Fluorescence imaging


Fluorescence imaging is a type of non-invasive imaging technique that can help visualize biological processes taking place in a living organism. Images can be produced from a variety of methods including: microscopy, imaging probes, and spectroscopy.
Fluorescence itself, is a form of luminescence that results from matter emitting light of a certain wavelength after absorbing electromagnetic radiation. Molecules that re-emit light upon absorption of light are called fluorophores.
Fluorescence imaging photographs fluorescent dyes and fluorescent proteins to mark molecular mechanisms and structures. It allows one to experimentally observe the dynamics of gene expression, protein expression, and molecular interactions in a living cell. It essentially serves as a precise, quantitative tool regarding biochemical applications.
A common misconception, fluorescence differs from bioluminescence by how the proteins from each process produce light. Bioluminescence is a chemical process that involves enzymes breaking down a substrate to produce light. Fluorescence is the physical excitation of an electron, and subsequent return to emit light.

Attributes

Fluorescence mechanism

When a certain molecule absorbs light, the energy of the molecule is briefly raised to a higher excited state. The subsequent return to ground state results in emission of fluorescent light that can be detected and measured. The emitted light, resulting from the absorbed photon of energy hv, has a specific wavelength. It is important to know this wavelength beforehand so that when an experiment is running, the measuring device knows what wavelength to be set at to detect light production. This wavelength is determined by the equation:
Where h = Planck's constant, and c = the speed of light. Typically a large scanning device or CCD is used here to measure the intensity and digitally photograph an image.

Fluorescent dyes versus proteins

Fluorescent dyes, with no maturation time, offer higher photo stability and brightness in comparison to fluorescent proteins. In terms of brightness, luminosity is dependent on the fluorophores’ extinction coefficient or ability to absorb light, and its quantum efficiency or effectiveness at transforming absorbed light into fluorescently emitting luminescence. The dyes themselves are not very fluorescent, but when they bind to proteins, they become more easily detectable. One example, NanoOrangeTM, binds to the coating and hydrophobic regions of a protein while being immune to reducing agents. Regarding proteins, these molecules themselves will fluorescence when they absorb a specific incident light wavelength. One example of this, green fluorescent protein, fluoresces green when exposed to light in the blue to UV range. Fluorescent proteins are excellent reporter molecules that can aid in localizing proteins, observing protein binding, and quantifying gene expression.

Imaging range

Since some wavelengths of fluorescence are beyond the range of the human eye, charged-coupled devices are used to accurately detect light and image the emission. This typically occurs in the 300 – 800 nm range. One of the advantages of fluorescent signaling is that intensity of emitted light behaves rather linearly in regards to the quantity of fluorescent molecules provided. This is obviously contingent that the absorbed light intensity and wavelength are constant. In terms of the actual image itself, it is usually in a 12-bit or 16-bit data format.

Imaging systems

The main components of fluorescence imaging systems are:

Types of microscopy

A different array of microscope techniques can be employed to change the visualization and contrast of an image. Each method comes with pros and cons, but all utilize the same mechanism of fluorescence to observe a biological process.

Disadvantages

Overall, this form of imaging is extremely useful in cutting-edge research, with its ability to monitor biological processes. The progression from 2D fluorescent images to 3D ones has allowed scientists to better study spatial precision and resolution. In addition, with concentrated efforts towards 4D analysis, scientists are now able to monitor a cell in real time, enabling them to monitor fast acting processes.

Future directions

Developing more effective fluorescent proteins is a task that many scientists have taken up in order to improve imaging probe capabilities. Often, mutations in certain residues can significantly change the protein's fluorescent properties. For example, by mutating the F64L gene in jellyfish GFP, the protein is able to more efficiently fluoresce at 37oC, an important attribute to have when growing cultures in a laboratory. In addition to this, genetic engineering can produce a protein that emits light at a better wavelength or frequency. In addition, the environment itself can play a crucial role. Fluorescence lifetime can be stabilized in a polar environment.
Mechanisms that have been well described but not necessarily incorporated into practical applications hold promising potential for fluorescence imaging. Fluorescence resonance energy transfer is an extremely sensitive mechanism that produce signaling molecules in the range of 1-10 nm.
Improvements in the techniques that constitute fluorescence processes is also crucial towards more efficient designs. Fluorescence correlation spectroscopy is an analysis technique that observes the fluctuation of fluorescence intensity. This analysis is a component of many fluorescence imaging machines and improvements in spatial resolution could improve the sensitivity and range.
Development of more sensitive probes and analytical techniques for laser induced fluorescence can allow for more accurate, up-to-date experimental data.