Size (molecular Weight)
Most fluorophores are organic small molecules of 20 - 100 atoms (200 - 1000 Dalton - the molecular weight may be higher depending on grafted modifications, and conjugated molecules), but they are also much larger natural proteins: Green fluorescent protein (GFP) is 26 kDa and several phycobiliproteins(PE, APC...) are ≈240kDa.
Fluorescence particles are not considered as fluorophores (quantum dot: 2-10 nm diameter, 100-100,000 atoms)
The size of the fluorophore might sterically hinder the tagged molecule, and affect the fluorescence polarity.
Read more about this topic: Fluorophore
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