Designing Micro- and Nanoswimmers for Specific Applications

Abstract

Self-propelled colloids have emerged as a new class of active matter over the past decade. These are micrometer sized colloidal objects that transduce free energy from their surroundings and convert it to directed motion. The self-propelled colloids are in many ways, the synthetic analogues of biological self-propelled units such as algae or bacteria. Although they are propelled by very different mechanisms, biological swimmers are typically powered by flagellar motion and synthetic swimmers are driven by local chemical reactions, they share a number of common features with respect to swimming behavior. They exhibit run-and-tumble like behavior, are responsive to environmental stimuli, and can even chemically interact with nearby swimmers. An understanding of self-propelled colloids could help us in understanding the complex behaviors that emerge in populations of natural microswimmers. Self-propelled colloids also offer some advantages over natural microswimmers, since the surface properties, propulsion mechanisms, and particle geometry can all be easily modified to meet specific needs. From a more practical perspective, a number of applications, ranging from environmental remediation to targeted drug delivery, have been envisioned for these systems. These applications rely on the basic functionalities of self-propelled colloids: directional motion, sensing of the local environment, and the ability to respond to external signals. Owing to the vastly different nature of each of these applications, it becomes necessary to optimize the design choices in these colloids. There has been a significant effort to develop a range of synthetic self-propelled colloids to meet the specific conditions required for different processes. Tubular self-propelled colloids, for example, are ideal for decontamination processes, owing to their bubble propulsion mechanism, which enhances mixing in systems, but are incompatible with biological systems due to the toxic propulsion fuel and the generation of oxygen bubbles. Spherical swimmers serve as model systems to understand the fundamental aspects of the propulsion mechanism, collective behavior, response to external stimuli, etc. They are also typically the choice of shape at the nanoscale due to their ease of fabrication. More recently biohybrid swimmers have also been developed which attempt to retain the advantages of synthetic colloids while deriving their propulsion from biological swimmers such as sperm and bacteria, offering the means for biocompatible swimming. In this Account, we will summarize our effort and those of other groups, in the design and development of self-propelled colloids of different structural properties and powered by different propulsion mechanisms. We will also briefly address the applications that have been proposed and, to some extent, demonstrated for these swimmer designs.

Figure 1

Fabrication and control of tubular microjets. SEM image of (A) a tubular microjet fabricated by the roll-up method, (B) a tubular microjet fabricated by electrodeposition method, and (C) a nanojet synthesized based on heteroepitaxically grown layers. (D) Different types of swimming behavior. (E) Folding and unfolding of thermoresponsive microtubes leads to a variation in propulsion velocities. (F) An eight-coil magnetic setup used for 3-D control of microjets. (G) Light regulated velocity control of microjets. Panel A reprinted with permission from ref (39). Copyright 2016 John Wiley and Sons, Inc. Panel C reprinted with permission from ref (24). Copyright 2011 John Wiley and Sons, Inc. Panel D reprinted with permission from ref (20). Copyright 2009 John Wiley and Sons, Inc. Panel E reprinted with permission from ref (23). Copyright 2014 John Wiley and Sons, Inc. Panel G reprinted with permission from ref (28). Copyright 2011 John Wiley and Sons, Inc. Panel F reprinted with permission from ref (27). Copyright 2013 AIP Publishing.

Figure 2

Environmental and biomedical applications of microjets. (A) Schematic for the degradation of polluted water (Rh6G) into inorganic products. (B) Scheme of graphene-oxide (GOx) microbots for lead decontamination and recovery. (C) Degradation of Rh6G (blue diamonds). Black dots and squares are controls. (D) Removal of lead by GOx micromotors. (E) Lifetime activity of microjets for 24 h. (F) SEM (A) and optical (B) images of a microjet with CAD cells. (G) Optical microscopy image of a nanotool drilling into a HeLa cell. Scalebar represents 10 μm. Panels A and C reprinted with permission from ref (35). Copyright 2013 American Chemical Society. Panels B and D reprinted with permission from ref (40). Copyright 2016 American Chemical Society. Panel G reprinted with permission from ref (25) Copyright 2012 American Chemical Society. Panel F reproduced with permission from ref (31), 2010 Royal Society of Chemistry.

Figure 3

Self-phoretic Janus microswimmers. (A) SEM image of silica–Pt Janus particles. (B) External magnetic fields to guide particles. (C) Magnetic caps used to transport paramagnetic particles. (D) Transport of cargo by single/double Janus particle configuration. Panels A and C reproduced with permission from ref (44). Copyright 2012 American Chemical Society. Panel B reproduced with permission from ref (45). Copyright 2015 Intech. Panel D reproduced with permission from ref (46). Copyright 2011 Royal Society of Chemistry.

Figure 4

Self-phoretic Janus microswimmers near surfaces. (A) Silica–Pt Janus particles confined near a surface reorient on addition of H₂O₂. A second vertical step has similar effect. (B) Phase portrait of a Janus particle near a surface showing the steady-state at the cap-parallel position. (C) (a) Approach, reorientation, and guiding of a Janus particle at a vertical 800 nm step. (b) Numerically calculated steady-state distribution of the reaction products. (D) Self-assembly of Janus microswimmers around an asymmetric gear. (E) Dependence of angular velocity of the microgears on the number of Janus particles. Panels A–C reproduced with permission from ref (47). Copyright 2016 Nature Publishing Group. Panels D and E reproduced with permission from ref (48). Copyright 2016 John Wiley & Sons, Inc.

Figure 5

Propulsion at nanoscale. (A) Janus mesoporous silica nanomotor: (a) TEM image and schematic of the nanomotor, (b) tracking trajectories and (c) MSD plots of the nanomotors. (B) Au–Pt nanomotor: (a) schematic of the self-electrophoresis of Au–Pt nanomotor; DLS measurement indicates (b) a left shift of relaxation time and (c) enhancement of translational diffusion with increasing H₂O₂ concentration. (C) (a) Schematic of a stomatocytes nanomotor, (b) MSD of the platinum-filled stomatocytes before and after the addition of H₂O₂, and (c) size distribution of platinum-filled stomatocytes before (blue) and after (red) the addition of H₂O₂. Inset shows the same measurements for stomatocytes without Pt. Panel A reprinted with permission from ref (51). Copyright 2015 American Chemical Society. Panel B reprinted with permission from ref (52). Copyright 2014 American Chemical Society. Panel C reprinted from ref (53) with permission. Copyright 2012 Nature Publishing Group.

Figure 6

Enzyme-powered micro- and nanomotors. (A) Bubble propulsion of (a) tubular microjet and (b) Janus mesoporous cluster motor modified with catalase. (B) Self-propulsion of Janus hollow mesoporous nanomotors powered by various enzymes, catalase, urease, and glucose oxidase (GOx). (C) Urease powered hollow capsule: (a) motion control by manipulating the enzymatic activity, (b) “on”/“off” motion control by addition of inhibitor and DTT, and (c) repeated motion control up to 8 cycles. Panel A, part a, reproduced with permission from ref (37). Copyright 2010 American Chemical Society. Panel A, part b, reprinted with permission from ref (56). Copyright 2015 Royal Society of Chemistry. Panel B reproduced with permission from ref (57). Copyright 2015 American Chemical Society. Panel C reprinted with permission from ref (58). Copyright 2016 American Chemical Society.

Figure 7

Sperm and bacteria powered biohybrids. (A) Bull spermatoozen trapped within microtubes. (B) Sperm cell coupling (i), transport (ii), approach to the oocyte membrane (iii), and release (iv). (C) MC-1 bacteria with (bottom) and without liposomes (top). (D) Janus particles with specific cell adhesion of E. coli to the metal cap. (E) Beads (30 μm) with attached Serratia marcescens. (F) Examples of E. coli swimming with 2 μm and 600 nm Janus particles. Panels A, D, and F reprinted with permission from refs ( and 76). Copyright 2016 John Wiley and Sons, Inc. Panel B reprinted with permission from ref (69). Copyright 2016 American Chemical Society. Panel C reprinted with permission from ref (72). Copyright 2014 American Chemical Society. Panel E reprinted with permission from ref (75). Copyright 2012 Springer Science+Business Media, LLC.