Structure property relationships and excited state photophysics of carbon dots for photothermal and photodynamic applications

Carbon dots (CDs) are versatile light-responsive nanomaterials whose tunable electronic structure, surface chemistry, and biocompatibility make them promising for photothermal and photodynamic applications across biomedicine, environmental remediation, agriculture, and sensing. This review provides a critically integrated account of how precursor selection, carbon-core structure, heteroatom doping, and surface passivation govern the excited-state behaviour of CDs and thereby determine photothermal heat generation and reactive oxygen species (ROS) production. Particular emphasis is placed on photophysical and photochemical principles relevant to this field, including absorption pathways, non-radiative relaxation, intersystem crossing, triplet-state formation, and the balance between Type I and Type II ROS mechanisms. Recent advances in tumour ablation, antimicrobial phototherapy, pollutant degradation, photoimmunotherapy, plant disease management, and vector control are evaluated. Persistent barriers to translation, such as shallow optical penetration, batch-to-batch variability, inconsistent performance metrics, incomplete mechanistic understanding, and limited long-term safety data, are critically examined together with emerging strategies to improve reproducibility, standardization, and clinical applicability. Unlike previous reviews that primarily summarize individual applications, this review integrates carbon-dot synthesis, excited-state photophysics, structure–property relationships, and photothermal/photodynamic mechanisms with multifunctional applications within a single mechanistic framework, while identifying the key challenges and future research directions required for their practical translation. Carbon dots enable integrated photothermal and photodynamic functionalities governed by excited-state dynamics. Heteroatom doping and surface engineering modulate intersystem crossing and energy dissipation pathways. Structure–property relationships dictate photothermal efficiency and ROS generation performance. Applications extend from cancer therapy to environmental remediation and smart agricultural systems. Standardized photochemical metrics and scalable synthesis remain critical challenges for translation.