Nanostripe controversy in new twist

A simmering controversy over whether certain nanoparticle structures are merely instrument artefacts has boiled over into a bitter dispute, with a senior scientist alleging that he has become the victim of a personal vendetta, something that is strongly denied by researchers on the other side of the argument.

In 2004, Franceso Stellacci, then at the Massachusetts Institute of Technology in the US, published scanning tunnelling microscope (STM) images of gold nanoparticles on which ligands had apparently arranged themselves into discrete parallel stripes. Other researchers, however, suggested that the ‘stripes’ were in fact most likely an artefact produced by the microscope. A critique of the 2004 paper was published in 2012, by a team led by Raphaël Lévy at the University of Liverpool in the UK.

Stellacci, now at the Swiss Federal Institute of Technology in Lausanne, published subsequent papers which, he says, addressed each of the concerns and demonstrated unequivocally that the stripes are indeed a genuine phenomenon.

Results just in

Now, a team led by Julian Stirling of the University of Nottingham in the UK has published a detailed analysis of Stellacci’s data together with new experiments. The researchers conclude: ‘Although these data can indeed be interpreted in terms of stripe formation, we show that the reported results can alternatively be explained as arising from a combination of instrumental artefacts and inadequate data analysis techniques.’

There is nothing personal whatsoever and there is no reason why there should be anything personal

The team carried out a series of numerical analyses and simulations that suggested that the striped patterns in the earlier images could be caused by a ‘feedback’ issue with the STM; as the tip travels over the surface the current is constantly altered to maintain the distance of the tip from the surface. Where the surface is particularly lumpy, the tip can ‘oversteer’, creating a ripple effect. To confirm this possibility researchers imaged ‘smooth’ nanoparticles with no surface ligands present, to show how stripes can appear to be present in the final image.

The team also called into question the interpretation of subsequent images, where a frequency analysis technique called power spectral density was used to demonstrate that ligands did arrange themselves into stripes. The researchers modelled randomly distributed particles over a surface to show that ‘stripes’ could be discerned by frequency analysis if they were specifically searched for.

Stirling, now based at the National Institute of Standards and Technology in the US, stresses that he is not a nanoparticle expert nor a chemist, but that his expertise is in data analysis and modelling. ‘I saw no evidence of striped nanoparticles when I looked through the data,’ he concludes.

‘Personal war’

Stellacci, however, remains adamant that he has already answered all the questions raised by his critics. ‘The older papers and the new ones show stripes,’ he says. ‘I think the controversy is over. Since 2012, I have published four papers that fully addressed all of the concerns. It is very clear that they want to run a personal war against me. In 2012 they “proved” the images were wrong, but now have felt the need to publish a whole new paper with different scientific points. They are changing the story all the time.’

Stellacci is not without support. Paolo Samorì of the University of Strasbourg in France says: ‘In my laboratory we have imaged Professor Stellacci’s particles and found that these particles indeed have stripes on them. The images show clear features that are invariant with imaging parameters (scan rate, scan angle, feedback loop, etc) and hence they can ascribe to true tip sample interactions.
Since seeing may be always argued, with Stellacci we decided we needed a more quantitative proof of the stripes existence.’

Samorì says that independent power spectral density analysis carried out in another laboratory ‘convincingly proves that images taken in my lab show the same features as images taken in three other top laboratories including Stellacci’s’. He adds that the new paper questioning the results is ‘confusing and full of overstatements’. ‘For me the controversy is closed; the images are reproducible across labs and are not artefacts.’

Lévy, a co-author of the new paper, is adamant that he is solely interested in establishing the scientific facts of the matter. ‘There is nothing personal whatsoever and there is no reason why there should be anything personal,’ he says. ‘It’s now 10 years since the first images were published and it has taken a considerable amount of energy for everyone to try and sort this out. We have a duty as scientists to discuss these things and to point out any problems if we see them.’

Stellacci has now prepared a new paper to answer his critics. ‘In my rebuttal I have fully proved that all of the arguments in their PLoS One paper are wrong,’ he says.