Laurdan

Laurdan is a polarity-sensitive lipid membrane fluorescent probe used to image lipid rafts (also known as lipid microdomains) in cell membranes, the absorption wavelength of Laurdan is 340-380nm and the emission wavelength is 440-490nm^{[1, 2, 3]}. Laurdan is membrane-embedded. Its hydrophobic tail can be inserted into the phospholipid bilayer of the cell membrane, while the polar head remains near the membrane surface, which can detect polarity changes and the presence of water molecules inside the membrane ^[4]. Laurdan is phase-sensitive. Its fluorescence emission in the liquid crystal phase will show a large red shift, while it will show a small red shift in the gel phase, which can detect the phase state and fluidity of the membrane ^[5]. Laurdan can measure the generalized polarization (GP) of the cell membrane. High GP is usually associated with low fluidity, low polarity or high cholesterol content of the membrane, while low GP is the opposite ^[6].

References:
[1] Horváth Á, Erostyák J, Szőke É. Effect of Lipid Raft Disruptors on Cell Membrane Fluidity Studied by Fluorescence Spectroscopy[J]. International Journal of Molecular Sciences, 2022, 23(22): 13729.
[2] Alvares S M, Dunn C A, Brown T A, et al. The role of membrane microdomains in transmembrane signaling through the epithelial glycoprotein Gp140/CDCP1[J]. Biochimica et Biophysica Acta (BBA)-General Subjects, 2008, 1780(3): 486-496.
[3] Goldys E M. Fluorescence applications in biotechnology and life sciences[M]. John Wiley & Sons, 2009.
[4] Jay A G, Hamilton J A. Disorder amidst membrane order: standardizing laurdan generalized polarization and membrane fluidity terms[J]. Journal of fluorescence, 2017, 27: 243-249.
[5] Bianchetti G, Azoulay-Ginsburg S, Keshet-Levy N Y, et al. Investigation of the membrane fluidity regulation of fatty acid intracellular distribution by fluorescence lifetime imaging of novel polarity sensitive fluorescent derivatives[J]. International Journal of Molecular Sciences, 2021, 22(6): 3106.
[6] Yu W, So P T, French T, et al. Fluorescence generalized polarization of cell membranes: a two-photon scanning microscopy approach[J]. Biophysical journal, 1996, 70(2): 626-636.

Laurdan是一种极性敏感脂膜荧光探针，用于对细胞膜中的脂筏（也称为脂质微域）进行成像，Laurdan的吸收波长为340-380nm，发射波长为440-490nm^{[1, 2, 3]}。Laurdan具有膜嵌入性，它的疏水尾部可以插入细胞膜的磷脂双层中，而极性头部留在膜表面附近，能够探测膜内部的极性变化和水分子存在情况^[4]。Laurdan具有相态敏感性，在液晶相中的荧光发射会显示较大的红移，而在凝胶相中则会显示较小的红移，能够检测膜的相态和流动性^[5]。Laurdan可测量细胞膜的广义极化（GP），高GP通常与膜的低流动性、低极性或高胆固醇含量相关，而低GP则相反^[6]。