Xanthene dyes are one of the oldest synthetic dyes. Fluorescein was first synthesized by von Baeyer in 1871. Since its discovery, it has been extensively studied. Fluorescein can exist as four different ionic forms (cationic form, neutral form, monoanionic form and dianionic form). Under physiological conditions (pH 7.4), fluorescein mainly exists as the dianionic form which grants it large quantum yield and excellent solubility in water. This renders fluorescein and its derivatives useful for studies in biological media. Cysteine (Cys) and homocysteine (Hcy) are of the few amino acids which contain sulfur and both of them are believed to be related to some diseases. Cardiovascular diseases, Alzheimer’s disease and neutral tube defects are reported to be associated with elevated levels of homocysteine in blood plasma. Low levels of cysteine are associated with slowed growth, hair depigmentation, liver damage, etc. The reason for Cys and Hcy being connected with diseases is still unclear. Herein, we designed and synthesized fluorescein-based dyes for detecting Hcy and Cys in the visible spectral region with the highest selectivity. Optical sensors in the near-infrared (NIR) spectral range have captured the attention of researchers interested in studying chemical and biological processes in live tissues because absorption and scattering by endogenous biomolecules are minimal. Three heptamehtine cyanine-based dyes were designed and synthesized for the detection or labeling of Cys and Hcy. Biocompatible hydrogels have been studied in many fields, e.g., drug delivery system, separation processes or sensors (fluorescent polymers). A fluorescein-based hydrogel with potential application for detection of Cys and Hcy were designed and underwent synthesized. Dibenzo-p-dioxin is a structural analog of xanthene but is notorious for the toxicicty of the halogenated members, such as 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD). Since polyhalogenated dibenzo-p-dioxins (PXDDs) have been shown to be a significant threat to public health, measuring the concentrations of PXDDs in the environment as well as understanding their mechanism of formation and environmental fate is needed. Here we designed a novel synthesis with control halogen substitution patterns in order to result in a large number of PXDDs
