Chemotactic Behavior of Catalytic Motors in Microfluidic Channels

Abstract

Chemotaxis is the ability of living systems to sense chemical gradients in their surroundings and react accordingly. Most prominent chemotactic behavior is exhibited by simple microorganisms that are able to migrate towards gradients of concentrations of the chemo-attractant, by activation of complex intracellular sensing cascades making use of specific chemoreceptors. [1] Classical demonstrations of this phenomenon were described by Engelmann, who observed the accumulation of Bacterium termo, a rod-shaped bacterium, in oxygen-rich surroundings of cells undergoing photosyn-thesis. [2] Yet, other bacteria can show completely different behavior, such as Spirillum tenue, which is repelled by high oxygen pressure, presenting an anti-chemotactic behavior. [3] Increasing research in the field of artificial micro-and nanomotors has revealed enormous advances and several similarities not only to biological motors, but also to bacteria. [4] Recently, several sources including magnetic field, [5] light, [6] temperature, [7] electrical stimuli, [8] or ultra-sound waves [9] have been employed for external control over the motion of microscopic motors. Enzymes and DNA used as motors showed attraction to chemical gradients, that is, chemotaxis. [10] Sen et al. reported the chemotactic behavior of Janus motors powered by polymerization reactions. [11] The same group studied the chemotaxis of bimetallic rods in a gradient of hydrogen peroxide solution when a H 2 O 2-soaked agarose gel was placed on a cover slip containing the nanomotors suspension. [12] The study was also performed using capillaries containing different concentrations of H 2 O 2 as chemo-attractant, similar to the classical experiments on bacterial chemotaxis. [13] The authors described that the motion of the self-propelled rods presented a slight bias directed towards the capillaries with higher H 2 O 2 concentration. Recently, Solovev et al. reported that catalytic tubular microjets can be attracted to specific targets by using capillary forces. [14] This effect takes place at the air-liquid interface and resembles the motion of water striders, induced by the meniscus climbing effect. There is thus the possibility that the capillaries used for the chemotaxis of nanorods may also have contributed to a physical attraction force (capillary force) in addition to the chemical attractive force provided by the H 2 O 2. To date, there is a lack of chemotactic studies in which the attraction of artificial motors within the bulk liquid only originates from the chemical source, thereby ruling out any other external factor, such as capillary forces. Moreover, the movement of living organisms and the chemotactic motion of bacteria varies depending on their size and shape. [15] Thus, a comparative study between artificial chemotaxis of two types of motors, with different shapes and size within the same system, is of great interest. Herein, we present the chemotactic attraction of two types of catalytic motors (tubular microjets [16] and Janus particles [5h]) towards high concentrations of hydrogen perox-ide, which is used both as a fuel and as chemical attractant, in a microfluidic device where the capillary forces that act at the air-liquid interface can be neglected. We quantified the deviation angle (opening angle b) the motors experience once the intersection of the channels is reached for different concentrations of the chemicals. Remarkably, although driven by different propulsion mechanisms, both types of catalytic motors, tubular microjets and spherical Janus beads, orient and deviate towards higher hydrogen peroxide concentrations. We observed that spherical motors are more sensitive to the gradient of the fuel imposed in the system, which is probably governed by the processes of translational and rotational diffusion of the catalytically active motors. Their "turn" is different in magnitude, making the shape of the artificial motors an important parameter in the movement through chemotaxis. We designed a three-inlet parallel flow device in a Y-channel geometry [17] (Figure 1 A and Experimental Section) where colloidal micromotors flow through the central channel , whereas the aqueous solutions (with or without H 2 O 2) flow in the other two side-channels. Microfluidics offers a high degree of control over the chemical environment where analytes, microorganisms, or particles are spatially localized. In addition, it enables the impact of various undesirable factors, such as capillary forces that may interfere with the results of the analysis, to be decreased. Since the fluid flows and channel geometries can be precisely controlled, microfluidics seems an excellent tool