**3.4 Advanced assembling**

The aforementioned fabrication methods: template-assisted electrochemical deposition, PVD, and rolled-up nanotech, are effective approaches for synthesizing micro-/nanomotors. However, to achieve more complex structures, the assembly technique must be developed. The construction of devices with multiple individual tiny parts is an extremely challenging task. The assembling approach plays an essential role in micro-/nanofabrication. It is a technique that combines miniaturized components to form a required device. The unique properties of the assembling make it applicable for the synthesis of micro-/nanomotors. Not only employing self-assembling of materials to synthesize the required devices, but also the desired elements can be embedded into micro-/nanomotors by incorporating them into materials as well.

bilayer. After five bilayers were coated, the formed particles were spread onto a glass wafer to form a monolayer that was then printed with a PDMS stamp loaded with a drop of dendritic Pt NPs suspension. The Pt NPs partially loaded hollow Janus micromotors were collected after the removal of the templates by using hydrofluoric (HF) acid. Polymer stomatocytes that are able to entrap Pt NPs into their nanocavities were reported by Wilson and collaborators, as shown in **Figure 13B** [37]. This approach applied the controlled transformation of spherical polymersomes into stomatocyte structures. Entrapment of the Pt NPs was achieved by adding nanoparticles to solvent-swollen polymersomes during the transformation process. H2O2 can get in touch with the catalytic Pt NPs by controlling the entrance of the stomatocytes to produce driving force for the directional motion of

*Catalytic Micro/Nanomotors: Propulsion Mechanisms, Fabrication, Control, and Applications*

The control of micro-/nanomotors is essential to meet various requirements in practical applications. The precise propulsion control of micro-/nanomotors is leading to advances in practical applications, and thus it is quite critical to put forward the controlling strategies for micro-/nanomotors. In the past years, scientists have realized the propulsion control of micro-/nanomotors by using different methods as

External magnetic field is the most common control source employed to direct

Wang et al. reported a multifunctional nanomotor with three segments (Au-Ni-Au), which was thrust by ultrasound and steered by the magnetic field. A concavity was also decorated at the end of the Au segment by the sphere lithography process to realize asymmetric geometry. The interaction between the magnetic field and the middle magnetic Ni segment produced a predefined and controllable movement of the nanomotor. The Ni segment was used to load and deliver magnetic particles

Magnetic orientation has proved to be extremely effective for achieving the required directionality of the self-assembled motors. Sputtering a layer of magnetic material on one side of the motors is widely applied in Janus capsule motors. The catalase-functionalized Janus capsule motor was coated with a layer of 5-nm-thick Ni before the deposition of Au. Such biocatalytic Janus capsule motors were capable of swimming in cellular media in the presence of H2O2 fuel and were steered by the applied magnetic field toward the targeted HeLa cells, as shown in **Figure 14B** [39]. It should be mentioned that the magnetic field is exclusively employed to steer the propulsion directionality of motors and is not strong enough to initiate the propul-

For electrodeposited nanowire/-rod and micro-/nanotube, a Ni part can be incorporated into the structure by using electrodeposition. A self-propelled segmented Pt/ Ni/Au/Ni/Au nanowire was taken as one of the earliest examples, as shown in **Figure 14C** [40]. The nanowire was magnetized transversely rather than

nanomotors can be realized by incorporating a paramagnetic or ferromagnetic part that can be magnetized by the magnetic field. Relying on the shapes of micro-/ nanomotors, the magnetic part can be introduced by either electrodeposition or PVD. The appropriately used magnetic material candidates in micro-/nanomotors

and guide the micro-/nanomotors. The predetermined trajectory of micro-/

along a predetermined route as well, as shown in **Figure 14A** [38].

the stomatocytes.

reported below.

**4.1 Magnetic control**

are nickel (Ni) and iron (Fe).

sion of motors by magnetic attraction.

**181**

**4. Controlling methods**

*DOI: http://dx.doi.org/10.5772/intechopen.90456*

Layer-by-layer (LbL) self-assembling is a nanofabrication strategy for multilayer formation by coating selective layers of oppositely charged materials. It is an easy-operation and low-cost process, which can encapsulate diverse materials, such as tiny inorganic compounds, colloids, macromolecules, and organic molecules together. The LbL process can be applicable for a wide range of solvent-accessible surfaces, allowing the application of different templates. Encapsulation of Pt nanoparticles enables the movement of the assembled multilayer structure to be driven in H2O2 solution. Taking advantages of simplicity, versatility, and low cost, the LbL assembling, primarily employing the electrostatic interaction between oppositely charged species, has been widely employed to synthesize various multilayer materials.

He's group firstly reported the combination of a colloid template-assisted LbL assembling with a microcontact printing method to synthesize platinum nanoparticles (Pt NPs) asymmetrically coated autonomous Janus micromotors, as shown in **Figure 13A** [36]. The SiO2 particles as templates were selectively dispersed into positively charged polyallylamine hydrochloride (PAH) solution and negatively charged polystyrene sulfonate (PSS) solution to form one polyelectrolyte

#### **Figure 13.**

*Schematic diagram of various types of controllable self-assembled micro-/nanomotors. (A) Synthesis process of Pt NPs-functionalized Janus capsule motors. (B) Selective and controlled encapsulation of Pt NPs inside artificial stomatocytes during shape transformation. Copyright 2012, ACS Publications. Copyright 2012, Nature Publishing group.*

*Catalytic Micro/Nanomotors: Propulsion Mechanisms, Fabrication, Control, and Applications DOI: http://dx.doi.org/10.5772/intechopen.90456*

bilayer. After five bilayers were coated, the formed particles were spread onto a glass wafer to form a monolayer that was then printed with a PDMS stamp loaded with a drop of dendritic Pt NPs suspension. The Pt NPs partially loaded hollow Janus micromotors were collected after the removal of the templates by using hydrofluoric (HF) acid. Polymer stomatocytes that are able to entrap Pt NPs into their nanocavities were reported by Wilson and collaborators, as shown in **Figure 13B** [37]. This approach applied the controlled transformation of spherical polymersomes into stomatocyte structures. Entrapment of the Pt NPs was achieved by adding nanoparticles to solvent-swollen polymersomes during the transformation process. H2O2 can get in touch with the catalytic Pt NPs by controlling the entrance of the stomatocytes to produce driving force for the directional motion of the stomatocytes.
