The essence of fluorescence

Like a hammer, fluorescent pigments are very rarely used with subtlety. High-visibility jackets and garish nightclub interiors are, on the face of it, their sole purpose. The colors hit you like a smack in the face, and that basically is the point. But there is another side to fluorescence, a distinctly darker side.

Fluorescent pigments convert ultraviolet (UV) light into visible light. Since UV light is invisible, a dark room bathed in UV light remains dark. Unless, that is, there is something fluorescent in the room. This will spring into view, appearing to glow for all the world as if lit from within.

Trawling around a pitch-black house with a portable UV light is like scuba diving without getting wet. Opening a desk drawer reveals that some seemingly mundane staplers and pens have a hidden life – they fluoresce mysteriously as if members of a stationary disco cult. An occasional postage stamp also lights up. Whole libraries of books and magazines loom into view. This latter effect is the result of the blue fluorescent pigment in modern paper designed to make it whiter than white.

More of this pigment is to be found in the bedroom. In the darkness, you encounter squids and eels, which are really shirts and socks glowing blue-white, again because of a fluorescent pigment imparted by washing powder.

A trip to the bathroom becomes a dreadful shock; those clean tiled surfaces are suddenly, like in some ghastly TV advert for bleach, full of yellow and blue stains. By switching on the room light they disappear, but now you know that these fluorescent pigments from soaps and bacteria are all over the place. Looking at your hands you see more – perhaps they are stained vivid orange, a fluorescent pigment that can be traced back to the orange you peeled earlier in the evening. If you need a drink at this stage, a gin and tonic will do little to calm you down. It glows bright blue in UV light because of the quinine in the tonic water that also fluoresces.

Such a plunge into the UV requires a portable UV light. These are easy to obtain precisely because so many professionals rely on them. Gemologists tell a good diamond from a dud using a UV light: fluorescent diamonds appearing to be milky, because of the internal production of light, have lower value. In contrast, finding fluorescence in bank notes ensures value, since it is there as an antifraud device, as it is on passports, credit cards, and all manner of identification cards.

Now, just like a story line lifted from the pages of a Philip K. Dick novel, fluorescent millimeter-sized microdots have been produced that can be sprayed onto any object and are invisible except under UV light. These dots, produced by DataDot Technology, uniquely identify an object and are being used successfully by BMW and other manufacturers to prevent the theft of automobiles for their parts. If this becomes standard practice, UV lights and magnifying glasses are likely to become essential kit for mechanics in the near future.

The discovery of the jellyfish green fluorescent protein, or GFP, has turned fluorescence into a vital biological tool. The cells of plants and animals can now be genetically engineered to fluoresce under certain environmental conditions or in the presence of certain proteins. This means that the inner working of cells and tissues can be interrogated using a microscope attached to a UV light. The GFP blue-green fluorescence is instantly recognizable, and can be seen without the aid of a microscope in new GFP transgenic animals, such as the green fluorescent pigs recently bred in Taiwan. Such genetic messing about may sound bizarre, but nature visited the theme of fluorescent organisms billions of years ago: fluorescent scorpions being one of the most puzzling evolutionary relics around.

Another fluorescent molecule beloved of biologists and materials scientists is nile red. This molecule will fluoresce with a different color depending on the polarity of its environment. Such behavior qualifies it for ‘smart material’ status, and means that the evolution of functionality and disease in biological tissues can be mapped in color.

Whether it be from a stamp on a postcard from Canada, a piece of seemingly nondescript rock, or a fungus growing slowly on the bark of tree, fluorescence is everywhere. It is capable of producing the exuberant brightness of a fluorescent tube as well as the exquisite subtleties of phosphorescent algae. Its aesthetic charm derives not just from its color and brightness but also from its role as a secret chemical agent. This is the essence of fluorescence.