Establishing a Model Precursor System: Over a Decade of Research on Carbon Dots from the Citric Acid‐Urea System

ABSTRACT Carbon dots (CDs) have attracted extensive interest due to their tunable photoluminescence, biocompatibility, and simple synthesis. Among various precursor systems, the citric‑acid‐urea (CA‐Urea) pair has emerged as a highly versatile and scalable platform with exceptional control over structural evolution and optical behavior. Over the past decade, systematic studies have revealed how precursor chemistry and reaction parameters govern carbon core formation, surface states, emission tunability, photothermal, photodynamic, and photocatalytic performance. These mechanistic insights have enabled advanced engineering strategies, including heteroatom doping, surface modification, supramolecular assembly, and hybridization, to produce multifunctional CDs with enhanced luminescence, catalytic activity, and targeted biological functions. This review summarizes key progress on the CA‐Urea model precursor system‐based CDs, highlighting structural control principles, functional diversification, and applications in LEDs, sensing, catalysis, bioimaging, and cancer therapy. By demonstrating how a simple chemical pair can yield highly tunable and high‐performance CDs, this work provides a foundational framework to accelerate the development and translational utilization of next‑generation CDs in energy, environmental, biomedical, and clinical fields.