to 700 nm.
• The position of the absorption band maximum for metal nanoparticles varies with their average size or diameter. The size of nanoparticles depends, in turn, on the concentration of metal ions in the original solution and on the length of time the reduction proceeds. Have students experiment with different (very dilute) solutions of silver and citrate ions in the reaction mixture and compare the color and absorption spectrum of the resulting colloidal silver solution.
• Colloidal silver nanoparticles are widely advertised for their antimicrobial and disinfectant properties in all sorts of commercial products, everything from socks and shoes to laundry detergents. Team up with the biology teacher for an integrated chemistry and microbiology lab activity to test the effectiveness of the colloidal silver in preventing the growth of microorganisms.

Answers to Prelab Questions

1. The average size of the bacterium Escherichia coli is 2 micrometers long. If the average size of silver nanoparticles is 12 nm in diameter, then how many silver nanoparticles would have to be lined up to equal the length of one E. coli?

   1000 nm = 1 μm
   1 μm = 1 x 10^–6 m
   1 x 10^–6 m x (1 nm/1 x 10^–9 m) = 1000 nm
   2 μm x 1000 nm = 2000 nm
   The length of one E. coli is 2000 nm.
   2000 nm/12 nm = 167 silver nanoparticles.
   
   Approximately 170 silver nanoparticles would have to be lined up to equal the length of one E. coli.

2. The diameter of the E. coli is 1 μm across. (a) What is the volume of one E. coli bacterium? (b) What is the volume of one silver nanoparticle? (c) How many silver nanoparticles would fit into one E. coli bacterium?

   Volume of a cylinder: V = πr²h
   Volume of a sphere: 4/3πr³
   Volume of E. coli = π(0.5 μm)²(2 μm) = 1.57 μm³
   Convert to nm = 1.57 x 10³ μm x (1000 nm/1 μm)³ = 1.57 x 10⁹ nm
   Volume of one silver nanoparticle = 4/3π(6)³ = 904 nm
   1.57 x 10⁹/904 = 1,737,606
   
   About 1.7 x 106 silver nanoparticles would fill an E. coli

3. Describe the Tyndall Effect and give an example of where it may occur.

   Answers may vary. The Tyndall Effect is the scattering of light by particles in a mixture. Fog is a mixture of water droplets in air. The water droplets scatter light causing the fog to appear hazy.

Answers to Questions

Observations

1. Describe the properties of the colloidal silver from step 6.

   The colloidal silver solution is a yellow-gold color. There are no solid particles visible. The liquid appears translucent rather than transparent

2. What happens to the beam of light as it passes through the solution in step 7?

   The beam of light becomes visible as it passes through the solution. This is known as the Tyndall Effect.

3. Describe any changes that occurred when water and sodium chloride were added to the silver solution in the test tubes, respectively.

   When water was added to the silver solution, it became less yellow. The solution became cloudy and white when sodium chloride was added.

Post-Lab Questions

1. Compare and contrast the properties of a true solution versus a colloid: particle size, settling behavior, filtration, and light scattering.

   A true solution has very small particles that will not settle out. The particles are too small to be separated by filtration. A true solution is transparent and will not scatter light. Colloids have a slightly larger particle size than those found in a true solution. The particles cannot be filtered out of the solution but may be by means of ultra filtration. Colloids do scatter light.

2. How is using a silver colloid as an antimicrobial agent more beneficial than using a chemical solution such as chlorine?

   Chlorine reacts with many organic compounds to form substances that may be harmful to the environment. Silver is a relatively safe material and will not react with other organisms or chemicals found in the environment in a harmful way. Silver nanoparticles are also very small and stable. They will not break down easily unless exposed to a high concentration of salt, as shown in this lab activity.

Discussion

The method of preparation for the silver nanoparticles in this activity involves a citrate reduction by citrate ions, which are a mild reducing agent.

The nanoparticles made in this activity consisted of silver but appeared gold. How is this possible? Solutions of metallic nanoparticles absorb visible light due to a unique phenomenon called Plasmon resonance. The yellow color of colloidal silver arises when incident light creates oscillations in conduction electrons on the surface of the nanoparticles, according to this theory, causing them to absorb electromagnetic radiation. The spectrum of the clear yellow colloidal silver solution is shown in Figure 1. The wavelength of the plasmon absorption maximum in a solvent can be used to estimate the size of the nanoparticles. The prepared silver colloid has a plasmon absorption band peak at 420 nm. Sodium chloride was added to the silver nanoparticles and caused a change in the material’s composition. The addition of the NaCl to the silver nanoparticles causes them to aggregate. The nanoparticles are kept in suspension by repulsive electrostatic forces between the particles, which are coated with nitrate ions. Adding salt shields the charges, allowing the particles to clump together and form larger aggregates.

References

Brett, D. W. A Discussion of Silver as an Antimicrobial Agent: Alleviating the Confusion: Ostomy Wound Management, Online January 2006, 52, No. 1. http://www.o-wm.com/article/5125 (accessed September 2010).

Redmond, et al. Photovoltage and Photocatalyzed Growth in Citratr-Stabilized Colloidal Silver Nanocrystal: J. Phys. Chem. C., 2007, 111, pp 8942–8947.

Solomon, S. D. Synthesis and Study of Silver Nanoparticles: Journal of Chemical Education, 2007; 8, 2, p 322.

Introduction

Imagine you are holding a silver sphere about the size of a gumball. If that sphere were shrunk to 10 nm in size, would it still look the same? Would it still be shiny and silver in color? Reducing a solid-phase particle down to the nanometer scale changes its physical and chemical properties. The properties of colloidal silver are a great example of this phenomenon.

Concepts

• Nanoscience
• Colloids vs. solutions
• Redox reactions

Background

Nanotechnology is the study of the preparation, characterization, and use of particles and structures that have dimensions between 1 and 100 nm. Nanoparticles have unique physical and chemical properties that are significantly different from the macroscopic properties of bulk solids. These properties are important in the applications of nanotechnology as well as in medicine. One of these properties is the ability to scatter light. A beam of light is invisible in clear air or pure water. However, if a beam of light is shone through a colloidal suspension, the path of light becomes visible such as headlights through fog.

Silver has impressive antimicrobial properties that have been proven useful since ancient times. In ancient Greece and Rome, silver coins were dropped into water functioning as a disinfectant. In 1884, a German obstetrician, Carl Sigmond Franz Crede (1819–1892), used 1% silver nitrate to prevent blindness in newborns due to post-partum infection. Today, silver is being used as a disinfectant in water sanitation systems in hospitals, hotels, and even aboard NASA space shuttles to sterilize recycled water.

Experiment Overview

The purpose of this experiment is to produce silver nanoparticles via a redox reaction of silver nitrate and sodium citrate. The properties of the silver nanoparticles will be investigated.

Materials

Prelab Questions

1. The average size of the bacterium Escherichia coli is 2 micrometers long. If the average size of silver nanoparticles is 12 nm in diameter, then how many silver nanoparticles would have to be lined up to equal the length of one E. coli bacterium?
2. The diameter of the E. coli? is 1 μm across. (a) What is the volume of one E. coli bacterium? (b) What is the volume of one silver nanoparticle? (c) How many silver nanoparticles would fit into one E. coli bacterium?
3. Give an example of where the Tyndall Effect may occur.

Safety Precautions

Silver nitrate is a corrosive solid that causes burns and is highly toxic. Avoid contact with eyes and skin. It will stain skin and clothing. Wear chemical splash goggles, gloves and a chemical-resistant apron. Wash hands thoroughly with soap and water before leaving the laboratory. Please follow all laboratory safety guidelines.

Procedure

1. In a 250-mL beaker, add 1.5 mL of 0.04 M silver nitrate solution using a graduated pipet and 150 mL of DI water.
2. Bring this solution to a boil using a hot plate at a medium-high setting.
3. Once the solution is boiling, add 3 mL of 2% sodium citrate.
4. Continue boiling the solution for approximately 10 minutes until the solution is a pale yellow color.
5. Turn off the hot plate and allow the beaker to cool until it is safe to handle. Place the beaker on a heat-resistant surface and allow the solution to cool to room temperature. Note: If necessary, add DI water to bring the solution volume up to 150 mL.
6. When the solution is cooled to room temperature, pour some of the “colloidal silver” into a smaller beaker and observe its properties.
7. Shine a laser pointer or a flashlight through the silver solution and observe the “path” of the light through the solution.
8. Pour some of the colloidal silver into two medium test tubes, filling each test tube about one-third full.
9. Mark the volume of solution in one of the test tubes and add an equal amount of deionized water to the test tube. Swirl gently to mix. Observe and record any changes.  
10. Mark the level of silver solution in the second test tube and add an equal volume of 1 M sodium chloride solution. Swirl gently to mix and observe what has happened.
11. Consult your instructor for appropriate disposal procedures. 

Student Worksheet PDF

12306_Student1.pdf