Light actuated hierarchical nanotree swimmer

Abstract

Artificial nano/micro motors, scaling with microorganisms in nature, are widely investigated for biomedical applications and environmental remediation, as well as for fundamental research models like non-equilibrium dynamic systems. A self-electrophoretic nanotree swimmer stimulated by sun light is presented. The swimmer has a hierarchical large head-slim tail morphology coupling n-type titanium dioxide nanowire as the nanotree branch with p-type silicon microwire as the nanotree trunk to construct a motile photoelectrochemical (PEC) circuit. The self-electrophoretic locomotion is dominated by the PEC reaction happening on the nanotree surface, but anodic and cathodic reactions are separated by the head-tail structure, which establishes a concentration gradient of products, resulting in a localized electric field that propels the swimmer. The prototype nanotree swimmer is successfully demonstrated in dilute hydrogen peroxide solution under near ultraviolet light. The unique head-tail bi-component qualifies orthogonal surface modification to selectively switch the swimming pattern from tail-forward locomotion to head-forward locomotion or from positively phototactic migration to negatively phototactic migration. The individual behaviour of prototype nanotree swimmer is characterized in both translation and rotation involved with propulsion mechanism verification and phototaxis programing, and the schooling behaviour is conducted in a macro scale and compared with natural green algae to confirm the advanced substrate-free unidirectional swimming. However, the nanotree swimmer is not perfect yet due to the involvement of cytotoxic peroxide and harmful ultraviolet light. The biocompatible issue is well addressed by fuel replacement and dye sensitization to upgrade the prototype swimmer. Hydroquinone/1, 4-benzoquinone redox couple (H2Q/Q) is selected to substitute for hydrogen peroxide to fuel the nanotree swimmer without changing the propulsion mechanism. Inspired by dye sensitized titania-based solar cells, dye cis – diisothiocyanato - bis (2, 2’ - bipyridyl - 4, 4’ - dicarboxylato) ruthenium (Ⅱ) bis (tetrabutylammonium) (N719) is used to sensitize the nanotree swimmer to harvest visible light in H2Q/Q solution, and the sensitization is characterized by simulated external quantum efficiency (EQE) pertinent to locomotion velocity and energy conversion efficiency in terms of wavelength respectively. The selective response to monochromatic light of N719 sensitized swimmer indicates the effective transformation of visible light to kinetic locomotion. In addition to the successful upgrade of prototype swimmer, two different dyes are selected to discriminate individual nanotree object by spectral sensitization. Dye 3-(5-(4-(diphenylamino) styryl) thiophen-2-yl)-2-cyanoacrylic acid (D5) is responsive mainly to blue light, while dye 5-carboxy-2-[[3-[(2,3-dihydro-1,1-dimethyl-3-ethyl-1H-benzo[e]indol-2-ylidene)methyl]-2-hydroxy-4-oxo-2-cyclobuten-1-ylidene]methyl]-3,3-dimethyl-1-octyl-3H-indolium (SQ2) absorbs red light the most. The discriminated nanotree swimmers are constructed together into a functional group which could be externally controlled separately yet simultaneously to complete a cooperative job. The synergistic construction method provides a promising scenario in the design of multifunctional nanorobots.