Light-driven micro/namomotors: Mechanisms and performances

Abstract

The micro/nanomotors are nanometer or micrometer-sized devices which can be activated by chemical reactions or external physical energy sources. The activated micro/nanomotors display rotation, rolling, shuttling, delivery, contraction, or collective behavior. Compared with the traditional nanotechnology for drug delivery or cargo transport with passive way (micro/nano particles have been moved by micro-fluid in body), biologically or chemically functionalized micro/nanomotors exhibit more attractive potential in chemistry, environment and biomedicine due to their controllable behavior. Over the past decade, great progress has been achieved in the field of micro/nanomotors, especially in fabrication and application of micro/nanomotors. Various shape and functional micro/nanomotors have been created, such as nanowires, microtubes, Janus spheres and helical springs. The operating mechanisms of such motors mainly are self-eletrophoresis, bubble-driven, light-driven, ultrasound-driven, magnetic field-driven or electrical field-driven. They are strongly related to the kind of energy sources, which can be chemical fuels (H2, HCl, I2, H2O, N2H4 and glucose) or a physical stimuli (light, ultrasound, magnetic and electrical). In addition, the applications of micro/nano motors range from environment to biology, such as drug delivery, DNA identification, ions sensing, degradation of biological and chemical warfare agents, oil remove and so on. As a result, micro/nanomotors have become an attractive and significant research field in nanoscience and nanotechnology. Light is the treasure from the nature, to power micro/nanomotors using renewable and environment-friendly light (especially sunlight) is an excited challenge. Besides, a series of unique operations can be easily realized using light to power micro/nanomotors, such as remote control, tunable speed and cyclic on/off motion, which make the light-driven micro/nanomotors to be one of the most attractive nanomachines. With the efforts of scientists over the years, light driven micro/nanomotors have been designed as tubes, Janus spheres, gears and irregular shapes and activated by different mechanisms, such as photocatalytic reactions, photothermal effects, photo-isomerization or photo induced deformation and so on. In fact, these devices are principally characterized according to the type of photo responsive materials. However, no matter what the materials are or how the structures of motors are, the most important and essential mechanism for the propulsion is the gradient field around motors which can be concentration, electrical, thermal or light gradient. Asymmetric motor structure or light field is the key to form such a gradient which could propel motors. Due to the excellent performances of light driven micro/nanomotors, they have been successfully applied in water cleaning, sensing and target therapy. Although the scientists in this research field achieve much valuable success, there still remain many challenges. Challenges result from expending the kinds of fuels, using sunlight to power motors, controlling the direction of motors with new methods, and exploring biocompatible light driven motors et al. In this review, we briefly introduce the micro/nanomotors, and then mainly present a review of the state-of-the-art in light driven micro/nanomotors including the basic design principles, types, operational mechanisms, motion characters and the environmental and biological applications of light driven micro/nanomotors. Finally, we summarize the challenges and outlooks in light driven micro/nanomotors.