Scientific claims should be supported by experimental evidence [1]

The previous three “stripy revisitedposts, including the guest post by Philip Moriarty, have focused mainly on the scanning probe microscopy images and their interpretation.

In this post and the next two, I want to address a problem which goes beyond the question of interpretation of data, namely the problem of important claims which are supported by barely any published evidence, or even in one case, no published evidence at all.

I will give three examples, this post contains the first example.

Example 1: Transmission Electron Microscopy

In the response to “Stripy Nanoparticles Revisited”, the authors state that:

The STM evidence produced was backed up by:

* Trasmission electron microscopy (TEM) […] [2 ] 

and later

In 2004, we presented a series of STM images whose interpretation was corroborated by […] TEM data. [ 2 ]

Reference [2] is Jackson et al, Nature Materials, 2004 and indeed, a similar claim is made:

Additional confirmation of the presence of ordered phase-separated domains was provided by transmission electron microscopy (TEM) images. In fact, in these images (see Supplementary Information, Fig. S2) we have found that there is an observable ring around the nanoparticles’ metallic cores consisting of discrete dots spaced ∼0.5–0.6 nm.

The entirety of the published TEM evidence that “backs up” or “corroborates” the STM is reproduced below:

TEM evidenceThe above can be described as “barely no data” because:

1) this is showing 3 particles in total; the “ring of dots” is shown on one particle in total.

2) there is no evidence that the “dots” seen in fig S2A corresponds to the stripes [the grid itself shows similar fluctuation in grey levels; the origin of the contrast is attributed to “adsorbed” ions (which ones? how does their electronic density compare to gold? controls with different ions?);

3) Fig S2b does not show anything;

4) Fig S2c does not show anything either, except arrows which are supposed to indicate the stripes.

5) No controls (i.e. images of non-stripy nanoparticles taken in similar conditions) are shown

To say that TEM corroborates the existence of stripes therefore corresponds to an extremely optimistic interpretation of a very small body of published evidence.

Update (21/12/2012): David A. Muller, professor of Applied and Engineering Physics at Cornell University, and the co-director of the Kavli Institute at Cornell for Nanoscale Science, said… (commenting at Douglas Natelson “Nanoscale views” blog)

Funny thing is the TEM image from their appendix, Fig S2a, that is cited as independent confirmation is also an instrumental artifact. The ring of black dots is the out of focus point spread function (basically a Fresnel fringe). This is a very common problem for casual users of a TEM who are looking for core-shell nanoparticle structures. By changing focus, either a dark or bright ring can be created. Going in to focus will make it go away. The focus of the image can be determined from a quick FFT of the amorphous background, and sure enough, the passband is about a factor of two off from the optimal defocus.

I guess one of the dangers of interdisciplinary work if you try to do everything yourself instead of forming teams of experts is that there are an awful lot of artifacts to learn about – sometimes the hard way.

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2 comments

  1. @Elias: thank you very much for your comment which opens several interesting areas of discussion.

    Let me clarify a very important point. My argument is that there is no evidence for the existence of stripes: it is not that there is no monolayer segregation in mixed SAMs. Segregation is well established on flat surfaces. In mixed peptide SAMs on gold nanoparticles, we have shown self-organization (non-random spatial distribution) using chemical cross-linking. Others had also previously provided indirect evidence of patches/self-organization on systems similar to those studied by Stellacci’s group (e.g. a few papers by Rotello on “templatable” nanoparticles and a few papers on nanozymes by Scrimin and Pasquato).

    The paper linked to above, Wang et al, strengthen rather than weaken my argument that the EM evidence presented to support the stripes is “barely any”. Wang et al use EM to look at self organization. This article follows from a seminal paper also by the Weinstock group in 2008 (J. Am. Chem. Soc. 2008, 130, 16480). Both articles use an heteropolytungstate cluster anion which features 11 W atoms (Z = 183) as a contrast agent to visualize the monolayer. The JACS 2008 paper was novel and exciting precisely because, for the first time, it became possible to visualize directly single molecules on nanoparticles using EM. Wang et al does not suggest the existence of stripes and indeed, quite correctly, the “response to “stripy revisited”” does not quote Wang et al at all.

    To summarize: Wang et al shows self organization (island growth, not stripes) and uses high electron density anions to provide contrast. Jackson et al does not provide any convincing EM evidence of the existence of stripes.

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