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Antibacterial Silver


Published on Dec 13, 2018

Abstract

Silver, a naturally occurring element, is non-toxic, hypoallergenic, does not accumulate in the body to cause harm and is considered safe for the environment. Many manufactured goods like washing machines, air conditioners and refrigerators are using linings of silver nanoparticles for their antimicrobial qualities. Sportswear, toys and baby articles, food storage containers, HEPA filters, laundry detergent etc. are made with silver nanoparticles. The medical field also is using products with silver nanoparticles, such as heart valves & other implants, medical face masks, wound dressings and bandages.

In this activity,

Visitors will understand that the increased surface to volume ratio of nano-scale particles can enhance chemical reactivity.

Introduction

Nanomaterials are the leading in the field of nanomedicine, bionanotechnology and in that respect nanotoxicology research is gaining great importance. Silver exhibits the strong toxicity in various chemical forms to a wide range of microorganism is very well known and silver nanoparticles have recently been shown to be a promising antimicrobial material. Analysis of bacterial growth showed that the toxicity of silver nanospheres is higher than that of gold nanospheres. In addition, no research has discovered any bacteria able to develop immunity to silver as they often do with antibiotics.

Bacteria depend on an enzyme to metabolize oxygen to live. Silver interferes with the effectiveness of the enzyme and disables the uptake of oxygen killing them. This process has the added benefit of not harming humans. A cell wall is present around the outside of the bacterial cell membrane and it is essential to the survival of bacteria. It is made from polysaccharides and peptides named peptidoglycan. There are broadly speaking two different types of cell wall in bacteria, called Grampositive and Gram-negative. The names originate from the reaction of cells to the Gram stain, a test long-employed for the classification of bacterial species. Gram-positive bacteria possess a thick cell wall containing many layers of peptidoglycan. In contrast, Gram-negative bacteria have a relatively thin cell wall consisting of a few layers of peptidoglycan.

Materials

• 2 Petri dishes with images of bacterial growth experiment

• 2 beakers of hydrogen peroxide (H2O2) working solution

• Silver casting pellets

• Silver powder

• 16 oz. squirt bottle of water

• Waste bucket

• The Sharper Image FresherLongerTM food storage container with Velcro loop “bacteria” inside

• Intact foam “silver” disk

• Divided foam “silver” disk

• 2 flipbooks of strawberry storage experiment

• Paper towels

Procedure:

Set-up:

1. Prepare 40 ml H2O2 working solution in each beaker by adding 20 ml tap water to 20 ml H2O2 stock solution. Lay out all other supplies.

Doing the demonstration:

1. Use the Petri dish images to introduce visitors to the concept that silver has antibacterial properties. Explain that the images are pictures of an experiment where bacteria were grown on plates with a center disk soaked in either a silver solution (silver nitrate) or water. The pale yellow area on the plates is E. coli growth. Ask visitors if they see a difference between the two images. The dark ring around the silver-soaked disk is a zone of inhibition where the silver prevented bacterial growth.

2. While silver’s antibacterial properties have been known for a long time, recent research has shown that nanoparticles of silver are more effective because given the same volume of small and large particles, smaller particles have greater surface area for chemical reaction. Drop a few silver casting pellets into one beaker of H2O2 working solution. Small oxygen bubbles will begin to form slowly as the silver surface catalyzes the breakdown of H2O2 into water and oxygen.

3. Add a few grains of silver powder (very little is needed to demonstrate the effect) to the second beaker of H2O2 working solution. Ask visitors to compare the two beakers to identify which one is producing oxygen bubbles at a faster rate. The silver powder grains have a higher surface to volume ratio than the silver sheet coating the penny, so a small amount creates a “fizzier” effect. Remove the silver-coated penny, rinse it off with water into the waste bucket, and dry with a paper towel for reuse. You can continue to add a few grains of silver powder to the second beaker each time you do the demo and empty the beaker at the end of the day.

4. Explain that one application of nano-silver is in food storage containers, where nano-silver is embedded in plastic to slow bacterial growth and keep food fresh longer. To demonstrate the principle, bring out the food storage container and attach the intact foam “silver” disk to the inside. Close the container and have a visitor shake it for 10 seconds. Open the container, remove the disk, and count how many “bacteria” (usually about 10-15) stuck to the disk as you detach them.

5. Next, attach the six pieces of the divided foam “silver” disk to the inside of the container. Close the lid and again have a visitor shake the box for 10 seconds. Open the container, remove each piece of the disk, and count how many total “bacteria” (usually around 30) stuck as you detach them. Ask visitors whether the large disk or smaller pieces were more effective – more “bacteria” stuck to the smaller pieces because there was more Velcro surface for the same size disk.

6. Tell visitors that the food storage container is a real product containing nano-silver, but that they can decide for themselves whether the product is effective based on the results of an experiment comparing strawberries stored in either a regular container or a nano-silver container. Have visitors flip through the flipbooks and try to guess which container is which before looking at the answer on the last page.

Clean-up:

1. Rinse silver casting pellets with water and dry for reuse; return to vial. Pour H2O2 working solutions down the drain and rinse beakers with water. Empty waste bucket down the drain.

2. Collect Velcro bacteria in Ziploc bag. Gather all supplies and return to storage.

Explanation:

The anti-bacterial properties of silver have long been recognized. In ancient times, people stored food and drink in silver containers to prevent spoiling; more newborn infants received silver drops in their eyes to fight infection. Silver interacts with bacteria via multiple targets, although the mechanisms are only partially understood: it reacts with sulfur groups in cell proteins, it induces structural changes to increase cell permeability, and once inside the cell, it binds to DNA. This broad range of results makes silver an effective anti-bacterial agent, since organisms are less likely to develop resistance when multiple simultaneous mutations are required. In addition, the bactericidal effects of both metallic silver and charged silver ions can occur at low concentrations of silver that are generally nontoxic to human cells. (Ingestion of too much silver can result in argyria, a permanent blue-gray discoloration of the skin and deep tissues.)

Recent research has focused on silver nanoparticles, which have enhanced reactivity due to a higher surface to volume ratio. The interaction of nano-silver with bacteria is size-dependent; nanoparticles less than 10 nm in size are more likely to attach to and penetrate the E. coli cell membrane. In addition, nanoparticles exhibit a multi-faceted surface structure (e.g. icosahedral or decahedral) that is different from and more active than larger bulk metal.

Nano-silver is being applied to many commercial products, including socks, washing machines, and band-aids, as well as food storage containers. In 2006, the U.S. Environmental Protection Agency declared that products containing silver nanoparticles and claiming anti-bacterial action would be regulated as pesticides. Subsequently, some companies, including The Sharper Image with its FresherLongerTM food containers, stopped advertising nano-silver content to avoid regulation. Compare early press releases that publicize the benefits of silver nanoparticles to current Sharper Image product literature that mentions only “specially treated” polypropylene. Regulation of nanoparticles in consumer products continues to be a controversial aspect of commercial nanotechnology. Critics of widespread nano-silver use suggest regulation is needed until potential health and environmental risks have been fully investigated, while proponents claim that existing research demonstrates that the threat is minimal.

What Could Go Wrong?

The silver coating on the pennies will wear off over time. Bubbles will appear more quickly on the penny as the coating wears off, but the silver powder should always produce a significantly faster reaction. Switch to a different penny when the coating has worn off to the point that it is no longer a convincing silver color.

Visitors can be very energetic in shaking the food storage container. Have them take two steps back before shaking to avoid hitting other visitors or knocking over the beakers of H2O2 working solution. However, very young visitors may not be able to shake the container vigorously enough to produce a dramatic difference in the number of “bacteria” that get stuck; you may want to give them some assistance.

Conclusion:

People have used silver for its antibacterial qualities for many centuries. However, Silver Nanoparticles have showed antibacterial activities more than silver. In addition to this various researchers have tried to enhance the antibacterial actions of Silver nanoparticles adopting various methods i.e. using capping agents while on synthesis, using a combination of light energy with nanoparticles, using a combination of ultrasound wave with nanoparticles, using a combination of electric field with nanoparticles etc.

References:

[1] Chen X and Schluesener H J 2008 Toxicol. Lett. 176 1-12

[2] Rai M, Yadav A and Gade A 2009 Biotechnol. Adv. 27 76-83

[3] Alt V, Bechert T, Steinrucke P, Wagener M, Seidel P, Dingeldein E, Domann E and Schnettler R 2004 Biomaterials 25 4383-4391





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