Research progress on lanthanide coordinated multicolor fluorescent polymeric hydrogels

Lanthanide-coordinated multicolor fluorescent polymeric hydrogels are a type of smart material that combines the unique optical properties of lanthanide ions with the advantages of a three-dimensional hydrogel network structure. In recent years, a significant number of typical luminescent lanthanide ions have been developed, including Eu3+ and Tb3+ ions. These ions possess the capability of emitting red and green fluorescence through the antenna effect with the ligand, respectively. By adjusting the molar ratio of Eu3+/Tb3+ ions in the hydrogel, the emission color of the hydrogel can be effectively adjusted to be any mixture of red and green colors. Due to the dynamic reversibility of coordination bonds and the tunability of the hydrogel network structure, lanthanide coordinated hydrogels exhibit tunable luminescent colors and deformational behaviors in response to external stimuli. Chemical and temperature stimuli are the most common methods for achieving multicolor luminescence in these hydrogels. Upon the introduction of chemical stimuli such as acids, bases, or metal ions, the initial coordination interactions between lanthanide ions and organic ligands within the hydrogel can be readily disrupted, thereby altering the luminescence color of the hydrogel. Similarly, temperature variations influence the random thermal diffusion of lanthanide ions as well as the state of the ligands, thus also affecting the lanthanide ion-ligand coordination interaction. Furthermore, the thermoresponsive fluorescence behavior of lanthanide-coordinated hydrogels can be enhanced by incorporating additional thermosensitive polymer chains copolymerized with other chromophores into the hydrogel system. Meanwhile, photothermal/electrothermal effects, light, and mechanical force are emerging as novel stimulus sources that offer more convenient, precise, and residue-free strategies for inducing fluorescent color changes in lanthanide-coordinated hydrogels. The outstanding stimulus responsiveness of these hydrogels also demonstrates their significant potential applications in fields such as information encoding or encryption, visual stimulus-responsive sensors, biomimetic actuators, biomedicine, and smart textiles. Especially in the field of information anti-counterfeiting, lanthanide coordinated hydrogels can achieve higher security levels and additional information storage capacity by employing multi-stimulus responsiveness, constructing 3D/4D encryption platforms, and establishing dual optical channels featuring structural color and fluorescence. Although considerable advancements have been made in the research on lanthanide-coordinated hydrogels, the practical application of these materials still faces numerous challenges. These challenges include broadening emission spectra, optimizing material properties, and expanding functionalities such as self-healing, shape memory, and conductivity. Additionally, integrating multi-response capabilities and advancing intelligent systems requires further development. Therefore, future research should focus on developing novel organic ligands and optimizing the hydrogel network structure design. Moreover, future sustainable development will rely on collaborative efforts across multiple disciplines, including deepening cooperation with biomedical fields to meet higher demands in biological imaging and dynamic monitoring, integrating mechanical engineering technologies to construct smart grippers combining rigid and flexible elements for precise manipulation, and leveraging big data and computer algorithms to achieve precise prediction and efficient design of material properties. Thereby, these materials are poised to become a vital component of future intelligent material systems. In general, this review presents the recent progress of lanthanide-coordinated multicolor fluorescent polymeric hydrogels. We systematically classify these hydrogels based on different stimulus-response mechanisms and analyze their individual luminescence regulation mechanisms. Additionally, we discuss recent advancements in the applications of these hydrogels in various fields. Lastly, we summarize the many challenges that these hydrogels face on their path to practical application and highlight that the coordinated optimization of optical, mechanical, and responsive properties remains the central bottleneck. Interdisciplinary convergence is thus pivotal to surmount these limitations and unlock their transformative potential in advanced domains such as bioimaging, adaptive soft robotics, and predictive smart materials.