Bimodal Underwater Motion Monitoring Using Room Temperature Phosphorescent Zwitterionic Hydrogels Enabled by Salting-Out Ion-Induced Crystallization

Hydrogels featuring excellent mechano-electric performance have been widely developed for intelligent robots. Nevertheless, realizing the accurate visualization of motion trajectories for intelligent control remains challenging, particularly in harshly interfering underwater environments. Herein, we present zwitterionic hydrogels that integrate perceptual capabilities with long-lived room temperature phosphorescent (RTP) visualization for underwater motion monitoring, enabled by salting-out ion-directed in situ crystallization of melamine (MA) and poly(vinyl alcohol) (PVA). The resulting supramolecular MA-SO₄^2- (Supra-MS) nanocrystals act as both RTP chromophores and a rigid matrix for on-demand RGB-based multicolor RTP emission. Notably, the hydrogel achieves the synergistic integration of high ionic conductivity (7.7 S/m), favorable mechanical properties (∼320% stretchability and 1.1 MJ m^-3 toughness), and persistent underwater RTP emission (retaining stability after immersion for 7 days). The as-prepared hydrogels enable bimodal real-time tracking of locomotion trajectories and swimmers' postural dynamics. This work advances RTP-integrated flexible electronics for applications in underwater scenarios.