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NBD Created 2026-10-20 Updated 2026-11-05
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
NBD and rhodamine are initially close together in the labelled vesicle lipid bilayer, allowing Förster resonance energy transfer and suppressing donor fluorescence. Mixing labelled lipid bilayer with unlabelled lipid bilayer dilutes the dyes, decreases energy transfer and increases the NBD signal.
The rise with the complementary SNARE proteins, but little rise with protein-free vesicles, therefore supports SNARE-dependent lipid mixing. It is consistent with membrane fusion, but does not by itself distinguish complete fusion from hemifusion or other causes of dye dilution. A separate content mixing assay with a leakage control would test merger of the aqueous compartments.