No abstract

There is a growing effort in the scientific community to design and fabricate versatile artificial nanomotors propelled by selfgenerated forces, because they have potential in the field of directed drug delivery, roving sensors, isolation and detection of targets, active biomimetic systems, and other emerging applications. [1] Inspired by the nanoscale linear biomotors (for example, kinesins), which can autonomously move in aqueous solution and are powered by spontaneous hydrolysis of biological energy units, substantial efforts towards the design of chemically powerful synthetic motors at the micro-and nanoscale have recently demonstrated the ability of converting chemical energy into autonomous motion based on a fuel solution (for example, aqueous hydrogen peroxide solution). [2][3][4] To explain the motion and energy transfer process in these chemically powered systems, several mechanisms, including bubble propulsion, [5] interfacial tension gradients, [6] self-electrophoresis, [7] self-diffusiophoresis, [8] osmotic propulsion, [9] ultrasound propulsion, [10] and polymerization reactions [11] were proposed.Among diverse synthetic microengines, chemically powered tubular micromotors prepared by the rolled-up technique and template electrosynthesis have displayed a high speed and the controllable directionality of the movement compared to bimetal nanorods or Janus microsphere motors. [12,13] These rocket-like microengines are capable of the pick-up, transportation, and release of various cargoes, including polymer particles, [14] nucleic acids, [15] cancer cells, [16] and bacteria. [17] However, they still have some inherent limitations, such as complex preparation technology, difficulty of surface modification, and poor biocompatibility or biodegradability. Moreover, it is required in many cases that synthetic motors can encapsulate, transport, and release targeted substances by themselves in an easy and controllable way and have good biocompatibility and biodegradability, particularly in both biomedical and environmental fields. Therefore, it still remains a challenge to develop new fabrication methods and expand the diversity of the building components.

We demonstrate the first example of a self-propelled Janus polyelectrolyte multilayer hollow capsule that can serve as both autonomous motor and smart cargo. This new autonomous Janus capsule motor composed of partially coated dendritic platinum nanoparticles (Pt NPs) was fabricated by using a template-assisted layer-by-layer (LbL) self-assembly combined with a microcontact printing method. The resulting Janus capsule motors still retain outstanding delivery capacities and can respond to external stimuli for controllable encapsulation and triggered release of model drugs. The Pt NPs on the one side of the Janus capsule motors catalytically decompose hydrogen peroxide fuel, generating oxygen bubbles which then recoil the movement of the capsule motors in solution or at an interface. They could autonomously move at a maximum speed of above 1 mm/s (over 125 body lengths/s), while exerting large forces exceeding 75 pN. Also, these asymmetric hollow capsules can be controlled by an external magnetic field to achieve directed movement. This LbL-assembled Janus capsule motor system has potential in making smart self-propelling delivery systems.

We describe an approach to modulating the on-demand motion of catalytic polymer-based microengines via near-infrared (NIR) laser irradiation. The polymer multilayer motor was fabricated by the template-assisted layer-by-layer assembly and subsequently deposition of platinum nanoparticles inside and a thin gold shell outside. Then a mixed monolayer of a tumor-targeted peptide and an antifouling poly(ethylene glycol) was functionalized on the gold shell. The microengines remain motionless at the critical peroxide concentration (0.1%, v/v); however, NIR illumination on the engines leads to a photothermal effect and thus rapidly triggers the motion of the catalytic engines. Computational modeling explains the photothermal effect and gives the temperature profile accordingly. Also, the photothermal effect can alone activate the motion of the engines in the absence of the peroxide fuel, implying that it may eliminate the use of toxic fuel in the future. The targeted recognition ability and subsequently killing of cancer cells by the photothermal effect under the higher power of a NIR laser were illustrated. Our results pave the way to apply self-propelled synthetic engines in biomedical fields.