Fluorescence spectroscopy is a method to study the properties of a sample based on its fluorescence properties. This technique measures the quantity of certain chemical products in the sample. Fluorescence is the process in which a substance absorbs light at a specific wavelength and then emits light at a longer wavelength. This method employs a light source to excites a molecule, resulting in the emission of light of a higher wavelength. This emitted light is aimed at a filter and then detected by a sensor to analyze and identify the molecule or any changes it undergoes. It can be analyzed to provide valuable information about the properties and behavior of any molecule. 

This technique is employed to investigate structural transformations in conjugated systems, aromatic molecules, and inflexible, planar compounds due to fluctuations in temperature, pH, ionic strength, solvent, and ligands. Fluorescence spectroscopy is highly sensitive since a single fluorophore can produce numerous detectable photons that can be excited and detected repeatedly. It is a powerful analytical tool used in a variety of fields, including chemistry, biology, physics, and material science.

Working of Fluorescence Spectroscopy

Figure 1: Fluorescence Process

Fluorescence spectroscopy involves illuminating a sample with monochromatic light, usually in the ultraviolet or visible range, which is absorbed by the sample compounds. This absorption causes the molecules to get excited from their ground state to an excited electronic state. Then the molecule returns to the lowest vibrational energy level of the excited state and then to the ground state by releasing energy in the form of a photon, resulting in fluorescence. This process is depicted in figure 1. The frequencies and intensities of these emitted photons are analyzed to determine the structure of the molecule's vibrational energy levels. By studying this information, it is possible to gain insight into the molecule's identity, quantity, any changes it may be undergoing, its interactions with other molecules in the same sample, and more.

Fluorescence Spectrum

The spectrum obtained after performing fluorescence spectroscopy is called the fluorescence spectrum. When molecules are excited by a constant light source, a steady-state fluorescence spectrum is obtained and the emitted photons or intensity is detected as a function of wavelength. 

There are different types of fluorescent spectrums: fluorescence emission spectrum, fluorescence excitation spectrum and fluorescence absorption spectrum.

A fluorescence emission spectrum is obtained by scanning the emission wavelength while the excitation wavelength is held constant to get a plot of intensity versus emission wavelength. In the case of excitation spectra, they are highly specific to a single emitting species or wavelength.

By fixing the emission wavelength and scanning the excitation monochromator wavelength, the fluorescence excitation spectrum is obtained. This technique provides details on the wavelengths at which the light is absorbed and emitted by the sample at a particular emission wavelength. 

All absorbing species present in a sample are measured by the absorption spectrum. 

The emission and excitation spectra of a fluorophore are mirror images of each other, with the emission spectrum typically occurring at higher wavelengths (lower energy) than the excitation or absorption spectrum. Fluorescence spectroscopy is a sensitive technique compared to absorption spectroscopy in terms of limits of detection and molecular specificity.

Molecules and materials that exhibit fluorescence

Fluorescent compounds exist in various sizes and shapes. Certain compounds, like chlorophyll and certain amino acid residues (tryptophan, phenylalanine, and tyrosine), exhibit intrinsic fluorescence. On the other hand, some molecules are artificially synthesized as stable organic dyes or tags to be incorporated into non-fluorescent systems. Organic fluorescent dyes generally contain aromatic rings and pi-conjugated electrons, and their emission wavelength ranges from UV to near-IR depending on their size and structure.

Some of the fluorescent molecules and materials are:

• Amino acids (Trp, Phe, Tyr)
• Base pair derivatives (2-AP, 3-MI, 6-MI, 6-MAP, pyrrolo-C, tC)
• Quantum dots
• Chlorophylls
• Fluorescent Proteins (FPs)
• Rare earth elements (lanthanides)
• Semiconductors
• Single Walled Carbon Nanotubes (SWCNTs)
• Solar cells
• Organic dyes (fluorescein, rhodamine, N-aminocoumarins and derivatives of these)
• Phosphors
• Pigments, brighteners
• Others

Some of the other commonly used fluorescent samples include fluorescent proteins, phosphors, semiconductors, and rare earth elements. Polymers with conjugated aromatics or dienes also frequently possess fluorescent characteristics. 

Applications of Fluorescence Spectroscopy

Fluorescence spectroscopy is used to study the structure and dynamics of proteins. Researchers can monitor changes in the protein's conformation and folding by labeling specific amino acids in a protein with fluorescent probes. This technique can also be used to study protein-protein interactions and protein-ligand binding.

It is used in medical diagnostics to detect and monitor diseases such as cancer and diabetes. For example, fluorescent probes can be used to detect cancer cells in tissue samples or monitor glucose levels in diabetic patients.

Fluorescence spectroscopy is a valuable tool in drug discovery, particularly in the identification and characterization of small molecule inhibitors. It is also used in material science to study the properties of polymers, nanoparticles, and other materials. 

It is used to monitor environmental pollutants, such as polycyclic aromatic hydrocarbons (PAHs) and heavy metals. Fluorescence spectroscopy is also used in forensic science to detect and analyze trace amounts of evidence, such as fibers and biological fluids.

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