Our lives are full of machines, and the idea of machines taking over the world is no longer just fiction, it's already happening. Current machines are relatively large, but many scientists want much smaller machines: machines made out of molecules. Micro-machines can change everything from medicine to materials science, and molecular processes play an important role in it, but controlling motion at the microscopic scale requires some physical and chemical knowledge.
The basic function of the machine is to input some energy into at least one moving part, each with a different function. These parts combine to produce useful movement as an output, which is known as doing work. Making machines smaller has some obvious advantages, such as being able to transport them more easily so that they move more precisely. In 1959, physicist Richard Feynman spoke of "the problem of manipulating and controlling things on a small scale," where "small ranges" are machines made up of one or more molecules.
Twenty years later, nanotechnology pioneer Eric Drexler stumbled upon a copy of Feynman's lecture on machines. He further developed some ideas and published a paper in 1981 called Molecular Engineering. Drexler imagines that molecule-sized machines can manipulate reactants of chemical processes at the atomic scale and even build new materials from molecules. This will be a huge breakthrough! Think about how engineers over the past few decades managed to shrink electronic components and turn building-sized computers into mobile phones. Shrinking mechanical components could usher in a similar revolution, but the challenges of building nanoscale machines are very different from those encountered before.
At the molecular scale, machines don't behave as we're used to on a daily scale, and the "nuts" and "bolts" of molecules can't be easily unscrewed without careful design. There are van der Waals forces between molecules, and their effect of attraction together is much greater than the effect of friction on ordinary nuts and bolts. Another problem is that getting the components of the molecular machine to move the way you want it to be tricky, and the thermal motion (noise) of nearby molecules can also cause the components to move randomly. Finally, most molecules are joined together by chemical bonds, with different kinds of chemical bonds, which tend to be fairly rigid and do not allow free movement between two parts, whereas machines often rely on relative motion between parts.
Olefins
So to build molecular machines, engineers have to figure out how to take advantage of so-called mechanical bonds. In mechanical bonds, the shapes of the molecules are interlocked together, and there is no structure of a covalent bond between the two interlocked molecules, but only by breaking the covalent bond of one of the molecules can the mechanical bond between the molecules be broken, making them completely separated. As early as the 1960s, scientists had created such connecting molecules, which are called thyons. But they are rare, and difficult to produce for scientific research, let alone anything practical.
It wasn't until 1983 that the French chemist Jean-Pierre Souzh made an unexpected discovery. Sowazh was originally studying chemical reactions driven by ultraviolet light, one of which involves attaching itself to C-shaped molecules of copper ions. While modeling the reaction, he realized that by tuning the method, he could produce more thyonosines from these molecules than ever before.
The trick is to have the copper ions bind to the inside of the ring molecule, and then a C-shaped molecule can cross the ring and attach to the same copper ion. In a suitable environment, another C-shaped molecule can chemically bond with the first molecule to form an interlocking loop, and finally the copper ion is ejected. In this way we get two molecular rings in a mechanical bond structure. These rings can rotate freely relative to each other, just like machines. Sovaz even extended this process to the manufacture of knotted chemicals and more complex chains.
In 1994, the Sowazh team found a way to use solfrons with sandwiched copper ions to spin one ring around another. Because the rings are not uniform, if the charge of the ions changes, they adjust to a more stable electrical position. So when that copper ion is stripped of its electrons at the center of the chemical reaction, one of the rings will rotate 180 degrees; if the copper ion recapsulates the electron, it will twist back again. If we want to build molecular machines with rotating parts, it's really important to master this movement.
Rotane
Around the same time, the British chemist Fraser Stodart made progress on different chemical mechanisms, making a molecular machine called rotane. Back in 1991, Stoddart's team made an almost closed ring of atoms missing electrons. They also make a rod-like molecule with two electron-rich loci and a massive silicon-based end cap. When they are put together, electrostatic gravity connects the ring to the rod, where a chemical reaction closes the ring.
Although the positively charged ring is attracted to the negatively charged position on the shaft, it is not connected more tightly by chemical bonds. In addition, the ring can also jump between two negatively charged points on the axis, while the massive silicon-based end cap prevents it from falling out. Using this principle, they built a molecular elevator that could raise itself by a few nanometers.
Constantly spinning
Sovanzh's rings can rotate in response to inputs, but cannot provide a continuous, controlled output like a motor. However, in 1999, the team of Dutch chemist Bernard Felinga achieved this goal. They developed a bifacial molecule that acts a bit like a motor blade. As we mentioned earlier, thermal noise makes it tricky to control how molecular components move, but Ferringa's molecules are based on two methyl groups that are designed to rotate in only one direction. Each time a pulse of ultraviolet light hits one of these methyl groups, it absorbs the light and converts it into kinetic energy, which then rotates around the axis.
In 2011, Fehringa and his team went a step further, using this technique to build a nanocar with four rotating wheels.
Sovarz, Stodart and Fehringa use clever design and special environments to solve the problems we encounter when using basic molecular machines. In 2016, their efforts won the Nobel Prize in Chemistry.
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Source: Vientiane Experience
Edit: Garrett