Surfaces – Take a sheet of A4 paper. Fold it in half. Now fold it in half again. One more time please. Look at the sheet that you have got now. Is it smaller? Yes, the external dimensions are now one eighth of the original sheet size (they are now A7). Does it have the same surface area? Yes, it is still an A4 sheet of paper, but the surface is now internal, i.e. inside the folded sheet. Metal-organic frameworks are similar, they are characterized by internal porosity. Like Swiss cheese, they have pores and their internal surface area is much bigger than their size suggests. Much, much bigger in fact. Every atom a MOF is exposed to the internal surface, MOFs are true ‘all surface – no bulk’ materials. 1 gram of a MOF can have a surface area of a football field!
Light – Take a bright, but small lamp. A bike light is ideal. Hold it at the stretch of skin where your thumb and your index finger meet. Look at the light transmitted through your hand on the other side. It looks red, doesn’t it? White (normal) light, is in fact made up of all colours at once, but only red light goes through your skin, the rest is absorbed. You have just neatly demonstrated the ‘near-infrared window’. Whilst human beings are quite opaque (light absorbing), red light is able to travel through the skin relatively deep without being absorbed.
Gold – Take a lump of gold (a small nugget will do) and start chopping it up into small pieces. Just kidding. But if you were to do chop and grind the gold finely enough you will find it changes colour from golden/yellow to deep red. At the nanoscale, the optical properties of gold change because the surface electrons oscillate collectively – a phenomenon known as surface plasmon resonance. Moreover, they are able to convert light energy (especially infrared light) into heat.
Treating cancer using surfaces, light and gold – Imagine we grow the MOF around a gold nanoparticle, so we have ‘nano-Kinderegg’ with a gold core and a MOF shell. The MOF only consists of surfaces and pores, so like a sponge we can put stuff inside it and ‘squeeze’ it out. The ‘squeezing’ can be done by applying heat from inside the core-shell nanoparticle. In our case we put anti-cancer drugs inside the MOF pores and squeeze them out remotely using light energy, which is converted into heat inside the MOF particles. By applying this principle, we can transport transport drug molecules safely around the body and only release them on demand only where we shine our infrared light. So let’s pew the laser and start working!