Sensors for Continuous Measuring of Sucrose Solutions Using Surface Plasmon Resonance

Round 1

Reviewer 1 Report

A sensor based on surface plasmon resonance (SPR) and optical fibers to determine the concentration of sucrose has been proposed in this manuscript. The sensor has been made with a hemispherical prism, a layer of MgF2 and another of Ag or Al with the Kretschmann configuration and they are modulated in intensity. Authors have carried out a numerical simulation with these sensors to show how they can detect small changes in the refractive index depending on the concentration of sucrose where the device is immersed. The sensors have an alarm system which is triggered when the sucrose concentration is insufficient or excessive. The author has made many changes to this article, bringing it up to publication level. I suggest that this paper can be published in Applied Science.

Author Response

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Reviewer 2 Report

The authors propose two new sensors based on surface plasmon resonance (SPR) and optical fibers to determine sucrose concentration in products such as beverages, honey, condensed milk, etc., in real-time during the fabrication process or when the product has been manufactured. My suggestions to the technical parts are as follows:

(1) Since the SPR owns the highest resolution, the authors only claim 10^−4 RIU. What are the reasons?

(2) WinSpall software package was used to calculate the multilayer systems through Fresnel equations for surface plasmon analyses. The wave optics is not accurate enough for SPR simulation. Besides the validation, the finite difference time domain (FDTD) is a must for demonstration.  

(3) The authors claim that “Our proposed sensor works with only one laser, only one photodiode, and only two optical fibers, for this reason, it only needs one hole in the upper side of the tank or container (if the tank has to be closed) for the solution we want to measure.” I do not agree with any SPR novelties the authors proposed. The one wavelength and one detector are only selected to be operated in one specific parameter under resonance or non-resonance of SPR. If the different SPR layers own specialties, the FDTD must be executed for verifications. 

Author Response

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Reviewer 3 Report

In this manuscript, the authors have performed a sucrose sensor based on SPR. They have performed two substrates, checking the plasmon shifting depending on the sucrose concentration. The manuscript is well written and easy to follow. I just have a few comments:

• How many times did they perform the experiments? Figures 3, 4, 6 and 7 have to include the error.
• How reproducible are the measurements?
• How many uses have the material without losing the SPR properties?

Author Response

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Round 2

Reviewer 2 Report

I was asking about the FDTD simulation because of the penetration depth and propagation loss. The low sensitivity may come from these issues. The SPR is a mature technology, and I cannot see that the authors demonstrate any novelties in this manuscript.

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

In my opinion, in the introduction too much space is devoted to description of the role of sugar in a food industry instead of consideration of different optical techniques for sugar concentartion measurement. I think, there are plenty of optical sensors based on frustrated total internal reflection os surface plasmon resonance which can be used for continuous measurements of sugar concentartion.

The development of the sensor itself is described very poorly. The choice of materials and thicknesses of layers is not justified, i mean, why namely MgF2 and Ag/Al were choosen and how did the authors determine its thicknesses. 

Any experimental data is absent in the paper.

The usability of working angle 87 degrees is questionable.

Reviewer 2 Report

The authors present a numerical model of two SPR configurations that allows measuring the sucrose level. In this study, the authors present a basic analysis of their configurations and calculate some typical parameters such as sensitivity and resolution. However, I believe that the quality of this work is low. For this reason, I suggest rejecting the submission. In addition, I include some comments and suggestions that could help to improve the quality of this work:

1. I believe that the abstract could be improved. Please try to include the novelty of the manuscript and the most important results like sensitivity, LOD, Resolution, etc.
2. The manuscript presents an alternative to measuring the sucrose concentration based on a plasmonic sensor. However, the authors do not include a revision of the state of art about refractive index sensors based on SPR. Please include a brief revision of different configurations. I suggest the following manuscripts:
   10. 1016/j.proche.2016.07.020
   11. 3390/s20072111
   12. 1016/j.aca.2009.10.003
   13. 3390/photonics8050155
   14. 3390/bios10070077
   15. 1007/s11468-021-01477-8
3. It is not possible to understand what the novelty of the work is, in the literature configurations based on prisms have been used to measure changes in concentration in liquids, several experimental studies have even been done.
4. Please try to improve the quality of Figure 1. I suggest including colors and increasing the resolution of this image.
5. The authors do not include details about the numerical analysis. Please include the software or the mathematical model. In fact, the used methodology is unclear.
6. The authors include a brief description of each component on page 4. However, the authors need to include more information about the characteristics of each component. For example, what is the responsivity of the detector? What are the characteristics of the optical fiber?, etc.
7. Why did you need optical fiber in this configuration?
8. Please improve the discussion and analysis of each result. For example, the results illustrated in Figures 2, 3, and 4 lack a discussion. Please argue your results.
9. Please improve the discussion and analysis of each result. For example, the results illustrated in Figures 5, 6, and 7 lack a discussion. Please argue your results.
10. Please calculate the Limit of Detection (LOD) of your configurations.
11. Please include more details about the fabrication of this configuration, and how this configuration could be used in real applications.
12. Please include a comparison table between your sensors and other previously reported.
13. The authors claim that the proposed sensor could be used in real-time, however, they do not offer any support of this into the manuscript. Then, they need to modify the abstract and conclusions sections because they generate false expectations.
14. What are the losses when you try to couple the light between the optical fiber and the prism? Please include a section and present a numerical model to quantify this problem.

Reviewer 3 Report

The authors proposed a solution for sensors by using surface plasmon resonance.

1. I cannot get any novelty in this article. The main structure were composed a hemispherical, a layer with MgF2 and another layer of metal. However, the structure for sensor by the concept of SPR were already proposed several times in past research. such as [1]. 

[1] T. J. Yen, ACS Apppl. Nano Mater., 2021, 4, 2211-2217.

2. The quality of figure should be improved.

3. English writing should be improved.

4. The authors should compare the results to other previous articles.