Fundamental Aspects of Enzyme-Powered Micro- and Nanoswimmers

Abstract

Conspectus Self-propulsion at the nanoscale constitutes a challenge due to the need for overcoming viscous forces and Brownian motion. Inspired by nature, artificial micro- and nanomachines powered by catalytic reactions have been developed. Due to the toxicity of the most commonly used fuels, enzyme catalysis has emerged as a versatile and biocompatible alternative to generate self-propulsion. Different swimmer sizes, ranging from the nanoscale to the microscale, and geometries, including tubular and spherical shapes, have been explored. However, there is still a lack of understanding of the mechanisms underlying enzyme-mediated propulsion. Size, shape, enzyme quantity and distribution, as well as the intrinsic enzymatic properties, may play crucial roles in motion dynamics. In this Account, we present the efforts carried out by our group and others by the community on the use of enzymes to power micro- and nanoswimmers. We examine the different structures, materials, and enzymes reported so far to fabricate biocatalytic micro- and nanoswimmers with special emphasis on their effect in motion dynamics. We discuss the development of tubular micro- and nanojets, focusing on the different fabrication methods and the effect of length and enzyme localization on their motion behavior. In the case of spherical swimmers, we highlight the role of asymmetry in enzyme coverage and how it can affect their motion dynamics. Different approaches have been described to generate asymmetric distribution of enzymes, namely, Janus particles, polymeric vesicles, and non-Janus particles with patch-like enzyme distribution that we recently reported. We also examine the correlation between enzyme kinetics and active motion. Enzyme activity, and consequently speed, can be modulated by modifying substrate concentration or adding specific inhibitors. Finally, we review the theory of active Brownian motion and how the size of the particles can influence the analysis of the results. Fundamentally, nanoscaled swimmers are more affected by Brownian fluctuations than microsized swimmers, and therefore, their motion is presented as an enhanced diffusion with respect to the passive case. Microswimmers, however, can overcome these fluctuations and show propulsive or ballistic trajectories. We provide some considerations on how to analyze the motion of these swimmers from an experimental point of view. Despite the rapid progress in enzyme-based micro- and nanoswimmers, deeper understanding of the mechanisms of motion is needed, and further efforts should be aimed to study their lifetime, long-term stability, and ability to navigate in complex media.

Recent Advances in Nano‐ and Micromotors

“…Enzymes are the biological counterparts to catalytic metal surfaces and are therefore considered as engines for nano/micromotors . Enzymes can use a large variety of substrate molecules to convert chemical energy into locomotion . For instance, catalase became a replacement for Pt in microjet designs .…”

“…[81] Enzymes can use a large variety of substrate molecules to convert chemical energy into locomotion. [4] For instance, catalase became a replacement for Pt in microjet designs. [82] Catalase-carrying micromotors acquired forward mobility due to oxygen bubble generation when exposed to H 2 O 2 , but they demonstrated 10 × higher efficiency than their Pt counterparts (i.e., microtube-based micromotors presented speeds up to 220 and 20 µm s −1 , respectively).…”

“…Externally driven nano‐ and micromotors can be powered by light, ultrasound, as well as magnetic or electric fields . Alternatively, fueled nano‐ and micromotors translate chemical energy into locomotion typically via a catalytic conversion of fuel molecules on their surface . These various concepts evolved and advanced from simple colloidal systems to approaches with potential practical relevance in different areas of biomedicine including first evaluations in animal models, active drug delivery, or diagnostics or for environmental applications …”

“…[3] Alternatively, fueled nano-and micromotors translate chemical energy into locomotion typically via a catalytic conversion of fuel molecules on their surface. [4] These various concepts evolved and advanced from simple colloidal systems to approaches with potential practical relevance in different areas of biomedicine [5] including first evaluations in animal models, [6] active drug delivery, [7] or diagnostics [7] or for environmental applications. [8] In this Progress Report, an overview of the general developments in the area of nano-and micromotors in the past 24 months is provided (Scheme 1).…”

Enzymatic Micromotors as a Mobile Photosensitizer Platform for Highly Efficient On‐Chip Targeted Antibacteria Photodynamic Therapy

“…Janus microspheres asymmetrically functionalized with enzymes are known to self-propel in appropriate substrate solutions, [28] and diffusiophoresis is commonly invoked to explain such self-propulsion. [34] The presence of holes on the hollow mSiO 2 spheres naturally break the symmetry, yet how the enzymatic reactions lead to propulsion away from the hole is not intuitive. To understand this propulsion mechanism, we performed numerical modeling of an MHSTU particle that is uniformly covered with urease on the outer surface except for the region at the hole (modeling details are shown in the Experimental Section).…”