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Light at the end of the tunnel

Silicon Sponge – the light at the end of the tunnel against deadly diseases

Silicon and its wider family of materials make up 90% of the planets crust. That’s quite a large amount of material and it’s possible to take these and refine them into pure grade silicon. This is then sliced into thin silicon wafers. These wafers allow us to make devices used in semiconductor electronics – essential to integrated circuits — most computers, cell phones, and modern technology depend on it.

In my research I use silicon wafers with electrochemical methods to make porous silicon. Porous silicon means that the silicon has tiny holes throughout its entire structure just like a sponge! The difference is that the size of these pores is incredibly small – several nanometres in width, roughly one thousand times smaller than the width of a human hair.

The electrochemical process allows the user to tune the width of these pores and if you change the size of these pores it changes the material properties. The one that concerns me is how it behaves with light - the optical properties. If you remove silicon throughout the spongey layer, the refractive index of the porous silicon layer decreases.

If you make a higher refractive index layer (less spongey) followed by a lower refractive index layer (more spongey) then you can couple light into the less spongey layer because of total internal reflection – how fibre optic cables work. If we cut patterns into these spongey layers we can create waveguides or tunnels that guide the light down it.

The coupled light speeds through the higher refractive index layer, interacting with both silicon and holes. The light changes depending on what is in the holes. If certain chemicals or bio-molecules are present in the holes then the light that is guided through them will change. We detect this change of light which tells us there is something in the holes.

The future of this research is to tune the spongey layers to only trap certain molecules – that way if the light changes we know that specific molecule is there. The benefit of the sponge is the massive increase in the sensing power due to the surface area change which makes it more sensitive to small concentrations of molecule – exactly what is needed to detect things that are low in concentration but could be harmful, such as biomolecules that indicate early stages of cancers that go undetected with current bio-sensors.

Silicon sponge truly is the light at the end of the tunnel – especially if it can help us detect diseases before they progress to a more threatening stage in an individual.

Paul Clarkson

NanoDTC PhD Student Cohort 2013

Department of Chemistry

 Cover Image credit- Royal Palm Med Spa

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