Autonomous extraction and separation for rare-earth metals using self-propelled droplets with a pH-responsive function

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We have developed self-propelled droplets having the abilities to detect a chemical gradient, to move toward a higher concentration of a specific metal ion, and to extract it. Such abilities rely on the high surface activity of DEHPA in response to pH and the affinity of DEHPA for the dysprosium ion. We used two external stimuli as chemical signals to control droplet motion: a pH signal to induce motility, and metal ions to induce directional sensing. The oil droplets loaded with DEHPA spontaneously move around beyond the threshold of pH even in a homogeneous pH field. In the presence of a gel block containing metal ions, the droplets show directional sensing and their motility is biased toward higher concentrations. The metal ions can be arranged in decreasing order of directional sensing as Dy>>Nd>Y>Gd. Furthermore, the analysis of component by an atomic absorption spectrophotometer reveals that the metal ions can be extracted to the interiors of the droplets.