Weird Science

Chemiluminescence of Luminol in Organic Solvents

Polish ver­sion is here

The fol­lo­wing article was ori­gi­nally publi­shed in the jour­nal for edu­ca­tors Che­mia w Szkole (eng. Che­mi­stry in School) (5/2024):

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Ples M., Che­mi­lu­mi­ne­scen­cja lumi­nolu w roz­pusz­czal­niku orga­nicz­nym (eng. Che­mi­lu­mi­ne­scence of Lumi­nol in Orga­nic Solvents), Che­mia w Szkole (eng. Che­mi­stry in School), 5 (2024), pp. 28–29

Che­mi­cal Fla­sh­li­ght

Che­mi­lu­mi­ne­scence is a fasci­na­ting phe­no­me­non in which cer­tain sub­stan­ces emit light as a result of che­mi­cal reac­tions. Unlike flu­o­re­scence and pho­spho­re­scen­ce­—where light is emit­ted after absor­bing energy from an exter­nal source (for instance, when expo­sed to ultra­vio­let light)—che­mi­lu­mi­ne­scence occurs direc­tly due to che­mi­cal pro­ces­ses. It can invo­lve a variety of mecha­ni­sms, but gene­rally requ­i­res oxi­da­tion of a che­mi­cal com­po­und.

Che­mi­lu­mi­ne­scence invo­lves com­po­unds known as lumi­no­pho­res (or more pre­ci­sely, che­mi­lu­mi­no­pho­res) that can emit light when moving from a higher energy level to a lower one. This pro­cess often takes place in an alka­line envi­ron­ment and in the pre­sence of oxi­di­zing agents. The gene­ra­tion of light is asso­cia­ted with the reor­ga­ni­za­tion of elec­trons in the mole­cule, resul­ting in the emis­sion of radiant energy to the sur­ro­un­dings.

Che­mi­lu­mi­ne­scence natu­rally occurs in many orga­ni­sms. A well-known exam­ple invo­lves fire­flie­s—be­e­tles that inc­lude spe­cies found in Poland, such as the com­mon glow-worm Lam­py­ris noc­ti­luca, Phau­sis splen­di­dula, and Pho­spha­e­nus hemip­te­rus [1]. These insects are reco­gni­zed for their abi­lity to glow in the dark, using che­mi­lu­mi­ne­scence mainly to attract mates. Their glo­wing abi­lity results from the oxi­da­tion of luci­fe­rin by the enzyme luci­fe­rase. Other exam­ples inc­lude cer­tain marine bac­te­ria such as Alii­vi­brio fischeri (for­merly Vibrio fischeri), which emit blue light. This is rela­ted to the intri­gu­ing quo­rum-sen­sing phe­no­me­non, whe­reby strong bio­lu­mi­ne­scence occurs not in sin­gle bac­te­ria but only when they gather in lar­ger gro­up­s—one could say the bac­te­ria “re­co­gnize” each other [2].

Che­mi­lu­mi­ne­scence has been applied in many areas of tech­no­logy and science:

One of the most fre­qu­en­tly uti­li­zed che­mi­lu­mi­ne­scent sub­stan­ces in edu­ca­tion is lumi­nol, men­tio­ned above. Typi­cally, expe­ri­ments with lumi­nol are per­for­med in an aqu­e­ous envi­ron­ment, but che­mi­lu­mi­ne­scence is actu­ally much more effi­cient in solvents other than wate­r—as rea­ders can disco­ver by con­duc­ting the rele­vant expe­ri­ment.

What Do We Need?

The fol­lo­wing are requ­i­red for the expe­ri­ment:

Dime­thyl sul­fo­xide is a color­less liquid with a faint, some­what fish-like odor. It is used in che­mi­cal labo­ra­to­ries and in medi­cine, where it assi­sts the pene­tra­tion of other sub­stan­ces thro­ugh the skin. DMSO is also known for its strong solvent pro­per­ties, which make it an excel­lent medium for dis­so­lving many che­mi­cal com­po­unds.

Lumi­nol is a yel­low cry­stal­line solid (Photo 1) that, when oxi­di­zed, emits a cha­rac­te­ri­stic blue (in water) or blue-green (in other solvents) light.

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Photo 1 – Lumi­nol

Keep in mind that potas­sium hydro­xide is highly cau­stic. Lumi­nol may be an aller­gen and is suspec­ted of having car­ci­no­ge­nic pro­per­ties. Avoid con­tact with eyes and skin, and use appro­priate per­so­nal pro­tec­tive equ­ip­ment.

Potas­sium hydro­xide can be repla­ced with sodium hydro­xide (NaOH), and dime­thyl sul­fo­xide can be repla­ced with dime­thyl­for­ma­mide (DMF), altho­ugh the effect might be wea­ker. It is cru­cial that all sub­stan­ces and the con­ta­i­ner are com­ple­tely dry, because even a trace of moi­sture can thwart the expec­ted out­come.

Lumi­nol, or 3-ami­no­ph­thal­hy­dra­zide (Fig. 1), is rela­ti­vely expen­sive, but only a very small amo­unt is nee­ded for the reac­tion, mea­ning that even half a gram (approx. 0.02 oz) is eno­ugh for many trials. Howe­ver, any­one with some deter­mi­na­tion can syn­the­size it them­se­lves. In one of the pre­vious issues of Che­mia w Szkole, I descri­bed my own com­pre­hen­sive method for pro­du­cing this valu­a­ble sub­stance, star­ting from waste in the form of dispo­sa­ble labo­ra­tory glo­ves [3].

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Fig. 1 – Struc­tu­ral for­mula of lumi­nol

Expe­ri­ment

Place 25 cm3 (approx. 0.85 fl oz) of dime­thyl sul­fo­xide, 0.5 g (approx. 0.018 oz) of potas­sium hydro­xide, and a small amo­unt of lumi­nol into a flask. Vigo­ro­u­sly shake the mixture, and after a few dozen seconds it begins to emit a bri­ght glow (Photo 2).

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Photo 2 – Che­mi­lu­mi­ne­scence

The emit­ted light is blue-green, and it can last for seve­ral hours. Its bri­ght­ness is suf­fi­cient to read prin­ted text (Photo 3).

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Photo 3 – Che­mi­cal fla­sh­li­ght

I must admit that con­duc­ting this reac­tion in a small flask (espe­cially a volu­me­tric one) can easily evoke an asso­cia­tion with the phial gifted by Gala­driel to Frodo, said to con­tain the light of the star Eären­dil [4].

Expla­na­tion

The glow of the solu­tion is due to the oxi­da­tion of lumi­nol in an alka­line medium. When the reac­tion is per­for­med in a water-con­ta­i­ning envi­ron­ment, water acts as an inhi­bi­tor, so a strong oxi­di­zer (e.g., H2O2 or KMnO4) plus sui­ta­ble cata­ly­sts are requ­i­red. The quan­tum yield of che­mi­lu­mi­ne­scence in water is 10-3, mea­ning only one in every tho­u­sand lumi­nol mole­cu­les emits light; the rest of the energy is rele­a­sed as heat. In orga­nic solvents such as DMSO, the effi­ciency of this reac­tion is signi­fi­can­tly higher, and it can even pro­ceed under atmo­sphe­ric oxy­gen without the need for addi­tio­nal cata­ly­sts.

Refe­ren­ces

All pho­to­gra­phs and illu­stra­tions were cre­a­ted by the author.

The above text inc­lu­des minor edi­to­rial modi­fi­ca­tions com­pa­red to the ver­sion publi­shed in the jour­nal, aimed at sup­ple­men­ting and adap­ting it for online pre­sen­ta­tion.

Adden­dum

The result of the expe­ri­ment can be seen in the video:

Marek Ples

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