2, The University of Hong Kong, Hong Kong, , Hong Kong
Self-propelled nanorobots, as one of representative smart materials, attract more and more interests in the past decade for its potential application in biomedicine and environmental remediation as well as for a research model to study dynamic system. Here a hierarchical nano swimmer is fabricated in a large scale by top down etching in conjunction with bottom up synthesis, integrating n-type titanium dioxide (TiO2) with p-type silicon (Si). The swimmer owns a sophisticated tree structure with TiO2 nanowires as the branches and Si nanowire as the trunk. Powered by near ultraviolet light, the tree-structured swimmer could move spontaneously in dilute hydrogen peroxide solution through completing a photoelectrochemical reaction happening on separate tree surface. The self-locomotion is verified to be electrophoresis by light intensity manipulation, surface modification and other control experiments. What’s more, the tree-like swimmer could be aligned autonomously and tracking the incident light direction. The direction-sensitive nano swimmers imitates natural microorganisms capable of phototactic migration, either at individual level or at colonial level. Therefore, the nano swimmer could be well controlled externally including swimming speed and direction by regulating the incident light. Besides, efforts are also taken to address biocompatible issues, like hydrogen peroxide replaced by redox couple hydroquinone/p-benzoquinone and dye sensitization to enhance swimmer’s response to visible light. These concepts are successfully demonstrated by fuel replacement and delicately selected dyes like N719, D5 and SQ2. Furthermore, dye sensitized swimmers selectively respond to monochromic light, which could be exploited to develop a synergistic functional group. Hence the sensitization provides a promising scenario to make a multicomponent nanorobot with complex functions.