Fluorescence spectroscopy

One of the most important aspects of analytical chemistry is spectroscopy. The word “spectroscopy” comes from two Latin words, “spectrum” meaning appearance or form and “scopos” meaning to see, to watch. This field is widely used in chemistry, physics and other related fields to study substances but more specifically their emissions and properties.

Fluorescence Spectroscopy:

It is a technique where light of a specific frequency is passed through a sample. 

The unbound electrons in the excited state interact with the same chemical species. The study of these interactions is called fluorescence spectroscopy. 

The light will have lower energy than when it entered the sample, however its wavelength will be longer or shorter than that of the original source. This change in wavelength caused by an electron transition is called “Stokes shift”. Stokes shift is negative for any substance that absorbs light and positive for any substance that emits light.

Application of Fluorescence Spectroscopy:

Fluorescence spectroscopy is used in other fields such as internal quantum efficiency (IQE) for OLEDs, life science and astronomy. Fluorescence spectroscopy is widely used in polymers, fuel cells and biological products. Fluorescence spectroscopy has been used to measure the concentration of ions in electrolytes . The measurement of fluorescence can be used to determine the concentration of corrosive gases such as chlorine, ammonia, mercury and hydrogen sulphide. It is also used in detecting semiconductor doping concentrations. Some of other application of Fluorescence Spectroscopy are:

1) Food chemistry

2) Chemical imaging

3) Industrial processes

5) Protein characterization and quantification

6) Corrosion detection

7) Pharmaceutical excipient dissolution. 

8) Photobiology and noninvasive tumour imaging. 

9) Organometallic chemistry. 

10) Semiconductor manufacturing

11) Forensic applications etc. 

Difference between Fluorescence spectroscopy and UV Spectroscopy:  

In fluorescence spectroscopy the sample absorbs a small amount of light at an ultraviolet wavelength and the absorption causes a large change in energy (Stokes shift).  In UV-Visible spectroscopy the sample absorbs light and the absorption causes a small change in energy (Avogadro shift). The same molecule can have both fluorescence and UV-visible absorption. The molecule with fluorescence has a higher energy than that of the corresponding primary absorption peak.  Ultraviolet/Visible spectrometers are also called “linear” instruments or double beam instruments. UV/visible spectroscopy is also called “Absorption Spectrometry”. Fluorescence Spectrometers are also called “Non-linear” instruments or single beam instruments. 

Fluorescence Emission Line (FEL):

Fluorescence spectroscopy is used to determine the composition of substances. Fluorescence emission lines are a characteristic feature of fluorescent materials. A fluorescence emission line is an absorption line that has been shifted in frequency, in comparison to a standard reference spectra.  The emission lines occur at specific wavelengths because of the high speed transition from an excited electronic state to a ground electronic state . Alternatively, if an element gives off light through the creation of free radicals , then these radicals emit at short wavelengths and this type of “fluorescence” is called chemiluminescence .

Principles of Fluorescence Spectroscopy:

The first step in fluorescence spectroscopy is to excite the material with a specific wavelength of light, different for each chemical species.  In the second step, the substance absorbs a photon or higher energy particle and becomes an excited state.  The excited electron loses its excess energy by emitting an electromagnetic wave. This emission is called fluorescence . The wavelength emitted is usually longer than the excitation wavelength.

Fluorescence Spectroscopy Instrumentation:

Fluorescence spectroscopy uses ultraviolet or visible light as an excitation source and emits near-infrared (NIR) radiation. This near-infrared radiation is used to detect and read the fluorescence signature. The wavelength range is typically between 200 – 1300 nm with a peak at around 400 nm. The instruments are LabVIEW, Matlab, Excel,and OpenOffice.

To achieve fluorescence spectroscopy, special band pass filters are used. Such special filters have NIR emission properties and can be tuned for wavelengths between 200 and 1300 nm. Such filters are used in LabVIEW software to control the excitation source and detection of light emitted from detectors positioned at specific locations on the sample. 

Conclusion:

Fluorescence spectroscopy is a technique that studies the change in energy state of electrons in a substance due to excitation by light. Exciting the electrons by using UV or visible light, the electrons will lose their excess energy by emitting electromagnetic radiation, resulting in the emission of NIR light. When using fluorescence, only one photon is needed to liberate an electron from its ground state into an excited state. The emission itself does not need any external stimulus for example heat or electricity to be provided.

This technique is widely used in research and development of new products such as molecular fingerprinting and life science, medicine and technology (bioengineering).