Nitrogen-doped carbon dots as visible light initiators for 3D (bio)printing

Developing visible light initiators with certain water solubility can expand the application of photopolymerization technology into some emerging fields, such as 3D (bio)printing and biomedicine. In this study, two nitrogen-doped carbon dots (CD-1 and CD-2) were prepared and characterized by AFM, TEM, XPS and FTIR. Both of them have uniform sizes around 1-2 nm and abundant amino, carboxyl and hydroxyl groups on their surface, which make their UV-vis absorption spectra in water extend into the visible light region. Under irradiation with a 405 nm or 450 nm LED, they can generate active free radicals through redox reactions between photogenerated electrons/holes and oxygen, water or coinitiators to induce the polymerization of acrylate monomers. Notably, CD-2 containing a higher nitrogen content (36%), mainly in pyridinic and pyrrolic forms, exhibits significantly enhanced photoinitiation activity compared with CD-1 (17% N, mainly in the graphitic form). The photoinitiation efficiency of CD-2 combined with NPG (N-phenylglycine) is even higher than that of the classical commercial combination ITX/NPG. In addition, these CDs also present excellent biocompatibility, providing them with promising application prospects in 3D (bio)printing.