Chemical Education Today
Getting Students of All Ages Excited about Fluorescence I enjoyed the recent article by MacCormac, O'Brien, and O'Kennedy entitled “Classroom Activity Connections: Lessons from Fluorescence” (1). I think fluorescence is a great way to engage students in a topic that is, as the authors say, both “beautiful and fascinating”. I want to offer some comments to enhance their fine activity. Use of a yellow filter is an excellent way to extend the activity. It can be used to prove that fluorescent emission is different from the excitation light in that the yellow filter blocks the black light emission but passes the fluorescent emission. This demonstrates the so-called Stokes shift: that emitted light is usually of longer wavelength than the excitation light. Most transparent yellow plastic film will work well for this purpose, but my article cited by the authors (2) refers readers to a yellow filter material that is particularly good at blocking the emission from common black light bulbs peaked at around 375 nm. In fact, I was initially skeptical about the photo showing emission from green pepper in Figure 1 (and gracing the front cover of the issue), because the bright magenta color could be due to reflected light from the visible, violet component of the black light. We did confirm, using a fresh green pepper, that the beautiful magenta color is fluorescent emission because it is visible through a yellow filter that blocks the excitation wavelengths. Discussion of the effect of a yellow filter gives one more avenue into engaging the student's understanding of the concepts of light and its spectral components. I am skeptical that vitamin B-12 tablets mentioned in the article will give the fluorescent result described. I note that the supplementary material provided by the authors only mentions “vitamin B tablets”. Indeed, a vitamin B complex tablet that contains vitamin B-2 (riboflavin) will produce the spectacular yellow fluorescence mentioned in the article. In our testing it was not necessary to add vinegar to get excellent yellow emission from a crushed vitamin B complex tablet in water. We tested a vitamin B-12 tablet (containing cyanobalamin), and fluorescent emission was not readily apparent even after adjusting pH. Cyanobalamin is reported to be fluorescent but it absorbs and fluoresces in the UV, and the emission will not be visible to the human eye. Among the nonfood fluorescent materials, the authors mention stamps and minerals. There are minerals that respond very well to the longwave UV emission from common black lights, for example, the authors mention fluorite, which can be excellent under the black light. Aragonite and sphalerite are also specimens that can make a very good show under longwave, black light UV excitation. Many other minerals, including calcite, are much more impressive under shortwave UV excitation (254 nm), and postage stamps, to be sure those from the USA, generally respond much better to shortwave UV. I add a note about safety. Whenever working with lamps that emit some UV light, participants should always be instructed not to look directly at the light source, whether they are wearing eye protection or not, particularly when the person can get quite close to the lamp. In addition, for the kinds of UV light sources that are well matched to the activities described in ref 1 and similar articles (for example, a low-wattage, hand-held,
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black light; a 40-W, 48-in. black light; or a 4-W shortwave lamp), I recommend wearing polycarbonate lab glasses when working with these types of lamps. The polycarbonate lens material does a good job of blocking both UV wavelength ranges, and while shortwave UV is of extra concern regarding eye exposure safety, the human eye lens does fluoresce under longwave UV light giving a “foggy” appearance that detracts from the desired observations. Demonstrating that the polycarbonate lens acts as a UV filter is another point of engagement with the concepts of light and absorbance. Literature Cited 1. MacCormac, A.; O'Brien, E.; O'Kennedy, R. J. Chem. Educ. 2010, 87, 685–686. 2. Muyskens, M. J. Chem. Educ. 2006, 83, 765–768. Mark A. Muyskens* and Maurica S. Stewart Department of Chemistry and Biochemistry, Calvin College, Grand Rapids, Michigan 49546-4403, United States *
[email protected] DOI: 10.1021/ed1007124 Published on Web 12/16/2010
Fluorescence Using Turmeric The recent article describing the fluorescence of everyday items was fascinating (1): I had no idea so many everyday objects fluoresce. I have found an easy method to examine fluorescence without the use of UV light. It would be an interesting exercise to determine whether more everyday objects fluoresce when excited in the UV or in the visible. Turmeric contains the fluorescent compound (and antioxidant) curcumin and can be used in outreach activities from elementary education through high school. Turmeric is a spice readily available at grocery stores and a key ingredient in curry. Curcumin is excited with blue light. The appropriate blue light can be produced by using blue food coloring dissolved in water, Roscoe filters 384 (available from most music or theater supply stores for less than $7; see ref 2), or even transparent blue notebook covers (from office supply stores). Curcumin fluorescence is clearly visible and one can match the yellow color in another tube with yellow food coloring so that in visible light, both tubes appear the same. The signal is even bigger if the emission is also filtered and a solution of yellow food coloring (so concentrated it appears orange) or Roscoe filter 23 is used. The turmeric can also be spotted on nonfluorescent paper (not bleached coffee filters) and the spot identified by illumination with blue light. Several extensions are possible. Different food coloring can be placed in the first dish. If you choose to do this, it is probably more dramatic to start with an inappropriate color first and then to move to the blue. If students need practice reading transmission versus wavelength graphs, having them pursue the Roscoe Web site (2) to find the most appropriate sheets is a good exercise. (For those with good eyesight, a swatchbook of all the papers is also available; see ref 3).
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r 2010 American Chemical Society and Division of Chemical Education, Inc. pubs.acs.org/jchemeduc Vol. 88 No. 3 March 2011 10.1021/ed1007603 Published on Web 12/14/2010
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Journal of Chemical Education
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Chemical Education Today
Curcumin's fluorescence is environmentally sensitive, so its fluorescence in water is different than in rubbing alcohol. I have found it convenient to make a stock solution in rubbing alcohol and then dissolve the stock into the appropriate test solutions. Curcumin's fluorescence is also pH sensitive and this is another way to make pH a bit more interesting as the students can add such household acids or bases as vinegar, citric acid (available at some ethnic food stores), tartaric acid, lemon juice, baking soda, or toothpaste. Literature Cited 1. MacCormac, A.; O'Brien, E.; O'Kennedy, R. Classroom Activity Connections: Lessons from Fluorescence. J. Chem. Educ. 2010, 87, 685–686.
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Vol. 88 No. 3 March 2011
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2. Web Site for Roscoe Color Effects Filters. http://www.rosco.com/ us/filters/roscolux.cfm#colors (accessed Nov 2010). 3. Web Site for Acquiring a Roscoe Swatchbook. http://www.rosco. com/sbreqs/index.cfm (accessed Nov 2010). Mark A. Milanick Medical Pharmacology and Physiology, Dalton Cardiovascular Research Center, University of Missouri, Columbia, Missouri 65211, United States
[email protected] DOI: 10.1021/ed1007603 Published on Web 12/14/2010
pubs.acs.org/jchemeduc
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r 2010 American Chemical Society and Division of Chemical Education, Inc.
