Substituted Poly(Vinylphosphonate) Coatings of Magnetite Nanoparticles and Clusters

Round 1

Reviewer 1 Report

This article is devoted to the use of substituted polyvinylphosphonic acid as a functional shell of magnetite nanoparticles. In my opinion, the article is written logically, some conclusions of the author require a more correct use of terms. A set of modern methods of analysis is used. In conclusion, I believe that the article is suitable for publication in the journal "Magnetochemistry" after significant corrections have been made.

See my comments below:

1.      In the introduction, the authors provide insufficient justification for the choice of polyvinylphosphonic acid as a stabilizing agent. The arguments "this compound has not been used before" or "many strong reactive groups" do not justify the choice of coating agent.

2.      Line 138: In the case of in situ syntheses, the difficulty arises in the presence of low molecular weight similar synthesis products in the final product. How was their removal controlled? It is not clear what is the reason for the need to leave the final product for magnetic separation overnight, since this can lead to the oxidation of magnetite? Under what conditions was the final product dried?

3.      Line 166: Indicate how many particles in the micrograph were taken into account when estimating the size

4.      Line 268: “Besides iron and oxygen, both samples showed significant amounts of phosphorus and gadolinium/erbium as well, proof that the coating is stable and that an adsorption of the rare earth ions has taken place”. The authors carry out the sorption of rare-earth metal ions to assess the functional ability of the obtained coated magnetic nanoparticles. However, the authors give neither the values ​​of the sorption capacity of nanoparticles, nor the sorption conditions, nor comparisons with existing analogous magnetic media. In addition, the presence of phosphorus in elemental analysis does not indicate the stability of the coating, but only the fact of functionalization and the presence of the selected ligand. The stability of a coating can be assessed in experiments on the influence of changing conditions (for example, 3R or dispersion or UV influence) according to, for example, IR spectroscopy, indicating the presence or absence of bonds between the core and shell, or from the values ​​of zeta potentials, in the case of on colloidal stability.

5.      Line 271: “FTIR spectroscopy of MNP 7, 8 and 9 (Figures 1 and S2) show bands at ca. 550-570 cm-1 , which is proof that magnetite has formed.” The presence of these bands means the formation of Fe-O bonds, which may also be present in other iron oxides, for example, in magnetite, but do not indicate the formation of magnetite.

6.      Line 275: “however there is no n(C-H) band at >3000 cm-1 275 , which means that no methylvinylphosphonate monomers have attached to the MNP.”  There is a broad band in the spectrum in this region. How did the authors determine that it does not apply to C-H?

7.      Line 278: “The spectra of 7, 9a, 9b, 9c and 9d show no great differences, which means that the attachment of rare earth ions underwent without changes of the structure of the coating.” The absence of bonds in the IR spectra that would relate to the bonds of the selected ions and phosphonate groups indicates physical, and hence weak, sorption of ions. Otherwise, the bands related to bonds with ions should have been. In addition, the authors do not indicate which particular bands can be attributed to the bond between magnetite and the phosphonate coating?

8.      Line 291: “Hence the amount of rare earth metal ions that are bound to the MNP by PMVP is likely relatively low”. Previously, the authors pointed to the successful sorption of ions on the surface of magnetic nanoparticles. Please give the value of the maximum sorption capacity in relation to each of the selected metal ions.

9.      Line 295: “XRD by itself cannot distinguish between magnetite and maghemite, but 295 taking into account the results from FTIR, magnetization and XPS measurements it is 296 clear that the nanoparticles are magnetite.” XRD makes it possible to distinguish magnetite from nonstoichiometric magnetite or maghemite by calculation (Gorski et al., 2010, Determination of nanoparticulate magnetite stoichiometry by Mössbauer spectroscopy, acidic dissolution, and powder X-ray diffraction: A critical review). On the other hand, IR does not allow accurate identification of magnetite, the given values of saturation magnetization can correspond to maghemite, and XPS allows one to estimate only surface ions, while in the case of nonstoichiometric magnetite, the structure can be “magnetite in a maghemite shell”.

10.   Line 342: “Since the pH during the reaction does not change or is maybe even more acidic due to the RE salts, this means that the RE ions bind very strongly to the phosphonate groups, because they were able to replace hydrogen despite the neutral or low pH.” This strong bond with the phosphonate groups should have been visible in the IR spectra. Can its absence indicate a rather small amount of ions adsorbed on the surface, and hence a low sorption capacity of the obtained compounds?

11.   Line 354: “This is important if the MNP are further used for medical applications, where RE metals could dissolve and show toxicity if they are not tightly bound to the MNP.” The authors could have judged the strength of the bond in desorption experiments, but they did not carry out such experiments.

12.   Line 409: “TEM and EDX (Figures 3, S5 and S6) of the samples showed that magnetite clusters formed in each case.” TEM micrographs show large spherical particles. How do the authors prove that it was the clusters that formed? How many single separated particles of what size do clusters consist of?

13.   Line 417: Bands at ca. 585-625 cm-1 don’t indicate magnetite

14.   Line 453: “Together with the other characterization (especially FTIR) as well as previous observations [30] it can however be concluded that the clusters consist of magnetite.” See my comment 9.

15.   Line 457: how can authors distinguish between clusters and aggregations?

Author Response

Please see the attachment.

Author Response File: Author Response.doc

Reviewer 2 Report

The submitted manuscript reports an effective fabrication of magnetite nanoparticles/clusters and their coating by polyvinylphosphonates. According to the authors, the utilized coatings have been applied for the first time to magnetite nanoparticles/clusters and provide them great stability in the water solution. I consider the findings interesting and worth of publication. The manuscript is well written, the presented results are convincing and supported by a wealth of experimental data.

I have only minor comments to the authors regarding the figures.

1 Figure 1 – I suggest the enlargement of the insets (size distributions).

2Figure 2 – Please, check the x-label of the plots.

Author Response

Please see the attachment.

Author Response File: Author Response.doc

Reviewer 3 Report

This paper describes the fabrication and polymerised coating of magnetite nanoparticles and nanoparticle clusters using chemical routes. The authors describe the fabrication procedures and the characterisation of their particles using a number of techniques to study the crystalline structure, particle/cluster size, chemical and magnetic properties of their particles and clusters. 

Overall the paper is well written and organised, with the results being discussed to outline the effects of preparation conditions and procedures. The paper is generally scientifically sound and is of interest to the scientific community. 

The authors claim to be the first to have prepared phosphonate coated magnetite nanoparticles in a one-pot polyol reaction procedure. I have found a paper in the literature which appears to have performed such a procedure:

G. Thomas et al., “Efficient functionalization of magnetite nanoparticles with phosphonate using a one-step continuous hydrothermal process”, Dalton Trans, 45, 10821 - 10829 (2016).

The introduction provides a nice summary of the paper and appears to be well documented with ample references. The paper provides a detailed study and is of broad interest for researchers in the area and is worthy of publication in Magnetochemistry. 

There are a few issues which I feel the authors should address before a recommandation can be made. I will outline these below:

1. Line 65: EDTA should be defined.
2. Line 69: Despite authors comments to the contrary, there are some references to the use of phosphonates as a coating for magnetic particles in the literature.
3. I cannot find a citing for ref. 70 in the text.
4. Line 126: The authors list a number of data pertaining to various NMR lines. However there is no explanatory text. I would recommend some explanation to this as the data appears a little out of place within the text in this way.
5. Line 226: The authors state that it is possible to obtain coatings with additional functionality, though no mention of what that functionality may be. 
6. As a general point I find the reading of the results to be a little bit hard going, with frequent references to figure either in the text or within the supplementary materials. This makes for difficult reading. I also find that the reference to results is somewhat patchy, with results being given ad hoc for some samples and not for others. This form of selective analysis makes following the arguments difficult and a little tedious. I would propose some re-editing of the results and discussion section to facilitate reading the text. 
7. Reference to scheme 3 should be made within the text, such as for example at the end of line 377.
8. I find the treatment of the magnetic data to be rather understated. In an article of magnetic nanoparticles the data is nearly discussed. The variation of the saturation magnetisation can also be related to the particle density within the clusters. There is little in the way of reasoning and no in-depth discussion of the effects of particle and cluster size with respect to the magnetic data.
9. Line 468: There is no section 4, so the Conclusions should be section 4 and not 5. 

Author Response

Please see the attachment.

Author Response File: Author Response.doc

Round 2

Reviewer 1 Report

I thank the authors for their detailed and correct response to a large list of my questions and propose to accept the article for publication.