Catalytically powered dynamic assembly of rod-shaped nanomotors and passive tracer particles

Abstract

Nano-and microscale motors powered by catalytic reactions exhibit collective behavior such as swarming, predator-prey interactions, and chemotaxis that resemble those of biological microorganisms. A quantitative understanding of the catalytically generated forces between particles that lead to these behaviors has so far been lacking. Observations and numerical simulations of pairwise interactions between gold-platinum nanorods in hydrogen peroxide solutions show that attractive and repulsive interactions arise from the catalytically generated electric field. Electrokinetic effects drive the assembly of staggered doublets and triplets of nanorods that are moving in the same direction. None of these behaviors are observed with nanorods composed of a single metal. The motors also collect tracer microparticles at their head or tail, depending on the charge of the particles, actively assembling them into close-packed rafts and aggregates of rafts. These motor-tracer particle interactions can also be understood in terms of the catalytically generated electric field around the ends of the nanorod motors.dynamic interactions | self-assembly | colloidal transport T he dynamic interactions between moving objects, in particular their response to external stimuli and their communication with each other, govern their collective behavior on many length scales. Schooling of fish and flocking of birds are good examples of emergent phenomena that are orchestrated by communication between individuals in a large group. In these systems, macroscale organization is typically driven by nearest neighbor interactions that follow simple rules. To reach the level of organization seen in such living assemblies, fast and precise (in terms of distances, angles, and velocities) communication and control are required from the members. It is now straightforward to create computational models from which such dynamic structures emerge, but artificial systems that mimic behaviors as complicated as fish schooling have very rarely been realized experimentally in macroscopic engineered systems (1). On the other hand, self-assembly at the nano-and molecular levels already demonstrates a certain level of complexity and has furthered our understanding of dynamic interactions at small scales (2, 3).There are already many examples of particle assembly driven by local forces or externally applied fields. Externally applied light, magnetic, electric, and acoustic fields can drive symmetric particles into ordered arrays (4-7). Colloidal Janus particles selfassemble into complex structures by various mechanisms (8-12). However, in these examples the particle aggregates hardly approach the complexity of assemblies of living organisms; the interactions are passive responses to local forces and external fields with very limited interparticle communication or active response to the behavior of nearest neighbors.Interactions between active particles, on the other hand, can more closely mimic those of living organisms (13-17). Powered particles generate signals, t...

Electrodeposited conductive polymers for controlled drug release: polypyrrole

“…These miniaturised systems can be divided, depending on their motion and actuation into two main types: micro/nano motors and micro/nano pumps [70,71,73].…”

“…The electron transfer is compensated by the generation and consumption of protons on the surface of Pt and Au segments, respectively. The movements of positively charged ions create an electroosmotic flow on the nanorods/liquid interface and drag the electrolyte solution by viscosity forces, thus causing the movements of nanorods in the reverse direction by speeds that are up to ≈40 μm s −1 [71,73]. In addition, other proposed catalytic nanomotors have used different propulsion methods, such as self-diffusiophoresis (spontaneous motion of dispersed particles in a fluid induced by a concentration gradient) [75] and bubble ejection [76].…”

“…It was demonstrated that the incorporated molecules can be released by using chemical stimuli [90] and the pH change of surrounding media [73,91] or by using an external stimulation factor, such as UV light [92] and magnetic fields [93]. Theoretically, it could be possible to electrodeposit a PPycontaining drug on the nanomotor side that is then coated with Mg.…”

Unusual swelling of a polymer in a bacterial bath

“…by inserting long polymers into two-dimensional Bacillus subtilis suspensions [19,24,46]. Another complementary realization is by exposing colloidal model chains [62] to artificial colloidal microswimmers [11,63,64].…”

Sequential Tasks Performed by Catalytic Pumps for Colloidal Crystallization

“…Artificial swimmers such as self-propelled active colloids and catalytic Janus particles also provide nice examples of collective motility, swarming and dynamic self-assembly [1,3,5,6,[8][9][10][11][30][31][32][33][34][35][36][37][38][39][40][41][42][43][44]. Microfabricated catalytic pumps can be used to manipulate particles in solution and to form crystals of colloids [45][46][47][48][49][50].…”