Valorization of selenium‐enriched Lactobacillus biomass into selenium‐doped carbon dots via hydrothermal conversion: physicochemical properties and biological properties

BACKGROUND Selenium‐enriched microbial fermentation residues contain both carbonaceous material and biologically incorporated selenium but are often underutilized. Developing sustainable strategies to convert these residues into value‐added functional nanomaterials is important for waste minimization and circular bioeconomy. This study aimed to directly synthesize selenium‐doped carbon dots (Se‐LCDs) from selenium‐enriched Lactobacillus fermentation residues through a one‐step hydrothermal process and evaluate their physicochemical properties, antioxidant activity, biocompatibility, and potential techno‐economic advantages. RESULTS Hydrothermal treatment at 200 °C for 12 or 20 h produced quasi‐spherical Se‐LCDs with mean diameters of 8.5 ± 0.5 and 2.3 ± 0.7 nm, respectively. Longer treatment decreased the measured Se content and particle size while increasing nitrogen retention. Both samples showed excitation‐dependent fluorescence, with maximum emission at 420 nm under 340 nm excitation, and ABTS radical‐scavenging rates of 37.99% (Se‐LCD1) and 36.76% (Se‐LCD2). No obvious inhibition zones were observed against the three tested bacterial strains. Cell‐viability responses depended on cell type and dilution: high‐dose treatments reduced viability in some mouse‐cell conditions, whereas increased viability was observed in HUVEC and HT‐29 cells. These effects are consistent with possible contributions from particle size, surface functional groups, and heteroatom composition, although the underlying cellular mechanisms were not directly tested. A preliminary scenario‐based estimate suggested lower precursor costs when Se‐enriched biomass was used instead of purified organoselenium reagents. CONCLUSION This study demonstrates a sustainable route for valorizing selenium‐enriched microbial residues into Se‐LCDs with antioxidant activity and biocompatibility, providing a promising platform for sustainable bioprocessing, renewable bioresource utilization, and circular bioeconomy.