Next-Generation Green Carbon Dots: Toward Standardized Synthesis and Purification for Translational Nanomedicine—A Comprehensive Review

Plant-derived green carbon dots (CDs) have gained significant attention as sustainable and biocompatible nanomaterials for biomedical applications. However, critical gaps remain in understanding how synthesis conditions and purification strategies jointly influence their functional performance. This review systematically evaluates recent advances in the green production of CDs from plant biomass, with a focus on the interplay between precursor chemistry, processing routes, and post-synthesis refinement in governing optical properties, colloidal stability, and biological reliability. We critically compare current green synthesis approaches, including hydrothermal and solvothermal methods, microwave- and ultrasound-assisted techniques, carbonization, and chemical oxidation, in terms of scalability, reproducibility, and compliance with green chemistry principles. Unlike previous reviews, this work provides a unified and in-depth analysis of downstream purification methods such as dialysis, centrifugation, electrophoresis, and size-exclusion chromatography, highlighting their decisive role in achieving batch-to-batch consistency, photoluminescence stability, and translational robustness. This review further consolidate emerging insights into structure–property–function relationships, clarifying how size distribution and surface chemistry modulate reactive oxygen species generation, biodistribution, and therapeutic efficacy. Special emphasis is placed on photodynamic therapy, where plant-derived CDs exhibit unique potential as metal-free and photoactive therapeutic agents. By identifying unresolved challenges, including non-standardized purification protocols, incomplete mechanistic understanding of luminescence, and limited systematic optimization studies, this review proposes strategic directions for rational material design and quality control. Overall, it bridges synthesis, purification, and application perspectives to accelerate the reliable integration of green carbon dots into next-generation nanomedicine.