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NEWS ARTICLE:

Amongst the strange rubber tubes and glassware in the Chemistry labs at Canterbury University a ‘radical’ new technology stirs. Alison Downard’s team is using the chemistry of ‘radicals’ to attach molecules to carbon substrates. Their aim is to create ‘chemically well defined surfaces’ by controlling the type and position of attached molecules. 'smart’ materials made in this way would open up a plethora of applications from molecular electronics to sensing devices for disease detection. This is not a new concept. More than twenty years and millions of scientist’s hours have been spent developing self-assembled monolayers (SAM’s) on gold substrates. The idea is that you evaporate a layer of gold onto a smooth substrate then attach groups of different functionality in defined areas over the surface. “Although there’s a lot of really neat things about these self assembled monolayers on gold there is an underlying difficulty that limits their applications…the interaction between the self-assembled monolayer and the gold surface isn’t that stable” The SAM tends to lift off particularly on reuse and in biological media. Problems such as this have thwarted progress. Remarkably few practical devices have been manufactured using this technology.

“We started to think about whether you could form layers that are more strongly attached…and that is when we turned to carbon. With carbon there are methods of attaching things to the surface, which result in a C-C or C-N bond and these are both a lot stronger than the bonds attaching the SAM’s to gold.”

Radical reactions

A radical is a highly reactive chemical species containing one or more unpaired electrons that will bond to almost anything they come into contact with. They can be formed in solution using electrolysis. The following diagram shows the reduction of aryl diazonium salts – one of the reactions Alison’s group uses to create radicals that bond to the carbon surface.

Reduction of aryl diazonium salts to form a radical that then bonds to the carbon surface.

An electron from the negative electrode cleaves the bond between N 2 and the rest of the molecule leaving a single electron desperate to find a partner. It pairs up with another radical in the carbon surface and Oila! There you have the beginnings of a structured surface. R can be a number of things, which makes the technique versatile.

Carbon to build on

To build these self-assembling monolayers you need an atomically smooth carbon surface so that nano-scale features are easily discernable from surface irregularities. It also needs to be thermally stable, hard, non-corrosive and reactive. None of the existing forms of carbon fitted the bill so Alison’s group made their own. Called ‘pyrolysed photoresist films’ (PPFs), they have all the desirable features: low cost, easy reproducibility (perfect for mass production) atomic smoothness and good reactivity. This is how it was done: 15mm 2 squares of silicon wafer were coat ed with a 7μm film of photoresist ( a light sensitive organic material) . They were then heated to 1100ºC in a furnace filled with inert nitrogen gas so that everything in the photoresist gets burnt out except for the carbon. You are left with a smooth 1.5μm pyrolised photoresist film.

Experimental set up

Voltammogram showing the different current-voltage relationships of the modified and unmodified carbon surfaces.

Structure of surface-attached films

SAM’s on gold will assemble in a very well ordered manner if the experiments are done properly but radical chemistry is much harder to control. Radicals can react together to form multilayers. Very little is known about the structures they form.

“This is where we’ve been focussing our efforts, trying to work out what our films look like…it’s a bit of a gnarly problem”

“One of the techniques we’ve used is the AFM scratching method”. The idea is to dig a trench through the surface layer with the sharp tip of an Atomic Force Microscope (AFM) probe and measure its depth. The underlying carbon is scratched to a relatively small and reproducible depth so by running the AFM tip (this time in tapping mode) over the surface again you can build up a nano-scale depth profile of the trench and from this work out the thickness of the attached layer.

Top: AFM image of a trench dug into the attached layer using the sharp tip of an AFM probe. Bottom: Depth profile of the trench.

Alison's films are around 7nm (five monolayers) thick.

The next question is whether the films are close packed or sparsely arranged. The resolution of the AFM isn’t good enough to see individual molecules. Once again redox chemistry provides a solution. Molecules can be attached to the surface, which undergo their own redox reactions. By measuring the charge required to carry out the oxidation or reduction of molecules on the surface it is possible to work backwards to the number of molecules attached to the surface and use this along with the film thickness and area, (which are already known) to estimate the film density. The results suggest that layers are loosely packed - around one fifth that expected for close packed layers. Other types of experiment s confirmed this .

Patterning functional groups over the surface

For most potential applications the self-assembled layers need to be modified to define areas of different functionality. So far Alison’s team has managed to attach gold nano-particles, single molecules and proteins to the self-assembled layer. Patterning different functional groups over the surface has proved a real challenge. The fundamental problem is that radicals are so reactive they bond to almost everything in their vicinity. It is difficult to modify one area without changing the rest. First they tried blocking off areas with photo-resist masks like the electrical engineers do to define features on computer chips (see the article on Maan Alkaisi) but to no avail. It was impossible to remove the last traces of photoresist after modification. Next they tried metal masks but the modifier went all over the mask forming an anti-corrosion layer that stopped it dissolving away. They decided to change tack. They used the AFM tip to scratch nm scale tracks in the attached layer and then filled them in with different sorts of molecules.

AFM scratching method for modifying the attached layer.

AFM image of a modified surface.

This proved successful, although in some experiments the modifiers attached all over the film rather than just in the AFM tracks so this technique has its limits too.

Compared to the Self Assembled Monolayers on gold “our layers seem to be very stable …but the disadvantage is that the chemistry is difficult to control.” Alison’s team is onto the case. It is early days yet but Alison is confident that this ‘radical’ new technology has a lot to offer.

Possible Applications

Alison’s new technique could be used in any of these applications.