Programmable artificial phototactic microswimmer

Journal name:
Nature Nanotechnology
Volume:
11,
Pages:
1087–1092
Year published:
DOI:
doi:10.1038/nnano.2016.187
Received
Accepted
Published online

Abstract

Phototaxis is commonly observed in motile photosynthetic microorganisms. For example, green algae are capable of swimming towards a light source (positive phototaxis) to receive more energy for photosynthesis, or away from a light source (negative phototaxis) to avoid radiation damage or to hide from predators. Recently, with the aim of applying nanoscale machinery to biomedical applications, various inorganic nanomotors based on different propulsion mechanisms have been demonstrated. The only method to control the direction of motion of these self-propelled micro/nanomotors is to incorporate a ferromagnetic material into their structure and use an external magnetic field for steering. Here, we show an artificial microswimmer that can sense and orient to the illumination direction of an external light source. Our microswimmer is a Janus nanotree containing a nanostructured photocathode and photoanode at opposite ends that release cations and anions, respectively, propelling the microswimmer by self-electrophoresis. Using chemical modifications, we can control the zeta potential of the photoanode and program the microswimmer to exhibit either positive or negative phototaxis. Finally, we show that a school of microswimmers mimics the collective phototactic behaviour of green algae in solution.

At a glance

Figures

  1. Schematic design and structure characterization of a Janus artificial microswimmer
    Figure 1: Schematic design and structure characterization of a Janus artificial microswimmer

    a, Schematic of a Janus artificial microswimmer. An array of TiO2 nanowires (yellow) is grown on a silicon nanowire (pink). Platinum (black) nanoparticles, which serve as the electrocatalyst, are attached to the surface of the silicon nanowire. On illumination, photoexcited minority carriers drive the PEC reaction on the nanotree surface and generate charged PEC products. The electric field generated by unbalanced ions propels the charged Janus nanotree. b, False-coloured SEM image of a Janus nanotree forest prepared on a silicon substrate. c, TEM image of an individual Janus nanotree. Insets: selected area electron diffraction patterns of a TiO2 nanowire (inset, upper right) and silicon nanowire (inset, lower left) indicate the single-crystalline nature of both materials.

  2. Individual Janus nanotree migration under ultraviolet illumination.
    Figure 2: Individual Janus nanotree migration under ultraviolet illumination.

    a,b, Superimposed images of sequential frames show the migration of individual Janus nanotrees under global illumination in 0.1% H2O2 (a) and a mixture solution of 1,4-benzoquinone and hydroquinone (1 mM:10 mM) (b). Arrows indicate migration direction. c, A typical pristine Janus nanotree migration speed in 0.1% H2O2 under chopped light exposure. Scale bars, 10 µm.

  3. Chemically treated Janus nanotree migration.
    Figure 3: Chemically treated Janus nanotree migration.

    ac, Superimposed images of sequential frames show the migration of chemically treated Janus nanotrees. Arrows indicate the migration direction. AEEA-treated Janus nanotrees migrate in a tail-forward direction, whereas platinum- and CSPTMS-treated Janus nanotrees migrate in a head-forward direction. d, Janus nanotree speed scales linearly with light intensity. Platinum- and CSPTMS-treated nanotrees migrate in a head-forward direction, which is presented as negative speed. The light-intensity-normalized migration velocities of different nanotrees are shown in the plots (95% confidence interval). Scale bars, 10 µm.

  4. The Janus nanotree self-aligns with the illumination direction and nanotree navigation.
    Figure 4: The Janus nanotree self-aligns with the illumination direction and nanotree navigation.

    a, Schematics of the nanotree alignment mechanism with side illumination. Asymmetry of the reaction speed between TiO2 nanowires on the illuminated side and those on the shaded side produces an unbalanced H+ distribution and an electric field E perpendicular to the axis of the nanotree. Because the TiO2 head is positively charged, the electric force F rotates the nanotree and pushes the TiO2 head away from the light source. b, Angular speed ω scales linearly with illumination intensity. Upper inset: plot showing that ω modulates with the illumination angle θ and is fitted by ω = C sin(θ + θ0) + ω0. Lower inset: a superimposed image of sequential frames of an individual pristine nanotree rotation following the rotating light source. Scale bar, 10 µm. c, The trajectory of a pristine nanotree spells ‘nano’, navigated by light (Supplementary Movie 5).

  5. Programmable phototaxis of an individual Janus nanotree by chemical treatment.
    Figure 5: Programmable phototaxis of an individual Janus nanotree by chemical treatment.

    a,b, Superimposed images of sequential frames indicate that the pristine and AEEA-treated Janus nanotrees migrate in a tail-forward direction and exhibit positive phototaxis. c, The CSPTMS-treated Janus nanotree migrates in a head-forward direction and exhibits negative phototaxis. d, The platinum-nanoparticle-decorated Janus nanotree migrates in a head-forward direction and shows positive phototaxis. Scale bars, 10 µm.

  6. Schooling of artificial microswimmers, compared with natural green algae.
    Figure 6: Schooling of artificial microswimmers, compared with natural green algae.

    a, Sequential images of the green algae E. gracilis suspension in aqueous solution with illumination from the right side demonstrate positive phototaxis. b,c, Sequential images of the pristine and CSPTMS-treated Janus nanotree suspension in H2O2 solution with ultraviolet illumination from the right side demonstrate positive (b) and negative (c) phototaxis.

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Author information

Affiliations

  1. Department of Chemistry, The University of Hong Kong, Pokfulam 999077, Hong Kong

    • Baohu Dai,
    • Jizhuang Wang,
    • Ze Xiong,
    • Xiaojun Zhan,
    • Wei Dai &
    • Jinyao Tang
  2. Department of Mechanical Engineering, The University of Hong Kong, Pokfulam 999077, Hong Kong

    • Chien-Cheng Li &
    • Shien-Ping Feng

Contributions

B.D. and J.T. conceived and designed the experiments. B.D., J.W., Z.X., X.Z. and W.D. fabricated the devices and performed the measurements. C.-C.L. and S.-P.F. helped with zeta potential measurements. B.D. and J.T. co-wrote the paper. All authors discussed the results and commented on the manuscript.

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The authors declare no competing financial interests.

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