Green Biosynthesis of N,K Codoped Carbon Quantum Dots From Cissus Quadrangularis With Tunable Multistate Emission for Fluorescence Bioimaging Applications

The development of sustainable and biocompatible fluorescent nanomaterials is essential for advancing next‐generation bioimaging and sensing technologies. This work reports an economical green synthesis of carboxyl‐functionalized, N,K codoped carbon quantum dots (cq‐hCQDs) derived from Cissus quadrangularis via a facile hydrothermal approach. The resulting cq‐hCQDs possess a quasispherical morphology (~3.23 nm), hybrid sp ² ‐sp ³ carbon architecture, and abundant ─COOH surface functional groups, enabling excellent hydrophilicity, aqueous stability, and chemical reactivity, as evidenced from HRTEM, XPS, NMR, FTIR, and zeta potential. The cq‐hCQDs exhibit enhanced visible‐light absorption with a narrowed bandgap (~2.85 eV) and strong, tunable fluorescence dominated by π –π * transitions, alongside contributions from surface and defect states introduced by heteroatom doping. A plausible mechanism is proposed in which the synergistic electronic modulation may contribute to enhanced emission intensity through multiple radiative recombination pathways. Notably, the synthesized cq‐hCQDs exhibited potential as fluorescent bioimaging probes, providing high‐contrast visualization of cellular structures in Funaria hygrometrica moss and Ficus religiosa plant tissues. Furthermore, phytotoxicity analysis validates marginal biological impact, with >75% cell survivability.