In fluorescence labeling, NBD denotes a nitrobenzoxadiazole fluorophore used in fluorescent derivatives, including labeled lipids. Its emission depends on local surroundings, so changing brightness can report an environmental change as well as a change in the number of labels.
For example, comparing a labeled lipid in aqueous solution and in a lipid bilayer requires accounting for different emission efficiencies. A fixed-illumination measurement of fluorescence alone is not an absolute concentration measurement unless that response is calibrated. [ref]
https://www.thermofisher.cn/cn/en/home/references/molecular-probes-the-handbook/tables/amine-reactive-environment-sensitive-fluorophores.html. Thermo Fisher Molecular Probes Handbook, Table 1.13 note: NBD derivatives have solvent-dependent emission and quantum yields
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