Fluorescent “Gummy Worm” Polymers

Introduction

Fluorescent polymer gel looks like glowing gummy worms! Prepare “gummy worm” polymers containing three different types of dyes and observe their fluorescence when exposed to an ultraviolet (black) light.

Concepts

• Polymers
• Cross-linking
• Fluorescence

Materials

Safety Precautions

Dye solutions will easily stain hands and clothing; avoid contact of all chemicals with skin and clothing. Do not look directly at the black light; its high-energy output can be damaging to eyes. “Gummy worm” polymers are not edible—do not taste or ingest any materials in the lab. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Please review current Safety 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. Polymer gel products obtained in this activity may be disposed of in the trash according to Flinn Suggested Disposal Method #26a. Excess solutions may be rinsed down the drain with excess water according to Flinn Suggested Disposal Method #26b.

Prelab Preparation

Sodium Alginate Solution, 2%: Measure 2.0 g of sodium alginate into a 250-mL beaker or Erlenmeyer flask. Add 100 mL of distilled or deionized water and a stir bar. Stir on a magnetic stirrer for about one hour or until the solid dissolves. For best results, mix the dry polymer with water and allow the mixture to sit overnight to give a uniform solution. The solution will be thick and viscous.

Beaker 1: Pour approximately 25 mL of tonic water into one of the 100-mL beakers.
Beaker 2: Add 10–12 drops of 1% fluorescein solution to the second 100-mL beaker.
Beaker 3: Add 10–12 drops of 1% rhodamine B solution to the third 100-mL beaker.

Procedure

1. Add 25 mL of calcium chloride solution to Beaker 1 (see the Prelab Preparation section). Stir with a stirring rod.
2. Add 50 mL of calcium chloride solution to each Beakers 2 and 3. Stir each solution with a clean stirring rod.
3. Draw up a pipet-full of sodium alginate solution into a clean, jumbo pipet. Slowly squeeze the pipet bulb and add the sodium alginate solution in one continuous stream into Beaker 1.
4. Repeat step 4, adding sodium alginate into Beakers 2 and 3.
5. Wait about 1–2 minutes for products to form.
6. Turn off the classroom lights and obtain a black light.
7. Using forceps, grasp a polymer “worm” from Beaker 1. Turn on the black light and hold the “worm” up to the black light. Have students record all observations about color, texture and appearance of the “worm.”
8. Repeat step 7 with polymer “worms” from Beakers 2 and 3.

Student Worksheet PDF

12420_Student1.pdf

Teacher Tips

• This kit contains enough materials to perform the activity as written at least 25 times: 10 g of sodium alginate powder, 500 mL of calcium chloride solution, 30 mL of fluorescein solution, 30 mL of rhodamine B solution and 10 oz. of tonic water.
• This experiment may be done as a teacher demonstration or as a student activity. The sodium alginate solution must be prepared before class by the teacher. It takes at least two hours to form.
• The calcium alginate polymer gel obtained by adding sodium alginate to calcium chloride is nontoxic. Pass the polymer “worms” around the classroom to allow students to observe their texture and properties. Remind students however that the polymer “gummy worms” are NOT edible. Never taste or ingest any materials in the laboratory.
• A polymer gel is a type of colloidal solution consisting of a cross-linked polymer network swollen in a liquid medium such as water. The properties of a gel depend on the interaction of these two components. The liquid component, or solvent, prevents the polymer matrix from collapsing into a hard, insoluble mass. The polymer matrix helps to retain the solvent.
• The difference in the colors of the worms under the classroom lights compared to the black light is most obvious in a completely darkened room.
• The fluorescent intensity of the polymer “worms” will increase as they soak in the dye solutions.
• The calcium chloride/dye solutions may be reused and will last all day, even a whole week; however, some evaporation will occur. The solutions will keep for an extended period of time if the beakers are covered with Parafilm^®.
• The tonic water does not have to be carbonated for fluorescence to occur—it will still fluoresce even if it goes flat.
• As an extension, some interesting fluorescent color patterns will form if the worms are soaked (or dipped) in all three solutions for varying amounts of time. Have fun and experiment with combinations of the dye solutions.
• This activity may be extended by testing the solubility of the polymer in metal salt solutions. Sodium alginate will precipitate with most polyvalent metal ions and gives colored gels with transition metal cations such as Fe³⁺ or Co²⁺.
• Help students build connections between chemistry and food science by having them conduct a kitchen or grocery store search for foods containing sodium alginate or other alginate food additives.

Answers to Questions

1. Compare and contrast the “worms” from Beakers 1, 2 and 3 before and after they were exposed to the black light.

The colors of the worms before and after the black light was applied were as follows:

2. Describe the appearance, form and texture of the polymer “worms.”

The product seems to float in the calcium chloride–dye solutions and appears smooth, flexible, semi-solid, translucent and gel-like. The polymer “worms” have the consistency of gummy candy.

3. What causes the calcium alginate polymer “worms” to appear swollen and translucent?

The calcium alginate polymer is hydrophilic and thus readily absorbs water. Water causes the polymer to swell and to appear translucent.

4. Fluorescence occurs when a substance absorbs a photon from a light source. The energy from that photon causes an electron to move to an “excited” state (higher energy level). As that electron returns to its ground state, it releases another photon with a particular wavelength. Explain how this relates to the colorful glow that was seen when the worms fluoresced.

The glow is caused by the energy that is released by an “excited” electron returning from a high energy level to a lower energy level. If the photon that is released at this time has a wavelength that is within the visible spectrum, then we can see the colorful glow it causes.

5. Polymer solutions form solid gels when numerous long-chain polymer molecules interact to build a three-dimensional “network.” Explain how calcium ions bind alginate molecules together to form a network.

Calcium ions are divalent, with a +2 charge. Each metal cation can bind to at least two –CO₂^– groups via ionic bonds. If the two carboxylate groups are on different (adjacent) polymer molecules, then the effect of adding divalent cations is to tie together many different polymer molecules into a large, three-dimensional network. Note: Studies have found that the alginate polymer acts like a giant chelating ligand (similar to EDTA) and that each Ca²⁺ ion is bound to four –CO₂^– groups.

Discussion

Sodium Alginate Polymer

The word polymer is derived from two Greek words, polys (many) and meros (part). Polymers are large, chain-like molecules that contain many copies of one or two “repeating units,” called monomers, which have been joined together by a chemical reaction. It is not unusual to have thousands of monomer units in a single polymer molecule. Because of the enormous size of polymer molecules and the flexibility of polymer chains, many polymers have unique and useful properties. Polymers can be formed into fibers, drawn out into thin films, or molded into a variety of solid objects. Many polymers will swell up in contact with water to give gels, with properties that appear to be intermediate between those of a solid and a liquid. The properties of a polymer depend on the chemical nature of the monomer, the length of the polymer chain and how the monomers are joined together. Many biological molecules and materials, such as DNA, proteins, starch, cellulose and wood, are examples of natural polymers.

Sodium alginate is a natural polymer obtained from kelp and seaweed, brown algae belonging to the phylum Phaeophyta. The polymer is a principal component of the cell wall in brown algae, comprising up to 40% of the dry weight of large species such as giant kelp. Worldwide, about 25 million pounds of sodium alginate are produced each year for use in the food, textile, medical, and pharmaceutical industries. The giant kelp Macrocystic pyrifera is the principal source of sodium alginate harvested in the United States. This is the largest seaweed in the world, growing at a rate of 50 cm a day. A single attached plant can be as large as 65 m in length.

Sodium alginate is a polysaccharide composed of thousands of oxidized sugar “units” joined together to form an ionic polymer. The repeating units are six-membered rings containing negatively charged –CO₂^– groups. The C-1 carbon atom of one ring is connected via an oxygen atom to the C-4 carbon atom of the next ring in the polymer chain (see Figure 1).

The presence of ionic –CO₂^– side chains, as well as numerous –OH groups, make this natural polymer extremely hydrophilic or “water-loving.” The resulting solution is thick, viscous, and smooth. Sodium alginate is used as a thickening agent in many processed foods, including ice cream, yogurt, cheese products, cake mixes and artificial fruit snacks. The nontoxic food additive absorbs water, helps to emulsify oil and water components, and gives foods a smooth texture. Replacing the sodium ions in sodium alginate with calcium ions leads to cross-linking between the polymer chains and gives an insoluble gel, calcium alginate. Each Ca²⁺ ion can bind to at least two carboxylate groups in the polymer. If the two –CO₂^– groups are on different (adjacent) polymer molecules, then the effect of adding divalent cations is to tie together or cross-link individual polymer molecules into a large, three-dimensional network. The cross-linked polymer swells up in contact with water to form an insoluble gel. Studies have shown that the polymer behaves like a giant chelating ligand (similar to EDTA), and that each Ca²⁺ ion is bound to four –CO₂^– groups. Calcium alginate has a number of uses in the medical and pharmaceutical industries. It is used to make wound dressings, dental impression materials, as a radiography agent, and to diagnose and treat intestinal or gastric diseases.

Fluorescence

Luminescence is the emission of radiation (light) by a substance as a result of absorption of energy from photons, charged particles, or chemical change. It is a general term that includes fluorescence, phosphorescence, and chemiluminescence, to name just a few special types. Fluorescence is different from other types of luminescence in that is it restricted to phenomena in which the time interval between absorption and emission of energy is extremely short. Therefore, fluorescence only occurs in the presence of the exciting source. This is different from phosphorescence, which continues after the exciting source has been removed. In this activity, the exciting source is the UV black light.

In fluorescence, when a light source is shined on a material, a photon is absorbed. The energy from the photon is transferred to an electron that makes a transition to an excited electronic state. From this excited electronic state, the electron naturally wants to relax back down to the ground state. When it relaxes back down to the ground state, it emits a photon (symbolized by the squiggly arrow in Figure 2). This relaxation may occur in a single step or in a series of steps. If it occurs in a single step, the emitted photon will be the same wavelength as the exciting photon. If the relaxation occurs in a series of steps emitting a photon along the way, the emitted photon will have a greater wavelength (lower energy) than the exciting photon.

If the emitted photon’s wavelength is in the visible portion of the spectrum, we observe a colorful, glowing effect. Emission of this form is termed fluorescence. This process is practically instantaneous so the fluorescence is observed as soon as the exciting source is present, and it disappears as soon as the exciting source is removed. The fluorescent glow is brighter than the color of the solution seen under normal fluorescent lights because light is being emitted from the solution, not just transmitted through it.

References

Portions of this activity were adapted from Polymers, Flinn ChemTopic™ Labs, Volume 21; Cesa, I., Ed.; Flinn Scientific Inc.: Batavia, IL (2006).