rhodamines

Rhodamines are a family of synthetic fluorescent dyes based on the xanthene core. They are typically cationic at physiological pH due to amine substituents and are valued for bright fluorescence and relatively high quantum yields.

Chemically, rhodamines consist of two benzene rings fused to a central xanthene ring. The core is readily modified to produce a wide range of dyes, including rhodamine B, rhodamine 6G, rhodamine 110, and rhodamine 123. Rhodamine 6G is widely used as a laser dye; silicon-rhodamines (SiRs) and other derivatives extend emission into the red and near-infrared and are popular for live-cell imaging.

Spectroscopically, rhodamines typically absorb in the blue-green region (roughly 500–560 nm) and emit in the green-to-orange range (roughly 520–600 nm). Many rhodamines have high fluorescence quantum yields and brightness. Some derivatives can exist in a nonfluorescent spirolactam form and become fluorescent upon ring opening, enabling turn-on probes.

Functionalization with reactive groups such as NHS esters and sulfonates improves bioconjugation and water solubility, enabling labeling of antibodies, nucleic acids, and proteins. The dyes are commonly used in fluorescence microscopy, confocal imaging, flow cytometry, and fluorescence in situ hybridization, and rhodamine 123 is widely used as a mitochondrial membrane potential indicator.

Considerations include photostability under illumination and potential cytotoxicity at high concentrations. The availability of many derivatives with varied lipophilicity, charge, and spectral properties makes rhodamines versatile for multicolor imaging and fluorescence assays.