An overview of humic acid-based carbon dots: synthesis, characteristics and applications

Humic acid is one of the most abundant natural organic carbon sources worldwide. In China alone, coal mining generates hundreds of millions of tons annually, yet most ends up as low-grade fertilizer or is simply discarded-high-value utilization remains minimal. Humic acid-based carbon dots (HACDs) offer a viable route to upgrade this cheap bulk byproduct into functional materials, aligning resource efficiency with green chemistry principles. HACDs are straightforward to synthesize, low-cost, water-soluble, and readily functionalized and have driven growing interest in sensing, catalysis, bioimaging, and optoelectronics. That said, the gap between proof-of-concept and practical application remains substantial. Quantum yields are generally modest, emission is largely confined to the blue region, batch-to-batch reproducibility is questionable, scalable production has barely been addressed, long-term in vivo fate is poorly understood and chronic toxicity data are essentially unavailable. This review surveys the synthesis, structural modulation and application landscape of HACDs, with particular attention to the structure-property relationships linking carbon cores, surface groups, and optical/catalytic performance. Based on functionalization strategies, we categorize the literature into three classes: surface-engineered sensing platforms, heteroatom-doped catalytic and photothermal systems, and metal-coordinated theranostic platforms. For each, we examine the design rationale and key limitations, aiming to offer practical guidance for performance optimization. More broadly, tackling these challenges is not just about improving HACDs themselves, but about establishing a transferable framework for turning biomass waste into functional nanomaterials, a goal that extends well beyond this specific material system.