Current organic fluorophores used as labeling reagents for biomolecule conjugation have significant limitations
in photostability. This compromises their performance in applications that require a photostable fluorescent reporting
group. For example, in molecular imaging and single molecule microscopy, photostable fluorescent labels are important
for observing and tracking individual molecular events over extended period of time. We report in this paper an
extremely photostable and highly fluorescent phthalocyanine dye, IRDyeTM 700DX, as a near-infrared fluorescence
labeling reagent to conjugate with biomolecules. This novel water-soluble silicon phthalocyanine dye has an isomericly
pure chemical structure. The dye is about 45 to 128 times more photostable than current near-IR fluorophores, e.g.
Alexa Fluor"R"680, CyTM 5.5, CyTM 7 and IRDyeTM 800CW dyes; and about 27 times more photostable than
tetramethylrhodamine (TMR), one of the most photostable organic dyes. This dye also meets all the other stringent
requirements as an ideal fluorophore for biomolecules labeling such as excellent water solubility, no aggregation in high
ionic strength buffer, large extinction coefficient and high fluorescent quantum yield. Antibodies conjugated with
IRDyeTM 700DX at high D/P ratio exist as monomeric species in high ionic buffer and have bright fluorescence. The
IRDyeTM 700DX conjugated antibodies generate sensitive, highly specific detection with very low background in
Western blot and cytoblot assays.