The formation mechanism and properties of carbon quantum dots prepared from guaiacyl/syringyl lignin and catechyl lignin

Abstract

Unlike the conventional three-dimensional network structure of guaiacyl/syringyl (G/S) lignin, catechyl lignin (C-lignin) has a homogeneous linear structure. Different structures of lignin may affect the formation mechanism of lignin-based carbon quantum dots (L-CQDs). To explore this issue, this study investigated the physicochemical properties and synthesis processes of L-CQDs derived from both G/S lignin and C-lignin. Firstly, G/S lignin, naturally coexisting G/S lignin and C-lignin (G/S–C lignin), and C-lignin were extracted from different shells and then hydrothermally synthesized into COFS-CQDs, COSS-CQDs, and CBS-CQDs, respectively. All L-CQDs exhibit blue fluorescence, as analyzed by fluorescence spectroscopy, and the fluorescence intensity of COSS-CQDs is higher than that of COFS-CQDs and CBS-CQDs. The Raman and XPS analyses further indicate that COSS-CQDs possess the largest sp² conjugation domain size, which generally enhances their fluorescence emission. The distinct properties of the L-CQDs are attributed to the different formation processes of the three types of L-CQDs. G/S lignin and G/S–C lignin can depolymerize to form over ten species of aromatic small molecules (e.g., phenols, aldehydes, and ketones) under hydrothermal conditions, while C-lignin only yields three types of low-abundance small aromatic molecules, as revealed by GC-MS analysis. These depolymerized molecules influence the growth of conjugated domains in the carbon cores, resulting in differences in the fluorescence intensity of the derived L-CQDs. This study elucidates the critical role of the lignin structure in forming conjugated domains within L-CQDs, accordingly providing a theoretical foundation for the rational design of L-CQDs with high fluorescence performance.