Green Light to Medicine! Fluorescence: from a Mysterious Phenomenon to Medication
For a long time people didn’t comprehend such unusual phenomena as lights appearing in the sky and self-glowing lighting of some minerals, insects and plants. Currently we know how it works and furthermore, how one can induce and control this unstable process. Read about fluorescence which was firstly marked during an experiment with rotten fish and eventually turned into a tool for treatment.
Humans started making the first discoveries of this phenomena many years ago. The great philosopher Aristotle, who lived in Ancient Greece, studied fluorescence when observing a process of fish rotting. More than 1,000 years ago Chinese sages already knew about minerals that could glow in the dark — back then non-organic luminophores (a chemical matter, which turns adsorbed energy into light) were discovered.
In 1555 Swiss scholar Konrad Jensen wrote a manual called "Lunariae," in which defined plants that glow at night. Then in 1570 Nicolás Monardes, a Spanish physician, wrote about unusual peculiarities of crassula tincture, which becomes golden yellow when affected by transmitted light, and blue in reflective light. In the 17th century Athanasius Kircher in his book "The Great Art of Light and Shadow" described glowing water in a goblet made of crassula.
A significant breakthrough in fluorescence research was made in the 19th century when John Herschel discovered that quinine affected by ultraviolet radiation can glow. It is known that quinine is used for malaria treatment during epidemics that spread in many countries in the colonial war period. A solution consisting of water and quinine was named tonic water. Now one can easily find it at a drugstore. It is far easier to dissolve quinine in alcohol than in water. This chemical peculiarity is likely to explain how gin and tonic appeared. This cocktail was popular in England thanks to its prophylactic effect.
The main secret of fluorescence phenomenon was made by George Gabriel Stokes in 1852. The scientist proved that a fluorescent material absorbs light with some wave-length (colored in a certain hue) and then "transforms" it into a light with another wave-length characteristics and color. He made the following experiment: put a piece of blue glass, which transmits only blue light, a cup of tonic and a cup of white wine in a line. When looking at sun light through the wine, tonic and glass, it seems that the glass of tonic glows. If one pours water instead of tonic one will not see any light.
A rule made by the scientist was later named after him — according to the Stokes' Theorem, the photo luminescent wave length is longer than a wave length of excitation light. In other words, the energy of luminescent photons is always less intensive than the energy of excitation light photons.
In his manual, "Light: Its Causes and Its Effects," Antoine Becquerel classified fluorescence: he defined such phenomena as phosphorescence generated by heating (diamond, fluorite), tribo-phosphorescence (pressure, breakup), electric phosphorescence, spontaneous bio-phosphorescence (is provoked by chemical changes in biological objects) and photo- phosphorescence.
Thanks to quantum theory and optics we have enhanced our knowledge in this field since Stokes' and Becquerel’s time. As of now, fluorescence is more than a natural phenomenon — it is used in science, medicine and everyday life. From the mid-1950s we started widely using such things as fluorescent tubes, radio detectors, computer and TV screens, shop signs, road signs, watermarks, lasers and various sensors.
It is well-known that many biological objects are fluorescent. One can easily observe how some of them glow. For instance, Vaadhoo — one of the islands of the Maldives — is famous for fluorescent plankton, which can be seen on beaches at night. Other biological objects have hardly visible light, as we cannot see it without special devices. Do you know that leaves of some plants have chlorophyll that fluoresces with red color?
Modern devices and methods allow us to detect and measure fluorescent parameters of cell organelles, molecules and even their parts. It gives an opportunity to receive unique information about molecular organization and activities of biological systems, compare signals from one segment and look for reasons of various defects.
Today more and more optical research methods are invented like fluorescent agents that are crucially important for this field.
These agents are divided into two categories — organic and non-organic fluorophores. Organic ones are compounds of a studied biological object and also those that were implemented into a system from outside. Practically, some "in vitro" processes are based on fluorescein which is used to color something. However, it cannot go through a cytoplasmic membrane without any assistance. One can transfer it into a cell by using fluorescein diacetate which doesn’t fluoresce but penetrates into a hydrophobia membrane’s barrier. Esterase, a hydrolase enzyme, eliminates acetyl groups within the cell. As a result, fluorescein "comes" into cells.
Apart from organic ones there are also nonorganic fluorophores. The discovery of quantum effect in semiconducting nano-scale particles helped in vitro diagnostics become more popular. These nanoparticles (or quantum dots) consist of pairs of elements — III/V (CdS, CdSe) or II/VI groups (InAs). These particles have unique peculiarities: the width of forbidden band gap of nanoparticles depends on their size. It means that the colors of emergent light vary depending on size of particles of one matter. For instance, lighting of "large" clusters is related to the red band while a smaller particle has a spectrum which is closer to the blue one.
One can apply quantum dots to visualization of tissues and also to anticancer therapy: due to high-energy emission, quantum dots produce electrons that destroy cancer cells by ionizing water surrounding them. Cancer cells are less viable than healthy ones; that is why they die first.
However quantum dots cannot work as panacea. Unfortunately, it is an illusion that their small size makes them able to penetrate into all biological barriers. In fact, they are toxic, which means that they cannot be used for the purposes of in-vitro diagnosis and therapy. Many scientists try to develop safe coverage for quantum dots but it makes them less effective. Furthermore, this coverage dissolves in organisms, and toxic elements become uncovered again.
However modern science is making progress. There are other agents that encourage attention researchers — bioconjugates. These are groups of organic compounds and atoms of transition metals like lanthanoide ions, gold and silver’s atomic clusters. One of the features of such compounds is sensibilized fluorescence, which is a process when their organic part absorbs light energy, and nonorganic part produces emerging light. The fluorescence of such conjugates is much more in comparison with a "common" fluorescence of non-organic fluorophore ions. This peculiarity is very important for cell research. Previously it was mentioned that many bio objects have such phenomena as luminescence but its duration is less than luminescence of non-organic complexes, as one can register this phenomenon without any drawback.
The other, completely different, type of sensibilizers is non-organic crystal matrices. They include ions that activate luminescence. Their main benefit is features of the matrix consisting of transparent zirconium or hafnium oxide. Unlike organic elements, these matrices are resistant to chemical interaction with bio molecules of organisms, and their mechanical strength is very high. Thanks to these features nothing can affect luminescent characteristics of activating ions, as they look like sitting in protected chambers. Thanks to the fact that particles of zirconium and hafnium oxides don’t interact with bio molecules, one can safely inject them into a human body. In 2015 nanoparticles of hafnium oxide were approved by the U.S Food and Drug Administration (the FDA) for use in radiotherapy. Like all heavy elements they produce high-energy electrons under the influence of X rays. It means that such systems combine advantages of quantum dots and organic compounds of transition metals.
Using zirconium and hafnium oxides as bio matrices for fluorescent diagnostics is a young but promising scientific trend. As a member of the International Laboratory "Solution Chemistry of Advanced Materials and Technologies" I can say that our team is also interested in this field.
Photo credit: aetherforce.com
