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Ion-exchange enabled synthetic swarm


Active matters are out-of-equilibrium systems that convert energy from the environment to mechanical motion. Non-reciprocal interaction between active matters may lead to collective intelligence beyond the capability of individuals. In nature, such emergent behaviours are ubiquitously observed in animal colonies, giving these species remarkable adaptive capability. In artificial systems, however, the emergence of non-trivial collective intelligent dynamics remains undiscovered. Here we show that a simple ion-exchange reaction can couple self-propelled ZnO nanorods and sulfonated polystyrene microbeads together. Chemical communication is established that enhances the reactivity and motion of both nanorods and the microbeads, resulting in the formation of an active swarm of nanorod–microbead complexes. We demonstrate that the swarm is capable of macroscopic phase segregation and intelligent consensus decision-making.

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Fig. 1: The exchange communication between different microswimmer species and the model system used in this study.
Fig. 2: The ion-exchange interaction between ZnO nanorod and sulfonated PS.
Fig. 3: 3D velocity field and pH profile of the ZnO nanorod–sulfonated PS system.
Fig. 4: The emergence of swarming behaviour of ZnO–sulfonated PS mixture in experiment and simulation.
Fig. 5: Sequential snapshots of macroscopic particle population of ZnO–sulfonated PS mixture in glass petri dishes.

Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request. Source data are provided with this paper.


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This work was supported in part by the Innovation and Technology Commission (HKSAR, China) to the State Key Laboratory of Synthetic Chemistry, and the Hong Kong Research Grants Council General Research Fund (grant nos. GRF17305917, GRF17303015 and GRF17304618), the Seed Funding for Interdisciplinary Research (University of Hong Kong), the URC Strategic Research Theme on New Materials (University of Hong Kong), the Science Technology and Innovation Programme of Shenzhen (JCYJ20170818141618963), the Shenzhen-Hong Kong Innovation Circle Programme (SGDX2019081623341332) and the National Natural Science Foundation of China (no. 11874397).

Author information

Authors and Affiliations



C.W. and J.T. conceived and designed the project. C.W., J.D., L.G. and J.L. prepared the samples. C.W., J.D. and J.T. conducted most of the measurements and analysis. X.L. helped with COMSOL simulation. M.Y. performed the coarse-grained simulation and analysis. C.W. and J.T. wrote most of the manuscript. J.D. and M.Y. participated in the manuscript revision. All authors discussed the results and commented on the manuscript.

Corresponding authors

Correspondence to Mingcheng Yang or Jinyao Tang.

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Competing interests

The authors declare no competing interests.

Additional information

Peer review information Nature Nanotechnology thanks the anonymous reviewers for their contribution to the peer review of this work.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Supplementary information

Supplementary Information

Supplementary Figs. 1–14, Discussion and Table 1.

Supplementary Video 1

A. Self-propulsion of the ZnO nanorod in water. B. Assembly of sulfonated PS in water. C. Chemotaxis of sulfonated PS towards the Zn2+ source.

Supplementary Video 2

A. Synergistic interaction between ZnO nanorods and sulfonated PS. B. Attraction, rotation and expulsion of sulfonated PS.

Supplementary Video 3

A. Motion of sulfonated PS with an immobilized ZnO nanorod. B. The sulfonated PS depletion region formation around a ZnO nanorod. C. Interaction between a CaCO3 microcube and sulfonated PS. D. Interaction between a MIL-101 (Fe) nanoparticle and ZnO nanorod. E. Interaction between Janus Pt-TPM and a PS microsphere.

Supplementary Video 4

A. 3D pH profile near a ZnO nanorod during the interaction between a ZnO nanorod and sulfonated PS B. 3D trajectory of sulfonated PS when interacting with a ZnO nanorod fixed on the substrate.

Supplementary Video 5

A. Complex formation with an active ZnO nanorod and sulfonated PS. B. Cluster formation of ZnO nanorod–sulfonated PS. C. Attraction and alignment behaviour of free sulfonated PS. D. Large-scale swarming behaviour of the ZnO nanorod–sulfonated PS mixture.

Supplementary Video 6

A. Simulation of a ZnO–sulfonated PS system with chemical communication and electroosmosis interaction. B. Simulation of a ZnO–sulfonated PS system without chemical communication. C. Simulation of a ZnO–sulfonated PS system without electroosmosis interaction. D. Simulation of a ZnO–sulfonated PS system in circular confinement.

Supplementary Video 7

A. Phase segregation of active ZnO–sulfonated PS in a glass petri dish. B. Diffusion of passive PS microspheres in a glass petri dish. C. Active Pt-TPM Janus microspheres in a glass petri dish. D. Active ZnO nanorods in a glass petri dish. E. Phase segregation of active ZnO–sulfonated PS in a glass petri dish with 10 μM EDTA. F. Phase segregation of active ZnO–sulfonated PS in a glass petri dish with a sulfonated PS-coated quartz plate and a blank quartz plate for comparison. G. Phase segregation of active ZnO–sulfonated PS in a glass petri dish with fixed ZnO nanorods loaded agarose disc plate and a blank agarose disc for comparison.

Supplementary Video 8

Quorum decision-making of ZnO–sulfonated PS in four cookie moulds.

Source data


Source Data Fig. 2 Figure 2c, The migration speed of the ZnO nanorod shown, where the shaded speed spikes correspond to the transient ZnO assembly with sulfonated PS. Figure 2e, Relationship of the speed of sulfonated PS with their separation distance from the ZnO nanorod. The error bars represent the standard deviation of the speed from multiple particles (n = 50).

Source Data Fig. 3

The power-law dependence between ion flux density and interparticle distance, where the error bars are the standard deviation of multiple sulfonated PS (n = 20).

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Wu, C., Dai, J., Li, X. et al. Ion-exchange enabled synthetic swarm. Nat. Nanotechnol. 16, 288–295 (2021).

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