On the Formation of Carbonaceous By-Product Species in Spray Flame Synthesis of Maghemite Nanoparticles

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript titled “On the Formation of Carbonaceous By-Product Species in 2 Flame Spray Synthesis of Maghemite Nanoparticles” aims on the synthesis of materials by SFS in two distinct ways, and their subsequent parameter were studied.

The idea seem to be interesting for the readers of the journal, however the manuscript in its present work is not acceptable.

1. The novelty of this work must be cited carefully in the abstract and conclusion
2. The abstract is too long and could be shortened
3. The introduction must be revised, particularly from line 98 to 120.
4. The methods and materials: (2.1 and 2.2 could be revised and authors could summarize the procedures and schemes can be applied. The section in its present form is too hard to read and understand the numerous steps and complexes phenomenon, please revise and be synthetic in writing the methods.
5. All sections need to revised to show only the necessary and interesting informations. In some case the previously published results or conclusions are not necessary to repeat its.
6. Conclusion is not acceptable, may be there is an error? 4. Conclusion and not 3.Conclusion. The maximum of word in conclusion could be 300 word and no more.
7. References can be reduced to 60 or 70, the 90 references are suitable for review and not research article.

Author Response

The manuscript titled “On the Formation of Carbonaceous By-Product Species in 2 Flame Spray Synthesis of Maghemite Nanoparticles” aims on the synthesis of materials by SFS in two distinct ways, and their subsequent parameter were studied.

The idea seem to be interesting for the readers of the journal, however the manuscript in its present work is not acceptable.

Comment 1: The novelty of this work must be cited carefully in the abstract and conclusion.
Response 1: We revised the abstract as well as the conclusion. Now, the most important quantitative and qualitative findings of our study are outlined there.

Comment 2: The abstract is too long and could be shortened
Response 2: Given the abstract revision, the abstract was shortened to 188 words. 

Comment 3: The introduction must be revised, particularly from line 98 to 120.
Response 3: The introduction was revised in major points. Now, the introduction covers the most important information considering the precursor system and burner types utilized. This way, we introduce the hypothesis of our study in greater detail.

Comment 4: The methods and materials: (2.1 and 2.2 could be revised and authors could summarize the procedures and schemes can be applied. The section in its present form is too hard to read and understand the numerous steps and complexes phenomenon, please revise and be synthetic in writing the methods.
Response 4: The materials and methods section was challenging to read due to the disorganized presentation of information, which primarily contained information that would have been more appropriately situated in the introduction. This has been adapted, and consequently, the introduction now contains relevant information focusing the study hypothesis, while the materials and methods section is more organized.

Comment 5: All sections need to revised to show only the necessary and interesting informations. In some case the previously published results or conclusions are not necessary to repeat its.
Response 5: All sections were revised considering the speech and solely most information is presented now in the results and discussions section.

Comment 6: Conclusion is not acceptable, may be there is an error? 4. Conclusion and not 3.Conclusion. The maximum of word in conclusion could be 300 word and no more.
Response 6: The conclusion was condensed to its most salient findings, resulting in a final length of 279 words.

Comment 7: References can be reduced to 60 or 70, the 90 references are suitable for review and not research article.
Response 7: We removed some redundant references (i.e. references for Raman spectra and ATR-FTIR spectra), which caused a considerable reduction of the number of references.  

Reviewer 2 Report

Comments and Suggestions for Authors

The work studied the formation of carbonaceous by-product species in flame spray synthesis of maghemite nanoparticles. It shows interesting results and can be considered for accepting after proper revision.

1. Fig 8, the authors should provide not only BET data, but also nitrogen adsorption–desorption isotherms data to reveal the pore structure type and characteristics of the material.
2. The conclusion part is too long, if it is to discuss it can be displayed in the main text, the conclusion should only show the most important part, as concise as possible.
3. What is the amount of various reagents and drugs used in the material preparation process? What is the yield? Relevant details should be described.
4. Relevant papers in the last three years need to be cited.

Author Response

The work studied the formation of carbonaceous by-product species in flame spray synthesis of maghemite nanoparticles. It shows interesting results and can be considered for accepting after proper revision.

Comment 1: Fig 8, the authors should provide not only BET data, but also nitrogen adsorption–desorption isotherms data to reveal the pore structure type and characteristics of the material.
Response 1: We agree that corresponding isotherms could offer valuable insights into the samples' pore structures. Unfortunately, we did not determine the entire adsorption-desorption isotherms in our initial approach, and therefore, we had to subject the samples again to BET measurements. We expect that the BET measurements will be completed within the next few days and will supplement the data as soon as possible.

Comment 2: The conclusion part is too long, if it is to discuss it can be displayed in the main text, the conclusion should only show the most important part, as concise as possible.
Response 2: We shortened the conclusions and shifted the findings of minor relevance to the main text. Now the conclusion covers 279 words.

Comment 3: What is the amount of various reagents and drugs used in the material preparation process? What is the yield? Relevant details should be described.
Response 3: We now provide more details about the materials utilized as well as their amount in the material and method section. In the introduction, we also devoted a few sentences to the relevance of the precursor solution applied.

Comment 4: Relevant papers in the last three years need to be cited.
Response 4: We have updated our references and removed some older ones, taking into account the Raman and ATR-FTIR spectroscopy. For references regarding SFS studies using SpraySyn1 and SpraySyn2 as burner types, we now exclusively refer to the most recent studies (2024 and later). 

Reviewer 3 Report

Comments and Suggestions for Authors

Thank you for the opportunity to review your study, which provides valuable insight into byproduct formation in flame spray synthesis (SFS) and its impact on nanoparticle purity and properties. Below are my detailed comments and suggestions for improvement:

Strengths of the manuscript

Relevance and originality: The topic is highly relevant to the field of nanomaterial synthesis, particularly to those working on iron oxide nanoparticles and their applications in the biomedical and catalytic fields.

Methodological rigor: The combination of TGA-DSC-MS, ATR-FTIR, Raman spectroscopy, and TEM provides a comprehensive characterization of carbonaceous byproducts.

Using two experimental approaches: Comparison of the SpraySyn1 and SpraySyn2 burners is a useful complement to understanding the effects of burner configuration on product purity.

Main areas for improvement

1. Clarity in research objectives

The introduction states that the study aims to “contribute to the understanding of byproduct formation in SFS”. However, it would be useful to specify whether the focus is more on mechanistic aspects (how these byproducts are formed) or on practical mitigation strategies to reduce them.

Please clearly define how your findings could influence future process optimizations or nanoparticle applications.

2. Methodological clarifications

The thermophoretic sampling methodology (TS-TEM) is briefly described, but lacks details on:

How the sampling positions were selected.

The temporal resolution of sampling and its impact on capturing transient combustion conditions.

Potential sources of error or bias introduced by TS-TEM.

For TGA-DSC-MS, please provide clearer justification for the chosen temperature ramp and atmosphere (air vs. argon) to ensure reproducibility.

3. Data interpretation and discussion

In Figure 4 (Raman spectral analysis):

The authors claim that the D and G bands of carbon may overlap with the vibrations of maghemite. While this is plausible, could you validate this with reference spectra of pure maghemite?

Your claim would be stronger if you performed a deconvolution of the Raman spectra to separate the possible contributions of carbon and iron oxides.

In Figure 5 (TGA-DSC-MS analysis):

The interpretation of the mass loss at 225 °C and 270 °C is attributed to carbonate decomposition. However, could it also be due to precursor residues or partial oxidation of adsorbed organic compounds? Further evidence from XPS or TOF-SIMS would be valuable.

The hypothesis about oxygen stoichiometry (𝜙 < 1) and its impact on carbon deposits is interesting, but lacks direct validation. Could you experimentally vary 𝜙 and confirm its role in carbon deposition?

Minor technical issues

The captions of several figures do not sufficiently explain the experimental conditions. For example, Figure 2 does not clarify the exposure time used for flame imaging.

There are typographical errors and formatting inconsistencies, such as misplaced citations (e.g., "see [15]"). A thorough review is recommended.

Overall, this study provides important insights into byproduct formation in SFS, but further clarification and experimental validation could improve the robustness of the conclusions. We encourage the authors to address these points to improve the impact and clarity of their work.

Author Response

Thank you for the opportunity to review your study, which provides valuable insight into byproduct formation in flame spray synthesis (SFS) and its impact on nanoparticle purity and properties. Below are my detailed comments and suggestions for improvement:

Strengths of the manuscript

Relevance and originality: The topic is highly relevant to the field of nanomaterial synthesis, particularly to those working on iron oxide nanoparticles and their applications in the biomedical and catalytic fields.

Methodological rigor: The combination of TGA-DSC-MS, ATR-FTIR, Raman spectroscopy, and TEM provides a comprehensive characterization of carbonaceous byproducts.

Using two experimental approaches: Comparison of the SpraySyn1 and SpraySyn2 burners is a useful complement to understanding the effects of burner configuration on product purity.

Main areas for improvement

1. Clarity in research objectives

Comment 1: The introduction states that the study aims to “contribute to the understanding of byproduct formation in SFS”. However, it would be useful to specify whether the focus is more on mechanistic aspects (how these byproducts are formed) or on practical mitigation strategies to reduce them. Please clearly define how your findings could influence future process optimizations or nanoparticle applications.

Response 1: We have thoroughly revised the introduction to ensure it provides a comprehensive overview of the study's background and scope. The revised introduction now includes a dedicated text section at the end (study scope), where we provide a concise explanation of the study's hypotheses and the relevance of our work.

2. Methodological clarifications

Comment 2: The thermophoretic sampling methodology (TS-TEM) is briefly described, but lacks details on:
How the sampling positions were selected.
The temporal resolution of sampling and its impact on capturing transient combustion conditions.

Response 2: In the introduction, the paper's background and hypotheses are explained in a more structured way, which should contribute to a better understanding of the 5 cm HAB sampling position. In addition, the material and methods section now explains in greater detail what temporal resolutions our TS-TEM device enables, and how it differs from other samplers known from the literature.

Comment 3: Potential sources of error or bias introduced by TS-TEM.

Response 3: This is a valuable consideration that should definitely be mentioned. In the discussion, we highlight that since TS-TEM is an intrusive technique, experimental biases cannot be excluded. Most importantly, TS devises introduce some cooling of the gas phase, which could favor the formation of carbon by attenuating local oxidation processes.

Comment 4: For TGA-DSC-MS, please provide clearer justification for the chosen temperature ramp and atmosphere (air vs. argon) to ensure reproducibility.

Response 4: We now describe the TGA-DSC-MS methodology in more detail in the materials and methods section. Wow we emphasize that a) we used a low temperature ramp of 7.5 K min^-1 to avoid the superposition of thermal events, and b) that measurements were performed under oxygen access vs. oxygen restriction to identify the origin of thermal events (oxidative decomposition events are sensitive to O₂ availability, whereas physical mechanisms such as physical desorption and evaporation are not).      

3. Data interpretation and discussion

Comment 5: In Figure 4 (Raman spectral analysis):
The authors claim that the D and G bands of carbon may overlap with the vibrations of maghemite. While this is plausible, could you validate this with reference spectra of pure maghemite?
Your claim would be stronger if you performed a deconvolution of the Raman spectra to separate the possible contributions of carbon and iron oxides.

Response 5: We appreciate the valuable suggestions regarding Raman spectroscopy. Unfortunately, we lack sufficient reference data for pure maghemite or pure carbon to conduct a sophisticated calculation of weight-contributions using the Raman data. We have considered the possibility of addressing your suggestion anyway, by utilizing reference data from literature instead. Therefore, we proved whether our references (references [41] & [49]) provide any reliable information about maghemites’ relative peak intensities. However, the maghemite spectra in [41] and [49] appear to be without baseline correction in the high-wavenumber region, which hinders the use of relative intensities in conjunction with wavelength positions. Therefore, we have to still consider the Raman data from a qualitative perspective in Figure 4b.

Comment 6: In Figure 5 (TGA-DSC-MS analysis):
The interpretation of the mass loss at 225 °C and 270 °C is attributed to carbonate decomposition. However, could it also be due to precursor residues or partial oxidation of adsorbed organic compounds? Further evidence from XPS or TOF-SIMS would be valuable.

Response 6: Given to your suggestion we now consider a new reference [56] in the TGA-DSC-MS section to improve our claims considering the presence of precursor residues. We now outline that INN residues should yield certain mass losses at ~135 °C and ~155 °C which is however, not recognizable neither in the mass evolution nor in the DSC signal. We appreciate the reviewers’ methodological suggestion. Unfortunately, we do not have proficiency with XPS and/or TOF-SIMS in our laboratories.

Comment 7: The hypothesis about oxygen stoichiometry (? < 1) and its impact on carbon deposits is interesting, but lacks direct validation. Could you experimentally vary ? and confirm its role in carbon deposition?

Response 7: Our department did not conduct a SFS study on the systematic adaptation of ? to address its influence on the carbon formation in SFS. However, as now stated more clearly in the introduction, a corresponding study was conducted by Strobel and Pratsinis. In their experimental setup, carbon depositions were avoided by utilizing ? <1.

Minor technical issues

Comment 8: The captions of several figures do not sufficiently explain the experimental conditions. For example, Figure 2 does not clarify the exposure time used for flame imaging.
Response 8: We specified and revised all figure captions and table titles.  

Comment 9: There are typographical errors and formatting inconsistencies, such as misplaced citations (e.g., "see [15]"). A thorough review is recommended.
Response 9: We reviewed the manuscript for typos and mistakes in the speech.  

Overall, this study provides important insights into byproduct formation in SFS, but further clarification and experimental validation could improve the robustness of the conclusions. We encourage the authors to address these points to improve the impact and clarity of their work.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

Revised manuscript can be accepted as is.