Covalent organic frameworks based dual-emission materials for ratiometric fluorescence sensing: A review on design strategies, mechanisms, and applications

Covalent organic frameworks (COFs), characterized by high specific surface area, tunable pore structures, and excellent stability, provide an ideal platform for developing high-performance ratiometric fluorescence sensors. By measuring the intensity ratio of two emission signals, these sensors offer built-in self-calibration, overcoming the limitations of single-signal probes affected by environmental interference, and thus improving sensitivity, selectivity, and reliability in detecting trace analytes in complex samples. This review systematically outlines construction strategies for dual-emission COF-based ratiometric sensors, such as intrinsic backbone dual-emission, doping-induced dual-emission, and hybrid/heterostructure-induced dual-emission. For each approach, advantages, limitations, and development directions are discussed. Key structural factors (e.g. topology, π–π stacking, donor–acceptor motifs, pore environment, and crystallinity) and their roles in integrating luminescent units are discussed to explain how they collectively influence dual-emission performance and stability. Common construction challenges and corresponding mitigation strategies are also summarized to enhance sensor reliability and efficiency. The review further elaborates on relevant sensing mechanisms, including excited-state intramolecular proton transfer (ESIPT) and Förster resonance energy transfer (FRET), as well as summarizes the interrelationships of construction strategy-response mode-sensing mechanism. Performance advantages and recent applications in environmental monitoring, food safety, and biomedical analysis are highlighted. Despite their promise, practical use of these sensors still faces challenges in signal controllability and environmental adaptability. Based on current limitations, this review suggests future directions: precise control and mechanistic study of dual-emission behavior, enhancing signal reliability in real samples and enabling device integration, and data-driven material design for performance optimization. Through collaborative advances, dual-emission COF-based sensors are expected to evolve into versatile detection platforms for environmental, clinical, and food safety applications, promoting the practical adoption of next-generation sensing technologies. • Dual-emisson COFs-based ratiometric fluorescence sensors design strategies are summarized by emission differences. • Dual-emisson COFs-based ratiometric fluorescence sensors and their luminescent mechanism are described. • Applications of dual-emisson COFs-based ratiometric sensors in environmental, food and biomedical analysis are reviewed. • Challenges and future trends,including signal reliability and device integration, are outlined.