A highly stable and water-soluble fluorescent dye for fluorescence imaging of living cells

A highly stable and water-soluble bis-benzoxazole-based fluorescent dye (BBT-mPEG) is prepared by the treatment of 2,5-bis(benzoxazol-2-yl)thiophene-3,4-diol (BBT) with methoxypolyethylene glycol (mPEG-OH, average M_w = 350). BBT-mPEG exhibits a blue emission at λ_em = 445 nm in water upon excitation with 365 nm light, and a moderate fluorescence quantum yield (ϕ_f = 0.33, in H₂O) is obtained using quinine sulfate (ϕ_f = 0.557, in H₂SO₄) as the reference. It is found that BBT-mPEG shows good photo-stability and large Stokes shift (Δλ = 80 nm) in aqueous solution in entire pH range (pH = 1-14). Moreover, it is also found that BBT-mPEG exhibits low cytotoxicity and enhanced fluorescence in cells; using HeLa cells as a prototype, a clear cellular imaging is obtained.