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Self-Healing Luminescent Hydrogels

A new study by a team of researchers from China has explored the development of hydrogels with self-healing and luminescent properties. Their research has been published in the journal ACS Applied Polymer Materials.

Study: Ultrastretchable Luminescent Nanocomposite Hydrogel with Self-Healing Behavior. Image Credit: Gorodenkoff/Shutterstock.com

Lanthanide Luminescent Hydrogels

The integration of phosphors and elastomers has facilitated the development of some flexible devices reported in current literature, but their service life can be limited by repeated use and physical damage. For this reason, research has focused on the fabrication of novel soft luminescent materials with stretchable and self-healing properties.

Recently, lanthanide luminescent hydrogels have become a focus of research due to their wide range of potential applications in several fields, including luminescence sensing, information encryption, bioengineering, and digital displays. These novel materials possess unique physical and optical properties.

Lanthanide coordinating complexes are formed by lanthanide ions and their corresponding ligands. The ligands absorb energy, transferring it to the lanthanide ions, increasing their sensitivity and facilitating luminescence. However, in aqueous solutions, they can be easily quenched by water, which can impact their luminescence through the excitation of O-H bonds.

A strategy to overcome this drawback is to incorporate lanthanide ions into hydrogels. Whilst this overcomes the challenges with hydration and retaining luminescence, directly introducing lanthanide complexes into hydrogels by using them as their cross-linking points leads to poor mechanical performance.

Stable, strong cross-linking causes the mechanical strength of hydrogels, whilst stretchability and self-healing properties require dynamic, reversible cross-linking interactions in the materials. Therefore, creating lanthanide luminescent hydrogels which possess superior mechanical strength, stretchability, and self-healing properties remains challenging.

Developing a Soft, Stretchable, Self-healing Lanthanide Luminescent Hydrogel

In the study published in ACS Applied Polymer Materials the researchers have demonstrated a facile fabrication method to construct soft, strong, ultra-stretchable luminescent hydrogel which possesses superior self-healing properties. The material possesses color-tunable luminescence.

The author’s novel approach involves exploiting the synergistic effect of lanthanide complexes and aminoclay. Aminoclay is a layered magnesium orthosilicate with the chemical structure R8Si8Mg6O16(OH)4. Dispersion of transparent aminoclay particles is achieved by the exfoliation in the water of the protonation of amino groups.

Electrostatic interactions between the carboxyl and amino groups of the aminoclay in water loads modified organic ligands onto the aminoclay. Lanthanide ions are then introduced to create a lanthanide-ligand complex along with in situ copolymerization of an acrylamide monomer to construct the nanocomposite hydrogel.

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The authors have stated that their novel strategy confers three benefits. Firstly, the hydrogel’s mechanical properties are improved by hydrogen bonds between the aminoclay and the acrylamide polymer due to the presence of amino groups and the aminoclay’s high dispersibility in water. Secondly, embedding lanthanide ions between the aminoclay layers helps to avoid interference by water molecules. Thirdly, the hydrogel possesses superior room-temperature self-healing due to dynamic and reversible lanthanide-ligand coordination and the presence of hydrogen bonds.

Mechanical and Luminescence Performance

Observations of mechanical stress tests demonstrated that the nanocomposite hydrogel possessed superior strength. Increasing nanocomposite content improves this property. However, when the content was higher than 7.5 mM, the hydrogel’s stock solution precipitates and became opaque. Control tests were performed to verify the roles of aminoclay in the hydrogel. Experimental samples of pure hydrogel without aminoclay displayed significantly reduced tensile stress.

Tensile tests confirmed that the constructed hydrogels can withstand stretching up to twenty times their original length. The authors have attributed this behavior to the dynamic lanthanide-ligand coordination interaction. Energy was dissipated during the deformation process due to de-crosslinking of the lanthanide-ligand complex. Loading-unloading tensile tests were performed at different strains in the research. Results indicated that the nanocomposite hydrogel possesses superior fatigue resistance.

Self-healing at room temperature is achieved without the need for external stimuli. The lanthanide complex is used as a noncovalent linking point, with damage repaired by dynamic and reversible interaction between both metal and ligand. Additionally, the hydrogen-bond interactions between donors and acceptors in the polymer chains contribute to this ability. This was demonstrated by cutting the hydrogel block and then allowing it to repair itself for 12 hours at room temperature.

Photoluminescence properties of the prepared hydrogels were investigated in the research. The authors noted that the nanocomposite hydrogel’s luminescence efficiency was significantly higher than a pure lanthanide-ligand hydrogel without the incorporation of aminoclay. Additionally, luminescence intensity was significantly higher. The excited-state lifetime of ions in the novel hydrogel was increased, along with the absolute quantum efficiency.

The authors have stated that there are two possible reasons for the increase in luminescence performance. Firstly, the hydrogel network’s hydration may restrict the free movement of water particles avoiding the quenching of lanthanide ions. Secondly, it could be due to the localization of lanthanide-ligand complexes between aminoclay layers, which protects them from the interference of water molecules. Multi-color luminescence was achieved by altering the molar ratio of ions.

In Summary

A novel, stretchable, self-healing lanthanide luminescent hydrogel has been demonstrated in the research. A facile fabrication method has been used by the researchers to create a robust hydrogel, with a possible application as optical building blocks for emerging technologies such as wearable tech, soft machines, and human-machine interfaces.

Further Reading

Li, B., Li, Z., & Li, H (2022) Ultrastretchable Luminescent Nanocomposite Hydrogel with Self-Healing Behavior [online] ACS Appl. Polym. Mater | pubs.acs.org. Available at: https://pubs.acs.org/doi/10.1021/acsapm.1c01531

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Reginald Davey

Written by

Reginald Davey

Reg Davey is a freelance copywriter and editor based in Nottingham in the United Kingdom. Writing for News Medical represents the coming together of various interests and fields he has been interested and involved in over the years, including Microbiology, Biomedical Sciences, and Environmental Science.

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