User:Caorduno/Catalytic Nanowire Motor



A catalytic nanowire motor is a bimetal (e.g. Au/PT) device propelled by the electrocatalytic decomposition of a chemical fuel. Most current catalytic nanowire nanomotor research involves the reaction of splitting hydrogen peroxide into water and oxygen by the action of a catalyst, such as platinum: 2H2O2 → 2H2O + O2.[1][2] This phenomenon leads to autonomous translational motion of the nanorod, with platinum end moving forward.

Besides the nanorod other more advanced designs have been explored, leading to the more general concept of artificial catalytic nanomotors.

Self driven systems


The mechanism of movement of camphor boats provides a relatively simple example of a self-driven system by surface energy. Their movement is accredited to an interfacial tension difference between water and air caused by the dissolution of camphor. [3]

Smaller systems using the same principle have been studied. One of the first reported involved the movement of polydimethylsiloxane (PDMS) boats of 9 mm in diameter with platinum rudders. In aqueous solutions of hydrogen peroxide, the platinum catalyzed the decomposition of hydrogen peroxide to convert chemical energy into mechanical energy.[4] The decomposition of hydrogen peroxide results in water and oxygen. In such systems the propulsion is attributed to the rupture of oxygen bubbles creating a propulsive force driving the PDMS boat forward at the air/water interface.

Strong efforts have been made in order to scale down such a concept down to the nano/micro scale, but as scientists report, different physical forces dominate; making it difficult to move nanoscale objects in a controlled fashion.[5]

Design
There are two important variables to be considered on the design. First, asymmetry i.e. confining the reaction to one section of the nanomotor, sets up a gradient along the shape, generating a force difference between the ends. Second, the placement of the catalyst onto the nanomotor backbone dictates the type of motion exhibited, which may be rotational, translational, or combinations of the two.[1]

Asymmetric structures are naturally found in biological motors, to make an analogy between these and artificial catalytic nanomotors: biological motors utilize asymmetrically placed ATP to gain conformal changes, thereby leading to autonomous motion;[6] catalytic nanomotors rely upon an asymmetrically placed catalyst that leads to self-induced electric fields, reaction product concentration gradients, and so on to gain autonomous mobility.

Nanorod
A bimetallic nanorod approach has been explored extensively, since it is easy to impart catalytic asymmetry to the rod. The chemical reaction is confined on one end of the road, causing a gradient along the length, generating a force at the end. The heterogeneous nanorod is the simplest example of a mechanism that leads to translational motion.

The nanorods can be fabricated following a template-directed electroplating (TDEP)[7], which will be explained later.

Rotary Nanorod
The design of a rotary silicon–platinum nanorod nanomotor can be achieved using a technique known as Dynamic Shadowing Growth (DSG). These rotary nanomotors consist of a silicon backbone that is generated using oblique angle deposition (OAD). Figure 3 illustrates the fabrication procedures. By rotating the substrate to an angle θ≈86° silicon nanorods are fabricated at an angle β<θ due to the self-shadowing effect, which will serve as the backbones for the nanomotors.[8] Then a thin platinum catalyst layer is deposited onto one side of the silicon nanorod backbones by rotating the substrate back to θ=0°. This process effectively creates rotary Si/Pt nanorod nanomotors.



L-shaped Nanorods
L-shaped nanorods were designed to break the symmetry of the nanorod and to determine the direction of the driving force applied to catalytic nanomotors when immersed in hydrogen peroxide solution. The shape of these nanomotors allows for the observation of their orientation and motion direction in solution.[8] As shown in Figure 4, the L-shape is achieved by first growing a short section of silicon nanorods using OAD, as discussed above, and then rotating the substrate 180° azimuthally to grow a long section of Si nanorods. Once this process is complete, a thin layer of platinum is deposited at the upper side of the long Si nanorods. It has been shown that when these nanomotors are placed into a solution of 5% hydrogen peroxide, they all rotate from the long arm to the short arm demonstrating that the L-shaped nanomotors are pushed from the platinum side. Therefore, the chemical reaction taking place at the platinum side of the nanomotor produces a propelling force that pushes the nanorod away from the reaction site.



Rolling Nanospring
An interesting example of a complex nanomotor structure that DSG can design is the rolling nanospring. The idea is roughly the same as for the L-shaped nanorods discussed above, but instead of rotating the substrate azimuthally by 180°, after depositing a section of silicon nanorod and coating its top side with a thin layer of silver, the substrate is rotated azimuthally by 90°, and a similar section of Si–Pt nanorod structure is deposited again; repeating the deposition procedure three times results in the creation of a spring-like structure, as in Figure 5. As shown, the silver deposited on each section of the nanospring contributes the force supplied to moving the nanomotor. When these objects are placed into a 5% solution of hydrogen peroxide, they exhibit rolling translational motion.[8] From force analysis, there will be a net downward force, as well as a net torque, since the actions of all of the forces are in four different planes as shown in Figure 5.



Template-directed electroplating (TDEP)
Mallouk and Sen’s group at Pennsylvania State University and Ozin’s group at the University of Toronto both designed heterogeneous nanorod structures through template-directed electroplating (TDEP) method. The method uses a membrane with nano sized channels as a mold for the design of the nanorod.[7][9] The method is relatively inexpensive and easy to perform. It also allows for different metals to be electroplated in succession, leading to a bimetal rod. Initially gold and platinum were used, but later the gold, platinum, and nickel combination was explored, as we it will be seen later, for controlling purposes. It is reported the catalytic decomposition of hydrogen peroxide occurred more efficiently using platinum as catalyzer.[10][11] After the fabrication of the nanorods they are freed by removing the conductive silver backing with nitric acid and the sacrificial template with a strong base, sodium hydroxide. The rods are washed with deionized water and ultracentrifuged to achieve a neutral pH.



Dynamic shadowing growth (DSG)
With TDEP only one type of motion, translational or rotational is achieved, since the TDEP can only produce coaxial nanostructures. If the mass of the deposited structures is well balanced, in principle, only translational motion may be observed.[12] To create a wider variety of structures with the ability to achieve several types of motion, a dynamic fabrication technique must be utilized: termed dynamic shadowing growth (DSG); this method is a vapor deposition technique and it allows for a flexible design of catalytic nanomotors.[8]

In this technique a material is melted until vapor is released into a vacuum chamber. This vapor condenses onto a substrate and forms a thin film. If the substrate is tilted at a large angle θ>70° with respect to the vapor incident direction, then an interesting phenomenon known as self-shadowing occurs; this method is known as oblique angle deposition (OAD).[13] The dynamic shadowing growth technique combines substrate rotation and the shadowing effect. This method is effective for the design of catalytic nanomotors since making heterogeneous nanostructures is simple, and just by alternating the material being deposited in the chamber, intrinsically, the DSG can place the catalyst asymmetrically on the nanomotor backbone.

Propulsion Mechanism
We will now look into what makes the nanorods move and why the move with the platinum end forward. Different theories have been formulated to explain the propulsion mechanism. These interpretations include self-electrophoresis, bubble propulsion, interfacial tension gradients, and reaction product gradients. The concepts involved address tension forces, viscous drag, and electrochemistry. All explanations revolve around the gradients of reaction products created by the asymmetrically distributed catalyst and are closely related to diffusiphoresis.

Diffusiphoresis
Diffusiphoresis arises due to concentration gradients of reaction products in solution that may cause motion depending upon the interaction of the products with the particle surface. Through van der Waals and dipole forces, reaction molecules interact with the surfaces of objects in solution.[1] If a concentration gradient exists in the solution, the system is not in equilibrium, and therefore, some action must take place in order for equilibrium to occur. If the solutes interact with the surface of a nearby particle, then the particle will move. Catalytic nanomotors are propelled by diffusiophoresis, since, by design, they create concentration gradients in the solution through a catalytic reaction.

Self-Electrophoresis


Self-electrophoresis been proposed as a mechanism for moving nanorods through a solution of hydrogen peroxide.[14][15] A self-induced electric field occurs as a result of an oxidation-reduction reaction (redox reaction); the reaction releases charged products into the solution surrounding the conductive material, which generates an electric field. The strength of the field depends upon the conductive rod.

For the Au–Pt nanorod motor, the platinum acts as the cathode by reducing hydrogen peroxide and consuming protons while the gold end of the nanorod acts as the anode generating protons. The chemical reaction results in a proton gradient in the solution therefore the rod becomes an electric dipole.[14][15][16] The protons then move past the rod to the platinum section (cathode), where they are consumed. The motion of the protons drags the fluid adjacent to the rod, and causes a slip velocity, propelling the rod through the solution.

An application of the self-electrophoresis is to design micrometer-sized pumps. These pumps exploit the process of self-electrophoresis differing only in the fact that the metals now are stationary. The slip velocity caused by the migration of protons propels Au–Ag nanorods through the solution, but if the metals are stationary, only the fluid moves, and this flow in the direction of the cathode effectively creates a pump. Figure 7 illustrates the redox reaction that occurs with a silver island placed atop an Au thin film. The generated protons migrate toward the silver and create an electroosmotic flow.[16] Kline et al. have successfully created pumps using self-electrophoresis at the micrometer scale.[17]



Bubble Propulsion


Another mechanism suggests the propulsion is due to oxygen bubbles rupture. Autonomous motion has been observed for macroscopic objects moving on the surface of hydrogen peroxide at the air/hydrogen peroxide interface.[18] Whitesides et al. created small, <1 cm, plates with platinum placed on one side, perpendicular to the solution surface. The catalytic reaction of hydrogen peroxide generates oxygen bubbles that nucleate on the catalyst (Pt) surface, aggregate, and eventually rupture, causing linear and rotational motion of the plates.[18] Such a propulsion mechanism was also observed by Ozin et al. when studying the motion of the Au–Ni nanorods moving in a solution of hydrogen peroxide.[19]

Interfacial Tension Gradients
Asymmetric interfacial tension gradients have been proposed to propel nanomotors through a solution of hydrogen peroxide. By solving the diffusion–convection equation regarding oxygen production, a linear dependence of the velocity with respect to the oxygen production speed can be determined, and observations have verified this calculation.[14] The interfacial tension is lower at the platinum end of the rod where oxygen is being produced at a constant rate. On the other hand the gold surface becomes hydrophobic, as result of being covered by oxygen bubbles, resulting in an even lower interfacial tension at the gold end and propelling the platinum end forward.

Temperature Gradient
Other gradient forces as in local heating generated by the exothermic hydrogen peroxide reaction were determined to be negligible because their contribution to velocity was less than a micron/second.[20]

Bioelectrochemical Propulsion
Organic catalytic nanomotors have been studied. Mano and Heller report man-made machines exhibiting locomotion via glucose–oxygen reactions.[21] The group fabricated a carbon fiber with one end containing bilirubin oxidase, which reduces oxygen, and the other end contains glucose oxidase, which catalyzes the oxidation of glucose. The flow of current through the carbon fiber runs from a glucose-oxidizing microanode to an oxygen-reducing microcathode. The drag of ions through the solution propels the fiber along the water–oxygen interface at a speed of 1 cm/s. The motility is reduced as time passes and the solution becomes contaminated with reaction produces. These fibers have been shown to exhibit linear and rotational motion. This reaction mimics metabolism in biological organisms. Developing organic catalytic nanomotos will allow for application that take place in vivo.

Movement Analysis
Nanowires will exhibit random Brownian motion when hydrogen peroxide for the platinum catalyst is absent. In the presence of dilute solutions of hydrogen peroxide a new force is observed. This force causes the nanorods to move in a non-Brownian fashion,[5] platinum end forward, as the platinum end is decomposing hydrogen peroxide. Asymmetry in the system is also necessary because a gold–platinum–gold nanorod does not move.

Motion Control
One of the short-falls with the catalytic nanorods is the poor directional control of the motor.[5] A key challenge in the design of catalytic nanomotors is controlling their autonomous motion i.e. designing a machine that converts energy into a controlled motion.

Incorporating Magnetic Segments.


A facile method to control the motion of autonomously moving Pt/Au nanorods is by incorporating magnetic segments into the wire. Nickel can be easily electroplated to form nanomotors with magnetic segments. The use of magnetism to align nanoscale–mesoscale objects has been previously reported by Meyer and Whitesides.[22][23] Optical microscopy techniques showed how a 550 Gauss strength magnetic field influenced the orientation of the rod. The magnetic field allowed the micron-scale control over the direction of motion. Figure 9 shows the trajectory plot of one striped nanorod tracing out the letters “PSU” accompolished by steering the nanorod remotely with a strong magnet as it moves autonomously by the catalytic decomposition of hydrogen peroxide. The trajectory plot in Fig. 9 demonstrates our ability to control these nanorods with micron-scale precision using magnets.

Chemotactic Behavior
Catalytic nanowire motors display a chemotactic behavior in the presence of a gradient of the fuel concentration, with a directed movement and increased speed toward higher peroxide concentrations.[24] Such behavior resembles the movement of living organisms toward a chemical attractant. Controlling and modulating the local fuel level may thus be used for guiding, initiating, or slowing the motion.

Recent efforts have demonstrated the use of light or heat to initiate and control the motion.[25]

Efficiency and power improvement
Efficient energy conversion is crucial for extending the scope of catalytic nanomotors to diverse operations and realistic conditions. Recent efforts have illustrated the ability to increase the velocity, force, lifetime, and versatility of synthetic nanomotors by exploring new motor and fuel compositions.[26] For example, we demonstrated that the incorporation of carbon nanotubes (CNT) into the platinum segment of catalytic nanowire motors leads to a dramatically enhanced speed and power.[27] The resulting nanomotors are capable of moving autonomously at speeds approaching 100 body lengths per second, representing the world’s fastest synthetic nanomotors. Such improvement reflects the increased electrochemical reactivity of the CNT component toward the hydrogen peroxide fuel. The substantial improvement in power and efficiency of catalytic nanomotors should extend their scope to diverse and demanding operations and realistic conditions.



Tailoring the fuel composition has also facilitated a dramatic speed enhancement. For example, adding a second component (hydrazine) to the peroxide fuel solution greatly increases the average speed of the Au/ Pt CNT nanowires to over 94 m/s.[25]

New environments and fuels
Chemically powered catalytic nanomotors currently operate only within a very narrow range of environments (low ionic strength aqueous solutions) and limited fuels (hydrogen peroxide, hydrazine). This limitation currently precludes many potential applications of artificial nanomotors, particularly biomedical ones. Extending the scope of synthetic nanomotors to diverse operations and wide range of environments would require the identification of new fuel sources.

One may induce motion using biomolecules present in body fluids as potential fuel precursors. For example, the high (mM) concentration of glucose in body fluids can be coupled with glucose-oxidase functionalized motors for biocatalytic generation of the peroxide fuel. Appropriate surface coatings may facilitate operation in media of higher ionic strength.

Microfabricated Motors


One of the potential applications of catalytically driven motors is in microelectromechanical devices (MEMS). Catchmark capitalized on the concept of the platinum– gold nanorods by designing gold gears (150 μm in size) with platinum spots on the teeth to produce the first rotating devices driven by the catalytic decomposition of hydrogen peroxide.[10] Rotation occurs because the platinum catalysts are located on the teeth of the gear and hang over the edge. The gear rotates with platinum end forward. The approximate rotational speed of the gear is one rotation per second but the translational speed of the gear (∼390 μm/s) is approximately 400 times faster than the Pt/Au nanord which is in agreement with the scaling of the device from the nanoscale to the microscale. Catchmark was able to control hydrophobicity of the gold through different wet chemistries, thereby demonstrating that a hydropbobic gold surface is necessary for movement to occur. This is in agreement with the interfacial tension gradient concept in that a hydrophobic gold surface is necessary to observe platinum end forward movement.

Microchannel networks transportation




Catalytic nanomotors offer great promise for creating powerful on-chip microsystems powered by autonomous transport. By transporting analytes or cargo without bulk fluid flow, such nanomotors may eliminate the need for external pump or power common to pressure-driven or electrokinetic flow-based microfluidic devices and may address the challenge of fluid transport in nanofluidic systems. A group of scientists is currently designing a chip-based nanoscale transport and distribution systems (Figure 12). Such engineered transport highways will rely on directed motion of nanowire motors and cargo manipulations (e.g., precise loading and release) along predetermined traffic tracks. The improved power and speed of our catalytic nanomotors have enabled demanding microchip applications.[28] For example, in an initial proof of concept, they demonstrated the magnetically guided nanomotor motion within microchannel networks, its selective sorting in microchip intersections, and the pickup and transport and release of “heavy” cargo along predetermined paths. (Figure 13) The latter relied on the incorporation of magnetic segments to enable dynamic loading and transport of magnetic sphere cargo.

Sen’s group also reported recently on the movement of sphere-loaded functionalized Pt/Au/Ni/polymer nanowires in free hydrogen peroxide solutions.[29] Coupling of the cargo spheres onto the nanomotors was accomplished through an electrostatic force between a negatively charged polypyrrole segment and a positively charged polystyrene sphere, or via a more selective linkage of a streptavidin-coated microsphere to nanowire functionalized with biotin-terminated disulfide (Figure 14).

Figure14-CatalyticNanomotors.jpg

Biomedical
One of the latest evolution in nanorod motors led scientists to imagine some of the possible biomedical applications of these devices. Specifically, when it was discovered that they were able to synthetically mimic magnetotactic bacteria because the magnetic moment of magnetotactic bacteria is on the order of the magnetic moment of a Pt/Ni/Au/Ni/Au striped nanorod.[30]

Roving Sensors
Scientists have already created devices which are sensitive toward hydrogen peroxide. The next step is the design of devices that are powered by hydrogen peroxide but have built in sensors for a molecule of interest. A library already exists for tethering sensing molecules to inorganic devices so the concept is most definitely within grasp.[20]