Interaction of lignin and xylan in the hydrothermal synthesis of lignocellulose-based carbon quantum dots and their application in in-vivo bioimaging

The complex interaction of lignin, cellulose, and hemicellulose in the hydrothermal degradation progress of lignocellulose, has led to uncertainty in the hydrothermal synthesis of lignocellulose-based CQDs (LC-CQDs). This makes it difficult to identify the specific formation mechanism of LC-CQDs. To simplify the reaction system and comprehensively describe the formation of LC-CQDs, both lignin and hemicellulose, the main hydrothermal degradation products of lignocellulose, were used as precursor to simulate and explore the synthesis of LC-CQDs at different time intervals (2-12 h). First, different lignin models were employed for preparing CQDs to determine the key lignin structure that govern CQDs formation. G-type lignin-model based CQDs were shown to have higher fluorescence intensity than H- and S-type. Then, G-type lignin model and hemicellulose model (xylan) were used simultaneously hydrothermal to prepare LC-CQDs. The analysis shows that the carbon nucleus preferentially formed by the lignin provides growth sites for small molecules degraded from hemicellulose, which gradually grow around the carbon core over time, thus forming a "sunflower" structure of CQDs. The presence of a lignin model could effectively guide the small molecules toward CQDs formation instead of carbonization. Additionally, the CQDs exhibit good in-vivo imaging performance.