Comparative Study on the Effect of Protonation Control for Resistive Gas Sensor Based on Close-Packed Polypyrrole Nanoparticles
Round 1
Reviewer 1 Report
The authors fabricated a NH3 gas sensor that was made of PPy NPs. The following questions should be revised for improving the quality of the paper.
What id the function of PVA for preparation of PPy NPS? The explanation about the sensors prepared at pH=1 had high sensitivity was not clear for reading. The authors should clearly explain the results of Fig. 7a. Does the porosity of the PPy NPS film affect the response/recovery times of the sensors that were made of PPy NPs film? The long-term stability of the sensor should be studied.Author Response
COMMENT
The authors fabricated a NH3 gas sensor that was made of PPyNPs. The following questions should be revised for improving the quality of the paper.
REPONSE:
We highly appreciate your positive comments on our manuscript.
COMMENT 1) What is the function of PVA for preparation of PPyNPs?
REPONSE:
Thanks for your valuable comment about function of PVA for generation of PPyNPs. A water-soluble PVA was dissolved in distilled water as a dispersion aid and formed complex with Fe3+ by and ion-dipole interaction in an aqueous medium. The PVA/Fe3+ complex allowed the fabrication of PPyNPs at the reactive site of the complex where the monomer came into contact the Fe3+.[R1]
[R1] Small 2010, 6, 679.
COMMENT 2) The explanation about the sensors prepared at pH=1 had high sensitivity was not clear for reading.
REPONSE:
Thanks for your valuable comment unclear expression about high sensing ability of the electrode at pH 1. The electrode treated at pH 1 shows large resistance change amount during detection of the analyte due to it has many holes in the structure. In addition, it also shows a fast response speed because there are more charge carriers (hole) than other electrodes.
Revised parts in the manuscript
(1) The sentence “The more protonated PPyNPs (treated at pH 1) have more holes in the chain structure, indicating a higher degree to change in external electron flow.” was inserted in the revised manuscript. (see Line 229, Page 11)
(2) The sentence “The numerous charge carriers induce the rapid transfer of electrons from the analyte and reduce the response time to <20 s.” was changed to “The numerous charge carriers (hole) at pH 1 induce the rapid transfer of electrons from the analyte and reduce the response time to < 20 s.” in the revised manuscript. (see Line 234, Page 11)
COMMENT 3) The authors should clearly explain the results of Fig. 7a.
REPONSE:
Thanks for your valuable comment about ambiguous illustration of Figure 7a. As you mentioned, we corrected expression about sensing mechanism and pH effect on the response amount.
Revised parts in the manuscript
(1) The paragraph about Figure 7a modified as follows: “Owing to the reduction characteristics during detection, the NH3 molecules generate electrons. These electrons are transferred to the PPyNPs, thereby reducing amount of the charge carriers (hole) in the polymer chain. Therefore, the resistance of the sensor electrode increases owing to the p-type semiconductor characteristics of the polymer. The sensor electrodes exhibit highly sensitive toward NH3 down to 10 ppb due to the uniformly decorated nanoparticles on the IDA electrode. However, response amounts of the electrodes display increase with reducing pH of solution. The more protonated PPyNPs (treated at pH 1) have more holes in the chain structure, indicating a higher degree to change in external electron flow. On the other hand, the electrode treated at pH 13 shows smallest amount of resistance change even though it has the smallest diameter. Thus, a reduction in the amount of holes, rather than a reduction in the surface area, has a greater effect on response change.”. (see Line 223, Page 11)
COMMENT 4) Does the porosity of the PPyNPs film affect the response/recovery times of the sensors that were made of PPyNPs film?
REPONSE:
Thanks for your valuable comment about porosity effect in response/recovery times. As you mentioned, we think there may be a change in resonse/recovery times depending on the size of the pores in the film. However, we don't think the pore size of the film will change significantly due to the small change in the size of the particles due to pH. Instead, the chemical structure of the material changes response/recovery times, resulting in a large change in sensor performance due to changes in electrical properties.
COMMENT 5) The long-term stability of the sensor should be studied.
REPONSE:
Thanks for your valuable comment about long-term stability test of the sensor electrode. As you mentioned, we added NH3 sensing performance of the electrodes for 15 days. As shown in Figure R1, the electrodes present similar value of responses without a sharp decline.
Figure R1. Normalized resistance changes of the electrodes with periodic exposure to 0.1 ppm of NH3 for 15 days (black: pH 1; red: pH 7; blue: pH 13).
Revised parts in the manuscript
(1) The sentence “Furthermore, the sensor electrodes maintain their sensing ability during the exposure to 0.1 ppm of NH3 over 15 days (Figure 8).” was added in the revised manuscript. (see Line 250, Page 11)
(2) Figure R1 was inserted as Figure 8 in the revised manuscript.
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The revised parts were highlighted by yellow background color. Thank you for your critical and valuable comments again.
Sincerely,
Prof. Junseop Lee
Author Response File: 
Author Response.pdf
Reviewer 2 Report
This manuscript describes a pH effect on the behavior of polypyrrole nanoparticles. This seems like it should be report so I think this manuscript should be published. But I'm not so sure it's ready now.
I'm confused by what the actual effect of acid is. How does it affect the structure of polypyrrole? Does it affect the oxidation potential? This confusion begins with figure 1. The oxidized and reduced states have the same number of protons. As I see the reaction, the concentration of A- would seem to affect the oxidation potential but not the proton concentration. I would also expect the identity of A- to affect the oxidation potential. Why don't they measure oxidation potentials? Why do we get N+ in the XPS spectrum
I'm also unclear on how pH was varied. It's pretty easy to get very high and very low pHs with strong acids and bases but you need a buffer for pH 7. Is the polypyrrole itself part of the buffer system?
And the same for spin coating. I thought spin coating has to involve a solution or a suspension. In what were the particles suspended for the spin coating? Is whatever solvent that is used then removed by evaporation?
I'm guessing this is more of a theoretical study since I would expect the proposed sensor to be affected by moisture but I may be wrong. This is not addressed.
Author Response
Reviewer 2.
COMMENT
This manuscript describes a pH effect on the behavior of polypyrrole nanoparticles. This seems like it should be report so I think this manuscript should be published. But I'm not so sure it's ready now.
REPONSE:
We highly appreciate your positive comments on our manuscript.
COMMENT 1) I'm confused by what the actual effect of acid is. How does it affect the structure of polypyrrole? Does it affect the oxidation potential? This confusion begins with figure 1. The oxidized and reduced states have the same number of protons. As I see the reaction, the concentration of A- would seem to affect the oxidation potential but not the proton concentration. I would also expect the identity of A- to affect the oxidation potential. Why don't they measure oxidation potentials? Why do we get N+ in the XPS spectrum.
REPONSE:
Thanks for your valuable comment about ambiguous illustration of chemical structure change of the polymer chain. First, the errors in the Figure 1 were corrected and presented as shown in Figure R2. Several groups have conducted research to control electrical performance of conducting polymer by treating with different pHs.[R2, R3] In this study, polypyrrole, one of conducting polymer, was treated in a different pH solution achieve different electrical performance. Since the above method is commonly used as a chemical control method, measuring chemical properties using oxidation potential was not carried out. As shown in the revised Figure R2, the amount of positive nitrogen is increased when the pH value is reduced, so chemical structural changes have been demonstrated using XPS.
Figure R2. Chemical structures of PPy in different chemical states.
[R2] J. Mater. Chem. C 2015, 3, 10616.
[R3] Org. Electron., 2014, 15, 641.
Revised parts in the manuscript
(1) The contents of Figure 1 have been modified to Figure R2 in the revised manuscript.
COMMENT 2) I'm also unclear on how pH was varied. It's pretty easy to get very high and very low pHs with strong acids and bases but you need a buffer for pH 7. Is the polypyrrole itself part of the buffer system?
REPONSE:
Thanks for your valuable comment about pH control of the aqueous solutions. The pH value of DI water is ca. 5.5 so used it as pH 5.5 solution. And we used pH 7 solution using commercial pH 7 buffer (Samchun Co.). Other pH values were controlled using NaOH and HCl.
Revised parts in the manuscript
(1) The sentence “HCl and NaOH were used for realizing acidic and basic conditions, respectively.” was changed to “Except pH 7, HCl and NaOH were used for realizing acidic and basic conditions, respectively. The pH 7 solution is used commercial phosphate buffers (Samchun Co., Korea)”. (see Line 84, Page 2)
COMMENT 3) And the same for spin coating. I thought spin coating has to involve a solution or a suspension. In what were the particles suspended for the spin coating? Is whatever solvent that is used then removed by evaporation?
REPONSE:
Thanks for your valuable comment about particle decoration on the IDA electrode. We used spin-coating method to introduce PPyNPs on the electrode surface.[R4] In detail, 0.1 wt% PPyNP contained aqueous solution is used. 10 µL of the solution was deposited on the electrode and following spin-coating was conducted (1000 rpm, 60 s). Then, the electrode was dried at 60 °C in an inert atmosphere for 6 h to obtain good electrical ohmic contact between the PPyNPs and electrodes.
In the experiment we used DI water as a solvent. Therefore, we think water has evaporated sufficiently during the drying process.
[R4] J. Phys. Chem. C 2010, 114, 18874.
COMMENT 4) I'm guessing this is more of a theoretical study since I would expect the proposed sensor to be affected by moisture but I may be wrong. This is not addressed.
REPONSE:
Thanks for your valuable comment about moisture effect in sensing measurement. As you mentioned, moisture greatly affects sensing performance. Generally, the sensing signal of the electrode is interrupted when the humidity increases. So we conducted sensing of the electrode with the same humidity of 20%.
Revised parts in the manuscript
(1) The sentence “Sensing performance was evaluated under ambient conditions (ca. 20% relative humidity).” was inserted in the revised manuscript. (see Line 102, Page 3)
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The revised parts were highlighted by yellow background color. Thank you for your critical and valuable comments again.
Sincerely,
Prof. Junseop Lee
Author Response File: Author Response.pdf
Reviewer 3 Report
The authors present an electrochemical platform with varying polypyrrole nanoparticles synthesis to develop sensitive gas sensors. The main objective of the study is to study the effect of PPy protonation on gas sensing. The manuscript can be considered after addressing the following points.
- Can the authors elaborate on why pyrrole was used in place of other conductive polymers?
- Could the authors describe how the polypyrrole was prepared? Did the authors do any distillation of the stock solution before use? Usually, the pyrrole monomers ordered are partially polymerized in the solution (turns brown) and will need distillation before use so that reproducible results are obtained. I would suggest the authors describe the protocol ina way that it can be reproduced.
- One of the important aspects of gas sensors is how quickly the sensors regenerate or behave if the direction of concentration is changed. I would suggest the authors show a continuous curve where the sensors are exposed to increasing concentration of gas followed by a decreasing concentration of gas. The authors should then plot the increasing and decreasing concentration curve on the same plot and comment on slope, reproducibility, and trend.
Author Response
Responses to Reviewer’s Comments
Reviewer 3.
COMMENT
The authors present an electrochemical platform with varying polypyrrole nanoparticles synthesis to develop sensitive gas sensors. The main objective of the study is to study the effect of PPy protonation on gas sensing. The manuscript can be considered after addressing the following points.
REPONSE:
We highly appreciate your comments on our manuscript.
COMMENT 1) Can the authors elaborate on why pyrrole was used in place of other conductive polymers?
REPONSE:
Thanks for your valuable comment about reason for using pyrrole as a monomer of the nanoparticle. First, we used pyrrole to manufacture conducting polymer nanoparticles. Synthesis of conducting polymer nanoparticles using pyrrole can be uniformly fabricated using monodisperse method.[R1] However, when trying to fabricate nanoparticles using other monomers, uniform particles could not be manufactured. So we used pyrrole.
[R1] ACS Nano 2011, 7, 10139.
COMMENT 2) Could the authors describe how the polypyrrole was prepared? Did the authors do any distillation of the stock solution before use? Usually, the pyrrole monomers ordered are partially polymerized in the solution (turns brown) and will need distillation before use so that reproducible results are obtained. I would suggest the authors describe the protocol in a way that it can be reproduced.
REPONSE:
Thanks for your valuable comment synthetic method of PPyNPs. Monodisperse method is easy to synthesize polypyrrole nanoparticles in a simple way, as presented in experimental section (part 2.2). As you said, additional information (such as rotating speed and reaction time) were inserted for reproducibility of the particles. The biggest advantage of monodisperse method is that reproducible conductive nanoparticles can be synthesized in an easy way.
As you mentioned, it is important to increase purity through post processing of monomer in polymer synthesis. However, this study confirmed that uniform nanoparticles are fabricated under certain conditions even if particles are synthesized without additional post-processing of monomers. Perhaps, partially polymerized pyrrole, which is not removed through post-processing, is not part of the fabricating process and is removed from the washing process.
Revised parts in the manuscript
(1) The part 2.2 is changed as following: “PPyNPs were prepared using PVA, FeCl3, and pyrrole in distilled water. A 0.5 g sample of PVA in distilled water (50 ml) was magnetically stirred (300 rmp) at 25 °C for 1 h. After the dissolution of PVA in distilled water, 0.02 mol of FeCl3 was added into the aqueous PVA solution with 300 rpm. After 1 h of equilibration, 0.06 mol of pyrrole monomer was rapidly added into the aqueous PVA/FeCl3 solution and stirred for 4 h. After the completion of polymerization, the resulting nanoparticles were separated from the dispersion by centrifugation and washed several times with water to remove the impurities. The remaining PPyNP precipitate was dried in a vacuum oven at room temperature for 6 h.”
COMMENT 3) One of the important aspects of gas sensors is how quickly the sensors regenerate or behave if the direction of concentration is changed. I would suggest the authors show a continuous curve where the sensors are exposed to increasing concentration of gas followed by a decreasing concentration of gas. The authors should then plot the increasing and decreasing concentration curve on the same plot and comment on slope, reproducibility, and trend.
REPONSE:
Thanks for your valuable comment about reproducibility of the sensor electrode. As you mentioned, we measured sensing ability of the electrode with decreasing NH3 concentration. Figure R1 shows that the sensing ability of the electrode remains constant regardless of the direction of change in NH3 concentration. Moreover, we also compared response and recovery times of the electrode with direction of concentration changes. As shown in Figure R2a, the response time of the sensor electrode is similar (19 s) in both cases. However, recovery time for backward direction (60 s) presents a little longer than forward direction (49 s) due to exposure to large amount of NH3 molecules during detection (Figure R2b). Therefore, PPy-based sensor electrodes have good reproducibility for environmental changes.
Figure R1. Normalized resistance changes of pH 1 based-electrode upon sequential exposure to decrease concentrations of NH3.
Figure R2. (a) Response and (b) recovery times of pH 1 based-electrode toward 0.1 ppm of NH3 depending on the direction of increase (black) and decrease (green) in concentration of NH3.
Revised parts in the manuscript
(1) Figure R1 was inserted as Figure 7d in the revised manuscript.
(2) Figure R2 was added as Figure 7e and f in the revised manuscript.
(3) The paragraph “Moreover, sensing performance with decreasing NH3 concentration also measured to confirm reproducibility of the electrode. Figure 7d shows that the sensing ability of the electrode remains constant regardless of the direction of change in NH3 concentration. In, addition, we also compared response and recovery times of the electrode with direction of concentration changes. As shown in Figure 7e, the response time of the sensor electrode is similar (19 s) in both cases. However, recovery time for backward direction (60 s) presents a little longer than forward direction (49 s) due to exposure to large amount of NH3 molecules during detection (Figure 7f). Therefore, PPy-based sensor electrodes have good reproducibility for environmental changes.” was added in the revised manuscript. (see Line 239, page 11)
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The revised parts were highlighted by yellow background color. Thank you for your critical and valuable comments again.
Sincerely,
Prof. Junseop Lee
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
The authors have positively revised the questions. The paper was suggested to be accepted.
Author Response
Thanks.
Reviewer 2 Report
Reduction is the gain of electrons. Protonation is adding a proton (H+) to a structure. They are different. The authors write as if the two are the same in their manuscript. They need to correct this. The bottom structure in figure 1 is protonated. The top structure is not protonated. There is no oxidation or reduction shown here. The oxidation states for the elements in the two structures are the same.
I was always taught that the experimental should contain all the detail needed by others to reproduce the reported experiments. The authors present their spin coating procedure in their response to review comments but haven't added it to the manuscript where it is need if someone wishes to reproduce their experiments.
Could it be that the protonated polypyrrole is acidic and therefore has a nigher affinity for basic ammonia than unprotonated polypyrrole?
I've lost confidence in the authors' knowledge of the basic chemistry of polypyrrole so I'd like to see this described here, albeit briefly. I don't know what they mean by doping, polarons, bipolarons, etc. but I do know the difference between oxidation and reduction.
Author Response
There is nothing specific the request for further review of my primary review,
but only criticism of the answers.
Reviewer 3 Report
The authors have addressed all my concerns and I think the manuscript can be accepted.
Author Response
Dear Editor
First of all, we sincerely appreciate you for reviewing our manuscript (ID: applsci-715215) “Comparative Study on the Effect of Protonation Control for Resistive Gas Sensor Based on Close-Packed Polypyrrole Nanoparticles”. We have revised the manuscript accordingly, and detailed correction is listed below point by point:
1) I was always taught that the experimental should contain all the detail needed by others to reproduce the reported experiments. The authors present their spin coating procedure in their response to review comments but haven't added it to the manuscript where it is need if someone wishes to reproduce their experiments.
According to the reviewer 2’s comment, we added detailed method of particle decoration on the electrode surface.
Responses to Reviewer’s Comments
Reviewer 2.
COMMENT 1) I was always taught that the experimental should contain all the detail needed by others to reproduce the reported experiments. The authors present their spin coating procedure in their response to review comments but haven't added it to the manuscript where it is need if someone wishes to reproduce their experiments.
REPONSE:
Thanks for your valuable comment about particle decoration on the electrode. We have explained the detailed method as follows.
“We used spin-coating method to introduce PPyNPs on the electrode surface.[R1] In detail, 0.1 wt% PPyNP contained aqueous solution is used. 10 µL of the solution was deposited on the electrode and following spin-coating was conducted (1000 rpm, 60 s). Then, the electrode was dried at 60 °C in an inert atmosphere for 6 h to obtain good electrical ohmic contact between the PPyNPs and electrodes. In the experiment we used DI water as a solvent. Therefore, we think water has evaporated sufficiently during the drying process.”
But we didn't reflect this in the revised manuscript, so we reflected it this time.
[R1] J. Phys. Chem. C 2010, 114, 18874.
Revised parts in the manuscript
(1) The sentences “To obtain a uniformly coated sensor electrode array, spin-coating was conducted. In detail, 0.1 wt% PPyNP contained aqueous solution is used. 10 µL of the solution was deposited on the electrode and following spin-coating was conducted (1000 rpm, 60 s). Then, the electrode was dried at 60 °C in an inert atmosphere for 6 h to obtain good electrical ohmic contact between the PPyNPs and electrodes.” were added in the revised 2.4 part. (see Line 91, Page 3)
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The revised parts were highlighted by yellow background color. Thank you for your critical and valuable comments again.
Sincerely,
Prof. Junseop Lee
Author Response File: 
Author Response.doc
