A nanomotor is a molecular or nanoscale device capable of converting energy into movement. It can typically generate forces on the order of piconewtons.^[1][2][3][4]

While nanoparticles have been utilized by artists for centuries, such as in the famous Lycurgus cup, actual research into nanotechnology did not come about until recently. In 1959, Richard Feynman gave a famous talk entitled "There's Plenty of Room at the Bottom" at the American Physical Society's conference hosted at Caltech. He went on to wage a scientific bet that no one person could design a motor smaller than 400 µm on any side.^[5] The purpose of the bet (as with most scientific bets) was to inspire scientists to develop new technologies, and anyone who could develop a nanomotor could claim the $1,000 USD prize.^[5] However, his purpose was thwarted by William McLellan, who fabricated a nanomotor without developing new methods. Nonetheless, Richard Feynman's speech inspired a new generation scientists to pursue research into nanotechnology.

Nanomotors are the focus of research for their ability to overcome microfluidic dynamics present at low Reynold's numbers. Scallop Theory is the basis for nanomotors to produce motion at low Reynold's numbers. The motion is achieved by breaking different symmetries. In addition, Brownian motion must be considered because particle interaction can dramatically impact the ability of a nanomotor to traverse through a liquid. This can pose a significant problem when designing new nanomotors. Current nanomotor research seeks to overcome these problems, and by doing so can improve current microfluidic devices or give rise to new technologies.

Nanotube and nanowire motors

In 2004, Ayusman Sen and Thomas E. Mallouk fabricated the first synthetic and autonomous nanomotor.^[6] The two micron long nanomotors were composed of two segments, platinum and gold, that could catalytically react with diluted hydrogen peroxide in water to produce motion.^[6] The Au-Pt nanomotors have autonomous, non-Brownian motion that stems from the propulsion via catalytic generation of chemical gradients.^[6][7] As implied, their motion does not require the presence of an external magnetic, electric or optical field to guide their motion.^[8] By creating their own local fields, these motors are said to move through self-electrophoresis. Joseph Wang in 2008 was able to dramatically enhance the motion of Au-Pt catalytic nanomotors by incorporating carbon nanotubes into the platinum segment.^[9]

Since 2004, different types of nanotube and nanowire based motors have been developed. Most of these nanomotors use hygrogen peroxide as fuel, but some notable exceptions exist.^[10][11] These silver halide and silver-platinum nanomotors are powered by halide fuels, which can be regenerated by exposure to ambient light.^[11] Some nanomotors can even be propelled by multiple stimuli, with varying responses.^[12] These multi-functional nanowires move in different directions depending on the stimulus (e.g. chemical fuel or ultrasonic power) applied.^[12] In Dresdan Germany, rolled-up microtube nanomotors produced motion by harnessing the bubbles in catalytic reactions.^[13] The bubble-induced propulsion enables motor movement in relevant biological fluids, but typically requies toxic fuels such as hydrogen peroxide.^[13] This has limited their in vitro applications. Further research into catalytical nanomotors holds major promise for important cargo-towing applications, ranging from cell sorting microchip devices to directed drug delivery. Such in-vivo applications of microtube motors were described for the first time by Joseph Wang and Liangfang Zhang using gastric acid as fuel.^[14]

Enzymatic nanomotors

Recently, there has been more research into developing enzymatic nanomotors and micropumps. At low Reynold's numbers, single molecule enzymes could act as autonomous nanomotors.^[15] Ayusman Sen and Samudra Sengupta demonstrated self-powered, autonomous micropumps can enhance particle transportation.^[16][17] The proof-of-concept demonstrates that enzymes can be successfully utilized as an "engine" in nanomotors.^[18] It has since been shown that particles themselves will diffuse faster when coated with active enzyme molecules in a solution of their substrate.^[19][20] Further, it has been seen through microfluidic experiments that enzyme molecules will undergo directional swimming when exposed to a substrate gradient.^[21][22] This remains the only method of separating enzymes based on activity alone. Developing enzyme-driven nanomotors promises to inspire new biocompatible technologies and medical applications.

A proposed branch of research is the integration of molecular motor proteins found in living cells into molecular motors implanted in artificial devices. Such a motor protein would be able to move a "cargo" within that device, via protein dynamics, similarly to how kinesin moves various molecules along tracks of microtubules inside cells. Starting and stopping the movement of such motor proteins would involve caging the ATP in molecular structures sensitive to UV light. Pulses of UV illumination would thus provide pulses of movement. DNA nanomachines, based on changes between two molecular conformations of DNA in response to various external triggers, have also been described.

See also