This is a guest post by Philip Moriarty, Professor of Physics at the University of Nottingham
Since the publication of the ACS Nano and Langmuir papers to which Mathias Brust refers in the previous post, I have tried not to get drawn into posting comments on the extent to which the data reported in those papers ‘vindicates’ previous work on nanoparticle stripes by Francesco Stellacci’s group. (I did, however, post some criticism at ChemBar, which I note was subsequently uploaded, along with comments from Julian Stirling, at PubPeer). This is because we are working on a series of experimental measurements and re-analyses of the evidence for stripes to date (including the results published in the ACS Nano and Langmuir papers) and would very much like to submit this work before the end of the year.
Mathias’ post, however, has prompted me to add a few comments in the blogosphere, courtesy of Rapha-z-Lab.
It is quite remarkable that the ACS Nano and Langmuir papers are seen by some to provide a vindication of previous work by the Stellacci group on stripes. I increasingly feel as if we’re participating in some strange new nanoscale ‘reimagining’ of The Emperor’s New Clothes! Mathias clearly and correctly points out that the ACS Nano and Langmuir papers published earlier this year provide no justification for the earlier work on stripes. Let’s compare and contrast an image from the seminal 2004 Nature Materials paper with Fig. S7 from the paper published in ACS Nano earlier this year…
Note that the image on the right above is described in the ACS Nano paper as “reproducing” high resolution imaging of stripes acquired in other labs. What is particularly important about the image on the right is that it was acquired under ultrahigh vacuum conditions and at a temperature of 77K by Christoph Renner’s group at Geneva. UHV and 77 K operation should give rise to extremely good instrumental stability and provide exceptionally clear images of stripes. Moreover, Renner is a talented and highly experienced probe microscopist. And yet, nothing even vaguely resembling the types of stripes seen in the image on the left is observed in the STM data. It’s also worth noting that the image from Renner’s group features in the Supplementary Information and not the main paper.
Equally remarkable is that the control sample discussed in the ACS Nano paper (NP3) shows features which are, if anything, much more like stripes than the so-called stripy particles. But the authors don’t mention this. I’ve included a comparison below of Fig. 5(c) from the ACS Nano paper with a contrast-enhanced version. I’ll leave it to the reader to make up their own mind as to whether or not there is greater evidence for stripe formation in the image shown on the right above, or in the image shown on the right below…
Finally, the authors neglect any consideration at all of convolution between the tip structure and the sample structure. One can’t just assume that the tip structure plays no role in the image formation mechanism – scanning probe microscopy is called scanning probe microscopy for a reason. This is particularly the case when the features being imaged are likely to have a comparable radius of curvature to the tip.
I could spend quite a considerable amount of time discussing other deficiencies in the analyses in the Langmuir and ACS Nano papers but we’ll cover this at length in the paper we’re writing.