Carbon Nanotubes and Carbon Quantum Dots for Sustainable Agriculture

Carbon-based nanomaterials have emerged as promising tools for addressing major challenges in sustainable agriculture, including declining soil fertility, climate change, resource inefficiency, and increasing environmental pollution. Among these materials, carbon nanotubes (CNTs) and carbon quantum dots (CQDs) have attracted considerable attention because of their unique physicochemical properties and diverse interactions with plants and soil systems. This review provides a comprehensive comparative analysis of CNTs and CQDs, emphasizing how their structural characteristics govern environmental fate, plant uptake, physiological responses, and stress mitigation mechanisms. CNTs primarily function as one-dimensional nanostructures that improve soil properties, facilitate nutrient delivery, and immobilize environmental contaminants, whereas CQDs, owing to their ultrasmall size, excellent water dispersibility, and intrinsic fluorescence, actively regulate plant metabolism, photosynthesis, nutrient acquisition, and antioxidant defense. Their distinct transport pathways, rhizosphere interactions, and subcellular localization are critically evaluated alongside recent advances in synthesis, surface functionalization, and physicochemical modification. The review further summarizes current evidence regarding their roles in enhancing tolerance to heavy metal toxicity, salinity, and drought stress through modulation of reactive oxygen species scavenging, osmotic regulation, ion homeostasis, and stress-responsive signaling pathways. Potential phytotoxicity, environmental persistence, ecological risks, and green synthesis strategies are also discussed to provide a balanced assessment of their agricultural applications. Emerging opportunities for synergistic CNT–CQD composite systems are discussed, together with major knowledge gaps and future research priorities, including machine learning-assisted nanomaterial design, multi-omics characterization, long-term field validation, and life cycle-based risk assessment. This review establishes a conditional, context-dependent structure–behavior–function conceptual framework that provides theoretical guidance for the rational design and safe implementation of carbon nanomaterials in next-generation sustainable agriculture.