Review Reports - Highly Selective and Stable Electrochemical Sensor for Hydrogen Peroxide—Application in Cosmetics Quality Control

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

In this study, a simple and reliable voltammetric sensor based on a glassy carbon electrode (GCE) decorated with rhodium (Rh) nanoparticles for hydrogen peroxide (H₂O₂) is presented. The authors emphasize its excellent analytical properties, including wide linear concentration ranges, low detection limits, and high selectivity toward the analyte of interest. In addition, the developed sensing platform has been successfully applied to quantify H₂O₂ concentration in commercial samples from the cosmetic industry.

The manuscript is written in fluent English, fits within the scope of Chemosensors, is well structured, and the discussion corresponds to the data presented. To further improve the quality of the work, it is necessary to clarify certain details.

Introduction, page 2, line 56. Scientific Committee on Consumer Safety is misspelled.
Section 3.2, electrochemical characterization, lines 189-192. Can the redox-behavior of the reversible probe be compared with the data from the literature?
Section 3.2, electrochemical characterization, lines 201-203. Why do the cathodic and anodic peak currents shift toward the more positive and more negative potentials, respectively, with increasing scan rate?
Section 3.2, page 6, Equation 3. Please correct the notation.
Section 3.2, amperometric sensing of the analyte. Why did the authors used PBS (pH = 7.0) as the base electrolyte? What would be the effect of different pH values of the base electrolyte solution on the amperometric response of the fabricated Rh/GCE platform? Can the authors compare this with the data given in Section 3.5 (Table 1, various H₂O₂ sensing platforms)?
Section 3.2, Figure 4a. Why is there a current output (defined anodic and cathodic maxima) presented in an electrochemical cell without the presence of the analyte?
It would be beneficial to introduce the mechanism of H₂O₂ reduction on the fabricated sensor.
It would be beneficial to include more citations within the Results and Discussion section.

Author Response

Authors responses to the critical notes of Reviewer #1.

Comments 1: In this study, a simple and reliable voltammetric sensor based on a glassy carbon electrode (GCE) decorated with rhodium (Rh) nanoparticles for hydrogen peroxide (H₂O₂) is presented. The authors emphasize its excellent analytical properties, including wide linear concentration ranges, low detection limits, and high selectivity toward the analyte of interest. In addition, the developed sensing platform has been successfully applied to quantify H₂O₂ concentration in commercial samples from the cosmetic industry.

Response 1: First of all, the authors would like to express their gratitude for the reviewer’s time and effort spent in reviewing their manuscript. All changes performed during the revision process are highlighted.

Comments 2: Introduction, page 2, line 56. Scientific Committee on Consumer Safety is misspelled.

Response 2: We are thankful for the reviewer’s note. The abbreviation was corrected in the revised MS.

Comments 3: Section 3.2, electrochemical characterization, lines 189-192. Can the redox-behavior of the reversible probe be compared with the data from the literature?

Response 3: Authors are thankful for this insightful question. Studies on the behaviour of modified electrodes by cyclic voltammetry (CV) in the presence of 5 mM redox probes (potassium hexacyanoferrates or Ru hexaamino chlorides) is a commonly accepted method for the determination of electrochemically accessible surface area (ECSA). The reactions occurring at the electrode surface are diffusion controlled, i.e. the peak current linearly depends on the square root of the scan rate. There are numerous examples of similar studies performed with nanostructured electrodes, e.g.

Comments 4: Section 3.2, electrochemical characterization, lines 201-203. Why do the cathodic and anodic peak currents shift toward the more positive and more negative potentials, respectively, with increasing scan rate?

Response 4: We are thankful for this theory-related question. With the increase of the scan rate, the reaction transforms from quasi-reversible to irreversible, which is manifested in the rise of the peak-to-peak separation, DE. Electrochemical reactions showing DEp > 60/n mV, but DEp < 200/n mV can be classified as quasi-reversible, whilst for DEp > 200/n mV, the reaction becomes irreversible. Here n denotes the number of electrons exchanged during the redox process. Thus, in our case, the reaction exhibits quasi-reversible behaviour at low to moderate scan rates, and becomes irreversible at scan rates exceeding 75 mV/s.

Comments 5: Section 3.2, page 6, Equation 3. Please correct the notation.

Response 5: Authors are very grateful to this reviewer for pointing out their technical mistake. The Randles-Sevcik equation was corrected, and the notations were amended (lines 213, 216).

Comments 6: Section 3.2, amperometric sensing of the analyte. Why did the authors used PBS (pH = 7.0) as the base electrolyte? What would be the effect of different pH values of the base electrolyte solution on the amperometric response of the fabricated Rh/GCE platform? Can the authors compare this with the data given in Section 3.5 (Table 1, various H₂O₂ sensing platforms)?

Response 6: The authors thank the reviewer for this theory-related question. Usually, the analysis is carried out under mild reaction conditions – neutral medium, and ambient temperature. This principle directed the choice of the buffering medium. The variation of pH would affect processes in which protons participate in the rate–limiting stage. In such a case, one would observe a linear shift of the peak potential with pH change dE/dpH = ±60 z/n, (here n is the number of electrons, z is the number of protons, and 60 mV is the Nernstian coefficient) i.e. the peak potential will shift per 1 pH unit with 60 mV for 1 electron-1 proton coupled redox reaction. We did not have this in mind; the sensitivity was just high enough for the analytical purposes.

Table 1 compares the operational parameters of different sensing platforms under specifically optimised conditions. As it can be seen, the acidic medium is in use either when the protonated form of the modifier is more active – e.g. the poly-L-lysine, row 2, or when the composite might be damaged at neutral medium – RGO/CuFe₂O₄/CPE catalyst will be destroyed at pH = 7.0 due to the formation of iron hydroxides. No such events are expected in our case.

Comments 7: Section 3.2, Figure 4a. Why is there a current output (defined anodic and cathodic maxima) presented in an electrochemical cell without the presence of the analyte?

Response 7: With gratitude to the reviewer for this intuitive question! Most of the electrochemical techniques require a basis for comparison – a baseline or a background voltammogram. Cyclic voltammetry can be considered a method for diagnosing capacitive or redox behaviour at the electrode-electrolyte interface. Often, in the absence of redox–active species, the appearance of peaks is an indication of the occurrence of surface phenomena – adsorption or desorption. There are a series of voltammetric methods for the ECSA determination based on the adsorption or desorption peaks in some electrolytes. For example, ECSA of Au electrodes can be calculated based on the deep desorption peak at ca 0.9 V in 0.5 M H₂SO₄. At this potential, the chemisorbed on the surface oxygen is desorbed. Thus the appearance of these humps on the CVs of Rh-modified glassy carbon electrode might be a result from surface phenomena, which did not impede the electroanalytical process.

Comments 8: It would be beneficial to introduce the mechanism of H₂O₂ reduction on the fabricated sensor.

Response 8: Authors thank the reviewer for this suggestion. A tentative equation of the electrochemical reduction of hydrogen peroxide is provided in the Introductory section – eq. 2.

Otherwise, the mechanism of H₂O₂ is a complex process that is highly dependent on the nature of the electrode material, see e. g. X. Cai, E. E. L. Tanner, C. Lin, K. Ngamchuea, J. S. Foord and R. G. Compton, Physical Chemistry Chemical Physics 2018 Vol. 20 Issue 3 Pages 1608-1614. DOI: 10.1039/C7CP07492A. Its elucidation requires support from mechanistic studies and/or computational data, which falls outside the scope of the work.

Comments 9: It would be beneficial to include more citations within the Results and Discussion section.

Response 9: Thank you for the suggestion. The “Results and Discussion” section contains 9 references, supporting discussion. This is almost 1/3 of the references cited in this work.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The paper deals with the development of a selective and stable electrochemical sensor for hydrogen peroxide, applied to the monitoring of the quality of cosmetics and pharmaceuticals..

The paper approaches a timely and interesting topic, given the significance of sensitive, performant methods, including electrochemical techniques, applied for the control of the antimicrobial agents’ level. All sections present relevant informations, and graphics are sound. The results begin with the scanning electron microscopy studies, revealing trace amounts of oxygen alongside Rhodium deposits on the glassy carbon support. Nevertheless, surface oxides were reported to fall below the detection limits. The electrochemical characterization in the presence of the ferri/ferrocyanide redox probe exhibits a quasi-reversible redox process and the presence of Rhodium modifier enhances electro-activity. The reaction at surface proved diffusion-controlled. Furthermore, the enhancement of the cathodic peak was linked to the analyte reduction, and correlated to the hydrogen peroxide concentration. The detailed study of the analytical parameters revealed excellent linearity, sensitivity in the range of micromolar and good specificity in the presence of interferents. The sensor was characterized by stability and good precision revealed by RSD, at inter- and intra-assay. The analysis of real samples was performed with very simple sample pre-treatment, accuracy at low operating potential.

The comparison with the standardized titrimetric techniques and with results obtained in other studies is welcome.

The developed sensor avoided the drop-casting technique, that could lead to “coffee ring”, impacting the performances and reproducibility. The single-step electrodeposition technique of a monometallic catalytically active phase has been reported as an advantageous one.

The Conclusions are sound and reveal the main findings and advantages of the non-enzyme sensor.

The paper presents a detailed, sound study and is recommendable for publication.

Nevertheless, some very minor modifications would be required, before preparing the final version.

Please, use "the" in some cases such as:

When the electrode was stored in air at room temperature. (better than "when electrode")

...the analytical performance of the electrode is strongly affected by the intrinsic activity

Fig. 7b presents the authentic record of the electrode signal.

Allong….please correct as "along"

Please, use comma when case:

Among the various types of modified electrodes, Rh/GCE is one of the most attractive sensing materials because of its good electrocatalytic activity, well-defined robust structure, and excellent stability.

As evident from the data, the analytical characteristics reported in the present study are comparable to, or even superior

European official methods of analysis. . (please use period only once at the end of the phrase).

In the Abstract, satisfactory recovery rates are mentioned. If available, it would be welcome to mention them in the manuscript as values /interval. Or, the Abstract should be modified accordingly.

Please, reformulate the phrase more clearly: The residual catalytic activity was monitored by measurement of the signal in the presence of 0.5 mM H2O2 an average of once every 10 days

Suggestion.

The residual catalytic activity was monitored by measurement of the signal in the presence of 0.5 mM H2O2. The result was reported as an average of measurements, performed once every 10 days

The residual catalytic activity was monitored by measurement of the signal in the presence of 0.5 mM H2O2, as an average of determinations performed once every 10 days (Fig. 6)

Author Response

Authors' responses to the critical notes of Reviewer #2.

Comments 1: The paper deals with the development of a selective and stable electrochemical sensor for hydrogen peroxide, applied to the monitoring of the quality of cosmetics and pharmaceuticals.

The comparison with the standardized titrimetric techniques and with results obtained in other studies is welcome.

The Conclusions are sound and reveal the main findings and advantages of the non-enzyme sensor.

The paper presents a detailed, sound study and is recommendable for publication.

Nevertheless, some very minor modifications would be required, before preparing the final version.

Response 1: First of all, the authors would like to express their gratitude for the reviewer’s time and effort spent reviewing their manuscript. All changes performed during the revision process are highlighted.

Authors are grateful for the reviewer’s helpful suggestions. All recommendations aiming to improve the readability of the MS text, were accepted with deep gratitude.

Comments 2: Please, use "the" in some cases such as:

When the electrode was stored in air at room temperature. (better than "when electrode")

...the analytical performance of the electrode is strongly affected by the intrinsic activity

Fig. 7b presents the authentic record of the electrode signal.

Allong….please correct as "along"

Please, use comma when case:

As evident from the data, the analytical characteristics reported in the present study are comparable to, or even superior

European official methods of analysis. . (please use period only once at the end of the phrase).

In the Abstract, satisfactory recovery rates are mentioned. If available, it would be welcome to mention them in the manuscript as values /interval. Or, the Abstract should be modified accordingly.

Response 2: The Authors appreciate this comment from the reviewer. The Abstract was modified as follows: “Furthermore, Rh/GCE has been successfully used to measure H₂O₂ concentration in hair dye and antiseptic solution, obtaining results with high precision.” (lines 22-23).

Comments 3: Please, reformulate the phrase more clearly: The residual catalytic activity was monitored by measurement of the signal in the presence of 0.5 mM H2O2 an average of once every 10 days

Suggestion.

The residual catalytic activity was monitored by measurement of the signal in the presence of 0.5 mM H2O2. The result was reported as an average of measurements, performed once every 10 days.

The residual catalytic activity was monitored by measurement of the signal in the presence of 0.5 mM H2O2, as an average of determinations performed once every 10 days (Fig. 6)

Response 3: The authors accept with gratitude the linguistic help from the reviewer.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript reports a nonenzymatic electrochemical sensor for hydrogen peroxide based on rhodium electrodeposited onto a glassy carbon electrode. The authors describe a one-cycle cyclic-voltammetric deposition from RhCl3 in 0.1 M HCl, characterize particle size and coverage by SEM/EDS, and evaluate performance in PBS at pH 7 using CV and amperometry. Careful revision is needed to address methodological transparency, statistics, figure reporting, and several points of interpretation before the work can be reliably evaluated for publication:

The deposition protocol is given as a single CV cycle in 2% RhCl3, but charge passed, deposition time, concentration units for “2%,” and Rh loading are not reported.
The manuscript states reversible or quasi-reversible behavior, yet ΔEp values of 138 mV (Rh/GCE) and 265 mV (bare) at 50 mV s⁻¹ are inconsistent with reversible 1e⁻ behavior.
he electroactive area is derived using the Randles–Sevcik equation, but the concentration of Fe(CN)6 used is not specified in the text
LOD and LOQ are said to use 3σ/S and 10σ/S, but the method for σ estimation, number of blank replicates, time base, and stability of baseline are not given.
Selectivity is assessed against a small panel at 10× concentration and several common ions, but no tests are shown for electroactive species common in cosmetics, such as ascorbate, sulfite, nitrite, parabens, or phenolics beyond salicylic acid
Long-term stability is assessed on two electrodes over three months with no error bars and no surface re-characterization.
For hair dye oxidant, the dilution protocol is described, but density, mass fraction assumptions, and propagation of uncertainty to the reported 5.91% are not provided.
EDS is conducted at 5 kV with qualitative statements about surface oxides. At this accelerating voltage and on nanostructures, quantification is uncertain.
The comparison table mixes different electrolytes, pH, potentials, and electrode types, which complicates claims of superiority, and some entries are not strictly cosmetics.
The text argues that oxygen reduction starts around −0.4 V, so measurements at −0.1 V are interference-free. This may be true for bare GCE, but catalytic Rh may shift kinetics. No oxygen-purged control is shown.
Sensitivity is reported per geometric area, though an electroactive area is also reported. The basis for normalization is not explicit.

Comments on the Quality of English Language

A light technical edit would improve precision. Examples: change “The data are fascinating because the increases in the current response…” to “The increased current and decreased ΔEp indicate improved electron-transfer kinetics.” Replace “active widely used oxidizing agent” with “widely used oxidizing agent.” Keep tense consistent in Methods (past) and Results (past for observations, present when stating interpretations).

Author Response

Authors responses to the critical notes of Reviewer #3

Comments 1: The deposition protocol is given as a single CV cycle in 2% RhCl3, but charge passed, deposition time, concentration units for “2%,” and Rh loading are not reported.

Authors agree with the reviewer’s critique and apologise for this technical oversight. The type of the % was specified in the text as 2% w/v (Please see lines 151, 153-156).

Comments 2: The manuscript states reversible or quasi-reversible behavior, yet ΔEp values of 138 mV (Rh/GCE) and 265 mV (bare) at 50 mV s⁻¹ are inconsistent with reversible 1e⁻ behavior.

Response 2: We thank the reviewer for their note, we did agree that the calculated peak-to peak separations indicate quasi-reversible and irreversible behaviour, respectively, as it was stated in the MS text : pls. see lines 186-189, v1 (193-196 in the revised version): “The peak-to-peak potential separation (ΔE_p) between the anodic and cathodic peaks is 138 mV and 265 mV for the modified and bare electrode, respectively, demonstrating a quasi-reversible electrochemistry of the redox process over Rh-modified, and irreversible – over the bare GCE”. The text of the MS never claimed reversible behaviour.

Comments 3: The electroactive area is derived using the Randles–Sevcik equation, but the concentration of Fe(CN)6 used is not specified in the text.

Response 3: Authors are thankful for this critical note. The concentration of the redox probe was provided in the Experimental section (lines 120-122).

Comments 4: LOD and LOQ are said to use 3σ/S and 10σ/S, but the method for σ estimation, number of blank replicates, time base, and stability of baseline are not given.

Response 4: This critical note is appreciated by the authors. The dispersion of the blank current was calculated on the basis of 3 to 5 independent measurements. Three of these recordings were shown in the supplementary materials. The very low noise level is the reason for the low sigma values.

Comments 5: Selectivity is assessed against a small panel at 10× concentration and several common ions, but no tests are shown for electroactive species common in cosmetics, such as ascorbate, sulfite, nitrite, parabens, or phenolics beyond salicylic acid.

Response 5: Thank you for this insightful question. Indeed, some cosmetic products – creams, emulsions, lotions, contain phenolic antioxidants, ascorbate, and parabens. The samples chosen in this study, however, are a hair dye developer, and antiseptic solution with no additives. The interferents were selected based on the common ingredients of the developer, as listed by the producers of hair dyes. Neither phenols, nor parabens can be commonly found in the developers of most hair dyes. In addition to that, none of the proposed substances – phenols, ascorbate, etc. would survive in the presence of 3 - 6% hydrogen peroxide, and therefore, they will not contribute to the electrode response at the selected working potential of -0.1 V. In the presence of peroxide, phenolics would polymerize after being oxidized. Concerning ascorbate, even on Au-nanoparticles modified glassy carbon (an electrocatalyst of the oxidative process), its electrochemical oxidation is negligible at an applied potential of 0.0 V in neutral medium, and its presence will definitely not affect the electrode response.

Comments 6: Long-term stability is assessed on two electrodes over three months with no error bars and no surface re-characterization.

Response 6: Authors appreciate the reviewer’s comment. Error bars were added to the Figure 6.

The electrochemical surface area, ECSA, was determined periodically by redox probe method, as it is described in the MS. It was found that after more than 3 months of use/storage, the modified electrode surface decreased by 9.5-9.7 %, which is in a good agreement with the remaining activity, Fig.6. One can hypothesise that the loss of activity is due to the ECSA decrease due to catalyst poisoning during measurements, or its departure from the surface.

Comments 7: For hair dye oxidant, the dilution protocol is described, but density, mass fraction assumptions, and propagation of uncertainty to the reported 5.91% are not provided.

Response 7: The authors thank the reviewer for this typically analytical question. Two different mass fractions were used for the determination: 3% and 6 % w/w of the developer, as well, as 3% w/v of the pharmaceutical product. The density of the latter is nearly 1.01 g/cm³ at room temperature, whilst the 20 Vol developer has a density of 1.04 g/ml. All these were taken into account upon calculation. The propagation of uncertainty is reported to be 5.91 ± 0.05 %. (line 321 - 322).

Comments 8: EDS is conducted at 5 kV with qualitative statements about surface oxides. At this accelerating voltage and on nanostructures, quantification is uncertain.

Response 8: We agree with the reviewer regarding the uncertainty in quantification. When it comes to light elements like oxygen, quantification is significantly hindered not only by the acceleration voltage but also by its low molecular weight, as the measurement error is much higher for light elements. We hypothesise that the trace amounts of oxygen detected by EDS are due to O₂ chemisorbed on Rh, which is a common condition for all noble metals.

Comments 9: The comparison table mixes different electrolytes, pH, potentials, and electrode types, which complicates claims of superiority, and some entries are not strictly cosmetics.

Response 9: Authors understand the viewpoint of the reviewer, and appreciate their comment. Indeed, hydrogen peroxide analysis can be performed in a wide spectrum of real life situations, and the choice of the analytical technique is dependent to a large extent on the concentration range of the analyte. For example – UV-spectrometry at 240 nm is fine for H₂O₂ at milimolar levels, but not suitable for micromolar range. Titration can be done at higher than milimolar concentrations etc. The Table summarizes different electroanalytical approaches for H₂O₂assay under a variety of conditions. In some of the examples, the electrode modifier manifests the highest activity under the specified conditions, i.e. Table 1 compares the operational parameters of different sensing platforms under specifically optimised conditions. As it can be seen, the acidic medium is in use either when the protonated form of the modifier is more active – e.g. the poly-L-lysine, row 2, or when the composite might be damaged at neutral medium – RGO/CuFe₂O₄/CPE catalyst will be destroyed at pH = 7.0 due to the formation of iron hydroxides.

In our case, we have demonstrated a combination of: a simple preparation procedure, low energy expenditure, practically interference-free response, minimum sample pre-treatment, good sensitivity, stability, reproducibility, reasonable detection limit and linear dynamic range. Moreover, the analysis can be done under mild reaction conditions-neutral medium and room temperature.

Comments 10: The text argues that oxygen reduction starts around −0.4 V, so measurements at −0.1 V are interference-free. This may be true for bare GCE, but catalytic Rh may shift kinetics. No oxygen-purged control is shown.

Response 10: Authors thank the reviewer for the comment. In our previous studies, even on excellent electrocatalysts of oxygen reduction reaction – e.g. Pt, Pd, Au, the ORR started not earlier than at -0.3 V vs the same reference electrode, and in neutral medium.

Comments 11: Sensitivity is reported per geometric area, though an electroactive area is also reported. The basis for normalization is not explicit.

Response 11: Authors are thankful for the reviewer’s critical note and apologise for this technical oversight. The sensitivity value was corrected in the revised MS (lines 20, 243).

Comments 12: A light technical edit would improve precision. Examples: change “The data are fascinating because the increases in the current response…” to “The increased current and decreased ΔEp indicate improved electron-transfer kinetics.” Replace “active widely used oxidizing agent” with “widely used oxidizing agent.” Keep tense consistent in Methods (past) and Results (past for observations, present when stating interpretations).

Response 12: Authors are grateful for the reviewer’s helpful suggestions. All recommendations aiming to improve the readability of the MS text, were accepted with deep gratitude. The English was improved by a professional interpreter.

Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

Comments and Suggestions for Authors

Thank you for considering comments and suggestions. The manuscript has been improved. I recommend the publication of the revised version.

Author Response

Comments of Reviewer 3 to the authors:

Thank you for considering comments and suggestions. The manuscript has been improved. I recommend the publication of the revised version.

Authors' response:

Authors gratefully appreciate the positive and highly supportive comments that the quality of the work is substantially improved. Thank you for your valuable feedback on the manuscript.

Comment from the Editor to the authors:

Dear authors,

Regarding the comments of Reviewer 3 related to reversibility of electrochemical processes at the GC surface and looking to voltammograms shown in Fig.2, it is difficult to “observe” an irreversible reaction particularity for the bare GC surface.
Other parameters besides the peak-to-peak potential separation (ΔEp), such as the anodic and cathodic peak current intensity ratio, are also an indicator in kinetics studies. Thus, I recommend the authors to provide a better explanation on this subject after careful reading of the publications:
- A Practical Beginner’s Guide to Cyclic Voltammetry, Noémie Elgrishi, Kelley J. Rountree, Brian D. McCarthy, Eric S. Rountree, Thomas T. Eisenhart, and Jillian L. Dempsey*, A Practical Beginner’s Guide to Cyclic Voltammetry, J. Chem. Educ. 2018, 95, 197−206, DOI: 10.1021/acs.jchemed.7b00361.
- An Introduction to Cyclic Voltammetry, Gary A. Mabbott, Journal of Chemical Education, Volume 60 Number 9 September 1983.
We look forward to hearing from you.
Kind regards,
José Ribeiro

Authors' response to Editor's comments:

Dear Prof. Ribeiro,

First of all, thank you very much for the recommended reading! The excellent work of J. Dempsey and co-workers is a part of our suggested readings in the Electroanalysis course. There is no discrepancy between our explanation and the electrochemical reversibility criteria discussed in the paper. The Ip,a-to Ip,c ratio is not a universal indicator for the electrochemical reversibility, due to its strong dependence on the reaction mechanism. In the case of a single-step one-electron redox process, as it is in our case, the electrochemically quasi-reversible reactions are characterised by a ΔEp which is greater than 60 mV, but lesser than 200 mV, while still conserving the “duck shape” of the cyclic voltammogram.

Irreversible behaviour is considered when the electron transfer is slow, leading to a significant separation between the cathodic and anodic peak potentials (ΔEp >200 mV) in the cyclic voltammogram. One of the peaks, e.g., the oxidative one, might be small or absent because the reduced product undergoes a subsequent irreversible chemical reaction that consumes it.

In our case, no subsequent reactions take place.

All the information discussed above we learned from the works of E. Laviron in previous studies, when determining the heterogeneous electron transfer rate constant, k_s, and from the book of Allen Bard and Larry Faulkner, listed below:

A.J. Bard, L.R. Faulkner, “Electrochemical Methods – Fundamentals and Applications, Second Edition; J. Wiley and Sons, 2001;
E. Laviron, J. Electroanalytical Chemistry vol. 39 (1972) 1;
E. Laviron, J. Electroanalytical Chemistry vol. 52 (1974) 355;
E. Laviron, J. Electroanalytical Chemistry vol. 52 (1974) 395;
E. Laviron, J. Electroanalytical Chemistry vol. 63 (1975) 245;
E. Laviron, J. Electroanalytical Chemistry vol. 101 (1979) 19;

Moreover, the Reviewer 3 has accepted our comments in round 1 and recommended the publication of the revised version.

Based on the above, we consider that any changes in the MS text concerning the reversibility part could be misleading to the readers.

We would like to draw your attention to the fact that the Reviewer 3 is the only referee who insisted on improving the English. At round 1 revision we have used the services of professional interpreter. Now, according to Reviewer 3 the English is fine and does not require any improvement.