Visible light, known as the visible spectrum, is the section of the electromagnetic spectrum that is visible to the human eye. Visible light occupies the portion of the electromagnetic spectrum between frequencies of approximately 400 terahertz (THz ) to 750 THz, and contains photon energies of 36 to 72 kcal/mole. The wavelength of visible light is approximately between 400 nm and 800 nm. Among them, the longest wavelength is red, and the shortest is purple. Visible absorbing dyes are one of the functional dyes that have been studied more in the field of dye chemistry in recent years.
Fig.1 Visible light shows different colors in different wavelength ranges
Fluorescent dyes are defined as materials that both absorb and emit strongly in the visible region of the spectrum and have a wide range of applications in electroluminescence, fluorescent probes, bioimaging, anti-counterfeiting and other fields. Many fluorescent dyes with specific emission and excitation spectra have been developed. Nowadays, fluorescent dyes that absorb and emit light at specific wavelengths throughout the visible spectrum have been commercialized, such as BODIPY dyes, rhodamine dyes and fluorescein dyes.
Boron dipyrromethene (BODIPY) fluorescent dyes have excellent photophysical properties, such as large molar extinction coefficients, high fluorescence quantum yields, and high photo- and chemical-stability. In addition, BODIPY is biocompatible, easy to synthesize and modify. The luminescence of BODIPY dyes can basically cover the entire visible region, with most of the emission wavelengths less than 600 nm and around 500 nm. Because of the ability to adjust the wavelength through molecular modification, BODIPY dyes have become a class of fluorescent dyes of great interest and have received widespread attention in many fields.
Fig. 2 BODIPY Dyes and their maximum absorption and emission wavelengths
Rhodamine (RhB) dyes have excellent photostability, high molar extinction coefficients, and high quantum yields. They are used extensively in biotechnology application such as fluorescent labels for proteins, nucleic acids, lipids and carbohydrates. Among them, the absorption and emission wavelengths of Rhodamine B, Rhodamine 6G, and Rhodamine 101 are in the visible light region. The absorption spectrum peak of Rhodamine B dye in ethanol solvent at room temperature is at 560 nm, and the fluorescence spectrum peak is at 600 nm. The absorption of Rhodamin 6G ranges between 440 nm and 570 nm, with a peak at 530 nm and an emission peak around 550 nm. The fluorescence region of Rhodamin 6G is from 550 nm to 590 nm with the maximum wavelength at 565 nm. Rhodamine 101 is also a fluorescent dye, which has an excitation peak at 569 nm and an emission peak at 590 nm.
Fig. 3 Chemical structures of Rhodamine B, Rhodamine 6G and Rhodamine 101
Fluorescein, an organic fluorescent dye, is one of the most popular dyes used in biological research because of its low cost and ease of use. Fluorescein dye has a maximum absorption wavelength of 494 nm and a maximum emission wavelength of 521 nm, as well as fluorescein isothiocyanate (FITC) with peak excitation and emission spectra of approximately 495 nm and 519 nm.
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