Photochemistry for an Oscar? In the movie Avatar, it plays a central role, although I must admit I didn’t see it listed in the credits… The movie is set on a planet called Pandora, and at night time, the forests of Pandora light up to give some really beautiful cinema, all in 3D! This article explains the glow in the trees, insects, inhabitants and just about everything else on Pandora at night time. Back on Earth, we’re familiar with this glow too!
When some chemicals react, they can give off or require huge amounts of energy in doing so, as existing bonds are ripped apart and new ones form. This energy is in the form of heat – reactions can give off heat (exothermic) or require heat to proceed (endothermic). More unusually, the can give off large amounts of energy in the form of light. This light is called chemiluminescence. In photochemistry, we are usually concerned with providing molecules with light to activate a reaction. With chemiluminescence, it’s the other way around – a chemical reaction results in the emission of light. The classic demonstration of chemiluminescence is with a compound called, appropriately enough, luminol. Here’s a short Youtube video on it (with a rather excited chemist).
So what is happening?
Let’s look closer at the luminol reaction. When hydrogen peroxide (e.g. from household bleach) is added to luminol, in the presence of base and a catalyst (such as iron(III) which gets involved in the oxidation), 3-aminophthalate is formed. But the energy involved in the oxidation of luminol by the peroxide results in the phthalate having an electronically excited state. The releases this excess energy by emission of light, giving the blue colour observed.
Applications of chemiluminescence
One of the most common observations of chemiluminescence, as any inhabitant of Pandora will know, is bio-chemiluminescence, or bioluminescence, which is where natural world has exploited the use of chemiluminescence. The most commonly known example of this is the firefly (Photinus), which uses a reaction similar to that of luminol, and an enzyme, luciferase, in place of the peroxide, along with magnesium ions to produce a glow (the colour depends on the type of fly).
As well as Pandora, back on Earth, Irish swimmers came across some beautiful examples of bioluminescence off the coast at Killiney when “spectacular green neon flashes” in the sea were observed by swimmers as they swam through water. This was determined to be the plankton Noctiluca scintillans, which is reported to be known as “Sea Ghost” or “Fire of Sea”.
Given that emission spectroscopy is such a versatile analytical tool, it is perhaps no surprise that chemiluminescence has several potential applications in the area of chemical analysis. The intensity of luminescence is proportional the the concentration of reactant. In principle, analysis does not need the same level of instrumentation as emission spectroscopy – which needs a light source to excite the sample and emission my be detectable by eye. Therefore it can be used in crude analytical tests. Luminol is used to test for blood at crimescenes – a luminol spray on any suspected blood traces results in the iron in the blood catalysing luminol chemiluminescence, and glows for up to a minute after being sprayed.
But there is more scope for its use. Two problems to its adoption as an analytical technique are that the quantum yield of emission can be low, which means that at low concentrations, the detection may be difficult. In addition to potentially poor sensitivity, long lived emission (such as those observed in glow sticks and luminol), which makes for great demonstrations, means that response time is unnecessarily long. Some work on both of these areas is advancing. Coupling the chemiluminescence interactions with metal nanoparticles harnesses the surface plasmon resonance effect, where the emission from the chemiluminescence couples or resonates with the electron density of the nanoparticles, which enhance the signal, with reports of a 4 – 10 fold increase. The efficient transfer of energy from the excited state of the reagent to the nanorparticles also significantly reduces their lifetime, hence the lingering glow. Readers interested in this work are referred to Aslan and Geddes, given below.
This being said, chemiluminescence is already in use to study a wide range of medicinal and environmental-related compounds in a technique that couples chemiluminescence with liquid chromatography (HPLC-CL). The reagents used in this technique include the now familiar luminol, which is used to investigate lipid hydroperoxides, neurotransmitters such as dopamine (which enhance the chemiluminescence), and environmentally relevant species such as organophosphorus reagents (e.g. diclorvos). The always familiar Ru(bpy)32+, which as well as everything else can exhibit chemiluminescence, undergoing reduction by analytes in high energy electron transfer reactions to produce the excited state. This system has been used to study nitrosamines, N-methylcarbamates pesticides in pear and apple samples and domoic acid which can be a factor responsible for shellfish poisoning. There are several set-ups possible for interfacing the chemiluminescence set-up with the HPLC; most simply by having an injection point after separation for the chemiluminescent reagent prior to the emission detector. A very detailed review of the use of chemiluminescence in medicinal, food and environmental analytes is that by Gámiz-Gracia et al.
Finally, it is worth noting that a lot of gas-phase reactions result in chemiluminescence. This is the basis of gas analysers, for example the nitrogen monoxide analyser. Nitrogen monoxide reacts with ozone to produce an excited state nitrogen dioxide which emits in the far visible/infrared region. The extent of luminescence can be related tot he initial concentration of NO.
Your very own magical world
If you want to set up your own version of Pandora, which cost James Cameron $250M, you can do it for a few euros, by buying some glow sticks and dotting them around. Their glow lasts for a few hours, so all you need is a little imagination during this time…
“Lights in Sea are Natural“, Irish Times, www.irishtimes.com, 18 October 2009
K. Aslan and C. D. Geddes, “Metal-enhanced chemiluminescence: advanced chemiluminescence concepts for 21st century“, Chem. Soc. Rev., 2009, 2556 – 2564.
Laura Gámiz-Gracia, Ana M. García-Campana, José F. Huertas-Pérez, Francisco J. Lara, “Chemiluminescence detection in liquid chromatography: Applications to clinical, pharmaceutical, environmental and food analysis—A review“, Anal. Chim. Acta, 2009, 640, 7 – 28.