A crash course on the synthesis of particles 50,000 times smaller than the width of a human hair

I am now over six weeks into my project so I thought I’d give you a brief overview of what I’ve been up to and what I’ve learnt. The aim of my project was to synthesise metal and metal-oxide nanoparticles and then to encapsulate them in metal-organic frameworks (MOFs), a class of ultra-porous materials, for catalysis of oxidations and other reactions. This all sounds a bit complicated so why bother? Well, the current method for oxidations in industry involves heating under high pressures with steam so these materials would provide a less energy intensive route to the same product. Encapsulation of the nanoparticles also improves their reusability and in some cases their activity too.

So, I’ve been attempting to synthesise nanoparticles of late transition metals such as cobalt oxide, copper and platinum. How big is a nanoparticle? Well, as their name suggests, typically they are a few nanometres across. To give you an idea of scale, this is 50,000 times smaller than the width of a human hair or just larger than the width of a DNA helix. That’s pretty small right?! Consequently, they are quite tricky to synthesise and handle. I’ll give you an idea of what I’ve been struggling with.

There are many published methods on the synthesis of metal and metal-oxide nanoparticles. They all painstakingly detail the types of reagents and synthetic conditions used but none of them mentioned just how tricky these guys can be to deal with once you’ve made them. They are so small that they do not settle out of solution once they’ve been made, they remain in suspension. This is called a colloid. For an everyday example, milk is also a colloid: a suspension of very fine particles of fats and proteins in water. Milk doesn’t settle out in your fridge, does it? That would be a bit disgusting! My problem was that I had to somehow get these particles out of the colloid.

Filtration does not work for separating colloids as the particles are so small they pass through the gaps between the cellulose strands in the paper. However, centrifugation was found to be successful. This is where you spin the colloid so fast that the slightly heavier solid particles are forced to the outside of the tube by centrifugal forces. These are the same forces that push you off a roundabout if you spin too fast. Although this method works, it is very time consuming typically taking a few days for the samples to be ready for analysis.

Cobalt oxide nanoparticles

Another potentially obvious question is what do nanoparticles look like? I’m not quite sure what I initially anticipated but all the nanoparticles I have made are various coloured powders. The particles aggregate into more visible clumps when dry that then disperse into ultra-fine particles in solution. For example, my cobalt oxide nanoparticles were black and are shown above. I’ve shown a couple of different metal nanoparticle solutions below to show the different colours.

Different nanoparticles in solution

But how can you determine how small these nanoparticles are? A normal microscope can’t be used to look at these particles as they are too small. A light microscope can typically focus down to the micrometre scale which is still 1000 times too large for the study of nanoparticles. Instead, a transmission electron microscope (TEM) can be used. The TEM is an exciting piece of kit, you feel a bit like a mad scientist in a film when you’re using it! Imagine a giant microscope, maybe 2 m high, in a cold dark room illuminated by red light so you can see the images. The TEM fires a beam of electrons at the material. The nanoparticles are so small that the electrons can pass through. Different materials have different electron densities so will allow different numbers of electrons to pass through and hit the fluorescent screen behind. So, different materials can be distinguished and a shadow image of the particles can be taken. I’ve shown a couple of TEM images below of cobalt oxide and platinum nanoparticles.

Cobalt oxide nanoparticles, about 20 nm across

So, after a bit of a struggle I’ve managed to make a selection of metal and metal-oxide nanoparticles. The next step of course will be to encapsulate these in MOFs. This will no doubt come with its own struggles and discoveries but I’m looking forward to the challenge.

Platinum nanoparticles, about 5 nm across



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