1. IYNT 2017 NANJING Team BULGARIA
Problem 8
Tonic water in UV light
Lachezar Dimitrov
BULGARIA

2. The problem
Tonic water glows brightly when exposed to an ultraviolet black light bulb. It is however easy to quench the glow of tonic water by adding salt. Investigate this effect. What other common substances glow under UV light and how can one influence their glow?
We will see why tonic water glows under UV light, how this is stopped and which other common items exhibit this property.
With this presentation, we aim to explain why tonic glows on ultra-violet light and how this property is stopped. We've made measurements to see how different factors affect this property.

3. Why Does it glow?
Tonic water contains quinine fluorescent alkaloid. Its presence is due the bitter lemon like flavor it gives to the beverage. H
Tonic water exhibits an interesting property- it glows brightly blue when exposed to UV light. This phenomenon is called fluorescence and we will look at this effect in details .Why it exhibits this property? There is an alkaloid in the tonic water called quinine that gives the bitter taste of the beverage. It is also used in medicine to treat malaria and babesiosis but in high doses is associated with ear problems. The main source for it is the cinchona officinalis bark, a flowering plant in the tropic forests. As you can see quinine contains amine nitrogen which makes it an organic base so it can exist in protonated and unprotonated form. Only the protonated form is fluorescent. As tonic water is acidic because of the citric acid added in it we have intensely fluorescent solution.
Quinine ion
Tonic water
Tonic water under UV
Cinchona officinalis
Toxicologycentre.com

4. Introduction to fluorescence
Fluorescence is emission of visible light by chemical that absorbs electromagnetic radiation. Such a chemical is called fluorophore. Every fluorescent molecule has a wavelength spectrum in which it absorbs and emits light.
Fluorescence is emission of light by a substance that has absorbed electromagnetic radiation. [Fluorescence fundamentals] UV light excites electrons in the fluorescent molecule. Electrons absorb high-energy photons and move on higher atomic orbitals, so called "excited levels". These levels are not stable and electrons come back to the ground state. This process is relaxation and then energy is released by emitting a photon. The energy of the re-emitted photons is lower and the light spectrum is different- observe it as visible light. Fluorophores usually contain several aromatic groups (like quinine). Every single fluorophore has determined spectra for excitation and for fluorescence. For example quinine is excited with 350nm wavelength and emits at 450nm. [Quinine spectrum]On this picture absorption and emission spectrums for quinine are shown.
Quinine fluorescent spectrum

5. Quenching
Quenching is the process of blocking fluorescence of fluorophores. Quinine is quenched by collisional quenching mechanism in which excited fluorophore gives its energy to the quencher particle which relaxes by giving off heat.
quenched original
Quenching is a process that decreases or stops entirely fluorescence of a given substance. There are a lot mechanisms but we will look at this that quench quinine - collisional quenching. The quencher, which is molecule or ion, makes complex with the fluorophore and absorbs the energy from its excited form, but after this relaxes non-radiatively. [Quenched tonic water]This happens when we add salt to the tonic, because chloride ions are good quencher for quinine. We made a video of tonic water has been quenched with sodium chloride solution under UV light.
Tonic water quenched with chloride ions

6. Sodium chloride solution
Tonic water

7. SET-UP In our experiments we used: -tonic -table salt
-sulfuric acid and sodium hydroxide
-potassium iodide
-vitamin b3 supplement
-highlighters
-UV lamp and spectrophotometer
These are all things we used for our experimental part.

8. Measurements
The environment is critical for quinine fluorescence. As the solution goes more alkaline the fluorescence intensity decreases.
[Fluorescence graphic base neutral acidic]We have made measurements to determine the intensity of tonic fluorescence in a different environment - acidic, basic and neutral. The values shown are non-dimensional, all samples are exited with in 350nm, and the emitted light is most intense just over 450nm but we showed the full emission spectrum. We can see that the sample with acid being added is more fluorescent than the others. The original tonic is less acidic and glows less intense, the neutral sample moreover and the basic one doesn’t glow at all.

9. Original tonic and quencher
Original tonic and quencher. Very little amount of chloride ions can stop the fluorescence almost entirely.
Now we will see how chloride ions affect the fluorescence. [Graphic chloride-original comparison] In this graphic we compare the intensity of original tonic with those of a sample with 0.1 mol sodium chloride per liter sample. This is 0.1M concentration. At wavelength of 457nm where the sample emits most light it is 17 times less intense with quencher, compared to the original tonic.

10. Comparison between samples with different quencher concentration
Comparison between samples with different quencher concentration. As quencher concentration increases the fluorescence intensity decreases.
Now we will compare tonic samples with different amounts of quenchers. [Graphic tonic water different conc. NaCl] As you can see fluorescence intensity decreases upon increasing quencher concentration. This is because the more chloride ions are in the solution, the more likely is excited quinine molecule to give its energy to them instead of emitting light.

11. Area below the curve vs. the molar concentration of NaCl
Now we will compare tonic samples with different amounts of quenchers. [Graphic tonic water different conc. NaCl] As you can see fluorescence intensity decreases upon increasing quencher concentration. This is because the more chloride ions are in the solution, the more likely is excited quinine molecule to give its energy to them instead of emitting light.
There is a clear power correlation between them

12. There are the samples we used. Only acidic and original tonic fluoresce.
acid added original neutral basic chloride ions different concentrations
We see the samples we used for measurements. [Picture of the samples] You can see that only the acidic and the original solution fluoresce because the others have very low light intensity to be observed.

13. Other fluorescent materials
Highlighters are common fluorescent items. They can contain two different fluorophores- fluorescein and pyranine. We will find out which one is present in our fluorescent ink by trying quenchers for each of them.
Highlighters show very nice fluorescence so they are good object for our research. The yellow ones may contain fluorescein or pyranine fluorophore. We want to know which one is used so we found that iodide ions are good quenchers for fluorescein and amines like niacin (which is vitaminb3) can quench pyranine.
Highlighter under normal light and UV
Extract from fluorescent marker

14. Highlighter ink quenching
We added two different quenchers for each fluorophore that may be in the marker extract – niacin (vitamin b3) for pyranine and iodide ions for fluorescein.
Iodide ions quench this ink but niacin doesn’t. This shows that fluorescein is used as fluorescent dye.
We mixed samples of fluorescent ink with iodide and niacin to see which one will have an effect. [Ink extract b3 iodide] As you can see the sample with iodide doesn’t glow at all. The one with vitamin b3 fluoresce a little bit orange, because the vitamin slightly changes the color but it is important to see that no quenching occurs. Now we know that the ink we used contains only fluorescein fluorophore that is quenched by the iodide but not by the vitamin b3.
iodide ions niacin control sample

15. Other fluorescent materials
reflective vest
notebook
There is a couple of domestic fluorescent materials we can see in our homes. Some of them are shown. [Fluorescent materials]
highlighter ink
household detergent

16. Conclusion
As conclusion we can say that quinine fluorescence is sensitive property that can be easily affected by environment and presence of quencher. We saw that by increasing quencher's concentration the fluorescence intensity decreases and decreasing the pH makes tonic more fluorescence. We found that the fluorophore in our markers is fluorescein that is quenched by iodide but not by vitamin b3.

17. Thank you for your attention! 謝謝