Antimicrobial Silver

In this lab, students will make a silver colloidal mixture. Silver solution will be reduced and stabilized with sodium citrate in a boiling water bath. The colloidal dispersions formed will be poured into Petri dishes, soaked up into filters, and placed on top of a bacterial agar plate. The silver colloid should be yellow in color. Silver colloidal particles are 20-50 nm in size. If the silver nanoparticles prevent bacterial growth, there should be a ring on inhibition around where the filters soaked with nanoparticles are placed on the bacterial agar plate.

If you have any questions about this lab, please contact Joe Muskin at (217) 265-6481 or by email at jmuskin@uiuc.edu.

Purpose:

This lab was developed to test claims that silver nanoparticles have antimicrobial effects. Many products on the market have nanoparticles of silver in them. These include socks, clothing, and food containers. One advertising claim is that the silver nanoparticles inhibit the growth of bacteria and, hence, reduce odors or spoilage.

Materials:

  • 1 mM silver nitrate
  • E. coli bacterial culture
  • 1% sodium citrate
  • Coffee filter/filter paper
  • Small test tube
  • Q-tip swab
  • 250-mL beaker
  • Small containers for soaking filter paper
  • Disposable transfer pipets
  • Incubator
  • Agar plate
  • Hot plate
  • Scissors
  • Tongs
  • Tweezers
  • Test tube rack
Various laboratory equipment for use within the lab.

Click on the image above to open a full sized version.

Experimental Procedure

  1. Make silver nanoparticles.
    1. Add 2 mL of 1mM silver nitrate to a small test tube.
    2. Place the test tubes in a 250-mL beaker of boiling water for 10 minutes.

    3. Add 5 drops of 1% sodium citrate to the test tube.

    4. Continue to heat until the silver turns a yellowish color. (should take ~15 minutes).

    5. Remove test tubes and set in a test tube rack to cool.

  2. Test antimicrobial effects.
    1. Cut filter paper into small squares about 2 cm across.
    2. Place filter paper squares in a small container and pour the test tube of silver nanoparticles over the squares and let the filter paper squares soak for about 10 minutes (or overnight).

    3. Divide and label the bottom of an agar plate into sections
    4. Put 1 to 2 drops of bacterial culture on the agar plate using a 1-mL disposable transfer pipet and spread the drops of bacteria culture on the agar plate using a cotton swab.

    5. Place your nanoparticle soaked filter paper square and your control(s) in the designated areas and incubate your agar plate for 24 hours at 37°C.

Downloadable Materials:

  • Activity Guide

    Instructors guide containing all relevant information to conduct the lab.

  • Presentation

    Presentation for teacher to introduce the activity to students.

  • Procedure

    Procedure with pictures of each step.

  • Student Handout

    Student laboratory handout containing the procedure as written steps. This material is contained in the Activity Guide.

  • SEM Images

    SEM images and descriptions of bacterial cells with and without exposure to silver nanoparticles.

Safety:

Goggles, gloves and aprons should be worn as in all chemistry laboratory activities. The hot water baths should be handled with care to avoid burns. Any liquids spilled on skin can be washed off with water.

This lab uses Escherichia coli, E. coli, a gram-negative rod-shaped bacterium that is part of the normal intestinal fauna in mammals. Some strains, particularly O157:H7, are pathogenic to humans; most strains, however, are benign. Because of the ease of culturing E. coli, it has become the "workhorse" of microbiology. Strains for use in laboratories and classrooms are derived from E. coli that grows very well on Petri dishes but very poorly in intestines. The E.coli used in this lab is non-pathogenic and likely wouldn't live in a human intestine, even if ingested in large amounts.

Acknowledgements:

Thanks to Scott Robinson and Cate Wallace of the Microscopy Suite, part of the Imaging Technology Group at the Beckman Institute of Advanced Science and Technology, University of Illinois Urbana-Champaign for SEM images.

Thanks to William Metcalf for providing safe bacterial cultures and expertise.

Thanks to Martha Atwater and Kate Carroll for helping organize, design and edit teaching resources.

Developers

Barbara Hug, Joe Muskin, Matt Ragusa, and Janet Wattnem