Catalytic nanotransporters for nanotechnology applications outside biological systems
(Nanowerk Spotlight) The catalytic conversion of chemical to mechanical energy is ubiquitous in biology, powering such important and diverse processes as cell division, skeletal muscle movement, protein synthesis, and transport of cargo within cells. Catalytic 'engines' will be key components of active micron- and sub-micron scale systems for controlled movement, particle assembly, and separations. A few days ago we took a look at catalytic nanomotors - sophisticated molecular-size biomotors have evolved in nature - and described an example where researchers supercharged their nanomotors by inserting carbon nanotubes into the gold and platinum nanowires (Speeding up catalytic nanomotors with carbon nanotubes). Today we show an example where catalytic nanomotors can, in principle, be tethered or coupled to other objects to act as the engines of nanoscale assemblies. Additionally, an object that moves by generating a continuous surface force in a fluid can, in principle, be used to pump the fluid by the same catalytic mechanism. Thus, by immobilizing these nanomotors, a group of scientists have developed micro/nanofluidic pumps that transduce energy catalytically.
"Until recently, catalytic micro- and nanomotors have been more or less unknown outside biology" Dr. Ayusman Sen explains to Nanowerk. "For nanotechnology researchers, catalyzed movement on the nanoscale is a fairly new phenomenon and there is much to be learned from nature's motor systems. There is a good possibility that unexpected applications will arise from exploratory research."
Sen, a professor and head of the Department of Chemistry at Pennsylvania State University goes on to point out that – freed from the constraint of biological reactions – nanoscientists could envision using much simpler fuels than ATP, and much more accessible catalysts than enzymes.
"By analogy to biological systems" he says, "we can project some obvious applications of catalytic nanomotors, including: (a) engines for micro/nanoscopic machines, (b) chemotactic roving sensors, (c) delivery vehicles for molecules and nanoparticles, and (d) formation of patterns or arrays by autonomous local deposition of materials."
Sen's group has demonstrated that one can build nanomotors 'from scratch' that mimic biological motors by using catalytic reactions to create forces based on chemical gradients. These motors are autonomous in that they do not require external electric, magnetic, or optical fields as energy sources. Instead, the input energy is supplied locally and chemically.
In recent work that was reported in the April 17, 2008 online edition of Nano Letters ("Catalytic Motors for Transport of Colloidal Cargo"), Sen's group demonstrated the attachment of a prototypical cargo: polystyrene microspheres to platinum/gold nanomotors which can then be transported:
Cargo attachment by (a) electrostatic interaction between the negative polypyrrole (PPy) end of a platinum/gold/polypyrrole motor and a positively charged polystyrene (PS) amidine micro-sphere and (b) biotin-streptavidin binding between the gold tips of platinum/gold rods functionalized with a biotin-terminated disulfide and streptavidin-coated cargo. (Reprinted with permission from American Chemical Society)
"Several research groups have harvested biomotors and reported their ability to carry loads in vitro" says Sen. "However, replicating their sophisticated motor function outside biology is a daunting challenge, due to their inherent complexity. Moreover, the lifetime of such devices is limited because biomotors, being tied to the precisely tuned physiology of the cell, degrade rapidly outside of the cellular support system. Simply scaling down macroscale motors is not viable due to limitations in fabricating complex architectures at the mesoscale and the profound differences between the familiar macroscale regime and highly overdamped aqueous mesoscopia."
Sen's group and others have designed motors powered via the asymmetric placement of onboard catalyst. For example, the PennState researchers have previously reported the motility of micrometer sized platinum-gold rods in aqueous hydrogen peroxide solutions via self-electrophoresis. The object creates its own electric field via bipolar decomposition of hydrogen peroxide ("Catalytically Induced Electrokinetics for Motors and Micropumps") – hydrogen peroxide is oxidized at the platinum surface and reduced at the gold surface. These reactions generate an imbalance in the proton concentration along the axis of the rod. The protons, which must move from platinum to gold to maintain charge balance, give rise to an electric field, causing the particle to undergo self-electrophoresis. In simple terms, the proton flux is like a paddle that pushes water along the side of a canoe.
A schematic illustrating self-electrophoresis. Hydrogen peroxide is oxidized to generate protons in solution and electrons in the wire on the platinum end. The protons and electrons are then consumed with the reduction of H2O2 on the gold end. The resulting ion flux induces motion of the particle relative to the fluid, propelling the particle towards the platinum end with respect to the stationary fluid. (Image: Dr. Sen)
In their recent work, Sen's team presents the first quantitative study of a nonbiological nanomotor’s cargo-carrying capability, elucidating systematic trends in motor speed as a function of cargo radius and demonstrating that magnetic steering elements can overcome the enhanced rotational diffusion of
a large spherical cargo (and the biased rotation of an asymmetric rod-sphere doublet) to enable persistent directed motion.
Sen notes that, while the drag on regular shapes like spheres and cylinders is well-known, the shape of the motor-cargo doublet is unique. "We have computed the drag on the rod-sphere doublets using a boundary integral equation and thereby demonstrate that doublets with largest-radius cargos exhibit an anomalously large motility."
The motor function is not disrupted due to the presence of passive cargo although a decrease in speed was observed. In addition, motors with nickel segments can overcome both Brownian orientational fluctuations and biased rotation of the rod-sphere doublet to enable persistent steerable uniaxial motion in an external magnetic field. The scientists are thus able to control even Brownian motion, at the microscopic level.
Transmission optical microscopy images of a platinum-nickel-gold motor pulling 1.05 µm radius polypyrrole microsphere cargo, showing the trajectory over 4 seconds. Images were captured at 1000x magnification. The top pane shows the trajectory in the absence of the field while the bottom pane shows the trajectory in the presence of the field. The external magnetic field suppresses the rotation of the doublet. (Reprinted with permission from American Chemical Society)
Sen says that many interesting applications can be envisioned for such cargo bearing motors in the mesoscale. "For instance, cargo can be concentrated at desired regions. This may find application in bottom-up assembly of colloids or for delivery of materials at a specific location whereupon further binding events may be triggered depending on cargo surface functionality."
Sen's work, which is the first examples of nano/microscale objects outside biological systems that move through catalysis, also reveals that chemotaxis (i.e. the movement by a cell or organism in reaction to a chemical stimulus) does not require a sophisticated 'temporal sensing' mechanism commonly attributed to bacteria. Rather, the nanoparticles move up a fuel gradient through catalysis; a straightforward extension is movement towards or away from a signaling molecule – a promoter or an inhibitor of the catalytic reaction.
This behavior provides a novel way to direct particle movement towards specific targets, even while allowing them to sample a large region of fluid by apparently diffusive motion. Sen says this discovery is potentially important in the design of 'smart' autonomous nano-robots, which could move independently in the direction they are needed, perhaps by harvesting energy from glucose or other abundant fuels in biological or organic systems.
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