**2. Propulsion mechanisms**

Whitesides and his colleagues firstly reported the motion of a millimeter-scale object, which was composed of a piece of platinum (Pt)-coated porous glass filter mounted on a thin polydimethylsiloxane (PDMS) plate using a stainless steel pin, as shown in **Figure 2** [10]. The assembled object was immersed into hydrogen peroxide (H2O2) solution. Pt catalyzes H2O2 decomposition to generate oxygen (O2) bubbles releasing from its surface, which reversely induced a recoil force to propel the object moving forward. This is the foundation in this research field. In micro-/ nanoscale regime, only asymmetric particles can realize autonomous propulsion. Based on this, researchers focused on two aspects: shape and material compositions of micro-/nanomotors for breaking the symmetry, and various types of micro-/ nanomotors were invented. Meanwhile, there were different mechanisms proposed to explain the propulsion phenomena, depending on the shapes (e.g., wires, rods, Janus spheres, and tubular jets) and material compositions of micro-/nanomotors [4–17].

#### **2.1 Dielectrophoresis**

biomolecular protein motors with fascinating abilities to harness energy from living environments for autonomous motion *in vivo* as described above. Inspired by the fantasy of naturally occurring motor proteins, researchers paid great attentions into synthetic micro-/nanomotors in the past decades. In particular, led by pioneering contributions of Sen and Mallouk's team and Ozin's group, current work mainly focuses on the exploration of high-efficiency and high-speed synthetic micro-/ nanomotors that are able to convert chemical energy into autonomous propulsion

*(A) Schematic diagram of the architecture of the bacterial flagellar motor. (B) The twin heads of kinesin motor protein alternately bind to the microtubules so that the protein motor moves forward. (C) Schematic diagram of ATPase. Copyright 2012, Elsevier. Copyright 2004, IEEE Xplore Digital Library. Copyright 2000,The Royal*

*Smart Nanosystems for Biomedicine, Optoelectronics and Catalysis*

The research of synthetic self-propelled micro-/nanomotors has rapidly developed in last few decades [4–9]. Several advanced developments and excellent contributions had been made in this field. Although the bright future of this research area can be expected, some major existing challenges are still remained to be solved. The design, fabrication, and characterization of functional micro-/nanomotors require some innovative approaches and ideas to realize. Fabricating micro-/ nanomotors with individual functional parts, smartly and precisely controlling motors are still extremely challenging. Hereby, a complete understanding of the physiochemical mechanism is necessary. To realize better control of micro-/ nanomotors in the future, an industrial level of functional micro-/nanomachinery could be achieved. Despite of the significant development and advances in micro-/ nanomotors, challenges are still remained to find specific relevant applications, such

[4–6].

**168**

**Figure 1.**

*society Publishing.*

as biologically compatible fuels, etc.

In the past decades, a variety of micro-/nanomotors have been envisioned to explore the concept of self-electrophoresis propulsion, especially micro-/nanowires, rods, and Janus spheres. In self-electrophoresis, micro-/nanomotors produce a locally distributed electric field through chemical gradients and propel forward in

#### **Figure 2.**

*(A) Schematic diagram of the propulsion of a millimeter-scale object. A thin plate (1–2 mm in thickness and 9 mm in diameter) was assembled from PDMS in a desired shape, and specified faces were rendered as hydrophilic by plasma oxidation. A 2 2 mm2 piece of porous glass filter (one side covered with Pt) was mounted on the PDMS plate by using a stainless steel pin. (B) A diagram illustrating self-assembly by capillary interactions. Copyright 2002, Wiley Online Library.*

respond to this self-produced electric field, and they do not respond to an external electric field.

Electrophoresis describes the movement of micro-/nanoscale particles in a fluid. In dielectrophoresis, if a micro-/nanoparticle would realize autonomous selfpropulsion, the micro-/nanoparticle must contain at least two different metals and acts as a self-contained electro-chemical cell. For instance, a micro-/nanorod is composed of Pt and gold (Au), Au serves as the cathode and Pt is the anode, as shown in **Figure 3** [6].

The oxidation of H2O2 at the Pt side (anode) of the rod generates negatively charged electrons (*e*) and positively charged protons (H+ ). The generated protons migrate along the double layer surrounding the rod and the electrons internally migrate along the rod from the Pt side to the Au side (cathode) of the rod; thus, H2O2 can be converted into water (H2O) and O2. The continuous chemical reactions result in a net flow of electrons from the anode to the cathode as well as the migration of protons to the cathode, producing a proton gradient along the axis of the rod. The electron flow produces a negatively charged rod that responses to the gradient, which propels the rod forward to the proton-rich environment previously occupied by Au [6].

In self-electrophoresis, the charged micro-/nanoparticles propel forward in a self-generated electric field resulting from an uneven distribution of ions. The velocity *v* of the particle is related to the self-produced electric field (*E*), zeta potential (*ζ*) of the particle, permittivity (*ε*), and viscosity (*μ*) of the medium, as shown below:

$$w = \frac{\zeta\_e eE}{\mu} \tag{1}$$

is produced along the surface of the micro-/nanomotor. As the reaction products reach a certain point, the local concentration is higher and the products start to diffuse away from the catalyst, which in turn produces a force leading to the

*Schematic illustration of a Pt-Au nanorod representing the dimensions used in the calculation of interfacial*

*force. The parameter f is the length ratio of Pt. Copyright 2004, ACS Publications.*

*Schematic diagram of a micro/nanomotor propelling under the diffusiophoresis propulsion mechanism. Copyright 2009, Wiley Online Library. Copyright 2011, Frontiers Journals of Higher Education Press.*

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

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

Surface tension gradient along an interface can result in an imbalanced force and further produce flow, which is well known as the "Marangoni effect." This motion mechanism has been employed to explain the propulsion of the micro-/nanomotors,

In this model, it is proposed that the velocity *v* of micro-/nanomotors is linearly proportional to the surface tension of the solution, as shown in the following equation:

*<sup>μ</sup>*DL <sup>∝</sup>k<sup>γ</sup> (2)

*<sup>v</sup>* <sup>¼</sup> SR<sup>2</sup> γ

and the model was firstly reported by Crespi, Mallouk, and Sen, as shown in **Figure 5** [13]. As H2O2 is decomposed at the Pt side of Au-Pt nanorods to produce H2O and O2, leading to an interfacial tension created near the surface of Pt is lower due to a larger quantity of O2 generated. The surface tension difference between the Pt side and the Au side produces a force to thrust the micro-/nanomotor

movement of the micro-/nanomotor.

**2.3 Interfacial tension**

**Figure 4.**

**Figure 5.**

propelling forward.

**171**

#### **2.2 Diffusiophoresis**

Self-diffusiophoresis is a propulsion phenomenon in which the movement of particles is induced by a concentration gradient of the reaction products. This propulsion mechanism is more commonly employed in spherical Janus micro-/ nanomotors, as shown in **Figure 4** [11, 12]. In this system, the catalyst (Pt) is located at one side of the micro-/nanomotor; the reaction products (H2O and O2) preferentially accumulate at the site of the catalyst. Hence, a concentration gradient

#### **Figure 3.**

*Schematic representation of dielectrophoresis (bipolar electrochemical) mechanism for the propulsion of an Au-Pt micro-/nanomotor in the presence of H2O2. The mechanism involves an internal electron flow from one end to the other end of the motor, along with the migration of protons in the double layer surrounding the motor. Copyright 2006, ACS Publications.*

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

#### **Figure 4.**

respond to this self-produced electric field, and they do not respond to an external

The oxidation of H2O2 at the Pt side (anode) of the rod generates negatively

In self-electrophoresis, the charged micro-/nanoparticles propel forward in a self-generated electric field resulting from an uneven distribution of ions. The velocity *v* of the particle is related to the self-produced electric field (*E*), zeta potential (*ζ*) of the particle, permittivity (*ε*), and viscosity (*μ*) of the medium, as

> *<sup>v</sup>* <sup>¼</sup> <sup>ζ</sup> *<sup>ε</sup><sup>E</sup> μ*

Self-diffusiophoresis is a propulsion phenomenon in which the movement of particles is induced by a concentration gradient of the reaction products. This propulsion mechanism is more commonly employed in spherical Janus micro-/ nanomotors, as shown in **Figure 4** [11, 12]. In this system, the catalyst (Pt) is located at one side of the micro-/nanomotor; the reaction products (H2O and O2) preferentially accumulate at the site of the catalyst. Hence, a concentration gradient

*Schematic representation of dielectrophoresis (bipolar electrochemical) mechanism for the propulsion of an Au-Pt micro-/nanomotor in the presence of H2O2. The mechanism involves an internal electron flow from one end to the other end of the motor, along with the migration of protons in the double layer surrounding the motor.*

migrate along the double layer surrounding the rod and the electrons internally migrate along the rod from the Pt side to the Au side (cathode) of the rod; thus, H2O2 can be converted into water (H2O) and O2. The continuous chemical reactions

result in a net flow of electrons from the anode to the cathode as well as the migration of protons to the cathode, producing a proton gradient along the axis of the rod. The electron flow produces a negatively charged rod that responses to the gradient, which propels the rod forward to the proton-rich environment previously

). The generated protons

(1)

In dielectrophoresis, if a micro-/nanoparticle would realize autonomous selfpropulsion, the micro-/nanoparticle must contain at least two different metals and acts as a self-contained electro-chemical cell. For instance, a micro-/nanorod is composed of Pt and gold (Au), Au serves as the cathode and Pt is the anode, as

charged electrons (*e*) and positively charged protons (H+

*Smart Nanosystems for Biomedicine, Optoelectronics and Catalysis*

Electrophoresis describes the movement of micro-/nanoscale particles in a fluid.

electric field.

shown in **Figure 3** [6].

occupied by Au [6].

**2.2 Diffusiophoresis**

shown below:

**Figure 3.**

**170**

*Copyright 2006, ACS Publications.*

*Schematic diagram of a micro/nanomotor propelling under the diffusiophoresis propulsion mechanism. Copyright 2009, Wiley Online Library. Copyright 2011, Frontiers Journals of Higher Education Press.*

#### **Figure 5.**

*Schematic illustration of a Pt-Au nanorod representing the dimensions used in the calculation of interfacial force. The parameter f is the length ratio of Pt. Copyright 2004, ACS Publications.*

is produced along the surface of the micro-/nanomotor. As the reaction products reach a certain point, the local concentration is higher and the products start to diffuse away from the catalyst, which in turn produces a force leading to the movement of the micro-/nanomotor.

#### **2.3 Interfacial tension**

Surface tension gradient along an interface can result in an imbalanced force and further produce flow, which is well known as the "Marangoni effect." This motion mechanism has been employed to explain the propulsion of the micro-/nanomotors, and the model was firstly reported by Crespi, Mallouk, and Sen, as shown in **Figure 5** [13]. As H2O2 is decomposed at the Pt side of Au-Pt nanorods to produce H2O and O2, leading to an interfacial tension created near the surface of Pt is lower due to a larger quantity of O2 generated. The surface tension difference between the Pt side and the Au side produces a force to thrust the micro-/nanomotor propelling forward.

In this model, it is proposed that the velocity *v* of micro-/nanomotors is linearly proportional to the surface tension of the solution, as shown in the following equation:

$$w = \frac{\text{SR}^2 \text{y}}{\mu \text{DL}} \propto \text{k}\chi\text{}\tag{2}$$

where S, R, *γ*, *μ*, D, L, and k are the O2 evolution rate, radius of micro-/ nanomotor, surface tension of solution, viscosity of solution, diffusion coefficient and length of micro-/nanomotor, and constant, respectively.

### **2.4 Acoustophoresis**

One of the potential applications of micro-/nanomotors is to be used in diagnostics and biomedicine; there is a demand to develop propulsion mechanisms which can be biocompatible. Ultrasound operates in a range of frequency above 20 kHz, which is biocompatible [5]. Hence, ultrasound is a promising technique for propelling micro-/nanomotors (wires/rods and tubular jets) in biomedical applications. The micro-/nanomotor propelled by ultrasound was firstly reported by Sen's group in 2012, as shown in **Figure 6** [5]. In this system, metallic microrods are suspended in water surrounded by an acoustic chamber. A vertical standing wave levitates the microrods to a plane at the midpoint of the cell resulting from the lowest pressure. In that plane, the metallic microrods behave axial propulsion at speeds up to 200 μm/s in water. The microrods also assemble into patterns in the nodal plane resulting from nodes and antinodes among the plane. The composition of the microrods was observed to effectively affect their propulsion, with only metallic microparticles demonstrating fast axial motion.

#### **2.5 Thermophoresis**

Temperature gradient could introduce the motion of micro-/nanoparticles (micro-/nanowires, rods, and Janus spheres). This phenomenon is called thermophoresis or the "Soret effect." Recently, the propulsion of micro-/ nanomotors induced by self-generated temperature gradient has been investigated, as shown in **Figure 7**.

alternating current (AC) magnetic field to heat up and down permalloy-capped SiO2 particles in solution and viewed autonomous movement, as shown in

*Self-thermophoretic microparticles. (A) Au-capped SiO2 microspheres undergoing autonomous propulsion resulting from the "Soret effect" in a defocused laser beam. (B) Permalloy-capped SiO2 particles moving by selfthermophoresis in an AC magnetic field. Copyright 2010, APS Publications. Copyright 2012, ACS Publications.*

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

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

Short heat pulses have also been applied to modulate the speed of micro-/ nanomotors. Au-Pt nanowires subjected to elevated temperatures were observed to propel forward extremely faster than those at room temperature. For instance, an average speed of 45 μm/s was obtained for nanowires at 65°C compared with 14 μm/ s at 25°C. The speed increasing could be resulted from the reduction of the solution's viscosity and the temperature dependency of the electrochemical process. The utilization of heat pulses is an extremely reversible process, after incorporated with magnetic steering, which will enable a more advanced spatial and temporal control,

Bubble propulsion is possibly the most commonly studied mechanism in the field of micro-/nanomotors, which can be suitable for motors with any shapes as long as the motors are decorated with catalysts. The motion of the motors is produced by the releasing of micro-sized bubbles from the decomposition of fuel catalyzed by the catalyst. The most extensively studied instances of bubblepropelled micro-/nanomotors are those decorated with Pt as a catalyst to decom-

Bubble propulsion originates from the spontaneous decomposition of a fuel stimulated by a catalyst into micron-sized gas bubbles, whose detachments from the micro-/nanomotor's surface produce a recoil force to thrust the movement of the motors in the direction away from the catalyst. These motors, whose size ranging from a few micrometers to hundreds of micrometers, can realize powerful movement with considerable speed. In bubble-propelled micro-/nanojets, Solovev and his colleagues reported that the speed of the micro-/nanojets was related to the bubble's detachment frequency and radius, with the deviations at large values

with the capability to modulate both the direction and the speed.

pose H2O2 into H2O and O2 bubbles, as shown in **Figure 8** [16, 17].

**Figure 7B** [15].

**Figure 7.**

**2.6 Bubble propulsion**

**173**

Jiang et al. studied self-thermophoresis at the single particle level (**Figure 7A**) in 2010 [14]. Janus silica (SiO2) microspheres half-coated with Au were irradiated in water by using a defocused laser beam at 1064 nm. Absorption of laser by the thin Au layer generated heat, resulting in a local temperature gradient (2 K across particles) which produced thermophoresis. After that, Baraban et al. applied an

#### **Figure 6.**

*Self-acoustophoresis mechanism: asymmetrically shaped metallic microrods are triggered by an ultrasonic standing wave at MHz frequency. Copyright 2012, ACS Publcations.*

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

#### **Figure 7.**

where S, R, *γ*, *μ*, D, L, and k are the O2 evolution rate, radius of micro-/ nanomotor, surface tension of solution, viscosity of solution, diffusion coefficient

One of the potential applications of micro-/nanomotors is to be used in diagnostics and biomedicine; there is a demand to develop propulsion mechanisms which can be biocompatible. Ultrasound operates in a range of frequency above 20 kHz, which is biocompatible [5]. Hence, ultrasound is a promising technique for propelling micro-/nanomotors (wires/rods and tubular jets) in biomedical applications. The micro-/nanomotor propelled by ultrasound was firstly reported by Sen's group in 2012, as shown in **Figure 6** [5]. In this system, metallic microrods are suspended in water surrounded by an acoustic chamber. A vertical standing wave levitates the microrods to a plane at the midpoint of the cell resulting from the lowest pressure. In that plane, the metallic microrods behave axial propulsion at speeds up to 200 μm/s in water. The microrods also assemble into patterns in the nodal plane resulting from nodes and antinodes among the plane. The composition of the microrods was observed to effectively affect their propulsion, with only metallic

Temperature gradient could introduce the motion of micro-/nanoparticles

nanomotors induced by self-generated temperature gradient has been investigated,

*Self-acoustophoresis mechanism: asymmetrically shaped metallic microrods are triggered by an ultrasonic*

*standing wave at MHz frequency. Copyright 2012, ACS Publcations.*

Jiang et al. studied self-thermophoresis at the single particle level (**Figure 7A**) in 2010 [14]. Janus silica (SiO2) microspheres half-coated with Au were irradiated in water by using a defocused laser beam at 1064 nm. Absorption of laser by the thin Au layer generated heat, resulting in a local temperature gradient (2 K across particles) which produced thermophoresis. After that, Baraban et al. applied an

(micro-/nanowires, rods, and Janus spheres). This phenomenon is called thermophoresis or the "Soret effect." Recently, the propulsion of micro-/

and length of micro-/nanomotor, and constant, respectively.

*Smart Nanosystems for Biomedicine, Optoelectronics and Catalysis*

microparticles demonstrating fast axial motion.

**2.4 Acoustophoresis**

**2.5 Thermophoresis**

as shown in **Figure 7**.

**Figure 6.**

**172**

*Self-thermophoretic microparticles. (A) Au-capped SiO2 microspheres undergoing autonomous propulsion resulting from the "Soret effect" in a defocused laser beam. (B) Permalloy-capped SiO2 particles moving by selfthermophoresis in an AC magnetic field. Copyright 2010, APS Publications. Copyright 2012, ACS Publications.*

alternating current (AC) magnetic field to heat up and down permalloy-capped SiO2 particles in solution and viewed autonomous movement, as shown in **Figure 7B** [15].

Short heat pulses have also been applied to modulate the speed of micro-/ nanomotors. Au-Pt nanowires subjected to elevated temperatures were observed to propel forward extremely faster than those at room temperature. For instance, an average speed of 45 μm/s was obtained for nanowires at 65°C compared with 14 μm/ s at 25°C. The speed increasing could be resulted from the reduction of the solution's viscosity and the temperature dependency of the electrochemical process. The utilization of heat pulses is an extremely reversible process, after incorporated with magnetic steering, which will enable a more advanced spatial and temporal control, with the capability to modulate both the direction and the speed.

#### **2.6 Bubble propulsion**

Bubble propulsion is possibly the most commonly studied mechanism in the field of micro-/nanomotors, which can be suitable for motors with any shapes as long as the motors are decorated with catalysts. The motion of the motors is produced by the releasing of micro-sized bubbles from the decomposition of fuel catalyzed by the catalyst. The most extensively studied instances of bubblepropelled micro-/nanomotors are those decorated with Pt as a catalyst to decompose H2O2 into H2O and O2 bubbles, as shown in **Figure 8** [16, 17].

Bubble propulsion originates from the spontaneous decomposition of a fuel stimulated by a catalyst into micron-sized gas bubbles, whose detachments from the micro-/nanomotor's surface produce a recoil force to thrust the movement of the motors in the direction away from the catalyst. These motors, whose size ranging from a few micrometers to hundreds of micrometers, can realize powerful movement with considerable speed. In bubble-propelled micro-/nanojets, Solovev and his colleagues reported that the speed of the micro-/nanojets was related to the bubble's detachment frequency and radius, with the deviations at large values

conducted without the requirements of expensive instruments and harsh working environments. Therefore, micro-/nanostructures with diverse dimensions can be grown using this method, especially the micro-/nanowire, micro-/nanorod, and

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

Template-assisted electrochemical deposition, as shown in **Figure 9**, utilizes the pores of a membrane template to grow the required wires and tubes comprising of different materials [18]. Each pore of the template functions as a reactor in which the desired structure is synthesized. Membrane templates commonly used for the synthesis of micro-/nanomotors are track-etched polycarbonate (PC) membranes and porous alumina (AAO) membranes. Relying on the properties of the material and the chemistry of the pore wall, the micro-/nanomotors can be either hollow or solid. Membrane template-assisted electrodeposition provides a relatively low-cost and powerful approach for synthesizing micro-/nanowires, micro-/nanorods, and

For membrane template-assisted electrodeposition, a layer of Au or silver (Ag) is firstly coated on one side of the membrane by physical vapor deposition (PVD) to play a role of the working electrode. Afterwards, the membrane is assembled in a Teflon plating cell with flat aluminum (Al) foil located against the metal layer to work as a conductive contact for subsequent electrodeposition. Commonly, a sacrificial layer of Ag or copper (Cu) is firstly grown, followed by sequential growth of different required metals. The Ag or Au backing and the sacrificial layer are etched away by chemical etchant or are removed by mechanical polishing. By removing the alumina (Al2O3) membrane in sodium hydroxide (NaOH) solution, the nanowires or nanotubes can be released and obtained after successive rinsing and

The Wang's and Pumera's research groups combined electrodeposition widely applied in the growth of nanowire-based micro-/nanomotors with the bubblepropelled tubular micro-/nanojets, as shown in **Figure 10** [19–24]. Two geometries (either cylindrical or conical) can be obtained, entirely relying on the type and geometry of porous template (either PC or AAO). In addition to such metals, the incorporation of polyethylenedioxythiophene (PEDOT), polyaniline (PANI), and polypyrrole (PPy) polymers with Pt generates catalytic microjets. On the other hand, the wall can be incorporated with molecularly imprinted polymers (MIPs) so that alternative recognition cavities can be implemented for the selective separation

*Membrane template-assisted electrochemical deposition of micro-/nanomotors: (a) coating of Au or Ag backing on the membrane template, (b) electrochemical deposition of the sacrificial layer, (c) sequential electrochemical deposition of desired elements, and (d) removal of the backing and sacrificial layer and dissolution of the*

tubular micro-/nanoengines.

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

tubular micro-/nanoengines.

centrifugation.

of biomolecules.

**Figure 9.**

**175**

*membrane. Copyright 2015, ACS Publications.*

#### **Figure 8.**

*Examples of bubble propelled micromotors. (A) Schematic illustration of a tubular micro-/nanomotor's movement by the bubble propulsion mechanism. (B) Janus Pt-SiO2 spheres moving in H2O2. Copyright 2014, RSC Publications. Copyright 2009, AIP Publications.*

resulted from the potential collisions between bubbles, which diminished the distance traveled by the bubbles within the tube and limits the detaching stage. They concluded that the dynamics of micro-/nanojets is influenced by their shapes, the fuel composition, and the viscosity of the medium.
