The article reports on the colloidal and biochemical properties of nanoparticles capped with mixed self-assembled monolayers of an alkanethiol (MUA) and a thiolated PEG. I would tend to disagree with the authors remark (abstract first sentence) that "nanoparticles have been traditionally modified with a single monolayer" as there have been many mixed monolayers studies but they have rarely quantified their systems as thoroughly as Comenge et al are doing here.
Figure 2 and associated discussion is particularly interesting. It shows that while composition of the monolayer is a linear function of the fraction of added ligands (something which is often assumed but rarely measured) , the ratio at the surface is closer to a surface area ratio than a molecular ratio. In other words, for a 50% molecular ratio in solution, there is a 39% surface ratio which corresponds to a 15 times excess of the MUA since the molecular footprint of the MUA is much smaller. There might be many hypotheses for why this is the case and the one proposed by the authors, though a little vague, has merits. I wonder if ligand exchange might be playing a role and whether the ratio might vary over long period of times. I think that the information on the length of incubation is currently missing. In a follow up work, it might be worth checking the composition at different time points.
Figure 2 also shows that while the composition varies linearly, the physicochemical properties do not. This is attributed to a conformation transition of the PEG ligand from a mushroom conformation to an extended conformation. This is confirmed by DLS and the rest of the paper explores the consequences of this transition for colloidal stability and protein adsorption. That discussion could be maybe improved by comparing the space available per PEG molecule at the transition composition with the radius of gyration of a free PEG molecule of the same size.