A simple method for the preparation of multi-color carbon quantum dots by using reversible regulatory color transformation.

It is known that polychromatic carbon quantum dots (CQDs) can be obtained by doping and surface modification. The layer-wise synthesis of blue and green emitting CQDs (with typical sizes between 3 and 6 nm) is described here by adding oxalic acid and by introducing polycarboxy groups. By changing the external environment, the emission of CQDs can be adjusted in the blue-green spectral region (469-527 nm) under photoexcitation at 405 nm. The findings presented here provide new directions for the reversible regulatory transformation of polychromatic CQDs. The luminescence also is affected by a variety of conditions such as surface defects, degree of polymerization, polarity of external solutions, and hydrogen bonding which is studied in some detail. Graphical abstract Schematic presentation of the blue-green fluorescent transformation of the green carbon quantum dots(G-CQDs) to blue carbon quantum dots(B-CQDs).