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Powerful nanoengine built from coated nanoparticles

Expanding polymer-coated gold nanoparticles. Credit: Yu Ji/University of Cambridge NanoPhotonics

Discussions of complex molecular machine systems or nanorobots navigating through water frequently raise the issue of whether nanoscale engines can be powerful enough. Scientists at UK’s Cavendish Laboratory have provided one response. A hat tip to KurzweilAI for showcasing this University of Cambridge news release “Little ANTs: researchers build the world’s tiniest engine“:

Researchers have built a nano-engine that could form the basis for future applications in nano-robotics, including robots small enough to enter living cells.

Researchers have developed the world’s tiniest engine – just a few billionths of a metre in size – which uses light to power itself. The nanoscale engine, developed by researchers at the University of Cambridge, could form the basis of future nano-machines that can navigate in water, sense the environment around them, or even enter living cells to fight disease.

The prototype device is made of tiny charged particles of gold, bound together with temperature-responsive polymers in the form of a gel. When the ‘nano-engine’ is heated to a certain temperature with a laser, it stores large amounts of elastic energy in a fraction of a second, as the polymer coatings expel all the water from the gel and collapse. This has the effect of forcing the gold nanoparticles to bind together into tight clusters. But when the device is cooled, the polymers take on water and expand, and the gold nanoparticles are strongly and quickly pushed apart, like a spring. The results are reported in the journal PNAS [abstract].

“It’s like an explosion,” said Dr Tao Ding from Cambridge’s Cavendish Laboratory, and the paper’s first author. “We have hundreds of gold balls flying apart in a millionth of a second when water molecules inflate the polymers around them.”

“We know that light can heat up water to power steam engines,” said study co-author Dr Ventsislav Valev, now based at the University of Bath. “But now we can use light to power a piston engine at the nanoscale.”

Nano-machines have long been a dream of scientists and public alike, but since ways to actually make them move have yet to be developed, they have remained in the realm of science fiction. The new method developed by the Cambridge researchers is incredibly simple, but can be extremely fast and exert large forces.

The forces exerted by these tiny devices are several orders of magnitude larger than those for any other previously produced device, with a force per unit weight nearly a hundred times better than any motor or muscle. According to the researchers, the devices are also bio-compatible, cost-effective to manufacture, fast to respond, and energy efficient.

Professor Jeremy Baumberg from the Cavendish Laboratory, who led the research, has named the devices ‘ANTs’, or actuating nano-transducers. “Like real ants, they produce large forces for their weight. The challenge we now face is how to control that force for nano-machinery applications.”

The research suggests how to turn Van de Waals energy – the attraction between atoms and molecules – into elastic energy of polymers and release it very quickly. “The whole process is like a nano-spring,” said Baumberg. “The smart part here is we make use of Van de Waals attraction of heavy metal particles to set the springs (polymers) and water molecules to release them, which is very reversible and reproducible.”

The team is currently working with Cambridge Enterprise, the University’s commercialisation arm, and several other companies with the aim of commercialising this technology for microfluidics bio-applications.

The authors note that various actuators used to turn energy sources into actual movement in microscopic machines all suffer from various shortcoming, especially being very slow (on the order of seconds) and only generating weak forces (on the order of piconewtons). To store large amounts of energy that can be released at greater than MHz frequencies, they turn to spherical 60-nm-diameter gold nanoparticles coated with poly(N-isopropylacrylamide (pNIPAM). This polymer undergoes a coil-to-globule transition at 32 °C, with the result that it is very hydrophilic and swells with water below that temperature, and becomes very hydrophobic and expels water above that temperature. Heating due to plasmonic resonance triggered with a laser causes the nanoparticles to collapse within a microsecond to 400-nm diameter aggregates comprising on average about 40 nanoparticles separated from each other by less than 4 nm. Cooling the solution below the transition temperature re-swells the pNIPAM, producing nanoparticles coated with a layer of pNIPAM 40 nm thick.

The authors note that these ‘ANTs’ can be repeatedly recycled between cold, isolated, inflated and hot, aggregated, deflated states. They also note that addition of salt or ethanol, or other manipulations, allows tuning the size of the clusters from 50 to 1000 nanoparticles.

The authors estimate the amount of elastic energy in the aggregated state as sufficient to drive an expansion force of ~5 nN (nanonewtons), four orders of magnitude greater than the typical Brownian forces in solution of 1 pN. They note this represents a force to unit weight ratio nearly a hundred times greater than any motor or muscle, due to very large van der Waals attractions between the gold cores in the collapsed pNIPAM state. The authors note that the next challenge for adapting this powerful reversible expansion and contraction to nanomachinery requires configurations that provide directional forces. We of course would like to see atomically precise power sources, but it will be interesting to see what can be achieved with ANTs powering MEMS, NEMS, microfluidics, and various types of nanomachinery.
—James Lewis, PhD

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