What is fluorescence?

We are well aware of the fact that light interacts with matter. Light interacts with cola in a different way to, say, carbonated spring water. Visible electromagnetic radiation is detected by the rods and cones in our retinas so that we can appreciate the difference between water and cola. Plants absorb the sun's radiation in the process known as photosynthesis - this is a different way in which light interacts with matter but the important word in describing these interactions is "Absorb".

The story of how matter interacts with light does not end with absorbance. A small set of materials are also able to fluoresce. This means that after taking light in (the absorbing process) they are able to emit the light a small fraction of a second later; they send back out light of a different color.

What are some good examples of fluorescence? Well actually there are very few examples to speak of. However, a good analogy of what is taking place in the emission process is the charging and discharging of glowstars. When I was a boy I had a collection of glow-stars on my bedroom ceiling. I would charge up my glowstars (allow my glowstars to absorb) by exposing them to my bedroom lamp. When I switched off the light the stars would glow; I could then watch them and eventually fall asleep. Fluorescence spectroscopy allows me to measure the colors of the light that are given off by a sample that has already been charged by absorption.

The Fluorometer

A diagram of the fluorescence spectroscope or fluorometer is given below. Each "monochromator" is simply a device used to select a specific color or "wavelength of light".

Why Fluorescence?

Fluorescence efficiency (Quantum yield) is an intrinsic property of a material. Hence we can monitor the presence of a given "fluorophore" (something that fluoresces). Furthermore, fluorescence efficiency and the fluorescence spectrum may often change depending on the fluorophore's environment, or the material's conformation or morphology. Here are some specific examples.

A set of polymers having a conjugated backbone (alternating single and double bonds) are able to both conduct electricity and to fluoresce. These polymers have found commercial application in organic light emitting diodes that can be ink-jet printed onto flexible conductive substrates. The efficiency of these devices has been improved by studying the fluorescence of the polymer. The fluorescence efficiency has improved as a result of chemical modification and fluorescence measurement but, as well, the alignment of the polymer chains has been investigated through use of spectroscopy. The alignment of these chains, resulting in increased order has led to improved conductivity in such devices. These conjugated polymers are also being used today as a host matrix for photovoltaics (solar cells).

Tryptophan is a naturally occurring amino acid that fluoresces. If the tryptophan amino acid is buried in a hydrophobic (oily) portion of a protein its fluorescence will be different when compared to a tryptophan that is exposed to the surrounding solvent, water. Hence, at a simple level one can obtain information about the structure of a protein. More importantly the tryptophan fluorescence may be quenched by the presence of a substrate that the protein is acting upon. The intensity of the fluorescence can be used to understand how the protein is functioning, and how its conformation is changing as it functions.

Many fluorescence tags are available that are selective in their ability to bond to substances of (biological) importance. In flow cytometry, for example, fluorescent tags stick only to diseased cells inside a blood sample. By measuring the amount of fluorescence the number of diseased cells in a patient's sample can be determined.

Overview of my research

Projects

Organic Bulk Heterojunction Solar Cells Optimization

Conjugated Polymer Nanoparticles

Interaction and energy transfer between SWNT