Muscle-like Artificial Molecular Actuators for Nanoparticles

Muscle tissue performs crucial contraction/extension motions that generate mechanical force and work by consuming chemical energy. Inspired by this naturally created biomolecular machine, artificial molecular muscles are designed and synthesized to undertake linear actuation functions. However, most of these muscle-like actuators are performed at large ensembles, while to realize the nanoscale actuation at the single- to few-molecule level remains challenging. Herein, we developed an artificial muscle-like molecular actuator that can reversibly control the proximity of the attached nano-objects, gold nanoparticles, within the single-molecule length level by its stimuli-responsive muscle-like linear contraction/extension motion. The molecular actuation motion is accompanied by an optical signal output resulting from the plasmonic resonance properties of gold nanoparticles. Meanwhile, the thermal noise of the muscle-like molecular actuator can be overcome by integrating the optical signal over a sufficiently long period. Artificial molecular muscles are arising as a species of man-made molecular materials with muscle-like contraction/extension capability under external stimuli. Some efforts have been made in macroscopic molecular muscles by integrating the actuation motions of molecular collections. However, to undertake nanoscale actuation tasks by artificial molecular muscles remains rarely explored, although its realization is intriguing and vital for the evolution of artificial molecular machines toward nanoactuators and even nanorobots in the future. In this work, we designed and artificially synthesized a muscle-like molecular actuator based on [c2]daisy chain rotaxane, which could be used as a linear actuator to reversibly manipulate the nanoscale distance between two gold nanoparticles. Our study offers new perspectives in the underexploited functions of artificial molecular machines, especially those working at the single- to few-molecule level. An artificial molecular actuator was designed and constructed for the reversible manipulation of nanoparticle dimers. This nanosized linear actuator can undertake contraction/extension between the attached nanoparticle dimers, thus actuating the gap distance in a controlled and reversible manner. A platform based on dark-field microscopy has been also constructed for the real-time detection and optical signal output at a single-particle level. This strategy is promising for the potential application of artificial molecular machines in single-molecule devices.