Professor Compton, Oxford University, talks to NJC about his latest work describing the behaviour of gold substrates towards the underpotential deposition (UDP) of thallium. He has found that while UDP of hallium is observed both on gold macroelectrodes and nanoparticles, the property is lost when the size of the nanoparticles becomes too small.

How did you get interested in this research project?

At first glance thallium(I) may seem an unusual choice of analyte to study the size-dependent properties of gold nanoparticles via measurements of the underpotential deposition (UPD) of thallium on gold! This work arose as a result of our previous studies on fabricating random arrays of multi-metal electrodes (consisting of gold, silver and palladium nanoparticles supported on carbon microspheres) to demonstrate proof-of-concept of truly combinatorial electrochemistry. In these studies thallium was chosen because it was selectively detected at the gold nanoparticles (ca. 50-60 nm in diameter) but not at the other metal nanoparticles on the electrode in the potential range used. In a separate project we had also deposited much smaller (ca. 10 nm) gold nanoparticles on multiwalled carbon nanotubes (MWCNTs) and found that this system was ultra-sensitive for the detection of arsenic(III). Therefore we decided to see if the smaller gold nanoparticles (supported on MWCNTs) could offer a similar improvement in the detection of Tl(I) and thus be incorporated into a multi-metal electrode to offer further improvements for combinatorial electrochemistry. To our surprise we could not see any Tl(I) UPD on the smaller (10 nm) gold nanoparticles and realised that this might be a good way to investigate the size-dependent properties of these nanomaterials.

What is the most important result in the paper?

The UPD of Tl(I) is observed on bulk gold surfaces and on gold nanoparticles that are ca. 50 nm in diameter, allowing the sensitive electroanalytical detection of this particular analyte. However, smaller gold nanoparticles of ca. 10 nm in diameter do not exhibit any signs of Tl(I) UPD. This is one example where attempting to make ever smaller nanoparticles does not offer any advantage and is in fact rather disadvantageous!

What are the implications of the results you present in this paper?

It is now well-established that the properties of nanomaterials can differ markedly from those of the bulk material. For instance, gold nanoparticles have been shown to be catalytic in certain reactions whereas bulk gold is not. There is a great deal of interest in studying the influence of the size and shape of nanoparticles on their properties, and our understanding of these effects is, currently, still rather unclear. In most cases, using smaller nanoparticles is shown to be advantageous in catalytic or analytical systems, in part due to their much larger surface area compared to bulk material and also possibly due to their higher surface energy in the nanoparticle form than the bulk material, which may impart a higher degree of reactivity/catalysis. This paper shows one example of where the behaviour of a material, in this case gold, is strongly dependent on the size of the material used, and that "going smaller" is not always "better".

Are there any particular challenges facing future research in this area?

Apart from the obvious challenges in finding ways to control the size and shape of nanomaterials there is, in my opinion, a subtler, philosophical challenge in this area. There is an admirable motivation within certain areas of the nanoscience community to try to develop and improve existing systems currently at the macro- or micro-scale by using nanomaterials. Thus, most of the nanoscience research that gets published is focussed on demonstrating the advantages of using nanomaterials in a vast range of different applications and fields of research. This has generated a lot of excitement-and dare I say, "hype"-both within the scientific community and the wider media about the possible future benefits of nanotechnology. However, it is also reasonable to assume that there has probably been a lot of interesting (and potentially important) research carried out that did not show any improvements by using nanomaterials and was, therefore, discarded and never considered worth submitting for publication. As scientists, (and also as journal referees and even editors) we sometimes have to remind ourselves that a "negative" result can, in certain cases, be just as valuable as a "positive" result in furthering our scientific understanding of the world around us.