Methylene Blue Optical Fiber Sensor Filled with Calcium Alginate Hydrogel

Round 1

Reviewer 1 Report

Authors designed, fabricated and tested an optical fiber sensor based on the Fabry-Perot cavity optical structure for the concentration measurement of the water contaminant methylene blue (MB). The sensor builds on the properties of the calcium alginate hydrogel produced by exposing sodium alginate to calcium chloride solution . Methylene blue adsorption on calcium alginate changes its swelling properties and refractive index resulting in changing the cavity length and thus leading to the interference spectrum drift. In a series of tests, authors investigate the interference spectrum stability in time, response of the spectrum change (peak wavelength shift) to the changes in MB concentration, influence of cavity length on the sensor sensitivity, influence of the sodium alginate concentration used for calcium alginate film preparation on response characteristics and the dynamic response of the sensor to the change in MB concentration. Properties of the developed sensor are then compared with properties of other MB sensors described in literature.

Comments:

- Description of the sensor fabrication and tests carried out with the new sensor are pretty exhaustive and present a good basis for comparison with other sensors or measurement methods for MB contamination. An important piece of additional information would be the lifetime of a fabricated sensor when measuring contamination, in both continuous and intermittent mode, which is not mentioned in the manuscript. Could authors please supply that?

- L15, L286: are the five decimal places in 2.45869 really relevant?

- In the reference list, DOI for the Ref. 34 is not correct (leads to a Soviet paper from 1974 concerning properties of garnets)

- In the Table 2 comparing MB sensors, I'd suggest to include the paper of Hu et al., Sensors 2020, 20(19):5506, doi:10.3390/s20195506, further the paper of Al-Gethami et al., Nanomaterials 2022, 13(1):97, doi:10.3390/nano13010097, and the paper of Sadrolhosseini et al., Optics Communication 2023, 529, doi:10.1016/j.optcom.2022.129057, all concerning sensors measuring MB in water.

Quality of English language of the paper is quite high (as far as I can judge it), there are only some missing words (e.g., L121: the verb is missing, probably prepared/manufactured), some typos (should be: L132: groups, L196: length) and some not-so-suitable prepositions/verbs (e.g., L159: monitored for 2000 s; L234: maybe "attained" instead of "got"), but these instances are very few. Unfortunately, I'm not a native English speaker.

Author Response

Thans a lot for you suggestion. The attachment is the response letter. Please check it.

Author Response File: Author Response.docx

Reviewer 2 Report

Title: Methylene Blue Optical Fiber Sensor Filled by Calcium Alginate Hydrogel

In my honest opinion, this paper has many applications in both the educational and industrial contexts. The article is well structured in a classical sense (introduction, materials and methods, results and conclusion). In addition, it has both a good scientific sound and easy reading due to a correct gradation of bibliography, concepts, experiments, and analysis of results. However, I have two major comments that I would like the authors to clarify.

[MAJOR COMMENTS]

[MAJOR COMMENT 1 -lines 134-142-] Calibration of the device described (micron Optics SM125) is key since its results and conclusions may vary if the measurements are not true. Were the calibrations for this experiment carried out by you? Did any external service perform them? How? Do you have a calibration certificate in accordance with the regulations? Please also keep in mind that the original factory calibrations are lost over time if the device is not new.

[MAJOR COMMENT 2 – Section 3.4.-] Why was the dynamic analysis performed with the 2% concentration of sodium alginate? Perhaps, the 2% obtained in the previous section (static analysis) is not optimal for a dynamic analysis. Why do you think so?

[MINOR COMMENTS/SUGGESTIONS]

[line 43] 2.55 x 10¹². Please add units. 10¹²? It is 10^-12.

[line 58] The coefficient of determination is denoted as R² and not as R as it erroneously appears in your manuscript.

[line 99] Please add units (in parenthesis) of the n_e.

Please put the units of the magnitudes of the formulas (in parentheses) in your manuscript.

[line 122] Adsorption or absorption?

[line 177] Sensor. S capital letter.

ENGLISH STYLE

Minor editing of English language required.

OVERALL RECOMENDATION

Accept after minor revision.

Minor editing of English language required.

Author Response

Thans a lot for you suggestion. The attachment is the response letter. Please check it.

Author Response File: Author Response.docx

Reviewer 3 Report

Dear Authors,

I have carefully read your document. It is quite interesting although, I've quit e a few questions and suggestions attached to the document. Most of them are to improve the quality of your work and others that require further explanation or analysis.

Comments for author File: Comments.pdf

Minor issues

Author Response

Thans a lot for you suggestion. The attachment is the response letter. Please check it.

Author Response File: Author Response.docx

Round 2

Reviewer 3 Report

Dear Authors,

I've read your revision. It seems to be improved form the previous one. However, I still ahve a few issues with you i regard of the quality of the Figures caption. i.e. Fig. 1, the caption says nothing relted to the image but a sort of juvenile one. In Fig. 3. what sort of info can we gather form this caption?

In your Eq(1), you named each one of the variables and in here, you point out that ne is the calcium alginate refractive index, what is the value for it in your analysis?

In section 2.3 from line 137 up to 143 the info that you're portraying in here is useless and make your research work looks cheap.

 "The light emited by the SM125" you mean a set of wavelengths if so, which ones?. The paragraph in lines 151 up to 163 is redundant. You use quite a few time the "methylene blue" that seems an odd undegrad sintaxys. The same occurs at 178-188.

Figures 4 & 5 offer little to non information. Whole set of waves are, I presume, shown lots of info but which one? a better way to display it is through a box diagram in which you can state the main differences among them based on concentration, wavelength, etc.

In Fig. 6 at which wavelength is was tested?

The paragraph in lines 226-250 need to be improved by far. You can say something in fewer lines than those.

In Fig.7 can you state what is the wavelength change for each one of the concentrations due to those are not clearly specified on either the graph or text.

Finally, why it is the opimun at 2%?, can you explain what is happening on your sensors from the point of view of physics and chemistry because there is no explanation on the text. 

In here you cannot use slang or Street-based English language. Ergo, you should avoid to use expresions such as got along the text.

In line 218, there is a masive typo.

Author Response

Thans a lot for you suggestion. The attachment is the response letter. Please check it.

Author Response File: Author Response.docx