A step forward for molecular machines

European chemists have achieved a long-standing goal in the field of molecular machines, developing a nano-scale motor that is efficiently powered by near-infrared light.

This miniature, controllable machine is just the newest development in a growing field that straddles chemistry and materials science, with applications in new materials, sensors and energy storage systems.

“Light-controlled artificial molecular machines hold tremendous potential to revolutionise molecular sciences,” the authors write in a paper in the journal Science Advances.

“Autonomous motion allows the design of smart materials and systems whose properties can respond, adapt, and be modified on command.”

Over the past decades, we have seen the revolutionary effect of miniaturising computer technology – but there is a parallel revolution occurring in the miniaturisation of machines.

Physicist Richard Feynman foresaw these advances as early as 1959, when he posed a visionary question: How small can we make machines?

Tiny machinery already exists in nature, of course: bacteria flagella propel themselves forward by spinning up to 60,000 times per minute. But in order to function properly, all of the flagellum’s components must fit together perfectly. So, Feynman asked, can humans find a way to build artificial machines with moving parts on the nanometer scale?

Spoiler alert: yes, we can.

Chemists first tackled the problem by attempting to link molecules together by mechanical bonds instead of the normal covalent bonds, and thus create advanced molecules with many moving parts. Early progress was made through the 1950s and 60s but soon petered out, with scientists struggling to produce enough molecules to justify the increasingly convoluted methods.

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Ben Feringa in 2019. Credit: Okinawa Institute of Science and Technology

The first molecular machine instead resulted from a different approach – photochemistry, which studies how light energy can be used to drive chemical reactions. Based on these principles and years of pioneering foundational research in linking molecules, Dutch chemist Ben Feringa performed the first successful demonstration of a molecular machine in 1999.

Normally, the movements of molecules are governed by chance, but Feringa forced a molecular “rotor blade” to continuously spin in a particular direction using pulses of ultraviolet light. This was a key step: molecular systems prefer to be in a state of equilibrium, occupying a lower energy state, but by inputting energy Feringa forced the system to do work.

Since then, further research has fabricated a range of tiny mechanical devices – for example, a 4WD nanocar with molecule motors as “wheels”; a motor that can rotate an object 10,000 times its size; a nanorobot that can grip and connect amino acids; and a “web” of molecular motors and polymers capable of storing light energy.

Feringa and colleagues were awarded the 2016 Nobel Prize in Chemistry for the design and production of the first molecular machines – and the field is still growing.

Molecular machines are now roughly at the same stage as the early machines of the industrial revolution. The 19th century world had no idea how the first steam engines and electric motors would revolutionise our society, and today we can only guess at the myriad ways molecular machines may become integral parts of our lives.

Since they can perform tasks on such a small scale, these machines could be put to work in fields as diverse as energy, medicine and materials.

For example, they could be used as molecular switches to activate drugs inside the body, which would be particularly useful in fighting antibiotic resistance; or, by mimicking the biological machinery in our cells, they may play an integral role in the rapidly developing field of synthetic biology. They could also be applied in molecular electronics, which aims to use molecules to form working components, such as wires, diodes and transistors.

“Autonomous motion allows the design of smart materials and systems whose properties can respond, adapt, and be modified on command.”

When used cooperatively, molecular machines could also form macroscopic materials able to dynamically change their properties, including gels, liquids, crystals and polymers.

But these applications are still on the distant horizon. First, researchers must overcome several long-standing challenges to create molecular machines that can adapt to specific environments, be precisely synchronised to act collectively, and operate with high efficiency and a durable energy input.

Energy input was the focus of this new research, led by Feringa’s lab at the University of Groningen in the Netherlands. Early molecular machines, such as Feringa’s molecular rotor blade, were powered by higher-energy ultraviolet light. But to use these machines in biological applications, they need to be powered by lower-intensity light that won’t be harmful to surrounding materials.

This study demonstrates that molecular motors can be powered by near-infrared light by attaching an extra component to the motor: a miniature “antenna”. The antenna absorbs two low-energy photons instead of one high-energy one, then passes the energy onto the motor.

Credit: Nong Hoang and Lukas Pfeiffer

“This is a direct transfer of the excited state, very similar to the way in which two strings on a guitar will resonate when one of them is struck,” explains Maxim Pshenichnikov, co-author from the University of Groningen. “If you know how it works, it becomes really simple. But the chemical design was certainly not trivial.”

“For the system to work, the energy levels of the antenna and the motor had to be closely tuned,” adds Lukas Pfeifer, a postdoctoral researcher also at the University of Groningen who carried out much of the research. “It also needed a linker that allows the antenna to be attached without interfering in the motor’s rotation.”

This advance is particularly exciting to those who have worked in the field for decades, including Feringa.

“After many years of designing molecular motors, being able to overcome the need for high-energy UV light to power these molecular rotary motors is like a dream come true,” he says.

“I feel that our results represent an important milestone in the design of artificial molecular motors and offer many prospects for future applications, ranging from responsive materials to biomolecular systems.”

The field is still rife with challenges for molecular designers to sink their teeth into, but the next decades of research may produce an explosion of new technologies.

Right now, we at least have an answer to Feynman’s initial question – how small can we make machines? At least a thousand times thinner than a strand of hair.

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