Many chemists are actually thinking a lot about making very tiny small machines out of molecules. But how you can move a machine of microscopic scale, which you can barely touch. Well, you have to have some chemical knowledge to control motion on a microscopic scale machine. Thing is this tiny machines about the size of a molecule could revolutionize everything from medicine to materials science, where molecular processes play a big role.
Let’s discuss first thing fast, What is a Machine?
A machine is basically any device that takes some Energy input into at least ONE MOVING PART each with DISTINCT FUNCTION. So, in the end, those parts come together to produce a useful motion as an output, called work.
Now, There are some obvious advantages to making machines smaller, like being able to transport them more easily and make them move more precisely. In 1959 Nobel Prize-winning physicist Richard Feynman talked about THE PROBLEM OF MANIPULATING AND CONTROLLING THINGS ON A SMALL SCALE. By small, we are talking about a few millionths of a millimetre small machines made up of one or few molecules.
Twenty years later, nanotechnology pioneer Eric Drexler came across a transcript of Feynman’s lecture on the machine. Then he developed some of the ideas further and in 198, he published a paper called Molecular Engineering An Approach To The Development Of General Capabilities For Molecular Manipulation.
Eric Drexler imagined molecule-sized machines that could manipulate the reactants of chemical processes on an atomic scale. Not only that he even imagined that those machines could build new materials from the molecules. If you ask me my opinion over that idea I would say That is ASTONISHING, But how to do that?
Engineers have managed to shrink electrical components over the last few decades, like turning computers that of the size of a building into cell phones. Shrinking mechanical components could unlock a similar kind of revolution. BUT building NANOSCALE machines come with totally different challenges than the ones that many engineers deal with.
For starters, when you get down to the size of molecules, objects don’t act the way we are used to on everyday scales. Like without careful design, a molecular nut and bolt couldn’t be twisted apart easily. The electrostatic forces between the molecules, known as Van Der Waals forces, would attract them together a lot more than frictions affects ordinary nuts and bolts. Another problem is that it is trickier to get the components of a molecular machine to move the way you want.
A tiny molecule of air bumping into a piston in your car engine doesn’t really change the way it moves, But that same air molecule might send a machine flying or even destroy it. Even if the damage is not that extreme, the constant bombardment from nearby molecules, known as thermal noise, could make the components move around randomly. That could make controlling their motions pretty difficult even though that’s what we need to do for molecular machines to be useful.
Finally, most molecules are linked together with chemical bonds, which form because of electrical attractions between molecules. There are different kinds of chemical bonds, but they tend to be fairly rigid and don’t allow for free movement between the two parts and that my boy frustrating, because that the kind of moment every machine rely on! For example, imagine a bunch of water molecules locked into the crystal structure of an ice cube or even clumped together in liquid water. Each negatively charged oxygen atom is attracted to the positively charged hydrogen atoms of nearby water molecules forming hydrogen bonds between them. So to build molecular machines engineers have to figure out a way to utilize a mechanical bond, which our basic chemistry textbook maybe didn’t mention.
In a mechanical bond, the shape of the molecules links them. The individual parts of each molecule are not strongly attracted to one another, but they can’t separate entirely without breaking the chemical bonds between the atoms within one of the molecules. Kind of like how your key can’t accidentally come off your keyring even though they are not physically connected. Scientists had created linked molecules like this in the early 1960s. They were called catenanes chains of two or more connected rings of atoms.
Researchers knew that catenanes existed, but they were rare and really difficult to produce for scientific studies. But in 1983 French chemist Jean Pierre Sauvage made an unexpected discovery. Sauvage was originally studying chemical reactions that were driven by ultraviolet light and one of those processes involved C-shaped molecules that attached themselves to copper ions, While modelling the reaction, he realized that by tweaking the method, he could produce catenanes from those molecules in much larger numbers than ever before.
The whole process starts by getting a copper ion to bond to the inside of a ring-shaped molecule. Then, a C-shaped molecule can thread through the ring and attach to the same copper ion. In the right kind of environment, another C-shaped molecule can chemically bond to the first one creating a second interlocking ring. The final part of Sauvage’s chemical process was to pop that copper ion out. And voila: two molecular ring in one mechanically bound structure. Those ring can freely rotate to one another, just like we want in a machine. Sauvage even extended the process to make knotted chemicals and more complicated chains.
To set things in motion, in 1994 Sauvage’s team found a way to use that catenane with a sandwiched copper ion to rotate one of the rings around the other. Because the rings are not uniform they will adjust to more electrically stable positions if the charge of that ions changes. So when that copper ion gets an electron ripped off in a chemical reaction, one of the rings will rotate 180 degrees. It will twist back if the copper ion recaptures an electron. This motion is really important to master if we want to build molecular machines with rotating parts, like something with a molecular propeller that can swim through a liquid.
Around the same time, Chemist James Fraser Stoddart was making progress with a different chemical mechanism. James Fraser Stoddart was well acquainted with the laws of attraction, i.e Positively charged chemical structures are attracted to negatively charged ones. With that, Stoddart’s team created a molecular machine called a rotaxane, a ring linked onto a thread. Back in 1991, Stoddart’s group made a nearly closed ring of atoms with a lack of electrons. They also made a rod-shaped molecule with two electron-rich sites and bulky silicon-based end cap. When they were put together electrostatic attraction made the ring thread onto the axle, where it could be closed off to form a complete ring with a chemical reaction. Although the positively charged ring was attracted to the negatively charged ring was attracted to the negatively charged sites on the axle, it was not locked in place too tightly with chemical bonds. Because we are talking about molecules here, when the ring had a certain amount of heat energy then it had the energy to move around. So the researchers could make the ring hop between the two negatively charged spots on the axle, while those bulky group kept it from sliding off.
In 1994, Stoddart got even more precise and created two chemically different sites on the axle structure based on molecules called benzidine and biphenol group. Those groups have different electric and chemical properties depending on the acidity or the pH of the surrounding environment. In an acidic environment, the benzidine group becomes positively charged, repelling a ring so it sits on the biphenol group.
So basically, this researchers figured out how to control a ring’s movement on an axle in multiple ways. Stoddart’s group also used the principle behind these axles to make a molecular elevator that can raise itself a few nanometers and even a molecular muscle that can stretch and contract like our muscle cells.
Now lots of components in normal machines like the cogs in a watch or wheels on a car rely on continuously rotating elements. Sauvage’s ring could rotate in response to an input, but couldn’t provide a continuous controlled output like a motor.
In 1999, the organic chemist Ben Feringa and his group in the Netherlands achieved just that a continuously rotating machine. They developed a double-sided molecule that acted a bit like motor blades. As we have mentioned thermal noise makes it tricky to control how a molecular component moves. But Feringa’s molecule was based on two methyl groups that were designed to only rotate one way around. Every time a pulse of UV light hits one of the methyl groups, it absorbs the light and converts it into kinetic energy. The hit methyl group then rotates around an axis and bends over the other methyl group until it snaps past, so it is blocked from spinning backwards. And You Got The World’s First Molecular Motor. As if that was not cool enough, in 2011 Feringa and his group took it even further and used this technique to build a nano car with four rotating wheels.
Fraser Stoddart, Jean-Pierre, Sauvage and Ben Feringa used clever designs and special environment to solve some of the problems we were having with very basic molecular machines. In 2016, Fraser Stoddart, Jean-Pierre Sauvage, Ben Feringa collectively awarded The Nobel Prize in Chemistry for their work.
We have just begun exploring other machines, we might be able to make on the nanoscale and we know there are plenty of options because nature has been building them for billions of years. Like right now in your body, supper complex molecular machines made of proteins are doing all kinds of things to keep you going. Like your myosins walk along tracks of muscle fibre pulling them to help you contract your muscles. In other cells like sperms or certain bacteria have built-in molecular motors to make their flagella spin around, so they can move through fluids. Those are two of many examples, so scientists have plenty of inspiration for future inventions.
Some researchers have proposed that molecular machines could be used to deliver drugs in the body. For Example, mesoporous particles have lots of little holes that release their contents in response to ultrasound waves. Filled with the right drugs, we could load these particles onto a molecular transport machine to dose tumour with cancer-fighting molecules.
Other researchers have developed a gel with those molecular motors we mentioned, by attaching them to a tangle of long chains of molecules called polymers. When you shine a light on the material or heat it up, the motors reel in the fibres like fishing line, which shrinks the volume of the gel. Because those motors are storing energy in the form of those bundled up molecules, if we could find a way to extract the energy back out, this could be a step towards a new kind of Solar battery.
All that said, we have a long way to go before we were building molecular machine factories or anything beyond these basic experiments. It’s still tricky to make these tiny machines in large quantities and there may be other problems with making a bunch of individually developed components work together. But after more research and with more science thinkers, we might have molecular mechanics in our scientific toolkits and machines to help us at every scale of life.
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