Hydrogels—soft, water-rich polymer networks—have evolved far beyond their early biomedical uses. By integrating dynamic chemical motifs, researchers have created stimuli-responsive fluorescent hydrogels that can change their optical and mechanical properties when exposed to light, heat, pH shifts, mechanical stress, or chemical agents. These materials bridge molecular-scale events with visible, measurable responses, offering ultrafast signaling, high sensitivity, and compatibility with complex systems.
Fluorescence is introduced into hydrogels through covalent linking or physical doping of emissive species. Traditional dyes such as rhodamine, fluorescein, and coumarin offer high intensity in dilute, well-dispersed states. Embedding them in hydrogels prevents aggregation-caused quenching by maintaining separation in the hydrated network. In some designs, these dyes act as passive reporters; in others, the chromophores themselves undergo structural changes under stimuli, shifting emission wavelength or intensity.
Several classes of dynamic motifs underpin this field. Photoswitches like azobenzenes, diarylethenes, and spiropyrans undergo reversible light-driven isomerization, altering polarity, dipole moment, and conjugation. Li and Zhao combined diarylethene units with lanthanide complexes to create hydrogels whose fluorescence resonance energy transfer (FRET) could be toggled by light. Spiropyran’s versatility extends to pH, temperature, and mechanical responsiveness, enabling hydrogels that mimic plant phototropism or change emission under compression.
Aggregation-induced emission luminogens (AIEgens) offer a counterpoint to traditional dyes by becoming brighter when aggregated. Tang’s group embedded a triphenylamine-based AIEgen into polymer networks, producing hydrogels with polarity-dependent emission gradients. Multi-stimuli systems have been built by combining AIE cross-linkers with dynamic covalent motifs, yielding materials responsive to temperature, light, and chemical triggers. Carbon nanodots with AIE behavior add solvent-responsive color changes, expanding design flexibility.
Lanthanide complexes, particularly Eu³⁺ and Tb³⁺, provide sharp emission lines, long lifetimes, and large Stokes shifts. Coordinating these ions with suitable ligands in hydrogels overcomes water-induced quenching. Chen and Lu have integrated lanthanides with AIEgens to produce actuators whose color and shape change under multiple stimuli, using stepwise growth to add layers with distinct responses.
Host–guest chemistry, especially with cyclodextrins and cucurbiturils, offers reversible, selective binding that can modulate fluorescence. Systems combining coumarin photodimerization with cyclodextrin inclusion achieve light-driven phase transitions and emission changes. Other designs use host–guest release of quenched fluorophores to create rewritable fluorescent patterns for secure information storage.
Applications span information security, soft robotics, and sensing. In encryption, hydrogels can encode data in fluorescence patterns that appear only under specific conditions, then self-erase to prevent leakage. Time-dependent systems couple pH-responsive fluorophores with enzyme-driven reactions to create “time-locked” messages. Multiple encryption layers are possible by combining fluorescence tunability with shape memory, origami structuring, or crystalline phase programming, enabling complex anti-counterfeiting features.
For bionic actuation, bilayer hydrogels exploit differential swelling to bend or twist under stimuli, emulating plant or animal movements. Incorporating multicolor fluorescence into these actuators allows simultaneous visual signaling. Single-layer films with stimulus gradients can also bend or crawl under light, as shown in spiropyran-based peptide amphiphile gels.
In sensing, the high water content and permeability of hydrogels enable rapid analyte diffusion, improving response time and sensitivity over solid-state films. AIE-based hydrogels have been used to detect Fe³⁺ corrosion products on steel surfaces, with reversible quenching upon chelation. Glucose-responsive PEG hydrogels incorporating diboronic acid–anthracene dyes have demonstrated stable, long-term fluorescence changes correlating with glucose concentration, suitable for continuous monitoring.
Challenges remain in achieving full-spectrum tunability, simplifying synthesis, and balancing ease of fabrication with complex, adaptive behavior. Integrating multiple dynamic motifs through constitutional dynamic chemistry offers one route forward. Advances in fabrication precision, coupled with computational design and AI-driven optimization, could accelerate the deployment of these materials in aerospace, robotics, and beyond, where responsive, multifunctional components can offer both performance and security advantages.