Reviewer #1.
This is important and high-impact work at the level required for publication in Nature Materials. The controlled self-assembly of nanoparticles and nanorods via polymer stabilizing layers is a relatively new and promising route to complex structures with tunable collective optical and electronic properties. Newer still are strategies that take advantage of amphiphilic self-assembly, analogous to block copolymer self-assembly. Of the few examples that have recently appeared (including those mentioned below), this is the first involving self-assembly of nanorods, and the first truly comprehensive study that demonstrates control over the morphology of hybrid assemblies. The fact that wide ranging morphologies are obtained via simple changes in solvent and water content, and that optical properties can be varied systematically via water-induced self-assembly, make this particularly attractive system a true candidate for real device applications.
Publication of this manuscript in Nature Materials is recommended, although the following corrections should be made: the following references to previous examples of amphiphilic self-assembly of polymer-stabilized nanoparticles should be cited, along with text to put the current work in better context:
1-1. Shan, J.; Nuopponen, M.; Jiang, H.; Viitala, T.; Kauppinen, E.; Kontturi, K.; Tenhu, H. Macromolecules 2005, 38, 2918.
2. Shan, J.; Chen, J.; Nuopponen, M.; Viitala, T.; Jiang, H.; Peltonen, J.; Kauppinen, E.; Tenhu, H. Langmuir 2006, 22, 794.
3. Zubarev, E. R.; Xu, J.; Sayyad, A.; Gibson, J. D. J. Am. Chem. Soc. 2006, 128, 4958.
4. Zubarev, E. R.; Xu, J.; Sayyad, A.; Gibson, J. D. J. Am. Chem. Soc. 2006, 128, 15098.
5. Erhardt, R.; Zhang, M.; Boker, A.; Zettl, H.; Abetz, C.; Frederik, P.; Krausch, G.; Abetz, V.; Muller, A. H. E. J. Am. Chem. Soc. 2003, 125, 3260.
Reviewer #2
Overall I support the acceptance of this manuscript enthusiastically. The results are fascinating and analyzed with depth. The authors, however, need to give thought to the following issues before they submit their paper again.
1-2. I do not quite buy the planar brush and cylindrical brush explanation for the misaligned end joining of the pom-pom triblocks in Figure 3c. At water volume fraction of 30%, the PS chains should be totally collapsed and there should be no brushes to talk about.
2-2. A paper by the Liu group (Yan, Liu et al. 2004) on preparation and phase segregation of nanotube multiblocks contains similar concept and not as good results in phase segregated structures. The paper should be cited.
Yan, X., G. Liu, et al. (2004). "Preparation and phase segregation of block copolymer nanotube multiblocks." J Am Chem Soc 126(32): 10059-66.
Reviewer #3
This manuscript describes the preparation and characterization of amphiphilic gold nanorods. These materials form very interesting structures through self-assembly and this work is likely to be of interest to a broad readership. However, there are a number of unanswered questions that should be resolved before the article is published. The majority of the structural observations that are made concern assemblies observed after solvent removal, but there is no discussion of possible surface/sample preparation effects. More detail on sample preparation needs to be included. Possible alternative explanations for the observed behaviors should be considered
Also, in my opinion, while there might be some comparison to very specific copolymer systems, the repeated analogies to block copolymers are too limiting--shouldn't all amphiphiles be subject to self-assembly under these principles? These are very interesting structures/assemblies regardless of their resemblance to ABA triblock copolymers.
More specific questions/comments:
1-3. Effects of mica surface on particle self-assembly are not discussed.
2-3. Effects of solvent evaporation on particle assembly in TEM and SEM samples are not discussed.
3-3. p. 5, line 1: Is there any way to quantify the number of PS chains bound per end in a slightly less rough fashion?
4-3. Is there any possibility that loss of the outer CTAB leaflet plays a role in modes of aggregation where the nanorods interact side-to-side (i.e., in THF mixtures)? Excess CTAB could form micelles in water and be lost during dialysis? (no experimental description of what MW cut-off dialysis tubing used is included)
5-3. p. 7, paragraph 2: It is stated that "PS chains were pulled to the ends of the bundle, preventing further side-to-side aggregation of the NRs." It is unclear why this would happen. Also, it is unclear why this would prevent further aggregation when further aggregation under similar circumstances is observed in Figure 1e (4:1 THF/water)
6-3. p. 7-8: Why does the probability of ring formation decrease as water concentration increases at Cw > 6%? The explanation given, "The probability of ring formation decreased because the increase in free energy due to the loss of conformational entropy resulting from ring formation was larger than the energy gain due to the linking of two chain ends," seems to be a generic one that does not specifically address this particular system. The observed L/Lp ratios seem to provide another measure of the behavior and not a causal explanation. It seems like increasing water concentration should disfavor the formation of free PS chain ends and should lead to increased ring formation, especially if the solvent mixture becomes a better one for the CTAB head groups. It also seems like there should be a nanorod concentration dependence and perhaps a time dependence (solvent evaporation rate in TEM grid preparation) for ring formation? The explanation for this behavior should be clarified.
7-3. p. 9, paragraph 2: Several things are unclear about the explanation of the observed increase in interparticle distance as water concentration increases. What evidence is there for the wetting of the CTAB surface of the nanorods by PS chains?
8-3. Why would the chains wet the charged surface rather than remain solvated by DMF-enriched solution? Could the observed behavior also be explained by the gradual deswelling of the PS chains: at low water concentrations, the PS chains would form a pom-pom-like assembly (as described by the authors) with chains coiled between the ends of nanorods and also extending out into solution; at higher water concentrations, the PS chains would be more confined to the volume between the nanorod ends, thus increasing the distance between the rod ends. It is not clear why kinks form at even higher concentrations (and it is not clear from the small sample of rods shown in Fig 3c how frequently kinks are observed), but the schematic shown in Figure 3h seems like it would unfavorably increase PS-solvent interactions over other possible arrangements.
9-3. Also, because these samples were observed by TEM, does sample evaporation rate (and perhaps ambient humidity) have any effect on the observed spacings?
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