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May 4, 2009
Writing With Triazoles
I’ve written before about the copper-catalyzed triazole formation (often referred to as “click chemistry”). It’s turned into a very useful way to stick all sorts of molecules and structures together, and is showing up in materials science, biochemistry, organic synthesis and other fields.
Now Fraser Stoddart’s lab has a new variation on the technique, using atomic force microscopy (AFM) equipment. If you’re not familiar with that machinery (invented in the 1980s), it’s rather startling. An AFM rig uses a very fine metal tip (fine, as in “down to one atom or so at the end” fine), which is brought down very close to a solid surface. And that’s close as in “within the size of a molecule or so” close. Once you’re ranged in, you can run these tips around in any direction you choose, and a lot ingenious measurements can be obtained. Both modern surface and solid-state chemistry live off this family instruments, with good reason.
One thing you can imagine doing is lowering some sort of active catalyst down near the surface and doing chemical reactions. If you want to tear up the surface below in some controlled fashion, that’s a bit easier, through straight oxidation or reduction. Forming bonds is a bit trickier, but that’s been achieved with some palladium reactions. Now Stoddart’s group has gotten it to work with the triazole chemistry, and in very straightforward fashion.
If you take an azide-functionalized silicon wafer (and these are well known), you can then dissolve some acetylene compound up in ethanol and put a drop of it on the surface. And lowering an AFM tip which has simply been coated with copper metal down to the surface is enough to initiate the reaction. As the tip moves, it “writes” a path of triazoles. The conditions are very mild, the resolution of the lines is very high (down to about 50 nanometers wide), and it turns out that the reaction is so fast that the tip can be moved at relatively high speed.
This opens up a potential way to stick all sorts of molecules to solid surfaces. There are a lot of ways known to do that, of course, but this one could have some real advantages. The selectivity and high resolution seen here could allow for very dense and complicated arrays of complex molecules to be laid down. Since the triazole reaction is compatible with all sort of biomolecules, this could provide a way to produce functionalized chips that would currently be rather hard (or nearly impossible) to make. And now that we can make them, we can start thinking up unusual things to do with them.
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