Nitrogen doping on fluorescent carbon dot nanoparticles synthesized from lychee exocarps for applications in UV tube down-conversion and pH-responsive curcumin delivery

Abstract

Fluorescent carbon dot (CD) nanoparticles are gaining importance as a luminous material and drug carrier. However, their poor resistance to water, oxygen, and chemical reagents limits their use in UV down-conversion coating. Therefore, utilizing a solid substrate to encapsulate CDs can help preserve the original optical properties of CDs. In this work, lychee exocarps were used as the carbon source for CD preparation via a hydrothermal method. The as-synthesized CDs were intentionally doped with various nitrogen sources to regulate the color emission behavior of CDs. Additionally, a widely used water-soluble polyvinyl alcohol (PVA) was applied as a solid-phase dispersion matrix for CDs to obtain CDs@PVA films with good fluorescence. The fabricated CDs produced multi-color visible light upon UV stimulation, demonstrating excellent photostability, and exhibit a relatively high absolute quantum yield of 7.20%. On the other hand, the CDs possess an outstanding ability to carry drugs via covalent bonding. In this study, nitrogen-doped CDs with amine groups on their surfaces were reacted with carbonyl groups in the structure of curcumin (Cur) through a Schiff base reaction to obtain a pH-sensitive CD drug delivery system. The imine bond in the structure of CDs–Cur breaks under acidic conditions, releasing Cur, with a maximum release rate of 81%. Additionally, CDs–Cur significantly damaged the structural integrity of bacteria, exhibiting remarkable antibacterial effects against Escherichia coli (E. coli). Therefore, luminescent CDs with good pH stability and biocompatibility successfully open a new avenue as a drug delivery system for anticancer therapy.