Drexel-SDP GK-12 ACTIVITY Subject Areas: Chemistry, Physical Science, Numbers & Operations, Problem Solving Activity Title: Color Switching Molecules the Magic of Fluorescence The green plastic tops of these grocery products appears nearly the same in room light, but examination under UV light reveals that one exhibits strong yellow fluorescence. 2008 Matthew D. Cathell Grade Level: 6 (6 8) Time Required: 60 minutes
Group Size: 4-5 students Expendable Cost per Group: <$5 Summary: Using common grocery store materials, students learn about the process of fluorescence. Students test various materials for fluorescent properties under an ultraviolet lamp (or black light ). This activity may accompany lessons on physics (light and sound) or astronomy, or whenever concepts of light and color are taught. Engineering Connection: Fluorescence is a process frequently used in science and engineering. A fluorescent molecule will glow (give off light) when it is struck by light of a different color. Often, UV light (which we cannot see) will cause the fluorescent molecule to glow with a visible color (which we can see). Fluorescent molecules are often used to track the movement of larger molecules in cells. Fluorescence is also used to measure the amount of a chemical present in a solution, and to monitor chemical reactions. Keywords: fluorescence, light, ultraviolet (UV) light, wavelength Educational Standards: Science: Math: Learning Objectives: After this lesson, students should be able to: Materials List: Each group needs: access to a UV lamp or black light (can be shared among groups) clear glass test-tubes or vials water, vinegar and/or acetone as solvents (depending on fluorescent material to be dissolved, see Safety Issues) To share with the entire class (to demonstrate fluorescence): a new $5, $10 and/or $20 bill postage stamps 2
water-soluble fluorescent dye (preferred), such as Atomic Glow dye from stevespanglerscience.com, or an organic-soluble dye such as fluorescein, Nile red, or other fluorescent dyes (see Safety Issues) tonic water laundry detergent (with and without bleach alternative) vitamin tablets Introduction / Motivation: See the student handout pages. Vocabulary / Definitions: Word absorption fluorescence emission Definition In the context of fluorescence, absorption refers to the light that the fluorescent molecule takes in. This absorbed light has a higher energy than the light that the fluorescent molecule gives out, or emits A process in which a molecule in an object gives off visible light of a higher wavelength (different color) than the light shining on it (often UV light) In the context of fluorescence, emission refers to the light that the fluorescent molecule gives out. The emitted light has a lower energy than the light that that fluorescent molecule took in, or absorbed Procedure: Background for the teacher: Fluorescence is a scientific topic that is frequently not discussed until the advanced high school or college level. However, the phenomenon of fluorescence is familiar to students of all ages (they ve all seen how their white sneakers and clothing glow under a black light). This activity is meant to introduce them to the most fundamental understanding of what fluorescence means, and why it is observed. This activity is best used when the students have been introduced to the concept of light as a wave of radiation. They should be aware that different colors of light have different wavelengths and different energies. They should also understand that what we see as visible light is actually only a small portion of a much larger spectrum that includes ultraviolet radiation, X-rays, infrared radiation, microwaves, and radio waves. 3
One critical idea that students should understand is that a fluorescent molecule takes in higher energy light and gives off lower energy light. This is because no process is 100% efficient, and some of the energy is lost to other non-fluorescent processes (like heat). This is a very important concept that will come up frequently in more advanced science classes. It also helps students understand why light bulbs get hot most of the electricity flowing through a normal incandescent light bulb is actually turned into heat, instead of light, giving incandescent bulbs poor efficiency (and making them rather wasteful of energy). A point of confusion for the students may arise from the fact that the UV lamp or black light gives off a violet-colored light. This violet light is not the UV radiation, which is invisible! It just happens that UV lamps also give off some visible light in the violet and blue regions of the color spectrum, along with ultraviolet radiation. This should be clarified for the students. Before the activity: The following handout pages contain information about fluorescence that can be printed for the students. They should read this information (either as a homework assignment or in class) before performing the activity. 4
NAME DATE What is it? The Facts About Fluorescence Chances are, you ve heard the words fluorescence or fluorescent. Maybe you ve used it to describe certain colors of crayons or markers, or a particular kind of light bulb. Fluorescence does relate to both color and light, but how? Fluorescence (be careful not to spell it flourescence) is a chemical process that happens when some chemical molecules interact with light. When light hits these fluorescent molecules, the molecules take in (or absorb) the light. The molecules become excited by the light (yes, that s the scientific way of describing it!) because they ve received the light s energy. These excited molecules then give off (or emit) that energy by glowing with light of a different color! For instance, a fluorescent molecule might take in blue light, but give off red light. However, a fluorescent molecule will not absorb red light and emit blue light. We ll talk about the reason why on the next page. Most molecules are not fluorescent and do not undergo the fluorescent process. For instance, plain water does not fluoresce. Fluorescent molecules typically have a chemical structure with lots of doubled-bonded atoms (making them stiffer and less bendable). What does color have to do with it? As you might know, light is a just a form of radiation that our eyes can see. We can think of light radiation as a kind of wave. Different colors of light have different sizes of wavelengths (the distance between the peaks or valleys of a wave). To the right is a model of the colors of the rainbow spectrum. See how the waves of violet light are closer together than the waves of red light? The closer and tighter the waves are, the more energy they have. This means that violet light has the most energy of any visible color, and red light has the least energy of any visible color. Our eyes are only able to see a very limited set of radiation waves (only the ones from the rainbow). If the wavelengths are too small or too big, we can no longer see them. We call waves that are smaller than those of violet light ultraviolet radiation. We call waves that are bigger than those of red light infrared radiation. Which one has more energy? 5
If you said ultraviolet radiation has more energy than infrared radiation, you re right! Remember, the smaller the waves, the higher the energy! Along with the colors of the rainbow, the sun gives off ultraviolet (or UV) radiation. Even though we can t see this high-energy radiation, it s what gives us sunburns! Ultraviolet radiation and fluorescence Many fluorescent molecules will absorb invisible ultraviolet light and turn it into visible light. For instance, the chemical quinine will take ultraviolet light and transform it into blue light. Fluorescence always involves taking in (absorbing) higher energy light and giving off (emitting) lower energy light. Now you understand why we said earlier that a fluorescent molecule cannot absorb red light and emit blue light. Red light has less energy than blue light. Again, fluorescent molecules must take in higher energy light than they give off. Fluorescence all around you As we said previously, most molecules do not fluoresce. However, there are still plenty of things we use every day that have fluorescent molecules inside. In this activity, we will be investigating some of common household products that contain fluorescent chemicals. Some fun fluorescent facts The word fluorescence is named after a mineral called fluorite, which happens to fluoresce. Many other minerals do, as well. A fluorescent light bulb is much more efficient than a regular incandescent bulb. This means that a fluorescent bulb uses less energy to make the same amount of light as an incandescent bulb. Most of the electricity flowing through an incandescent bulb gets turned into heat not light which is why incandescent bulbs get so hot! It only takes a fluorescent molecule 5 to 20 nanoseconds (a nanosecond is one billionth of a second, or 0.000000001 second) to absorb higher energy radiation and transform it into lower energy radiation. That s really fast! 6
With the students: 1. As a warm-up activity, you can give a demonstration of the hidden fluorescence in money and postage stamps. Rather than passing these materials around the classroom, have each group come to a demonstration table to observe! Slide some relatively new cash bills ($5 and up) under the UV light (the ones with larger, off-centered Presidents heads will always work). You should observe a differently-colored glowing strip in each bill. $5 blue $10 orange $20 green $50 yellow $100 red Postage stamps also frequently have fluorescent markings that can be seen in UV light. Ask the students to consider why these markings are placed on money and stamps (it s to hinder counterfeiting). 2. Next, have the students put some water in a small glass or vial. Have them confirm that the water does not fluoresce under the UV lamp. Contrast this with some tonic water. The tonic water will glow a bright blue color because of the presence of the fluorescent compound quinine (which gives tonic its bitter taste). If the tonic has been left open for a period of time to allow it to go flat, it will be undistinguishable from water until it is placed under UV light. Tonic water (left) contains the chemical quinine, which fluoresces under UV light, while plain water (right) exhibits no fluorescence. 2008 Matthew D. Cathell 7
3. Have the students look at vitamin tablets under UV light. They may notice that the tablets have fluorescent speckles. Vitamin pills can have a variety of fluorescent and non-fluorescent compounds packed together with a binder material. Have students investigate an unbroken tablet, one that has been broken in two pieces, and one that has been crushed into a fine powder. If a vitamin B-12 tablet is available, dissolve it in vinegar and observe the color of solution fluorescence. 4. Laundry detergents often contain fluorescent compounds. This is especially true of detergents with bleach alternative whitening agents. The fluorescent compounds generally are excited by UV light and emit violet-blue light. This has the effect of making white fabric appear even whiter. Have students compare two detergents (one with bleach alternative and one without) under the UV light. Both will likely fluoresce strongly. In order to differentiate between the two, it may be necessary to accurately dilute each with equal amounts of water. With enough dilution, the detergent with extra whitening agents will likely fluoresce noticeably more than detergent without whitening agents. Regular laundry detergent (left) and a detergent with additional whitening agents (right) have differing amounts of fluorescence, when equally diluted in water. 2008 Matthew D. Cathell 5. Finally, have students examine solutions made a fluorescent dye. A number of different dyes are available that fluoresce with different colors. These materials can be purchased in solution or in solid form. Some dyes are water-soluble, while other dissolve in organic solvents like acetone (follow the instructions with your particular compound, see Safety Issues). Have the students observe the dye solutions under the UV light. If the solutions are concentrated enough, they will have noticeable colors even in regular room lighting, and will change to a different color under UV light. The solutions can also be diluted until they appear colorless in room lighting, but still fluoresce under UV light. 8
Dilute solutions of fluorescent dyes fluorescein (left) and Nile red (right) are indistinguishable in room light (top), but fluoresce with different colors under UV light (bottom) 2008 Matthew D. Cathell Safety Issues: ***SPECIAL NOTE IF ORGANIC-SOLUBLE FLUORESCENT DYES ARE USED*** Several commonly available fluorescent dyes, including fluorescein and Nile red, have poor solubility in water. These dyes work best in solvents like acetone. If organic-soluble dyes are used in this activity, it is more appropriate as a demonstration rather than an investigation performed by the students. Organic solvents can cause serious injury, particularly if splashed into the eyes. Even if students are not performing the experiment themselves, extreme care must be taken to ensure safety of both students and instructor. Strict adherence to safety eyewear is essential. Water-soluble dye is greatly preferred for this experiment. Students should also be reminded to never taste any of the materials, and to promptly wash their hands after handling the chemicals. 9
Most battery-powered black lights do not emit dangerous amounts of ultraviolet light. However, more powerful high intensity UV lamps (for laboratory use) can cause eye injuries to students if they gaze directly into the light. Students should be instructed to not look directly at the light source, but rather, to simply shine the light on their liquid samples and observe the effects. Students should be instructed that the vitamin tablets are to be used to observe fluorescence and are not to be consumed. Troubleshooting Tips: Depending on the strength of your UV light source, it may be necessary to dim the room lights in order to best observe fluorescence. If you have solid fluorescent powders, note that a tiny amount goes a long way (a few specks of powder can easily dye an entire bottle of solvent). Assessment: The following questions may be used to assess student understanding of fluorescence and this activity. 1. A fluorescent compound glows a yellow color in ultraviolet light. Which one of the following is FALSE? a. the yellow light being emitted is of a lower energy than the light exciting the molecule b. the compound is being excited by red or orange light c. the fluorescent compound probably has double bonds and a rigid chemical structure d. the fluorescence cycle is taking 5 to 20 nanoseconds Answer: b. The compound is not being excited by red or orange light, which are less energetic than yellow light. Fluorescence only occurs when higher energy radiation is absorbed and lower energy radiation is emitted. 2. What is the difference between the sizes of waves of the rainbow of colors (red, orange, yellow, green, blue and violet). Which has the longest waves, and which has the shortest? 3. What negative effect can ultraviolet radiation have on the human body? 10
Activity Extensions: Students can use fluorescent solutions (such as tonic water) to paint designs on paper. When the tonic dries, the hidden designs can be revealed under UV light. Be aware that some papers have fluorescent compounds themselves, which make the paper appear whiter. This activity would work well with brown paper bags. Have the students research how flowers and other plants use fluorescence to their advantage. Other: A water soluble dye for this activity can be obtained at http://www.stevespanglerscience.com/product/atomic-glow Author: Matthew D. Cathell Owner: Drexel University GK-12 program, Engineering as a Contextual Vehicle for Science and Mathematics Education, supported in part by National Science Foundation Award No. DGE-0538476 Copyright: 2008 Drexel University GK-12 program 11