Reactive Aerosol Jet Printing of Ag Nanoparticles: A New Tool for SERS Substrate Preparation

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

In this study, the authors present an approach for fabricating surface-enhanced Raman spectroscopy (SERS) substrates using reactive aerosol jet printing as an additive manufacturing technique. The resulting coatings were characterized through X-ray diffraction and scanning electron microscopy, which revealed the formation of nanoporous crystalline silver agglomerates partially covered by residual matter. The authors report that the as-prepared SERS substrates exhibit good SERS activity, demonstrating an enhancement factor for rhodamine detection. While this work provides valuable insights for advancing trace chemical detection technologies, several critical issues must be addressed before it can be considered suitable for publication in Coatings.

1. In Introduction section, the authors stated: “Conventional detection methods, while effective, often lack sensitivity and specificity or they rely on complicated and highly-costing analysis techniques...” However, these “conventional methods” should be explicitly defined establish a clear baseline for comparison. Additionally, later in the text, the author mentions: “In this context, Surface-enhanced Raman spectroscopy (SERS) has emerged as a powerful tool to overcome these limitations, offering unparalleled molecular fingerprinting capability with high sensitivity.” Does SERS truly address the issues of traditional methods, and what is the underlying principle behind this solution? It is recommended that the author provide a detailed discussion on this matter.
2. In this paper, the authors demonstrate the effective detection of rhodamine (R6G) through substrate fabrication based on r-AJP. What is the approximate lowest detection concentration achieved? Compared with traditional preparation methods, what level of improvement has been attained? It is recommended that the authors include comparative analysis and further discussion on this aspect.
3. In Figure 4, the authors demonstrated the UV-Vis spectrum for detecting plasmonic peaks in SERS substrates. The authors stated: “with a larger width at half height, corresponding to a wider distribution of the nanoparticle dimensions.” I am confused about this description. Could the authors provide more explanation regarding why the absorption peak exhibits broadening?

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The work under consideration proposes an upgraded concept of aerosol jet printing of Ag nanoparticles. To achieve this, the authors suggest advanced reactive aerosol printing, bringing the overall technique closer to large-scale, repeatable fabrication of SERS-active substrates, for example, within additive manufacturing. While the work is of high scientific interest, several questions should be addressed before the paper is accepted:

1) While the authors use the term 'ultra-low' in relation to analyte concentration detection, their work only demonstrates experiments down to 10⁻⁸ M of R6G, which is far from the ultra-low criterion.

2) The statement that 'residual matter did not interfere with Raman analyses' is only valid for current investigations with rhodamine 6G and must be confirmed independently for the target analyte, since detecting R6G is not interesting from a practical perspective. It is just a model system. Nevertheless, such a strong claim requires solid proof.

3) The claim that 'reactive aerosol jet printing has never been reported yet' is not accurate, as it has already been demonstrated by Liperi Paolo in a thesis (https://www.politesi.polimi.it/handle/10589/215397) in collaboration with Gibertini Engenio in 2022/23.

4) The SERS experiments, especially the results shown in Figure 5(c), suggest that experiments involving intermediate numbers of Ag NPs layers (between 5 and 10) are necessary to determine the maximum detection efficiency, since the dependence of the Raman band intensity on the number of Ag NPs layers is non-monotonic.

In summary, several issues should be addressed during the minor revision.

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

General Concept Comments:  

The article proposes a novel approach for SERS substrate fabrication by utilizing Reactive-Aerosol Jet Printing (r-AJP) as an innovative additive manufacturing technique. Demonstrated the viability of the r-AJP technique for the deposition of nanoporous silver coating on a glass slide, owing to the fast precipitation of Ag nanoparticles by ascorbic acid oxidation. Computational fluid dynamics- CFD simulation suggested that homogeneous mixing of the two aerosols occurred after the recombination zone (Y fit joint), promoting Ag seeds formation. The findings of the study demonstrate the potential of r-AJP as a scalable and cost-effective fabrication strategy for next-generation SERS sensors

The manuscript demonstrates a clear technical novelty and was presented in a well-structured manner, integrating reactive chemistry with aerosol jet printing for nanostructured SERS sensors. With minor revisions to enhance clarity and rigor, the research study would be a valuable contribution to both the SERS sensor and additive manufacturing communities.

Specific Comments: 

I would like to suggest the following improvements to strengthen the article: 

1. In Line 21, the abbreviation CFD needs to be defined before using it 
2. In Line 177, the abbreviation QUICK needs to be defined before using it 
3. In Line 279, Figure 1. (b) and (c), the sub-labels are confusing, and Figure 1 (b)-3 is not labeled in the figure caption. It would be better to update the sub-labels appropriately for better representation. 
4. In Line 302, Figure 2 (b) is not labeled with the respective layers. 
5. In Line 339, Figure 3: I suggest using a different bright color for the sub-labels (a-i), as it is difficult to interpret. 
6. In Line 414 of Figure 5, “For figure (b) and (c), standard deviation is represented by the colored shaded region.”  Can you correct the sub-label (c)  with the correct one?
7. I suggest including a section on how the printing process parameters, such as nozzle size, nozzle-to-substrate distance, flow rates, and printing speed, were optimized during the deposition of the Ag nanoporous coatings process, as they play a crucial role in the r-AJP process. 
8. I suggest including a paragraph on the potential limitations of scaling r-AJP to other materials or complex geometries with micro-nano structures. 
9. In Line 430, I suggest rewriting the sentence  "we think this work could pave the way…" in a more formal tone for better representation. 

Comments for author File: Comments.pdf

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

After careful evaluation, I recommend accepting this manuscript in its current form.