Strategies for aqueous triplet-triplet annihilation upconversion using nanostructured materials

Achieving photon upconversion in aqueous environments remains a major challenge from a materials design perspective, largely due to oxygen quenching, poor solubility of many chromophores, and the presence of competing deactivation pathways. Among the available approaches, triplet-triplet annihilation upconversion (TTA-UC) is particularly appealing because it operates under low excitation power densities. However, despite its maturity in organic solvents and solid-state systems, the implementation of TTA-UC in water has proven far more demanding. This review examines recent progress in aqueous TTA-UC from the standpoint of nanostructured material design. Rather than focusing solely on the photophysical mechanism, we discuss how different material architectures have been engineered to enable efficient upconversion in water by controlling molecular confinement, interfacial environments, and oxygen accessibility. The main classes of aqueous-compatible platforms such as nanocapsules, micelles, liposomes, microemulsions, hydrogels, nanoparticles, supramolecular assemblies, and metal-organic frameworks, are critically compared. Particular emphasis is placed on identifying the key structure-property relationships that determine triplet energy transfer, annihilation efficiency, and operational stability in aqueous media. By contrasting the performance and limitations of the different nanostructured systems, the review highlights general design strategies that are relevant across application areas including photomedicine, photocatalysis, optogenetics, and chemical sensing. Finally, remaining challenges and open questions are discussed, especially in relation to scalability, long-term robustness, and realistic operating conditions. Overall, this review aims to provide a coherent materials-oriented framework that can guide the rational development of photon upconversion systems capable of operating efficiently in water. This review explores how triplet-triplet annihilation upconversion (TTA-UC) enables the conversion of low-energy light into higher-energy emissions in aquatic environments. Furthermore, various encapsulation and supramolecular strategies that stabilize these systems against the challenges of oxygen and solubility are highlighted. Finally, key nanostructures and applications in biomedicine, photocatalysis, and optogenetics are discussed, highlighting current advances and future opportunities. • Design strategies for photon upconversion in water. • Nanostructures control oxygen quenching and triplet diffusion. • Capsules, micelles and silica frameworks compared. • Structure–property rules for efficient aqueous upconversion. • Applications in bioimaging, photocatalysis and sensing.