Electrospinning-Derived PLA/Shellac/PLA Sandwich—Structural Membrane Sensor for Detection of Alcoholic Vapors with a Low Molecular Weight

Round 1

Reviewer 1 Report

In the manuscript authors describe a membrane that become transparent when it is exposed to alcohol vapours. Thanks to this property it should be used as optical gas sensor. Fig. 5 clearly shows the behaviour of this polymeric film. In the reviewer opinion there are some major issues that have to be deeper clarified, explained or investigated to be worth to be published. Here below a list of some points that in the reviewer opinion should be explained:

First of all, reviewer does not understand if the variation of light passing thru the membrane is collected by a photoresistor or by a photodiode. Usually photodiode provides a current proportional to the photons absorbed and electronic circuits (or just a load resistance) is used to give voltage as ouput. On the other hand, photoresistor acts as a resistance that changes in accordance with the light intensity. The responses are reported as resistance changes. Please provide the model of photodiode or photoresistor used and the relative electronic circuits (if any). The equation (6) reports the formula used to calculate the resistance variations and considered as sensor response. By the equation, when no light is detected the resistance R is equal to R0 and the delta R is to R_PR/R₀ * 100 rather than zero. Usually the equation used to normalize in the [0,1] range is (R₀-R)/(R₀-R_PR). Concerning the discrimination between ethanol and methanol, actually the dynamic response of sensor may depend on many factors such as the concentration and/or the flow rate, for this reason response time is usually used to discriminate different compounds only in system having microfluids components which provide constant rate flow. Furthermore, authors should take into consideration that methanol is more volatile than ethanol and has a higher vapour pressure. Usually to compare the responses similar concentration ranges should be considered for the analytes. Reviewer does not understand if the system for measurements was placed in a glove box or on open air. In the first case, the authors should report the influence of humidity on the sensor performance. Finally, in order to be used as sensors, it would be interesting to have a characterization of the membrane responses when exposed to different concentrations of ethanol and methanol in order to calculate the sensitivity of the proposed system. Saturated vapour pressure for ethanol and methanol are very high concentration for gas sensing application and so the detection of these VOC at these concentrations has low relevance for real applications.

Author Response

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Reviewer 2 Report

line 56, page 2, the collecting target can be any material bearing an opposite charge induced from the spinneret. this is given a piece of evidence here as well when the authors have also exposed the detached spun PLA membrane from the foil for second and third layers spinning and it was working well as expected. hence the grounded collectors are typically used however they can be replaced with oppositely charged/chargeable substrates. I recommend the authors reading the following link where the fibers are accumulated on a surface which is oppositely chargeable due to the different dielectric constant encountering an external electric field. https://www.mdpi.com/2079-4991/8/10/803 

Can you possibly explain the role of the lower molecular weight of alcoholic materials in the introduction section?

When the average fibre diameter of PLA is in microscale, it is better to call them: electrospun fibres /microfibers.

line 226 page 8, how the uniformity of the reshaped shellac layer have been evaluated?

line 227 page 8, how the reduced light scattering has been measured?

 

Author Response

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Reviewer 3 Report

This paper describes an electrospun membrane sensor for detecting alcohol vapors. In general, this manuscript requires a significant amount of revision before it is suitable for publication. Some suggestions for improvement:

The introduction does not explain why we need an alcohol vapor sensor. Is this a well-known pollutant? Is this needed for human sensing? Sensors should be used either to quantify the analyte concentration or to detect a threshold. It seems that this application is aimed toward the latter. If this is the case, please discuss the alcohol vapor concentration threshold that should be detected and provide a justification. Additionally, how does the ability to detect a given threshold correspond to the layer thicknesses of the membrane? It is unclear if a photodiode or a photoresistor was used in the experimental setup. Labels should be added to Figure 4 to delineate where PLA is and where shellac is. Are XRD measurements going to be included?  What did FTIR analyze, and why is this being included? What is the importance of these findings?

Author Response

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Round 2

Reviewer 1 Report

Reviewer carefully reads the authors reply and the amended manuscipt, however some aspects are still not completely clear.

1) Authors confirm that they used a photoresistor (5506 made by the Shenzhen RISYM Electronic Co. Ltd). The manuscript still extensively refers to it as a photodiode ( line 129, 135,137,143,276,279,282,288,289,290 and 297). Please replace in the text photodiode with photoresistor.

2) Authors say that "The experiments were carried out in a glove box with a constant temperature and humidity. Therefore, we consider the factors such as the temperature and humidity as consistent variables". In order to be used as sensor, it is very troublesome if the proposed sensing material can be used only in a glove-box apparatus. For this reason, in the reviewer opinion authors should investigate and report if the variation of humidity affects the response of the sensor. This information is crucial to understand if this material can be utilized in a quotidian scenario.  

3) Authors say that "We tested the impact of alcoholic vapor concentration on the response of the sandwiched membrane, and found that for certain concentration, the difference is not obvious". Since the difference is not obvious please at least indicate the range of concentration in which the membrane produced a measurable response. Actually limit of detection is a second crucial parameter to understand to potentialities of a material as gas sensor. Actually detection of vapours only at saturated pressure is quite useless in applicative scenarios.

Author Response

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Reviewer 2 Report

1- The authors have addressed some critical concerns, however, I couldn't understand the reason that they used the expression of '' uniform coating'' when a portion of the PLA/shellac/PLA  membrane was dissolved within the composite membrane while changing the composite membrane into composite film;, have they controlled the uniformity of the dissolved layer?  If not, I strongly recommend changing the misleading expression of ''uniform coating'' (line 158, 237, 348)  to the gap-filling layer. Besides, the dissolving Shellac membrane (as the middle layer) while facing a solvent can change into a thin film, not a coating layer. 

 2- In the abstract section, they have suggested that the composite film can be reusable: '' In the meantime, the PLA/shellac/PLA sandwich-structural membrane can be used again after evaporating the alcoholic vapor at an elevated temperature.'' Please rewrite this sentence showing that this is a suggestion and has not been evaluated in this work, e.g. '' the PLA/shellac/PLA composite film can be potentially reused .... ''.

Author Response

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Reviewer 3 Report

The revised manuscript is not an improvement upon the previous submission. The motivation behind sensor development and how one would be used is still unclear. There are many things in the world that can be hazardous to human health, but sensors may not be needed because individuals are unlikely to come into contact with some substances. Under which conditions are humans exposed to these vapors? How many people are affected by this, and at what cost? Would sensors be integrated into closing? Nose implants? Factory equipment?

There is still no discussion of a threshold concentration for sensing or how the concentration affects response time.

The ability to detect a given threshold at a given membrane thickness should be quantified.

Additionally, the authors keep flipping between "photoresisor" and "photodiode". There are inconsistencies throughout the paper and between the responses to reviewers and the paper.

There seem to be no changes to Figure 4, even though the authors noted that the requested changes were made.

In general, the authors did not address my previous comments in the revision.

Author Response

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Round 3

Reviewer 1 Report

In the reviewer's opinion, authors satisfactorily responded and amended the manuscript that is worth to be published in the present form.

Author Response

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Reviewer 3 Report

This manuscript has been improved by addressing recommendations from previous reviews.

i would recommend a change in language in the following description of Fig. 12:
"The results are shown in Fig. 12. From the figure, one can see that the smallest concentration of methanol (ethanol) in water is 10 wt% (10 wt%). Although the data are not as large as those for 100% of methanol (ethanol), the change of resistance is pretty obvious."

"Pretty obvious" is a meaningless descriptor for interpretation of scientific results. You might consider adding statistics or providing language quantifying how 10 wt% methanol/ethanol compares to 100 wt%.

Additionally, the authors should state which shellac content was used for this experiment.

Author Response

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This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

See attached file.

Comments for author File: Comments.pdf

Author Response

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Reviewer 2 Report

The paper needs to be rewritten. The contents are poorly organized and difficult to follow. For example, Results and Discussion section includes contents that supposed to go to material & methods section. Therational for the experimental design should explain clearly. For example, why do the authors used shellac in place of other resin? Why laser was used? What was the relationship between laser and photo resist?

Author Response

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Round 2

Reviewer 1 Report

Response to response #1 (abbreviated RR1 in the future). It would strengthen the manuscript to see a discussion of how the authors plan to use the time to make selective measurements, and to show that these measurements can be made selectively.

RR2. The manuscript would be strengthened if the authors include the justification provided to the reviewer in the manuscript for the reader.

RR3. The authors should include the thickness measurement in the manuscript.

RR4. This is fine.

RR5. This is fine.

RR6. The authors should include this data in the manuscript.

RR7. I strongly object to this data being included in the manuscript. The XRD is featureless and therefore cannot be used to characterize the membranes.

Reviewer 2 Report

The study fabricated PLA/shellac/PLA sandwich-structural membranes via an electrospinning approach, and found that the PLA/shellac/PLA sandwich-structural membrane exhibited fast response to the alcoholic vapors with low molecular weight, especially for methanol vapor. Though the topic has interest to sensor community, but the study requires further study to qualify the membrane to be used as gas sensor.

Major issues:

In previous research, K Ma et al. 2018 designed PCL/shellac/PCL nanofiber membranes. The author should addressed why PCL was not used in place of PLA. What was the advantage of using PLA over other electrospun material. Degradation of shellac under moist environment is big concern for the designed membrane application as sensor. This study conducted a short-term study (600 sec). A long-term study is required to evaluate the efficacy of designed structure as Alcoholic vapor sensor. SEM image clearly shows non-uniform distribution of shellac at the center layer of the membrane. The study specifically Figure 10 needs to be repeated with multiple samples with consistent shellac thickness.

Minor issues:

1. Combine Fig 9a and 9b, since there is no difference between the groups.

The name and company of photoresistor is missing in line 130. Fig 6. ,Fig 7 and corresponding discussion should be moved to introduction. Equations 1-5 and their discussion can be moved to introduction or create a separate heading titled theory under section 2 to discuss. of samples tests for each experiment is missing.