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 degrees 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. (C) 2018 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.