Comparative Analysis of Raman Signal Amplifying Effectiveness of Silver Nanostructures with Different Morphology

Round 1

Reviewer 1 Report

The reviewed work includes topics related to the use of silver nanostructures for Raman signal amplification. Due to the wide applications of Raman spectroscopy in practice, the amplification of the analytical signal can contribute to the further development of this method. The topic undertaken is very important and can be an important step in the development of this method. However, in my opinion, there are many shortcomings in the work. Abtract of the paper is very good and presents in a clear and to-the-point manner the subject undertaken. However, in my opinion, it lacks presented in a very brief way the main conclusions of the experiments carried out. I would very much appreciate the addition of literally one or two sentences presenting the main conclusions of the work. The same is the case with the introduction, but here I miss the methods of nanostructures of silver. The authors in the paper focused mainly on the characterization of silver nanostructures omitting completely the analytical aspect. It is true that the SARS spectrum was presented, but the aspect of which of the bands present in the molecule was intensified was completely ignored. In addition, the analysis of only one compound of hers in my opinion is insufficient to state the intensification of analytical signals. Therefore, in order for the work to be published it should be supplemented with examples of analysis of other compounds. 

Author Response

Thank you for your detailed review of our article.

The abstract has been completed with missing results. Please, see the first page. We also added information about methods of nanostructures in the introduction. In the SERS section, we added one more analyte and described molecular bands and whole investigations in more detail. All changes in the article are marked with a turquoise marker.

Author Response File: Author Response.docx

Reviewer 2 Report

The authors describe the characterisation of silver particles that are of potential interest as SERS substrates. The work is well-done and the conclusions are largely supported by the data. Figure 5 and the accompanying discussion (lines 149-170) are the heart of the paper and this part could be usefully amplified. 

Figure 5 should include a solution Raman spectrum of Nile Blue, so that it is clear that the molecule is unchanged on adsorption onto the SERS substrate. It should also include a spectrum of the 10^-6 M solution to demonstrate that there is an enhancement on adsorption. The units of intensity in Figure 5 are stated to be "a.u." are these the same for all three parts? I.e. are the relative intensities meaningful? If not, then this has to be addressed, otherwise comparing the three substrates is meaningless. When the intensities can be compared, the authors should include the peak height of the 590 cm-1 band (or another of their choice) in Table 1. The authors are to be applauded for showing the variation in the Raman measurements by the grey outline.

There is a more general question, in that the fabrication method is very complex, requiring ion beam bombardment at a central facility. The authors should comment on how they could make the fabrication simpler or more accessible. They should also comment on whether the substrates are re-useable.

 

Minor points

Line 76: Although the procedure is detailed in ref [43], the authors should provide a brief description (a couple of sentences or so) of the method.

As an aid to the reader including the structure of Nile Blue in Figure 5 would be welcome.

 

Author Response

Thank you for your detailed review of our article and good feedback.

The authors describe the characterisation of silver particles that are of potential interest as SERS substrates. The work is well-done and the conclusions are largely supported by the data. Figure 5 and the accompanying discussion (lines 149-170) are the heart of the paper and this part could be usefully amplified.

We have expanded this part of the article. For example, we added one more analyte, tables, and discussed results in more detail.

Figure 5 should include a solution Raman spectrum of Nile Blue, so that it is clear that the molecule is unchanged on adsorption onto the SERS substrate. It should also include a spectrum of the 10^-6 M solution to demonstrate that there is an enhancement on adsorption. The units of intensity in Figure 5 are stated to be "a.u." are these the same for all three parts? I.e. are the relative intensities meaningful? If not, then this has to be addressed, otherwise comparing the three substrates is meaningless. When the intensities can be compared, the authors should include the peak height of the 590 cm-1 band (or another of their choice) in Table 1. The authors are to be applauded for showing the variation in the Raman measurements by the grey outline.

We added native Raman spectrum of analytes in a solid state, solutions of the minimum detected concentrations, and 10^-6 M as well. We also changed the “a.u.” to “counts” in the graphs, because it is really important in article context. The peak height was also added to the new table 2.

There is a more general question, in that the fabrication method is very complex, requiring ion beam bombardment at a central facility. The authors should comment on how they could make the fabrication simpler or more accessible. They should also comment on whether the substrates are re-useable.

The ion-track technology used in this work to create the SERS substrates is well suited for research purposes because it allows controlling template pore diameter very simple. The pore diameter is very important for obtaining spatially separated silver structures because this parameter influences the diffusion-limitation processes in the pores on the silver structures growing stage that is necessary for growth of dendrites. In this work the presented results made it possible to determine that it is necessary to use templates with pore diameters of 320, 800, and 1000 nm for the growth of spatially separated structures. To obtain such pore diameters, classical lithography is also well suited. Therefore, in the future, this technique can be used to obtain ordered arrays of such structures. Obtaining ordered silver nanostructures will not influence EF because “hot spots” arranged randomly. However, ordered structures are still relevant because it will increase the analytical usefulness of SERS active substrates.

Unfortunately, at the moment our substrates are not reusable. But in the future, we plan to carry out works on the possibility of their cleaning from analytes.

Minor points

Line 76: Although the procedure is detailed in ref [43], the authors should provide a brief description (a couple of sentences or so) of the method.

The relevant information has been added to the methods section.

As an aid to the reader including the structure of Nile Blue in Figure 5 would be welcome.

Please, see the changes in the figure.

Author Response File: Author Response.docx

Reviewer 3 Report

The authors demonstrated the possibility of using the method of growing silver nanostructures in a limited volume of silicon oxide pores on a silicon substrate. Technically, the authors employ the experiment method with apparent authority, and the results are interesting for the readers. However, the necessary elucidation of the mechanism and references are absent in the manuscript to explain the obtained results. Therefore, I think a minor revision is needed to accommodate the high-quality requirements of this Journal. 

1. Silver is easily oxidized in the air. Please briefly elaborate on what difference if gold is used to replace the silver in the investigated structure.
2. Many articles related to Sunflower-like Dendritic Silver SERS structures have been published (e.g., DOI:10.1039/C4RA11151F and Electrochimica Acta 192 (2016) 15–21). The novelty and advantage of the fabricated structures should be clarified in more detail.
3. Line 101 states, “the power of which was fixed at 60 μW.” Is it 60mW or 60μW? Please check it and give an available range of Laser power in the text.
4. As mentioned in the abstract, the optimal shape of silver nanostructures increases the Raman cross section. However, the mechanism of SERS in the investigated structure is based on localized surface plasmon resonance (SPR). SPR effect depends on the size, shape, and surrounding medium. For example, the dendritic nanostructures were formed due to the edge effect of SPR and diffusion-limited aggregation. It is a lack of discussion on this effect in this work. To enrich the background of plasmonic effect, several references are suggested to quote in the introduction section (e.g., Nanomaterials, 2021, 11 (8), 2097; Scientific Reports, 2021, 11 (1), 1-17).
5. if possible (not necessary), please provide the electric field distribution related to one of the fabricated structures in the revised manuscript.

Author Response

Thank you for your detailed review of our article.

The authors demonstrated the possibility of using the method of growing silver nanostructures in a limited volume of silicon oxide pores on a silicon substrate. Technically, the authors employ the experiment method with apparent authority, and the results are interesting for the readers. However, the necessary elucidation of the mechanism and references are absent in the manuscript to explain the obtained results. Therefore, I think a minor revision is needed to accommodate the high-quality requirements of this Journal. 

1. Silver is easily oxidized in the air. Please briefly elaborate on what difference if gold is used to replace the silver in the investigated structure.

 

Yes, this is true. However, we used samples with silver half a year after they were obtained. We did not notice any difference with freshly prepared samples. However, after 2 years, a significant degradation of properties was observed, including a noticeable change in the morphology of the structures. As for similar structures with gold, no degradation was observed.

 

1. Many articles related to Sunflower-like Dendritic Silver SERS structures have been published (e.g., DOI:10.1039/C4RA11151F and Electrochimica Acta 192 (2016) 15–21). The novelty and advantage of the fabricated structures should be clarified in more detail.

 

The information was added. Please, see the introduction. We mentioned the article and noted the expansive DUV method used there. On the other hand, in our work we used swift-heavy ions. That is also expansive, but from one silicon wafer we can obtain hundreds of samples without lithography. We think this is novelty of our approach.

 

1. Line 101 states, “the power of which was fixed at 60 μW.” Is it 60mW or 60μW? Please check it and give an available range of Laser power in the text.

 

That’s the right value. More detail explanation was provided in updated method section.

 

1. As mentioned in the abstract, the optimal shape of silver nanostructures increases the Raman cross section. However, the mechanism of SERS in the investigated structure is based on localized surface plasmon resonance (SPR). SPR effect depends on the size, shape, and surrounding medium. For example, the dendritic nanostructures were formed due to the edge effect of SPR and diffusion-limited aggregation. It is a lack of discussion on this effect in this work. To enrich the background of plasmonic effect, several references are suggested to quote in the introduction section (e.g., Nanomaterials, 2021, 11 (8), 2097; Scientific Reports, 2021, 11 (1), 1-17).

 

The work Nanomaterials, 2021, 11 (8), 2097 was cited in the introduction – number [33]. But the second one Scientific Reports, 2021, 11 (1), 1-17) we didn’t find. It is not enough the information (no authors name, DOI). The nearest articles for this data are “Morphological convergence and adaptation in cave and pelagic scale worms (Polynoidae, Annelida)” and “Niche specificity and functional diversity of the bacterial communities associated with Ginkgo biloba and Panax quinquefolius”. Obviously, these are not the articles that were meant.

 

 

1. if possible (not necessary), please provide the electric field distribution related to one of the fabricated structures in the revised manuscript.

 

We already provided this for dendrite. Please, see the article “Self-organized spatially separated silver 3D dendrites as efficient plasmonic nanostructures for surface-enhanced Raman spectroscopy applications”, J. Appl. Phys. 126, 233105 (2019); https://doi.org/10.1063/1.5129207

Unfortunately, for crystals and “sunflowers” we do not have the same data.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

I accept of paper publishing in current version.