Calcein

Calcein (CAS No.: 1461-15-0) is a widely utilized fluorescent xanthene dye functioning as a calcium-sensitive probe and self-quenching indicator in biochemical and biophysical research. Exhibiting excitation and emission maxima near 495/515 nm, calcein displays fluorescence properties that are strongly modulated by pH and the presence of divalent and trivalent metal ions, forming fluorescent complexes particularly with calcium, magnesium, zinc, copper, and other metals, which underlies its use as a complexometric indicator in EDTA-based titrations and fluorometric metal ion quantification. Due to its inability to permeate intact cellular membranes, calcein is commonly applied in assays evaluating membrane integrity, cell viability, and liposomal or vesicular leakage, where its concentration-dependent self-quenching enables sensitive detection of membrane disruption. In addition, it serves as a model hydrophilic compound in drug delivery studies to assess encapsulation efficiency and release kinetics. In biological systems, calcein has been extensively employed for imaging mineralization processes, including visualization of bone formation and skeletal development in model organisms such as zebrafish embryos. Its functional mechanism relies on metal ion chelation and environment-sensitive fluorescence modulation rather than interaction with a specific molecular target or signaling pathway, and it is typically used at micromolar to millimolar concentrations depending on assay format, probe loading strategy, and detection sensitivity requirements in vitro or in vivo.