What are the types of fluorophores?

Fluorophores are classified into 3 categories: organic dyes, biological fluorophores, and quantum dots. Organic dyes such as fluorescein, rhodamine, and AMCA were the original fluorescent compounds used for research purposes. Derivatives of organic dyes have been created to improve their solubility and photostability. Since they are small compounds, organic dyes benefit over biological fluorophores for bioconjugation techniques since they can be crosslinked to antibodies without interrupting function.   

Biological fluorophores were first used when GFP was cloned from jellyfish and used as a gene expression reporter. The advantages of using these types of fluorophores is that expression plasmids can be injected into either bacteria, organs, cells, or whole organisms. This is done to generate expression of that particular fluorophore either alone or fused to a target protein. The drawbacks of biological fluorophores are that they are time consuming. Additionally, since they express high amounts of light producing proteins, this can create reactive oxygen species and cause toxicity. Another disadvantage is that the size of the fluorescent protein can change the biological function of the cellular protein in which the fluorophore is fused. Lastly, these types of fluorophores don’t have the photostability and sensitivity that synthetic fluorescent dyes provide. 

Quantum dots are nanocrystals (2-50 nm) that were designed to regulate the spectral characteristics of the flour. When quantum dots become excited, they emit fluorescence at wavelength based on the size of the particle. Smaller quantum dots emit higher energy than larger quantum dots; emitted light changes from blue to red as size of the nanocrystal increases. Quantum dots are more photostable than other types of fluorophores, and can remain fluorescent for up to 4 months. Moreover, quantum dots can be coated to be used in applications like protein labeling. The drawbacks is that there is cell toxicity possibly due to breakdown of particles, and quantum dots are not cost efficient.