Fluorescein

Fluorescein is a low-molecular-weight synthetic dye that absorbs blue light at 470–490nm and fluoresces green-yellow at 520–530nm^[1]. Fluorescein can reversibly toggle among four interconvertible ionic states depending on pH: cation, neutral, monoanion and dianion. Each state possesses a distinct, non-overlapping absorption maximum (490nm, 472/453nm, 434nm, 437nm), enabling its use as a four-state, reversible molecular switch^[2]. Fluorescein’s absorption maxima, fluorescence spectra, and protolytic equilibria are also temperature-dependent. After the temperature rises between 4℃ to 80℃, the absorption peak of fluorescein shifts slightly to the red, its acidity weakens, and both the overall spectrum and ionization equilibrium will change slightly^[3].

References:
[1] Kawahara A, Hikichi T, Kitaya N, et al. Adenosine agonist regulation of outward active transport of Fluorescein across retinal pigment epithelium in rabbits. Exp Eye Res. 2005 Apr;80(4):493-9.
[2] Margulies D, Melman G, Shanzer A. Fluorescein as a model molecular calculator with reset capability. Nat Mater. 2005 Oct;4(10):768-71.
[3] Sjöback R, Nygren J, Kubista M, et al. Absorption and fluorescence properties of fluorescein. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy

Fluorescein是一种低分子量合成染料，可吸收470–490nm蓝光并在520–530nm发出黄绿色荧光^[1]。Fluorescein可根据pH在四种可逆互变的离子态间切换：阳离子、中性、单阴离子和二阴离子；每种状态具有互不重叠的吸收峰（490nm、472/453nm、434nm、437nm），因此可作为四态、可逆分子开关^[2]。荧光素的最大吸收波长、荧光光谱和质子解离平衡也受温度影响。当温度在4℃至80℃之间升高时，荧光素的吸收峰会略微向红移动，其酸性减弱，整体光谱和电离平衡也会略有变化^[3]。