**4. Controlling methods**

**3.4 Advanced assembling**

them into materials as well.

multilayer materials.

**Figure 13.**

**180**

*Nature Publishing group.*

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

*Smart Nanosystems for Biomedicine, Optoelectronics and Catalysis*

employing self-assembling of materials to synthesize the required devices, but also the desired elements can be embedded into micro-/nanomotors by incorporating

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

He's group firstly reported the combination of a colloid template-assisted LbL

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

*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,*

assembling with a microcontact printing method to synthesize platinum

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 reported below.

## **4.1 Magnetic control**

External magnetic field is the most common control source employed to direct and guide the micro-/nanomotors. The predetermined trajectory of micro-/ 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 are nickel (Ni) and iron (Fe).

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

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 propulsion of motors by magnetic attraction.

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

whole inner surface of the microtube before electrodeposition of Pt, resulting in the microtube is magnetized along the tube axis. A simplified Ni/Pt alloy inner layer obtained by the co-deposition of a Ni/Pt layer can display both magnetic and catalytic properties. Unfortunately, the speed of microtube with a simplified Ni/Pt alloy inner layer in diluted H2O2 is hugely decreased because of the reduced catalytic area. For striped nanotube with different elements placed longitudinally, it can also be magnetized longitudinally due to the Ni portion has a larger dimension along the axis of the tube, demonstrating behavior similar to that of magnetotactic bacte-

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

Regarding rolled-up microtube, an additional Fe layer can be integrated into the microtube during the deposition process to realize the magnetic control. The longitudinally magnetized rolled-up microtube can monitor the direction of an external field and orient itself accordingly. A magnetized Fe-contained microtube was studied to be able to selectively pick up and deliver paramagnetic beads in the absence of an external magnetic field. The ability of a Pt Janus particle to deliver cargo has been reported by Sanchez and fellows. To realize the better control of the propulsion of catalytic Janus motors and the cargo delivery process, magnetic caps consisting of (Co/Pt) multilayers were incorporated into the structure by PVD. The magnetic caps were envisioned to align the magnetic moment along the main symmetric axis of the cap, enabling direct manipulation of the Janus motor as well as superparamagnetic cargoes delivery by using an external magnetic field, as shown in **Figure 14E** [42]. Precise control of magnetic Janus particles is further reported by

The individual control of microjet in a closed-loop manner and 3D propulsion

Ultrasound not only provides energy for the motion of micro-/nanomotors but also offers an alternative manner for controlling self-propelled motors. Using ultrasound to guide micro-/nanomotors and as rapid "stop/go" switching of micro-/

The movement direction of nanomotors can be reversed by varying the power of the ultrasound field. Fast and reversible transitions between aggregated and freemoving states of nanomotors in H2O2 were obtained in response to switching between on and off ultrasound states, as shown in **Figure 15A** [44]. The generation of bubbles can be disrupted by the ultrasound field. Wang et al. demonstrated the reversible control of the propulsion of PEDOT/Ni/Pt microengines by changing the applied voltage of the external transducer which produces the ultrasound field. The authors demonstrated extremely fast changes (< 0.1 s) in the motor speed and reproducible "on/off" activations that were faster than those by using other reported methods for stopping the propulsion of microjets, as shown in

nanomotors in respond to "on/off" of ultrasound are investigated.

control were the next steps to be considered. Recently, Misra's and Sanchez's research groups presented the accurately closed-loop control of microjet [43]. The authors reported precisely point-to-point closed-loop control by applying weak magnetic fields (2 mT). Another study demonstrated precise control when a flow was employed against and along the propulsion direction of the microjet. An electromagnetic setup consisting of two sets of orthogonal arrays of electromagnetic coils with a Fe core in conjunction with two microscopic systems was employed to guide the movement of microjet in 3D space, as shown in **Figure 14F** [43]. Microjet overcomes vertical forces, such as vertical flow buoyancy forces, and interaction forces with O2 bubbles, and thus it is able to drive downwards and swim upwards

ria, as shown in **Figure 14D** [41].

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

relative to reference positions.

**4.2 Acoustic control**

**Figure 15B** [45].

**183**

sorting beads between the channels in microchip devices.

#### **Figure 14.**

*Micro-/nanomotors controlled by the magnetic field. (A) Schematic diagram of an Au–Ni–Au metal alloy propelled by ultrasound and steered by the magnetic field. (B) Magnetically steered movement of Janus capsule motors toward targeted HeLa cell sheets. (C) SEM image of Pt/Ni/Au/Ni/Au nanowire. (D) SEM/energydispersive X-ray (EDX) elemental analysis of Au/Ni/Pt nanotube. (E) Scheme representing the magnetic steering of Janus micromotors. (F) Remote control of micro/nanojets by magnetic field. Copyright 2013, ACS Publications. Copyright 2014, ACS Publications. Copyright 2005, Wiley Online Library. Copyright 2013, ACS Publications. Copyright 2012, ACS Publications. Copyright 2013, AIP Publications.*

longitudinally, resulting from the scale of the electrodeposited Ni part was smaller than the diameter of the wire. Magnetized nanowire can orient its net magnetic moment parallel to an external magnetic field, resulting in precise steering by manipulating the orientation of the magnetic field. Experimental results proved that the magnetic field could only direct the nanowires without changing their speed. The steered propulsion of self-propelled Au/Ni/Au/Pt-CNT nanorod as well as the delivery of magnetic microbead cargoes by it in microchannel networks was reported by Burdick et al. [20].

Magnetic steering of the electrodeposited microtube could also be achieved by additional electro-deposition of Ni. For conical microtube, Ni is grown to cover the

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

whole inner surface of the microtube before electrodeposition of Pt, resulting in the microtube is magnetized along the tube axis. A simplified Ni/Pt alloy inner layer obtained by the co-deposition of a Ni/Pt layer can display both magnetic and catalytic properties. Unfortunately, the speed of microtube with a simplified Ni/Pt alloy inner layer in diluted H2O2 is hugely decreased because of the reduced catalytic area. For striped nanotube with different elements placed longitudinally, it can also be magnetized longitudinally due to the Ni portion has a larger dimension along the axis of the tube, demonstrating behavior similar to that of magnetotactic bacteria, as shown in **Figure 14D** [41].

Regarding rolled-up microtube, an additional Fe layer can be integrated into the microtube during the deposition process to realize the magnetic control. The longitudinally magnetized rolled-up microtube can monitor the direction of an external field and orient itself accordingly. A magnetized Fe-contained microtube was studied to be able to selectively pick up and deliver paramagnetic beads in the absence of an external magnetic field. The ability of a Pt Janus particle to deliver cargo has been reported by Sanchez and fellows. To realize the better control of the propulsion of catalytic Janus motors and the cargo delivery process, magnetic caps consisting of (Co/Pt) multilayers were incorporated into the structure by PVD. The magnetic caps were envisioned to align the magnetic moment along the main symmetric axis of the cap, enabling direct manipulation of the Janus motor as well as superparamagnetic cargoes delivery by using an external magnetic field, as shown in **Figure 14E** [42]. Precise control of magnetic Janus particles is further reported by sorting beads between the channels in microchip devices.

The individual control of microjet in a closed-loop manner and 3D propulsion control were the next steps to be considered. Recently, Misra's and Sanchez's research groups presented the accurately closed-loop control of microjet [43]. The authors reported precisely point-to-point closed-loop control by applying weak magnetic fields (2 mT). Another study demonstrated precise control when a flow was employed against and along the propulsion direction of the microjet. An electromagnetic setup consisting of two sets of orthogonal arrays of electromagnetic coils with a Fe core in conjunction with two microscopic systems was employed to guide the movement of microjet in 3D space, as shown in **Figure 14F** [43]. Microjet overcomes vertical forces, such as vertical flow buoyancy forces, and interaction forces with O2 bubbles, and thus it is able to drive downwards and swim upwards relative to reference positions.

### **4.2 Acoustic control**

Ultrasound not only provides energy for the motion of micro-/nanomotors but also offers an alternative manner for controlling self-propelled motors. Using ultrasound to guide micro-/nanomotors and as rapid "stop/go" switching of micro-/ nanomotors in respond to "on/off" of ultrasound are investigated.

The movement direction of nanomotors can be reversed by varying the power of the ultrasound field. Fast and reversible transitions between aggregated and freemoving states of nanomotors in H2O2 were obtained in response to switching between on and off ultrasound states, as shown in **Figure 15A** [44]. The generation of bubbles can be disrupted by the ultrasound field. Wang et al. demonstrated the reversible control of the propulsion of PEDOT/Ni/Pt microengines by changing the applied voltage of the external transducer which produces the ultrasound field. The authors demonstrated extremely fast changes (< 0.1 s) in the motor speed and reproducible "on/off" activations that were faster than those by using other reported methods for stopping the propulsion of microjets, as shown in **Figure 15B** [45].

longitudinally, resulting from the scale of the electrodeposited Ni part was smaller than the diameter of the wire. Magnetized nanowire can orient its net magnetic moment parallel to an external magnetic field, resulting in precise steering by manipulating the orientation of the magnetic field. Experimental results proved that the magnetic field could only direct the nanowires without changing their speed. The steered propulsion of self-propelled Au/Ni/Au/Pt-CNT nanorod as well as the delivery of magnetic microbead cargoes by it in microchannel networks was reported by Burdick et al. [20]. Magnetic steering of the electrodeposited microtube could also be achieved by additional electro-deposition of Ni. For conical microtube, Ni is grown to cover the

*Micro-/nanomotors controlled by the magnetic field. (A) Schematic diagram of an Au–Ni–Au metal alloy propelled by ultrasound and steered by the magnetic field. (B) Magnetically steered movement of Janus capsule motors toward targeted HeLa cell sheets. (C) SEM image of Pt/Ni/Au/Ni/Au nanowire. (D) SEM/energydispersive X-ray (EDX) elemental analysis of Au/Ni/Pt nanotube. (E) Scheme representing the magnetic steering of Janus micromotors. (F) Remote control of micro/nanojets by magnetic field. Copyright 2013, ACS Publications. Copyright 2014, ACS Publications. Copyright 2005, Wiley Online Library. Copyright 2013,*

*ACS Publications. Copyright 2012, ACS Publications. Copyright 2013, AIP Publications.*

*Smart Nanosystems for Biomedicine, Optoelectronics and Catalysis*

**Figure 14.**

**182**

#### **Figure 15.**

*(A) Scheme representing controlling of acoustically propelled nanowire toward a HeLa cell. (B) Scheme representing ultrasound-modulated bubble propulsion of chemically powered microtubes. Copyright 2013, ACS Publications. Copyright 2014, ACS Publications.*

> shown in **Figure 17B** [48]. NIR light irradiation could rapidly generate a thermal gradient that enables reversible movement of the Au nanoshell-functionalized polymer multilayer rockets. The rockets exhibit "on/off/on" cycles in response to an adjustable NIR irradiation, along with "go/stop/go" motions. The directional movements of the rockets were terminated as the NIR irradiation is "off" and were resumed upon switching on the NIR light. Accounting for the straight propulsion behavior of NIR-propelled polymer multilayer rockets, the "on/off" NIR switching realizes the precisely predefined route of the polymer multilayer rockets. In addition, the rockets were sustained with negligible damage under 30 times of NIR

*(A) Microengine's propulsion controlled by light. (B) NIR light-switchable motion of NIR propelled polymer multilayer rockets. Copyright 2011, Wiley Online Library. Copyright 2010, Wiley Online Library.*

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

Thermal control of the propulsion of artificial micro-/nanomotors has proved to

The temperature of the solution could be controlled by two Peltier elements in

The propulsion of self-propelled micro-/nanomotors could be adjusted by tuning the fuel level or chemical stimuli. In the presence of fuel concentration gradient, micro-/nanomotors can drive themselves along the gradient toward a region with a higher fuel concentration. As such, monitoring the fuel concentration and distribution can be used to direct and modulate the propulsion of micro-/nanomotors. In addition to fuel concentration, the propulsion of micro-/nanomotors is influenced by the presence of certain other chemicals. For instance, the movement of Au/Pt nanomotors was reported to be extremely accelerated upon the addition of Ag ion because of the under-potential deposition of Ag on the nanomotors, which

be applicable for both micro-/nanowires and microtubes. The speed of Pt-Au nanowires was substantially increased upon exposure to elevated temperatures. Similar phenomenon was discovered for bubble-propelled microtubes, which has

connection with a direct current (DC) power supply placed below the sample containing microjets. By heating up the system to a physiological temperature, microjets increase their efficiency and are propelled at extremely low concentrations of H2O2, as shown in **Figure 18** [49, 50]. In addition, soft micromotors consisted of flexible thermo-responsive polymeric microjets could reversibly fold and unfold in an accurate manner resulted from the temperature change of the solution in which they are dispersed, thereby allowing them to rapidly initiate and terminate multiple times in response to the radius of curvature accordingly. The employment of stimuli-responsive materials would be ideal for the future designs of

been used to compensate the effect of decreasing the fuel level.

irradiation and were highly durable.

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

**4.5 Thermal control**

**Figure 17.**

smart micro-/nanomotors.

**4.6 Chemical control**

**185**

#### **Figure 16.**

*Rotation of micro/nanomotors by applying AC voltages to multiple electrodes: (A) schematic diagram of experimental setup of quadruple electrodes and (B) images of one end fixed (left) and free (right) rotating Au nanowires, respectively. Copyright 2005, APS Publications.*

#### **4.3 Electric control**

Metallic micro-/nanomotors can perform controllable rotation resulting from rotational torque in an electric field provided by applying AC voltages to multiple electrodes, as shown in **Figure 16A** [46]. In addition, by applying AC electric fields to strategically designed microelectrodes, the propulsion of metallic micro-/ nanomotors could be tuned by dielectrophoretic force. They could be driven to chain, accelerate and align in certain directions, as well as to disperse, concentrate and assemble into complex scaffolds, as shown in **Figure 16B**.

#### **4.4 Light control**

Using the light to guide the movement of micro-/nanomotors was also reported. For example, Solovev and collaborators studied the control of microjets by using a white light source, as shown in **Figure 17A** [47]. This process was mediated through the illumination of the fuel solution above Pt-patterned Si surfaces, which generates a local decrease of the surfactant and H2O2 concentration. Although white light could be applied to switch off the motion of the microjets, shorter wavelengths were attributed to suppress the production of microbubbles faster than longer wavelengths. The phenomenon can be reversible, and thus a nonactive microjet is triggered by dimming the light source. Nevertheless, the "on/off" process is not immediate since it demands a few seconds to completely terminate or to reach a constantly maximum speed.

He and his colleagues recently reported the near infrared (NIR) light-switching "on/off" propulsion of Au nanoshell-functionalized polymer multilayer rockets, as

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

#### **Figure 17.**

*(A) Microengine's propulsion controlled by light. (B) NIR light-switchable motion of NIR propelled polymer multilayer rockets. Copyright 2011, Wiley Online Library. Copyright 2010, Wiley Online Library.*

shown in **Figure 17B** [48]. NIR light irradiation could rapidly generate a thermal gradient that enables reversible movement of the Au nanoshell-functionalized polymer multilayer rockets. The rockets exhibit "on/off/on" cycles in response to an adjustable NIR irradiation, along with "go/stop/go" motions. The directional movements of the rockets were terminated as the NIR irradiation is "off" and were resumed upon switching on the NIR light. Accounting for the straight propulsion behavior of NIR-propelled polymer multilayer rockets, the "on/off" NIR switching realizes the precisely predefined route of the polymer multilayer rockets. In addition, the rockets were sustained with negligible damage under 30 times of NIR irradiation and were highly durable.

### **4.5 Thermal control**

**4.3 Electric control**

**Figure 15.**

**Figure 16.**

*Publications. Copyright 2014, ACS Publications.*

*Smart Nanosystems for Biomedicine, Optoelectronics and Catalysis*

**4.4 Light control**

constantly maximum speed.

**184**

Metallic micro-/nanomotors can perform controllable rotation resulting from rotational torque in an electric field provided by applying AC voltages to multiple electrodes, as shown in **Figure 16A** [46]. In addition, by applying AC electric fields

*Rotation of micro/nanomotors by applying AC voltages to multiple electrodes: (A) schematic diagram of experimental setup of quadruple electrodes and (B) images of one end fixed (left) and free (right) rotating*

*(A) Scheme representing controlling of acoustically propelled nanowire toward a HeLa cell. (B) Scheme representing ultrasound-modulated bubble propulsion of chemically powered microtubes. Copyright 2013, ACS*

Using the light to guide the movement of micro-/nanomotors was also reported. For example, Solovev and collaborators studied the control of microjets by using a white light source, as shown in **Figure 17A** [47]. This process was mediated through the illumination of the fuel solution above Pt-patterned Si surfaces, which generates a local decrease of the surfactant and H2O2 concentration. Although white light could be applied to switch off the motion of the microjets, shorter wavelengths were attributed to suppress the production of microbubbles faster than longer wavelengths. The phenomenon can be reversible, and thus a nonactive microjet is triggered by dimming the light source. Nevertheless, the "on/off" process is not immediate since it demands a few seconds to completely terminate or to reach a

He and his colleagues recently reported the near infrared (NIR) light-switching "on/off" propulsion of Au nanoshell-functionalized polymer multilayer rockets, as

to strategically designed microelectrodes, the propulsion of metallic micro-/ nanomotors could be tuned by dielectrophoretic force. They could be driven to chain, accelerate and align in certain directions, as well as to disperse, concentrate

and assemble into complex scaffolds, as shown in **Figure 16B**.

*Au nanowires, respectively. Copyright 2005, APS Publications.*

Thermal control of the propulsion of artificial micro-/nanomotors has proved to be applicable for both micro-/nanowires and microtubes. The speed of Pt-Au nanowires was substantially increased upon exposure to elevated temperatures. Similar phenomenon was discovered for bubble-propelled microtubes, which has been used to compensate the effect of decreasing the fuel level.

The temperature of the solution could be controlled by two Peltier elements in connection with a direct current (DC) power supply placed below the sample containing microjets. By heating up the system to a physiological temperature, microjets increase their efficiency and are propelled at extremely low concentrations of H2O2, as shown in **Figure 18** [49, 50]. In addition, soft micromotors consisted of flexible thermo-responsive polymeric microjets could reversibly fold and unfold in an accurate manner resulted from the temperature change of the solution in which they are dispersed, thereby allowing them to rapidly initiate and terminate multiple times in response to the radius of curvature accordingly. The employment of stimuli-responsive materials would be ideal for the future designs of smart micro-/nanomotors.

#### **4.6 Chemical control**

The propulsion of self-propelled micro-/nanomotors could be adjusted by tuning the fuel level or chemical stimuli. In the presence of fuel concentration gradient, micro-/nanomotors can drive themselves along the gradient toward a region with a higher fuel concentration. As such, monitoring the fuel concentration and distribution can be used to direct and modulate the propulsion of micro-/nanomotors. In addition to fuel concentration, the propulsion of micro-/nanomotors is influenced by the presence of certain other chemicals. For instance, the movement of Au/Pt nanomotors was reported to be extremely accelerated upon the addition of Ag ion because of the under-potential deposition of Ag on the nanomotors, which

**5. Applications**

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

**5.1 Cargo delivery**

**Figure 20** [4].

**Figure 20.**

**187**

*protein cargo. Copyright 2009, ACS Publications.*

There are many current and potential applications, resulting from the great advances in cargo-towing force, propulsion control, and lifetime of synthetic micro-/nanomotors. The wide range of potential applications of micro-/nanomotors covering different fields requires specific functionalization strategies in each kind of application. Herein, the functionalization of micro-/nanomotors for four main categories of applications is reported as follows: cargo delivery, environmental

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

Cargo delivery is one of the most important envisioned applications of micro-/ nanomotors. Relying on the properties of cargoes, tailored methods are desired for their corresponding delivery. A general process of how to synthesize micro-/ nanomotors and employ them to deliver cargo molecules has been shown in

For the cargo delivery by micro-/nanomotors, the cargo could simply be connected to the motors by magnetic attraction. The delivery of drug-loaded magnetic poly(D,L-lactic-co-glycolic acid) (PLGA) microparticles has been studied by both chemically propelled, as shown in **Figure 21A** [52–54], and magnetically driven micro-/nanomotors. For charged cargoes, electrostatic interaction between cargoes and micro-/nanomotors could be applied for the pick-up process. A common scheme introducing charged portions into micro-/nanomotors is to incorporate a negatively charged polymer part. Sen et al. reported that a PPy part was incorporated to a nanowire via electropolymerization, which could be attached to oppositely charged polystyrene amidine cargo via electrostatic interaction, as shown in **Figure 21A**. A photo-chemically triggered cargo unloading manner was proposed for cargoes loaded nanowires via electrostatic interaction. An additional Ag portion

*Electrodeposited aptamer-functionalized micro/nanomotors for selective loading, deliver, and unloading of a*

remediation, chemical sensors, and biomedical applications.

**Figure 18.**

*Micro/nanojet's propulsion controlled by temperature. Copyright 2011, ACS Publications. Copyright 2013, RSC Publications.*

#### **Figure 19.**

*Micro/nanomotor's motion controlled by chemical gradient in microfluidic channel. Copyright 2013, Wiley Online Library.*

introduces differences in surface and catalytic properties. Hydrazine (N2H4) is another chemical stimulus observed to be effective to accelerate the propulsion of Au/Pt-CNT nanomotors. For bubble-propelled micro-/nanomotors, surfactants are significantly critical to the mobility of motors, resulting from they can stimulate bubble production and detachment.

Solovev et al. reported that the production of large microbubbles from small ensembles of microjets generated a chemophoretic attraction force and a capillary force that pulled other microjets into the swarm. A more complicated experiment was demonstrated by Baraban et al., who reported a controllable manner to study the chemotactic behavior of Janus motors and tubular microjets in microfluidic channels. Both types of motors move toward the gradient of the fuel without the influence of capillary forces, as shown in **Figure 19** [51].

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