Christmas wishes for nanoparticle synthesis

Back in August The Baran Laboratory blog posted some thoughts on yields and the need for qualitative assessment of reactions. I agree with their points, particularly about only believing in “0%, 25%, 50%, 75%, or quantitative” yields.

Unfortunately I can’t recall ever reading a paper on nanoparticle synthesis where the authors report a yield. It is difficult to define the yield of a nanoparticle because, unlike molecules, nanoparticles have a distribution of sizes and shapes which makes the concept of molecular weight and consequently yield somewhat hazy. Still, it’d be useful if chemists reported the mass of dry nanoparticles obtained per batch. Practically, it’s quite straightforward and I think it would be a handy metric for assessing reactions.

Their post inspired me to write some Christmas wishes for the field of nanoparticle synthesis. Here we go:

Reporting centrifugation speeds in relative centrifugal force rather than rotations per minute

Centrifugation is the nanoparticle equivalent of column chromatography. Different size and shape particles sediment at different rates so by centrifuging them you can achieve separation. Typically papers report revolutions per minute (rpm), but the relative centrifugal force (RCF) is a more useful number as it’s this force which causes the particles to sediment at different velocities and separate. RCF is dependent on not just the angular velocity (r, i.e. rpm) but also the radius i of the centrifuge rotor:

RCF = i r^2 / g

where g is acceleration due to gravity. Our centrifuge has the option to set this instead of rpm. As an example, if some authors used a centrifuge at 2000 rpm and then your centrifuge has a rotor with a radius n times larger, the RCF will be n times higher and you’ll need to either reduce the rpm or centrifuging time. But you probably won’t know what rotor they had so you’ll have to guess and waste time working it out for yourself…

Characterisation of what’s been washed away during washing/purification procedures

This is obvious to me but no one does it. If you have to centrifuge, decant the supernatant off the sediment, then wash the product multiple times, what are you getting rid of? I want to know.

Representative electron microscopy

I want big, high resolution images with good contrast. Close ups of particles of interest are fine but I also want to see lower magnification shots showing representative samples of the product. A paper claiming to make nanoparticle X but only one image showing just a few of X? Then I won’t bother trying to reproduce it. Electron microscopy leads me on to…

Histograms!

I love histograms and I want to see them showing size distributions of your product. At least 50 particle measurements, preferably a hundred or more. Rather than simply stating that your product “is monodisperse”, actually give statistical data like the standard deviation to back up your claims. I also like seeing ratios of shape X to shape Y. If you used ImageJ to automatically measure the particles, then include what algorithm and parameters you used so that others can reproduce it.

Papers that do what they claim

If you claim to make nanoparticle X, you should be making at least 75% X by mass. I don’t have a problem with other crud as long as it’s a minority product and you acknowledge that it’s there.

I’d be a very happy boy if I got all of these granted…

MSci Project Part 1: Quantum Dots

I don’t start my PhD until October so I won’t be posting much about it for a couple of months. In the mean time, I thought it would be nice to talk about what I did for my final year research project as part of my MSci degree.

The aim was to synthesise (core-shell and ternary) quantum dots using microfluidic reactors. It sounds complicated, but really it’s quite straight forward! An explanation of it all in one post would be rather long so I’m going to break it down into two posts, starting with quantum dots and then moving on to microfluidic reactors.

What are Quantum Dots?

Quantum dots are nanoparticles—particles only a few billionths of a metre in size—made from semiconductors. Semiconductors are materials whose electrical conductivity is midway between that of insulators and conductors. They are the foundation of modern electronics and without them we wouldn’t have components like transistors and diodes which are essential building blocks of the technology we use every day.

All materials have particular physical properties—such as the melting point or density—that are independent of how much of the material you have. For example, if you measured the melting point of a material, cut it in half, then remeasured the melting point, the melting point would not change. Properties like these are called intensive properties.

Imagine you had a piece of semiconductor and repeatedly measured an intensive property, such as melting point, then cut it in half. You would expect intensive properties to stay the same, regardless of the amount of material. However, if you carried on doing this for quite some time—so that your semiconductor was just a few billionths of a metre across—you would find that its properties would start to change: properties which were intensive become extensive and dependent on how much of the material you have. Chemists take can advantage of this phenomenon to tune the properties of semiconductors for particular applications by controlling the particle size.

Making Quantum Dots

Rather than breaking down macro- or microscopic bits of semiconductor to make nanoparticles (“top-down”), chemists usually make quantum dots from individual atoms (“bottom-up”). This is most commonly achieved by injecting the appropriate reagents into a hot solvent. The quantum dots spontaneously form in the hot solvent and are left to grow to the desired size.

The photo below is of some cadmium selenide quantum dots that I made last year. I think it’s a wonderful example of their size-dependent properties.

CdSe Quantum Dots
CdSe quantum dots fluorescing under UV light.

Each vial contains quantum dots that were removed from the reaction vessel at regular intervals. The vial on the far left hand side contains quantum dots grown for 30 seconds and the vial on the far right hand side contains quantum dots grown for 3 hours. The mean size of the particles grown for 30 seconds and 3 hours was 2.8 nm and 4.2 nm respectively, so the nanoparticle size increases from left to right.

The colour arises from a process called fluorescence. The vials are sat on top of an ultraviolet lamp which causes the quantum dots to fluoresce and emit light, the wavelength of which is dependent on the size of the quantum dots.

These unique optical properties make quantum dots very attractive for use in solar cells, displays and even in medical imaging. The trouble is that high-quality quantum dots are quite tricky to make, especially on an industrial scale. In part 2, I’ll talk a bit more about the applications of quantum dots, what microfluidics is and why it’s great for making quantum dots. If anyone has any questions, please don’t hesitate to ask!