Fluorescence

Fluorescence is an optical phenomenon wherein a material emits light in response to some external stimulus. Normally, the fluorescent light that is emitted is of a specific color or group of colors that is released when the material is bombarded with light in some other part of the color spectrum.

Certain minerals have a characteristic fluorescence pattern when hit with white light or ultraviolet light. Fluorite and calcite are two examples of fluorescent minerals. There are also many organic dye molecules with useful fluorescent properties. These molecules absorb light energy from external sources, and this energy causes some excitation of the electron orbitals in a process called pi-bonding. When the excited pi-bonds relax back to a lower energy state, photons of a specific wavelength are emitted in the process, giving rise to the fluorescent light. These organic dyes can be characterized by the wavelengths of light that they absorb (excitation wavelengths), and the wavelengths of light that they emit (emission wavelengths). The excitation and emission wavelengths are properties of each dye that are highly specific and reliable. Organic dyes tend to degrade over time as they are bombarded with light in a process called photodegradation. During photodegradation, the excited pi-bonds can break, rather than relaxing into their lower energy state. Organic fluorescent dyes have been in use for many years.

More recently, it has been discovered that very small particles of certain semiconductor materials also fluoresce, and the color of the fluorescence is dependent only on the size of the particles. These materials are referred to as quantum dots. Quantum dots absorb energy from a range of wavelengths, but the energy is not taken into pi-bonds. Rather, the fluorescence results from quantum mechanical interactions within the material. Smaller particles emit light on the blue end of the spectrum, whereas larger particles emit light on the red end of the spectrum. Because light energy is not absorbed into fragile pi-bonds, the rate of photodegradation is much lower for quantum dots compared with organic dyes, and thus the fluorescent signals are brighter and more durable.

There are a number ways in which fluorescence plays into forensic investigations. Biological materials sometimes have a characteristic fluorescent property that facilitates quick identification under UV examination. Semen stains, for example, may be identified by their characteristic fluorescence under ultraviolet light examination. Fluorescence of other biological samples can be brought about by chemical treatment to make their detection easier. Fingerprints can be treated with fluorescent powders to permit identification and detection even of relatively faint (latent) or degraded prints. Likewise, application of highly fluorescent materials, such as spy dust, permits tagging and tracking of suspects or agents across fairly wide areas by following the path of dispersal of the fluorescent agent as they drag and redistribute an unseen powder with their shoes or clothing. Certain chemicals, such as explosives and nerve agents, can sometimes be traced in the environment from their characteristic fluorescent spectral patterns. Examination of microscopic fibers for fluorescence can produce evidence linking suspects to crime scenes or other physical locations.

Fluorescence is a tool that allows evidence that would normally be invisible to come to light. It is a source of evidence only found with careful examination by those who are aware of its latent powers.

SEE ALSO Chemical and biological detection technologies; Confocal microscopy; Microscopes; Semen and sperm.