Water-soluble, lignin-derived carbon dots with high fluorescent emissions and their applications in bioimaging

We report the synthesis of water-soluble and environmentally friendly fluorescent carbon dots (CDs) using the renewable resource lignin as a precursor, which is a waste emitted from pulp and paper industries and lignocellulosic biofuel production, using a compressed liquid CO₂ antisolvent method followed by carbonization and chemical oxidation with an acid mixture (H₂SO₄:HNO₃ = 1:3, v/v) at 90 °C for 8, 16, and 24 h. The as-synthesized CDs emitted bright yellow fluorescence (FL) under argon laser excitation at 458 nm with excitation-wavelength-independent and excitation-wavelength-dependent FL emission characteristics. Moreover, the CDs exhibited excellent stability in a highly concentrated NaCl solution, photostability with pH-dependent FL emission activities, and cellular imaging capability with very low cytotoxicity. Notably, the FL emission intensities of the as-synthesized CDs increased with longer reaction times, indicating high degree of oxidation on the CD surface and consequently, high quantum yield of 13% was achieved. Therefore, the as-synthesized CDs can be used as promising fluorescent probes for bioimaging applications in vitro and in vivo.