Update 3 (10/06/2013): Francesco Stellacci has now confirmed that there are no images of the ‘non-striped’ nanoparticles used as a control. How do the authors know that their structure is any different from the ‘striped’ ones of the same composition therefore remains a mistery.
Update 2 (18/05/2013): Some raw data have been released but those corresponding to the ‘control’ / ‘non-striped’ nanoparticles of the same composition are missing. The raw data confirm that the stripes shown are an artefact.
Update 1 (04/03/2013): Philip Moriarty has requested from Francesco Stellacci the data that underpin the paper discussed below and they have not been forthcoming. Key data are missing from the paper… and are not available on request.
Cho et al reported the “Ultrasensitive detection of toxic cations through changes in the tunnelling current across films of striped nanoparticles” in the last issue of Nature Materials.
A gap between two electrodes is filled with a film of striped nanoparticles. In the presence of cations, e.g. mercury ions, the ions are trapped by the stripes, leading to an increase in the conductance. The results offer promise for sensing of environmental pollutants as highlighted in the Northwestern press release and blog coverage.
Stripy and non-stripy nanoparticles are compared in terms of ion selectivity and sensitivity using conductance measurements. The binding of the ions by the stripes is modeled with quantum mechanical calculations.
The stripes are supposed to come from the phase separation of binary mixtures of hexanethiol (HT) and alkanethiols terminated with 1, 2 or 3 ethylene glycol (EG) units (EG1, EG2 and EG3). These stripy nanoparticles have not been described before… and in fact they remain to be described.
There is strictly no experimental result in the article indicating the existence of stripes for the HT/EG1 and HT/EG3 binary mixtures. For the HT/EG2 mixture, there is one image of ~5 nanoparticles (and the stripes are, of course, aligned perpendicular to the scanning direction, etc; the image does not fulfill the criteria to be considered artifact-free).
The article compares the conductance of stripy and non-stripy of exactly the same composition. The non-stripy are obtained by ligand exchange (the concentration is not specified). It is unclear why ligand exchange should lead to non-stripy, but, in any case, the article does not present any scanning probe microscopy images of the non-stripy so we just have to take the authors word for it (we should be getting used to it by now).
Those nanoparticle films were then “reinforced” by ligand exchange with dithiols, i.e. that some of the molecules that are supposed to form the stripes are replaced. How many are left? We don’t know for sure since there are no measurement. A quick back-of-the-late-night-blog calculation suggests that it may be as many as none. The dithiol is added at a 5 mM concentration (volume non-specified, let say 10 microL) for 40 minutes to a film of about 1011 nanoparticles. That’s more than 100 000 dithiols per nanoparticle, i.e. largely enough to replace all of the maybe-stripe-forming molecules.
Cho, E., Kim, J., Tejerina, B., Hermans, T., Jiang, H., Nakanishi, H., Yu, M., Patashinski, A., Glotzer, S., Stellacci, F., & Grzybowski, B. (2012). Ultrasensitive detection of toxic cations through changes in the tunnelling current across films of striped nanoparticles Nature Materials, 11 (11), 978-985 DOI: 10.1038/nmat3406