Abstract
This paper summarizes some of our results on the application of oligothiophenes as fluorescent markers for biological studies. The oligomers of thiophene, widely known for their semiconductor properties in organic electronics, are also fluorescent compounds characterized by chemical and optical stability, high absorbance and quantum yield. Their fluorescent emission can be easily modulated via organic synthesis by changing the number of thiophene rings and the nature of side-chains. This review shows how oligothiophenes can be derivatized with active groups such as phosphoramidite, N-hydroxysuccinimidyl and 4-sulfotetrafluorophenyl esters, isothiocyanate and azide by which the (bio)molecules of interest can be covalently bound. This paper also describes how molecules such as oligonucleotides, proteins and even nanoparticles, tagged with oligothiophenes, can be used in experiments ranging from hybridization studies to imaging of fixed and living cells. Finally, a few multilabeling experiments are described.
Highlights
Staining of biological samples has always been a powerful technique, whose milestones are represented by: the staining of neuronal tissues with silver salts developed by Golgi in 1873 [1] that allowed him to visualize the structure of neurons; the coloration of bacteria developed by Gram in 1884 [2] that allows to classify bacteria based on the physical properties of their cell walls; and the method of Giemsa that allowed him to recognize cells infected with malaria parasites [3] and is used nowadays to visualize chromosomes
In principle the labeling of biological molecules with visible or fluorescent dyes can alter the functioning of the colored molecules, for instance by modifying their lipophilicity or sterically interfering with receptors or substrates; the chemistry behind the labeling process may be incompatible with the nature of the biomolecule, the properties of the fluorescent dyes such as emitted color, quantum yield, stability to bleaching and so on can be important for the experiment to be conducted
We studied the fluorescence behavior of each oligonucleotide either alone and in presence of an oligonucleotide complementary to the region of the hairpin loop, at different temperatures, to check if the oligothiophene emission was altered by the conformation and by the proximity to the dabcyl used as a fluorescence quencher, like in the right panel of Scheme 2
Summary
Staining of biological samples has always been a powerful technique, whose milestones are represented by: the staining of neuronal tissues with silver salts developed by Golgi in 1873 [1] that allowed him to visualize the structure of neurons; the coloration of bacteria developed by Gram in 1884 [2] that allows to classify bacteria based on the physical properties of their cell walls; and the method of Giemsa that allowed him to recognize cells infected with malaria parasites [3] and is used nowadays to visualize chromosomes. In principle the labeling of biological molecules with visible or fluorescent dyes can alter the functioning of the colored molecules, for instance by modifying their lipophilicity or sterically interfering with receptors or substrates; the chemistry behind the labeling process may be incompatible with the nature of the biomolecule, the properties of the fluorescent dyes such as emitted color, quantum yield, stability to bleaching and so on can be important for the experiment to be conducted For all these reasons it is important to have a variety of families of fluorescent markers to count on. This mini-review focuses on some applications derived from our recent works on the exploitation of oligothiophenes This class of molecules is, since long, widely studied for its use in semiconductors in devices such as field-effect transistors or solar cells [6,7], but their fluorescence properties and synthetic flexibility make them robust markers with emission in the full range of UV-Vis spectra [8]. Their polarizability makes them prone to sense their microenvironment and suitable to be used in recognition of biochemical process and for the realization of biosensors
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