Hello everyone, so nice to bring an update to you from the emerald isle. Some things have stayed the same since my last post… Ireland is still a beautiful place with amazing nature to visit, Guinness still tastes good and the Irish still put too much sauce on their “chips”… However, other things have changed now that the figurative dust, which accompanies any great move, has settled. In other words, the work rhythm in the lab has kicked into second gear and I am progressing towards the realisation of my first assay to screen for marine bio-toxins in shellfish. We are currently trying to exploit a physical phenomenon called “localised surface plasmon resonance” for this which is in accordance with the title of my PhD: Localised Surface Plasmon Resonance imaging for multiplex smartphone testing of marine toxins. I know that is quite a mouthful but when you break it down to its bare essentials it is quite simple. First lets start with the simplified form of the phenomenon, surface plasmon resonance or SPR. SPR is basically the interaction between light (photons) and electrons. The funny thing about these two very very small things is that they are both very different and almost the same at the same moment. As you all have most likely seen at least one time: E=MC2=hv or in other word, energy is the mass (M) of an object times the speed of light2 (C2) AND is the speed at which that object vibrates (v) (which is related to its wavelength and frequency) times a constant value (planck constant h). This beautiful equation shows that all things that have energy have a mass and that all objects that have a mass vibrate and must have a certain wavelength, frequency at which they do that. This basically means that all things have a dual character… we are all both mass (particles) and vibration (waves)…. I know it sounds crazy…I bet none of you feels like he or she is a wave….but this dual character becomes very clear when objects get very small, like electrons or photons (light). At that point these things can kind of change into each other. And that is what happens with SPR…. Let’s imagine a package of light (a photon) that hits a free electron (meaning an electron that can freely move in its environment like a conductive electron in copper or in gold). Now these things (considering E=MC2=hv) both have a mass and wavelength, frequency at which they vibrate. Since both things are very, very small, the frequencies at which they vibrate come close to each other… very close… so close that, if the light (with a certain wavelength) hits the electrons at a certain angle the electrons start to oscillate with the light i.e. moving in the same frequency. This is much like when you hit the string of a guitar and you don’t cover the string next to it… it will start to vibrate as well… The effect this oscillation has on the light hitting the electron is that it transfers its energy to the free electrons which will start to vibrate and form a polariton (an electron with the character of a wave) which will move away from its point of origin.
Figure 1: Check out this video that explains polaritons
Of course this energy transfer will cause the intensity of the light to go down at that specific wavelength and angle (and for completeness at another combination of those two parameters as well). Thus if you look at the entire spectrum of light, at the same angle, you will see a drop in intensity for that wavelength of light, say all light blue light disappears if you shine blue light onto a conductive surface at a fixed angle. This is SPR. Now, the important part is coming up… the angle of light changes when it travels through different media, we all know this because it is why fish always look bigger in the water than out. It is also why we miss when we try to catch them with a spear or our hands (if you didn’t know, try :-)). It’s also why we can see a rainbow when the sun shines and it rains at the same time… The sunlight has many wavelengths and they all change angle differently when travelling through a different media (water drops) thus breaking apart and allowing to see all the colours out of which it is built up…. A thing I can admire many many times here in Ireland :-)…
And it is that change of angle when traveling through different media that can be used to detect things with SPR.. Imagine again a photon hitting an electron, only this time there is a toxin molecule in the way. So this time the light will slightly change direction when it hits the electron. This means that the same light as before, same wavelength and initial angle, will hit that electron at another angle. Because of this it does not have the same momentum ( the same v from E=hv…) this means it will not oscillate with the electron. However, another wavelength of light, which will also change angle when travelling through that toxin molecule, will have the right momentum. Thus we will see a shift in the typical wavelength that has an intensity drop. And it is this shift; the different colour that disappears when the toxin binds that can be used to detect the toxin. Because now that the toxin was caught by the antibody that was on the gold from the beginning will change the angle of the light and dark blue light will disappear instead of light blue….
Figure 2: SPR explained
Okey, I hope I haven’t lost you there and it is clear what SPR is. Now it gets a little more complicated. Because, we want to detect only the toxin, and not something else, moreover we don’t want to use angle resolved SPR, because angle measurements are complicated and so we want to get rid of the angle problem…
First off I must be sure that it is the toxin that binds to the gold and not something else. For this we must fix some binding element to the gold that can specifically bind the toxin, and only the toxin… For this we are currently trying to use special, very specific antibodies that were previously developed. Apart from this we are also looking at other, novel, cheaper and more durable binding elements with a longer shelf life, that might just do the trick as well.
Than we must ensure ourselves that the signal we want to observe (a change in wavelength of the absorbed light after binding) is not overshadowed by other SPR phenomena at other places from things that have nothing to do with what we want to observe (read too much background noise because another wavelength at another angle causes SPR as well). In order to do that we decided to use LSPR instead of SPR. LSPR is Localised Surface Plasmon Resonance meaning that the SPR phenomenon happens only very close to the gold surface, which greatly reduces the chance the binding event will get overshadowed by random things happening further away. Moreover, if the particle that is used is much smaller than the length between two peaks in the wavelength that is used, the angle at which this light (wavelength of 600-1000 nm) hits the particle (diameter of max 150 nm) becomes irrelevant….(imagine a small ball getting hit by a wave in the sea, no matter the direction of the wave the impact it has on the ball is of the same magnitude as long as the wave is much bigger as the ball and all waves have the same wavelength). This means that no complicated instrumentation is needed to keep the angle equal at all times!
This can be achieved by using gold nanoparticles instead of a flat gold surface. There exist many of these particles in different forms (nanospheres, belts, wires, rods, mushrooms, cubes, cages etc. etc.) and I am currently trying to navigate my way through the advantages and disadvantages of all these different particles and am trying to synthesise several of them to test out which would work better.
Finally we do not just want to detect one toxin but several at the same time… and this with a smartphone based device… This means that the phone must try and distinguish which change in wavelength intensity comes from which toxin. In order to do that we want to use smartphone adapted imaging techniques. This basically comes down to finding a way that the smartphone will understand that a certain area it looks at is toxin A and another is toxin B. But don’t worry; I will leave the details of that for another time J.
SO this was a little introduction to LSPR and my work in the lab at queens university…A wonderful world of particles, light and binding molecules that all interact to (hopefully) help us detect toxins in real time with a smartphone based device.
Next to this work I also prepared a poster for the RAFA convention, to which I am very eager to go and meet up with some fellow ESRs… yep Aris, looking forward to see you at RAFA!!! J The poster I prepared is basically just an idea. It proposes a new way to classify biosensors and is designed to help end-users to better find their way into this complex and rapidly developing sector or, expressed into a Dutch saying, “help you to see the trees in the forest”… (do you know that one Vincent and Gina?)
I call the system The Bio End user Sensor Tree or BEST tree. It basically uses a decision tree that is based on simple questions like: Which compound do you want to detect? do you have laboratory experience? Or Do you want your sensor to be portable? Answering these questions will narrow done the choice of sensors and finally direct you to an open source e-document where your options will be summarized. Available as well as possible upcoming sensors are presented and a short conclusion/ recommendation is made. The Tree is also on facebook and is open for everyone, end-users, scientists and companies to interact and share their ideas. I hope that the BEST tree will help to promote some very interesting new sensors and help to develop this growing interesting sector. I also hope to convince others, at the RAFA convention, that the BEST tree can be a very useful tool if further developed (currently only the marine biotoxins branch has been thoroughly worked out). Since the completion of this tree is an enormous task it would be great to meet other biosensor enthusiasts at RAFA and plan to further grow the tree together to a fully comprehensive system….
Figure 3: The BEST tree an interactive platform
Okey everyone, I think that that is it for today. I hope I haven’t been too boring and I hope that you will all join the BEST tree discussion on Facebook or try out the actual tree on iBoske (which is really only a start of an open sourced project and so not at all fully comprehensive and open to all updates and suggestions)….Finally I hope to have convinced some of you that Physics, and Biology marry well and are super fun J
Hasta la pasta