GEORGE L. GILBERT Denison University Granville. Ohio 43023
A Convenient Lecture Demonstration of Fluorescence SUBMlTTED BY
Hugh D. Burrows University of ife Ile-lfe,Nigeria CHECKED BY
Darrell D. Axtell Eastern Montana College Billings, MT 59101
states often differs considerably from their ground state behavior. For example, the fluorescent SIstate of 2-naphthol (pK, 2.5-3.4) is a much stronger acid than the ground state (pK, 9.5).2 In view of the potential hazards of naphthols, quinine sulfate may be used instead. While the fluorescence is less dramatic, the materials used are very much safer. For a quinine sulfate demonstration. 10-20 me of auinine trihvdrate is dissolved in about 150 cm" of ethanol, and the solution is irradiated with the UV lamu. There is no sienificant fluoresrc.nrc i11 this caw: 11oww1.r.upon additiw~t ~ r ' i nc ~q ~ ~VI,IJIIIV al oidilutt suliurirwid. vale l h e tlucrre~cenved t h r i~rotwuucd quinine is ohserved. This can be compared with the fluorescence of the beverage tonic water, whose bitterness and lu-
In introducina molecular fluorescence in courses in vhotochemistry, spe&oscopy, or analytical chemistry i t is Galuable to have a demonstration showing clearly its characteristics, particularly that the emitted light is at longer wavelengths just occur in the blue region of the spectrum. Other readily than the absorbed light and that the fluorescence decays very available compounds whose aqueous solutions show emission rapidly when the source of excitation is removed. For a clear intense enough to be observed under these conditions include visual demonstration, it is worthwhile contrasting solutions eosin (yellow fluorescence), fluorescein (sodium salt, ereen giving a visible fluorescence with nonfluoresceut solutions or solutions whose fluorescence does not fall in the visible region . and, therefore, cannot be detected by the human eye. onstrates the way in which solvent polarity may affeEt fluoI have found that a particularly convenient way of showing rescence characteristics involves the use of the ammonium salt this is to use systems whose acidic and basic forms have difacid (ANS). This comof 8-anilino-l-naphthalenesulphonic ferent fluorescent characteristics. Such systems include the pound is only weakly fluorescent in water, hut shows a strong l-naphthols and 2-naphthols. (WARNING: See footnote lJ1 fluorescence in nonpolar solvents, and has consequently been In alkaline solutions the naphtholate anion emits around used as a probe for polarity, including that of binding sites in 400450 nm, while under acidic conditions the naphthol emits biological systems.5 A few milligrams of ANS are dissolved in a t about 350 nm.2 A demonstration suitable for a reasonable 150 c m h f water (CAUTION: Use plastic gloves).' Upon sized lecture room can he made bv dissolvine about 50 me of irradiation with the UV lamp there is little observable fluo2-naphthol (CAUTION: Use plastic gloves)' in 500 cmj of rescence. However, upon addition of a nonpolar solvent such 30% methanollwater in a tall glass beaker.VSolutions should as 1,4-dioxane, an attractive green fluorescence is ohserved, be freshly prepared as they tend to become colored on the intensity of which increases with decreasing polarity of standing.) The solution is then irradiated with a suitable ulthe dioxane-water mixture. traviolet lamp. (The UV lamps used for thin layer chromatography are particularly good for this, with the lamp mounted Acknowledgment vertically alongside the heaker.) Under acid conditions no The author is very grateful to the checker, Dr D. D. Axtell, emission is observed since the naphthol form is fluorescing in for his valuable comments on the original draft of the mauuthe ultraviolet region. However, upon addition of a few drops script and particularly for pointing out the potentially carciof sodium hydroxide solution, a clear blue fluorescence is nogenic nature of 2-naphthol. ohserved. The fluorescence is visible even in a fairlv well lit lecture room. On blacking out the exciting light-e.g., by vlacine a book in front of it-the fluorescence is seen to decav Warning! These compounds are all potential health hazards, both by skin contact and by ingestion, and great care should be exercised "inst&taneously." The fluorescence lifetime of 2-naphtholatk in their use. >-Naphthol, in particular, is a potential carcinogen and anion is. in fact. about 8 nsec4 Quite a vivid wav of demoushould only be handled in very small quantities. strating this emi'ssion is to add thesodium hydroxfde dropwise See, for example, Barltrop, J. A,, and Coyle, J. D., "Excited States from a burette while stirrina the solution with a maenetic in Organic Chemistry," Wiley, London, 1975, p. 50. stirrer. The anion luminescence is then observed in theform Methanol is present to stop the naphthol precipitating out in acid of blue fluorescent "ribbons." solution. Weller, A., 2. Phys. Chern. Neue Folge, 17, 224 (1958). See, for example, Stryer, L., J. Mol. BioL, 13, 482 (1965).
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228
Journal of Chemical Education
