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Review of 'The Role of PEG Conformation in Mixed Layers: From Protein Corona Substrate to Steric Stabilization Avoiding Protein Adsorption.'

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Interesting observations about the conformation of PEG molecules in mixed SAMs
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Competing interests:
The first author, Dr Joan Comenge, is currently a Marie Curie Fellow associated with my group at the University of Liverpool. I was not however involved in any way with this work which was carried prior to him joining Liverpool.

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The Role of PEG Conformation in Mixed Layers: From Protein Corona Substrate to Steric Stabilization Avoiding Protein Adsorption.

 Victor,  Joan Comenge (corresponding) (2015)
Although nanoparticles have been traditionally modified with a single ligand layer, mixture of ligands might help to combine different functionalities and to further engineer the NP surface. A detailed study of the competition between an alkanethiol (11-mercaptoundecanoic acid) and SH-PEG for the surface of AuNPs and the resultant behaviors of this model nanoconjugate is presented here. As a result, the physicochemical properties of these conjugates can be progressively tuned by controlling the composition and especially the conformation of the mixed monolayer. This has implications in the physiological stability. The controlled changes on the SH-PEG conformation rather than its concentration induces a change in the stabilization mechanism from electrostatic repulsion to steric hindrance, which changes the biological fate of NPs. Importantly, the adsorption of proteins on the conjugates can be tailored by tuning the composition and conformation of the mixed layer.
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    Review information

    10.14293/S2199-1006.1.SOR-MATSCI.A0Z6OM.v1.RCXLCP

    This work has been published open access under Creative Commons Attribution License CC BY 4.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Conditions, terms of use and publishing policy can be found at www.scienceopen.com.

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    Review text

    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.

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