Abstract
Background/Objectives: Carbon dots (CDs) are fluorescent carbon-based nanomaterials attracting considerable attention recently because of their high photostability, tunable luminescence characteristics, low toxicity, and versatile surface chemistry. In this study, Hematoxylin (HT), a naturally occurring phenolic aromatic compound used as a histological staining dye, abundant in hydroxyl functional groups, was selected as a sustainable precursor for CD synthesis. The intrinsic fluorescence and favorable carbonization characteristics of HT are also value-added advantages for CDs. Methods: The use of citric acid (CA) during synthesis improves fluorescence behavior, colloidal stability, and biological performance of CDs. HT-based CDs prepared via the bottom-up synthesis method employing the hydrothermal technique as multifunctional fluorescent materials were also evaluated for potential bioimaging and light-activated antimicrobial applications. Results: The obtained HT-based CDs displayed good aqueous dispersion properties. HT CDs had a hydrodynamic diameter of 115 ± 1 nm and showed a fluorescence emission intensity of 5181 a.u. at λem = 329 nm when excited at λex = 300 nm. In contrast, HT:CA CDs prepared at a 1:1 HT-CA weight ratio had a hydrodynamic diameter of 310 ± 19 nm and exhibited a significantly higher fluorescence emission intensity, e.g., 31,840 a.u. at λem = 332 nm when excited at λex = 300 nm. Antioxidant studies of HT CDs and HT CDs using four different assays revealed that the antioxidant capacity of HT was partially retained and exhibited a concentration-dependent response. Notably, HT:CA CDs demonstrated significantly enhanced antimicrobial performance under UV-A irradiation compared to HT CDs, with bacterial cell viability calculated as 13 ± 4% for HT:CA CDs and 59 ± 2% for HT CDs. Moreover, HT CDs and HT:CA CDs demonstrated significantly enhanced cytocompatibility on the L929 fibroblast cell line, high cell viability up to 200 μg/mL, versus toxic HT molecules even at 100 μg/mL. Conclusions: HT-based CDs, particularly HT:CA CDs, demonstrated favorable fluorescence, antioxidant, antimicrobial, and cytocompatibility properties, supporting their potential application as multifunctional fluorescent materials for bioimaging and light-activated antimicrobial applications.