Motion-Based Detection of Lanthanides(III) Using Self-Propelled Droplets

The directional and controllable transportation of self-propelled chemical objects in response to chemical signals in environmental media holds considerable promise for diverse applications. We investigated the chemotaxis of oil droplets loaded with surfactants to detect spatial gradients of lanthanide­(III) ions, among which Dy³⁺ and Tm³⁺ were the most effective chemoattractants for steering droplets toward the targets. Patterns within a chemotactic index of the lanthanide series exhibited a convex tetrad effect and a breakpoint at Gd³⁺. The Jørgensen–Kawabe equation, which is based on the refined spin-pairing energy theory, quantitatively demonstrated the tetrad effect. The self-propelled droplets served as a motion-based detection mechanism for lanthanides­(III).