Buckets full of oil measured by fibre made with a magic laser


Oh dear god, not again…

Yup! I published another paper! And this one’s totally open-access and available to all! 😀

Are you somehow more chirpy and excitable than last time?

Yes!! This is the last paper from my PhD and is the sort of final ‘here’s what I made’ paper.

And you’ve still not found anyone else to do these ‘interviews with myself’ things.

Nope, besides it was pretty popular last time. The public love you!

 *sound of head banging on a table*


Fine, what’s this one about then?

Well, I’m glad you asked. This paper is on the use of a coated fibre optic sensor for sensing of crude oil contamination in water. Specifically in this paper, I look at the sensitivity of the sensor to toluene, one of the components of crude oil.

Are these the same coatings from the last time we did this?

Yes. No. Well, some of them.

Okay, so what are the coatings then?

The coating is made up of molecules called ‘Calixarenes’ (calix for short) which I’ve written a little about before.

They are quite intresting little molecules that can be designed to have all kinds of functionality but in this case we’re using them as buckets. They have 4 benzene rings which form a bucket like shape. Other people have shown that when exposed to gasses such as benzene or toluene, the gas becomes temporarily trapped inside the ‘bucket’.

You and I have different definitions of the word “interesting”.

They are called calixarenes because a man called David Gutsche thought they looked a bit like a greek calyx vase

Calix Pot

That looks nothing like the vase! What you have there is two totally different pictures. The only way those are going to look the same is with quite a lot of alcohol, and possible some illegal substances.

Yeah I’ll give you that one, I think Dr Gutsche was reaching a bit.

So you have these molecules, how do they fit with the fibre optic sensor thing?

Okay, well first I need to explain the sensors a bit. Do you understand how fibre optics work?

Are those the things that make the porn appear on my computer faster?

Right, so fibre optics work on the basis of totally internally reflecting light from one end to the other. The telecoms industry spent the latter half of the 20th century working on fibres to transmit information long distances with little interference and noise.

Good, static is really distracting.

In fact, pretty much all of the research into fibre optics was designed around making sure that the signal/light going in at one end is the same as the light that comes out the other. What we wanted to do was the exact opposite. We needed fibres that are sensitive (at specific points) to their surroundings.

This has been the focus of a significant amount of work and is the subject of a great review paper by my colleague. The TL;DR version is : we shoot a laser at the fibre until a small section becomes sensitive to its surroundings.

“..shoot a laser..” – is that really the most technical you can manage?

That process really needs about two blog posts all to itself so for now, yes. You’ll just have to trust me.

Okay, so what does the magic ‘laser’ do to the fibre that makes it sense things outside itself? 

Well, the modification to the fibre makes a short section become very sensitive to the refractive index of the surroundings. The total internal reflectance of the fibre is very dependant on the refractive index different between the glass fibre and its surroundings.

“Refractive index”? don’t think you can use fancy words and get away without explaining them.

Err.. okay – very simply (understatement) you can think of the refractive index as the speed at which light travels through an object. A pure vacuum has a refractive index of 1, in vodka it’s 1.36 – this means that light moves 1.36 times slower in vodka (understandably).

Because of math (yup, that’s all you’re getting!) the reflectivity of a surface is determined by the difference in refractive index between where the light is coming from and the surface it’s hitting. So if that difference changes then the amount of light reflected changes and this in a fibre this creates a change in the light passing through it.

Okay, magic laser light, and something about bouncing mirrors… Got it.

Yeah, close enough. So if for example, I put my thumb on the fibre, a portion of the light passing through the fibre will no longer totally internally reflect and be lost to its surroundings. The calix things are then just a fancy extension of this refractive index detection.

Air has a refractive index of ~1.0, the calixarene I’m using is about 1.46 and the stuff I’m trying to sense (toluene) is around 1.49. So as the calix buckets fill with toluene its refractive index moves from 1.46 to 1.49 which changes the light inside the fibre. The more toluene in the buckets, the more the refractive index changes which we can measure and then use to deterring  exactly how much toluene is in the surrounding sample.

And this is what your PhD was on?


Err, wait… didn’t you say this was the “‘here’s what I made’ paper”?

Yes, well things change. My PhD was on using this sensor to detect toluene in air. I actually made a sensor for water – which is way more useful.


Well, for starters, it was waaaaaaay easier. Measuring stuff in air is ridiculously hard – without wanting to get too deep into some crazy more maths (seriously, I spent an entire chapter of my thesis on this) there are basically more bits in water to detect, making it all a lot easier.

Water vs air

But more importantly, I realised that the biggest weakness of the sensor could be used as a strength by looking at water sampling.

Weakness? Don’t tell me you’re admitting for a moment that your sensor isn’t perfect!

Well those calix buckets (the bit that absorbs the toluene) have a problem – like normal buckets they’re not very fussy about what goes in them, provided that it fits. Basically, any molecule that looks a bit like toluene fits in the calix and produces a response on the sensor.

HAHAHAHa…!! So you actually made a sensor that detects everything?! Well done *slow clap*

Thanks, that’s very kind of you…

And no, the calix buckets are semi-selective – they won’t for example, react much to small molecules, because they are so small they don’t get held by the buckets. Big molecules just don’t fit in the first place, leaving a relatively small family of aromatic molecules.

So, surely people are more interested in detecting the actual hazardous chemical rather than saying “ooh yup, there’s something a bit like that here…”?

Well, yes, but I’d also argue that they are looking at it all wrong. Instead of lamenting how terrible calix buckets are, why not look for an application where only being semi-selective is a good thing.

For example. Oil spills!

I’m pretty sure people like to be very selective about their ability to detect oil spills. Particularly if it’s near cute animals.

Yes, fair point. But crude oil is made up of a wide range of nasty chemicals that, when liberally spread by BP (other polluting companies are also available) over the ocean, get very diluted and hard to detect – especially underwater.

So what if, instead of designing a sensor looking for one of the nasty chemicals, why not design a sensor that looks for all of them at once and combines the signal. Kind of like looking to see if the ocean has fish by making a fish sensor, instead of a sensor that only detects herring.

Boring fishing

That’s what I made – a sensor that responds to a range of nasty chemicals, equally enabling it to detect something like an oil spill at lower concentrations, rather than just looking for one of them individually.

That’s not the dumbest idea I’ve ever heard.

Thank you! It’s obviously only a proof of principle device and needs a lot more work but its enough to demonstrate the idea. Using fibre optics and having the sensing region theoretically a long distance from the expensive reading equipment is uniquely suited to undersea sensing. I’m optimistic that we might be able to find someone interested in developing it further.

What, like Greenpeace? Do they fund university projects?

Well not that I’m aware of but someone’s got to be interested in it! Think of all the cute feathered sea birds it would help save if we could track undersea oil leaks without actually sampling the water with real buckets (seriously they do this)…

Also there are… *cough* other markets…


Well, as it detects oil I guess it could be used to find more oil.

Did you say “find more oil”?

Errr… maybe?

So the choices for future funding are some nice (but probably poor) altruistic bird saving charity heroicly helping wildlife from the evil oil polluting companies. Or, helping the brave oil polluting companies find more oil?

I think this is one of those morally flexible technologies. Probably best not to dwell on it.

Riiiight, you done now then?

Yup, I think that covers it all.

Well done, have a cookie.



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