Regular readers will remember a recent post where I documented the critical peer review of 20 influential (more than a 1000 citations) articles. I reviewed them initially in a Twitter thread and then also reproduced my comments on PubPeer.
Wolfgang Parak, corresponding author of one of these 20 papers (Cytotoxicity of colloidal CdSe and CdSe/ZnS nanoparticles), has kindly responded to my comments. I reproduce his response below (with his authorisation). The full discussion is here. Wolgang is doing the right thing. I hope other authors will follow suit; especially authors of papers where more serious concerns where raised such as here, here, here, here, here, here, here and here.
thank you very much for your comments about our article. In general I agree with your points. There were essentially two shortcomings you pointed out.
1) At low nanoparticle concentration there are less adherent cells after incubation than before incubation. In fact, at low nanoparticle concentrations there should be no effect, and thus the number of adherent cells before and after incubation should be equal (R = 1). However, cells proliferate. Thus, in case cells proliferate during incubation, there would be more cells after than before incubation (R > 1). In order to get rid of this effect we incubated the cells in SATO medium, which should stop cell proliferation. However, in fact the use of SATO medium also causes loss of some adherent cells. We pointed this out in our manuscript: “Exchanging the serum-containing cell culture medium to serum-free SATO medium resulted in the detachment of a significant fraction of cells, i.e., R < 1 even for c(Cd) = 0. Therefore, we assume that there are no toxic effects due to Cd in the first region of low Cd concentration and that the value of R < 1 can be simply explained by the effect of the SATO medium”. Thus, we did the control, that there is an effect due to changes of serum supplemented medium to the SATO medium which is supposed to stop cell proliferation. In retro-perspectives the conditions could have been better optimized, in getting culture conditions where there is no cell proliferation, but there is also no loss of cells during the incubation period. In addition, had I to plan the study today again, I would use a different assay which probes cell metabolism, instead of using cell adhesion as quantifier for toxicity. There are a variety of excellent assays, which also have different sensitivity, see for example: ” X. Ma, R. Hartmann, D. Jimenez de Aberasturi, F. Yang, S. J. H. Soenen, B. B. Manshian, J. Franz, D. Valdeperez, B. Pelaz, N. Feliu, N. Hampp, C. Riethmüller, H. Vieker, N. Freese, A. Gölzhäuser, M. Simonich, R. Tanguay, X.-J. Liang, W. J. Parak, “Colloidal Gold Nanoparticles Induce Changes in Cellular and Subcellular Morphology”, ACS Nano 11, 7807−7820 (2017)”. The reason why at that time we used the adhesion assay was that we were afraid that some metal ions may interfere with colorimetric viability assays. In a previous work (C. Kirchner, M. George, B. Stein, W. J. Parak, H. E. Gaub, M. Seitz, “Corrosion protection and long-term chemical functionalization of gallium arsenide in aqueous environment”, Advanced Functional Materials 12, 266-276 (2002)) we had observed that the MTT assay interferes with As ions released from GaAs surfaces and thus at that time we decided against the use of a biochemical assay.
2) The patch clamp experiment, in which ion currents were measured with and without presence of nanoparticles was merely quantitative. We thought at this time that it would be a good add-on to the paper, using one more complementary technique apart from the detachment assay to probe for toxic effects. In literature at that time there were reports suggesting the use of quantum dots for cell labelling. In our detachment assay we had shown, that at high concentration quantum dots are toxic to cells. Our motivation was to show, that one can use low quantum dot concentrations, which are enough to label cells, but which are low enough not to cause acute toxicity. Thus, we chose one concentration of quantum dots, which is enough to label cells, and showed that with this concentration ion channel currents were not affected. In retro-perspectives with some additional work we could have made also a more quantitative investigation, with a variation of quantum dot concentration, to see at which concentration ion channels are affected.
I hope with this I could comment on your two major points.
(Wolfgang Parak, email@example.com)