Marine hijiki-derived carbon dots enable fluorescent bioimaging and enhance rice seedling growth

In this study, we present a green and scalable hydrothermal strategy for synthesizing multifunctional carbon dots (CDs) from the edible marine alga Hizikia fusiformis . The resulting hijiki-derived CDs (H-CDs) exhibit well-defined optical and structural properties, including strong blue emission (λₑₘ ≈ 407 nm), a quantum yield of 8.7%, an extended fluorescence lifetime (15.8 ns), ultrasmall size, and abundant surface functionalization. These features confer high aqueous stability and compatibility with biological environments. Biological evaluation demonstrates that H-CDs are cytocompatible and efficiently internalized by HeLa cells, producing stable intracellular fluorescence suitable for bioimaging. In addition, H-CDs exhibit strong inhibitory activity against sphingomyelin synthase isoforms (SMS1 and SMS2), with inhibition levels comparable to those of established inhibitors. A plausible interaction mechanism involving multivalent surface interactions, including electrostatic binding and substrate mimicry, is proposed to explain this activity. In plant systems, H-CDs significantly enhance rice ( Oryza sativa ) seed germination and early seedling development, as reflected by increased root and shoot growth. Confocal imaging confirms their uptake and localization within metabolically active seed tissues, particularly in the bran and aleurone layers. At the molecular level, selective upregulation of OsDGAT1 suggests stimulation of lipid biosynthesis pathways associated with early-stage energy metabolism, while unchanged OsACBP4 expression indicates limited impact on lipid transport. Our work advances plant nanobiology by showing that biomass-derived carbon dots interact with plants at physiological and molecular levels, linking uptake to metabolic regulation and growth, and enabling sustainable platforms for bioimaging, metabolic control, and agriculture.