State-of-the-Art Optical Microfiber Coupler Sensors for Physical and Biochemical Sensing Applications

Round 1

Reviewer 1 Report

Dear Authors,

in my opinion, the manuscript titled: "State-of-the-art. optical microfiber coupler sensors for physical and biochemical sensing applications"
is not sufficient to be accepted for publication as a review in the BIOSENSORS journal.
My mine comments focus on the topic which is too broad for the review, and as the consequence, the review lacks many important items.
Secondly, this review is based on many very old papers.
Furthermore, this manuscript includes many self-citation.

Author Response

Dear reviewer,

Thank you for your careful review with regard to our manuscript “State-of-the-art optical microfiber coupler sensors for physical and biochemical sensing applications”.

Our manuscript systematically reviews the background, principle, fabrication and recent applications of microfiber coupler based physical and biochemical sensors. As a microfiber based device, microfiber coupler connects fiber optics and nanotechnology and realizes enhanced light-matter interaction, which is a benefit for optical sensing applications. We focus on the versatile sensing capability that includes biosensing of the microfiber coupler combining with other techniques, which considered as a “broad topic”. We have highlighted the remarkable advances and have pointed out the challenges and opportunities of this technique, hoping it is helpful for future work.

In the part of “operation principle”, we have cited a few old but classic papers, such as Ref. [1] and Ref. [2] trying to present the working fundamentals behind the sensing application. Also, to exhibit the development of microfiber coupler based sensors, we have cited some early works, such as Ref. [3]. Besides, the manuscript is built up with a lot of reference from recent advances of microfiber coupler sensors, to show the potential for high performance physical and biochemical sensing applications. With the combination of classic papers and recent advances, future readers may have a relatively comprehensive understanding of microfiber coupler sensors.

As you pointed out the self-citation, this may be a misunderstanding. The name abbreviation of one of the co-authors named Yuanfang Zhao is Zhao Y, which is the same as another researcher’s named Yong Zhao. Yong Zhao is of the Northeast University, Shenyang, China, working with fiber optic sensors. The related literatures are [4] and [5].

Thank you for your careful review and comments on our manuscript again.

Yours sincerely,

Maolin Dai on behalf of all authors

[1] Szu-Wen, Y.; Tzong-Lin, W.; Cheng Wen, W.; Hung-Chun, C. Numerical modeling of weakly fused fiber-optic polarization beamsplitters. Part II: the three-dimensional electromagnetic model. J. Lightwave Technol 1998, 16, 691-696.

[2] Danh, T.; Kee, K.; Sang, S. Single-mode fiber directional couplers fabricated by twist-etching techniques (stabilization). IEEE J. Quantum Elect 1981, 17, 988-991.

[3] Henning, M. Fiber optic couplers for protein detection. Sens Mater 2002, 14, 339-345.

[4] Zhao, Y.; Peng, Y.; Chen, M.-q.; Xia, F.; Tong, R.-J. U-shaped microfiber coupler coated with polyvinyl alcohol film for highly sensitive humidity detection. Sens. Actuators A Phys 2019, 285, 628-636.

[5] Peng, Y.; Zhao, Y.; Hu, X.-g.; Chen, M.-q. Humidity sensor based on unsymmetrical U-shaped twisted microfiber coupler with wide detection range. Sens. Actuators B Chem 2019, 290, 406-413.

Author Response File: Author Response.pdf

Reviewer 2 Report

The manuscript „State-of-the-art optical microfiber coupler sensors for physical and biochemical sensing applications" presents a review on sensors employing as a detection element a microfiber coupler (MFC). The review is enough comprehensive and it can attract the interest of researchers looking for new sensitive optical sensors. However, on my view, this review is not suitable for the publication in Biosensors because the part dealing with biosensors, as chemical sensors implementing biotransducers, is very short in comparison with parts dealing with physical and chemical sensors. Thus, I strongly recommend to submit this review e.g. into a journal Sensors (MDPI). Even in such a case, the manuscript needs some technical modifications such as:

1. It is necessary to describe what fiber output signals are used in sensors (shift or intensity of spectral shift)
2. Abstract has to generally summarize the article content, i.e. the MFC fabrication, employment in physical sensors of refractive index temperature, magnetic fields, ….. to inform a reader what he can find in the article.
3. Instead of biochemical sensor use a term biosensor which is defined by IUPAC.
4. It is not necessary to describe the traditional optical fiber (rows 28-30). They are well known.
5. There are some inconsistences in the manuscript. Thus, it is not generally true that the coupler waist or tip has the subwavelength dimension. Light is not transmitted only in evanescent waves but in modes with very strong evanescent waves.
6. Tab. 1 is  not clear. It is not explained what is the sensing object and sensing materials. Usually, the detected parameter and optical transducer are used, instead.
7. In row 112 the term moldable is not correct, because fiber material is only softened to a viscosity enabling the elongation and sintering.
8. Explain please, all abbreviations when using them for first time (e.g. row 178 and PMF).
9. I would expect some short description how xerogel detection membranes were applied on MFCs in part Chemical sensors.
10. In part Biosensors all techniques for the biotransducer immobilization should be reviewed not only the layer-by-layer self-assembly technique.
11. I would appreciate some evaluation of the sensor stability, robustness, mechanical properties, etc. The comparison e.g. with label free fiber-optic biosensors would be very useful.

I strongly recommend to carefully check English of the manuscript, e.g. using present perfect tense (e.g. row 596, "high specificity and fast response time have also investigated") instead of present perfect passive ("high specificity and fast response have also been investigated") , using of archive terms such as "mayhap", etc.   Check please, copyrights for the images taken from original papers.

Author Response

Dear reviewer,

Thank you for your careful review and constructive suggestions with regard to our manuscript “State-of-the-art optical microfiber coupler sensors for physical and biochemical sensing applications”. Our manuscript tries to systematically review the background, principle, fabrication and recent advances of microfiber coupler based physical and biochemical sensors. As a microfiber based device, microfiber coupler connects fiber optics and nanotechnology and realizes enhanced light-matter interaction, which is a benefit for optical sensing applications. We focus on the versatile sensing capability that includes biosensing of the microfiber coupler. Compared to physical sensing application, there are less investigations of microfiber coupler in the biosensor field, however, there is a chance that it can be developed as high-performance label free biosensors in future work. We have highlighted the remarkable advances and have pointed out the challenges and opportunities of this technique, hoping it is helpful for future work.

Based on your comments, we have improved our manuscript point-by-point and have mentioned the respective places in the revised manuscript in red color wherein such changes have been made. In the next page, the amendments for each suggestion are provided to clearly show the changes.

Thank you for your careful review and comments to help us improve our manuscript again.

Yours sincerely,

Maolin Dai on behalf of all authors

Suggestion 1

It is necessary to describe what fiber output signals are used in sensors (shift or intensity of spectral shift)

Response 1

In revised manuscript, we have added the content regarding to the modulation of signals in row 172. The added content as follows:

When the MFC has developed as sensors to detect ambient changes or biomolecule, both the intensity modulation [24,68] and wavelength modulation [23,55] can be employed to estimate the environmental parameters. However, the intensity modulation mayhap affected by the stability of the sensing system, such as the performance of light source or ambient disturbance. As a consequence, tracking of wavelength shift is more suitable for high-performance sensing applications, which has used in most MFC sensors investigations.

Suggestion 2

Abstract has to generally summarize the article content, i.e. the MFC fabrication, employment in physical sensors of refractive index temperature, magnetic fields, ... to inform a reader what he can find in the article.

Response 2

We have added the related content in abstract part (row 19) in the revised manuscript.

In this review, the fabrication and operation principle of MFC are elaborated, recent advances of MFC based sensors for scientific and technological applications have comprehensively reviewed.

Suggestion 3

Instead of biochemical sensor use a term biosensor which is defined by IUPAC.

Response 3

We have replaced the term “biochemical sensor” with “chemical sensor and biosensor”. The changed places are in row 20, 22, 484, 489 and 657 in the revised manuscript.

Suggestion 4

It is not necessary to describe the traditional optical fiber (rows 28-30). They are well known.

Response 4

We have removed the related content about traditional optical fiber in the revised manuscript.

Suggestion 5

There are some inconsistences in the manuscript. Thus, it is not generally true that the coupler waist or tip has the subwavelength dimension. Light is not transmitted only in evanescent waves but in modes with very strong evanescent waves.

Response 5

When the diameter of microfiber fiber reaches hundreds of nanometers or several micrometers, we call the microfiber has a subwavelength diameter. Usually, in the cited literatures, the diameters of MFCs are several micrometers. Therefore, we call the MFC has a subwavelength diameter. And we have changed the expression in several places, to explicit the characteristic of “light is transmitted in modes with very strong evanescent waves”.

In row 41, “As a consequence, … near the fiber surface” has been changed to “When the diameter…light-matter interaction”.

In row 61, we have added the content “a large portion of” in the revised manuscript as mentioned below.

The subwavelength diameter of the coupler waist region results in a large portion of evanescent field, which offers the incubator for mode coupling of the odd supermode and even supermode.

Suggestion 6

Tab. 1 is not clear. It is not explained what is the sensing object and sensing materials. Usually, the detected parameter and optical transducer are used, instead.

Response 6

We have changed “sensing object” to “detected parameter” and “sensing material” to “optical transducer” in Table 1.

Suggestion 7

In row 112 the term moldable is not correct, because fiber material is only softened to a viscosity enabling the elongation and sintering.

Response 7

We have changed the phrase “makes it moldable” to “makes it softened and able to be stretched and elongated”.

Suggestion 8

Explain please, all abbreviations when using them for first time (e.g. row 178 and PMF).

Response 8

I have explained the abbreviation of PMF in row 188, and have deleted the explanation of PMF in row 403. Thank you for your careful review.

Suggestion 9

I would expect some short description how xerogel detection membranes were applied on MFCs in part Chemical sensors.

Response 9

The gel is coated utilizing the dip coating method, the following content has been added the operation description in row 500.

For those gel-coated MFCs, the gel is immobilized on the surface of microfiber by the dip-coating method. For this method, a motor controlled translation stage has employed to pass a drop of the gel solution through the fiber surface, a single layer of coating has formed after each operation. Therefore, the thickness of the gel coating can be well controlled.

Suggestion 10

In part Biosensors all techniques for the bio-transducer immobilization should be reviewed not only the layer-by-layer self-assembly technique.

Response 10

In previous works of MFC biosensors, the bio-receptors are immobilized directly on fiber surface by physical absorption method or covalent coupling method. For physical absorption method, the bio-receptors are directly immobilized on fiber surface via physical forces. The operation is simple and the bio-activity is slightly impacted. However, the stability is not good. For covalent coupling method, the layer-by-layer self-assembly is used to form the bio-transducer layers in previous works. The bio-receptor is bonded to the bio-transducer via covalent coupling. Therefore, we elaborate this method with example. We have added the following content in row 581 to express these two immobilization methods.

For the immobilization of bio-receptor, physical absorption method and covalent coupling method have employed in previous works of specific label-free MFC biosensor. For physical absorption method, the biomolecules are absorbed on the fiber surface by physical forces, such as a polar bond, van der Waals forces, hydrogen bond and so on. As is a kind of physical absorption with the merits of easy operation and less influence on biomolecule, the treatment of fiber surface is not needed, which is established in Ref. [75]. For covalent coupling method, the fiber surface has treated to immobilize the bio-transducer. Then the bio-receptor is coupled to the bio-transducer to realize specific recognition of analyte. In this way, the bio-activity of biomolecules may be affected by the reaction between the groups. The bio-transducer is immobilized by layer-by-layer self-assembly method in previous works [77, 80].

Suggestion 11

I would appreciate some evaluation of the sensor stability, robustness, mechanical properties, etc. The comparison e.g. with label free fiber-optic biosensors would be very useful.

Response 11

Thanks for your suggestions. We have added the content to discuss about the stability, regeneration, and reproducibility of MFC biosensor. The added content is in row 635.

The stability, regeneration, and reproducibility are key factors for a practical biosensor, especially for MFC based biosensors since MFC is easily affected by ambient disturbance. In Ref. [80], the real time response curve has measured to show the reaction kinetics. The dip wavelength before and after bio-reaction are constant, that shows the stability of the sensor. In Ref. [77], the biosensor is utilized for 8 cycles to show the stability, reusability and regeneration. After 8 cycles, the sensing performance without significant losses has obtained. Also, from the real time response transmission spectrum shown in Figure.12 (c), the stability is provided by valuing the response intensity before and after bio-reaction. The reproducibility is investigated in Ref. [75]. 15 MFC samples have fabricated, and the sensing performances for each sample are slightly different. These mayhap caused limited precision in the fabrication.

Author Response File: Author Response.pdf

Reviewer 3 Report

As a review paper, it is propriety written, as a Reviewer, I have few questions which do not decrease the value of this paper but should be considered by the authors

1. Figure 2 should be extended to other methods of manufacturing MFC or should be deleted, it is very well described in the literature.
2. Equation 1 and description, authors give only two RI, depending on the scale of tapering in in process of manufacturing MFC it should be using a n_eff (in same cases core and cladding still exist and fulfill the propagation role) it should be changed
3. In all article, authors describe different use/application of MFC there is no information about fibers that were used for fabrication, such information  should be placed (SMF, MMF, working in what wave range)
4. Equation 8 is not clear described: ∆???f is described but (????) is not described, for people who are not connected with this field it can make some problems it should be extended, all equations should be described in detail
5. This same with equation 9, is the ∆???f that same as in equation 8
6. Line 392 - what kind of PMF fiber was used
7. References 43, 63,63,64 reference to a co-author, also cited in the article

Author Response

Response Letter

Dear reviewer,

Thank you for your careful review and constructive suggestions with regard to our manuscript “State-of-the-art optical microfiber coupler sensors for physical and biochemical sensing applications”. Our manuscript tries to systematically review the background, principle, fabrication and recent applications of microfiber coupler based physical and biochemical sensors. As a microfiber based device, microfiber coupler connects fiber optics and nanotechnology and realizes enhanced light-matter interaction, which is a benefit for optical sensing applications including label free bio-sensing. We have highlighted the remarkable advances and have pointed out the challenges and opportunities of this technique, hoping it is helpful for future work.

Based on your comments, we have improved our manuscript point-by-point and have mentioned the respective places in the revised manuscript in red color wherein such changes have been made. In the next page, the amendments for each suggestion are provided to clearly show the changes.

Thank you for your careful review and comments to help us improve our manuscript again.

Yours sincerely,

Maolin Dai on behalf of all authors

Suggestion 1

Figure 2 should be extended to other methods of manufacturing MFC or should be deleted, it is very well described in the literature.

Response 1

We have removed Figure 2.

Suggestion 2

Equation 1 and description, authors give only two RI, depending on the scale of tapering in in process of manufacturing MFC it should be using a n_eff (in same cases core and cladding still exist and fulfill the propagation role) it should be changed.

Response 2

Actually, in Equation 1 and description, the n₂ describes the effective refractive index of microfiber. I have changed the n₂ to n_eff to avoid misunderstanding.

Suggestion 3

In all article, authors describe different use/application of MFC there is no information about fibers that were used for fabrication, such information should be placed (SMF, MMF, working in what wave range).

Response 3

In the previous works, all the MFCs is made of single mode fiber (SMF). Due to the ultra-broad band operation characteristics, the MFC can be used in common wave range. It depends on the light source.

I have added the content “In general, the MFC is made by single mode fiber (SMF)” in row 99.

Suggestion 4

Equation 8 is not clear described: ∆???? is described but (????) is not described, for people who are not connected with this field it can make some problems it should be extended, all equations should be described in detail.

Response 4

The ∆_???? is the effective refractive indices difference between two supermodes. This has been described in row 256 in the revised manuscript. The term “?_???” is a writing mistake.

Suggestion 5

This same with equation 9, is the ∆?_??? that same as in equation 8.

Response 5

The ∆_???? is the same as equation 8. And we have changed slightly the description in row [285-288]

Suggestion 6

Line 392 - what kind of PMF fiber was used.

Response 6

In the literature, the author did not mention the specification of the PMF.

Suggestion 7

References 43, 63,63,64 reference to a co-author, also cited in the article.

Response 7

We have not cited any reference to any of co-authors. As you pointed out the self-citation, this may be a misunderstanding. The name abbreviation of one of the co-authors named Yuanfang Zhao is Zhao Y, which is the same as another researcher’s named Yong Zhao. Yong Zhao is of the Northeast University, Shenyang, China, working with fiber optic sensors. The related literatures [63], [64] are listed as follows.

[63] Zhao, Y.; Peng, Y.; Chen, M.-q.; Xia, F.; Tong, R.-J. U-shaped microfiber coupler coated with polyvinyl alcohol film for highly sensitive humidity detection. Sens. Actuators A Phys 2019, 285, 628-636.

[64] Peng, Y.; Zhao, Y.; Hu, X.-g.; Chen, M.-q. Humidity sensor based on unsymmetrical U-shaped twisted microfiber coupler with wide detection range. Sens. Actuators B Chem 2019, 290, 406-413.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Dear Authors,

I am satisfied with the improvements which you introduced into the manuscript.